Synthetic Metals, 33 (1989) 141 - 169 141

HIGH-TEMPERATURE SUPERCONDUCTING CUPRATES: SUBSTITUTIONAL AND RELATED STUDIES

A. V. NARLIKAR, S. K. AGARWAL and C. V. NARASIMHA RAO National Physical Laboratory, Hillside Road, New Delhi-110012 (India)

Abstract

Prominent characteristics of high Tc cuprates, making them different from conventional superconductors, are unusually small coherence length, anisotropy effects and the presence of weak links. These aspects provide novel features such as the observation of different kinds of Josephson phenomena in bulk samples, unusual magnetization behaviour, etc. However, they impose serious restrictions on the fabrication of thin film tunnel devices and on J¢ values for magnetic applications. Their crystal structure with low dimensional character, dependence on oxygen stoichiometry and the mixed valence character of the consti tuents make various studies relating to their processing, substitution, high pressure, etc. quite rewarding. Apart from superconductivity, some of the above structural features give rise to different kinds of luminescence phenomena. A concise account is presented of some studies relating to many of the above aspects carried out in this laboratory.

1. Introduct ion

With the advent of high-temperature ceramic superconductors the field of superconductivity has received an unprecedented impetus. The pioneering discovery of a superconducting critical temperature Tc of about 30 K of L a - B a - C u - O by Bednorz and Miiller [1] in itself was an extraordinary feat, but more than that it was a major breakthrough in the type of materials that were being explored for high Tc values, as it was inconceivable to think of ceramics in this context . Extensive research carried out subsequently has led to three prominent cuprate systems, namely Y - B a - C u - O , B i - S r - C a - C u - O and T1-Ba-Ca-Cu-O, which exhibit superconductivity above the liquid nitrogen temperature of 77 K, their Tc values being in the range 80 K to 125 K. Of these, the first includes a host of materials formed by replacing Y by other rare earths (La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb) wi thout any substantial change in the parental Tc of 90 K of the 1 - 2 - 3 compound [2].

Despite extensive experimental and theoretical efforts, the under° standing of high-temperature superconductivity in ceramic materials remains

0379-6779189/$3.50 © Elsevier Sequoia/Printed in The Netherlands

142

in a fluid state and contrary views have been expressed whether, except for their high T c values, there is anything really unusual about these materials. Unusual or not, the ceramic superconductors do possess some inherently distinct features, which make them behave differently from the conventional superconductors. Broadly, these features are: (i) small coherence length ~; (ii) anisotropy effects; and (iii) the presence of weak links. In contrast to conventional superconductors with Tc values of 10 K to 20 K and coher- ence length in the range 50 A to 500 A, the new materials possess a much smaller coherence length of 3 A to 30 A. The reason for this may be at tr ibuted to a small Fermi velocity (VF) and a higher T¢ through the relation ~ = 0.18 ¢i VF/hBT c which follows from the conventional microscopic theory. The smaller VF value is essentially a result of a smaller carrier con- centration (n) and a higher effective carrier mass (m) in the new materials.

The anisotropy effects stem from unusual crystal structures {Fig. 1) which these cuprate superconductors possess. Two relevant aspects common to all are an or thorhombic unit cell with long c-axis and highly conducting Cu--O networks in the a-b plane. Both the effective carrier concentrat ion and the carrier mobil i ty in the a-b plane are expected to be larger than along the c-direction, resulting in

~± \p , , , ] ~n±/

where Pn is the normal state resistivity and the symbols II and ± respectively correspond to parallel and perpendicular configurations with respect to the a-b plane. Since the ratio Pn±/Pn, ~- 250 for the 1 - 2 - 3 compound, the coherence length in the a -b plane is about 6 times larger than along the c-axis. The estimated values for ~ll and ~± are respectively around 15 A and 3 A. Similar anisotropy exists for other structures shown in Fig. 1. Intimately linked with the coherence length are the type II parameters Be1 and Be2 which in ceramic superconductors also exhibit anisotropy; Be21f > Be2± and Bclll < Bcl ± [3, 4]. This aspect, in turn, is reflected in the anisotropy of the critical current density J~. Hall measurements have also revealed anisotropies both with respect to the carrier densities and their sign [51.

An unusually small value of the coherence length, particularly along the c-axis, causes the transport of pair current across various kinds of inhomo- geneities and crystalline boundaries in ceramic superconductors to be governed by the Josephson tunnelling process (an important feature which is absent in conventional superconductors). As a consequence of this, both intergrain and intragrain connections in ceramic superconductors can serve as either proximity regions or weak links having properties significantly different from superconducting grains. This may lead to the bulk material exhibiting various Josephson tunnelling effects, a pronounced field dependence of J¢, field slippage occurring at a field much lower than B¢I, including low field magnetic hysteresis and an anomalous temperature dependence of B¢2, etc. [3]. A small coherence length leading to the weak

143

I

(a)

0 La

El Cu

O o

Y Ba2Cu 30,7_y

(b)

%

~.':0 0 = Vacancy "" 0 = Oxygen

• = Copper

~ o 4 J ~ : Bi 11 '

Bi • r •

S r ) • '

Bi 2 SI3.xCa x Cu 2 084y TIzBoz CaCu208

( c ) ( d )

--Ca --Cu -- Ba

i~oo -- T, - - 1 " 1

• • ~ - - B o

- - C u • • ~ - - C o

- - C u

. ! - - Cu - - B o

- - T I

- - Cu - - C a

- - C u

TlzBa2CazCu3Oio

Fig. 1. Crystal structures of compounds containing Cu--O networks: (a) (La, Sr)2CuO4; (b) YBa2Cu307; (c) Bi2CaSr2Cu2Os; and (d) T12CaBa2Cu20 s.

links and anisotropy aspects mentioned earlier make the critical current density of the polycrystalline materials strongly texture-sensitive and pose a formidable challenge in the realization of acceptable Jc values for technologi- cal exploitation.

Another aspect, again linked with the crystal structure, which makes ceramic superconductors different from conventional ones, is the fact that substitutional effects in them, with respect of their Tc, are strongly site dependent. Whereas the incorporation of magnetic impurities in the con-

144

ventional superconductors, by and large, has a deleterious effect on T¢, the same may not have so pronounced an effect in the cuprate super- conductors. On the other hand, nonmagnetic dopants in the new materials can drastically lower Tc values if they occupy Tc-sensitive sites of the Cu--O networks. Similarly, the oxygen stoichiometry, particularly in La- and Y- based cuprates, can have devastating effects on the crystal structure and the critical temperature. The overall behaviour of the new materials is largely influenced by the mixed valence character of Cu.

The oxygen content being so crucial, the materials synthesized under different thermal conditions, high pressures or varying degrees of com- pactions possess different Tc values. Interestingly, as with many other oxide systems, the oxygen vacancies can serve as host lattice defects leading to the various types of luminescence phenomena exhibited by these ceramic superconductors.

