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Local transport properties of PrFeAsO0.7 using FIB micro-fabrication technique
Physica C 470 (2010) 1473–1476
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Local transport properties of PrFeAsO0.7 using FIB micro-fabrication technique K. Shirai a,b, H. Kashiwaya a,*, S. Miura a,b, M. Ishikado a,c,d, H. Eisaki a,d, A. Iyo a,d, I. Kurosawa b, S. Kashiwaya a a
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan Graduate School of Science, Division of Mathematical and Physical Sciences, Japan Women’s University, Mejirodai, Bunkyo, Tokyo 112-8681, Japan c Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Naka, Ibaraki 319-1195, Japan d JST, Transformative Research-Project on Iron Pnictides (TRIP), 5, Sanbancho, Chiyoda, Tokyo 102-0075, Japan b
a r t i c l e
i n f o
Article history: Available online 15 May 2010 Keywords: Iron-pnictide superconductor Transport Anisotropy
a b s t r a c t To evaluate the anisotropy of single crystals with a size of several tens of micrometers, we develop a technique for measuring the local transport properties along both the ab-plane and the c-axis using microsize devices fabricated by a focused ion beam process. We applied this technique to identify the resistivity anisotropy (cq = qc/qab) of the iron-pnictide superconductor PrFeAsO0.7 synthesized under high-pressure. The obtained value of cq 120 at 50 K indicates the presence of strong anisotropy, which is comparable to the anisotropy measured by other methods. The weak temperature dependence detected for the normalized resistivity along the c-axis (less than 10% difference at 280 K and 50 K) suggests a peculiar interlayer transport mechanism for PrFeAsO0.7. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The discovery of a family of iron-pnictide superconductors has renewed interest in the physical properties and mechanisms of unconventional superconductors [1]. The critical temperature (Tc) of as high as 56 K observed in SmFeAsO1xFx is the highest among the non-cuprates superconductors [2]. The crystal structures of the iron-pnictides characterized by a stack of superconducting Fe2As2 layers sandwiched between block layers are similar to those of the cuprate superconductors. This similarity suggests that the two-dimensionality of the electronic states is an important requirement for the onset of high-Tc superconductivity. On the other hand, previously reported experimental data on (Ba,K)Fe2As2 (‘1 2 2’, Tc 28 K) based on Hc2 measurements indicated nearly isotropic features in the temperature range of 10–27 K [3]. The compatibility of isotropic electronic states and high-Tc superconductivity may remove the bottleneck in several possible applications of superconductors, such as power supply cables. Therefore, the exact evaluation of the anisotropy of iron-pnictides is an important issue. Here we present a method of evaluating the local transport properties of small single crystals along both the ab-plane and the c-axis using a microsize device fabricated by a focused ion beam (FIB) process. We have applied this method to evaluate the transport anisotropy in oxygen-deficient PrFeAsO0.7 single crystals grown by a high-pressure synthesis (HPS) method. This compound
* Corresponding author. Tel.: +81 298 61 5568; fax: +81 298 61 5569. E-mail address: [email protected] (H. Kashiwaya). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2010.05.141
has so-called ‘1 1 1 1’ structure with typical Tc of 54 K [4,5]. Although techniques for growing large crystals are rapidly improving [6], the domain size still tends to be limited to few-hundred micrometers. Therefore, previous evaluations of the anisotropy relied on the anisotropy of Hc2 or the Montgomery technique [7–10]. Furthermore, even in single-domain crystals, unexpected errors may still appear owing to irregularities or microscopic cracks. The present method has the capability of excluding the effect of lm-scale imperfectness. In this paper, we present details of the fabrication process and preliminary results on the local transport properties of PrFeAsO0.7 crystals above Tc. 2. Experimental Single crystals of oxygen-deficient PrFeAsO0.7 were prepared by the HPS method using belt-type anvil apparatus. Since details of the growth conditions have been described elsewhere [4–6], we mainly focus on the device fabrication process in the following. Small pieces of single crystals with typical sizes of 10–100 lm width and 20 lm thickness were selected from the polycrystalline bulk. The single crystals were cleaved to obtain fresh surfaces. Then, each piece was pasted on a SrTiO3 substrate and then an Au electrode was deposited using a metal mask (Fig. 1a); a contact resistance of about 1 mX/lm2 can usually be achieved. The sample was mounted on an FIB system (Seiko Instruments Inc. SMI-9200) soon after the deposition. The etching and transport measurements were repeated alternately by the following procedure. First, the PrFeAsO0.7 single crystal was necked to form a constricted shape using a Ga+ beam
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Fig. 1. Schematic diagrams of the fabrication process of s-shaped devices: (a) Au contact deposition and patterning, (b) FIB etching process from the top, (c) FIB etching process from the side, (d) SIM image of the s-shaped device used in the present study (sample A). The intersectional area of the constricted area is about 5.6 lm2.
