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Twins and properties of classical and high-Tc superconductors
Materials Science and Engineering, A164 (1993) 449-453
449
Twins and properties of classical and high-Tc superconductors V. S. Bobrov and M. A. Lebyodkin Institute of Solid State Physics, Russian Academy of Science, 142432 Chernogolovka, Moscow District (Russian Federation) (Received August 30, 1992)
Abstract Data on the influence of twins and other crystalline defects on the properties of classical and high- Tc superconductors are presented. It is concluded that the increase in Tc and H c due to the low temperature twinning of classical superconductors is related to the formation of localized superconducting states in the region of twin boundaries. A crossover was observed of the temperature dependence of the critical current I~(T) near T~ of YIB2C3OT_x ceramics and single crystals. It was shown that in the case of ceramics this is possibly connected with the existence of Josephson junctions in the grain boundary region. The reasons which may cause deviation of I¢(T) in Y1B2C307_~ single crystals from linearity are discussed, in particular a possible analogy between the influence of twin boundaries on the properties of high- Tc and classic superconductors.
1. Introduction Defects of the crystalline structure may affect various physical properties of materials. In classical superconductors the influence of defects is mainly related to a change in the Ginzburg-Landau parameter and in the pinning conditions of magnetic flux vortices [1, 2]. The influence of defects on the critical temperature Tc and thermodynamic magnetic field He, which are fundamental parameters for superconductivity [2], is insignificant in classical superconductors. The exception is a remarkable increase in T¢ and H c upon low temperature deformation twinning [3-8]. An increase in Tc (by approximately one order of magnitude less than that for deformational twins) was also observed in bicrystals with the interface orientation close to that of twins (see for example ref. 9 and the review ref. 10). It was suggested that twin boundaries give rise to localized superconducting states. Two fields of research have appeared, dealing with stimulation action of both growth and deformational twins on superconductivity. Twins are a typical defect of high-Tc superconductors (HTSCs) [11] and receive much attention from researchers [7, 8, 12-18]. The possibility has not been excluded that, as in classical superconductors, twins may affect the formation of HTSC properties in a temperature range near Tc. Note, however, that defects may also decrease the superconductivity. In particular, interfaces (grain boundaries in ceramics and films) may act as a Josephson junction and limit the critical current density Jc [19].
In this paper we present data for the influence of twins and other crystalline defects on the properties of classical and high- Tc superconductors.
2. Experimental details The superconducting properties of high purity single crystals of niobium, tin, indium and lead (RRR > 1000) and of less pure niobium and rhenium (RRR~-100) were studied directly under low temperature deformation. Y1B2Cu307_x ceramic samples annealed in oxygen ( x ~ 0 ) had Tc~93-96 K. To deform them without damage they were mechanically treated under high pressure. The orthorhombic single crystals of Y~B2Cu307-x ( Tc ~ 70-90 K, x < 0.3) were in the form of platelets (approximately 3 mm x 3 mm x 0.01 mm) with (a, b) basal plane. As-grown crystals were used along with oxygen-annealed crystals ( Tc -~ 90-93 K). The superconducting properties were controlled by measuring the differential magnetic susceptibility and electrical resistance R (standard four-probe technique). In the case of Y - B a - C u - O (YBCO) single crystals, silver contacts were placed on the (a,b) plane. The temperature was measured with an accuracy of 10 -2 K. The Ic(T) dependence was determined using R(T) curves at different current values and I - V characteristics at different temperatures at the level 0.1 pV. The results obtained by these two methods coincided well. Elsevier Sequoia
450
V. S. Bobrov, M. A. Lebyodkin
/
3. Results
3.1. Classicalsuperconductors Low temperature twinning of niobium and tin leads to a noticeable increase in T~ and H~ (Figs. 1 and 2). Higher values of A HJH~ compared with A Td T~ may be evidence of singularities of the electronic state in the twin region. In the case of H~2 it may be connected with an increase in the Ginsburg-Landau parameter [2]. The increase in T~ and /arc upon twinning was less prominent if twinning was accompanied by appreciable dislocation glide. After heating the deformed samples exhibit a decrease ("anneal") in and/arc. A T~ values after heating to 300 K correspond approximately to the data for growth twins [9, 10] and may thus be associated with heating-reconstructed twin boundaries. Dislocation glide of niobium under compression at 100-200 K or bending at lower temperatures weakly affected T~ and H~ (Figs. 3 and 4). No change in T~was exhibited by indium and lead after dislocation glide in the range to 1.4 K. Thus dislocation and point defects arising under low temperature deformation in the absence of twinning do not lead to a noticeable increase in T~ and H~. Among the examined metals T c
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.
