Flux patterns of monofilamentary Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ tapes at various temperatures

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1 Supercond. Sci. Technol. 12 (1999) Printed in the UK PII: S (99) Flux patterns of monofilamentary Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ tapes at various temperatures M R Koblischka, T H Johansen, H Bratsberg and P Vase Department of Physics, University of Oslo, PO Box 1048, Blindern, 0316 Oslo 3, Norway Nordic Superconductor Technologies A/S (NST), Priorparken 878, DK-2605 Brøndby, Denmark Received 28 October 1998 Abstract. By means of magneto-optic imaging, flux patterns of monofilamentary, silver-sheathed Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ (Bi-2223) tapes are obtained at various temperatures between 12 K and 77 K. Above 50 K, the flux distributions are found to be nearly homogeneous, implying a uniform current flow. On decreasing the temperature, the observed flux patterns develop indications of granularity. This effect is most pronounced at the lowest temperature investigated. From these observations we can deduce the temperature dependence of the transport current density, j trans, and of the current density of the grains, j grain. We show that the appearance of granularity in the flux patterns at low temperatures can be explained by assuming a very steep temperature dependence of the intragranular current density, being large at low T and decreasing rapidly with increasing T until at about 50 K, j grain j trans. The absence of granularity above 50 K shows that the current flow is here dominated by the transport currents. 1. Introduction While silver-sheathed Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ (Bi-2223) tapes are an important material for practical applications of high-t c superconductors [1], these materials also show some interesting behaviours which require a new basic understanding of granular superconductors. The tapes consist of thin, about 10 µm wide platelet-like grains, where the platelets align within a misorientation angle of 5 to 10 with the c direction perpendicular to the plane of the tape, and the a and b directions are oriented at random from platelet to platelet [2]. This grain alignment in conjunction with a high density is responsible for the high critical currents achieved in the tapes. In general, in a granular superconductor there are two different current densities contributing to the flux distribution: the intergranular or transport current density, j trans, flowing throughout the entire sample, and the intragranular current density, j grain, circulating inside the grains. One of the features mentioned above is e.g. the anomalous position of the central peak in magnetization loops of the tapes, observed in both mono- and multifilamentary tapes [3 5]. This anomalous peak position is very pronounced at low temperatures, but shifts towards zero field on increasing the temperature. Furthermore, a pronounced hysteresis is observed in transport measurements performed Present address: Superconductivity Research Laboratory, International Superconductivity Technology Center, , Shibaura, Minato-ku, Tokyo 105, Japan. at low temperatures, e.g. the measured transport current density is larger in increasing external field, H a, than in decreasing field [6]. This effect is seen in all granular high-t c materials, and was explained by trapped flux inside the well superconducting grains [7, 8]. In Bi-2223 tapes, at higher temperatures this effect diminishes and at above 50 K no apparent hysteresis of j trans is observed [9]. This leads to the puzzling situation that the superconductor behaves granular at low temperatures, but the granularity vanishes with increasing temperature. In polycrystalline YBa 2 Cu 3 O 7 δ (YBCO) and other granular high-t c superconductors, just the opposite behaviour is observed: The grains are strongly coupled at low temperatures, and on increasing T, the coupling strength is reduced [10, 11]. Any integral measurement technique is in principle not capable of separating the two current contributions from each other. Only from measurements of heavily bent tapes, where the bending to a very small diameter breaks most of the connections between the grains, can one obtain some information about j grain [5, 12]. This problem can be resolved by means of a temperature-dependent local investigation of the resulting flux patterns. Among the possible observation techniques, magneto-optic (MO) imaging [13 15] offers the best properties, e.g. a high magnetic resolution combined with the possibility to perform the investigations on a relatively large area of the tape and using an intact tape with the silver sheathing in place. Magneto-optical visualization of flux distributions is very sensitive to any kind of structural defects as shown e.g. in [16] and [17]. Several experiments /99/ $ IOP Publishing Ltd 113

