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Crystal growth and anisotropic superconducting properties of Sr2VFeAsO3
Physica C 484 (2013) 16–18
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Crystal growth and anisotropic superconducting properties of Sr2VFeAsO3 T. Katagiri, T. Sasagawa ⇑ Materials and Structures Laboratory, Tokyo Institute of Technology, Kanagawa 226-8503, Japan
a r t i c l e
i n f o
Article history: Accepted 2 March 2012 Available online 12 March 2012 Keywords: Sr2VFeAsO3 Single crystals Anisotropy Irreversibility line Critical current density
a b s t r a c t We have succeeded in growing single crystals of the Sr2VFeAsO3 superconductor, which are large enough to experimentally elucidate the anisotropic physical properties in this system. Using the obtained single crystals, superconducting properties under magnetic fields parallel to the c-axis (H//c) and ab-plane (H//ab) have been evaluated. It is found that the irreversibility field/temperature, below which the superconducting critical current density (Jc) becomes finite, is highly anisotropic in Sr2VFeAsO3: the irreversible region is reduced by approximately half for the case of H//c as compared with H//ab. The suppression of the irreversibility temperature by H//c is found to be the largest among the Fe-based superconductors and comparable to the highly anisotropic Bi–Cu-based high-Tc superconductors. The rapid decrease of Jc with increasing H//c and T in the irreversible region is also observed. Therefore, there are pronounced fluctuations in the mixed state of Sr2VFeAsO3, suggesting that the vortices are in the pancake state as discussed in the Bi–Cu-based high-Tc superconductors. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of LaFeAsO1 xFx [1], Fe-based superconductors have been actively studied, and various new compounds have been found, which include Ae(Fe, M)2As2 (Ae = Ca, Sr, Ba, M = Co, Ni) [2–4], AFeAs (A = Li, Na) [5,6], and FeSe [7]. All these Fe-based superconductors have layered crystal structures which consist of FeAs superconducting layers and additional blocking layers. In case of AeFe2As2, the blocking layers are one atomic-thick alkalineearth, while LnFeAsO1 xFx (Ln = lanthanides) has several times thicker Ln(O, F) blocking layers. Since it has been reported that LnFeAsO1 xFx compounds have more anisotropic physical properties than AeFe2As2 [8], it seems that the blocking layer plays fundamental roles for the material-dependent anisotropy in the Fe-based superconductors as similar in the Cu-based high-Tc superconductors. In this regard, it is extremely interesting to examine the anisotropic properties in the recently discovered Sr2VFeAsO3 [9] as it has considerably thick perovskite-type Sr2VO3 blocking layers (its crystal structure is shown in the inset of Fig. 1). To the best of our knowledge, however, there has been no experimental report as to the normal and superconducting properties of Sr2VFeAsO3 as a function of the lattice orientation. This is mainly because of the lack of the sizable single crystals of this compound.
⇑ Corresponding author. Address: R3-37, MSL, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503, Japan. Tel./fax: +81 45 924 5366. E-mail addresses: [email protected] (T. Katagiri), [email protected]. ac.jp (T. Sasagawa). 0921-4534/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2012.03.005
In this study, we have succeeded in growing Sr2VFeAsO3 single crystals, which are large enough to perform magnetic, transport, and other measurements with respect to anisotropy. A self-flux technique was applied for the crystal growth, and the growth conditions including the flux composition were optimized. Using the obtained single crystals, we performed magnetic measurements in the superconducting states. The irreversibility line, which is the closing point of the magnetization hysteresis and corresponds to the onset of the critical current density, was determined under magnetic fields both parallel and perpendicular to the c-axis. The obtained magnetic phase diagram revealed the highly anisotropic nature of the mixed state in Sr2VFeAsO3. The extended reversible (Jc 0) region, in addition to the rapid suppression of Jc in the irreversible region, under magnetic fields parallel to the c-axis indicates that vortex fluctuations are by far larger in Sr2VFeAsO3 than any other Fe-based superconductors. These results together with the highly anisotropic layered structure of Sr2VFeAsO3 are reminiscent of the Bi–Cu-based high-Tc superconductors, and are suggestive of the exotic mixed state such as the formation of pancake vortices. 2. Experimental Single crystals of Sr2VFeAsO3 were grown by a flux technique using FeAs as a self-flux. The FeAs precursor was prepared from elemental Fe and As at 900 °C for 24 h in a vacuum. The resultant FeAs was mixed with Sr, SrO, and V2O5 powders at a molar ratio of 5:3:1:1. The mixture was loaded in an alumina crucible to prevent it from reactions with the quartz tube. Then, the powdercontaining alumina crucible was sealed in an evacuated quartz
T. Katagiri, T. Sasagawa / Physica C 484 (2013) 16–18
Fig. 1. X-ray diffraction pattern from the cleaved surface of a Sr2VFeAsO3 single crystal. The inset shows the crystal structure and the grown single crystal.
