Electron*tunnelinginto epitaxial

films of Nd,-,CeXCu04+

M. Homma, S. Tanda, and T. Nakayama
Department of Applied Physics, Hokkaido

University, Sapporo 060, Japan(Received 29 June 1992; accepted for publication 6 August 1992)
We have performed tunneling experiments on planar junctions A1/A10~/Nd,.+Ce,Cu04~,
This is the first experiment using epitaxial films with the planar junction for electron-doped highr, Nd,-,Ce,CuO,-,,.
The temperature dependencies of the gap features were measured, inwhich the phonon structures were clearly observed in the normalized conductance curve. Theseresults show accord with the phonon peaks in the phonon density of states obtained recently byneutron inelastic scattering experiments.
The tunneling experiments serve as a useful technique
to obtain information about gap features and/or phonon
densities of the states of superconductors.’ In addition,these can provide insight into the mechanism responsible
for electron pairing. For these reasons, a great deal of tunneling experiments have been performed on high T, oxide
superconductors (HTSC). There are, however, still unclear points about the pairing mechanism and whether theHTSC has the BCS-like energy gap. The main difficulty inthe tunneling experiments for the HTSCs is due to its shortcoherence length; namely, it is shorter than the distance
between Cu-0 planes. In the case of Nd,-,Ce,CuO,(NCCO),2 the coherence length c along the c axis is 3.4 A(Ref. 3) and the distance between the Cu-0 planes is-6.06 A. Since the tunneling proves the region from thesurface close to the coherence length, the results are susceptible to the surface conditions.
In this letter, we report the results of tunneling experiments on planar junctions of the NCCO epitaxial films
prepared by the method of molecular beam epitaxy(MBE). The results of (d1/&‘)i/, ( -d21/dV2) --V
curve show clearly fine structures, and some peaks are inaccord with the phonon peaks in the phonon density ofstates obtained from neutron inelastic scattering experiments.4 In addition, there are in accord with the peaks ofa2F(o) derived from the point-contact tunneling experiments on polycrystalline NCCO by Huang et aL5 We alsofound specific features on NCCO, such as the existence oflarge zero-bias conductance, linear background conductance, and gap broadening.
The NCCO epitaxial films ( 1000 A) were prepared on
the SrTiO, (100) substrate by MBE. Knudsen cells wereused for the sources of Nd, Cu, and Ce. Each of the deposition rates was monitored by the quartz crystal oscillator
method throughout the deposition. Oxygen gas was flowed
toward the substrate from the pinholes of the pipe locatedbeneath the substrate, and the typical oxygen pressure waslop4 Torr around the substrate. To oxidize fully, the films
were post-annealed in air at 900 “C for 2 h and thenquenched to room temperature. Reduction treatment,
which introduces oxygen deficiency (y-0.04), is necessary
in order to obtain superconductivity in the case of NCC0.6
These films were reduced in an Ar atmosphere (0.4 Torr,with O2 pressure less than lop6 Torr) at a temperaturefrom 550 to 700 “C for 15-20 min. It should be noted that
this reduction is extremely sensitive to time, pressure, tem1724

Appt. Phys. Lett. 61 (14), 5 October 1992

perature, and film thickness.’ X-ray analysis shows only
sharp (001) peaks, indicating the films were grown withthe axis preferentially oriented normal to the substrate.
The junctions were fabricated in the following manner:
first, Al was deposited on the NCCO films and then oxidized in air for several minutes. The thickness of the insulator is approximately 30 A, which was extrapolated fromthe surface roughness measurements. The Al film was,again, deposited on top, with the thickness of approximately 800 A as counterelectrodes. Typical junction resistance was between 30 and 100 fl and the area was 0.25
x0.5 mm.’ Since the planar junctions have a lower current
density than that of the point-contact method or the STM,
the planar junctions are free from higher current density,
causing local heating. The additional features are that it is
not difficult to observe the fine structures in the conductance curves because of its high stability and reproducibility. The measurements of differential resistance were performed by the standard ac modulation technique in thetemperature range from 1.5 to 77 K. The differential conductance was obtained by inverting these differential resistances.
In a BCS superconductor (not restricted to thephonon-mediated pairing mechanism), the quasiparticle
density of states at T=O is given by the normalized conductance(qj(f),;-;q(E&l”)-(1)

As is well known, in the case of strong coupling superconductors, a phonon structure (or excitations relevant topairings) emerges in the normalized conductance as thedeviation from the BCS density of states.’ Such structures
are usually compared with the phonon density of statesobtained from neutron inelastic sc-attering experiments.
In Fig. 1, we present the observed results of conductances for sample A (planar junction Al/AlOdNCCO)
inthe temperature range from 1.5 to 77 K. Since the T, onsetof the tllm is 21.6 K, the drastic decrease of the conductance in the vicinity of zero-bias voltage apparently reflects
the presence of the energy gap. In addition, the small
humps in the observed conductance have appeared at 7,4.2, and 1.5 K. Our results indicate larger gap broadening
than the expected thermal broadening (0.09 meV at 1 K).