2 M R Koblischka et al have been carried out on Bi-2223 tapes, mainly focusing on the current flow in as-prepared tapes, both mono- and multifilamentary [16, 18, 19]. Furthermore, the flux patterns can be directly linked to the current flow in the samples as shown in [20]. In this paper, we present flux patterns of monofilamentary Bi-2223 tapes at various temperatures in order to study the differences in the flux distributions and hence, in the current flow. Based on these observations, we deduce the temperature dependence of the current density of the Bi-2223 grains. The paper is organized as follows. In section 2, we briefly discuss details of the MO imaging technique with respect to observations on tapes. In section 3, the flux patterns obtained at various temperatures are presented. Furthermore, we discuss general aspects of flux patterns in granular superconductors. From the MO images, we deduce the temperature dependence of the intragranular current density. Finally, in section 4, some conclusions are drawn. 2. Experimental procedure The magneto-optical (MO) visualization techniques are described in detail in [14], so a short summary suffices here. The field distribution is obtained by the Faraday effect, i.e. the rotation of the polarization plane of linearly polarized light which passes a magneto-optically active layer exposed to the magnetic field of the underlying superconductor. From fluxfree regions the light is reflected without rotation and thus cannot pass the analyser which is set in a crossed position with respect to the polarizer. In this way, the flux line lattice is imaged as bright areas, whereas the flux-free Meissner area stays dark. The images presented here are, therefore, maps of the z-component of the local magnetic field, B i,z. The magneto-optical imaging technique has two outstanding advantages; one of them is the capability to observe dynamic processes in the vortex lattice, the other is the possibility to perform experiments in a wide range of length scales, i.e. from whole samples down to individual grains. The spatial resolution of the MO technique depends on the type of indicator used. In the present experiment, we have employed a Bi-doped yttrium iron garnet (YIG) film with in-plane anisotropy with a thickness of 4 µm, half of which corresponds to the spatial resolution of our experiment. In order to obtain images with a relatively high contrast, which is especially important for the observations at elevated temperatures, an indicator film with a very high field sensitivity (better than 0.1 mt) was used. The images are recorded using an 8-bit Kodak DCS 420 charge-coupled device (CCD) digital camera ( pixels per frame) and subsequently transferred to a computer for processing and storage. In the MO apparatus the sample was mounted on the cold finger of an optical helium flow cryostat [21] using conductive carbon cement (CCC) [22] to ensure a good thermal contact. The magnetic field was applied perpendicular to the tape surface using a copper solenoid coil. All measurements were performed using monofilamentary silver-sheathed (Pb,Bi)-2223 tapes prepared by the powder-in-tube method with subsequent drawing and rolling [23]. For the MO measurements, we used a single piece of tape measuring externally (including the silver sheath) 8.2 mm in length, 3.8 mm in width and 90 µm in thickness. The silver sheath is 20 µm thick. For the MO studies the garnet indicator film (size 4 7 mm 2 ) is laid on top of the tape and carefully centred. Note that the visualization is carried out on an intact tape, i.e. the flux is imaged through the Ag sheathing. 