NBSIR 88-3090

A 1 1 1 0 2 flSBMtn

NAT'i INST OF STANDARDS & TECH R.I.C.

A1 11 02853409

BREAK JUNCTIONS I

Interim Report

October 1 , 1 986 to September 31 , 1 987

John Moreland, L F. Goodrich, J. W. Ekin, T. E. Capobianco,

and A. F. Clark

Electromagnetic Technology Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303-3328

MAY 1988

Stimulating America s Progress

1913-1988

-QC100

• U56

Work performed under Office of Naval Research contract N000-86-F-0109

NBSIR 88-3090

BREAK JUNCTIONS I

Interim Report

October 1 , 1 986 to September 31 , 1 987

John Moreland, L F. Goodrich, J. W. Ekin, T. E. Capobianco,

and A. F. Clark

Electromagnetic Technology Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303-3328

Work performed under Office of Naval Research contract N000-86-F-0109

MAY 1988

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
. —* >* > '

'•

I .

' •• • '

.

.

.
FOREWORD

This report summarizes the work performed during FY 87 by the Superconductor

and Magnetic Measurements Group at the National Bureau of Standards using

break junctions for tunneling spectroscopy of superconductors. It is intendednot only as an NBSIR but as the FY 87 interim report to ONR as well, since the

work was primarily funded by the Office of Naval Research.

Break junctions belong to a relatively new class of tunneling junctions

that rely on the spacing of two conductors with a one -nanometer vacuum, gas, or liquid insulator. As the name "break junction" implies, this is done

within the fracture of a conducting material permitting tunneling spectroscopy

of freshly exposed bulk surfaces. As is the case for all vacuum tunneling

techniques, this eliminates the need for thin-film depositions of samples or tunneling barriers traditionally employed in tunneling experiments. The break

junction technique is a result of recent developments at NBS on methods for

measuring the superconducting properties of bulk materials. Primarily our efforts have centered around multifilamentary superconductors for magnets to

be used for fusion, superconducting accelerators, and magnetic resonance

imaging. In particular, the break junction method was first used for

measuring the energy gap of Nb-Sn and Nb-Ti filaments extracted from the

matrix of a multifilamentary superconductor.

In this report we discuss recent improvements in the break junction

method and its uses for studying some of the more exotic superconducting

materials. Tunneling measurements of the superconducting energy gap function

and Josephson effect in the high-Tc materials La 1 85 Sr 0 15 Cu0 4 and YBa 2 Cu 3 Ov are presented.

CONTENTS

Page

FOREWORD iii

ABSTRACT vii

INTRODUCTION 1

ELECTRON TUNNELING MEASUREMENT OF THE ENERGY GAP IN A La-Sr-Cu-0

SUPERCONDUCTOR in Cryogenics 27, 227 (1987)

TUNNELING SPECTROSCOPY OF A La-Sr-Cu-0 BREAK JUNCTION: EVIDENCE FOR

STRONG COUPLING SUPERCONDUCTIVITY in Phys . Rev. B 35, 8711 (1987) 5

BREAK -JUNCTION TUNNELING MEASUREMENTS OF THE HIGH-TC SUPERCONDUCTOR

Y 1 Ba 2 Cu3 0x . § in Phys. Rev. B 35, 8856 (1987) 12

JOSEPHSON EFFECT ABOVE 77 K IN A YBaCuO BREAK JUNCTION in Appl. Phys.

Lett. 51, 540 (1987) 18

ELECTRON TUNNELING MEASUREMENTS IN LaSrCuO AND YBaCuO in High

Temperature Superconductors , edited by D . U. Gubser and M. Schluter,

Materials Research Society, Pittsburgh, 1987, pp . 73-75

ELECTRON TUNNELING MEASUREMENTS OF HIGH Tc COMPOUNDS USING BREAK

JUNCTIONS in Jap. J. Appl. Phys. 26., Supplement 26-3, 999 (1987) 29

ANOMALOUS BEHAVIOR OF TUNNELING CONTACTS IN SUPERCONDUCTING PEROVSKITE

STRUCTURES in Advances in Cryogenic Engineering (Materials) 34, edited

by A. F. Clark and R. P. Reed, Plenum, New York, 1988, pp . 625-632. . . .

PUBLICATIONS, PRESENTATIONS, AND PATENTS 46

APPENDIX - PATENT APPLICATION FOR METHOD AND APPARATUS FOR FORMING

MECHANICALLY ADJUSTABLE BREAK JUNCTIONS FOR ESTABLISHING AN ELECTRON

TUNNELING CURRENT 48

v

BREAK JUNCTIONS I

Interim Report

October 1, 1986 to September 31, 1987

John Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco,

and A. F. Clark

Electromagnetic Technology Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303-3328

Measurements of the tunneling current-voltage characteristics of

break junctions in conventional superconductors can be used to

determine their superconducting energy gap as a function of energy.

These results agree with those previously obtained using traditional

oxide tunneling barriers. Break junctions in some exotic superconductors, on the other hand, have anomalous current- voltage

characteristics compared to BCS predictions. Energy gaps and the

Josephson effect measured for the new high Tc materials YBaCuO

(Tc = 93 K) and LaSrCuO (Tc = 36 K) indicate that the samples are inhomogeneous with varying gap functions depending on the location

of the tunneling contact within the break junction fracture. Break

junction data for these materials are within the strong coupling

limits of BCS theory.

Key words: break junctions; electron tunneling; energy gap; high-Tc superconductors; LaSrCuO; YBaCuO; YBa2 Cu30y.g

Vll

INTRODUCTION

Electron tunneling has many uses in the field of materials science. We begin

with a short descriptive bibliography. There are several texts summarizing

developments over the last thirty years related to electron tunneling

phenomena. General overviews by Duke, [1] Hansma, [2] and Burstein and

Lundqvis t [3] outline various applications of the tunneling phenomena to

metals, semiconductors, superconductors, and surface adsorbates.

Electron tunneling between superconductors, in particular, is also reviewed by

several authors. Some introductions to tunneling spectroscopy and the

Josephson effect are presented by Scalapino and Rowell and McMillan in Parks.

[4]

Wolf [5] reviews some of the recent developments in tunneling

spectroscopy of superconductors including the proximity effect and its effect

on tunneling spectra and the superconductive energy gap function derived from

tunneling data.

The works mentioned above are mostly based on tunneling junctions fabricated

using a thin oxide tunneling barrier. The development of vacuum tunneling

methods has extended the uses of tunneling to include scanning capabilities

with atomic topographic resolution. The scanning tunneling microscope

invented by Binnig and Rohrer is reviewed in two volumes of the IBM Journal of

Research and Development. [6]

Recently application of vacuum tunneling junctions has been extended

to spectroscopy. In particular, the squeezable electron tunneling (SET)

junction invented by Hansma, Moreland, and Alexander [7] has been used to

study semiconductor band gaps and superconductor energy gaps. SET junctions

offer ultra-stable electromechanical systems for tunneling spectroscopy.

Break junctions were developed at NBS by Moreland, Ekin, and Clark [8] to

study superconductors in their native state without using thin film

depositions normally required for tunneling experiments. Both SET junctions

and break junctions have sufficient mechanical stability for measuring the

superconducting energy gaps. The patent application reproduced in the

appendix gives a detailed description of the break junction technique.

REFERENCES

[1] C. B. Duke, Tunneling in Solids . Academic Press, New York 1969.

[2] P. K. Hansma, Tunneling Spectroscopy . Plenum, New York, 1982.

[3] E. Burstein and S. Lundqvist, Tunneling Phenomena in Solids . Plenum. IVYork, 1969.

[4] R. D. Parks, Superconductivity . Deckker, New York, 1969.

[5] E. L. Wolf, Rep. Prog. Phys . 41, 1439 (1978).

[6] IBM J. Res. and Development 30, No. 4 and No. 5 (1986).

[7] J. Moreland and P. K. Hansma, Rev. Sci. Inst. .55 , 399 (1984).

[8] J. Moreland and J. W. Ekin, J. Appl. Phys. 5£, 3888 (1985).

1
ELECTRON TUNNELING MEASUREMENT OF THE ENERGY GAP

IN A La- Sr-Cu-0 SUPERCONDUCTORt

JOHN MORELAND and A. F. CLARK

ELECTROMAGNETIC TECHNOLOGY DIVISION

NATIONAL BUREAU OF STANDARDS

BOULDER, COLORADO

AND

H. C. KU| and R. N. SHELTON

AMES LABORATORY and PHYSICS DEPARTMENT

IOWA STATE UNIVERSITY

AMES, IOWA

The break junction technique has been used to determine the energy

gap of lanthanum-strontium-copper-oxide, one of the new high

critical temperature superconductors. The current-voltage

characteristics demonstrated a variety of tunneling behaviors. The

best characteristic indicating quasiparticle tunneling between

superconducting electrodes implied an energy gap of 7.0 meV ± 0.1

meV. Derivatives of other characteristics showed weak structure

indicating possible energy gaps up to 9 meV.

The break junction technique [1] has been used to determine the energy gap of

lanthanum-strontium-copper-oxide, one of the new high critical temperature

superconductors. [2] In this technique, a small piece of bulk material is

electromechanically fractured under liquid helium and the freshly fractured

surfaces adjusted to form a tunneling barrier with the helium as the

insulator. Extraordinarily precise mechanical adjustment permits the study of

electron tunneling phenomena between pieces of a bulk superconductor.

The material measured was a sintered metal powder ceramic manufactured at the

Ames Laboratory [3] and had the chemical form La^ 85^ r0 . 15^u®4 • It an onset critical temperature, Tc , of 36 K as determined from ac susceptibility

measurements. The pellet was cut with a diamond saw to provide small rods

(1 mm x 1 mm x 10 mm) for the break junction technique.

Preliminary results of this experiment are shown in Figure 1. The currentvoltage (I-V) characteristics in the figure demonstrate some of the variety of

the observed tunneling behavior for the various settings of the tunneling

barrier spanning the break. The uppermost I-V curve indicates quasiparticle

tunneling between superconducting electrodes. [4] The gap-sum voltage is 28

f Published in Cryogenics 2_7, 227 (1987).

$ Permanent address: Department of Physics, National Tsing Hua University,

Hsinchu, Taiwan 30043, Republic of China.

2
mV ± 0.5 mV. The gap-sum voltage equals four times the superconductive energy

gap, A/e, assuming identical material on either side of the break. This

implies that A = 7.0 meV ±0.1 meV, which is consistent with the high Tc of

this material. (The A for Nb 3 Sn, for example, is 3.4 meV for a Tc of 18 K.)

This I-V curve occurred only once during many adjustments of the junction.

The middle I-V curve occurred more often. Notice the increased deviation from

antisymetry and the washed out gap. Derivatives of these types of I-V curves showed weak structure indicating possible gap -sum voltages as high as 35 mV,

with corresponding energy gaps of about 9 meV. For these values of the energy

gap and the onset Tc , the ratio 2A/kgTc ranges from 4.5 to 5.8 as compared to

the BCS value of 3.4.

By far, the most common I-V characteristic was of the type shown at the bottom

of Figure 1 . These characteristics showed very little structure even in

derivative traces and appeared to closely follow a quadratic current

dependence on voltage.

Scanning electron microscope pictures of the fracture surfaces between which

the tunneling occurs show a rough surface with frequent voids and scattered

inclusions. It is probable that tunneling shifts from point to point and thus occurs between different phases of the material. The variability in the

tunneling I-V curves may thus be due to tunneling between materials with

different energy gaps and also perhaps across non- superconducting material as well

.

ACKNOWLEDGEMENTS

This work was supported at NBS by the Office of Naval Research, Grant #N000K86-F-0109. The Ames Laboratory is operated for the U.S. Department of Energy

by Iowa State University under Contract No. W-7405-ENG-82

.

REFERENCES

[1] J. Moreland and J. W. Ekin, J. Appl. Phys . 58, 3888 (1985).

[2] R. J. Cava, R. B. van Dover, B. Batlogg, and E. A. Rietman, Phys. Rev.

Lett. 58, 408 (1987)

.

[3] D. K. Finnemore, R. N. Shelton, J. R. Clem, R. W. McCallum, H. C. Ku, R. E. McCarley, S. C. Chen, P. Klavins , and V. Kogan, Phys. Rev. Rapid

Communications (submitted)

.