0003-6951/92/391724-03$03.00@I 1992 American Institute of Physics

1724Al/AlOx/NCCO

AllAlOxlNCCO-50

1,,:i:.:;.
Voltc&FIG. 1. Temperature dependence of dI/dV of sample A, planar junction
AI/AIO~Nd,-xCe,CuO, from 1.5 to 77 K up to 20 mV. The measured
voltage is the Al voltage relative to the NCCO tilm.

Dynes et a1.9 introduced a parameter I’ into the quasipartitle density of states, replacing E in Eq. ( 1) by E- iI’,
(2)

This phenomenological broadening originates from the effect of the quasiparticle lifetime due to inelastic scattering.
The best fitting for the normalized conductance [dl/dqi.s K/[d1/UJ2s K (not shown here) presents the valueof A= 1.5 meV and l?= 3.1 meV. The anomalously large
value of r is due to the fact that the value of [d1/&‘l,,K/
Id-l/dv125
K (at V=O) is finite and approximately 0.75 in ourcase. Namely, since a planar junction has a much larger
area than the point-contact method or the STM, a certain
percent of the contact area is probably in the normal state
due to surface inhomogeneity. As a result, the tunnelingcurrent through the normal state region enhances the conductance in the vicinity of zero-bias voltage. We have estimated the value of 2A/k,T,-3.6
using T, endpoint of 9.6
K and A= 1.5 meV, since the film used in our experiments
showed a broad superconducting transition. Although thisvalue is quite close to the BCS value 3.53, it involves ambiguity causing from the definition of T, and A.
Linear background conductance was observed even up
to 35 K. The explanation given by Varma et al. lo is basedon the quasiparticle density of states itself, in which thelifetime of the quasiparticle is proportional to the inverse of1E--ir 1, i.e., the density of states has the linear energy
dependence. However, it is not consistent with the resultson the BiSrCaCuO system, not yielding the linear background conductance.” In this connection, several explanations have been proposed, including the inelastic scattering
effect,12 the barrier height effect, and the charging effect. i3
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Appi. Phys. Lett., Vol. 61, No. 14, 5 October 1992

50

FIG. 2. The normalized conductance [dI/dl/l,,K/[dZ/df125 K of sample
B up to 70 mV. Arrows indicate the structures observed at the identicalvoltage both in positive and in negative voltage. The measured voltage is
the Al voltage relative to the NCCO film.

(mV>

D(E,I’)=%fE--ilY/[(E--ir)2-A2]“2j.

0Voltage (mV)

Figure 2 shows the result of the normalized conductance [GY/~V]~.~ K/[dI/&!v],s K of sample B with the voltage up to 70 mV. We have chosen the conductance ofnegative voltage as unity since it seems to converge to unity
more rapidly than that of positive voltage. The normalized
conductance shows some unusual features. First, there isno conductance overshoot and, in the negative voltage region, the value of the whole curvature is less than unity,indicating the slope of the normal state conductance isdifferent from that of the superconducting state. At thispoint, we have no explanation for this but the possibility
that the nature of the tunneling barrier changed after themeasurement of the superconducting state conductance.
Second, asymmetry was observed in the conductance, i.e.,
the tunneling current from Al to NCCO is larger than thatof the opposite direction. These characteristics have also
been observed for the case of hole carrier HTSC. Kirtley13
has given the explanation on this based on the hole-typecarrier. Since it is reported that the carrier of NCCO is theelectron,26 our results indicate that this is common forboth cases of hole and electron carrier HTSC.
The most striking feature of Fig. 2 is the observation ofthe structures in the normalized conductance at the same
voltage in the positive and negative direction. To clarify
this point, we have obtained the second derivative ( -d21/dV2), as shown in Fig. 3, in which the abscissa shows theenergy, V-A. Since the normalized conductance of sample B has no sharp peaks in it, the determination of theenergy gap by fitting Dynes’s equation is not appropriate.
For this reason, we assumed that the gap value of sample B
is 1.5 meV because samples A and B were fabricated on theidentical film. The peaks in ( -dLl/dV2),
which are large
and have the same energy in both positive and negative
voltage, are 15.0, 23.5, and 33.5 meV. The peaks at 15.0
and 33.5 meV show good agreement with the phonon density of states F(o) (short dashed line shown in Fig. 3)
obtained from the neutron inelastic scattering experimentsby Tralshawara et aL4 and with a2F(w) derived from theprevious tunneling experiment by Huang et al.’ The peaks
Homma, Tanda, and Nakayama