.
.
Classical and high-T,, superconductors
rhenium is especially noteworthy, since when it is deformed not only twins but also other defects strongly affect Tc (e.g. point defects [20]). We now discuss the question of localization of superconducting states in the twin region. The dependence I¢( T ) near T¢ has the form: t¢
I o(1 - T / L )
=
We observed [7] a crossover of the dependence on twinning of niobium: below the critical temperature T~0 of the starting sample r n = l , and in the range T~0~< T~< Tc rn= 1.5 (Fig. 5, see also Fig. 9) which is characteristic of thin superconducting films [2]. The linear dependence I~(T)-Hcl(T)-(1-TITs) corresponds to Silsbee's rule for bulk superconductivity. These data indicate the appearance of narrow (d<~ 2, where 2 is the penetration length) superconducting channels in the region of twin boundaries.
3.2. High-T~superconductors It was reported [7] that near T~ the dependence of the critical current I~(T) of YBCO ceramics has the form of eqn. (1) with rn= 1.5-2. This is qualitatively similar to the data for twinned niobium. Analogous dependences were observed in a number of studies of YBCO ceramics and films [21, 22]. However, in the case of ceramics and films there may be other factors besides twins giving rise to singularities of I~(T), e.g.
. .
.
.
.
.
.
.
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.
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Fig. 1. Critical temperature T~ as a function of strain e at low temperature twinning of (a) niobium and (b) tin; • after heating the samples to 300 K.
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-~. . . . . . . . . .
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Fig. 3. Tc vs. e at dislocation glide of (a) niobium (100-200 K) and (b) indium (1.4 K); • after heating the samples to 300 K.
o
o
~"1.1 II
o i
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/
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Fig. 2. Dependences on e at twinning of niobium: curve 1, upper critical field He2; curve 2, the field Hp (Hp= H¢l), at which the magnetic flux starts penetrating into the samples; • after heating the samples to 300 K.
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~-o-
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J
10
z [~1
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Fig. 4. H¢2 as a function of e at dislocation glide of niobium.
T/ S. Bobrov, M. A. Lebyodkin
/
the existence of Josephson junctions in the grain boundary regions and size effects in thin films. To determine the role of grain boundaries we studied the change in critical current density j, and its temperature dependence in Y ,B2Cu307 _ x ceramics as the intergrain contacts are broken by mechanical treatment under high pressure (Fig. 6) [23]. The initial and treated samples exhibit the dependence given by eqn. (1) near T,, where ti = 1.5-2, and at lower temperatures this dependence becomes linear (m = 1). It is known that eqn. (1) with m i= 2 may indicate that the current passes through Josephson junctions [2]. The data suggest that destruction of the intergrain contacts leads to degradation of the current-carrying ability of these junctions: i, drops by more than two orders, and the temperature range in which i,( T ) has the form of eqn. ( 1) broadens noticeably.
Fig. 5. Normalized temperature dependence of the critical current Z, of a niobium single crystal: curve 1, starting sample; curve 2, after low temperature twinning (T,,, is the critical temperature of the starting sample).