[4] J. R. Schrieffer, Theory of Superconductivity . Benjamin, New York, 19' . p. 78.

3
I I

I

I 1

I

I I

I I -I I

I I

I

—I

I—I—I I I I I I I I I I I I -60 -40 -20 O 20 AO 60

V O LT AG E (mV)

Fig 1. Current-voltage curves of a La-Sr-Cu-0 electron tunneling break

junction immersed in liquid helium at 4 K. The curves demonstrate the types

of tunneling characteristics obtained at different break junction barrier

settings. The junction resistance measured at +20 mV was 5 MQ for the upper

curve, 170 K£3 for the middle curve, and 8 MQ for the lower curve.

4
TUNNELING SPECTROSCOPY OF A La-Sr-Cu-0 BREAK JUNCTION: EVIDENCE FOR

STRONG - COUPLING SUPERCONDUCTIVITYt

John Moreland, A. F. Clark, and L.F. Goodrich

National Bureau of Standards

Electromagnetic Technology Division

Boulder, Colorado 80303

H. C. Ku^ and R. N. Shelton

Ames Laboratory and Physics Department

Iowa State University

Ames, Iowa 50011

Detailed structure in the quasiparticle tunneling has been observed

in La-Sr-Cu-0 superconductive tunneling junctions using the break

junction technique. Variability in the energy gap and associated

structure in the current-voltage curves is observed indicating

significant inhomogeneity in the superconducting properties. Large

energy gaps (7.0 - 9.0 meV) and deep structure in the conductance

derivatives are evidence for a strong coupling mechanism.

Quasiparticle tunneling has been observed in La-Sr-Cu-0 superconductive

tunneling junctions. [1-5] These measurements were on samples with onset

superconductive transition temperatures of Tc = 36 K to 35 K. The corre- sponding superconductive energy gaps determined from the measured I-V

characteristics ranged from A = 7 meV to 14 meV. This implies strong coupling

superconductivity in La-Sr-Cu-0 since 2A/kgTc ranges from 4.5 to 9.0 compared

to the BCS prediction of 3.4.

Break junction tunneling experiments, in particular, have the potential for

providing some of the most informative details of the quasiparticle states in

the new high Tc materials. This is because tunneling occurs across the

fracture of a bulk sample. [1,7] That sample may be a single crystal, polycrystal, or sintered pellet.

In this paper, we present additional evidence for strong coupling in

La-Sr-Cu-0. Derivatives of the I-V characteristics show that the quasiparticle tunneling conductance of a La-Sr-Cu-0 break junction can vary as i: : as 30% over voltages ranging from the gap edge at 2A/e to 50 mV. These

variations are large compared to those observed in superconductive - Pb

tunneling junctions, for example, where strong electron-phonon coupling can-;, up to 4% conductance deviations from the theoretical BCS density of states.

f Published in Phys . Rev. B 35, 8711 (1987).

t Permanent address: Department of Physics, National Tsing Hua Univorsitv.

Hsinchu, Taiwan 30043, Republic of China.

5
The material measured was a sintered metal powdered ceramic with the chemical

form La^ g 5 SrQ 15 CUO4 . It had an onset Tc of 36 K as determined from ac susceptibility measurements. [ 8 ] The pellet was cut with a diamond saw into

small rods (1 x 1 x 10 mm) for the break junction technique. All measurements

were performed using a break junction apparatus designed for dipping in liquid

helium storage dewars (T = 4 K) . The junction actuator was an electromagnet

of the type described by Moreland and Hansma. [9] We measured the magnetic

field in the vicinity of the junctions and found it to be less than a few

gauss. First derivatives of the I-V curves were taken using the standard

lock-in technique with a modulation voltage of 420 /zV.

As discussed in a previous paper [1] these junctions must be adjusted

(mechanically set and reset) many times to obtain I-V characteristics

indicative of quasiparticle tunneling. We believe this phenomenon is due

to sample inhomogeneity since break junction measurements on pure Pb and Sn

indicate low leakage quasiparticle tunneling independent of junction setting.

As a junction is set and reset tunneling probably shifts between different

points within the fracture, thus sampling different materials. Alternatively,

with continued adjustment, the surface of the fracture elements may have been

worn away exposing subsurface superconductor. The former conjecture is

supported by SEM micrographs of the fractured surfaces examined after the

junction I-V measurements. The fractured surfaces show scattered inclusions

and voids down to less than 0.1 /zm scales. The latter possibility is

supported by TEM micrographs [10] showing lattice distortion at the grain

boundaries of these materials that may destroy conductivity as well as superconductivity

.

Figure la shows the predominate type of I-V characteristic observed during

adjustments of the junctions. These characteristics showed very little

structure and appeared to closely follow a quadratic current dependence on voltage. Zeller and Giaever [11] attribute this kind of tunneling behavior to

the granular nature of a superconductor with insulating intergrain regions . A

model based on the remnant voltage caused by itinerant electrons residing on individual capacitive grains agreed well with their measurements on Sn

particles. They found that when the Sn particles were normal and less than

20 run in diameter, the junction conductance increased linearly with bias

giving rise to a zero bias resistance anomaly similar to that in figure la.

When the particles were superconducting, a step near the peak in the

superconducting density of states occurred in the conductance curves with

finite conductance at zero bias. This behavior has also been observed in

La-Sr-Cu-0 break junctions [1] as well as point contact [4,5] and scanning

tunneling microscopy experiments. [2,3] Another tantalizing observation made

by Zeller and Giaever was that the experimental Tc

'

s

of the small Sn particles were elevated from the bulk value.

The granular model, when applied to La-Sr-Cu-0, may be discrepant in that the

1 nm grain size measured in these materials is much larger than the 20 nm in

the study by Zeller and Giaver. However, as Kirtley et al . [2] point out, the

grain size may not be appropriate for comparison. Perhaps stoichiometric

variations (La versus Sr) preserve the crystal lattice but not local

superconductivity. Natural granularity provided by the layered perovskite

structure of La-Sr-Cu-0 may be another possibility.

6
Occasionally our break junction settings would result in I-V traces with the more familiar quasiparticle current curve. The results for one particularly

interesting setting are shown in figure lb. Notice the sloping curve indicating that at least one of the electrodes has the granular nature

discussed above, and the presence of a superconducting energy gap near zero bias spanning the voltage range from -7 mV to 7 mV. Assuming identical

material on either side of the junction then 4A/e = 14 mV or A = 3.5 meV

.

However, since previous measurements have resulted in higher gap values

(7 meV or larger) perhaps the proper assumption would be that one side of the

junction is granular normal with the other side having a full superconductive

energy gap value of 7 meV. Since this is more consistent with the current

picture we will assume SIN tunneling in what follows.

Figure 2 shows the first derivative (dynamic conductance) of figure lb.

Notice the central conductance minimum corresponding to the tunneling energy

gap flanked by symmetric maxima and minima as a function of voltage. We

estimate the normal conductance of the junction to be around 0.60 /mhos and

the structure to be 30% to 40% variations about this value. This compares

with measurements on Al-Al 203 -Pb junctions (Al normal) where conductance

variations outside of the gap reach only 4% of that in the normal state. [12]

Since Pb is a well known strong coupling superconductor, this implies a very

strong coupling mechanism in La-Cu-Sr-0.

Figure 3 shows the second derivative (d^V/dl^) 0 f figure lb. This curve was digitally generated from figure 2 which was used as input into the McMillianRowell computer program [13] for calculating a^F(w) , the frequency dependent

coupling parameter derived from tunneling conductance data. Generally,

extrema in the second derivative indicate coupling of tunneling electrons to

the normal modes of the materials comprising the junction. Some of these

modes may be responsible for the high Tc superconductivity in these compounds.

For example, preliminary analysis of this data using the McMillan-Rowell

program indicate that possible low energy phonon modes apparent in the second

derivative curve near 5 and 15 mV from the gap edge could very well account

for the high Tc of this material. This is assuming an ohmic normal state

conductance of 60 /nnhos . We caution, however, that our present apparatus

could not be used to measure the true normal state conductance of the break

junction. In addition, the sloping I-V curves made it difficult to accurate lv

estimate the normal state conductance. Since a normal state measurement is

required to accurately calculate phonon couplings, any resulting theoretical

Tc would be suspect.

The data presented here represents some of our "best" results in that the curves are close to being symmetric about zero bias. The majority of our < 1 . 1

1

.

»

showed a great deal of variability with a lot of detailed structure in the curves. All indications lead to the conclusion that we have more to learn

about the superconductivity in these materials, in particular, what is the

role of sample inhomogeneity.

7
ACKNOWLEDGEMENTS

This work was supported at NBS by the Office of Naval Research, Grant #N00014-

86-F-0109. Ames Laboratory is operated for the U.S. Department of Energy by

Iowa State University under Contract No. W-7405-ENG-82 . The authors wish to

thank K. E. Kilhstrom, Westmont College, Santa Barbara, California for

supplying us with the McMillan-Rowell program.

REFERENCES

[1] J. Moreland, A. F. Clark, H. C. Ku, and R. N. Shelton, Cryogenics 27 > 227

(1987).

[2] J. R. Kirtley, C. C. Tsuei, S. I. Park, C. C. Chi, J. Rozen, and M. W.

Shafer (unpublished)

.

[3] S. Pan, K. W. Ng, A. L. de Lozanne, J. M. Tarascon, and L. H. Greene

unpublished)

.

[4] M. E. Hawley, K. E. Gray, D. W. Capone II, and D. G. Hinks (unpublished).

[5] T. Ekino , J. Akimtsu, M. Sato, and S. Hosya (unpublished).

[6] E. L. Wolf, Rep. Prog. Phys . 41, 1439 (1978).

[7] J. Moreland and J. W. Ekin, J. Appl. Phys. 58, 3888 (1985).

[8] D. K. Finnemore , R. N. Shelton, J. R. Clem, R. W. McCallom, H. C. Ku, R.

E. McCarley, S. C. Chen, P. Klavins , and V. Kogan (unpublished).

[9] J. Moreland and P. K. Hansma, Rev. Sci. Inst. 55, 399 (1984).

[10] F. Mueller, Los Alamos National Laboratory private communication.

[11] R. Zeller and I. Giaever, Phys. Rev. 181 . 789 (1969).

[12] J. M. Rowell and L. Kopf, Phys. Rev. 137 . 907 (1965).

[13] W. L. McMillan and J. M. Rowell, Superconductivity, Decker, New York, ed.

by R. D. Parks, 1969, pp. 561-613.

8
La-Sr-Cu-0

4 K

Fig 1. I -V characteristics of a La-Sr-Cu-0 break junction immersed ir. 1: . helium at 4 K.

9
dl/dV

(/imhos)

VOLTAGE (mV)

Fig. 2. dl/dV vs. V for figure lb.

10
SECOND

DERIVATIVE

(arbitrary

units)

Fig. 3. d^V/dl^ vs. V-A/e for figure 2.

11
BREAK-JUNCTION TUNNELING MEASUREMENTS OF THE HIGH-TC SUPERCONDUCTOR

Y1 Ba2Cu309_ 5

t

J. Moreland, J. W. Ekin, L. F. Goodrich,

T. E. Capobianco, and A. F. Clark

National Bureau of Standards

Electromagnetic Technology Division

Boulder, Colorado 80303

J. Kwo , M. Hong, and S. H. Liou

AT&T Bell Laboratories

Murray Hill, NJ 07974

Current-voltage tunneling characteristics in a high critical

temperature superconducting material containing predominately

Y^Ba2 Cu309 _g have been measured using the break junction technique.

Sharp gap structure was observed, with the largest superconductive

energy gap measured to be A = 19.5 meV ± 1 meV, assuming a superconductor -insulator -superconductor junction. This energy gap

corresponds to 2A/kgTc = 4.8 at T = 4 K, for a critical temperature

of 93 K (mid-point of the resistive transition)

.