Many of the above-mentioned aspects have been manifested in studies on high Tc superconductors in the authors' laboratory which were initiated almost immediately after the preliminary results on La-based cuprates became known at the Valence Fluctuations Conference held during early January 1987 at Bangalore. A short consolidated account of the results, including those obtained in collaboration with other groups and laboratories, some of which have been already reported elsewhere, forms the mainstay of this paper.

2. Weak-link effects

Almost immediately after both La- and Y-based cuprates were synthesized in the laboratory, it became apparent that because of the way they had been processed (i.e., either by fusing and sintering together of const i tuent oxides at high temperatures, or by coprecipitation followed by sintering) the materials would be inhomogeneous, comprising super- conducting grains held together and separated by nonsuperconducting boundary regions. Since the samples showed no residual resistance at To, it was natural to think of a Josephson tunnelling process being dominant for the transport of pair current. This content ion was soon corroborated by systematic observations of d.c., a.c. and inverse a.c. Josephson effects on bulk samples.

2.1. D.c. and a.c. Josephson effects Studies on the I - V curves of bulk materials of YBa2Cu3OT-y (i.e., a

1 - 2 - 3 compound [6]) and BiCaSrCu20 x (i.e., a 1 - 1 - 1 - 2 compound [7]) at 77 K have revealed typical Josephson tunnelling characteristics. Both the zero voltage current with the critical current Ic of about 3 mA followed by quasiparticle tunnelling characteristics in the finite voltage regime for the 1 - 2 - 3 compound are depicted in Fig. 2. Also shown in the inset are the I--V characteristics for the 1 - 1 - 1 - 2 compound across a constricted

145

40 F- Z uJ 0

-40

O4 02

o -0.2

CD

-O.4

Bi Co Sr C u

F

-6 -4 -2 0 2 4 6 Voltage (mY)

YB%Cu3OT_y ~ - T=77K Y p ~ -

, "'o ~' i 5 20

- 8 0 - 4 0 0 4 0 8 0

Voltage [juV)

Fig. 2. I - V curves for YBa:Cu3OT-y bulk sample at 77 K. Inset depicts the character is t ics at 4.2 K for the BiCaSrCu20 x sample wi th cons t r i c t ion geomet ry .

geometry at 4.2 K. Such a configuration has been introduced to keep the I¢ value low and, more importantly, to reduce the number of junctions and thereby lower their randomness. The curve shows a zero voltage and a finite voltage pair current region, and a plateau region followed by a single particle tunnelling region. The finite voltage pair current region is understood to be a nonequilibrium effect analogous to that found for three-dimensional weak links in conventional superconductors [8]. The characteristic feature of the d.c. Josephson effect may be seen in the curves of the 1 - 2 - 3 compound where small magnetic fields have a substantial effect in lowering Ic. Self- induced steps, characteristic of the a.c. Josephson effect, are detected in the zero-field curve. These are smeared out by the application of a small external magnetic field. Figure 3 shows variation in the voltage change as a function of magnetic field for various biasing currents above Ic, found for a 1 -2 -3 compound. Due to the presence of a large number of random intergrain junctions, one does not observe the characteristic behaviour with well- defined periods, but instead a cumulative profile of all the junction results. However, if the sample is constricted to form a single bridge-like structure, the modulated behaviour of I c with magnetic field becomes more distinct, as may be seen for the 1 - 1 - 1 - 2 sample at 4.2 K (inset of Fig. 3).

For the a.c. Josephson effect studies, the sample was mounted inside the X-band waveguide, short circuited by a fixed plunger and was irradiated using a highly stabilised phase-locked microwave source of 9.965 GHz, having a frequency stability of 1 in 109 over a period of 2 h. Microwave- induced steps have been observed with increasing power levels at 4.2 K

146

z

u

BI Co Sr Cu 2 0 x o

o

J I i I l I , I

0 8 16 24 32

Solenoid cu~ent ~ A )

Bo2Cu 5 0 7 _ y T I 7 7 K I{~

L 58 / \ 2, 62

I ~ ] / J f_~"~ 4I I O'

-18 - L 2 -0 .6 0 0 .6 1.2 18

MAGNETIC FIELD (GAUSS)

<[

E v

h- z 0 Ill rY

r'F -4 ~_)

-8

VOLTAGE(juV)

Fig. 3. V - H curves at d i f fe ren t biasing cur ren t for a 1 - 2 - 3 c o m p o u n d at 77 K. Inset shows the magnet ic field m o d u l a t i o n of I c for the 1 - 1 - 1 - 2 sample at 4.2 K.

Fig. 4. I - V curves o f a 1 - 2 - 3 c o m p o u n d at 77 K for (A) zero and (B) 45 mV microwave p o w e r wi th F = 9.8795 GHz.

which are 20 pV apart, in conformity with the a.c. Josephson relation hf = 2 eV. Such studies have also been carried out on 1 - 2 - 3 samples which show that microwave-induced d.c. voltage is produced due to an inverse a.c. Josephson effect off-setting the zero-voltage position of the I - V curves. Also, the expected behaviour of I c with microwave power is indicated [ 6].

The I - V curves of bulk samples of the 1 - 2 - 3 compound, obtained in the presence of impressed microwaves or small d.c. magnetic fields, have revealed [9] oscillatory structures (Fig. 4) which seem to be very similar to those reported recently by Fulton and Dolan [10] for multiple, low capacitance, small area tunnel junctions of AI. These are explained as being due to tunnelling of individual electrons charging and discharging the junct ion capacitance, which results in discrete voltage jumps of e/C. The oscillations corroborate the presence of weak links in these materials, although the precise role of impressed microwaves and small magnetic fields in causing such oscillations, however, remains unclear.

147

2.2. Inverse a.c. Josephson e f fec t These studies [11 - 13] were carried out during the very beginning of

the advent of Y-based cuprates, in parallel with those of Chen et al. [14]. It is well known that both discretely quantised or continuously varying d.c. voltage can possibly result when unbiased Josephson junctions are subjected to microwave radiation. The induced voltage, in contrast to the diode rectification effect, shows polarity and is sensitive to small magnetic fields and varies with microwave power and frequency. It was realised that the observation of the effect would provide a direct confirmation of the presence of weak links in ceramic superconductors, and a systematic s tudy of microwave-induced Josephson voltage with increasing temperature might give some upper limit for Tc up to which the weak links persist. The work, which has been described in detail elsewhere [13], showed that in the so- called single phase 1 - 2 - 3 compound the microwave-induced Josephson voltage persisted up to 230 K, suggesting that some grains possibly remain superconducting until this temperature (Fig. 5). Similar observations were reported by Chen et al. [14] in their Y - B a - C u - O sample up to 240 K. From these studies it was concluded that a high Tc phase with a super- conducting onset temperature around 230 K perhaps coexists with the 90 K phase. In the course of the study of substi tuted samples of Y-based cuprates, it was found that although Tc for R = 0 was lowered by partial substi tutions of Ba by Ca or Sr, the samples showed significant resistance drops commencing at much higher temperatures, close to 300 K. Such resistance drops were reproducible but short lived with respect to thermal cyclings. The inverse Josephson effect studies of some of these Sr-doped samples revealed that the weak links persist up to temperatures of 288 to 299 K (inset to Fig. 5). The possibility of extra high Tc superconductivity above room temperature in the multiphase Y - B a - S r - C u - O system was

8 0 " l ~ - - ' ~ S - z Bo S r Cu30 x 60 I-\ FREQ : 40 L '~. 9.8843994G1-

> o ~ 70 rSO 23O 300

TEMP (K) Y B%Cu307 FREQ.: 9.872-~0 G Hi

N

70 no 150 190 230 270 310

TEMPERATURE (K)

Fig. 5. Temperature variation of microwave-induced voltage for compound. I n s e t i n d i c a t e s this variation for the Y - B a - S r - C u - O sample.

a Y - B a - C u - O

148

further corroborated by Ihara e t al. [15], who found nonlinear I - V charac- teristics in this material up to a temperature as high as 328 K. Again, the extra high T¢ phase was found to be short lived.