3. Results and discussion The tapes are always zero-field cooled to the desired temperature, and subsequently the external field is applied. In all images presented throughout this paper, flux is imaged as bright areas; well shielding areas are visualized as dark. In figure 1, we present the initial penetration of flux into a Bi-2223 tape at T = 12 K. In the upper part of these images, the edges of the MO indicator film are seen, having some irregularities. In images (a) to (d), the external field is raised from 15 mt (a) to 30 mt (b), 45 mt (c) and 60 mt (d). (e) presents the remanent state (i.e. µ 0 H a = 0 T) corresponding to (d). This experiment is similar to the initial flux penetration in an unbent tape as shown in [24] (figures 1(a) to (h)). On applying an external field, one sees that the edges of the sample are becoming bright (the local B z values are larger than the applied field outside the sample) due to the stray fields near the edges, typical for thin superconducting samples. Note that this field overshoot takes place at the edges of the tape core, and not at the real sample edges. Thin superconducting samples in perpendicular geometry (field applied perpendicular to the sample surface) show current flow everywhere in the sample [25 27]; and not only in the flux-penetrated regions as assumed in the original Bean model (= longitudinal geometry, i.e. field applied parallel to a long direction of the sample) [28]. This has important consequences for the flux distributions, as various structural defects within the tape core can easily alter the flux distributions [16, 17, 29], due to the large demagnetization effects. Vortices penetrate the sample starting from the edges as in the case of a thin, homogeneous superconducting strip [25 27]. It is remarkable that at low field values, the field pattern is quite uniform. This reflects clearly the better orientation and grain growth along the silver sheath [30]. When the flux enters deeper into the sample on increasing field, several defects are encountered which alter the flux pattern. Following the defect-classification scheme of [17], the present defects are of a size comparable to λ L, causing the typical plumes with a nearly parabolic shape. Close to the fully penetrated state, a black stripe is visible in the tape centre; indicating the border between the countercirculating currents in the sample (so-called discontinuity line (d line) of the currents) [31]. The remanent state (e) shows a d line in the centre which is now bright. This implies that the vortices are forced to rearrange themselves when reducing the external field; they move towards the sample centre in accordance with the model for the thin, homogeneous strip. As a consequence, the edges of the tape core get darker. If the external field surpasses a critical value, vortices of opposite polarity may become stable and enter the sample along the tape core. Annihilation of vortices of different polarity causes 114

3 Flux patterns of monofilamentary Bi-2223 tapes Figure 1. Initial flux penetration into a Bi-2223 tape, observed at T = 12 K. The flux patterns are visualized through the silver sheath of the intact tape. Field is imaged as bright areas, well shielding regions are represented dark. The exposure time is for all images 1 s. The scale bar is 2 mm long. (a) µ 0 H a = 15 mt. The two white arrows in each image denote the edges of the superconducting core. The tape covers practically the entire frame shown in the image. (b) 30 mt, (c) 45 mt, (d) 60 mt. At low fields, the flux penetration into the tape is relatively uniform, especially at the edges of the tape core along the silver sheath. From (b) on, several defects are seen which alter the flux patterns considerably. A d line (dark line) is visible in the centre of the tape. (e) shows the corresponding remanent state, i.e. µ 0 H a = 0T after applying a maximum field of 60 mt. The d line in the centre is now bright; and vortices of opposite polarity enter the sample starting from the core edges. At some defects (i.e. those which are very bright during the initial flux penetration), vortices of opposite polarity appear even inside the sample. the formation of a belt where the local field, B i,z equals zero. Note that MO imaging allows to detect the sign of the vortices by rotating the polarizer/analyser, however, the recorded intensity in our images is always