High-Temperature Superconductivity:
History and Outlook
Shoji TANAKA
Director General
Superconductivity Research Laboratory
1-10-13 Shinonome Koto-ku, Tokyo 135-0062, JAPANHISTORY OF HIGHTEMPERATURE SUPERCONDUCTIVITY

0.010

After superconductivity was
discovered by Onnes in 1911, manysuperconductors were discovered
and the critical temperature roseyear by year. More than 2000 superconducting materials had been
discovered by 1975, and the critical temperature had reached 22.3
K with the discovery of Nb3Ge in
1973. After that, however, nohigher critical temperature wasobtained for more than 10 years.
When the BCS theory in 1957
provided an elegant explanation forsuperconducting phenomena,
many scientists believed that higher
critical temperatures could not bereached without finding new superconducting phenomena. Alreadyin 1954 Fröhlich 1) had proposed a

0.008

ρ(Ωcm)

0.006

0.004
7.5 A/cm2
2.5 A/cm2
0.5 A/cm2
0.002

0

0

10

20

30
T(K)

40

50

60

Fig. 1 Low-temperature Resistivity of a Ba-doped LaCuO3 Sample
with x (Ba)=0.75, Recorded for Different Current Densities

model of high-temperature super-

2
.5
0
0
-1

Magnetic Susceptibility ( 10- 4 emu/g )

-2

-2

-3

-4

19

21

23

25

27

T( K)
-6

-8

Fig. 2 Temperature
Dependence of theMagnetic Susceptibility
in Ba-doped La2CuO4

H=1.0 0e

- 10

- 12
0

5

10

15
Temperature( K)

20

25

30

conductivity in charge density wave systems,
and in 1964 Little 2) proposed a so-called excitonic superconductivity model. No such superconductors, however, have actually been found.
Sleight 3) reported in 1975 that superconductivity was seen in BaPb(1-x)BixO3 and that thecritical temperature of this material, which canbe as high as 13 K, changes with the Bi/Pb ratio. This was just the beginning of research on
the oxide superconductors. Tanaka and others
4)
immediately studied this material and foundthat the carrier concentration in it is more thanan order of magnitude smaller than that in ordinary metals. At that time there was a little
bit of hope that the superconductivity seen inthis material is non-BCS superconductivity, buttoday it is still thought to be BCS superconductivity.
In the early 1980s investigators all over
the world began looking for new types of superconductivity. In Switzerland a new superconductor, PbMo6S8 was found by Chevrell. It
had high upper magnetic critical field, but its
critical temperature was still only about 16 K.
In Japan a government project called “New Superconducting Material” started in 1984, andin the United States a new conference on the“Material and Mechanism of Superconductivity” was held in 1985.
Bednorz and Müller5) found early in 1986
that in Ba-doped LaCuO3 the temperature dependence of the conductivity in the transition
region from the normal state to the superconducting state changes with the current density(Fig. 1) and they pointed out this might indicate the possibility of high-temperature superconductivity. Tanaka, Kitazawa, Uchida, andTakagi 6) at the University of Tokyo immediately
started to explore this possibility and at the endof 1986 reported that measurements of resistivity and diamagnetism confirmed that thecritical temperature for superconductivity in Badoped La2CuO4 was as high as 30 K (Fig. 2).
Furthermore, in early 1987 Tanaka 7) pointed

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Fig. 3 OCMG-processed Gd-Ba-Cu-O Bulk (Single-domain 48 mm ø sample)
Mixing ratio Gd123:Gd211=10:5
Trapped magnetic field 2.2T at 77K

Fig. 4 Characteristics of High-Jc Bi2223 Conductor

Space insertion for theabsorption of heat contractionforce
XLPE cable

Filled-core fine-strand former
for eddy current reduction

Cooling system
for terminations
Spiral pitch adjustment
for AC loss reduction

150mm duct
3 splitting structure
Short connection of 3 shielding layers
100m 3-phase HTS cable