tube. It was further encapsulated in the second evacuated quartz tube. All the weighing and mixing procedures were carried out in a globe box with the dried N2 gas. It was heated up to 1250 °C and cooled slowly down to 1150–1200 °C at a rate of 0.5–1 °C/h. To improve the size of crystals, the first crop of the tiny crystals were used as the seed crystals for the second growth. Using the good cleavage found in Sr2VFeAsO3, single crystals with flat shiny surfaces of 0.3 0.3 mm2 were easily separated from the flux as shown in the inset of Fig. 1. The X-ray diffraction pattern was measured with Cu Ka radiation by using a Bruker D2 PHASER diffractometer equipped with a LynxEye 1D-detector. Magnetization measurements were performed using a commercial SQUID magnetometer (Quantum Design, MPMS-XL5). The external magnetic field was applied parallel or perpendicular to the c-axis of Sr2VFeAsO3 single crystals. 3. Results and discussion It turned out that the use of the alumina crucible, the application of the double-quartz-tube technique, and the two-step crystal growth are essential to obtain sizable single crystals of Sr2VFeAsO3.
17
The X-ray diffraction pattern from the cleaved surface (0.3 0.3 mm2) of a single crystal is shown in the main panel of Fig. 1. It has only the (0 0 L) reflections, indicating that the cleavage takes place along the ab-plane. The c-axis lattice constant was determined to be 15.67 Å, which is in good agreement with the reported values for polycrystalline samples [9]. Shown in Fig. 2a are the magnetization curves as a function of temperature measured in the same Sr2VFeAsO3 crystal at 10 Oe of the magnetic field parallel to the ab-plane (upper) and the c-axis (lower). Within the experimental uncertainty, both of the magnetization curves are identical under this lower limit of the magnetic field, and thus give the onset of the thermodynamic superconducting critical temperature Tc of 27 K. It is noted that this value is lower than the maximum Tc of 37 K reported for polycrystalline samples. This will be attributed to oxygen nonstoichiometry in this system. While it is quite challenging to determine the oxygen deficiency x in our Sr2VFeAsO3 x crystals by a chemical technique, a rough estimate can be done from the reported relation between Tc and x in polycrystalline samples [10], and x 0.3 is inferred. Once the magnetic field is well above the lower critical field Hc1, the vortices are responsible for superconducting properties. Then, the onset of magnetic hysteresis turns from the critical temperature/field to the onset of the critical current density Jc, and the boundary in the magnetic phase diagram is called as the irreversibility line. We found that the irreversibility line in the Sr2VFeAsO3 crystal depended considerably on the field orientation with respect to the crystal axis, indicating the vortex state of this material is highly anisotropic. As indicated by arrows in Fig. 2b, the irreversibility temperature at 10 kOe is distinctly different between the fields along the ab-plane and the c-axis; the latter is significantly lower than the former. The same behavior was confirmed also by the magnetization measurements as a function of field at a fixed temperature (not shown here). Fig. 2c summarizes the irreversibility line in the Sr2VFeAsO3 crystal under H//ab and H//c. Now it becomes clearer that there are pronounced reversible (Jc 0) regions below the critical temperature both under H//ab and H//c, which should be compared with those reported for NdFeAs(O, F) and (Ba, K)Fe2As2 single crystals under H//c [8]. As for the anisotropy, the irreversible region is reduced by approximately half from H//ab to H//c in Sr2VFeAsO3. It would be useful to define the irreversibility anisotropy as cirr = Tirr(H)//ab/
Fig. 2. Temperature dependence of the magnetization curves in a Sr2VFeAsO3 single crystal under the magnetic fields parallel to the ab-plane or c-axis at 10 Oe (a) and 10 kOe (b). The magnetization curves under H//c are shifted downward for the easy comparison of the irreversibility temperatures, which are indicated by arrows. (c) The irreversibility lines plotted in the magnetic phase diagram for Sr2VFeAsO3 (circles), NdFeAs(O, F) (squares) [8], and (Ba, K)Fe2As2 (triangles) [8].