It is worth pointing out that the use of tunnelling effects to detect superconducting phases is not applicable to conventional superconductors, bu t is restricted to the high T c ceramics which contain weak links. However, the presence of random weak links is a major hurdle for utilising these new materials for Josephson tunnelling devices. Presently, efforts are being directed to form thin films of high Tc cuprates having grains with desired orientations.

3. Hall effect and low-field microwave absorption

Interestingly, the granular nature of the bulk materials is also reflected through Hall effect studies and the low-field microwave absorption experiments carried out using a conventional EPR set-up. The Hall measure- ments have been carried out using a 7 T superconducting magnet system with a variable temperature cryostat , both designed and fabricated in the laboratory. The Hall coefficient shows an anomaly across the super- conducting transition (Fig. 6), as has also been reported by others [16]. When the temperature is lowered below the onset-To the grains start expelling out the magnetic flux, with the result that the adjoining intergrain regions show an enhanced magnetoresistance. The carrier concentrat ion for 1 - 2 - 3 compounds estimated from these measurements is found to be around 1021/cm 3.

These materials have also been studied using the Varian E-109 X-band EPR spectrometer [17]. The samples containing impurity phases show a prominent Cu line with g-value anisotropy corresponding to gll and g± of the ion in the or thorhombic axial symmetry. Since the signal in carefully prepared samples was of considerably diminished intensity, its further s tudy did no t seem relevant for superconductivity.

A more interesting aspect of this s tudy is, however, the observation of the so-called zero-field signal. Unlike the EPR line, this signal is a non- resonant type of microwave absorption, that is, the condit ion g p ~ / = hv does no t hold here. The signal is a field derivative of the imaginary part of the magnetic susceptibility as a function of the externally applied field and hence its origin is linked with a change in X". Figure 7 shows a zero-field signal in the temperature range 78 K to 89 K for a 1 - 2 - 3 sample. The appearance of the signal coincides with Tc and its intensity grows with decreasing temperature. Its origin is linked with the field penetrat ion occurring at low fields in the weak links, i.e. the lower critical field Bcl of the weak links, causing a change in the susceptibility [18]. Since Be1 of the weak links is only about a few oersteds, the microwave absorption takes place at very low fields. When the field is swept and decreased back to zero, some of the magnetic flux may get trapped and the microwave absorption

149

5 . 0

4.54.0 q Y B°2Cu307-y

I.--

I0 ~0.ST Z LLJ

. . . . . . . . . . . . . . . . . . . . . . . .-".LL.~ ~:gT o.o _I I

, , 0 .0 1.2 2.4 3 .6 60 ~ 8o 90 ,oo ,;o ,20 ,3o ,4o TEMPERATURE(K) B(m T)

Fig. 6. Temperature variation of the Hall coefficient across the superconducting transition for a 1-2-3 compound.

Fig. 7. Temperature dependence of the intensity of zero-field signal in a 1-2-3 sample.

3 . 5 u

~ 2 , 5

2.(

<

may occur in the zero field. The method is found convenient for determining Tc and for estimating the size of the superconducting clusters.

4. Substi tutional effects

At tempts at complete or partial substitutions of any of the consti tuents of the high Tc cuprates are, in general, made with the three broad objectives: (i) to replace a toxic, less abundant or expensive componen t with a less hazardous, more abundant and cheaper one wi thout making any compromise on superconduct ing properties; (ii) to study any anomalous changes in Tc; and {iii) to explore the possibility of forming a new superconducting phase or system. Substi tutional studies have been carried out in 2 - 1 - 4 , 1 - 2 - 3 and 2 - 1 - 2 - 2 systems where one or more of the consti tuents have been substi tuted by isovalent or nonisovalent cations of the compatible ionic radii with respect to the site occupancy. These studies are generally complex and not free f rom ambiguities as regards to the site occupation. The site occupancy need not necessarily be unique and predetermined, and is control led by the valence state of the dopant and the process parameters. The problem can be best resolved through detailed neut ron diffraction. The nonisovalent cation substitutions in the parental s toichiometry inevitably

150

lead to changes in the oxygen stoichiometry which, particularly in 2 - 1 - 4 and 1 - 2 - 3 systems, can bring about or thorhombic to tetragonal (O-T) transformation and cause a rapid suppression in To. In both these systems, the oxygen stoichiometry and low-dimensional Cu--O networks are crucial for high T¢. Any substitutions other than for Cu, if they keep the crystal structure intact and do not interfere with these two features, have little influence in changing T¢. Both Bi- and Tl-based cuprates, on the other hand, are much less sensitive to oxygen stoichiometry and the role of cations other than Cu is not so clear.

Many substi tutions have been made in 1 - 2 - 3 systems where one or more of the cations have been replaced by Li, Cs, Mg, Ca, Sr, Ti, Zr, Nb, Ta, Cr, Fe, Co, Ni, Zn, Cd, Hg, A1, Si, Sn, Pb. Sb, Bi and Te. The system is vulnerable with respect to complete replacement of Y, Ba, or Cu by any of these cations, which results in complete destruction of superconductivity. The system can tolerate only partial substitutions although with noticeable reduct ion in T¢ (R = 0). Only trivalent rare earths have been found to fully substi tute for Y in a 1 - 2 - 3 system wi thout any significant change in Tc [2].

4.1. S tront ium and calcium substi tutions Isovalent substitutions of Sr and Ca at the Ba site in Y-based cuprates

in the nominal composit ions of 2 - 2 - 3 and 1 - 2 - 3 provide some interesting features [19, 20] different from many other substitutions. Samples of both series have shown noticeable resistance drops (Fig. 8), starting from room temperature down to abou t 240 K, followed by a plateau region and subsequent resistance drop leading to R = 0 at temperatures below 80 K. Such resistance drops were found to be reproducible but their magnitude gradually decreased with repeated thermal cyclings. In some of these samples the inverse a.c. Josephson effect studies, described earlier, indicated the presence of weak links up to room temperature, suggesting the possible

2 4 0 - 2 2 0 - 0 - "(2 Sa laSrG2Cu30e-y ( I )

"@- YZ BatsSro.2Cu3Os-y(E) 2 0 C 43- Y2 Ba Sr Cu3Os_y

180 - A - Y Bat25COO.75.Cu3 07_y

g ,40 £ w 120

ioo

~- 8(?