Current transformer

Thermal insulation
property at corner
Splitter box

Cooling systemfor cable

Termination
Transformer forvoltage apply

Specification
Voltage
Current
Capacity
LengthCable type
Cooling

Fig. 5 Superconducting Transmission Cable System & Design Concept

66kV
1000Arms
114MVA100m
3-phase, cold dielectric type
Closed circulation of
sub-cooled LN2

out the possibility of two-dimensional superconductivity due to the layer structure of thismaterial. At almost the same time, Anderson
8)
proposed the mechanism of the high-temperature superconductivity based on the twodimensional resonating valence band model.
At that time, many scientists began to thinkthat the superconductivity of Ba-doped La2CuO4
cannot be explained by the BCS theory.
In February of 1987 Chu and others 9)
found a new superconducting material,
YBa2Cu3O7, with a critical temperature above
90 K. Many scientists all over the world thenbegan to search for new superconducting materials with higher critical temperatures. On
March 15 of 1987 the American Physical Society held a special symposium on high-temperature superconductivity. Several thousand scientists gathered in the main hall of the Hilton
Hotel in New York City, where the enthusiastic
atmosphere of the symposium continued until
the dawn of the next day. One physicist called
this meeting a Woodstock in physics.
Tremendous advances were made in theoretical and experimental research, and newmaterials were found in quick succession. As aresult the critical temperature reached 112 K
in a Bi-compound, 126 K in a Tl-compound and
135 K in a Hg-compound. The physical properties of these compounds were also investigated very intensively, and it was confirmed thatin all cuprate superconductors the superconductivity occurred in very thin layers including
CuO2 planes. Many kinds of peculiar properties were observed, but except the d-wave symmetry of the superconductivity, most of them
are not explained with consistency. The mostpeculiar property may be the appearance of the
stripe structure observed recently in neutron diffraction experiments. In this structure, the electric charges exist in stripes that are separated
by the array of magnetically ordered stripes.
The physical properties in the normal stateof the high temperature superconductors are
so complicated that any theory can not yet explain them in a consistent way. Deeper understanding on “the strongly correlated system”
may be necessary in order to solve the originof the high temperature superconductivity.

OUTLOOK FOR THE FUTUREOF HIGH-TEMPERATURE
SUPERCONDUCTIVITY[1] Material developments
There have in the past ten years beenFig. 6 Cryogen-Free R&D Magnet

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posed, but even today there is no consensus
among theoretical physicists. There are so manyvarious kinds of interactions in such complicated systems-electron-phonon interactions,spin-spin interactions, charge density waves,spin density waves, and so on-that it may beconsidered we are just beginning to understandthe physics of these complex systems. Explaining this phenomenon clearly will take a longtime.

Refrigerator
Power lead
LHe tank
LN2 tankService port

Outer vessel
Supporter
SC coilRadiation shield
Super insulator[3] Applications

Persistentcurrent switch
VacuumeSince room temperature superconductivity has not been found yet, the applications ofhigh-temperature superconductivity 12) are restricted to temperatures around that of liquid

Railway Technical Research InstituteFig. 7 Superconducting Magnet for MAGLEV Train(1) Silver-sheathed Bi-Compound Tape
SilverBi-compound
layers(2) Tape of Next Generation (1-2-3 compounds)
Superconductor (axis aligned)

Pinning Centers
Buffer layer(axis aligned)

Substrate[Multi-layer Structure]

Fig. 8 Structure of Superconducting Tape

107

106
NbTi(4.2K)

105

Jc(A/cm 2 )many attempts to obtain high-temperature
superconductivity in materials other thancuprates. Superconductivity was observed inalkali-ion doped C60 at 33 K 10), and Akimitsu
very recently found superconductivity in MgB2
at of 39 K 11). But the superconductivity in boththese materials is explained by the BCS theory,
so it can be said at present that all the nonBCS superconductors belong to cuprate family. We therefore need to find new non-BCS
superconductors outside the cuprate family if
we raise the critical temperature beyond room
temperature. Room temperature superconductivity is still a dream of many scientists, but there
is no guideline to reach it at present. A little
bit of hope, however, may be found when we
consider organic compounds. The methods foranalyzing very complicated biological materials, like the human genome, are developing
very fast and it may soon be possible to analyze the interactions between complicated genomes and proteins. This would enable us tosimulate new organic materials on high-speed
supercomputers and evaluate the interaction
between electrons and molecules in new materials. This is a way to realize excitonic superconductivity suggested by Little in 1964. Thismeans the progress of future computing science will open a new way for science and technology of new superconducting materials.