Since the discovery of high critical- temperature (Tc — 90 K) superconductivity

in the Y-Ba-Cu-0 system, [1] a number of experimental techniques have been

used to study these new superconducting compounds. This letter reports

tunneling measurements in this high Tc superconductor using the relatively new

break- junction technique. [2] These measurements were made on a bulk sample

with the predominant superconducting phase Y^Ba2 Cu309 _^ (where 5 indicates an undetermined amount of oxygen deficiency) , with Tc onset at 95 K and R = 0 at

91 K. Variations of the energy gap, A, along with the gap structure were observed as the junction was set and reset; the largest energy gap was

19.5 meV ± 1 meV. For a critical temperature of 93K (mid-point of the

resistive transition) ,[ 3 , 4 ] this value of the energy gap corresponds to

2A/kgTc = 4.8, which is larger than would be expected from the BCS prediction

of 2A/kgTc = 3.5. This value is consistent with strongly coupled

superconductors

.

The break- junction technique used in this study has the potential for

providing details of the quasiparticle states in the new high Tc materials.

In a break junction, tunneling occurs across the fracture of a bulk sample.

A small piece of bulk material is electromechanically fractured under liquid

helium, and the freshly fractured surfaces are adjusted to form a tunneling

barrier with the helium as the insulator. A break- junction sample may be a single crystal, polycrystal, or sintered pellet. The break- junction method

f Published in Phys . Rev. B 3j>, 8856 (1987).

12
has been useful in providing tunneling data in the La-Sr-Cu-0 superconductor

system, [ 5 , 6 ] along with other tunneling studies using the point contact

technique [7,8] and the scanning tunneling microscope. [9,10] Unlike these

other tunneling techniques, the break- junction technique gives access to the

tunneling characteristics of the interior of bulk samples; it is not

restricted to a surface layer that may not be representative of the bulk

material

.

The sample with an overall composition of Y2 Ba2 Cu30x was prepared from Y2 O 3 , BaC03 , and CuO powders sintered in the pressed-pellet form. The synthesis of

the Y-Ba-Cu-Ox compounds is reported elsewhere. [4] The majority phase is the

superconducting Y^Ba2 Cu309 .§ phase recently identified, [3,11,12,13] with a small addition of the semiconducting phase Y2 BaCuC> 5

. The resistive (10% -90%)

transition temperature is 94 K - 92 K, and R = 0 at 91 K. Filaments with

dimensions of 1 x 1 x 20 mm were cut from a sintered 1 - inch disk for preparing

break junctions. The measurements were performed using a break- j unction

apparatus designed for dipping in liquid-helium storage dewars (T = 4 K)

.

First derivatives of the I-V curves were taken using the standard lock-in

technique with a modulation voltage of 1 mV.

The I-V characteristics of the Y;[Ba2 Cu309 _ g break junctions changed as the

junctions were mechanically set and reset. This is similar to the

characteristics observed in the La-Sr-Cu-0 system. [5,6] We think that this

phenomenon was due to sample inhomogeneity, since break junction measurements

on pure Pb and Sn indicate low- leakage, quasiparticle tunneling independent of

junction setting. As a junction was set and reset, tunneling probably shifted

between different points within the fracture, thus sampling different

materials

.

Figure 1 shows the predominant type of I-V characteristic and its associated

dynamic conductance curve. Two traces are shown corresponding to increasing

and decreasing bias voltage. This I-V curve shows little structure and is

nonlinear, with a nearly quadratic dependence (nearly linear dynamic

conductance trace) . This type of behavior has been observed in most of the

tunneling measurements of La-Sr-Cu-0. ' As pointed out by Kirtley et al .

,

[9] this is consistent with the model of Zeller and Giaever, [14] which treats

tunneling in a system of small superconducting grains separated by insulating

material. The nonlinearity of the I-V curve decreased at low junction

resistances (less than about 100 kfl) . This curvature is not apparent at low

bias in pure Pb or Sn break junctions, implying that its cause is inherent to

the high-Tc compounds Y-Ba-Cu-0 and La-Sr-Cu-0.

An I-V characteristic showing strong gap structure was obtained after

resetting the junction several times. The data shown in figure 2 indicate low

leakage tunneling between superconducting electrodes. Conductance peaks a; -39 mV and +39 mV clearly define a tunneling energy gap. If the materials

either side of the junction are identical, and the junction is formed in

superconductor- insulator-superconductor (SIS) configuration, then 4A/e

or A = 19.5 meV. This is consistent with previous interpretations of poi:.:

contact data on the La-Sr-Cu-0 system. [5-10] We have not observed a tunneling gap larger than the one reported, indicating that we are observi:.,

SIS tunneling. However, we cannot rule out that the actual junction mav

13
the superconductor- insulator-normal configuration, in which case the gap would

be nearly twice as large.

The peaks in the dynamic conductance (dl/dV) traces at the gap edges are broad

(between 16 meV and 24 meV wide at half maximum) as shown in figure 2. Also,

there is a strong negative peak in the dynamic conductance trace within 15 to

30 meV of the gap edge. If the normal state conductance varies smoothly from

low bias to high bias, this represents a large negative deviation from normal

state conductance. Additional structure may exist beyond the gap edges, but

was too small to be detected in these measurements.

In summary, a direct measure of the superconductive energy gap in Y^Ba2 Cu309 _§

gives us some information concerning the mechanism of superconductivity in

this high critical- temperature superconductor. The relatively large gap of

19.5 meV ± 1 meV gives a value of 2A/kgTc of 4.8, which is consistent with the

values of strongly coupled superconductors.

ACKNOWLEDGEMENTS

This work was supported at NBS by the Office of Fusion Energy, U. S. Department of Energy, Contract No. DE-AI01 - 84ER521 13 ; the Office of Naval

Research, Grant #N00014-86-F-0109 ; and by a grant from the National

Engineering Laboratory, National Bureau of Standards.

REFERENCES

[1] M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, Phys . Rev. Lett. 58[, 908 (1987).

[2] J. Moreland and J. W. Ekin, J. Appl. Phys. 58, 3888 (1985).

[3] R. J. Cava, B. Batlogg, R. B. Van Dover, D. W. Murphy, S. Sunshine, T. Siegrist, J. P. Remeika, E. A. Rietman, S. Zahurak, G. P. Espinosa, to be

published.

[4] S. H. Liou, M. Hong, J. Kwo , C. L. Chien, to be published.

[5] J. Moreland, A. F. Clark, H. C. Ku, and R. N. Shelton, Cryogenics 27 . 227 (1987).

[6] J. Moreland, A. F. Clark, L. F. Goodrich, H. C. Ku, and R. N. Shelton, to

be published.

[7] M. E. Hawley, K. E. Gray, D. W. Capone II, and D. G. Hinks , to be

published.

[8] T. Ekino, J. Akimtsu, M. Sato, and S. Hosya, to be published.

[9] J. R. Kirtley, C. C. Tsuei, S. I. Park, C. C. Chi, J. Rozen, and M. W.

Shafer, to be published.

[10] S. Pan, K. W. Ng, A. L. de Lozanne, J. M. Tarascon, and L. H. Greene, to

be published.

[11] C. H. Chen, D. J. Werder, S. H. Liou, J. Kwo, and M. Hong, to be

published.

[12] Y. Le Page, W. R. McKinnon, J. M. Tarascon, L. H. Greene, to be

published.

14
[13] P. M. Grant, R. B. Beyers, E. M. Engler, G. Lim, S. S. P. Parkin, M. L. Ramirez, V. Y. Lee, A. Nazzal, J. E. Vazquez, and R. J. Savoy, to be

published

.

[14] H. R. Zeller and I. Giaever, Phys . Rev. 181 . 789 (1969).

15
-200 -100 0 100 200

1.0

0.8

0.6

0.4

0.2

0

VOLTAGE, mV

Fig. 1 . Current-voltage characteristic and dynamic conductance (dl/dV)

characteristic of a YiBa2Cu3C>9 _ £ break junction immersed in liquid helium.

This trace is typical of that predominately seen in the sample.

16

Sri

‘AP/IP
CURRENT,

VOLTAGE, mV

Fig. 2 . Current-voltage characteristic and dynamic conductance (dl./dV

showing superconducting gap structure typical of the largest measured

Y2Ba2Cu309 . § break- j unction sample.

17

‘AP/IP
JOSEPHSON EFFECT ABOVE 77 K IN A YBaCuO BREAK JUNCTION t

John Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco and A. F. Clark

National Bureau of Standards

Electromagnetic Technology Division

Boulder, Colorado 80303

A. I. Braginski and A. J. Panson

Westinghouse R&D Center

Pittsburgh, Pennsylvania 15235

We have observed the Josephson effect in a YBaCuO break junction.

Critical currents as high as 10 mA were measured at 4 K for break

junctions with a point contact within the fracture of a sample. The

junction was susceptible to microwave radiation showing Shapiro

steps with the ratio of V/f of 2.04 ± 0.05 /IV/Ghz compared to the

pair tunneling value of h/2e = 2.068 /zV/GHz. These steps were clearly visible in the current -voltage characteristics at

temperatures up to 85 ± 5 K.

The Josephson effect has been observed in YBaCuO by several authors. So far

reported results have been point contact experiments [1-4] along with the

construction of a thin film SQUID by IBM that is based on junctions formed at

grain boundaries. [5] In this paper we present the first observation of the

Josephson effect using break junctions. Break junctions offer the possibility

of tunneling between freshly exposed surfaces of a fracture in a bulk sample.

The break junction apparatus and sample geometry is similar to that described

by Moreland and Ekin. [7] A break junction is pictured in figure 1. It has a superconducting filament mounted on a bending beam. The surface strain

developed by the beam is concentrated at the center of the filament so that

the filament can be fractured without exceeding the elastic limit of the beam.

The beam is bent using an electromagnet. Once the filament is fractured, the

beam is relaxed to form a tunneling contact within the fracture of the

filament. If the filament is broken in an inert environment, such as liquid

helium, then the tunneling contact should be "clean."

YiBa2 Cu30x ceramic pellets were prepared as described by Panson et al. [6] The

ceramic pellets were cut to form 1x1x10 mm^ bars, notched near their centers

to facilitate the breaking process. Four terminal contacts, a voltage and a current tap on either side of the notch in the samples, were used to measure

the current-voltage characteristics. Microwaves were coupled to the junction

using a coaxial cable with the shield removed in the vicinity of the junction.

f Published in Appl. Phys . Lett. 51 . 540 (1987).

18
Temperature was varied by lifting the break junction apparatus into the vapor

above the liquid helium level in a storage Dewar. Temperatures were measured

using a carbon resistance thermometer in the vicinity of the break junction.

We estimated ± 5 K uncertainty at higher temperatures due to variable

temperature gradients between the sample and the thermometer in the helium

vapor

.

Onset temperatures were measured on separate samples from the same batch used

for the break junction measurements. The resistive onset temperature was about 97 K ± 0.2 K with a transition width of about 3 K. The inductive onset

temperature was about 90 K ± 0.2 K with a transition width to full shielding

of about 10 K. [8]

Figure 2 shows the current-voltage characteristic of the YBaCuO break junction

immediately after fracturing the sample under liquid helium. Once the sample

was fractured the electromagnet used to bend the break junction substrates was de-energized so that the fracture collapsed to form a point contact. Notice a well defined critical current at zero bias with an I c of 10mA. The normal

state resistance measured at a voltage of 1 mV is 0.2 Q. The I C R product is

about 2 mV, well below the Bardeen-Cooper-Schrieffer theoretical prediction of

7rA/2e = 17 mV [^] (assuming a BCS A of 18 mV^) . Zimmerman [11] has observed

this theoretical limit in "clean" Nb point contacts formed under liquid

helium. The source of the reduced I CR product in YBaCuO is unknown at this

time, however, the experiment was magnetically unshielded. It is possible,

therefore, that remnant fields may have caused low I CR products.

Figure 3 is a series of current -voltage curves for a YBaCuO break junction for

various temperatures. In each case, 7.43 GHz microwave radiation was introduced into the helium storage dewar . Shapiro steps are visible from 4 K

to 85 K as shown in the figure. The voltage step size is very close to the

predicted relation for electron pair tunneling of 2eV = hf. Theoretically,

V/f should be 2.067 /IV/GHz for electron pair tunneling. We observed 2.04 ± 0.05 GHz for YBaCuO. The 90 K curve illustrates the junction's susceptibi 1 i t

v

to microwave radiation, but the observed Shapiro steps are ill defined

compared to those seen at lower temperatures.