2O 0 I l i l l l t l 1 1 , J I l J l I

.50 9 0 130 170 210 2 5 0 TEMPERATURE (K)

48O

44O

40O

360

320

28o

240 z

2oo~ 160

~20

80

4O Io

Fig. 8. R vs. T curves fo r va r i ous Y - B a - C u - O s a m p l e s d o p e d w i t h Sr a n d Ca.

151

presence of extra-high T¢ phases. The interconnectivity of such phases in resistance drops seem to deteriorate on thermal cyclings. Subsequent studies of Ihara e t al. [15] and Bhagat [21] again gave indications of super- conductivity at room temperature in Sr-doped samples.

4.2. F l u o r i n e s u b s t i t u t i o n

These early results stimulated considerable interest and gave impetus for further search for extra-high Tc superconductivity in this laboratory and elsewhere. In these materials, since oxygen is as important as Cu for super- conductivity, it was perceived that by substituting O by the highly electro- negative anion F, the critical temperature might change significantly. First results of partial replacement of O by F were reported by Ovshinsky e t al. [22], who synthesized YBa:Cu3F206_y by incorporating barium fluoride in a solid-state reaction. R = 0 was found to be at 155 K and for some of the samples the onset-To was around 300 K. However, subsequent efforts by others, including those at the authors' laboratory, using a chemical approach, did not result in any change in the usual Tc of 90 K. The negative result is generally attr ibuted to the failure in incorporation of F in the 1 - 2 - 3 lattice. This motivated us to look for other approaches and efforts were directed to introduce F through implantation. The resistivity curve [23] of a fluorine- implanted sample with a doping level of 1019 atoms per cm 3 is shown in Fig. 9. The striking feature of the Figure is the resistive drop at 300 K with width less than 2 K. R = 0 is, however, attained at about 89 K, which corresponds to the Tc of the starting 1 -2 -3 compound. In contrast to Sr- and Ca-doped compounds described above, the implanted sample was far more stable with respect to thermal cyclings, although eventually the high temperature resistive transition at 300 K faded out. Interestingly, unstable extra-high T¢ phases showing R = 0 at about 148 K were also seen in the ion-implanted studies carried out by Xian-Ren e t al. [24]. It is natural to ask what is the role of F in enhancing T c? We feel that F occupies either an interstitial site or any of the vacant O sites, probably the O (5) site. Because of its high electronegativity, F perhaps creates excess holes in either the p-

8O0

ATO0

zSO0

400

3o0

20C

IO0

0 7O

Y- B o - C u - O F

I00 125 150 175 200 225 250 275 3(]0 325

TEMPERATURE (K)

Fig. 9. R vs. T curve for the fluorine-implanted sample.

152

band of O or the d-band of Cu, which again get shifted to the oxygen band and thus the hole concentrat ion would go up and enhance the T¢.

4.3. Zinc substitutions in 2 - 1 - 4 and 1 - 2 - 3 compounds As compared to the search for extra-high Tc superconducting phases,

no less interesting are the situations where relatively small substitutions give rise to anomalously large depressions in To. Such substitutions are mainly at Cu sites and include both magnetic and nonmagnetic elements like Fe, Co, Ni, Cr, Zn, Ga and A1 [25 - 30]. Initially, the T¢ depression in most cases is gradual with dopant concentration, bu t beyond a certain substi tution level T¢ drops rather abruptly. As most of these elements are nonisovalent with respect to the divalent Cu, their incorporation at the Cu sites causes the oxygen stoichiometry to change and this eventually brings about O-T transformation. The abrupt fall in T¢ which takes place beyond a certain level of substi tution is a manifestation of the O - T transformation. In many ways Zn offers some peculiar features like (a) a fixed divalent state, (b) a completely filled d-shell, and (c) ionic radii (0.74 A) close to that of divalent Cu (0.72 A), which make its substitutional s tudy notably different from others. The results of Zn substitutions in 2 - 1 - 4 and 1 - 2 - 3 compounds are shown in Figs. 10 and 11 [31, 32]. In both the systems T¢ decreases linearly with Zn concentrat ion (Fig. 12) at the rate of 11 K and 15 K per atomic percent of Zn, respectively. This is considerably faster than for the other elements ment ioned above, where the rate of Tc depression prior to the O - T transformation is less than 5 K/at.%. The sudden drop in T c associated with the O - T transformation is not seen in the case of Zn, and in both 2 - 1 - 4

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~ ~ - ~ - - - s ~ ~ X = O, 04

6.0

5,0

E x . o . o 3 X= 0.02 4.0

(.~ X= 0.0 t

U~3.0 X= 0 .005 •

2.(1

1C

0 0 t0 20 30 40 50 60 70 80 90 t00

T E M P E R A T U R E (K)

Fig. 10. R vs. T plots for various Zn concentrations in Zn-doped La-Sr-Cu-O samples.

153

I0

9

8

7 E

E 6

U5

~ 4

a: 3

2

I

©

0

: 8 o 2 ~ X =0 120 (CUl-x Znx)3 07-y -~- X-O01

~ .^ . . --i'- X =0017 18 X=O 027

• -6- X- 0 033 16 X =0043

~ X =005 14 "~ i ~ 41.- x . 0057 ~

t " -u-- ×oc ,o ,2~

~ ~10 i

I0 20 30 40 50 60 70 80 '90 I00 I10 120 150

TEMPERATURE (K)

Fig. 11. R vs. T plots for various Zn concentra t ions in Zn-doped 1 - 2 - 3 samples .

I 00

eo~

t 80 ~ 70

~'6o

~ 5o

20

10

14 F • L a - S r - C u - O I- • Y - B a - C u - O / A

2

~ - ~ , ~ . o , 2 3 4 5

\ \ . . . . . . . . . ~ ' , ~ -o- Tc On.. , ~x,, • - , , ..41- Tc (R-O)

1 2 3 4 5 6 7

Zn conc. x (at%)

Fig. 12. Concentra t ion d e p e n d e n c e o f Tc (R = 0) and onset-Tc for 2 - 1 - 4 and 1 - 2 - 3 samples . Variat ion o f trans i t ion w i d t h A T c w i th concentra t ion is s h o w n in the inset.

and 1 - 2 - 3 systems the linear Tc decrease continues right up to about 2.2 at.% and 5.9 at.% of substitution respectively for Cu when the superconduc- tivity finally disappears. These are the smallest dopant concentrations known for the destruction of superconductivity of the two systems. Interestingly, over this substitution range the X-ray studies of the above samples have given no indication of the O-T transformation and, moreover, there is no noticeable change in the lattice parameters either; both these observations conform with the preliminary findings reported earlier [29, 30] . Strobel e t al. [33] found little change in the oxygen stoichiometry resulting from Zn substitution. These factors make the interpretation of the data for both the systems far less ambiguous.