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T. Katagiri, T. Sasagawa / Physica C 484 (2013) 16–18
Fig. 3. (a) Magnetization hysteresis loops in a Sr2VFeAsO3 single crystal at various temperatures. The magnetic field is parallel to the c-axis. (b) Magnetic field dependence of the critical current density Jc in a Sr2VFeAsO3 single crystal at 1.7–10 K.
Tirr(H)//c for the purpose of the quantitative comparison among different material systems. At 20 kOe, Sr2VFeAsO3 has cirr = 1.98, which is the largest among the other Fe-based superconductors whose data are available in literature; e.g. cirr = 1.20 for NdFeAs(O, F) and cirr = 1.02 for (Ba, K)Fe2As2. The extended reversible region and the highly field-direction dependent vortex state in Sr2VFeAsO3 are more like Cu-based high-Tc superconductors than its Fe-based family compounds. The observation indicates that there are significant fluctuations in the mixed state of Sr2VFeAsO3, which may come from the 2D pancake-like state of vortices, as similar to the Bi–Cubased high-Tc superconductors, rather than the conventional 3D rod-like state. The effect of the giant vortex fluctuations was also observed in the irreversible (Jc > 0) region. Fig. 3a shows the magnetization hysteresis curves at various temperatures under H//c in the Sr2VFeAsO3 single crystal. The magnetization hystereses DM were found to monotonously decrease with field and temperature, and thus there is no sign of the so-called ‘‘second peak’’ or ‘‘fish tail’’ effect which have often been observed in Fe-based and Cu-based superconductors [4,11,12]. From these magnetization hysteresis loops, Jc as a function of field was estimated based on the Bean model [13] as Jc(H) = 20DM(H)/{a(1 a/3b)}, where a and b are the dimensions of the rectangular sample (a < b) perpendicular to the field. Fig. 3b shows the obtained field dependence of Jc at various temperatures. It is noted that Jc at 5 K in the Sr2VFeAsO3 single crystal is 10 times lower than that in the typical Fe-based superconductors such as SmFeAsO1 xFx [14] and Ba(Fe, Co)2As2 [15], suggesting that the vortex pinning is very weak in Sr2VFeAsO3. Furthermore, Jc is rapidly suppressed with increasing temperature (notice that the vertical axis is logarithmic scale). Such weak vortex pinning in the irreversible region is also the consequence of the significant vortex fluctuations, which may results from the 2D pancake state of vortices. By analogy with the Bi–Cu-based high-Tc materials, these superconducting properties are not simply a consequence of its crystallographic anisotropy but may be involved in the exotic electronic states such as the confinement of superconductivity within the FeAs layers. In this regard, it should be profoundly interesting to characterize the anisotropy in the microscopic superconducting parameters (superconducting coherence length and magnetic penetration depth) in Sr2VFeAsO3. In the normal state above Tc, on the other hand, unconventional electronic properties such as the incoherent carrier transport (i.e. metallic in-plane whereas insulating outof-plane resistivity) will also be expected. Experimental confirmations