O-1
\
i
451
Classical and high-T,. superconductors
It may be concluded that when HTSC ceramics are deformed, the principal change in their properties is related to destruction of grain boundaries. The influence of other structural defects such as twins, dislocations and point defects is masked in ceramics by this stronger factor. To investigate the properties of high-T, superconductors related to twins and dislocations it is more expedient to use single crystals free of grain boundaries. In most of the investigated YBCO single crystals, twins formed a system of intersecting domains with directions of the type (110). Near T, the temperature dependence of the critical current had the form of eqn. (1) with m = 1.3-2 for all the investigated crystals. We present the data for one of the as-grown samples with a twinned structure in the form of parallel domains with a period of approximately 1 pm (Fig. 7). The electric current passed along domains in this sample. The corresponding dependence Z,(T) is shown in Fig. 8. Plotting these data on a logarithmic scale (Fig. 9) yields m = 1.4 + 0.1. Note that the observed I,( T ) for YBCO single crystals is similar to the data for niobium, containing deformational twins, and is observed for HTSC and niobium in approximately analogous ranges of a relative change in T: A T/T, = 3%-5% (compare Figs. 5 and 8). As mentioned above, in classical superconductors the behaviour of i,( T) near T, may be related to the existence in the twin region of localized superconducting states with increased T, values. Possibly the same situation is observable in the tested YBCO single crystals in the range T,, G T< T, (Fig. 8). Although this analogy between the data for classical and high-T, superconductors may imply similar mechanisms, one should, however, point out other factors that may affect i,(T) in HTSC single crystals: anisotropy of HTSC properties, fluctuation effects, presence of weak-link
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,
I
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Fig. 6. Temperature dependence of the critical current density j, of YBapCu,0,,9, ceramics: curve 1, sample prior to mechanical treatment; curve 2, after hydrostatic pressurization at T= 300 K and p = 4.5 GPa (the inset shows curve 2 magnified).
Fig. 7. Optical polarizing image of a surface region of a single crystal YBa,Cu,O,_, sample. The direction of the electric current and a schematic presentation of the measurements are given.
K S. Bobrov, M. A. Lebyodkin
452 I
,
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,
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, t.06
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Fig. 8. Normalized temperature dependence of the critical current Ic of a YBa2Cu3OT_x single crystal.
I0
Classical and high-T, superconductors
tivity is suppressed in the twin region. However, in [24] values of high critical current in HTSC films containing a large number of twins but no grain boundaries were reported. It may also be pointed out that even when the superconductivity is enhanced along twins (as in classical superconductors) we may have the opposite effect of weakening superconductivity and a decrease in j~ as the current flows across twin boundaries. This may be the result of the existence of energy barriers in the twin region [25]. In general, twins may influence HTSC properties in several ways, and investigations in this field are far from being completed. We hope that in the near future fresh data will be obtained for HTSC single crystals and films of different compositions with different twin structures, in particular for homogenized single crystals with an oriented twin structure. Of particular interest will be investigations of properties of HTSCs under mechanical action on the twin structure.
Acknowledgments l
We thank G. A. Emel'chenko and N. S. Sidorov for furnishing the samples. V. E. Ant•nov for cooperation and carrying out the investigations under high pressures and S. S. Shevaga for his assistance.
I
f A / t & I"IL •-~ 0.1
jpo/""
0.01
References
/ • 0.001 I l ~ i
"~"
O. 001
i
I
I I I I I
|
I
I
0.01
1-'r/m~
Fig. 9. Data of Figs. 5 and 8 on a logarithmic scale: o e , YBa2Cu3OT_ x single crystal; zx A, undeformed zx and deformed (A) niobium single crystal, ozx, T normalized to Too; o A , T normalized to Tc.
elements and inhomogeneity of the composition (especially oxygen).