positive. This generation of negative vortices during the reduction of the external magnetic field makes it very attractive to observe the flux patterns in reducing external magnetic field, as the entering negative vortices effectively scan the arrangement of structural defects [32]. Figures 2 to 5 present the flux patterns on a different section of the same tape observed at various temperatures. Within one series of images, the exposure time is kept fixed in order to allow for a direct comparison of the images as equal values of intensity imply equal values of B i,z. In the case when the detected intensity becomes too large for the camera setting, the observed saturated image is presented together with one created by using a shorter exposure time and/or contrast enhancement to allow comparison. In all series of images, always the same sequence of external fields is presented, i.e. an external field, H a of 150 mt is applied to the ZFC state, and subsequently, the field is reduced to 60 mt (a), 30 mt (b), 18 mt (c), 6 mt (d), and 0 mt (= remanent state) in (e). Figure 2 presents the flux pattern of the Bi-2223 tape at T = 12 K. The sequence (a) to (e) is recorded in decreasing external field, as effects of granularity are most easily detectable in this experimental procedure [17, 24]. Reducing the external field forces the vortices to rearrange: Flux moves towards the d line, and vortices are leaving the sample. First, the edges of the tape core turn dark; and also some dark areas appear within the core, indicating where flux is only very weakly pinned and has apparently left the sample. These areas act as easy channels for the motion of vortices. Further decrease of the external field reveals a very inhomogeneous flux pattern, caused by two effects, (i) the sample geometry (which reflects the properties of the transport currents; the flux is confined to the d lines in the remanent state) and by the presence of the weakly pinning channels (revealing the areas with high intragranular current density). This granular behaviour becomes more pronounced on further decreasing the external magnetic field. In (b), H a is reduced to 30 mt and in (c) to 18 mt. Now, we observe a structure of bright and dark regions pointing towards the tape edges. The bright areas are due to flux being trapped inside some Bi-2223 grains, whereas the dark areas emphasize where flux has left the sample, i.e. from areas with only weak bulk pinning. (d) shows the flux pattern at 6 mt, and (e) presents finally the remanent state. Here, we can observe two bright stripes along the edges of the tape core which are due to vortices of opposite polarity. In a thin superconducting sample, these vortices of opposite polarity can enter the sample before reaching the remanent state and annihilate with some trapped vortices. Due to this behaviour, the observation of flux patterns in reducing external magnetic field is extremely sensitive to the existence of weak pinning regions in a thin, granular superconductor and, therefore, ideally suited to study traces of granularity in an unknown superconducting sample. Figure 3 presents the same experimental sequence, but at T = 30 K. In principle, the patterns obtained are not much 115

4 M R Koblischka et al Figure 2. Flux patterns in decreasing external magnetic field, observed at T = 12 K. An external field, µ 0 H a = 150 mt, is applied to the ZFC state, and subsequently, the field is reduced to 60 mt (a), 30 mt (b), 18 mt (c), 6 mt (d) and 0 mt (= remanent state) in (e). Flux leaves the sample from some regions with apparently weaker pinning (darker areas). In contrast, also some areas are seen which can effectively trap the vortices (bright spots). In (d), vortices of opposite polarity appear along the edges of the tape core, and annihilation with pinned vortices takes place. Also, flux moves towards the bright current discontinuity line (see text) in the tape centre. In (e), negative vortices invade the sample. different from those taken at T = 12 K. Note, however, that the dark areas are now somewhat larger. Moreover, the remanent state (e) shows a much wider stripe of negative Figure 3. Flux patterns in decreasing external magnetic field, observed at T = 30 K. An external field, µ 0 H a = 150 mt, is applied to the ZFC state, and subsequently, the field