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Preparation and superconductivity of potassium-doped fullerene nanowhiskers
Materials Research Bulletin 48 (2013) 343–345
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Preparation and superconductivity of potassium-doped fullerene nanowhiskers Hiroyuki Takeya a,b,*, Ryoei Kato a, Takatsugu Wakahara a, Kun’ichi Miyazawa a, Takahide Yamaguchi a,b, Toshinori Ozaki a,b, Hiroyuki Okazaki a,b, Yoshihiko Takano a,b a b
National Institute for Materials Science, Tsukuba 305-0047, Japan Transformative Research Project on Iron Pnictides (TRIP), Japan Science and Technology Agency, Chiyoda-ku, Tokyo 102-0075, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 June 2012 Accepted 24 October 2012 Available online 2 November 2012
Superconductivity at 17 K was observed in a potassium (K)-doped fullerene nanowhisker (K-C60NW). The superconducting volume fraction of a full shielding volume was as high as 80% in the nominal composition of K3.3C60NW, while that of K-doped fullerene powder was less than 1% under the same heating condition at 200 8C. We report the superconducting behaviors of our newly synthesized K3.3C60NWs, such as the applied field dependence of magnetization and the critical current density of 3 105 A/cm2 at 5 K under 50 kOe, in comparison to those of K-doped fullerene powders. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Fullerenes A. Superconductors D. Defects
1. Introduction Various materials composed of carbon have been reported so far, such as graphite, diamond, carbon nanotube, fullerene (C60), fullerene-nanowhisker (C60NW),1 and, very recently, graphene as a new material for future devices. Such materials have been attractive due to their anomalous structures, conductance, chemical absorption and adsorption, and strength. Superconductivity in carbon materials has been one of the most interesting physical properties to date. Several superconductors have been reported, such as K-intercalated graphite KC8 (transition temperature, Tc = 0.15 K) [1,2], Ca-doped graphite CaC6 (Tc = 11.5 K), Bdoped diamond (Tc = 8 K) [3,4], K-doped fullerene K3C60 (Tc = 19.0 K) [5], and Cs-doped fullerene Cs3C60 (Tc = 38 K) [6]. Recently, Miyazawa et al. prepared various types of fullerenebased supramolecular materials, such as nanowhiskers (C60NW)1 [7,8], nanosheets, nanowires, and nanotubes composed of fullerene units, by using the liquid–liquid interfacial precipitation method (LLIP method). We have been exploring new superconductors and are currently focusing on carbon-based materials. If those forms of fullerene-based materials turn out to be a superconductor, a new application for flexible and light superconductors will have been developed. In this paper, we report on
* Corresponding author at: National Institute for Materials Science, Tsukuba 3050047, Japan. Tel.: +81 29 859 2318; fax: +81 29 859 2301. E-mail address: [email protected] (H. Takeya). 1 We use the terms C60NW and KxC60NW in this paper. 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.10.033
the intercalation of K in fullerene nanowhiskers (KxC60NWs)1 and their superconducting properties. 2. Experimental We prepared fullerene nanowhiskers, C60NWs, using the LLIP method [7]. The as-grown C60NWs were filtrated and dried up. The typical dimension of the C60NWs used in this experiment was 0.54 0.16 mm in average diameter and 4.43 2.63 mm in average length. About 10 mg C60NWs and an amount of 1.8 mg potassium (K) as the molar ratio of K/C60 = 3.3 were put together in a quartz tube (5 mm in inner diameter), followed by sealing the quartz tube under a vacuum condition at 3 10 3 Pa. We added a 10% excess amount of K over the crystallographically limiting composition K3C60 in consideration of the K adsorption on the surface of crystallites or associated with defects. The sample was heated at 200 8C for 1–36 h in an electric oven. Magnetization measurements were performed using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-5S). The information of the crystal structure was analyzed by powder X-ray diffraction (XRD). The shapes and microstructures of those samples were observed with a scanning electron microscope (SEM, Hitachi SU-70) and a transmission electron microscope (TEM, JEOL JEM-4010). Qualitative microanalysis was achieved with an energy dispersive X-ray analyzer (EDAX, Genesis). For comparison to K-C60NWs, K–C60 powders (particle size = 1–5 mm) were also used and evaluated through all procedures shown above in the same way. The nominal composition was used to indicate the sample names in the following discussion because the quantitative analysis of KxC60NWs was not reliable enough to determine the K/C60 ratio.