irradiated along the c-axis (Fig. 1b). The size dependence of the abplane transport properties can be deduced by changing the intersectional area of the necked region. Next, two slits were formed by irradiating the Ga+ beam along the ab-plane direction. These slits were designed to overlap in the c-axis direction, as schematically shown in Fig. 1c, so that the current direction was limited along the c-axis. This s-shaped device, usually referred to as an ‘intrinsic Josephson junction’ when made from cuprates [11,12], was used to evaluate the c-axis transport properties. Fig. 1d shows a scanning ion microscopy (SIM) image of the s-shaped device used in the present study. In spite of the surface degradation during the FIB etching process, the present method has several advantages as summarized below. First, the local transport properties can be measured using a crystal with a diameter of as small as several micrometers. In some cases, there may exist lm-scale cracks and irregularities in the crystals. Since the position of the constricted area can be selected arbitrarily, we can minimize the effect of irregularities in crystals by referring to SIM images. Secondly, the effect of surface degradation can be avoided because the present process does not rely on the heterojunction configuration. Thirdly, the transport properties along both the ab-plane and the c-axis can be measured in an identical region. Finally, the present method can be applied to a compound for which a thin film has not yet been successfully fabricated. This is in clear contrast to the conventional lithography process, which can only be applied to thin-film samples.
3. Results and discussion Measurements were performed on two PrFeAsO0.7 crystals, referred to as sample A and sample B. Figs. 2 and 3 show the temperature (T) dependences of the local resistivity along the ab-plane (qab) and c-axis (qc), respectively. For both samples, qab exhibits
Fig. 2. Temperature dependences for qab of PrFeAsO0.7 obtained in sample A and sample B.
K. Shirai et al. / Physica C 470 (2010) 1473–1476
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Fig. 3. Temperature dependences of qc obtained using s-shaped device made of PrFeAsO0.7.
similar temperature dependences to those obtained in other studies, but the absolute value for sample B is about five times larger than that for sample A. In the SIM image of sample B, we observed an irregularity, identified as a region with different contrast, inside the necked region. Also the absolute value of qab for sample A is almost equivalent to that obtained in Ref. [6]. For these reasons, we consider that the results for sample A are more reliable than those for sample B. Regarding the c-axis transport property, qc for sample A exhibits insulating (dqc/dT < 0) behavior even though qab has a metallic characteristic (dqc/dT > 0) in the entire temperature range. It is still unclear whether this feature is common to all samples because qc for sample B also exhibits a metallic characteristic. Fig. 4 shows a plot of the transport anisotropy cq(=qc/qab) above Tc. Assuming the relation c 2 2 cq qc =qab ¼ mc =mab ¼ ðHab c2 =H c2 Þ ¼ cH c2 ;
ð1Þ
the obtained value of cq of about 120 slightly above Tc is close to cHc2 ffi 5–9 for [1 1 1 1] compounds [6–10], and also to the anisotropy cq ffi 15 obtained by a band calculation [13]. The deviations probably reflect the electron filling, structure, and other details of each crystal. Comparing this value with those for cuprates, the anisotropy is comparable to typical values for YBa2Cu3O7+d (YBCO) and is about two orders of magnitude smaller than that for Bi2Sr2CaCu2O8+d (Bi2212). Thus, we conclude that the anisotropy of PrFeAsO0.7 is much larger than that of [1 2 2] systems and comparable to that of YBCO.