is reduced to 60 mt (a), 30 mt (b), 18 mt (c), 6 mt (d) and 0 mt (= remanent state) in (e). This is the same experimental sequence as in figure 2 to allow a direct comparison. The flux patterns are quite similar to those of figure 2, except that the lower critical current density leads to a somewhat easier flux penetration. Note e.g. that in (e) the stripe of negative vortices along the edge of the core is much broader than in figure 2(e). vortices along the edges of the tape core. All this is an indication that the current density of the grains is now somewhat lower than at 12 K, but the transport current density has not changed very much. 116

5 Flux patterns of monofilamentary Bi-2223 tapes Figure 4. Flux distributions in decreasing external field at T = 52 K. The field values are the same as in figures 2 and 3. In (a) and (b), the right half of the image presents the flux pattern after image processing; the left half shows the original saturated image. Still some dark areas can be detected, but as the magnitude of the intragranular currents is decreased considerably, these effects do not play a dominant role as at lower temperatures. In figures 4(a) to (e), we present the series at T = 52 K. Now, there is a considerable change as compared to the previous flux patterns. In (a) and (b), the intensity became too large for the camera setting. The left half of the image presents the real image for comparison with the rest of the series, and the right half shows the image obtained with a different exposure time and/or after image processing. These images clearly reveal that the flux patterns are now quite uniform. On a further decrease of the external field (c), (d), some features of granularity as observed at lower temperatures are still visible, but these are no longer dominant. Note that even in the remanent state (e) the bright stripe in the tape centre is now relatively broad, and as a consequence, there are no vortices of opposite polarity detectable in the sample. These observations clearly evidence that the two current densities are now of a comparable magnitude, and as a consequence, the flux pattern begins to look quasi-uniform as the disturbance of the intergranular current flow due to the intragranular currents diminishes. In figure 5, the observation temperature is further increased to 77 K. In this series, the flux patterns are found to be completely uniform, as there is no longer any detectable contribution of the intergranular currents. Even in the remanent state (e), just the bright stripe in the centre is observed, exactly like in a homogeneous thin strip. The flux patterns are, therefore, exclusively generated by the transport currents only. From these observations, we can deduce the temperature dependence of the intragranular critical current density, j c,grain. The MO images are in accord with the situation sketched in figure 6. Below 50 K, j c,grain > j c,trans,so traces of granularity appear in the MO images. The lower the temperature, the more pronounced the influence of j c,grain.at very low temperatures (12 K and below), the MO patterns are dominated by j c,grain, so the images look completely granular, i.e. the transport current flow is disturbed by many well shielded islands (= grains). Above 50 K, the two current densities are approximately equal to each other, i.e. j c,grain j c,trans. This implies that the flux patterns reveal mainly the flow of the transport currents, without being disturbed by islands of higher current density. At T = 77 K, only the transport currents are responsible for the observed flux patterns. A similar temperature dependence is also observed concerning the anomalous position of the central peak in Bi-2223 tapes. This effect becomes increasingly pronounced at low temperatures, whereas the peak position goes to 0 T above 50 K [4]. The temperature dependence of j c,grain resembles the one measured on Bi 2 Sr 2 CaCu 2 O 8+δ single crystals [33], with the remarkable difference that the current density is still large above 25 K, and decreases at about 50 K. This also reflects the somewhat smaller anisotropy of Bi-2223 as compared to Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ. This deduced temperature dependence of the intragranular current density poses immediately the question of whether there is bulk pinning inside the Bi-2223 grains at elevated temperatures above 50 K. This question can only definitively be answered by investigating the temperature behaviour of j c in Bi-2223 single crystals [34]. From the MO patterns, we can deduce that j c,grain j c,trans, but not judge the origin of j c,grain, as the interaction between the two current densities is still present in the tape, and does not allow us to study the properties of j c,grain alone. Also this will require measurements on Bi-2223 single crystals. Our observations also solve the problem that observations of transport current flow in Bi-2223 tapes revealed a 117