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/II mi

I

I010I

20I

I30w(meV)

Jil L,I

4050

FIG. 3. ( -d’I/dV2)
of sample B vs energy. The peaks are observed at15.0, 23.5, and 33.5 meV both in the positive voltage direction. Theabscissa is energy o( = V-h),where A= 1.5 meV, determined for sample A by fitting the Dynes equation. The short dashed line is the neutron
measurements of F(o) from Tralshawara ef al. (Ref. 4).

at 23.5 meV are close to the peak at around 22 meV inF(w). In a2F(o) of Huang et al. data, junction 1 has thepeak at around 20 meV but junction 2 does not. Although
the reason for this uncertainty is unknown, obviously it isinterpreted that both the lower and the extended higher
energy peaks in F(w) correspond to the vibrational modes
relevant to heavy atoms such as Nd and Ce, and those
relevant to the 0 atom or the Cu-0 plane, respectively.
To summarize, we have performed tunneling experiments on planar junctions of NCCO epitaxial films prepared by MBE. The obtained ( -&1/dV2)
curve haveshown peaks at the energy of 15.0, 23.5, and 33.5 meV,

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Appl. Phys. Lett., Vol. 61, No. 14, 5 October 1992

which are in accord with the phonon density of states obtained from the neutron inelastic scattering experiment by
Tralshawara et al. ,’ indicating the relatively strong
electron-phonon interaction. It suggests that phonons areplaying a role in pairing in the NCCO. However, this does
not rule out the possibility that other excitations are also
playing a role in the pairing. Clearly, tunneling and neutron data at high energy would be desirable.
The authors would like to thank Y. Nishida, NTT
Optical Electronics Laboratory, for experimental support
at the early stage of this work, and 1. Aoyama of Komatsu,
Ltd. Material Research Laboratory for experimental support on the MBE. We would also like to express sincere
thanks to Hiroshi Takahashi of the Eiko Engineering Company.
’W. L. McMillan and J. M. Rowell, in Superconductivity, edited by R. D.
Parks (Marcel Dekker, New York, 1969). Vol. 1, p. 561.
zY. Tokura, H. Takagi, and S. Uchida, Nature 337, 345 (1989).
‘Y. Hidaka and M. Suzuki, Nature 337, 720 ( 1989).
‘N. Tralshawara, J. F. Zasadzinski, L. Coffey, and Q. Huang, Phys. Rev.
B 44, 12 102 (1991).
5Q. Huang, J. F. Zasadzinski, N. Tralshawara, K. E. Gray, D. G. Hinks,J. L. Peng, and R L. Greene, Nature 347, 369 ( 1990).
‘H. Takagi, S. Uchida, and Y. Tokura, Phys. Rev. Lett. 62.1197 (1989).
‘S. Tanda, M. Honma, and T. Nakayama, Phys. Rev. B 43,8725 (1991).
*J. R. Schriefer, D. J. Scalapino, and J. W. Wilkins, Phys. Rev. Lett. 10,336 (1963).
‘R. C. Dynes, J. R. Garno, G. B. Hertel, and T. P. Orlando, Phys. Rev.
Lett. 53; 2437 (1984).
‘cc . M . Varma, P. B. Littlewood, S. Schmitt-Rink, E. Abraham, and A.
E. Runckenstein. Phvs. Rev. Lett. 63, 1996 (1989).
” S. Vieira, M. A.’ Rados, M. Vallet-Regi, and J. M. Gonzalez-Calbet,Phys. Rev. B 38, 9295 ( 1988); T. Hasegawa, M. Nantoh, and K. Kitazawa. Jpn. J. Appl. Phys. 30, L276 (1991).
“J R Kirtley and D. J. Scalapino, Phys. Rev. Lett. 65, 798 (1990).
I3J: R: Kirtley, Int. J. Mod. Phys. B 4, 201 (1990).

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