4. Conclusion We may conclude that in the case of classical superconductors it is twins that give rise to localized superconducting states with increased values of Tc and He. These states seems to arise in the deformational-twinboundary regions and are thus tied to a metastable situation. They depend on structural factors and stresses near the boundary regions. When discussing the influence of twins on the HTSC properties, various, often controversial, assumptions are made. Some authors suggest [15] that superconduc-
1 A. M. Campbell and J. E. Evetts, Critical Currents in Superconductors, Taylor and Francis, London, 1972. 2 P. G. De Gennes, Superconductivity of Metals and Alloys, W. A. Benjamin, New York, 1966. 3 G. K. Baranova, V. S. Bobrov, Ch. V. Kopetskii and G. Ya. Sal'nikov, JETP, 50 (1979) 132. 4 V. S. Bobrov and S. N. Zorin, JETPLett., 40 (1984) 1147. V. S. Bobrov, M. A. Lebyodkin and S. N. Zorin, Physica B, 165-166 (1990) 1515. 5 V. K. Aksenov, I. A. Gindin, V. I. Sokolenko and Ya. D. Star•dub•v, Fiz. Nizk. Temp., 11 (1985) 93. 6 A.M. Dolgin, Fiz. Nizk. Temp., 13 ( 1987) 1015. 7 V. S. Bobrov and M. A. Lebyodkin, JETP Lett., 47 (1988) 715. 8 I. N. Khlustikov and M. S. Khaikin, Pis'ma Zh. Eksp. Teor. Fiz., 38 (1983)191. 9 M. S. Khaikin and I. N. Khlustikov, Pis'ma Zh. Eksp. Teor. Fiz., 33 (1981) 182. 10 I.N. Khlustikov and A. I. Busdin, Adv. Phys., 36 (1987) 271. 11 V. K. Vlasko-Vlasov, M. V. Indenbom and Yu. A. Ossipyan, Soy. JETP Lett., 47(1989) 312. 12 A. A. Abrikosov, A. I. Buzdin, M. L. Kulik and D. A. Kuptsov, Int. J. Modern Phys., B2(1988) 1045. 13 A.F. Andreev, Pis'ma Zh. Eksp. Teor. Fiz., 46 (1987) 463. 14 V.L. Indenbom, Pis'ma Zh. Eksp. Teor. Fiz., 46 (1987) 144. 15 G. Deutscher, Physica C, 153-155 (2) (1988) 15. 16 G. Deutscher and K. Miiller, Phys. Rev. Lett., 59 (1987) 1745. 17 S. E. Inderhees, M. B. Salamon, N. Goldenfeld, J. P. Rise, B. G. Pazol and D. M. Ginsberg, Phys. Rev., 60 (1988) 1178.
V. S. Bobrov, M. A. Lebyodkin
/
18 M.M. Fang, V. G. Kogan, D. K. Finnemore, J. R. Clem, L. S. Chumbley and D. E. Farrel, Phys. Rev. B, 37 (1988) 2334. 19 D. Dimos, E Chaudhari, J. Maunhart and F. K. Le Goues, Phys. Rev. Lett., 61 (1988) 219; Phys. Rev. B, 41 (1991) 4038. 20 N. A. Tulina and N. I. Novokhatskaya, Fiz. Nizk. Temp., 1 (1975) 27. 21 H. Watanabe, Y. Kasai, T. Mochuku, A. Sugishita, I. Iguchi and E. Yamaka, Jpn. J. Appl. Phys., 26 (1987) L615.
Classical and high- T~ superconductors
453
22 S. B. Ogale, D. Dijkkamp, T. Venkatesau, X. D. Wu and A. Inam, Phys. Rev., 36 (1987) 7210. 23 V. E. Antonov, T. E. Antonova, O. I. Barkalov, V. S. Bobrov, A. M. Gromov, E. V. Kisterev, E. V. Konopleva, V. P. Korzhov, I. N. Kremenskaya, V. I. Kulakov and N. S. Sidorov, Superconductivity." Phys. Chem. Technol., 5 ( 1991 ) 683. 24 P. Chaudhari, E K. Le Goues and A. Segmiiller, Science, 238 (1987) 342. 25 S. N. Burmistrov and L. B. Dubovskii, Fiz. Tverd. Tela, 26 (1984) 1719.