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Fig. 2. Temperature dependence of magnetization warming under 20 Oe after zerofield cooling for K3.3C60NW and K3.3C60 powder. The axis of the ordinate is normalized by the sample weight (g) and applied magnetic field (Oe).
Fig. 1. SEM micrographs of (a) pristine C60NWs (upper) and (b) K3.3C60NWs (lower). The inset pictures are the TEM images, showing disordered nanopores.
3. Results and discussion SEM micrographs of C60NW before (a: upper) and after Kdoping (b: lower) are shown in Fig. 1. The shapes of C60NWs are mostly hexagonal prisms with the growth axis along the [0 0 1] direction [9]. C60NWs grow in a hexagonal crystal structure by the LLIP method, and the hexagonal structure then turns to the fcc structure while the solvent is drying up. The inset pictures in Fig. 1(a) and (b) are the images of the pristine and K-doped C60NWs observed using a transmission electron microscope (TEM) [9]. They show that C60NW contains disordered nanopores, which are not observed in C60 crystals. Fig. 2 shows the superconducting transitions of K3.3C60NW and K3.3C60 powders heated at 200 8C for 24 h, which were measured upon warming under 20 Oe after cooling in a zero field. The left and right ordinates of the graph set separately for K3.3C60NW and K3.3C60 are the magnetization normalized by the applied magnetic field and the weight of the samples, respectively. As seen in the figure, a large difference in the superconducting signals between the two samples was observed. The signal of the nanowhisker sample was 200 times larger than that of the powder sample. The XRD patterns of pristine C60, K3.3C60 powder, and K3.3C60NW are identified to be the fcc structure phase as shown in Fig. 3. According to the reported phase diagram of K–C60 [11], C60 and K3C60 exist in the x = 0–3 region of KxC60. In K-doped C60 materials, the only fcc phase of stoichiometric K3C60 is a superconductor. Two tetrahedral sites and one octahedral site ¯ per C60 in the structure (space group: Fm3m) are occupied with the three K atoms. It is known that the intensity ratio of XRD reflections is different in C60 and K3C60. The actual K-doping content might be estimated using the intensity ratio. The simulated intensity ratio
(311/111) (shown in the inset) was calculated assuming that the samples consisted of the mixture of C60 and K3C60. K seemed to be hardly intercalated into the K3.3C60 powder, while an approximate amount of 83% of the K3C60 phase was in K3.3C60NW, as estimated by the XRD intensity ratio. This is consistent with the change of the lattice parameters in C60, K3.3C60 powder, and K3.3C60NW, which was calculated to be 1.4180(5), 1.4180(2), and 1.4200(5) nm, respectively. These results prove that K was intercalated into the C60NWs. To investigate the reaction rate forming K3C60 in the NWs, the superconducting shielding fractions in K3.3C60NW were measured hourly during the heating process at 200 8C by using a SQUID magnetometer. The shielding volume fractions are normalized by the shape of the sample space. As shown in Fig. 4, the shielding volume fraction was saturated at 24 h. On the other hand, in the case of K3.3C60 powder, the superconducting volume fraction was less than 1% by heating at 200 8C for 24 h. Such a small volume fraction of the superconducting KxC60 has already been reported, for example, by Hebard et al. [5] and Murphy et al. [10]. Hebard et al. reported approximately 1% by heating at 200 8C for 36 h. Murphy et al. described that a 1–2% fraction was obtained by
Fig. 3. XRD patterns of pristine and K-doped C60 samples, K3.3C60 powder, and K3.3C60NW (nominal composition), with an fcc structure index. The inset shows the simulated intensity ratio (311/111) and observed intensity ratio of K3.3C60 powder and K3.3C60NW.
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powder. From the hysteresis of the M–H curves, DM (emu/cm3), we can estimate the critical current density Jc on the assumption of a critical-state model with the simple formula, Jc = 30DM/d, where d is the size of the sample (cm) [12]. Fig. 5(b) shows the Jc’s at 5 K as a function of applied magnetic fields calculated for the K3.3C60NW and the K3.3C60 powder, where a third root of the average volume was used for the nanowhisker size and an average particle size measured by SEM was used for the powder size. Both samples heated for K-doping were still kept in their original whisker and powder forms, indicating that there was no difference in their intergranular coupling, as observed using SEM. A possibility for the low Jc of the K3.3C60 powder is the low volume fraction of the superconducting K3C60 phase. The Jc of K3.3C60NW is much higher than that of K3.3C60 powder and is 3 105 (A/cm2) under 50 kOe. The Jc of K3.3C60NW is in the highest level so far among the reported Jc’s [13] and shows the flat applied field dependence from 20 kOe to 50 kOe, indicating a fairly large pinning force in the grain, which can be associated with the nanopores or defects in K3.3C60NW. Fig. 4. Formation of the superconducting phase (%) in K3.3C60NWs estimated by the shielding volume fractions vs. heating time (h) at 200 8C.