With these results, we are confronted with seemingly usual and unusual poin - contact behavior. On one hand there is reassuring evidence for the usual

pairing state associated with superconductivity. Low I C R products, on the

other hand, may indicate an unusually low energy gap compared to a BCS super

conductor. This may be due to a proximity contact instead of an assumed

"clean" high-Tc SS contact. These assumptions, however, contradict our observation of higher gap values when the junction is in the vacuum tunnel: re- configuration. Extremely high gap values (A = 80 meV) have been observed : Ng et al . [12] on samples broken at 77 K, and measured using a scanning

tunneling microscope. The discrepancy between the I C R products and measur. energy gaps may be due to small magnetic fields (less than one gauss) in th« vicinity of the junction or there may be a self limiting field of the poi:.-

contact. Alternatively, the coherence length may be short compared to t la- length of the microbridge in the sample fracture.

19
ACKNOWLEDGEMENTS

Work at Westinghouse was supported in part by Air Force Office of Scientific

Research Contract No. F49620-85-C-0043 . The authors acknowledge and thank J.

Zimmerman and R. Ono of the NBS Cryoelectronics group and J. S. Tsai for

helpful discussion. This work was supported by the Office of Naval Research,

Grant #N00014-86-F-0109 ; the Office of Fusion Energy, U. S. Department of

Energy, Contract #DE- AI01 - 84ER521 13 ; and by a grant from the NBS National

Engineering Laboratory.

REFERENCES

[1] J. S. Tsai, Y. Kubo , and J. Tabuchi , Phys . Rev. Lett. 58, 1979 (1987).

[2] J. S. Tsai, Y. Kubo, and J. Tabuchi, unpublished.

[3] D. Esteve, J. M. Martinis, C. Urbina, M. H. Deveret, G. Collin, P. Monod,

and M. Ribault, unpublished.

[4] W. R. McCrath, H. K. Olsson, T. Cleason, S. Erikson and L. G. Johansson,

unpublished.

[5] R. H. Koch, Materials Research Society Meeting, April 23-24, 1987,

Anaheim, California.

[6] A. J. Panson, A. I. Braginski, J. R. Gavaler, J. K. Hulm, M. A. Janocko,

H. C. Pohl , A. M. Stewart, J. Talvacchio, and G. R. Wagner, unpublished.

[7] J. Moreland and J. W. Ekin, J. Appl. Phys. 58, 3888 (1985).

[8] R. B. Goldfarb, Materials Research Society Meeting, April 23-24, 1987,

Anaheim, California.

[9] B. D. Josephson, Phys. Lett. 1, 251 (1969).

[10] Scaling A from that measured for strong -coup ling Nb-Sn where Tc = 18 K

and A = 3.4 meV. This is consistent with break junction measurements in

the "vacuum" tunneling configuration (see J. Moreland, J. W. Ekin, L. F. Goodrich, T. E Capobianco, A. F. Clark, J. Kwo , M. Hong, and S. H. Liou,

to be published in Phys. Rev. Bl 35)

.

[11] J. E. Zimmerman, Proceedings of the Applied Superconductivity Conference,

May 1-3, 1972, Annapolis, Maryland, IEEE, New York, p. 544.

[12] K. W. Ng, S. Pan, A. N. de Lozanne , A. J. Panson and J. Talvacchio,

unpublished.

20
GLUE JOINT

CONCENTRATING

STRAIN

Fig 1. Break junction (after Ref. 7).

21
CURRENT

(mA)

- 2-1012

VOLTAGE (mV)

Fig 2. Point contact Josephson current in a YBaCuO break junction at 4 K. The

junction was broken under liquid helium to form a clean superconducting point

contact

.

22
CURRENT

(mA)

7.0

6.5

6.0

5.5

VOLTAGE (fjLV )

Fig 3. Shapiro steps seen in break junction current-voltage characteristi

as a function of temperature. Notice that the steps are clearly visible

23

CURRENT

(mA)
ELECTRON TUNNELING MEASUREMENTS IN LaSrCuO AND YBaCuOt

JOHN MORELAND, J. W. EKIN, L. F. GOODRICH, T. E. CAPOBIANCO , AND A. F. CLARK

National Bureau of Standards

Electromagnetic Technology Division

Boulder, Colorado 80303.

The break junction technique, whereby vacuum tunneling occurs within

the fracture of a bulk sample is used to study the LaSrCuO and

YBaCuO perovskite superconductors. Structure in the current -versus - voltage characteristics is reminiscent of previous quasiparticle

curves obtained for BCS superconducting materials. Some curves have

anomalous qualities, including large dips in the junction

conductance with increasing voltage just above a well defined

tunneling gap edge, linearly increasing junction conductance with

applied bias, features occurring near 1, 3, 5 voltage intervals.

INTRODUCTION

Previous break junction measurements [1] on Sn, Nb , NbSn, Pb , and NbTi

demonstrate that this technique can be used in the same way as the more traditional oxide barrier junction technique for determining superconductive

energy gaps, electron phonon couplings, and the Josephson effect. We present

here break junction results for various bulk samples of the new high Tc perovskites cut from ceramic pellets. The advantages of this technique are that it can be applied to bulk materials directly, and that the freshly

exposed surfaces of the fracture remain pristine if the samples are broken in an inert environment (under liquid helium for example)

.

EXPERIMENT

The break junction apparatus is based on an electromagnet actuator for

precise control of the tunneling gap within the fracture of the samples.

Experiments can be done by dipping the electromagnet-sample assembly directly

into various cryogenic fluids (liquid He, Ne , or N2 ) or in a variable

temperature rig using He exchange gas to cool samples.

Samples were cut from ceramic pellets using a diamond wheel with ethanol

cutting fluid. They were then fixed with epoxy to bendable glass substrates.

LaSrCuO samples with Tc onsets of 36 K were prepared by Ku and Shelton. [2]

Single phase YBaCuO samples were prepared by Kwo , Hong, and Liou, [3] and by

Panson, et al. [4] with onset Tc

s above 90 K.

f Published in High Temperature Superconductors, edited by D. U. Gubser and M. Schluter, Materials Research Society, Pittsburgh, 1987, pp . 73-75.

24
RESULTS

Figure 1 shows several conductance -voltage curves for LaSrCuO break

junctions immersed in liquid He at 4 K. These curves are for samples taken

from the same batch. Even so, the tunneling results are variable not only

from sample to sample, but for different break junction settings of a single

sample. Figure la is derived from results obtained by Moreland, et al. [5]

showing classic quasiparticle behavior of a well defined tunneling voltage gap

similar to that expected for strong coupled BCS superconductor with a Tc of

36 K. Figure lb on the other hand, has a washed out gap with a linearly

increasing conductance. In figure lc the gap has disappeared. Figure Id

shows a reduced gap with strong variations (dips) in conductance as a function

of voltage above the gap edge.

Figure 2 shows similar gap structures for YBaCuO break junctions immersed in

liquid He at 4 K. As above, figures 2a through 2d have, respectively, a sharp

gap, washed out gap, no gap, and conductance dips at higher biases. Also

notice that the voltage gaps and dip locations scale roughly with Tc

. Figure

2a is from reference 6.

Preliminary data taken as a function of temperature show that the gap

structure disappears near Tc in both materials. However, this may not apply

to the data in figures Id and 2d since we did not vary the temperature in

these instances. Also what appears to be a Josephson current has been

occasionally observed in the YBaCuO break junctions. More definitive results

have been obtained by Tsai, Kubo , and Tabachi . [7] They have seen Shapiro

steps in YBaCuO "crack" junctions (very similar to break junctions) with the

appropriate spacing of hu>/ 2e when irradiated with microwaves.

DISCUSSION

Consider the area extent of the tunneling contact of a break junction. When

the fracture is relaxed enough to measure a tunneling current the fracture

surfaces do not perfectly mesh back together again. This is evident from the

high junction resistances (1 kO to 10 kH) when the surfaces are shorted

together. Realistic areas are, therefore, similar to those encountered using

point contacts of about 1 run [2] or less. We see that the break junction

technique is a local probe extending over a few atoms of the fracture. This

is consistent with the variability of the data presented above in that the

perovskite structure consists of alternating layers of insulating and

conducting platelets. Further, the individual conducting platelets can be

superconducting or semiconducting (or both). Zeller and Giaever [8]

demonstrated the effect of barrier granularity and the existence of mult i; . coulomb gaps on tunneling structures with results that are strikingly

to those shown in figures lb, lc, 2b, and 2c.

Assuming the data in figures la and 2a stems from tunneling between i : BCS superconductors, we conclude that the superconductive energy gap.

7 meV for LaSrCuO and 19 meV for YBaCuO. This gives a ratio of 2A/kgT(. <>* about 4.5 for the perovskites indicative of BCS pairing with strong • 1

« • • phonon coupling. In addition, the existence of strong conductance dij

•

25
gap edge seen in break junction measurements of both compounds is at least

consistent with the notion of strong coupling between electrons and soft

phonons. This could be responsible for the high Tc

s of these perovskites as discussed by Weber. [9] Alternatively, the conductance dips could be remnants

of a 1 , 3, 5,.. series of voltage steps caused by multiple particle tunneling

[10] or a single coulomb gap [11] intrinsic to each material.

ACKNOWLEDGEMENTS

This work was funded by ONR Grant #N00014-86-F-0109 and DoE Contract #DEAI01 - 84ER521 13 . The authors wish to thank AT&T, Westinghouse and Iowa State

University for providing samples.

REFERENCES

[1] J. Moreland and J. W. Ekin, J. Appl. Phys . 58, 3888 (1985).

[2] D. K. Finnemore, R. N. Shelton, J. R. Clem, R. W. McCallom, H. C. Ku,

R. E. McCarley, S. C. Chen, P. Klavins , and V. Kogan, Phys. Rev. B 35,

5319 (1987).

[3] C. H. Chen, D. J. Werder, S. H. Liou, J. Kwo and M. Hong, preprint, AT&T

Bell Labs.

[4] A. J. Panson, A. I. Braginski, J. R. Gavaler, J. K. Hulm, M. A. Janocko,

H. C. Pohl, A. M. Stewart, J. Talvacchio, and G. R. Wagner, preprint,

Westinghouse R&D Center.

[5] J. Moreland, A. F. Clark, H. C. Ku and R. N. Shelton, Cryogenics 27, 227

(1987).

[6] J. Moreland, J. W. Ekin, L. F. Goodrich, T. E. Capobianco, A. F. Clark,

J. Kwo, M. Hong, and S. H. Liou, to be published, Phys. Rev. B. [7] J. Tsai, Y. Kubo, and J. Tabuchi, to be published, Jap. J. Appl. Phys.

[8] H. R. Zeller and I. Giaever, Phys. Rev. 181, 789 (1969).

[9] W. Weber, Phys. Rev. Lett. 58, 1371 (1987).

[10] A. Kapiltuldnik, presented at APS March meeting, 1987.

[11] J. B. Barner and S. T. Ruggiero, preprint, Univ. of Notre Dame, Physics

Dept

.

26
Fig. 1. Conductance vs. voltage curves for LaSrCuO break junctions in li

He at 4 K.

27
100 mV

Fig. 2. Conductance vs. voltage curves

He at 4 K.

28

for YBaCuO break junctions in liquid
ELECTRON TUNNELING MEASUREMENTS OF HIGH Tc COMPOUNDS USING BREAK JUNCTIONS

t

JOHN MORELAND, L. F. GOODRICH, J. W. EKIN, T. E. CAPOBIANCO, and A. F. CLARK

National Bureau of Standards

Electromagnetic Technology Division

Boulder, Colorado 80303

We report on the break junction technique and its application to the

high Tc superconductors LaSrCuO and YBaCuO. In this technique, bulk

samples are fractured and the freshly fractured surfaces adjusted to

form a tunneling junction with vacuum or liquid helium as the

insulating barrier. Precise mechanical adjustment permits the study

of electron tunneling phenomena between pieces of a bulk

superconductor. The current voltage characteristics of these break

junctions are variable indicating sample inhomogeneity. However,

some junction settings result in the more familiar quasiparticle

signatures in the current voltage characteristics. Low leakage

junctions indicate the presence of a sharp superconductive energy

gap as well as large variations in junction conductance above the

gap edge in both materials

.