6 M R Koblischka et al Figure 6. Schematic drawing of the temperature dependence of j c,trans and j c,grain. In the hatched area, the magneto-optic images reveal granular behaviour, and j c,grain is considerably larger than j c,trans. Above 50 K, the two current densities are comparable in magnitude, and the flux patterns do not show effects of granularity. flow, which may be affected by only some well-shielded areas along the core edges. As these regions are more uniform than the tape centre, there will not be a large disturbance of the current flow. Another important issue is the still open question of whether an increase of j c,grain at the elevated temperatures would directly lead to an increase of j c,trans. An increase of j c,grain also implies that effects of granularity will be observed at these temperatures. It is not clear, however, whether this would be a positive effect for the transport current flow. However, for a further increase of the transport current density at 77 K the increase of the bulk pinning plays an essential role, as a high transport current density can only be maintained if also the intragranular current density is sufficiently large. This may be achieved by embedding foreign particles like carbon nanotubes [35], MgO nanorods [36], or intrinsic particles like (Sr,Ca) 2 CuO y [37] into the Bi-2223 grains. 4. Conclusions Figure 5. Flux distributions in decreasing external field at T = 77 K. The field values are the same as in figures 2, 3 and 4. In (a) and (b), the right half of the image presents the flux pattern after image processing; the left half shows the original saturated image. Note that the flux patterns reveal now a completely different character, as there is no detectable contribution from the intragranular currents anymore. more uniform behaviour than states generated by applying an external field [19]. If we apply an external magnetic field, we force the sample to shield itself against the external field, so regions with a high j c,grain will appear in the images. Applying a transport current brings the system in another situation. In this case we observe directly the transport current By means of MO imaging we have observed flux patterns in Bi-2223 tapes at various temperatures. At low T, clear signatures of granularity are observed which are due to a large intragranular current density. On increasing the temperature, j c,grain and j c,trans become of comparable magnitude, and as a consequence, we do not observe effects of granularity at e.g. 77 K. At this temperature, the obtained flux patterns are completely homogeneous, and generated only by the transport currents. These observations also reveal that bulk pinning is only very weak at elevated temperatures. The properties of the intragranular current density as deduced from the magneto-optic images are improved as compared to Bi 2 Sr 2 Ca 2 Cu 3 O 8+δ, reflecting the lower anisotropy of Bi A final clarification of these properties will only be possible by investigating Bi-2223 single crystals. 118