4. Conclusions In summary, we reported the preparation and properties of superconducting K-C60NWs by doping K into C60NW. The superconducting shielding volume fraction by heating at 200 8C for 24 h gave high values up to 80%, in contrast to the value of Kdoped C60 powder, which was lower than 1%. We believe that this contrasting difference of the superconducting shielding fractions in the two samples is associated with the defects promoting Kintercalation in K3.3C60NWs. We also observed the Jc of K3.3C60NW higher than 3 105 (A/cm2) at 5 K without a significant decrease up to 50 kOe. We will try to prepare the superconducting materials of longer K-doped C60NW fibers or other forms. Acknowledgments The authors are grateful to the Advanced Low Carbon Technology R&D Program (ALCA) of the Japan Science and Technology Agency for partial financial support. Partial support received from the Japan Society for the Promotion of Science is also acknowledged.
Fig. 5. (a) M–H curves and (b) Jc’s at 5 K for K3.3C60NW and K3.3C60 powder heated at 200 8C for 24 h.
References
heating at 225 8C for 24–48 h and the fraction then increased up to 35% by subsequent heat treatment at 230–265 8C for 21 days. The considerable difference of the superconducting volume fraction between KxC60NWs and K3C60 powder can be discussed on the basis of the cross-sectional analysis of C60NWs by TEM [9]. In comparison to C60 powders, C60NWs contain many defects, such as a core–shell structure, porous inner cores (nano-pores: the average size is 8.3 nm), and disordered domains. We believe that those defects assist in the reaction of K-doping and that the migration of K through the defects promotes the formation of K3C60. Fig. 5(a) shows the applied field dependence of the magnetizations (M–H curves) at 5 K for the two samples, as reported above, measured using a SQUID magnetometer. The magnetization hysteresis of K3.3C60NW is much larger than that of the K3.3C60
[1] N.B. Hannay, T.H. Geballe, B.T. Matthias, K. Andres, P. Schmidt, D. MacNair, Phys. Rev. Lett. 14 (1965) 225. [2] Y. Koike, H. Suematsu, K. Higuchi, S. Tanuma, Physica B+C 99 (1980) 503. [3] E.A. Ekimov, V.A. Sidrov, E.D. Bauer, N.N. Melfnik, N.J. Curro, J.D. Thompson, S.M. Stishov, Nature (London) 428 (2004) 542. [4] Y. Takano, M. Nagao, I. Sakaguhci, M. Tachiki, T. Hatano, K. Kobayashi, H. Umezawa, H. Kawarada, Appl. Phys. Lett. 85 (2004) 2851. [5] A.F. Hebard, M.J. Rosseinsky, R.C. Haddon, D.W. Murphy, S.H. Glarum, T.T.M. Palstra, A.P. Ramirez, A.R. Kortan, Nature (London) 350 (1991) 600. [6] A.Y. Ganin, Y. Takabayashi, Y.Z. Khimyak, S. Margadonna, A. Tamai, M.J. Rosseinsky, K. Prassides, Nat. Mater. 7 (2008) 367. [7] K. Miyazawa, Y. Kuwasaki, A. Obayashi, M. Kuwabara, J. Mater. Res. 17 (2002) 83. [8] K. Miyazawa, K. Hamamoto, S. Nagata, T. Suga, J. Mater. Res. 18 (2003) 1096. [9] R. Kato, K. Miyazawa, Diamond Relat. Mater. 20 (2011) 299. [10] D.W. Murphy, M.J. Rosseinsky, R.C. Haddon, A.P. Ramirez, A.F. Hebard, R. Tycko, R.M. Fleming, G. Dabbagh, Physica C 185–189 (1991) 403. [11] D.M. Poirier, D.W. Owens, J.H. Weaver, Phys. Rev. B 51 (1995) 1830. [12] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. [13] V. Buntar, H.W. Weber, Supercond. Sci. Technol. 9 (1996) 599.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Preparation and superconductivity of potassium-doped fullerene nanowhiskers Hiroyuki Takeya a,b,*, Ryoei Kato a, Takatsugu Wakahara a, Kun’ichi Miyazawa a, Takahide Yamaguchi a,b, Toshinori Ozaki a,b, Hiroyuki Okazaki a,b, Yoshihiko Takano a,b a b
National Institute for Materials Science, Tsukuba 305-0047, Japan Transformative Research Project on Iron Pnictides (TRIP), Japan Science and Technology Agency, Chiyoda-ku, Tokyo 102-0075, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 June 2012 Accepted 24 October 2012 Available online 2 November 2012