Nd123(Expected, 77K, B//c)
Y123877K, B//c)

104
Bi-2223(77K, B//c)103
Tl-1223(77K, B//c)[2] Theoretical Developments
After the discovery of the high-temperature superconductivity in cuprates, many kindsof theoretical models of the mechanism of thehigh-temperature superconductivity were pro-

102
0

2

4

6

8

10

12

14

Magnetic Field(T)Fig. 9 Magnetic Field Dependence of Critical Current (prospect)

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(b) Superconducting Tapes
The first generation of superconducting
tape using high-temperature superconductors
is the so-called Silver-Sheathed Bi-Compound
Tape. The Bi-compound usually used is
Bi2Sr2Ca2Co3O10, and the cross section of thetape is shown in Fig. 4. At 77 K the criticalcurrent of this tape is more than 110 A, andthe length of commercial tape is more than
1000 m. Trials of the actual applications of
this tape in various fields have already begun.
A high-current transmission cable is very important when large amounts of electric power
are sent to the central part of a big city. Because digging new underground tunnels is very
expensive, it is necessary to use the old tunnel
and new superconducting cables that can carrylarge amounts of power even though they havea small cross section. These trials are being
made in Detroit in the United States and in Tokyo, Japan. In Fig. 5 is shown the scheme ofthe superconducting transmission cable whichis under construction in Tokyo Electric Power
Company.
Another application is in the superconducting magnet for the pulling system used tomake large-diameter crystals of silicon. To prevent oxygen from the crucible of fused quartz
from entering the molten silicon, a strong magnetic field must be applied. The photographin Fig. 6 shows a magnet developed for thissystem by Toshiba and Sumitomo Electric Co.
The newest application trial is the developmentof a race-track-shaped magnet for the magnetically levitated transportation system(MAGLEV) train in Japan. As shown in Fig. 7,
this magnet is about 1 m long and 50 cm inwide, and at 20 K the strength of the mag-Biaxial In-plane Alignment
Homogeneity
High Production Rate
Thickening
Superconducting Layer
Y-123, Nd123, Sm123,
RE(mix)-123(Thickness; 1-10µm)

High StrengthNon-magnetic Materials
Thinning
Alignment Control
for Buffer Layer

Buffer Layer
YSZ, MgO, CeO2, NiO
ZrO2, BaZrO2, etc.
(Thickness; <3µm)
Substrate
Ni-Alloy(Hastelloy), Ni,
Ag, Ni-Cr Alloy, Zr, etc.
(Thickness; 25-125µm)

Fig. 10 Structure of Coated Conductor(a) Superconducting Bulk
Materials for superconducting bulks are
REBa2Cu3O7, where RE is Nd, Sm, Gd, or Y. The
bulk is made by using the quenched melt
growth (QMG) method in the case of
YBa2Cu3O7 and in other cases is made by using
the oxygen controlled melt growth (OCMG)
method under a low partial pressure of oxygen. The diameter of the circular plate of bulk
reaches more than 10 cm and crystal c-axis isperpendicular to the plate. In the bulk, thepinning force of magnetic flux in the superconducting state is very strong and at 77 K thecritical current density is more than 104 A/cm2.
The applications of the bulk are of two kinds.
(1) As the pinning force of the magnetic flux is
so strong, the outside magnetic field cannot
penetrate the bulk in the superconducting
state. This results in a strong levitation force
when the bulk is close to an ordinary permanent magnet, and at 77 K this force usuallyreaches 15 kg/cm2. This phenomenon can be
exploited to make friction-free flywheel-type
electricity storage systems that can store, say,
10 kWh and be used as emergency power supplies. (2) When the bulk is in the normal state,a magnetic field applied from outside is uni-

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formly distributed throughout the bulk. But
after the bulk is cooled to below the criticaltemperature, the magnetic field is quantizedand quantized flux is pinned by strong pinning
centers. Then when the external field is removed, the quantized flux is left inside andbehaves like a permanent magnet. The
strength of the trapped magnetic field in
GdBa2Cu3O7 bulk reaches more than 2 T at 77
K and more than 3.5 T at 30 K (Fig. 3). This is
several times the strength of the magnetic fieldof an ordinary permanent magnet.
One very fruitful application of this bulk
would be in a water-cleaning system using themagnetic separation effect. This system would
be more than 100 times as efficient as the magnetic-separation cleaning systems available today.

106

105
Generator

Generator

105

104
Bi-based (20K)
SMES

Current X Wire length (A•km)

nitrogen. As the refrigeration systems are welldeveloped recently, those applications at liquid
nitrogen temperature will greatly contribute to
the society.
The fundamental technologies for applications are classified into those of (a) superconducting bulk, (b) superconducting tapes and(c) superconducting devices.