of them become possible now with the single crystals of Sr2VFeAsO3 prepared in this study, and therefore they are currently under investigation. 4. Conclusions Sizable single crystals of Sr2VFeAsO3, large enough to perform anisotropy measurements, were successfully grown in this study. It was revealed that Sr2VFeAsO3 had the widest reversible (Jc 0) region and the largest anisotropy on the irreversibility line in the magnetic phase diagram ever found in Fe-based superconductors. In the irreversible region (Jc > 0), the vortex pinning and the resultant Jc property degraded rapidly with increasing temperature under H//c. All these observations indicated the existence of the gigantic fluctuations in the mixed state of Sr2VFeAsO3, for which the 2D pancake vortices were thought to be responsible. References [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130 (2008) 3296. [2] K. Sasmal, B. Lv, B. Lorenz, A.M. Guloy, F. Chen, Y.Y. Xue, C.W. Chu, Phys. Rev. Lett. 101 (2008) 107007. [3] M.S. Torikachvili, S.L. Bud’ko, N. Ni, P.C. Canfield, Phys. Rev. Lett. 101 (2008) 057006. [4] D.L. Sun, Y. Liu, C.T. Lin, Phys. Rev. B 80 (2009) 144515. [5] X.C. Wang, Q.Q. Liu, Y.X. Lv, W.B. Gao, L.X. Yang, R.C. Yu, F.Y. Li, C.Q. Jin, Solid State Commun. 148 (2008) 538. [6] D.R. Parker, M.J. Pitcher, P.J. Baker, I. Franke, T. Lancaster, S.J. Blundell, S.J. Clarke, Chem. Commun. (2009) 2189 (Cambridge). [7] F.C. Hsu, J.Y. Luo, K.W. Yeh, T.K. Chen, T.W. Huang, P.M. Wu, Y.C. Lee, Y.L. Huang, Y.Y. Chu, D.C. Yan, M.K. Wu, Proc. Natl. Acad. Sci. 105 (2008) 14262. [8] J. Kacmarcik, C. Marcenat, T. Klein, Z. Pribulova, C.J. Van der Beek, M. Konczykowski, S.L. Bud’ko, M. Tillman, N. Ni, P.C. Canfield, Phys. Rev. B 80 (2009) 014515. [9] X. Xhu, F. Han, G. Mu, B. Zeng, P. Cheng, B. Shen, B. Zeng, H.H. Wen, Phys. Rev. B 79 (2009) 220512. [10] F. Han, X. Zhu, G. Mu, P. Cheng, B. Shen, B. Zeng, H.H. Wen, Sci. China Ser. G 53 (2010) 1202. [11] C.J. Van der Beek, G. Rizza, M. Konczykowski, P. Fertey, I. Monnet, T. Klein, R. Okazaki, M. Ishikado, H. Kito, A. Iyo, H. Eisaki, S. Shamoto, M.E. Tillman, S.L. Bud’ko, P.C. Canfield, T. Shibauchi, Y. Matsuda, Phys. Rev. B 81 (2010) 174517. [12] B. Khaykovich, E. Zeldov, D. Majer, T.W. Li, P.H. Kes, M. Konczykowski, Phys. Rev. Lett. 76 (1996) 2555. [13] C.P. Bean, Rev. Mod. Phys. 36 (1964) 31. [14] N.D. Zhigadlo, S. Katrych, Z. Bukowski, S. Weyeneth, B. Puzniak, J. Karpinski, J. Phys. Condens. Matter. 20 (2008) 342202. [15] R. Prozorov, M.A. Tanatar, N. Ni, A. Kreyssig, S. Nandi, S.L. Bud’ko, A.I. Goldman, P.C. Canfield, Phys. Rev. B 80 (2009) 174517.