Superconductivity at 17 K was observed in a potassium (K)-doped fullerene nanowhisker (K-C60NW). The superconducting volume fraction of a full shielding volume was as high as 80% in the nominal composition of K3.3C60NW, while that of K-doped fullerene powder was less than 1% under the same heating condition at 200 8C. We report the superconducting behaviors of our newly synthesized K3.3C60NWs, such as the applied field dependence of magnetization and the critical current density of 3 105 A/cm2 at 5 K under 50 kOe, in comparison to those of K-doped fullerene powders. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Fullerenes A. Superconductors D. Defects
1. Introduction Various materials composed of carbon have been reported so far, such as graphite, diamond, carbon nanotube, fullerene (C60), fullerene-nanowhisker (C60NW),1 and, very recently, graphene as a new material for future devices. Such materials have been attractive due to their anomalous structures, conductance, chemical absorption and adsorption, and strength. Superconductivity in carbon materials has been one of the most interesting physical properties to date. Several superconductors have been reported, such as K-intercalated graphite KC8 (transition temperature, Tc = 0.15 K) [1,2], Ca-doped graphite CaC6 (Tc = 11.5 K), Bdoped diamond (Tc = 8 K) [3,4], K-doped fullerene K3C60 (Tc = 19.0 K) [5], and Cs-doped fullerene Cs3C60 (Tc = 38 K) [6]. Recently, Miyazawa et al. prepared various types of fullerenebased supramolecular materials, such as nanowhiskers (C60NW)1 [7,8], nanosheets, nanowires, and nanotubes composed of fullerene units, by using the liquid–liquid interfacial precipitation method (LLIP method). We have been exploring new superconductors and are currently focusing on carbon-based materials. If those forms of fullerene-based materials turn out to be a superconductor, a new application for flexible and light superconductors will have been developed. In this paper, we report on
* Corresponding author at: National Institute for Materials Science, Tsukuba 3050047, Japan. Tel.: +81 29 859 2318; fax: +81 29 859 2301. E-mail address: [email protected] (H. Takeya). 1 We use the terms C60NW and KxC60NW in this paper. 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.10.033
the intercalation of K in fullerene nanowhiskers (KxC60NWs)1 and their superconducting properties. 2. Experimental We prepared fullerene nanowhiskers, C60NWs, using the LLIP method [7]. The as-grown C60NWs were filtrated and dried up. The typical dimension of the C60NWs used in this experiment was 0.54 0.16 mm in average diameter and 4.43 2.63 mm in average length. About 10 mg C60NWs and an amount of 1.8 mg potassium (K) as the molar ratio of K/C60 = 3.3 were put together in a quartz tube (5 mm in inner diameter), followed by sealing the quartz tube under a vacuum condition at 3 10 3 Pa. We added a 10% excess amount of K over the crystallographically limiting composition K3C60 in consideration of the K adsorption on the surface of crystallites or associated with defects. The sample was heated at 200 8C for 1–36 h in an electric oven. Magnetization measurements were performed using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-5S). The information of the crystal structure was analyzed by powder X-ray diffraction (XRD). The shapes and microstructures of those samples were observed with a scanning electron microscope (SEM, Hitachi SU-70) and a transmission electron microscope (TEM, JEOL JEM-4010). Qualitative microanalysis was achieved with an energy dispersive X-ray analyzer (EDAX, Genesis). For comparison to K-C60NWs, K–C60 powders (particle size = 1–5 mm) were also used and evaluated through all procedures shown above in the same way. The nominal composition was used to indicate the sample names in the following discussion because the quantitative analysis of KxC60NWs was not reliable enough to determine the K/C60 ratio.