154

Apart from T¢ depression, the transition width in the two systems also increases systematically with Zn substitution. Interestingly, as may be seen from the inset of Fig. 12, the plot of the transition width with Zn concentra- tion falls on the same line for the two systems. Although several factors can cause broadening of the superconducting transition, the observed systematic increase of equal magnitude may be attr ibuted to disorder and composition fluctuations caused by Zn substitution. From similarities in ionic radii, Zn should be strictly confined to Cu sites in Cu--O planes, as with the 2 - 1 - 4 compound, or along the chains as well as the planes in the case of the 1 - 2 - 3 compound. One can put forth contradictory arguments for Zn to occupy octahedral or tetrahedral coordinations. In the 1 -2 -3 system Cu(2) sites (i.e., planar sites) are closer to the former, while Cu(1) sites (i.e., chain sites) are closer to the latter. Since the increase in the transition width with Zn substitution is identical in the two systems, it seems reasonable to conclude that Cu(2) sites would be preferred.

The anomalously large T~ decrease with Zn concentration can be understood in terms of its unique electronic structure. Since the d-shell of Zn is completely filled, there is considerably diminished overlap between its d-orbitals and the p-orbitals of O. Secondly, its nonfluctuating divalent oxidation state does not permit transfer or receipt of electrons or holes. Consequently, Zn on Cu sites virtually becomes an obstacle for charge carriers to move during conduction. Also, the concentration of holes which are known to be the charge carriers in oxide superconductors is reduced. These aspects cause a reduction in Tc and turn the nature of the resistive transition curves in both 2 - 1 - 4 and 1 -2 -3 systems from metallic to semi- conductor type as Zn is progressively added (Figs. 10, 11). Since the coherence length along the a-b plane has a value of about 24 A and 14 A respectively for 2 - 1 - 4 and 1 -2 -3 compounds, it would take only 2.2% and 5.9% of Zn at Cu sites to intercept the coherence of the superconducting state and quench superconductivity; this agrees with the experimental values. Finally, since these measurements have indicated that Zn substitutions in both 2 - 1 - 4 and 1 - 2 - 3 systems give rise to almost identical effects on super- conductivity, one can safely conjecture that the microscopic origin of pair condensation in both 30 K and 90 K superconductors is unlikely to be radically different [34]; the actual mechanism whether it is RVB, excitonic or even phononic, however, remains an open question.

4.4. Tin substitution: M6ssbauer studies The substitution of Sn at low doping levels in 2 - 1 - 4 and 1 - 2 - 3

systems is nowhere as deleterious as that of Zn. In contrast to the undoped 1 - 2 - 3 sample with onset-To = 91 K and Tc (R = 0) = 89 K, the behaviour of YBa2_xSnxCu3OT_y sample shows only a slightly broadened transition spread over 95 K to 82 K, which is not much changed when the Sn con- centration x is raised to about 0.3. At higher concentrations, however, O-T transformation would take place, presumably due to the change in the oxygen stoichiometry, which causes superconductivity to be rapidly

155

quenched. The marginal decrease in T¢ observed at low doping levels makes the site occupancy of Sn in the 1 - 2 - 3 system ambiguous. Purely from the viewpoint of ionic radii, Sn (either divalent or tetravalent) should prefer the two Cu sites and thereby significantly deteriorate To, in analogy to Zn substitution. This is contrary to observation. On the other hand, in the perovskite ABO 3 unit cell, the tetravalent B atom occupies the body centre position and, consequently, in the triple perovskite 1 - 2 - 3 one expects the tetravalent Sn to occupy the Y and Ba sites. But, the ionic radii of tetra- valent Sn is much smaller than that of Ba, so the possibility of Sn at the Ba site seems equally uncertain. In view of this, the actual occupancy of Sn at various cation sites may be metastable and thus controlled by the process parameters used in the synthesis of the sample. In an a t tempt to gain insight into this problem, MSssbauer absorption spectroscopy using Ca 119SNO3 and a constant acceleration drive has been carried out on samples of YBa:-xSnxCu3OT-y with x = 0.2 and 0.3 in the temperature range 290 K to 80 K, which includes the superconducting transition regime.

The results, which have been discussed in detail elsewhere [35, 36], show that the observed MSssbauer spectra cannot be fitted at all to a single Lorentzian lineshape, but instead two single lines, a quadrupole doublet along with a single line or two quadrupole doublets, can provide a reasonably good fit. This suggests that Sn does not occupy a single unique site but goes to two crystallographically inequivalent sites of the 1 - 2 - 3 unit cell.

The temperature variation of two line components 1 and 2 for the sample with x = 0.2 is shown in Fig. 13. The change in the respective intensities of the two components is small as the temperature is decreased from 290 K to 118 K, suggesting a relatively large value of the Debye tem- perature. If the temperature is lowered further to about 95 K, which is the onset-Tc of the sample, the intensity of component 1 increases while that of componen t 2 remains almost invariant. Across the superconducting transition region, i.e. from 95 K to 82 K, component 1 increases in intensity while component 2 remains unperturbed within the experimental accuracy (Fig. 14). Quadrupole double t and a single line and the two quadrupole double t fittings also gave essentially the same temperature variation as that of single line components 1 and 2 respectively. It is worth mentioning that for fitting the spectra with two quadrupole doublets, the splitting of the second component was constrained to be double that of the first [36]. The isomer shifts of both components in the two samples studied correspond to the tetravalent Sn ion regardless of the type of fitting used. It is, however, not possible from these measurements alone to unambiguously conclude which sites the Sn ions occupy, as equally good fits to the spectra axe obtained in different ways. Except for the Y site, which is eight-fold coordinated by an approximate cube of oxygen atoms and thus would not experience any significant efg (electric field gradient), all other cation sites, namely Ba, Cu(1) and Cu(2), are expected to experience varying efg values. Between Cu(1) and Cu(2) sites the former is expected to experience a larger efg. Various fittings described above can thus be interpreted in various ways.

156

Z 0 0 03 03

03 Z

n- l--

LO C >

_J Ld I n-

O . ,v+8 .0 +4 -0 0"0 - 4 " 0 -8"0

o.52 f

0.5 ' ~-~}..~_

~ a 4

m0.26 :?~t I t - " 0.18 ' ~"

I . , . = . i I

0.6 (o,

o.5tt 0.4

8O I00120 28O 300 VELOCITY (mm/s ) TEMP. (K)

Fig. 13. M6ssbaue r a b s o r p t i o n spec t ra of Sn (x = 0.2, see t e x t ) f i t t ed to two c o m p o n e n t s 1 and 2 a t d i f f e r en t t e m p e r a t u r e s .

Fig. 14. Va r i a t i on of the in t ens i ty o f the t w o c o m p o n e n t s as a f u n c t i o n o f t e m p e r a t u r e .

As MSssbauer studies of Eu in 1 - 2 - 3 samples, where Eu is subst i tuted for Y, have shown [37] that the intensity of the absorption line remains invariant across the superconducting transition, the single line component 2 in the above s tudy can be ascribed to Sn on the Y site which experiences little efg. The local vibrational modes on the Y site are insensitive to superconduc- tivity. On the other hand, component 1, which shows a temperature variation across the transition, may be related to Sn occupying either the Ba site or the Cu(1) site, i.e. the Cu site with larger efg. The vibrational modes at these sites are sensitive to superconductivity. The third type of fitting, namely the two quadrupole doublets, may arise if the Sn occupies both Cu(1) and Cu(2) sites, as suggested by Boolchand e t al. [38]. But, perhaps this does not apply to our samples, as only one of the quadrupole

157

doublets is found to show temperature dependence, whereas phonon modes at both the Cu sites are expected to be sensitive to superconductivity.