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Crystal growth and anisotropic superconducting properties of Sr2VFeAsO3 T. Katagiri, T. Sasagawa ⇑ Materials and Structures Laboratory, Tokyo Institute of Technology, Kanagawa 226-8503, Japan
a r t i c l e
i n f o
Article history: Accepted 2 March 2012 Available online 12 March 2012 Keywords: Sr2VFeAsO3 Single crystals Anisotropy Irreversibility line Critical current density
a b s t r a c t We have succeeded in growing single crystals of the Sr2VFeAsO3 superconductor, which are large enough to experimentally elucidate the anisotropic physical properties in this system. Using the obtained single crystals, superconducting properties under magnetic fields parallel to the c-axis (H//c) and ab-plane (H//ab) have been evaluated. It is found that the irreversibility field/temperature, below which the superconducting critical current density (Jc) becomes finite, is highly anisotropic in Sr2VFeAsO3: the irreversible region is reduced by approximately half for the case of H//c as compared with H//ab. The suppression of the irreversibility temperature by H//c is found to be the largest among the Fe-based superconductors and comparable to the highly anisotropic Bi–Cu-based high-Tc superconductors. The rapid decrease of Jc with increasing H//c and T in the irreversible region is also observed. Therefore, there are pronounced fluctuations in the mixed state of Sr2VFeAsO3, suggesting that the vortices are in the pancake state as discussed in the Bi–Cu-based high-Tc superconductors. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of LaFeAsO1 xFx [1], Fe-based superconductors have been actively studied, and various new compounds have been found, which include Ae(Fe, M)2As2 (Ae = Ca, Sr, Ba, M = Co, Ni) [2–4], AFeAs (A = Li, Na) [5,6], and FeSe [7]. All these Fe-based superconductors have layered crystal structures which consist of FeAs superconducting layers and additional blocking layers. In case of AeFe2As2, the blocking layers are one atomic-thick alkalineearth, while LnFeAsO1 xFx (Ln = lanthanides) has several times thicker Ln(O, F) blocking layers. Since it has been reported that LnFeAsO1 xFx compounds have more anisotropic physical properties than AeFe2As2 [8], it seems that the blocking layer plays fundamental roles for the material-dependent anisotropy in the Fe-based superconductors as similar in the Cu-based high-Tc superconductors. In this regard, it is extremely interesting to examine the anisotropic properties in the recently discovered Sr2VFeAsO3 [9] as it has considerably thick perovskite-type Sr2VO3 blocking layers (its crystal structure is shown in the inset of Fig. 1). To the best of our knowledge, however, there has been no experimental report as to the normal and superconducting properties of Sr2VFeAsO3 as a function of the lattice orientation. This is mainly because of the lack of the sizable single crystals of this compound.
⇑ Corresponding author. Address: R3-37, MSL, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503, Japan. Tel./fax: +81 45 924 5366. E-mail addresses: [email protected] (T. Katagiri), [email protected]. ac.jp (T. Sasagawa). 0921-4534/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2012.03.005
In this study, we have succeeded in growing Sr2VFeAsO3 single crystals, which are large enough to perform magnetic, transport, and other measurements with respect to anisotropy. A self-flux technique was applied for the crystal growth, and the growth conditions including the flux composition were optimized. Using the obtained single crystals, we performed magnetic measurements in the superconducting states. The irreversibility line, which is the closing point of the magnetization hysteresis and corresponds to the onset of the critical current density, was determined under magnetic fields both parallel and perpendicular to the c-axis. The obtained magnetic phase diagram revealed the highly anisotropic nature of the mixed state in Sr2VFeAsO3. The extended reversible (Jc 0) region, in addition to the rapid suppression of Jc in the irreversible region, under magnetic fields parallel to the c-axis indicates that vortex fluctuations are by far larger in Sr2VFeAsO3 than any other Fe-based superconductors. These results together with the highly anisotropic layered structure of Sr2VFeAsO3 are reminiscent of the Bi–Cu-based high-Tc superconductors, and are suggestive of the exotic mixed state such as the formation of pancake vortices. 2. Experimental Single crystals of Sr2VFeAsO3 were grown by a flux technique using FeAs as a self-flux. The FeAs precursor was prepared from elemental Fe and As at 900 °C for 24 h in a vacuum. The resultant FeAs was mixed with Sr, SrO, and V2O5 powders at a molar ratio of 5:3:1:1. The mixture was loaded in an alumina crucible to prevent it from reactions with the quartz tube. Then, the powdercontaining alumina crucible was sealed in an evacuated quartz
T. Katagiri, T. Sasagawa / Physica C 484 (2013) 16–18
Fig. 1. X-ray diffraction pattern from the cleaved surface of a Sr2VFeAsO3 single crystal. The inset shows the crystal structure and the grown single crystal.