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Fig. 2. Temperature dependence of magnetization warming under 20 Oe after zerofield cooling for K3.3C60NW and K3.3C60 powder. The axis of the ordinate is normalized by the sample weight (g) and applied magnetic field (Oe).
Fig. 1. SEM micrographs of (a) pristine C60NWs (upper) and (b) K3.3C60NWs (lower). The inset pictures are the TEM images, showing disordered nanopores.
3. Results and discussion SEM micrographs of C60NW before (a: upper) and after Kdoping (b: lower) are shown in Fig. 1. The shapes of C60NWs are mostly hexagonal prisms with the growth axis along the [0 0 1] direction [9]. C60NWs grow in a hexagonal crystal structure by the LLIP method, and the hexagonal structure then turns to the fcc structure while the solvent is drying up. The inset pictures in Fig. 1(a) and (b) are the images of the pristine and K-doped C60NWs observed using a transmission electron microscope (TEM) [9]. They show that C60NW contains disordered nanopores, which are not observed in C60 crystals. Fig. 2 shows the superconducting transitions of K3.3C60NW and K3.3C60 powders heated at 200 8C for 24 h, which were measured upon warming under 20 Oe after cooling in a zero field. The left and right ordinates of the graph set separately for K3.3C60NW and K3.3C60 are the magnetization normalized by the applied magnetic field and the weight of the samples, respectively. As seen in the figure, a large difference in the superconducting signals between the two samples was observed. The signal of the nanowhisker sample was 200 times larger than that of the powder sample. The XRD patterns of pristine C60, K3.3C60 powder, and K3.3C60NW are identified to be the fcc structure phase as shown in Fig. 3. According to the reported phase diagram of K–C60 [11], C60 and K3C60 exist in the x = 0–3 region of KxC60. In K-doped C60 materials, the only fcc phase of stoichiometric K3C60 is a superconductor. Two tetrahedral sites and one octahedral site ¯ per C60 in the structure (space group: Fm3m) are occupied with the three K atoms. It is known that the intensity ratio of XRD reflections is different in C60 and K3C60. The actual K-doping content might be estimated using the intensity ratio. The simulated intensity ratio
(311/111) (shown in the inset) was calculated assuming that the samples consisted of the mixture of C60 and K3C60. K seemed to be hardly intercalated into the K3.3C60 powder, while an approximate amount of 83% of the K3C60 phase was in K3.3C60NW, as estimated by the XRD intensity ratio. This is consistent with the change of the lattice parameters in C60, K3.3C60 powder, and K3.3C60NW, which was calculated to be 1.4180(5), 1.4180(2), and 1.4200(5) nm, respectively. These results prove that K was intercalated into the C60NWs. To investigate the reaction rate forming K3C60 in the NWs, the superconducting shielding fractions in K3.3C60NW were measured hourly during the heating process at 200 8C by using a SQUID magnetometer. The shielding volume fractions are normalized by the shape of the sample space. As shown in Fig. 4, the shielding volume fraction was saturated at 24 h. On the other hand, in the case of K3.3C60 powder, the superconducting volume fraction was less than 1% by heating at 200 8C for 24 h. Such a small volume fraction of the superconducting KxC60 has already been reported, for example, by Hebard et al. [5] and Murphy et al. [10]. Hebard et al. reported approximately 1% by heating at 200 8C for 36 h. Murphy et al. described that a 1–2% fraction was obtained by
Fig. 3. XRD patterns of pristine and K-doped C60 samples, K3.3C60 powder, and K3.3C60NW (nominal composition), with an fcc structure index. The inset shows the simulated intensity ratio (311/111) and observed intensity ratio of K3.3C60 powder and K3.3C60NW.
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powder. From the hysteresis of the M–H curves, DM (emu/cm3), we can estimate the critical current density Jc on the assumption of a critical-state model with the simple formula, Jc = 30DM/d, where d is the size of the sample (cm) [12]. Fig. 5(b) shows the Jc’s at 5 K as a function of applied magnetic fields calculated for the K3.3C60NW and the K3.3C60 powder, where a third root of the average volume was used for the nanowhisker size and an average particle size measured by SEM was used for the powder size. Both samples heated for K-doping were still kept in their original whisker and powder forms, indicating that there was no difference in their intergranular coupling, as observed using SEM. A possibility for the low Jc of the K3.3C60 powder is the low volume fraction of the superconducting K3C60 phase. The Jc of K3.3C60NW is much higher than that of K3.3C60 powder and is 3 105 (A/cm2) under 50 kOe. The Jc of K3.3C60NW is in the highest level so far among the reported Jc’s [13] and shows the flat applied field dependence from 20 kOe to 50 kOe, indicating a fairly large pinning force in the grain, which can be associated with the nanopores or defects in K3.3C60NW. Fig. 4. Formation of the superconducting phase (%) in K3.3C60NWs estimated by the shielding volume fractions vs. heating time (h) at 200 8C.