104
Laboratory
Magnet
Steel
Plant

103
Chemical
Reaction(Laboratory)

MRI
Levitation
Current
LimterChemical
Reaction(Laboratory)

Si single crystal
Magnetic Separator

102

Electric Power Cable

SMES

Laboratory
Magnet

MRI

103
Levitation

Current
Limter

Steel
Plant

102

Si single crystal
101

Magnetic Separator
Electric Power Cable

101

100
Y-based (77K)
Current Lead

100
1990

1995

Current Lead

2000

2005

10-1
2010

AGE

Fig. 11 Expectations of the Development of Superconducting Tape

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Magnetic field X Inner diamiter of Coil (T-cm)

Alignment Control for SC Layer
Prevention of Reaction
Suppression of Crack Formation
Homogeneity, Flatness
High Production Rate

netic field at the center of the magnet is 5 T. It
is hoped that this magnet can replace the magnet made of Nb-Ti wire and operated at 4.2 K,
since no quenching phenomena in it is expected
and this is very robust against electromagnetic
disturbance from outside.
Possible applications of superconducting
magnets of this type are appearing all the time:in the continuous casting systems in steel mills,
in high-power motors for ship propulsion systems, and in the superconducting magnetic
energy storage (SMES) system.
The development of the next generationof superconducting tape is underway in Japan,
the U.S., and Europe. This tape, shown in crosssection in Fig. 8, has three layers (metal substrate, oxide buffer layer, and superconducting
layer) and is called a coated conductor. The
superconducting layer is made of REBa2Cu3O7,
and the critical current density (Jc) and its magnetic field dependence expected from the results of testing the materials are shown in Fig.
9. The question of what kind of combinationof three kinds of layers is the most preferable
is quite serious at present as is shown in Fig.
10. If the critical current is to be large, the caxis of superconducting layer must be perpendicular to the surface of substrate. Therefore
the alignment of crystal grains in the a-b plane
becomes important, as the critical current decreases rapidly when the grain boundary angles
are more than 10 degrees.
As mentioned above, in the developmentsof the coated conductors there are still difficult
problems left unsolved, but this superconducting tape is expected to reach the market ataround the year of 2005 (Fig. 11).

one and zero, and logical circuits can be madeby combining SFQ devices. The most important features of this device are that its operation time is only a few picoseconds and its
power dissipation is only of the order of 1
nanowatt. As a result, the circuits comprisingthese devices have very high operating frequencies, of the order of 100 GHz, and consume
very little power, of the order of 1 mW. This
operation speed is almost 100 times faster thanthat of ordinary semiconductor circuits and thepower consumption is about 1/100 that of thesemiconductor circuits. Of course there aremany kinds of difficulties in the developmentof the circuits, as the operating principle of thistype of circuit is completely different from thatof semiconductor circuits and the design

method is not well established yet.
With regard to the production process, on
the other hand, the SFQ circuits have someadvantages over semiconductor circuits (Table1). The most important thing is that SFQ circuits with a 1-micrometer line width are goodenough that operating frequencies greater than
100 GHz can be obtained. This means that we
can use 10-year-old lithography equipment in
the semiconductor industry and that we will
be able to produce better characteristic circuitsin the future.
A SFQ circuit consisting of several thousand Nb junctions has already been tried andproven to work at very high frequencies at 4.2
K. High-Tc junctions have been made using
YBCO, and very recently high-quality Joseph-

SET

ø0

Presence of
SFQ
logic value="1"

Absence of
SFQ
logic value="0"

RESET

Control

Magnetic Field or Current

SQUID
dc-SQUID

SFQ: (Single Flux Quanta)

Superconducting
Weak Link

rf-SQUID

Fig. 12 Controlling SFQ with Use of SQUID(c) Superconducting Electronic
Devices
The most prominent phenomenon in superconductivity is Josephson tunneling, andmost applications of the superconductivity inelectronics are based on this phenomenon. The
most well known application is the superconducting quantum interference device (SQUID).
Its extremely high sensitivity to magnetic fields
is exploited in many fields: medical electronics,
mass production (e.g., as non-contact defectdetectors), and so on.
Another important application is the single
quantum flux (SFQ) device. As shown in Fig.
12, the principle operation of the SFQ device israther simple. The presence and absence of asingle magnetic flux quantum in the SQUID ring
respectively correspond to information signals