449
Twins and properties of classical and high-Tc superconductors V. S. Bobrov and M. A. Lebyodkin Institute of Solid State Physics, Russian Academy of Science, 142432 Chernogolovka, Moscow District (Russian Federation) (Received August 30, 1992)
Abstract Data on the influence of twins and other crystalline defects on the properties of classical and high- Tc superconductors are presented. It is concluded that the increase in Tc and H c due to the low temperature twinning of classical superconductors is related to the formation of localized superconducting states in the region of twin boundaries. A crossover was observed of the temperature dependence of the critical current I~(T) near T~ of YIB2C3OT_x ceramics and single crystals. It was shown that in the case of ceramics this is possibly connected with the existence of Josephson junctions in the grain boundary region. The reasons which may cause deviation of I¢(T) in Y1B2C307_~ single crystals from linearity are discussed, in particular a possible analogy between the influence of twin boundaries on the properties of high- Tc and classic superconductors.
1. Introduction Defects of the crystalline structure may affect various physical properties of materials. In classical superconductors the influence of defects is mainly related to a change in the Ginzburg-Landau parameter and in the pinning conditions of magnetic flux vortices [1, 2]. The influence of defects on the critical temperature Tc and thermodynamic magnetic field He, which are fundamental parameters for superconductivity [2], is insignificant in classical superconductors. The exception is a remarkable increase in T¢ and H c upon low temperature deformation twinning [3-8]. An increase in Tc (by approximately one order of magnitude less than that for deformational twins) was also observed in bicrystals with the interface orientation close to that of twins (see for example ref. 9 and the review ref. 10). It was suggested that twin boundaries give rise to localized superconducting states. Two fields of research have appeared, dealing with stimulation action of both growth and deformational twins on superconductivity. Twins are a typical defect of high-Tc superconductors (HTSCs) [11] and receive much attention from researchers [7, 8, 12-18]. The possibility has not been excluded that, as in classical superconductors, twins may affect the formation of HTSC properties in a temperature range near Tc. Note, however, that defects may also decrease the superconductivity. In particular, interfaces (grain boundaries in ceramics and films) may act as a Josephson junction and limit the critical current density Jc [19].
In this paper we present data for the influence of twins and other crystalline defects on the properties of classical and high- Tc superconductors.
2. Experimental details The superconducting properties of high purity single crystals of niobium, tin, indium and lead (RRR > 1000) and of less pure niobium and rhenium (RRR~-100) were studied directly under low temperature deformation. Y1B2Cu307_x ceramic samples annealed in oxygen ( x ~ 0 ) had Tc~93-96 K. To deform them without damage they were mechanically treated under high pressure. The orthorhombic single crystals of Y~B2Cu307-x ( Tc ~ 70-90 K, x < 0.3) were in the form of platelets (approximately 3 mm x 3 mm x 0.01 mm) with (a, b) basal plane. As-grown crystals were used along with oxygen-annealed crystals ( Tc -~ 90-93 K). The superconducting properties were controlled by measuring the differential magnetic susceptibility and electrical resistance R (standard four-probe technique). In the case of Y - B a - C u - O (YBCO) single crystals, silver contacts were placed on the (a,b) plane. The temperature was measured with an accuracy of 10 -2 K. The Ic(T) dependence was determined using R(T) curves at different current values and I - V characteristics at different temperatures at the level 0.1 pV. The results obtained by these two methods coincided well. Elsevier Sequoia
450
V. S. Bobrov, M. A. Lebyodkin
/
3. Results
3.1. Classicalsuperconductors Low temperature twinning of niobium and tin leads to a noticeable increase in T~ and H~ (Figs. 1 and 2). Higher values of A HJH~ compared with A Td T~ may be evidence of singularities of the electronic state in the twin region. In the case of H~2 it may be connected with an increase in the Ginsburg-Landau parameter [2]. The increase in T~ and /arc upon twinning was less prominent if twinning was accompanied by appreciable dislocation glide. After heating the deformed samples exhibit a decrease ("anneal") in and/arc. A T~ values after heating to 300 K correspond approximately to the data for growth twins [9, 10] and may thus be associated with heating-reconstructed twin boundaries. Dislocation glide of niobium under compression at 100-200 K or bending at lower temperatures weakly affected T~ and H~ (Figs. 3 and 4). No change in T~was exhibited by indium and lead after dislocation glide in the range to 1.4 K. Thus dislocation and point defects arising under low temperature deformation in the absence of twinning do not lead to a noticeable increase in T~ and H~. Among the examined metals T c
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• ~ • . % ~• # ' *__ ~,_, .....