tube. It was further encapsulated in the second evacuated quartz tube. All the weighing and mixing procedures were carried out in a globe box with the dried N2 gas. It was heated up to 1250 °C and cooled slowly down to 1150–1200 °C at a rate of 0.5–1 °C/h. To improve the size of crystals, the first crop of the tiny crystals were used as the seed crystals for the second growth. Using the good cleavage found in Sr2VFeAsO3, single crystals with flat shiny surfaces of 0.3 0.3 mm2 were easily separated from the flux as shown in the inset of Fig. 1. The X-ray diffraction pattern was measured with Cu Ka radiation by using a Bruker D2 PHASER diffractometer equipped with a LynxEye 1D-detector. Magnetization measurements were performed using a commercial SQUID magnetometer (Quantum Design, MPMS-XL5). The external magnetic field was applied parallel or perpendicular to the c-axis of Sr2VFeAsO3 single crystals. 3. Results and discussion It turned out that the use of the alumina crucible, the application of the double-quartz-tube technique, and the two-step crystal growth are essential to obtain sizable single crystals of Sr2VFeAsO3.
17
The X-ray diffraction pattern from the cleaved surface (0.3 0.3 mm2) of a single crystal is shown in the main panel of Fig. 1. It has only the (0 0 L) reflections, indicating that the cleavage takes place along the ab-plane. The c-axis lattice constant was determined to be 15.67 Å, which is in good agreement with the reported values for polycrystalline samples [9]. Shown in Fig. 2a are the magnetization curves as a function of temperature measured in the same Sr2VFeAsO3 crystal at 10 Oe of the magnetic field parallel to the ab-plane (upper) and the c-axis (lower). Within the experimental uncertainty, both of the magnetization curves are identical under this lower limit of the magnetic field, and thus give the onset of the thermodynamic superconducting critical temperature Tc of 27 K. It is noted that this value is lower than the maximum Tc of 37 K reported for polycrystalline samples. This will be attributed to oxygen nonstoichiometry in this system. While it is quite challenging to determine the oxygen deficiency x in our Sr2VFeAsO3 x crystals by a chemical technique, a rough estimate can be done from the reported relation between Tc and x in polycrystalline samples [10], and x 0.3 is inferred. Once the magnetic field is well above the lower critical field Hc1, the vortices are responsible for superconducting properties. Then, the onset of magnetic hysteresis turns from the critical temperature/field to the onset of the critical current density Jc, and the boundary in the magnetic phase diagram is called as the irreversibility line. We found that the irreversibility line in the Sr2VFeAsO3 crystal depended considerably on the field orientation with respect to the crystal axis, indicating the vortex state of this material is highly anisotropic. As indicated by arrows in Fig. 2b, the irreversibility temperature at 10 kOe is distinctly different between the fields along the ab-plane and the c-axis; the latter is significantly lower than the former. The same behavior was confirmed also by the magnetization measurements as a function of field at a fixed temperature (not shown here). Fig. 2c summarizes the irreversibility line in the Sr2VFeAsO3 crystal under H//ab and H//c. Now it becomes clearer that there are pronounced reversible (Jc 0) regions below the critical temperature both under H//ab and H//c, which should be compared with those reported for NdFeAs(O, F) and (Ba, K)Fe2As2 single crystals under H//c [8]. As for the anisotropy, the irreversible region is reduced by approximately half from H//ab to H//c in Sr2VFeAsO3. It would be useful to define the irreversibility anisotropy as cirr = Tirr(H)//ab/
Fig. 2. Temperature dependence of the magnetization curves in a Sr2VFeAsO3 single crystal under the magnetic fields parallel to the ab-plane or c-axis at 10 Oe (a) and 10 kOe (b). The magnetization curves under H//c are shifted downward for the easy comparison of the irreversibility temperatures, which are indicated by arrows. (c) The irreversibility lines plotted in the magnetic phase diagram for Sr2VFeAsO3 (circles), NdFeAs(O, F) (squares) [8], and (Ba, K)Fe2As2 (triangles) [8].