7 Flux patterns of monofilamentary Bi-2223 tapes Acknowledgments We thank L Půst (Wayne State University, Detroit) and A A Polyanskii (University of Wisconsin, Madison) for valuable discussions, and M E McHenry (Carnegie Mellon University) for information about the Bi-2223 single crystals. This work is financially supported by The Research Council of Norway. References [1] Martini L 1998 Supercond. Sci. Technol [2] Bulaevskii L N, Daemen L L, Maley M P and Coulter J Y 1993 Phys. Rev. B [3] Müller K-H, Andrikis C, Liu H K and Dou S X 1994 Phys. Rev. B Cimberle M R, Ferdeghini C, Flükiger R, Giannini E, Grasso G, Marrè D, Putti M and Siri A S 1995 Physica C Koblischka M R, Půst L, Galkin A, Nálevka P, Johansen T H, Bratsberg H, Nilsson B and Claeson T 1998 Phys. Status Solidi A 167 R1 Koblischka M R, Johansen T H, Bratsberg H, Půst L, Galkin A, Nálevka P, Maryško M, Jirsa M, Bentzon M D, Bodin P, Vase P and Freltoft T 1998 J. Appl. Phys [4] Koblischka M R, Půst L, Galkin A and Nálevka P 1997 Appl. Phys. Lett [5] Müller K-H, Andrikis C and Guo Y C 1997 Phys. Rev. B [6] Cesnak L, Melišek T, Kováč P and Hušek I 1997 Cryogenics and references therein [7] Evetts J E and Glowacki B A 1988 Cryogenics [8] Mishra P K, Ravikumar G, Chaddah P, Kumar S and Dasannacharya B A 1990 Japan. J. Appl. Phys. 29 L1612 [9] Hu Q Y, Schalk R M, Weber H W, Liu H K, Wang R K, Czurda C and Dou S X 1995 J. Appl. Phys Cave J R, Ramsbottom H D, Willen DWA,Nadi R, Zhu W and Paquette A 1996 Critical Currents in Superconductors Conf., IWCC 8 (Kitakyushu, Japan, 1996) ed T Matsushita and K Yamafuji (Singapore: World Scientific) p 279 [10] Schuster T, Koblischka M R, Reininger T, Ludescher B, Henes R and Kronmüller H 1992 Supercond. Sci. Technol [11] Koblischka M R, Schuster Th and Kronmüller H 1994 Physica C [12] Nálevka P, Jirsa M, Půst L, Galkin A, Koblischka MRand Flükiger R rd EUCAS Conf. (Veldhoven, 1997) (IOP Conf. Ser. 158) ed H Rogalla and DHABlank (Bristol: Institute of Physics) p 1161 [13] Hübener R P 1979 Magnetic Flux Structures in Superconductors (New York: Springer) [14] Koblischka M R and Wijngaarden R J 1995 Supercond. Sci. Technol [15] Schuster T, Koblischka M R, Moser N, Ludescher N and Kronmüller H 1991 Cryogenics [16] Polak M, Parrell J A, Polyanskii A A, Pashitski AEand Larbalestier D C 1997 Appl. Phys. Lett [17] Koblischka M R 1996 Supercond. Sci. Technol [18] Welp U, Gunter D O, Crabtree D O, Zhong W, Balachandran U, Haldar P, Sokolowski R S, Vlasko-Vlasov V K and Nikitenko V I 1996 Nature Welp U, Gunter D O, Crabtree G W, Luo J S, Maroni V A, Carter W L, Vlasko-Vlasov V K and Nikitenko V I 1995 Appl. Phys. Lett Pashitski A E, Polyanskii A A, Gurevich A, Parrell JAand Larbalestier D C 1995 Physica C Pashitski A E, Polyanskii A A, Gurevich A, Parrell JAand Larbalestier D C 1995 Appl. Phys. Lett Parrell J A, Polyanskii A A, Pashitski AEand Larbalestier D C 1996 Supercond. Sci. Technol Schuster Th, Kuhn H, Weisshardt A, Kronmüller H, Roas B, Eibl B, Leghissa M and Neumüller H-W 1996 Appl. Phys. Lett Koblischka M R, Johansen T H, Bratsberg H and Vase P 1998 Supercond. Sci. Technol Koblischka M R, Johansen T H, Bratsberg H and Vase P 1998 Supercond. Sci. Technol [19] Pashitski A E, Polyanskii A A, Gurevich A, Parrell JAand Larbalestier D C 1995 Appl. Phys. Lett [20] Pashitski A E, Gurevich A, Polyanskii A A, Larbalestier D C, Goyal A, Specht E D, Kroeger D M, DeLuca J A and Tkaczyk J E 1997 Science [21] Johansen T H, Baziljevich M, Bratsberg H, Galperin Y, Lindelof P E, Shen Y and Vase P 1996 Phys. Rev. B [22] Thermal conducting carbon glue, Leit-C, Neubauer Chemikalien, Münster, Germany [23] Bodin P, Han Z, Vase P, Bentzon M D, Skov-Hansen P, Bruun R and Goul J rd EUCAS Conf. (Veldhoven, 1997) (IOP Conf. Ser. 158) ed H Rogalla and D H A Blank (Bristol: Institute of Physics) p 1189 [24] Koblischka M R, Johansen T H and Bratsberg H 1997 Supercond. Sci. Technol [25] Brandt E H and Indenbom M V 1993 Phys. Rev. B Brandt E H 1996 Phys. Rev. B Schuster Th, Kuhn H, Brandt E H, Indenbom M V, Koblischka M R and Konczykowski M 1994 Phys. Rev. B [26] Zeldov E, Clem J R, McElfresh M and Darwin M 1994 Phys. Rev. B [27] McDonald J and Clem J R 1996 Phys. Rev. B [28] Bean C P 1962 Phys. Rev. Lett [29] Baziljevich M, Johansen T H, Bratsberg H, Shen Y and Vase P 1996 Appl. Phys. Lett [30] Grasso G, Hensel B, Jeremie A and Flükiger R 1995 Physica C [31] Schuster Th, Indenbom M V, Koblischka M R, Kuhn H and Kronmüller H 1994 Phys. Rev. B [32] Koblischka M R, Das A, Muralidhar M, Sakai N and Murakami M 1998 Japan. J. Appl. Phys. 37 L1227 [33] van Dalen AJJ,Griessen R and Koblischka M R 1996 Physica C [34] Chu S and McHenry M E 1998 J. Mater. Res [35] Fossheim K, Tuset E D, Ebbesen T W, Treacy MMJand Schwartz J 1995 Physica C Huang S L, Koblischka M R, Fossheim K, Ebbesen TWand Johansen T H 1997 Physica C [36] Yang P and Lieber Ch M 1996 Science [37] Koblischka M R, Huang S L, Fossheim K, Johansen THand Bratsberg H 1998 Physica C