Fig. 5. Temperature dependences of the normalized resistivity ri(T) qi(T)/qi (280 K) (i = ab, c) of PrFeAsO0.7 and Bi2212. Note that rc(T) for PrFeAsO0.7 exhibits only weak temperature dependence.
Fig. 5 shows the temperature dependences of the normalized resistivity ri(T) qi(T)/qi (280 K) (i = ab, c). For comparison, rab(T) and rc(T) for slightly underdoped Bi2212, measured by the present method, are also plotted in the figure. We found that the normalized resistivity rc(T) of PrFeAsO0.7 is anomalous. First, the temperature dependences of rab and rc are different in PrFeAsO0.7. If we refer to a naïve transport mechanism for two-dimensional superconductors in which coherent interplanar tunneling with repeated intraplanar incoherent scattering is assumed [14], the resistivities along the ab-plane and the c-axis are expected to exhibit similar temperature dependences although slight renormalization is required. Actually rab(T) and rc(T) for Bi2212 have similar temperature dependences. The features detected in PrFeAsO0.7 appear to be incompatible with a naïve mechanism. Secondly, the variation of rc(T) is reasonably small (less than 10% difference at 280 K and 50 K). This finding implies that a thermally independent factor dominates the scattering process in the c-axis direction. The weak temperature dependence is inconsistent with the conventional thermally assisted interlayer hopping mechanism through the insulating block layer. Thirdly, although details of the transport properties below Tc will be described separately [15], here we mention that the critical current along the c-axis below Tc (data not shown here) is largely suppressed compared with the value expected from the superconducting gap amplitude and the normal state resistance. This feature suggests that the c-axis scattering is a highly inspecular process [15]. This is different from the case of cuprates, in which the c-axis transport is a mostly specular scattering process with a subdominant inspecularity originating from the effect of the orbital shape in the block layer [16]. For these three reasons, we conclude that the c-axis transport mechanism of the iron-pnictide is anomalous and distinct from that in the cuprates. Clarifying the c-axis transport mechanism will reveal one of the requisites for the onset of high-Tc superconductivity. Therefore, it is important to confirm whether the present anomaly in c-axis transport is universal to all iron-pnictide superconductors. Detailed studies will be carried out in the near future. 4. Summary
Fig. 4. Temperature dependences of transport anisotropy (c = qc/qab) above Tc.
We have developed a technique for detecting the local transport properties of a single crystal with a diameter of as small as several micrometers. The measurements performed on high-pressure synthesized PrFeAsO0.7 single crystals reveal transport anisotropy with cq 120 slightly above Tc. We found that the c-axis transport
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characteristics are anomalous and distinct from those of cuprate superconductors. Acknowledgements This work was financially supported by Grants-in-Aid for Scientific Research (Nos. 21710100, 20540392 and 20221007) from JSPS, Japan, and also by Mitsubishi Foundation. References [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130 (2008) 3296. [2] Z.A. Ren, W. Lu, J. Yang, W. Yi, X.L. Shen, Z.C. Li, G.C. Che, X.L. Dong, L.L. Sun, F. Zhou, Z.X. Zhao, Chin. Phys. Lett. 25 (2008) 2215. [3] H.Q. Yuan, J. Singleton, F.F. Balakierev, S.A. Baily, G.F. Chen, J.L. Luo, N.L. Wang, Nature 457 (2009) 29. [4] H. Kito, H. Eisaki, A. Iyo, J. Phys. Soc. Jpn. 77 (2008) 063707. [5] K. Miyazawa, K. Kihou, P.M. Shirage, C.-H. Lee, H. Kito, H. Eisaki, A. Iyo, J. Phys. Soc. Jpn. 78 (2009) 034712.