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T. Katagiri, T. Sasagawa / Physica C 484 (2013) 16–18
Fig. 3. (a) Magnetization hysteresis loops in a Sr2VFeAsO3 single crystal at various temperatures. The magnetic field is parallel to the c-axis. (b) Magnetic field dependence of the critical current density Jc in a Sr2VFeAsO3 single crystal at 1.7–10 K.
Tirr(H)//c for the purpose of the quantitative comparison among different material systems. At 20 kOe, Sr2VFeAsO3 has cirr = 1.98, which is the largest among the other Fe-based superconductors whose data are available in literature; e.g. cirr = 1.20 for NdFeAs(O, F) and cirr = 1.02 for (Ba, K)Fe2As2. The extended reversible region and the highly field-direction dependent vortex state in Sr2VFeAsO3 are more like Cu-based high-Tc superconductors than its Fe-based family compounds. The observation indicates that there are significant fluctuations in the mixed state of Sr2VFeAsO3, which may come from the 2D pancake-like state of vortices, as similar to the Bi–Cubased high-Tc superconductors, rather than the conventional 3D rod-like state. The effect of the giant vortex fluctuations was also observed in the irreversible (Jc > 0) region. Fig. 3a shows the magnetization hysteresis curves at various temperatures under H//c in the Sr2VFeAsO3 single crystal. The magnetization hystereses DM were found to monotonously decrease with field and temperature, and thus there is no sign of the so-called ‘‘second peak’’ or ‘‘fish tail’’ effect which have often been observed in Fe-based and Cu-based superconductors [4,11,12]. From these magnetization hysteresis loops, Jc as a function of field was estimated based on the Bean model [13] as Jc(H) = 20DM(H)/{a(1 a/3b)}, where a and b are the dimensions of the rectangular sample (a < b) perpendicular to the field. Fig. 3b shows the obtained field dependence of Jc at various temperatures. It is noted that Jc at 5 K in the Sr2VFeAsO3 single crystal is 10 times lower than that in the typical Fe-based superconductors such as SmFeAsO1 xFx [14] and Ba(Fe, Co)2As2 [15], suggesting that the vortex pinning is very weak in Sr2VFeAsO3. Furthermore, Jc is rapidly suppressed with increasing temperature (notice that the vertical axis is logarithmic scale). Such weak vortex pinning in the irreversible region is also the consequence of the significant vortex fluctuations, which may results from the 2D pancake state of vortices. By analogy with the Bi–Cu-based high-Tc materials, these superconducting properties are not simply a consequence of its crystallographic anisotropy but may be involved in the exotic electronic states such as the confinement of superconductivity within the FeAs layers. In this regard, it should be profoundly interesting to characterize the anisotropy in the microscopic superconducting parameters (superconducting coherence length and magnetic penetration depth) in Sr2VFeAsO3. In the normal state above Tc, on the other hand, unconventional electronic properties such as the incoherent carrier transport (i.e. metallic in-plane whereas insulating outof-plane resistivity) will also be expected. Experimental confirmations