INTRODUCTION

The break junction technique [1] has been used to investigate the electron

tunneling properties of the newly discovered high Tc superconductors. [2-5]

We present here a summary of break junction measurements on LaSrCuO and

YBaCuO. Though some aspects of these tunneling results appear to have the

usual behavior when compared to BCS theory, anomalous characteristics are usually observed in these materials. Such characteristics may stem from the

granular nature of the samples tested and weak- link coupling between grains.

EXPERIMENT

Break Junction Apparatus

A break junction is pictured in Fig. 1. It has a superconducting filament

mounted on a bending beam. The surface strain developed by the beam is

concentrated at the center of the filament so that the filament can be

fractured without exceeding the elastic limit of the beam. The beam is Sr:.- using an electromagnet. Once the filament is fractured, the beam is relax,

to form a tunneling contact within the fracture of the filament. If t h<

f Published in Jap. J. Appl. Phys . .26, Supplement 26-3, 999 (1987).

29
filament is broken in an inert environment, such as liquid helium, then the

tunneling contact should be "clean."

Samples

Lai 85^ r 0 15 CUO4 ceramic pellets were prepared by Ku and Shelton at

Ames-Iowa State University [ 6 ], The Tc of this material was 35 to 36 K.

YiBa2 Cu30x ceramic pellets were prepared by Hong and Kwo [7] at AT&T, and

Panson, et al. [ 8 ] at Westinghouse . The Tc of these materials was 93 K.

RESULTS

Vacuum Tunneling in LaSrCuO

A summary of the types of LaSrCuO break junction results is shown in figure 2.

A variety of behavior can be seen as the junctions are set and reset (opened

and closed) . Figure 2a is indicative of SIS quasiparticle tunneling. The

energy gap, A, determined from the location of the peaks in the conductance - voltage curves is 7 meV. This is consistent with a strong coupling BCS theory

in that 2A/kgTc is about 4.5. Figure 2b shows washed out yet still

discernable gap structure. The most commonly observed characteristic is shown

in figure 2c. Linearly increasing junction conductance has been observed in

granular tunneling junctions by Zeller and Giaever [9] and attributed to the

existence of a distribution of Coulomb voltage gaps.

Large variations in the conductance above the gap edge are sometimes observed

in LaSrCuO break junctions as shown in figure 2d. This structure in the

current-voltage characteristic may be due to the electron coupling mechanism.

Alternatively the structure may stem from multiparticle tunneling or the

existence of a single Coulomb voltage gap as observed by Barner and Ruggerio

[ 10 ].

Vacuum Tunneling in YBaCuO

We have observed results for YBaCuO break junctions (see Fig. 3) that are similar to those for LaSrCuO break junctions. The energy gap scales with Tc

. The ratio 2A/kgTc = 4.8 at T = 4 K (the measured A was 19.5 meV)

.

Point Contact Tunneling in LaSrCuO

The fracture in a break junction sample can be adjusted to form a point

contact. Thus far, we have observed only hints of Josephson supercurrents in

LaSrCuO break junctions. A few junctions had small conductance peaks at zero bias

.

Point Contact Tunneling in YBaCuO

Superconducting point contacts have been observed with the break junction

technique in YBaCuO. Figure 4 shows a current-voltage characteristic for a YBaCuO break junction irradiated with microwaves (f = 7.43 GHz) at T = 85 K.

30
The spacing of the Shapiro steps is 2.04 ± 0.05 /iV/GHz , close to the

appropriate value of h/2e = 2.068 /zV/GHz for pair tunneling.

At higher biases we observed switching phenomena in the current-voltage

characteristics. Figure 5 shows such a characteristic for various

temperatures below Tc

. Notice the Josephson supercurrent, barely visible at zero bias, with an Ic of about 500 /iA at 4 K. Switching occurs with

increasing voltage (at higher voltages while increasing voltage and at lower

voltages while decreasing voltage) . As temperature increases from 4 K towards

Tc , the switching features in the junction characteristics move in from higher

biases and the hysteresis decrease, disappearing all together at Tc

. This

behavior is typical of heating effects in microbridges as sections of the

microbridge become normal with increasing current flow.

CONCLUSION

There is evidence for of an intrinsic energy gap that can be detected using

tunneling spectroscopy in the high Tc compounds LaSrCuO and YBaCuO . The

energy gap scales with Tc of the material. The energy gap decreases and

vanishes when approaching Tc from lower temperatures. We believe, therefore,

that the tunneling energy gap is quasiparticle in nature and not Coulombic.

REFERENCES

[1] J. Moreland and J. W. Ekin: J. Appl. Phys . 58(1985) 3888.

[2] J. Moreland, A. F. Clark, H. C. Ku, and R. N. Shelton: Cryogenics

27(1987) 227.

[3] J. Moreland, A. F. Clark, L. F. Goodrich, H. C. Ku, and R. N. Shelton:

Phys. Rev. B 35(1987), in press.

[4] J. Moreland, J. W. Ekin, L. F. Goodrich, T. E. Capobianco, A. F. Clark,

J. Kwo , M. Hong, and S. H. Liou: submitted to Phys. Rev. B.

[5] J. Moreland, J. W. Ekin, L. F. Goodrich, T. E. Capobianco, A. F. Clark:

Materials Research Society Meeting, April 23-24, 1987, Anaheim,

California

.

[6] D. K. Finnemore, R. N. Shelton, J. R. Clem, R. W. McCallom, H. C. Ku, R. E. McCarley, S. C. Chen, P. Klavins and V. Kogan: Phys. Rev. B 35(1987)

5319.

[7] S. H. Lion, M. Hong, J. Kwo, and C. L. Chein: AT&T Bell Laboratories.

[8] A. J. Panson, A. I. Braginski , J. R. Gavaler, J. K. Hulm, M. A. Janocko.

H. C. Pohl, A. M. Stewart, J. Talvacchio, and G. R. Wagner: submitted t< Phys. Rev. B.

[9] H. R. Zeller and I. Giaever, Phys. Rev. 181(1969) 789.

[10] J. B. Barner and S. T. Ruggiero: submitted to Phys. Rev. B.

31
GLUE JOINT

STRAIN

Fig. 1. Break junction (after ref. 1).

32
Fig. 2. Conductance -voltage curves for LaSrCuO break junctions

in liquid helium at 4 K (after ref. 5).

33
Fig. 3. Conductance -voltage curves for YBaCuO break junctions

in liquid helium at 4 K (after ref. 5).

34
2 i

i—

r

]—i—i—i—i—i—i—

r

Voltage (jjV)

Fig. 4. Current-voltage curve for a BaCuO break- junction poi:.*.

contact showing a Josephson current at 85 K (f - 7.4 3 Cii

35
Current

(mfl)

Fig. 5. Current-voltage curves for a YBaCuO break- j unction point

contact at various temperatures.

36
ANOMALOUS BEHAVIOR OF TUNNELING CONTACTS IN SUPERCONDUCTING PEROVSKITE

STRUCTURES

t

John Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco,

and A. F. Clark

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

Our break junction results for electron tunneling spectroscopy of

the perovskite superconductors La-Sr-Cu-0 and Y-Ba-Cu-0 are similar

to those obtained using thin film, scanning tunneling microscopy,

and point contact methods. Energy gap structures are sometimes

observed in the measured current voltage characteristics. More

often, however, the characteristics are anomalous when compared to

previous tunneling studies of BCS superconductors. The anomalies

include linearly increasing conductance with voltage, large

deviations in junction conductance above the gap edge, and junction

diode action. We discuss some possible explanations for these

observations

.

INTRODUCTION

Several techniques have been used for electron tunneling spectroscopy of the

perovskite superconductors La-Sr-Cu-0 (LSCO) and Y-Ba-Cu-0 (YBCO) (see Table

1). The purpose of this paper is to compare and contrast our results obtained

using the break junction technique to similar results obtained using thin

films, scanning tunneling microscopes, and point contacts. In general,

current-voltage characteristics show energy gap spectra with detailed

structure at high junction biases but the data are variable even for a given

technique on a given sample. In addition, current-voltage curves are generally nonlinear and show rectification.

Some of this tunneling behavior may be microstructural in origin and not

related to the electron coupling mechanism (phonons, plasmons, excitons, . We discuss three types of model microstructures that may exist in the vicin: - of the tunneling contact and their possible effects on tunneling current

voltage characteristics. The first model is based on the experiments done

Zeller and Giaever [1] on granular tunneling structures. This model is

adapted by Kirtley et al. [2] to perovskite superconductors. The second it is based on a series of superconducting tunnel junctions within the v :

• :

•

of the tunneling contact in the perovskite material as proposed bv K . . j

- i

f Published in Advances in Cryogenic Engineering (Materials) 36 . edited

by A. F. Clark and R. P. Reed, Plenum, New York, 1988, pp . 623-632.

37
[3] Finally, we propose a third model based on the laminar nature of a single

crystal grain of a perovskite material. In this model individual laminae form

a series -parallel network of superconducting junctions within a single grain

of the material.

EXPERIMENT

A break junction is described in ref. 4. It has a superconducting filament

mounted on a bending beam. The surface strain developed by the beam is

concentrated at the center of the filament so that the filament can be broken

without exceeding the elastic limit of the beam. The beam is bent using an electromagnet. Once the filament is fractured the beam is relaxed to form a tunneling contact within the fracture of the filament. This contact may be a through a thin insulating medium (vacuum, gas, or liquid) or the fracture can

be closed to form a point contact. Precise control of the tunneling gap

approaching picometer sensitivity is afforded by the electromagnet bending

assembly

.

Experiments can be done by dipping the electromagnet- sample assembly directly

into various cryogenic fluids such as liquid helium, neon, or nitrogen or in a variable temperature rig using helium exchange gas to cool the samples. If

the filament is broken in an inert environment such as liquid helium then the

tunneling contact is probably clean.

Samples were cut from calcined ceramic pellets using a diamond wheel and

ethanol cutting fluid. They were then fixed with epoxy to glass break

junction substrates. La l . 85 Sr0 . 15 Cu04 samples with Tc onsets of 36 K were prepared by Ku and Shelton. Single phase Y^Ba2 Cu30x were prepared by Kwo

,

Hong and Liou and by Panson et al. with onset Tc

'

s

above 90 K. Four terminal

contact to the samples was done using silver paint. The silver paint- sample

contact resistance was between 1 and 10 Q.

RESULTS

A summary of the tunneling results known to us at this time is shown in Table

1. Detailed experimental data can be found in the cited references. This

summary describes both qualitative features of the curves as well as characteristic voltages where structures in the tunneling spectrum occur. The

tunneling experiments fall into three categories - break junctions, STM and

point contact, and thin film junctions. The STM and point contact methods are similar in that the STM is used in a stationary mode and generally the STM tip

is touching the surface of the sample. A central observation in these experiments is that the tunneling results show variability even for measurements of

different points on the same sample. Linearly increasing conductance as a function of voltage is commonly observed. In addition, a staircase pattern

where the conductance increases in steps with increasing bias has been

reported along with other large variations in conductance with increasing

bias. These features in the conductance voltage curves may be occurring near harmonics of the fundamental gap voltage. Finally, the curves are asymmetric

to varying degrees about zero bias. This may be due to a semiconducting

38
sample surface that forms a Schottky barrier or to an asymmetric tunneling

barrier

.

Junction leakage is the ratio of the junction conductance at zero bias to

junction conductance just above any observed gap edge in the tunneling

spectrum. In most cases the leakage is high compared to that for oxide

barrier junctions of Pb , for example, where leakages on the order of 0.01 are possible. This may not be surprising since tunneling may be occurring through

a Schottky contact or narrow-band semiconductor on or within the material.

Also, point contact and STM experiments may have high junction leakage because

the counter electrode tip is pressed into the sample.

The electrode configuration is the inferred type of tunneling junction, i.e.

SS for superconductor- superconductor tunneling, SN for superconductornormal

metal tunneling or SS' for superconductor- superconductor tunneling between

different superconductors. The actual configuration is somewhat uncertain

since the Josephson effect has been observed in point- contact experiments

where the point is made of a normal metal, indicating that the junction is

actually inside the superconducting sample and not at the sample surface.