4. Conclusions In summary, we reported the preparation and properties of superconducting K-C60NWs by doping K into C60NW. The superconducting shielding volume fraction by heating at 200 8C for 24 h gave high values up to 80%, in contrast to the value of Kdoped C60 powder, which was lower than 1%. We believe that this contrasting difference of the superconducting shielding fractions in the two samples is associated with the defects promoting Kintercalation in K3.3C60NWs. We also observed the Jc of K3.3C60NW higher than 3 105 (A/cm2) at 5 K without a significant decrease up to 50 kOe. We will try to prepare the superconducting materials of longer K-doped C60NW fibers or other forms. Acknowledgments The authors are grateful to the Advanced Low Carbon Technology R&D Program (ALCA) of the Japan Science and Technology Agency for partial financial support. Partial support received from the Japan Society for the Promotion of Science is also acknowledged.
Fig. 5. (a) M–H curves and (b) Jc’s at 5 K for K3.3C60NW and K3.3C60 powder heated at 200 8C for 24 h.
References
heating at 225 8C for 24–48 h and the fraction then increased up to 35% by subsequent heat treatment at 230–265 8C for 21 days. The considerable difference of the superconducting volume fraction between KxC60NWs and K3C60 powder can be discussed on the basis of the cross-sectional analysis of C60NWs by TEM [9]. In comparison to C60 powders, C60NWs contain many defects, such as a core–shell structure, porous inner cores (nano-pores: the average size is 8.3 nm), and disordered domains. We believe that those defects assist in the reaction of K-doping and that the migration of K through the defects promotes the formation of K3C60. Fig. 5(a) shows the applied field dependence of the magnetizations (M–H curves) at 5 K for the two samples, as reported above, measured using a SQUID magnetometer. The magnetization hysteresis of K3.3C60NW is much larger than that of the K3.3C60
[1] N.B. Hannay, T.H. Geballe, B.T. Matthias, K. Andres, P. Schmidt, D. MacNair, Phys. Rev. Lett. 14 (1965) 225. [2] Y. Koike, H. Suematsu, K. Higuchi, S. Tanuma, Physica B+C 99 (1980) 503. [3] E.A. Ekimov, V.A. Sidrov, E.D. Bauer, N.N. Melfnik, N.J. Curro, J.D. Thompson, S.M. Stishov, Nature (London) 428 (2004) 542. [4] Y. Takano, M. Nagao, I. Sakaguhci, M. Tachiki, T. Hatano, K. Kobayashi, H. Umezawa, H. Kawarada, Appl. Phys. Lett. 85 (2004) 2851. [5] A.F. Hebard, M.J. Rosseinsky, R.C. Haddon, D.W. Murphy, S.H. Glarum, T.T.M. Palstra, A.P. Ramirez, A.R. Kortan, Nature (London) 350 (1991) 600. [6] A.Y. Ganin, Y. Takabayashi, Y.Z. Khimyak, S. Margadonna, A. Tamai, M.J. Rosseinsky, K. Prassides, Nat. Mater. 7 (2008) 367. [7] K. Miyazawa, Y. Kuwasaki, A. Obayashi, M. Kuwabara, J. Mater. Res. 17 (2002) 83. [8] K. Miyazawa, K. Hamamoto, S. Nagata, T. Suga, J. Mater. Res. 18 (2003) 1096. [9] R. Kato, K. Miyazawa, Diamond Relat. Mater. 20 (2011) 299. [10] D.W. Murphy, M.J. Rosseinsky, R.C. Haddon, A.P. Ramirez, A.F. Hebard, R. Tycko, R.M. Fleming, G. Dabbagh, Physica C 185–189 (1991) 403. [11] D.M. Poirier, D.W. Owens, J.H. Weaver, Phys. Rev. B 51 (1995) 1830. [12] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. [13] V. Buntar, H.W. Weber, Supercond. Sci. Technol. 9 (1996) 599.