The above studies indicate that phonons are involved in superconducti- vity. The different temperature dependences of the intensities of the two MSssbauer components clearly suggest the nonisotropic behaviour of the lattice rigidity of two corresponding sites at a temperature close to T c . This aspect is reflected in the low-temperature X-ray diffraction studies carried out on these samples. The temperature dependence of the orthorhombic strain is found to show [36] anomalous behaviour near the superconducting transition temperature {Fig. 15). The unit cell volume, however, does not show any clear anomaly. The interrelation between these observations and the mechanism of superconductivity is still to be established.

4.5. Zirconium substi tution Substitution of Zr at all the three cation sites has been explored and the

studies carried out reveal some interesting features. In some ways Zr should be analogous to Sn in 1 -2 -3 systems in that it may occupy the Cu sites, Y site or even the Ba site. Many nominal compositions have been tried to realise Zr substitutions at different cations sites [39]. These efforts, however, turned out to be futile in the sense that in almost all situations the material was found to be multiphase containing barium zirconate and unreacted zirconia, as revealed by X-ray diffraction studies. In the nominal composition when Cu was replaced partially by Zr and the sample was processed in the usual way, as followed for the 1 -2 -3 compound, the XRD revealed the presence of BaZrO3 along with the characteristic diffraction lines of the 1 -2 -3 compound as the major superconducting phase. Super- conducting transition curves with increasing Zr concentration are depicted

178

174

170

166

x

I o >

40'

t 20

I0

0 I I I I [ 70 80 90 IO0 IlO 120

TEMPERATURE (K)

Fig. 15. Plots of un i t cell vo lume ( top ) and the o r t h o r h o m b i c strain as a f u n c t i o n of temperature . Arrows indicate their respect ive r o o m temperature values. Solid l ine is on ly a guide to t he eye.

158

in Fig. 16. Double transitions were at tr ibuted to 1 - 2 - 3 phases differing in oxygen stoichiometry. The sample with the highest Zr concentrat ion showed semiconducting-like behaviour characteristic of the dominance of the BaZrO 3 phase intercepting the interconnectivity of superconducting grains. When samples were synthesized with Zr partially replacing Y, they showed a higher onset-Tc of 103 K and R = 0 at about 90 K (Fig. 17). Apart from the

, . . . t~oo ~ / I ~ ?,oo ~r " ~ - ~ = 1 =°° W I ~ ~ o o ~

'P~rt~! ~;; ; ; : ; ' ; ; ; : ; ; ; - ; 1 *o 'rtl ! ' P V I I 1 ~°

60 90 120 TE I;:ER;TUR E21:K) 24.0 270

Fig. 16. R vs. T plots for YBa2Cu3_xZrxOT_y.

= I ] "~-' YosZrosBozCu3 07-Y I 6°t I +Yo~Z,.Bo,co, o,_y i

,o r ,0 t . . . . . , . ~ ~ o 451- 45ob i~,~ "~'- i -lia::o

~o~ ~oo~ _ . ~ t ~ f ~ 4,~_

,op-,ooUI I 1' ~t '~ -~°i)il 1' o- "" " " ~ ' ' ~ ' '~;o' '~o ~-:o

TEMPERAT U R E (K)

Fig. 17. R vs. T plots for Y l -xZrxBa2Cu307-y samples.

159

presence of BaZrO3, the XRD showed diffraction lines different from the characteristic 1 - 2 - 3 pattern which were assigned to the superconducting phase 1 - 4 - 5 [40]. At tempts made to partially replace Ba and also both Y and Ba resulted in the formation of BaZrO3, the usual 1 - 2 - 3 and another phase identified as 3 - 3 - 4 [40]. Because of the presence of superconducting multiphases, the width of the transition curve was broad with R = 0 occurring at 68 K. In all these substitutions, Zr could not be incorporated in the system, presumably because of inadequate synthesis parameters, the importance of which is well manifested in Bi- and Tl-based cuprate systems where the sintering schedule has been proven to be crucial for realising phases with a range of Tc values.

4.6. Lead substi tution in Bi-based cuprates Superconductivi ty in the B i - C a - S r - C u - O system has also been

investigated. Various stoichiometries like 1 - 1 - 1 - 2 , 2 - 1 - 2 - 2 and 2 - 2 - 2 - 3 have been studied. These form multiphase systems, but with appropriate heat treatments the relative dominance of different phases can be changed. Basically, one notices two superconducting phases, one with Tc (for R = 0) of about 85 K and the other with a higher Tc of about 110 K, having an onset-To of about 125 K. Studies carried out here (Fig. 18) and elsewhere have shown that longer annealing times of a few hundred hours result in the formation of the higher T¢ phase, while 10 to 20 h of annealing suffices to form the lower Tc phase whose stoichiometry has been assigned to be 2 - 1 - 2 - 2 . Various nominal composit ions like 4 - 2 - 3 - 3 , 4 - 2 - 3 - 4 , 4 - 3 - 2 - 3 , etc. have been explored by Rao et al. [41] and some of these samples, interestingly, have given indications of extra high T¢ values above 200 K, although R = 0 is found to take place below 50 K. An extra high temperature resistance drop has also been noticed in the nominal 1 - 1 - 1 - 2

Bi 2 Ce Sr 2 Cu20 x BiCo SrCu20

/ B-52hrs. ~ 801- t I

f . . f :I I I ,oop 1 " / ~(~ ~40 f'T -,ot/ /

50 70 90 IIO 130 150 170

TEMPERATURE (K)

Fig. 18. R vs. T plots for various B i - C a - S r - C u - O samples. Inset shows a resistive d rop at ext ra-high t empera tu re .

160

composition, depicted in the inset of Fig. 18. The extra high Tc phases are generally ascribed to the intergrowth of superstructures possessing a higher c-parameter due to greater number of Cu--O planes.

The most extensively studied substitution in Bi-based cuprates is that of Pb, partially in place of Bi. While the majority of reported studies are centred around the higher Tc phase where Pb substitution is found to enhance To, in the present work attention is focused on the low Tc 2 - 1 - 2 - 2 phase formed after short-duration annealing. The results obtained reveal that up to about 15 at.% Pb substitution for Bi, T¢ decreases gradually at a very slow rate of less than 1 K/at.%. The transition width, however, becomes narrow in conformity with the results of others for the higher Tc phase [42]. XRD shows the effect of Pb addition is to reduce the ortho- rhombicity with respect to a- and b-parameters without any significant change in the c-parameter.

(a) (b)

(c)

Fig. 19. (a) SEM micrograph of Bi2CaSr2Cu20 x sample; (b) SEM micrograph of Pb-doped Bi2CaSr2Cu20 x sample; (c) SEM micrograph of Pb-doped Bi2CaSr2Cu:O x sample showing an individual crystallite.