This possibility may apply to all measurements and though the SN conf iguratior. seems appropriate, there may be SS tunneling instead.

The ratio 2A/kgTc should be equal to 3.5 for a BCS superconductor in the weak

coupling limit. Strong coupling BCS superconductors have ratios as high as

5.0. Here, A is the superconductive energy gap as measured from the location

of the conductance peaks in the tunneling spectrum. These peaks are presumably caused by peaks in the quasiparticle density of states. For SN tunneling

the first peak occurs at A/e. For SS tunneling the first peak occurs at 2A/e

.

Note that the ratios for SN tunneling may be half the reported values because,

as discussed above, the junction may be in the SS configuration. A spectral

pattern or sequence of peaks in the conductance -voltage curve sometimes occurs

at voltages near multiples of the energy gap. This phenomenon is generally

observed in oriented films where the tunneling direction is perpendicular to

the c axis of the layered structure. It has also been observed in bulk

samples where there may not be any grain orientation. Such harmonic structure

may be micros tructural in origin or may be due to the coupling mechanism for

electron pairing.

Some of the tunneling curves show a Josephson supercurrent at zero junction

bias. The type of Josephson effect is indicated in table 1. ac and dc

Josephson effects have been seen and SQUIDs have been made from YBCO filrrs

point contacts

.

39
Table 1. Summary of electron tunneling experiments on LSCO and YBCO

Description Junction

leakage c

Electrode

onf iguration

2A/kBTc Spectral

pattern

Josephson

effect

Ref.

LSCO break

j unction

0.05 SS 4.5 ** ** 9

LSCO break

j unction

0.05 SN 4.5 1,3,5 ** 10

Pt-Rh to LSCO

STM

0.05 SN 3. 5-6.

3

1,3,5 ** 2

Pt-Rh to LSCO

STM

0.50 SN 5-9   11

Au to LSCO

point contact

0.10 SN 8-14 **  ** 12

W to LSCO

thin- film STM

< 0.001 SN 8-18 1,3,5 ** 13

Pb thin- film to

LSCO thin film

0.10 SS' 8-18   13

Al to LSCO

point contact

0.80 SN 4-5 **  ** 14

Cu to LSCO

point contact

1.0 SN 4.5 1,2,3 ** 15

Al to LSCO

point contact

** SN   ac , dc 16

YBCO break

junction

0.10 SS 4.8 **  ** 17

YBCO break

junction

0.20 SN 4.7 1,3,5 ** 18

Pt-Ir to YBCO

STM

0.30 SN 4.3   19

Pb thin film

to YBCO

**  SN 10.5 1,2,3 ** 20

W to YBCO

point contact

< 0.001 SN 13 1,3,5 ** 21

40
Table 1. Summary of electron tunneling experiments on LSCO and YBCO (cont.)

Description Junction Electrode 2A/kgTc Spectral Josephson ref.

leakage configuration pattern effect

Pt-Rh to YBCO

STM

0.20 SN 20 ** 22

Nb to YBCO

point contact

** SS'   ac 23

YBCO crack

j unction

**  ss ** ** ac , dc 24

YBCO break

junction

** SS   ac 25

NbZr to YBCO

point contact

**  SS' ** ** ac 26

YBCO to YBCO

point contact

** ss   ac 26

YBCO thin- film to

YBCO thin- film

**  ss ** ** dc SQUID 27

YBCO to YBCO

point contact

** not reported

** ss ** ** ac SQUID 28

DISCUSSION

It is difficult at this time to decide what are good data or which technique

is the most viable for electron tunneling spectroscopy of the superconduct: .• energy gap in the perovskite superconductors. The biggest problem probable

stems from the inhomogeneity in these samples and/or the transport anisot ro; . of the individual crystals in a polycrystalline matrix. Most if not all of

the tunneling techniques probe small areas of the sample and may be sens it : to material variations approaching atomic scales. Some caution should hi- taken, therefore, when interpreting the results. The local Tc may not be

bulk value determined resistively or inductively. Also, Tc may be pro.

dependent. Pressure within the samples induced by the point contact m.iv

result in an anomalously high A especially if the load is distributed ov< r small contact area. Presumably for high resistance break junctions (R • !M

and film contacts this is not a problem since the induced pressure should

,

41
smaller. The break junction technique has the additional appeal of probing

freshly exposed surfaces within a fractured material.

Large features in the conductance -voltage curves occurring near the given

voltage sequences may be related to fundamental modes that participate in the

electron pairing mechanism. For example, Weber has calculated the phonon

density of states and electron coupling parameters for LSCO.^ He finds that

low energy (soft) phonons could be responsible for the high Tc in LSCO of

36 K. Though this calculation is pushing the limits of the Eliashberg

formalism, there is evidence for the existence of these soft modes in the

tunneling data. As Tc' s become larger and larger, however, the soft phonon

theory becomes less appealing. Further, there is the possibility that the

microstructure near the tunneling contact may be causing the features in the

conductance -voltage curves. The three following models, or combinations

thereof, may account for the data.

Granular Model

In the granular model the superconductor is actually divided into superconducting grains isolated from each other by insulating tunneling barriers.

This model was first proposed by Zeller and Giaever^ for granular Sn

junctions. They observed a linearly increasing conductance with voltage and

attributed this to the existence of a distribution of coloumbic energy gaps

due to the capacitance of individual grains. The grain size must be about

20 run or less in order to cause such a zero bias anomaly. If the particles

are identical, or their size distribution is peaked, i.e. if there is a single

coulomb gap associated with charging the isolated grains, then the

conductance -voltage curves may show a staircase pattern in the 1,3,5 sequence.

This effect has been seen by Barner and Ruggiero in granular Ag tunnel

junctions. [30]

The 1 fim grain size typical of the perovskite superconductors seems much to

large to cause these tunneling spectrum features. A linearly increasing

conductance curve may therefore indicate intergranular inhomogeneity.

Further, peaks in the conductance -voltage curve occurring at a 1 , 3, 5,...

sequence may indicate an intrinsic regular inhomogeneity.

Multiparticle Model

This model, proposed by Kapultulnik, [3] assumes that the grains are oriented

to from a series array of junctions near a primary tunneling contact. If this

series is in a NSSS... configuration then one might expect that the

conductance peaks would occur at a 1 , 3, 5,... voltage sequence. If the

series is SSSS... then peaks might occur at a 2 , 4, 6,... voltage sequence.

However, for a series array of junctions with similar resistances one would

expect the array to switch to the normal conducting state at a voltage

corresponding to the sum of the gaps resulting in an apparently large

tunneling energy gap and not in a staircase 1, 3, 5,... pattern.

42
Laminar Model

We propose another model that leads to conductance steps in a 2 , 6, 10,...

sequence, thus reconciling some of the tunneling measurements of A.

Essentially the proposed microstructure would consist of a complex tunneling

matrix with parallel superconducting laminae connected to each other, the

point contact, and the surrounding grains by tunneling junctions. This

structure may be manifested within a single crystal grain, for example as a layered perovskite, with superconducting layers separated by high dielectric

insulating barriers.

Let R^i be the resistance between adjacent laminae within a grain. Let

be the resistance between the surrounding grains and a single lamina, and let

Rp^ be the resistance between the point contact and several laminae of the

grain. These laminae form the primary junction of the contact. Further let us assume that R]_]_ is much larger than Rg]_ and Rpi- Under these conditions we expect steps in the conductance to occur at a 2 , 6, 10,... sequence. The

first step occurs at 2A/e because the primary junction has the NISIN

configuration. By adjusting the point contact it should be possible to make

the primary junction nearly symmetric, that is, R^gl is roughly equal to RPp^.

This means that the primary laminae are at a voltage close to half the applied

bias. When they reach a voltage of 3A/e (V bias = 6A/e) then tunneling can

also occur through an adjacent laminae following a SISIN series. The key

point is that the voltage drop across peripheral adjacent laminae can not

exceed 2A/e if Rg]_ is much larger than R]^ so the next adjacent layer begins

to contribute to the conductance of the contact when the first adjacent layer

reaches 3A/e (primary laminae at 5A/e, V bias at lOA/e) and so on.

CONCLUSION

Which of these models (if any) apply to the perovskite superconductors? The

gap features of the tunneling spectra generally disappear upon warming

through Tc , whereas the zero bias anomaly persists, though thermally smeared

with increasing temperature. Also, the energy gap scales with the Tc

s of the

two compounds. These facts indicate that the staircase pattern in the

conductance -voltage curves is probably not caused by sample microstructure as in the granular model, but that granularity exists nonetheless. Perhaps thr

important energy gap measurement is not that of the largest tunneling gap :>u‘ that of the tunneling gap that is the common denominator of all tunneling

measurements

.

ACKNOWLEDGEMENTS

This work was supported by the Office of Naval Research, grant

#N00014- 86 -F-0109 ; the Office of Fusion Energy, U. S. Department of

contract #DE-AI01 - 84ER521 13 ; and by a grant from the National Bureau of

Standards National Engineering Laboratory.

43
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[2] J. R. Kirtley, C. C. Tsuei, S. I. Park, C. C. Chi, J. Rozen, and M. W.

Shafer, Local tunneling measurements of the high-Tc superconductor La2

_ x

SrxCu04 _y, Phys , Rev . B 35:7216 (1987).

[3] A. Kapiltulnik, American Physical Society March Meeting (1987).

[4] J. Moreland and J. W. Ekin, Electron tunneling experiments using Nb-Sn

break junctions, J. Appl. Phys. 58:3888 (1985).

[5] D. K. Finnemore , R. N. Shelton, J. R. Clem, R. W. McCallom, H. C. Ku, R.

E. McCarley, S. C. Chen, P. Klavins , and V. Kogan, Magnetization of

superconducting copper oxides, Phys , Rev . B 35:5319 (1987).

[6] S. H. Liou, M. Hong, J. Kwo , and C. L. Chien, AT&T Bell Laboratories.

[7] A. J. Panson, A. I. Braginski , J. R. Gavaler, J. K. Hulm, M. A. Janocko,

H. C. Pohl , A. M. Stewart, J. Talvacchio, and G. R. Wagner, submitted to

Phys. Rev. B.

[8] J. R. Schrieffer, Theory of superconductivity, Benjamin, New York (1964).

[9] J. Moreland, A. F. Clark, H. C. Ku, and R. N. Shelton, Electron tunneling

measurement of the energy gap in a La-Sr-Cu-0 superconductors, Cryogenics

27:227 (1987).

[10] J. Moreland, A. F. Clark, L. F. Goodrich, H. C. Ku, and R. N. Shelton,

Tunneling spectroscopy of a LaSrCuO break junction: evidence for strong

coupling superconductivity, to be published in Phys. Rev. B, June 1,

1987.

[11] S. Pan, K. W. Ng, A. L. deLozanne, J. M. Tarascon, and L. H. Greene,

Measurements of the superconducting gap LaSrCuO with a scanningtunneling microscope, Phys . Rev . B 35:7220 (1987).

[12] M. E. Hawley, K. E. Gray, D. W. Capone II, and D. G. Hinks , Energy gap in

LaSrCuO from point contact tunneling, Phys . Rev . B 35:7224 (1987).

[13] M. Naito, D. P. E. Smith, M. D. Kirk, B. Oh, M. R. Hahn, K. Char, D. B. Mitzi, J. Z. Sun, D. J. Webb, M. R. Beasley, 0. Fischer, T. H. Geballe,

R. N. Hammond, A. Kapitulnik, and C. F. Quate , Electron- tunneling studies

of thin films of high-Tc superconducting LaSrCuO, Phys . Rev . B 35:7228

(1987).

[14] T. Ekino, J. Akimitsu, M. Sato, and S. Hosoya, Electron tunneling study

of high-Tc superconductor LaSrCuO system, Aoyama - Gakvin University,

Setagaya, Japan.

[15] I. K. Yanson, L. F. Rybalchenko, V. V. Fijun, N. D. Bobrov, M. A.

Obolensky, N. B. Brandt, V. V. Moshalkov, Yu. D. Tretyakov, A. R. Kaul

,

and J. E. Graboi, Sov. J. Low Temp. Phys. 13:557 (1987).

[16] D. Esteve, J. M. Martinis, C. Urbina, and M. H. Devoret, Observation of

the ac Josephson effect inside a copper oxide based superconductor,

submitted to Europhysics Lett.