[6] M. Ishikado, S. Shamoto, H. Kito, A. Iyo, H. Eisaki, T. Ito, Y. Tomioka, Physica C 469 (2009) 901. [7] Y. Jia, P. Cheng, L. Fang, H. Luo, H. Yang, C. Ren, L. Shan, C. Gu, H.-H. Wen, Appl. Phys. Lett. 93 (2008) 032503. [8] J. Jaroszynski, F. Hunte, L. Balicas, Youn-jung Jo, I. Raicevic, A. Gurevich, D.C. Larbalestier, F.F. Balakirev, L. Fang, P. Cheng, Y. Jia, H.H. Wen, Phys. Rev. B 78 (2008) 174523. [9] M.A. Tanatar, N. Ni, C. Martin, R.T. Gordon, H. Kim, V.G. Kogan, G.D. Samolyuk, S.L. Bud’ko, P.C. Canfield, R. Prozorov, Phys. Rev. B 79 (2009) 094507. [10] L. Balicas, A. Gurevich, Y.J. Jo, J. Jaroszynski, D.C. Larbalestier, R.H. Liu, H. Chen, X.H. Chen, N.D. Zhigadlo, S. Kaytrych, Z. Bukowski, J. Karpinski, arXiv:0809.4223v2. [11] Y.I. Latyshev, S.-J. Kim, T. Yamashita, JETP Lett. 69 (1999) 84. [12] S.-J. Kim, Y.I. Latyshev, T. Yamashita, Appl. Phys. Lett. 74 (1999) 1156. [13] D.J. Singh, M.-H. Du, Phys. Rev. Lett. 100 (2008) 237003. [14] N. Kumar, A.M. Jayannavar, Phys. Rev. B 45 (1992) 5001. [15] H. Kashiwaya, K. Shirai, T. Matsumoto, H. Shibata, H. Kambara, M. Ishikado, H. Eisaki, A. Iyo, S. Shamoto, I. Kurosawa, S. Kashiwaya, Appl. Phys. Lett. 96 (2010) 202504. [16] O.K. Anderson, A.I. Liechtenstein, O. Jepsen, F. Paulsen, J. Phys. Chem. Solids 56 (1995) 1573.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Local transport properties of PrFeAsO0.7 using FIB micro-fabrication technique K. Shirai a,b, H. Kashiwaya a,*, S. Miura a,b, M. Ishikado a,c,d, H. Eisaki a,d, A. Iyo a,d, I. Kurosawa b, S. Kashiwaya a a
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan Graduate School of Science, Division of Mathematical and Physical Sciences, Japan Women’s University, Mejirodai, Bunkyo, Tokyo 112-8681, Japan c Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Naka, Ibaraki 319-1195, Japan d JST, Transformative Research-Project on Iron Pnictides (TRIP), 5, Sanbancho, Chiyoda, Tokyo 102-0075, Japan b
a r t i c l e
i n f o
Article history: Available online 15 May 2010 Keywords: Iron-pnictide superconductor Transport Anisotropy
a b s t r a c t To evaluate the anisotropy of single crystals with a size of several tens of micrometers, we develop a technique for measuring the local transport properties along both the ab-plane and the c-axis using microsize devices fabricated by a focused ion beam process. We applied this technique to identify the resistivity anisotropy (cq = qc/qab) of the iron-pnictide superconductor PrFeAsO0.7 synthesized under high-pressure. The obtained value of cq 120 at 50 K indicates the presence of strong anisotropy, which is comparable to the anisotropy measured by other methods. The weak temperature dependence detected for the normalized resistivity along the c-axis (less than 10% difference at 280 K and 50 K) suggests a peculiar interlayer transport mechanism for PrFeAsO0.7. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction The discovery of a family of iron-pnictide superconductors has renewed interest in the physical properties and mechanisms of unconventional superconductors [1]. The critical temperature (Tc) of as high as 56 K observed in SmFeAsO1xFx is the highest among the non-cuprates superconductors [2]. The crystal structures of the iron-pnictides characterized by a stack of superconducting Fe2As2 layers sandwiched between block layers are similar to those of the cuprate superconductors. This similarity suggests that the two-dimensionality of the electronic states is an important requirement for the onset of high-Tc superconductivity. On the other hand, previously reported experimental data on (Ba,K)Fe2As2 (‘1 2 2’, Tc 28 K) based on Hc2 measurements indicated nearly isotropic features in the temperature range of 10–27 K [3]. The compatibility of isotropic electronic states and high-Tc superconductivity may remove the bottleneck in several possible applications of superconductors, such as power supply cables. Therefore, the exact evaluation of the anisotropy of iron-pnictides is an important issue. Here we present a method of evaluating the local transport properties of small single crystals along both the ab-plane and the c-axis using a microsize device fabricated by a focused ion beam (FIB) process. We have applied this method to evaluate the transport anisotropy in oxygen-deficient PrFeAsO0.7 single crystals grown by a high-pressure synthesis (HPS) method. This compound