--
_
_
•
~1.0
.
.
.
Classical and high-T,, superconductors
rhenium is especially noteworthy, since when it is deformed not only twins but also other defects strongly affect Tc (e.g. point defects [20]). We now discuss the question of localization of superconducting states in the twin region. The dependence I¢( T ) near T¢ has the form: t¢
I o(1 - T / L )
=
We observed [7] a crossover of the dependence on twinning of niobium: below the critical temperature T~0 of the starting sample r n = l , and in the range T~0~< T~< Tc rn= 1.5 (Fig. 5, see also Fig. 9) which is characteristic of thin superconducting films [2]. The linear dependence I~(T)-Hcl(T)-(1-TITs) corresponds to Silsbee's rule for bulk superconductivity. These data indicate the appearance of narrow (d<~ 2, where 2 is the penetration length) superconducting channels in the region of twin boundaries.
3.2. High-T~superconductors It was reported [7] that near T~ the dependence of the critical current I~(T) of YBCO ceramics has the form of eqn. (1) with rn= 1.5-2. This is qualitatively similar to the data for twinned niobium. Analogous dependences were observed in a number of studies of YBCO ceramics and films [21, 22]. However, in the case of ceramics and films there may be other factors besides twins giving rise to singularities of I~(T), e.g.
. .
.
.
.
.
.
.
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.
.
.
.
.
=
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b
._.o __•.-o
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=
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0
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ooooo a,=Aaa
.
1
-
-
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a o'S
e4~o
~
oo~ / .-t,"~ °/='
1.2 o/
1.0
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i
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i
2'0
5
Fig. 1. Critical temperature T~ as a function of strain e at low temperature twinning of (a) niobium and (b) tin; • after heating the samples to 300 K.
1.4-
-~. . . . . . . . . .
foo/° o,
0
F.u 1.0
b
o-
6
-
-°-
-
"a-
-°'-
-
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-
e
-
=
'
-
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=
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Fig. 3. Tc vs. e at dislocation glide of (a) niobium (100-200 K) and (b) indium (1.4 K); • after heating the samples to 300 K.
o
o
~"1.1 II
o i
10
/
8L-~]
2'0
Fig. 2. Dependences on e at twinning of niobium: curve 1, upper critical field He2; curve 2, the field Hp (Hp= H¢l), at which the magnetic flux starts penetrating into the samples; • after heating the samples to 300 K.
/o
3:1
~-o-
f"
I/ /
"1-
1.0
J
10
z [~1
2'0
3'0
Fig. 4. H¢2 as a function of e at dislocation glide of niobium.
T/ S. Bobrov, M. A. Lebyodkin
/
the existence of Josephson junctions in the grain boundary regions and size effects in thin films. To determine the role of grain boundaries we studied the change in critical current density j, and its temperature dependence in Y ,B2Cu307 _ x ceramics as the intergrain contacts are broken by mechanical treatment under high pressure (Fig. 6) [23]. The initial and treated samples exhibit the dependence given by eqn. (1) near T,, where ti = 1.5-2, and at lower temperatures this dependence becomes linear (m = 1). It is known that eqn. (1) with m i= 2 may indicate that the current passes through Josephson junctions [2]. The data suggest that destruction of the intergrain contacts leads to degradation of the current-carrying ability of these junctions: i, drops by more than two orders, and the temperature range in which i,( T ) has the form of eqn. ( 1) broadens noticeably.
Fig. 5. Normalized temperature dependence of the critical current Z, of a niobium single crystal: curve 1, starting sample; curve 2, after low temperature twinning (T,,, is the critical temperature of the starting sample).