[17] J. Moreland, J. W. Ekin, L. F. Goodrich, T. E. Capobianco, A. F. Clark,

J. Kwo, M. Hong, and S. H. Liou, Break- junction tunneling measurements of

the high-Tc superconductor YBaCuO, to be published in Phys. Rev. B, June

I, 1987.

[18] J. Moreland, J. W. Ekin, L. F. Goodrich, T. E. Capobianco, and A. F. Clark, Electron tunneling measurements in LaSrCuO and YBaCuO, Materials

Research Society, Vol. EA-11, June 1987.

44
[19] J. R. Kirtley, W. J. Gallagher, Z. Schlesinger, R. L. Sandstron, T. R.

Dinger, and D. A. Chance, Tunneling and infra-red measurements of the

energy gap in the high critical temperature superconductor YBaCuO

,

submitted to Phys . Rev. Lett.

[20] I. Iguchi, H. Watanabe, Y. Kasai, T. Mochiku, A. Sugishita, and E. Yamaka, Tunneling spectroscopy of YBaCuO compound, University of Tsukuba

Sakura-mura, Ibaraki , Japan.

[21] M. D. Kirk, D. P. E. Smith, D. B. Mitzi, J. Z. Sun, D. J. Webb, K. Char,

M. R. Hahn, M. Naito, B. Oh, M. R. Beasley, T. H. Geballe, R. H. Hammond,

A. Kapitulnik, and C. F. Quate , Point- contact electron tunneling into the

high-Tc superconductor YBaCuO, to be published in Phys. Rev. B, June 1,

1987.

[22] K. W. Ng, S. Pan, A. N. deLozanne, A. J. Panson, and J. Talvacchio,

submitted to LT 18 proceedings.

[23] J. S. Tsai, Y. Kubo , and J. Tabuchi, Josephson effects in the BaYCuO

compounds, Phys. Rev. Lett. 58:1979 (1987).

[24] J. S. Ysai, Y. Kubo and J. Tabuchi, All-ceramics Josephson junctions

operative up to 90 K, submitted to Jap. J. Appl. Phys.

[25] J. Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco, A. F. Clark,

A. I. Braginski , and A. J. Panson, Josephson effect above 77 K in a YBaCuO break junction, submitted to Appl. Phys. Lett.

[26] W. R. McGrath, H. K. Olsson, T. Claeson, S. Eriksson, and L. G.

Johansson, Observations of the ac Josephson effect in high-Tc YBaCuO

point contacts, submitted to Europhysics Letters.

[27] R. H. Koch, Materials Research Society meeting, April 23-24, 1987,

Anaheim, California.

[28] J. E. Zimmerman, J. Beal, and R. Ono , submitted to Appl. Phys. Lett.

[29] W. Weber, Phys. Rev. Lett. 58, 1371 (1987).

[30] J. B. Barner and S. T. Ruggiero, submitted to Phys. Rev. Lett.,

University of Notre Dame, Notre Dame, IN.

45
PUBLICATIONS .PRESENTATIONS, AND PATENTS

Papers Published in Referred Journals

J. Moreland, A. F. Clark, H. C. Ku, and R. N. Shelton, "Electron Tunneling

Measurement of the Energy Gap in a La-Sr-Cu-0 Superconductor," Cryogenics 27 . 227 (1987). ONR, NBS , DoE

J. Moreland, A. F. Clark, L. F. Goodrich, H. C. Ku, and R. N. Shelton,

"Tunneling Spectroscopy of a La-Sr-Cu-0 break junction: Evidence for Strong - Coupling Superconductivity," Phys . Rev. B 35 . 8711 (1987). ONR, NBS, DoE

J. Moreland, J. W. Ekin, L. F. Goodrich, T. E. Capobianco, A. F. Clark, J.

Kwo , M. Hong, and S. H. Liou, "Break Junction Tunneling Measurements of the

High-'j; c Superconductor Y 1 Ba 2 Cu3 0 9 . £ , " Rap. Comm. Phys. Rev. B 3j>, 8856 (1987).

ONR, NBS, ATT Bell Labs, DoE

J. Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco, A. F. Clark, A. I. Braginski , and A. J. Panson, "Josephson Effect above 77 K in a YBaCuO Break

Junction," Appl. Phys. Lett. 5_1, 540 (1987). ONR, NBS, Westinghouse , DoE

J. Moreland, L. F. Goodrich, T. E. Capobianco, J. W. Ekin and A. F. Clark,

"Electron Tunneling Measurements in LaSrCuO and YBaCuO," Materials Research

Society EA-11 . (1987). ONR, NBS

J. Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco, and A. F. Clark,

"Electron Tunneling Measurements of High Tc Compounds Using Break Junctions,"

Proc . 18th Int . Conf. on Low Temperature Physics, Kyoto, 1987, Jap. J. Appl.

Phys. 26, 999 (1987). ONR, NBS

J. Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco, and A. F. Clark,

"Anomalous Behavior of Tunneling Contacts in Superconducting Perovskite

Structures," Proc. Int. Cryogenic Materials Conf., St. Charles, 1987, Advances

in Cryogenic Engineering (Materials) 34 (accepted for publication) . ONR, NBS

Patents Filed

J. Moreland, J. W. Ekin, and A. F. Clark, "Break Junctions: Mechanically

Adjustable Tunneling Junctions for Establishing an Electron Tunneling Current

Across a Fracture in a Conducting Material," Department of Commerce patent

application no. 929,467 (filing date 11/12/86). ONR, NBS

Presentations

J. Moreland, A. F. Clark, J. W. Ekin, R. J. Soulen, Jr., J. H. Colwell, and W.

E. Fogle, "Subkelvin Break Junction Apparatus for Superconductive Tunneling,"

APS March Meeting, New York, March 16-20, 1987. ONR, NBS

46
J. Moreland, J. W. Ekin, L. F. Goodrich, and A. F. Clark, "Break Junction

Electron Tunneling in High Tc Materials," APS March Meeting, New York, March

16-20,1987. ONR, NBS

J. Moreland, L. F. Goodrich, T. E. Capobianco, J. W. Ekin, and A. F. Clark,

"Electron Tunneling Measurements In LaSrCuO and YBaCuO , " MRS Spring Meeting,

Anaheim, April 21-23, 1987. ONR, NBS

J. Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco, and A. F. Clark,

"Anomalous Behavior of Tunneling Contacts in Superconducting Perovskite

Structures," Proc . Int. Cryogenic Materials Conf., St. Charles, June 14-18,

1987. ONR, NBS

J. Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco, and A. F. Clark,

"Electron Tunneling Measurements of High Tc Compounds using Break Junctions,"

LT 18, Kyoto, August 20-26, 1987. ONR, NBS

J. Moreland, A. F. Clark, J. W. Ekin, R. J. Soulen, Jr., J. H. Colwell, and W.

E. Fogle, "Electron Tunneling Measurements of the Heavy Fermion Superconductor

UBe 13 using Break Junctions," YCS 18, Sendai, August 31 - September 3, 1987.

ONR, NBS

Awards

J. Moreland and A. F. Clark, Silver Medal Award from the Department of

Commerce, October 20, 1987, for "providing the world with the first detailed

measurements of the superconducting energy gap in the new high-critical

temperature superconductors." ONR, NBS

47
.
APPENDIX

by

John Moreland, J. W. Ekin, and A. F. Clark

METHOD AND APPARATUS FOR FORMING

MECHANICALLY ADJUSTABLE BREAK JUNCTIONS

FOR ESTABLISHING AN ELECTRON TUNNELING CURRENT

BACKGROUND OF THE INVENTION

This invention relates generally to electron

tunneling junctions and in particular to mechanically

adjustable break junctions for establishing an

electron tunneling current across a fracture in a conducting material.

With the advent of quantum mechanics, Frenkel

realized that electrical conduction due to the

overlap of the electron clouds surrounding two

electrodes could be detected if they were separated

by a distance of less than about two nanometers.

See, for example, J. Frenkel, Phvs . Rev

.

, 36 , 1604

(1930) . Later, in the early sixties, Giaever found

that a native oxide tunneling barrier satisfied this

requirement and was stable enough for electron

tunneling spectroscopy experiments. See, I. Giaever,

Rev . Mod . Phv

.

. 46, 245 (1974) . Such measurements

indicated that the oxide barrier was mechanically

stable to within a small fraction of an atomic

diameter (less than a picometer). Recently the oxide

barrier concept has been extended to materials that

grow inadequate native oxides with the development of

49
artificial tunneling barriers consisting of deposited

layers of oxidized metals. See, for example, D. A.

Rudman and M. R. Beasely, Appl . Phys. Lett. 36, 1010

(1980)

.

The scanning electron tunneling microscope

(STM) invented by Binnig and Rohrer has sparked new

interest in mechanically controlling tunnel barriers.

See, G. Binnig, H. Rhorer, C. Gerber and E. Weibel,

Appl. Phys. Lett. 40, 178 (1982). Mechanically

controlling relative displacements of two electrodes

for meaningful tunneling measurements can be a difficult task. Thermal as well as electromechanical

drifts can cause unwanted apparatus deformations.

However, experimentalists have been driven by the

prospect of tunneling into new materials that form

inadequate oxides by replacing oxide barriers with a vacuum, gas or liquid gap. Though the STM has been

used mainly to study surface topography, it has

recently been adapted for tunneling spectroscopy.

Squeezeable electron tunneling (SET) junctions

developed by Hansma, Moreland and Alexander (Patent

No. 4,566,023 incorporated by reference) have also

been used to establish ultrastable mechanically

adjustable tunneling barriers. This is facilitated

by the compact SET junction construction. In other

words, the transverse scanning mobility of the STM

was sacrificed for increased SET junction stability

necessary for higher resolution tunneling

spectroscopy.

In the initial work on piezo-electric systems

for mechanical tunneling barriers, Teague in his

thesis, "Room Temperature Gold-Vacuum-Gold Tunneling

Experiments" (North Texas State University, Denton,

Texas, 1978, available from University Microfilms)

incorporated herein by reference, proposes adjusting

thin-film microbridges using the surface strain

50
developed in a piezo-electric crystal. Break

junctions extend this idea to include bulk samples.

See, J. Moreland and J. W. Ekin, J. Appl . Phys. 58

(10), Nov. 1985, p. 3888, the whole of which document

is being incorporated by reference. This represents a technological break through in that, traditionally,

properties measured by tunneling spectroscopy were

usually inferred from measurements on thin films of

that material. Break junctions, on the other hand,

provide a straight forward process for achieving

tunneling measurements of freshly exposed bulk

materials without oxide barriers or thin film

depositions previously required for junction

stability.

SUMMARY OF INVENTION

Accordingly, it is an object of the present

invention to provide an apparatus capable of creating

break junctions for establishing electron tunneling

currents across a fracture in a conducting material.

Another object of the present invention is to

provide an apparatus capable of creating break

junctions without disturbing the chemical nature of

the material.

It is a further object of the present

invention to provide an apparatus capable of creating

break junctions by developing the necessary strain to

fracture conducting materials that are in bulk form

and are of varying degrees of brittleness.

Other objects and advantages of this invention

will become more apparent hereinafter in the

specification and drawings.

In accordance with the invention, a substrate

is laminated onto a surface of an elastic beam on either end to form a central unattached section. A

51
conducting material is then mounted over a gap in the

central unattached section of the substrate and

broken by elongating the surface of the beam to form

a break junction for establishing an electron

tunneling junction between fracture elements.

Breaking the conducting material in an inert

environment prevents oxidation of the freshly exposed

fracture surfaces. The spacing between the fracture

elements can then be adjusted to obtain a tunnel

junction by varying the elongation force applied to

the beam. Stable low leakage tunneling I-V

characteristics can be obtained sufficient for

tunneling spectroscopy of the conductor. In

addition, once tunneling is established, the addition

of chemically active impurities to the inert

environment can be detected as they react with the

surfaces of the fracture elements.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cross-sectional view of the

laminated substrate assembly before the conducting

material is applied to the assembly.

Fig. 2 is a cross-sectional view of the beam

bending configuration according to the present

invention before fracture.

Fig. 3 is a cross-sectional view of the three

point bending beam configuration according to the

present invention after the fracture.