* Corresponding author. Tel.: +81 298 61 5568; fax: +81 298 61 5569. E-mail address: [email protected] (H. Kashiwaya). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2010.05.141
has so-called ‘1 1 1 1’ structure with typical Tc of 54 K [4,5]. Although techniques for growing large crystals are rapidly improving [6], the domain size still tends to be limited to few-hundred micrometers. Therefore, previous evaluations of the anisotropy relied on the anisotropy of Hc2 or the Montgomery technique [7–10]. Furthermore, even in single-domain crystals, unexpected errors may still appear owing to irregularities or microscopic cracks. The present method has the capability of excluding the effect of lm-scale imperfectness. In this paper, we present details of the fabrication process and preliminary results on the local transport properties of PrFeAsO0.7 crystals above Tc. 2. Experimental Single crystals of oxygen-deficient PrFeAsO0.7 were prepared by the HPS method using belt-type anvil apparatus. Since details of the growth conditions have been described elsewhere [4–6], we mainly focus on the device fabrication process in the following. Small pieces of single crystals with typical sizes of 10–100 lm width and 20 lm thickness were selected from the polycrystalline bulk. The single crystals were cleaved to obtain fresh surfaces. Then, each piece was pasted on a SrTiO3 substrate and then an Au electrode was deposited using a metal mask (Fig. 1a); a contact resistance of about 1 mX/lm2 can usually be achieved. The sample was mounted on an FIB system (Seiko Instruments Inc. SMI-9200) soon after the deposition. The etching and transport measurements were repeated alternately by the following procedure. First, the PrFeAsO0.7 single crystal was necked to form a constricted shape using a Ga+ beam
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Fig. 1. Schematic diagrams of the fabrication process of s-shaped devices: (a) Au contact deposition and patterning, (b) FIB etching process from the top, (c) FIB etching process from the side, (d) SIM image of the s-shaped device used in the present study (sample A). The intersectional area of the constricted area is about 5.6 lm2.
irradiated along the c-axis (Fig. 1b). The size dependence of the abplane transport properties can be deduced by changing the intersectional area of the necked region. Next, two slits were formed by irradiating the Ga+ beam along the ab-plane direction. These slits were designed to overlap in the c-axis direction, as schematically shown in Fig. 1c, so that the current direction was limited along the c-axis. This s-shaped device, usually referred to as an ‘intrinsic Josephson junction’ when made from cuprates [11,12], was used to evaluate the c-axis transport properties. Fig. 1d shows a scanning ion microscopy (SIM) image of the s-shaped device used in the present study. In spite of the surface degradation during the FIB etching process, the present method has several advantages as summarized below. First, the local transport properties can be measured using a crystal with a diameter of as small as several micrometers. In some cases, there may exist lm-scale cracks and irregularities in the crystals. Since the position of the constricted area can be selected arbitrarily, we can minimize the effect of irregularities in crystals by referring to SIM images. Secondly, the effect of surface degradation can be avoided because the present process does not rely on the heterojunction configuration. Thirdly, the transport properties along both the ab-plane and the c-axis can be measured in an identical region. Finally, the present method can be applied to a compound for which a thin film has not yet been successfully fabricated. This is in clear contrast to the conventional lithography process, which can only be applied to thin-film samples.