161

SEM studies of both undoped (Fig. 19(a)) and Pb-doped 2 - 1 - 2 - 2 samples reveal the presence of dendrites; in the latter, needle-like crystal- lites are frequently seen (Fig. 19(b)). Both copper and lead oxides have been successfully used as flux to grow the single crystals and the presence of needle-shaped crystaUites is presumably due to the flux-like role of lead oxide used, as the EDAX analysis of the individual crystallites (Fig. 19(c)) failed to detect the presence of Pb.

5. High pressure studies

The effect of externally imposed static pressures at different tem- peratures on the superconductivity of these high Tc cuprates has been investigated. The high pressure facility used was a hydraulic cubic press with six tungsten carbide anvils which could be advanced synchronously with hydraulic rams. The reaction cell containing the superconducting disc was made of pyrophillite with a cylindrical cavity drilled through it. The sample could be subjected to a hydrostat ic pressure up to about 60 kbar and to a maximum temperature of 1400 °C. It should be mentioned that the experimental set-up did not permit the in situ low temperature characteriza- tion of the samples under high pressure, but was restricted to the application of pressure as part of the synthesis process where superconducting measure- ments could be made after removal from the pressure cell.

Superconducting sintered samples of La-, Y- and Bi-based cuprates exhibited varying response as regards to their Tc to the imposed pressures in the range of 20 to 60 kbars at ambient temperature. Superconducting 1 - 2 - 3 samples lost superconductivity completely after the application of pressure [43]. This effect is at tr ibuted to pressure quenching occurring in these samples, resulting in a significant loss of overall oxygen content, driving them to the tetragonal form. The samples, however, regained super- conductivi ty when they were oxygenated. Curiously, the behaviour of onset-T¢, mid point-T¢ and T¢ (R = 0) with pressure for these samples turned out to be very similar to the in situ pressure studies of 1 - 2 - 3 samples [44, 45], although the methodology in the two approaches was quite different. Studies were also carried out on reacted, nonsuperconducting samples which were compacted using the above facility at pressures ranging from 20 to 50 kbars, followed by oxygenation under identical conditions [46]. Because of the reduction in the porosi ty of the samples compacted at higher pressures, the T¢ values attained were expected to decrease with compacting pressure. The experimental results, however, revealed that Tc rose steadily towards 90 K with compaction. This was unders tood from the SEM studies of the samples which showed increased crack formation at higher pressures, the cracks serving as effective channels for oxygen flow during sintering.

In contrast to Y-based cuprates, the La-based superconductors, when subjected to pressures, did no t lose superconductivity completely, but

162

instead the Tc was reduced. The Tc of 33 K (R = 0) of the La-Ba-Sr-Cu-O sample was gradually decreased to about 28 K after applying a pressure of about 60 kbars. The onset-T¢ was increased to about 45 K and made the transition broader. The 2 - 1 - 2 - 2 Bi-cuprate also exhibited a gradual T¢ depression after the application of pressure. Interestingly, however, the pressure had a further effect of turning the metallic-type resistance behaviour to semi-metal-like (Fig. 20) [47] and with the sample subjected to the highest pressure, although it showed the transition, the R = 0 state was not attained at temperatures down to 62 K. Both the observed negative dR/ dT and decrease in T~ due to imposed pressure are believed to be manifesta- tions of ensuing disorder in the C u - O networks of 2 - 1 - 2 - 2 structures. Whether such an effect results from perturbation directly in the Cu--O networks or indirectly through disorders in other planar stacks of the structure is a moot question to be settled only through further experimentation.

16

o

~4 =

12

I I ° °ooa ao aoa oo o ooa o a

= = =~= ID • • =o =" • •

~ , . . _ . : . , . . :,. :. :. ~.:.;.,... ~ ' & * * • • • •

• = i f= = • ==== l= l =

• i

06 %, , o

• i z Q ~ • . •

J = 0.'~ "# /

0.~ = •

02 '

0 . 1 " / "

O0 ~ ~; ' ' ; ~ ' ' ~ o ' ' (50 8 0 IO0 120 140 I 0 I 0 2 0 0 2 2 0 2 2 6 0 2 8 0

TEMPERATURE(K)

0.9

0.8 \

0 .7

n

°o o

Bi2Col Sr2Cu20x • 0 K bor x 2 0 K bor

o 3 0 K bor

c, 4 0 K bar

o 5 0 K bar

Fig. 20. Normalised R vs. T c u r v e s for Bi2CaSr2Cu2Ox at d i f f e r e n t pressures . ,

6. Electroluminescence

The presence of oxygen vacancies, particularly in La- and Y-based cuprates, has proven to be crucial for high T c superconductivity. Interestingly, the oxygen vacancies as host lattice defects are found to be

163

equally vital for the phenomenon of luminescence in oxygen-dominated lattices and, consequently, it was felt that the ceramic cuprate super- conductors ought to exhibit the phenomenon of luminescence. Indeed Cook e t al. [48] did observe thermoluminescence in Ho-Ba-Cu-O and Gd-Ba-Cu-O systems excited by X-rays and, more recently, cathodolumi- nescence was also reported by Luff e t al. [49] in Y-Ba-Cu-O.

Both Y- and Bi-based cuprates have been studied for electrolumi- nescence [50, 51] by using an insulating transparent cavity and by applying an a.c. electric field of variable voltage and frequency. The samples have shown luminescence emission at a threshold value of voltage with brightness

40

32

e -

, 24

rn

8

0 I 300 I IO0

(a)

o yo4ZrosBa2 Cu 3 CT_y l • YosZro.sBa2 Cu 3 07_ Y /

"T I I 500 7O0 90O

Volts

1.6

• YO.4 Z{o.sB° 2 Cu3 0 7 - Y e / o Yo.~Zs..B%Cu3CT_y /

1.2 e y0.75 Z~sBa 2 Cu~0 ~

_3

2.69 2.77 2.85 2.9.'3 3,20 (b) Log V

Fig. 21. (a) Brightness B vs. excitation voltage V p]ots for different Yl_xZrxBa2Cu3OT_y samples ; (b) p lo ts o f In B vs. In V.

1 6 4

B at a fixed frequency increasing with applied field. Although the general behaviour observed was similar for all samples, the magnitude of the effect varied. Some typical curves for Zr-substituted samples of section 4.5 are shown in Fig. 21(a). Plots of In B versus In V {Fig. 21(b)) have been found to be linear, giving a power law relation of the form B = cV ~ where c is a constant and the numerical voltage exponent has a value close to 2. This is taken to be indicative of the bimolecular nature of the electroluminescence process involving recombinat ion of electrons with luminescence centres. On increasing the excitation voltage, the luminescence emission from the sample surface is found to be inhomogeneous, indicating a spatial distribu- tion of the light-emitting regions. The possible relevance of microstructural features like twin boundaries and grain boundaries and their interrelation with the light-emitting regions might prove rewarding. Interestingly, YBa2Zr3OT_y, which did not contain Cu, showed neither superconductivity nor electroluminescence. For samples with both Cu and Zr, the threshold voltage increased with Zr content and brightness decreased; the Tc for such samples was low. The charge-transfer process, which is important in both phenomena, seems to be hindered by Zr substitution. It is interesting that even for Bi-based cuprates [51] the electroluminescence brightness is found to be consistently larger for higher Tc samples. Further studies are clearly necessary in this area.