of them become possible now with the single crystals of Sr2VFeAsO3 prepared in this study, and therefore they are currently under investigation. 4. Conclusions Sizable single crystals of Sr2VFeAsO3, large enough to perform anisotropy measurements, were successfully grown in this study. It was revealed that Sr2VFeAsO3 had the widest reversible (Jc 0) region and the largest anisotropy on the irreversibility line in the magnetic phase diagram ever found in Fe-based superconductors. In the irreversible region (Jc > 0), the vortex pinning and the resultant Jc property degraded rapidly with increasing temperature under H//c. All these observations indicated the existence of the gigantic fluctuations in the mixed state of Sr2VFeAsO3, for which the 2D pancake vortices were thought to be responsible. References [1] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130 (2008) 3296. [2] K. Sasmal, B. Lv, B. Lorenz, A.M. Guloy, F. Chen, Y.Y. Xue, C.W. Chu, Phys. Rev. Lett. 101 (2008) 107007. [3] M.S. Torikachvili, S.L. Bud’ko, N. Ni, P.C. Canfield, Phys. Rev. Lett. 101 (2008) 057006. [4] D.L. Sun, Y. Liu, C.T. Lin, Phys. Rev. B 80 (2009) 144515. [5] X.C. Wang, Q.Q. Liu, Y.X. Lv, W.B. Gao, L.X. Yang, R.C. Yu, F.Y. Li, C.Q. Jin, Solid State Commun. 148 (2008) 538. [6] D.R. Parker, M.J. Pitcher, P.J. Baker, I. Franke, T. Lancaster, S.J. Blundell, S.J. Clarke, Chem. Commun. (2009) 2189 (Cambridge). [7] F.C. Hsu, J.Y. Luo, K.W. Yeh, T.K. Chen, T.W. Huang, P.M. Wu, Y.C. Lee, Y.L. Huang, Y.Y. Chu, D.C. Yan, M.K. Wu, Proc. Natl. Acad. Sci. 105 (2008) 14262. [8] J. Kacmarcik, C. Marcenat, T. Klein, Z. Pribulova, C.J. Van der Beek, M. Konczykowski, S.L. Bud’ko, M. Tillman, N. Ni, P.C. Canfield, Phys. Rev. B 80 (2009) 014515. [9] X. Xhu, F. Han, G. Mu, B. Zeng, P. Cheng, B. Shen, B. Zeng, H.H. Wen, Phys. Rev. B 79 (2009) 220512. [10] F. Han, X. Zhu, G. Mu, P. Cheng, B. Shen, B. Zeng, H.H. Wen, Sci. China Ser. G 53 (2010) 1202. [11] C.J. Van der Beek, G. Rizza, M. Konczykowski, P. Fertey, I. Monnet, T. Klein, R. Okazaki, M. Ishikado, H. Kito, A. Iyo, H. Eisaki, S. Shamoto, M.E. Tillman, S.L. Bud’ko, P.C. Canfield, T. Shibauchi, Y. Matsuda, Phys. Rev. B 81 (2010) 174517. [12] B. Khaykovich, E. Zeldov, D. Majer, T.W. Li, P.H. Kes, M. Konczykowski, Phys. Rev. Lett. 76 (1996) 2555. [13] C.P. Bean, Rev. Mod. Phys. 36 (1964) 31. [14] N.D. Zhigadlo, S. Katrych, Z. Bukowski, S. Weyeneth, B. Puzniak, J. Karpinski, J. Phys. Condens. Matter. 20 (2008) 342202. [15] R. Prozorov, M.A. Tanatar, N. Ni, A. Kreyssig, S. Nandi, S.L. Bud’ko, A.I. Goldman, P.C. Canfield, Phys. Rev. B 80 (2009) 174517.