Table 1 Comparison between Semiconductor and Superconductor circuits
Semiconductor

Superconductor

1. Line Width< 0.1µm

0.8µm

2. Lithography

after 2005

completed

3. Structure

Three Dimensional

Two Dimensional

> 20

≈ 10

5. WiringMulti-layered ; high resistanceheating

Multi-layered ; no resistanceno heating

6. Wafer

8 inchs ; surface roughness
several nm's

3 inchs ; surface roughness
several nm's

7. Design

4. Number of Marks

Efficient Tool ; Margin. large

Small Tool ; Margin. small

8. Frequency

5 GHz (2005)

50~100 GHz

9. Output≈ 1V≈ 0.1mV

Which is easier to make, semiconductor devices or superconductor devices?

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at 4.2K

at 40K

tronics will greatly be changed.
High-quality superconducting microwave
filters operating in the gigahertz region are usedin the base stations of wireless communication
systems, and some of them have already beencommercialized. Wireless communication is becoming increasingly important in the information society, and therefore it must play an important role in connection with the development of future software communication systems.

W = 4µm

W = 4µm

SUMMARY

X : 2.0 mV/div.
Y : 1.0 mA/div.

X : 1.0 mV/div.
Y : 0.5 mA/div.

Fig. 13 Typical Characteristics in Ramp-edge YBCO Josephson Junction

son junctions have been made by using an ionbombarded barrier. These types of junctions
are called ramp-edged junctions, and the typical characteristics of this type of junction are
shown in Fig. 13. This figure indicates that circuits consisting of this type of junction can workat 40 K.
In order to integrate the junctions, the distribution 1σ of the critical current through junctions is critically important and until now 8%of distribution in 100 junctions and 10% in1000 junctions have already been obtained.
The relation between possible circuits and the
distribution 1σ of junctions is shown in Fig. 14.
This figure shows that a high-speed (40 GHz)
sampler has already been made and that somecircuits of small-scale integration can be madein the very near future. If very inexpensive superconducting chips with excellent characteristics appear on the market, the features of elec-

20

Required σIc (%)

15

Circuit margin : 20%: 50% circuit yield: 99% circuit yield

Digital demos.

Digital sampler
10Counting A/DHigh dynamic range A/D

References

4X4 network switch
32bit DSP
5
MUX/DMUX

4bit DSP
128X128 switch

Spread spectrum MODEM
0
100

101

102

103

104

Number of junctions in circuit

Fig. 14 Relation between Possible Circuits and the Distribution of Junctions

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Fifteen years have already passed sincehigh-temperature superconductivity was discovered in 1986. Many new materials havebeen found and the critical temperature of superconducting materials has increased to 135
K. To obtain superconductivity at room temperature, however, we must find materials otherthan those in the cuprate family. It is very difficult to say when that can be done and who
can do it. The theoretical basis of high-temperature superconductivity is still uncertain andit must remain in the forefront of solid statephysics.
On the other hand, we can see the futureof the superconducting technologies moreclearly than ever and can expect the superconductivity industry to take off around the year
2005. It must play an important role in the
new industrial revolution now underway.

105

106

1) H. Fröhlich: Proc. Roy. Soc. A223, 296 (1954).
2) W.A. Little: Phys. Rev. A134, 1416 (1964).
3) A.W. Slight, J.L. Gillson and F.E. Bierstedt:
Solid State Commun. 17, 27 (1975).
4) T.D. Thanh, A. Kuma and S. Tanaka: Appl.
Phys. 22, 205 (1980).
5) J.G. Bednorz and K.A. Müller: Z. Phys. B64,
189 (1986).
6) H. Takagi, S. Uchida, K. Kitazawa and S.
Tanaka: Jpn. J. Appl. Phys. 26, L 123 (1987).
7) S. Tanaka: Jpn. J. Appl. Phys. 26, 218 (1987).
8) P.W. Anderson: Science 235, 1196 (1987).
9) C.W. Chu et al.: Phys. Rev. Lett, 58, 908(1987).
10) R.C. Haddon, A.S. Hebard, et al. : Nature 350,
320 (1991).
11) J. Nagamatsu et al.: Nature 410, 63 (2001).
12) As to Applications, see S. Tanaka; Proc. Int.
Conf. On M2SVI / Physica C 341-348, 31(2000).

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