7. Critical current and the nature of flux pinning

In this last section of the paper we consider the problem of current transport in high Tc materials and to do so one has to go back from where we started, namely their anisotropy and weak-link aspects. Although high temperature superconductors have been in existence for the last 2 years, the problem of realising acceptable values for the critical current density Jc in a high magnetic field for technological applications continues to remain one of the main challenges of high-temperature superconductivity research today. The question of Jc and flux pinning in high-temperature ceramic superconductors has previously been considered at length by the present authors [52, 53]; here we bring out some of the salient features.

An exceptionally small range of coherence and strong anisotropy with respect to the Cu--O networks are two important factors that severely restrict the critical current density Jc. The ceramic cuprates are extreme type-II superconductors the J¢ of which is primarily controlled by pinning of Abrikosov's flux vortices by inhomogeneities. The ideal situation for the opt imum interaction between a normal particle and the flux line core is that when the former is of radius ~. In the case of a single crystal, the anisotropy of ~ and Be2 lead to different Jc values for three different orientations of the mutually perpendicular field and current.

For H(c-axis)

Jc (a- or b-axis) = [Be 2 ~LI/8¢0] [ 1 -- B/B~2± ] (I)

165

For H(a- or b-axis)

J~(c-axis) = [B¢2~±/8¢0] [1 -- B/B¢211 ] (II)

For H(a-axis)

Jc(b-axis) = [Bc 2 ~11/8¢0] [ 1 -- B/Bc211 ] (III)

The anisotropy aspects make the orientation (III) most favourable for J~ (Fig. 22); in the other two orientations, J¢ is lower or it decreases rapidly with the applied field. The second restriction stems from what is termed the weak-link problem. The small range of coherence necessitates the current transport to be controlled by Josephson tunnelling where Jc decreases rapidly with a small imposed magnetic field. Since the range of coherence in the a-b plane is several times larger than in the c-direction, the current transport in the orientation (III) with respect to B is relatively free from the weak-link effect. Because of the above two restrictions, in polycrystalline samples the unfavourably oriented grains (orientations I and II) can become normal at low transport currents. To circumvent this problem, the desired texturing of grains in bulk samples is achieved by suitable heat treatments or, in the case of deposited thin films, by optimization of process conditions.

Some of the available Jc data on polycrystalline sintered compacts, single crystals and thin films of 1 - 2 - 3 compounds were discussed earlier [52]. In general, it is found that the J¢ of bulk materials is low, the maximum value in zero field at 77 K being 1.7 X 104 A/cm 2, obtained at Bell Laboratories by allowing the sintering temperature to rise slightly above the melting point. Jc at 77 K is strongly field dependent but, surprisingly, at low temperatures J~ is relatively insensitive to B, and over a field range it even exhibits a small increase with B. At a fixed field J¢ decreases with increasing temperatures, but the decrease is much faster than that known for conventional superconductors. The pinning curves obtained for both bulk samples and single crystals have shown [52] that the peak position occurs at a low reduced field and the peak moves to a higher value as the tem- perature is lowered. This suggests that the number of effective pinning centres tends to increase as the temperature is lowered.

Jc I H (c axis), J (a-or b-ares) H (a-or b-axEs], J (c-axis)

m H(a-ax~s),J(b-ax~s)

B ~

Fig. 22. Jc (B) curves for three orientations.

166

The zero-field Jc of thin films is exceptionally high, more than 106 A/cm 2, but in most cases Jc decreases exponentially, or very quickly with H. The situation is analogous to that of superfine filaments of conventional superconductors [54] where there is a pronounced surface pinning and no bulk pinning. For thin films deposited with favourable orientation [55], Jc values are higher and much less sensitive to B in conformity with the earlier discussion. Single crystals, in general, show J¢ values intermediate between sintered compacts and deposited thin films.

One of the possible reasons for the observed weak pinning in high Tc cuprates, as suggested by us [52], is the exceptionally rigid nature of the flux line lattice. Because of very large thermodynamic critical field of about 1 T for the 1 - 2 - 3 compound, the shear modulus C66 of the flux line lattice (FLL), given by [56]

Be2K2(2K 2 - - 1) C66 -- X 0.48(1 -- B/B~2) 2

4~[1 + 1.16(2K 2 -- 1)] 2

is about an order of magnitude larger than for a conventional high-field superconductor such as NbaSn. A rigid FLL is difficult to pin as the flux vortices are not elastically compliable to adjust themselves to the minimum energy positions at the pinning centres, unless the pinning forces exerted are sufficiently strong.

Many features of the observed J~ data can be unders tood in terms of the part played by the intergrain connections. If the overlap of the super- conducting order parameter • of the adjoining grains in the region of intergraln connect ion is appreciable (Fig. 23(a)), then the interconnection becomes a proximity connect ion which can provide effective pinning to vortex lines. On the other hand, if the overlap of the order parameter is small (Fig. 23(b)), which is more likely to happen at a higher temperature near To, the same intergrain connect ion becomes a weak link and Jc decreases rapidly with increasing B. The proximity pinning model, described elsewhere [52] , seems to provide a plausible explanation of the observed temperature dependence of Jc and the near-insensitivity of J~ (or even a slight increase) to B. In deposited thin films with grains having favourable orientation and the intergrain regions being narrow and free from contaminations, the intergraln boundaries are expected to serve as an effective proximity type of pinning

× × -tin/2 +dn/2 -a~/2 + tin/2

(a) (b)

Fig. 23. Schemat ic spatial dependence of the order parameter • at the i n t e r c o n n e c t i o n between the two adjacent superconduct ing grains: (a) weak l ink; (b) p rox imi ty connec t ion .

167

centres. This would account for the feeble field dependence of J¢ of thin films, reported recently [ 55 ].

Although Jc values of both bulk and thin films of high Tc materials are continually rising, the likelihood of the 1 - 2 - 3 compound being used in the form of wires and tapes for high-field applications at 77 K seems to be remote. The reason for this is not so much connected with the problem of developing flexible conductors (which in itself is no less challenging) but is intimately linked with the inherent question of whether acceptable Jc values in modest magnetic fields could be realised at 77 K. This working temperature is too close to the Tc of the 1 - 2 - 3 compound, so it poses a constraint to fulfilling the general requirement of attaining a short-sample current density of about 5 X l 0 s A/cm 2 for a bare wire in a 5 T field. The problem is aggravated further as, generally, it is found that the J¢ of long lengths of stabilized superconductors with insulation is about an order of magnitude lower than for the bare short sample. This leaves two obvious options, either to reduce the working temperature substantially or to enhance the T¢ further. Of the two, the latter approach is seemingly more challenging and in the long run might prove more rewarding.

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