3. Results and discussion Measurements were performed on two PrFeAsO0.7 crystals, referred to as sample A and sample B. Figs. 2 and 3 show the temperature (T) dependences of the local resistivity along the ab-plane (qab) and c-axis (qc), respectively. For both samples, qab exhibits
Fig. 2. Temperature dependences for qab of PrFeAsO0.7 obtained in sample A and sample B.
K. Shirai et al. / Physica C 470 (2010) 1473–1476
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Fig. 3. Temperature dependences of qc obtained using s-shaped device made of PrFeAsO0.7.
similar temperature dependences to those obtained in other studies, but the absolute value for sample B is about five times larger than that for sample A. In the SIM image of sample B, we observed an irregularity, identified as a region with different contrast, inside the necked region. Also the absolute value of qab for sample A is almost equivalent to that obtained in Ref. [6]. For these reasons, we consider that the results for sample A are more reliable than those for sample B. Regarding the c-axis transport property, qc for sample A exhibits insulating (dqc/dT < 0) behavior even though qab has a metallic characteristic (dqc/dT > 0) in the entire temperature range. It is still unclear whether this feature is common to all samples because qc for sample B also exhibits a metallic characteristic. Fig. 4 shows a plot of the transport anisotropy cq(=qc/qab) above Tc. Assuming the relation c 2 2 cq qc =qab ¼ mc =mab ¼ ðHab c2 =H c2 Þ ¼ cH c2 ;
ð1Þ
the obtained value of cq of about 120 slightly above Tc is close to cHc2 ffi 5–9 for [1 1 1 1] compounds [6–10], and also to the anisotropy cq ffi 15 obtained by a band calculation [13]. The deviations probably reflect the electron filling, structure, and other details of each crystal. Comparing this value with those for cuprates, the anisotropy is comparable to typical values for YBa2Cu3O7+d (YBCO) and is about two orders of magnitude smaller than that for Bi2Sr2CaCu2O8+d (Bi2212). Thus, we conclude that the anisotropy of PrFeAsO0.7 is much larger than that of [1 2 2] systems and comparable to that of YBCO.
Fig. 5. Temperature dependences of the normalized resistivity ri(T) qi(T)/qi (280 K) (i = ab, c) of PrFeAsO0.7 and Bi2212. Note that rc(T) for PrFeAsO0.7 exhibits only weak temperature dependence.
Fig. 5 shows the temperature dependences of the normalized resistivity ri(T) qi(T)/qi (280 K) (i = ab, c). For comparison, rab(T) and rc(T) for slightly underdoped Bi2212, measured by the present method, are also plotted in the figure. We found that the normalized resistivity rc(T) of PrFeAsO0.7 is anomalous. First, the temperature dependences of rab and rc are different in PrFeAsO0.7. If we refer to a naïve transport mechanism for two-dimensional superconductors in which coherent interplanar tunneling with repeated intraplanar incoherent scattering is assumed [14], the resistivities along the ab-plane and the c-axis are expected to exhibit similar temperature dependences although slight renormalization is required. Actually rab(T) and rc(T) for Bi2212 have similar temperature dependences. The features detected in PrFeAsO0.7 appear to be incompatible with a naïve mechanism. Secondly, the variation of rc(T) is reasonably small (less than 10% difference at 280 K and 50 K). This finding implies that a thermally independent factor dominates the scattering process in the c-axis direction. The weak temperature dependence is inconsistent with the conventional thermally assisted interlayer hopping mechanism through the insulating block layer. Thirdly, although details of the transport properties below Tc will be described separately [15], here we mention that the critical current along the c-axis below Tc (data not shown here) is largely suppressed compared with the value expected from the superconducting gap amplitude and the normal state resistance. This feature suggests that the c-axis scattering is a highly inspecular process [15]. This is different from the case of cuprates, in which the c-axis transport is a mostly specular scattering process with a subdominant inspecularity originating from the effect of the orbital shape in the block layer [16]. For these three reasons, we conclude that the c-axis transport mechanism of the iron-pnictide is anomalous and distinct from that in the cuprates. Clarifying the c-axis transport mechanism will reveal one of the requisites for the onset of high-Tc superconductivity. Therefore, it is important to confirm whether the present anomaly in c-axis transport is universal to all iron-pnictide superconductors. Detailed studies will be carried out in the near future. 4. Summary
Fig. 4. Temperature dependences of transport anisotropy (c = qc/qab) above Tc.