Fig. 4 is an exploded view of area A shown in

Fig. 3.

Fig. 5 is a set of I-V curves for a Nb-Sn

break junction immersed in liquid helium.

52
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now more particularly to the

preferred embodiment of the invention selected for

illustration. Fig. 1 shows the long side of the laminated substrate assembly designated generally by 10

used to break a conducting material. A glass cover

slip 14 and a glass microscope slide 12 are attached

at two sections designated by sections g, using an adhesive 16 such as an acrylonitrate adhesive. The

two sections g are thereby separated by a central

unattached section 15. As representative examples,

the glass microscope slide 12 and cover slip 14 may

each be 7 mm wide and 13 mm long. The cover slip 14

is typically 0.2 mm thick. The length of sections g

are each typically 5 mm. After the adhesive 16 has

cured, the cover slip 14 is scribed and broken by

bending the laminated substrate assembly 10 leaving a well defined substrate fracture 22. The fracture

may, for example, be on the order of 0.2 mm deep and

nominally 0.25 - 0.3- mm wide along the surface of the

laminated substrate assembly 10.

Conducting material in the form of filaments

18 are mounted across the substrate fracture 22

thereby forming a free central section 23 of the

filaments 18 over the substrate fracture 22 as shown

in Fig. 2. The filaments 18 are fixed to each side

of the laminated substrate assembly 10 with silver

paint 20 thereby providing a bending beam geometry

designated generally by 11. The silver paint 20 also

provides electrical contact with the filaments 18.

The cover slip 14 is broken before mounting the

filaments 18 in order to relieve internal stress

acquired when the cover slip 14 was glued to the

slide 12. The bending beam geometry 11 is then

mounted in an iron electromagnetic bender that

53
applies forces as shown in Fig. 3. See, for example,

J. Moreland et al Rev. Sci. Instrum. . 55 , 399 (1984),

incorporated herein by reference.

Referring now to Figs. 1 and 4 in which Fig. 4

shows an exploded view of area A of Fig. 3, the

central unattached section 15 serves to concentrate

the bending strain of the substrate assembly 10 near

the fracture 22 in the cover slip 14. In this way

the local strain at the fracture 22 can exceed the

elongation strain at which the filaments 18 fracture

thereby forming break junction 24 before the

laminated substrate assembly 10 fractures. Referring

now to Fig. 2, if the length of the central

unattached section 15 of the cover slip 14 is s and

the length of filament 18 along the free central

section 23 is d, then the strain concentration factor

is e = s/d. When s = 3 mm and d = 0.3 mm then

e = 10. Brittle materials typically fracture at

around a 1% strain. Therefore, with a concentration

factor of 10, the bending beam geometry 11 just

described only requires a 0.1% surface strain

developed by the bending force in order to fracture

the filaments 18. Typically, this corresponds to a force of about 100 N for the given- geometry. It is

important to remain below the elastic limit of the

glass slide 12 or else the fracture may not collapse

into a tunnel junction 24 when the bending force is

decreased. Furthermore, since the stiffness of glass

requires strong bending forces, the effects of

outside vibrations, which are relatively small

compared to the applied forces, are lessened.

However, other materials exhibiting these same

qualities may also be used effectively for both slide

12 and cover slip 14. These materials could also be

conducting materials as long as the slide 12 and the

54
cover slip 14 were electrically isolated from the

filaments 18.

In operation, the bending beam assembly 11 is

mounted on the end of a probe designed for experiments in a liquid helium storage Dewar. Once the

assembly 11 is immersed in liquid helium, the

filament 18 is broken by slowly increasing the force

applied to assembly 11. The force is then decreased

bringing the two broken ends of filament 18 towards

each other until a tunneling current can be seen in

the I-V trace on an oscilloscope. Thus, the bending

beam assembly 11 allows for the mechanical adjustment

of break junction 24.

Operation in liquid helium minimizes the

effect of thermal fluctuations on the mechanical

stability of break junctions 24 since thermal

expansivities become vanishingly small at

temperatures approaching absolute zero. Also liquid

helium provides some fluid damping. In addition,

breaking the filament 18 in liquid helium prevents

contamination of the freshly exposed fracture

surface. However, the bending beam assembly 11 will

also effectively form a break junction 24 in a vacuum, gas or other fluid as long as vibrations and

surface contamination can be otherwise controlled.

Optimum conditions could result in break junction

sensitivities to the relative displacements of the

fracture elements of better than one picometer.

Many materials can be manufactured in

filamentary form. Nb-Sn filaments from magnet wire,

for example, are easily adapted to break junctions

since the elongation and strength of Nb-Sn are such

that the filaments snap before the laminated

substrate exceeds its elastic limit or before the

filaments 18 slip.

55
Fig. 5 shows I-V curves that demonstrate the

tunneling nature of a Nb-Sn break junction formed in

liquid helium at 4°K. As the electromagnet current

applied to the glass slide increases, the bending

force and the surface strain of the glass slide

increase, thereby increasing the width of the break

junction and decreasing the normal conductance of the

junction. As can be seen in Fig. 5, the normal

conductance decreases exponentially with an estimated

increase in the surface strain of the glass beam. In

addition, the marked features of very little current

flow at low voltages (referred to as a tunneling

energy gap) in each of the curves are classic

signatures of electron tunneling in a superconducting

tunnel junction. These features can be used to

determine the superconducting energy gap of this

particular Nb-Sn filament (10 mV) and demonstrate the

spectroscopy capabilities of the break junction

method.

The dimensions given for the substrate

assembly 10 are for purposes of illustration only.

Dimensions will vary according to the materials used

in the substrate assembly 10 as well as the

conducting material to be broken. For example, a longer substrate assembly 10 could be used to develop

more strain for a given force if a conducting

material has a high elongation. Other alternatives

include replacing the silver paint contacts with

stronger attachment materials such as cryogenic

epoxies for samples with a high ultimate strength.

Another possibility would be to break thin films

evaporated directly onto laminated substrate

assemblies

.

Thus, although the invention has been

described relative to specific embodiments thereof,

it is not so limited and numerous variations and

56
modifications thereof will be readily apparent to

those skilled in the art in light of the above

teaching. It is therefore to be understood that

within the scope of the appended claims the invention

may be practiced otherwise than as specifically

described.

57
WHAT IS CLAIMED IS ;

1. An apparatus for mechanically forming

break junctions for establishing an electron

tunneling current across a fracture in a conducting

material comprising:

a beam;

a substrate fixably laminated to a surface of said beam at portions thereof and having a central unattached section of said substrate between

said beam and said substrate and wherein a substrate

gap is formed in said substrate along said central

unattached section;

a conducting material mounted across said

substrate gap and fixed to said substrate on both

sides of said substrate gap wherein a free central

section is formed over said substrate gap and wherein

a break junction is formed in said material along

said free central section wherein said central

unattached section serves to concentrate the strain

developed at the surface of said beam to exceed an

elongation strain- of said conducting material at

fracture of said conducting material when the surface

of the beam is elongated.

2. An apparatus as claimed in claim 1,

wherein said beam is made of an elastic material.

3. An apparatus as claimed in claim 2,

wherein said elastic material is glass.

4. An apparatus as claimed in claim 1,

wherein said substrate is glass.

5. An apparatus as claimed in claim 1,

58
wherein said substrate is fixably laminated to said

beam at said ends of said beam using an adhesive.

6. An apparatus as claimed in claim 1,

wherein said conducting material is fixed to said

substrate with an adhesive.

7. An apparatus as claimed in Claim 6,

wherein said adhesive is silver paint.

8. An apparatus as claimed in claim 1,

wherein said beam apparatus is completely immersed in an inert environment before elongation of the surface

of said beam.

9. An apparatus as claimed in claim 8,

wherein said inert environment is liquid helium.

10. A method for mechanically forming break

junctions for establishing an electron tunneling

current across a fracture in a conducting material

comprises the steps of:

a) providing a beam;

b) fixably laminating a substrate to a surface of the beam at portions thereof forming a laminated assembly having a central unattached

section of said substrate between the beam and the

substrate

;

c) forming a gap in said substrate

along said central unattached section;

d) mounting a conducting material

across the substrate gap;

e) fixably adhering the conducting

material to both sides of the substrate gap wherein a free central section of said conducting material is

formed over the substrate gap; and

59
f) elongating the laminated assembly

with the conducting material fixably adhered thereto

until a fracture is formed along said free central

section of said conducting material.

11. A method as claimed in claim 10, further

including the step of immersing the laminated

assembly with the conducting material fixably adhered

thereto in an inert environment after said step of

fixably adhering the conducting material and prior to

said step of elongation.

12. A method as claimed in claim 13, wherein

the inert environment is liquid helium.

13. A method as claimed in claim 10, further

including the step of adjusting the fracture until an electron tunneling current is established across the

fracture along said free central section.

14. A method as claimed in claim 11, further

including the step of adjusting the fracture until an electron tunneling current is established across the

fracture along said free central section.

15. A method as claimed in claim 10, wherein

the central unattached section serves to concentrate

the strain developed at the surface of the beam to

exceed an elongation strain of the conducting

material at fracture of the conducting material when

said step of elongation as defined in step g) occurs.

16. An apparatus for mechanically forming

break junctions for establishing an electron

tunneling current across a fracture in a conducting

material comprising:

60
a beam;

a substrate fixably laminated to a surface of said beam at portions thereof and having a central unattached section of said substrate between

said beam and said substrate and wherein a substrate

gap is formed in said substrate along said central

unattached section by bending said substrate; and

a conducting material mounted across said

substrate gap and fixed to said substrate on both

sides of said substrate gap wherein a free central

section is formed over said substrate gap and wherein

a break junction is formed in said material along

said free central section during the bending of said

beam.

61
ABSTRACT OF THE DISCLOSURE

A method and apparatus for forming mechanically adjustable break junctions for establishing an electron tunneling current across a fracture in a conducting material. A substrate is laminated onto

an elastic beam on either end thereby forming a central unattached section. The substrate has a gap

along the central unattached section. A conducting

material is mounted onto the substrate to form a beam

assembly. The beam assembly is elongated to form a fracture in the conducting material whereby the

central unattached section concentrates the strain

near the gap in the substrate. The assembly is then

mechanically adjusted until an electron tunneling

junction between the fracture elements is developed.

Prevention of any oxidation of the freshly exposed

fracture surfaces is accomplished by breaking the

conducting material in an inert environment.

62
FIG. Z ASSE1BLY 11

63
Nb-Sn/liquid helium/Nb-Sn

T= 4 K

v 2 nA

u

2 A

64
SBS -1I4A (REV. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 88-3090

2. Performing Organ. Report No, 3. Publication Date May 1988

4. TITLE AND SUBTITLE

BREAK JUNCTIONS I

5. author(S) John Moreland, L. F. Goodrich, J. W. Ekin, T. E. Capobianco, and A. F, Clark

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Office of Naval Research, Pasadena, CA

10. SUPPLEMENTARY NOTES

J Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gn i fi can t bi bl iography or literature survey, mention it here)

Measurements of the tunneling current-voltage characteristics of break

junctions in conventional superconductors can be used to determine their

superconducting energy gap as a function of energy. These results agree with

those previously obtained using traditional oxide tunneling barriers. Break

junctions in some exotic superconductors, on the other hand, have anomalous

current-voltage characteristics compared to BCS predictions. Energy gaps

and the Josephson effect measured for the new high Tc materials YBaCuO (T c

= 93 K) and LaSrCuO (Tc = 36 K) indicate that the samples are inhomogeneous with

varying gap functions depending on the location of the tunneling contact

within the break junction fracture. Break junction data for these materials

are within the strong coupling limits of BCS theory.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words t y break junctions; electron tunneling; energy gap; high T^ superconductors; l.isr

YBaCuO; YBa

2 Cu

3

0

7 _ 5

13. AVAILABILITY

| XI Unlimited

| | For Official Distribution. Do Not Release to NTIS

[jj] Order From Superintendent of Documents, U.S. Government Printing Office, Washington. D.C. 20402.

|_Xj Order From National Technical Information Service (NTIS), Springfield, VA. 2216'

14. NO. OF

PRINTED PAGE

15. Pn

* U.S. GOVERNMENT PRINTING OFFICE: 1988-575-842