O-1
\
i
451
Classical and high-T,. superconductors
It may be concluded that when HTSC ceramics are deformed, the principal change in their properties is related to destruction of grain boundaries. The influence of other structural defects such as twins, dislocations and point defects is masked in ceramics by this stronger factor. To investigate the properties of high-T, superconductors related to twins and dislocations it is more expedient to use single crystals free of grain boundaries. In most of the investigated YBCO single crystals, twins formed a system of intersecting domains with directions of the type (110). Near T, the temperature dependence of the critical current had the form of eqn. (1) with m = 1.3-2 for all the investigated crystals. We present the data for one of the as-grown samples with a twinned structure in the form of parallel domains with a period of approximately 1 pm (Fig. 7). The electric current passed along domains in this sample. The corresponding dependence Z,(T) is shown in Fig. 8. Plotting these data on a logarithmic scale (Fig. 9) yields m = 1.4 + 0.1. Note that the observed I,( T ) for YBCO single crystals is similar to the data for niobium, containing deformational twins, and is observed for HTSC and niobium in approximately analogous ranges of a relative change in T: A T/T, = 3%-5% (compare Figs. 5 and 8). As mentioned above, in classical superconductors the behaviour of i,( T) near T, may be related to the existence in the twin region of localized superconducting states with increased T, values. Possibly the same situation is observable in the tested YBCO single crystals in the range T,, G T< T, (Fig. 8). Although this analogy between the data for classical and high-T, superconductors may imply similar mechanisms, one should, however, point out other factors that may affect i,(T) in HTSC single crystals: anisotropy of HTSC properties, fluctuation effects, presence of weak-link
120 )
5w
,
I
2Z-
Fig. 6. Temperature dependence of the critical current density j, of YBapCu,0,,9, ceramics: curve 1, sample prior to mechanical treatment; curve 2, after hydrostatic pressurization at T= 300 K and p = 4.5 GPa (the inset shows curve 2 magnified).
Fig. 7. Optical polarizing image of a surface region of a single crystal YBa,Cu,O,_, sample. The direction of the electric current and a schematic presentation of the measurements are given.
K S. Bobrov, M. A. Lebyodkin
452 I
,
I
I
i
/
!
'~ 0.12 .u 0.081
Tco
Tc
0.04
0.00 0.98
,
, '1.00
,-o 8~.~.eoLo 1.02 '1.04
, t.06
T/Tco
Fig. 8. Normalized temperature dependence of the critical current Ic of a YBa2Cu3OT_x single crystal.
I0
Classical and high-T, superconductors
tivity is suppressed in the twin region. However, in [24] values of high critical current in HTSC films containing a large number of twins but no grain boundaries were reported. It may also be pointed out that even when the superconductivity is enhanced along twins (as in classical superconductors) we may have the opposite effect of weakening superconductivity and a decrease in j~ as the current flows across twin boundaries. This may be the result of the existence of energy barriers in the twin region [25]. In general, twins may influence HTSC properties in several ways, and investigations in this field are far from being completed. We hope that in the near future fresh data will be obtained for HTSC single crystals and films of different compositions with different twin structures, in particular for homogenized single crystals with an oriented twin structure. Of particular interest will be investigations of properties of HTSCs under mechanical action on the twin structure.
Acknowledgments l
We thank G. A. Emel'chenko and N. S. Sidorov for furnishing the samples. V. E. Ant•nov for cooperation and carrying out the investigations under high pressures and S. S. Shevaga for his assistance.
I
f A / t & I"IL •-~ 0.1
jpo/""
0.01
References
/ • 0.001 I l ~ i
"~"
O. 001
i
I
I I I I I
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I
I
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Fig. 9. Data of Figs. 5 and 8 on a logarithmic scale: o e , YBa2Cu3OT_ x single crystal; zx A, undeformed zx and deformed (A) niobium single crystal, ozx, T normalized to Too; o A , T normalized to Tc.
elements and inhomogeneity of the composition (especially oxygen).
4. Conclusion We may conclude that in the case of classical superconductors it is twins that give rise to localized superconducting states with increased values of Tc and He. These states seems to arise in the deformational-twinboundary regions and are thus tied to a metastable situation. They depend on structural factors and stresses near the boundary regions. When discussing the influence of twins on the HTSC properties, various, often controversial, assumptions are made. Some authors suggest [15] that superconduc-
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