We have developed a technique for detecting the local transport properties of a single crystal with a diameter of as small as several micrometers. The measurements performed on high-pressure synthesized PrFeAsO0.7 single crystals reveal transport anisotropy with cq 120 slightly above Tc. We found that the c-axis transport
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characteristics are anomalous and distinct from those of cuprate superconductors. Acknowledgements This work was financially supported by Grants-in-Aid for Scientific Research (Nos. 21710100, 20540392 and 20221007) from JSPS, Japan, and also by Mitsubishi Foundation. References [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130 (2008) 3296. [2] Z.A. Ren, W. Lu, J. Yang, W. Yi, X.L. Shen, Z.C. Li, G.C. Che, X.L. Dong, L.L. Sun, F. Zhou, Z.X. Zhao, Chin. Phys. Lett. 25 (2008) 2215. [3] H.Q. Yuan, J. Singleton, F.F. Balakierev, S.A. Baily, G.F. Chen, J.L. Luo, N.L. Wang, Nature 457 (2009) 29. [4] H. Kito, H. Eisaki, A. Iyo, J. Phys. Soc. Jpn. 77 (2008) 063707. [5] K. Miyazawa, K. Kihou, P.M. Shirage, C.-H. Lee, H. Kito, H. Eisaki, A. Iyo, J. Phys. Soc. Jpn. 78 (2009) 034712.
[6] M. Ishikado, S. Shamoto, H. Kito, A. Iyo, H. Eisaki, T. Ito, Y. Tomioka, Physica C 469 (2009) 901. [7] Y. Jia, P. Cheng, L. Fang, H. Luo, H. Yang, C. Ren, L. Shan, C. Gu, H.-H. Wen, Appl. Phys. Lett. 93 (2008) 032503. [8] J. Jaroszynski, F. Hunte, L. Balicas, Youn-jung Jo, I. Raicevic, A. Gurevich, D.C. Larbalestier, F.F. Balakirev, L. Fang, P. Cheng, Y. Jia, H.H. Wen, Phys. Rev. B 78 (2008) 174523. [9] M.A. Tanatar, N. Ni, C. Martin, R.T. Gordon, H. Kim, V.G. Kogan, G.D. Samolyuk, S.L. Bud’ko, P.C. Canfield, R. Prozorov, Phys. Rev. B 79 (2009) 094507. [10] L. Balicas, A. Gurevich, Y.J. Jo, J. Jaroszynski, D.C. Larbalestier, R.H. Liu, H. Chen, X.H. Chen, N.D. Zhigadlo, S. Kaytrych, Z. Bukowski, J. Karpinski, arXiv:0809.4223v2. [11] Y.I. Latyshev, S.-J. Kim, T. Yamashita, JETP Lett. 69 (1999) 84. [12] S.-J. Kim, Y.I. Latyshev, T. Yamashita, Appl. Phys. Lett. 74 (1999) 1156. [13] D.J. Singh, M.-H. Du, Phys. Rev. Lett. 100 (2008) 237003. [14] N. Kumar, A.M. Jayannavar, Phys. Rev. B 45 (1992) 5001. [15] H. Kashiwaya, K. Shirai, T. Matsumoto, H. Shibata, H. Kambara, M. Ishikado, H. Eisaki, A. Iyo, S. Shamoto, I. Kurosawa, S. Kashiwaya, Appl. Phys. Lett. 96 (2010) 202504. [16] O.K. Anderson, A.I. Liechtenstein, O. Jepsen, F. Paulsen, J. Phys. Chem. Solids 56 (1995) 1573.