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Growth and superconducting properties of F-substituted ROBiS2 (R=La, Ce, Nd) single crystals
Solid State Communications 178 (2014) 33–36
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Growth and superconducting properties of F-substituted ROBiS2 (R¼ La, Ce, Nd) single crystals Masanori Nagao a,b,n, Akira Miura a, Satoshi Demura b, Keita Deguchi b, Satoshi Watauchi a, Takahiro Takei a, Yoshihiko Takano b, Nobuhiro Kumada a, Isao Tanaka a a b
University of Yamanashi, 7-32 Miyamae, Kofu, Yamanashi 400-8511, Japan National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 7 October 2013 Received in revised form 17 October 2013 Accepted 21 October 2013 by P. Chaddah Available online 29 October 2013
F-substituted ROBiS2 (R¼ La, Ce, Nd) superconducting single crystals with different F concentrations were grown successfully using a CsCl/KCl flux. All crystals produced had a plate-like shape, with a welldeveloped ab-plane 1–2 mm in size. Electron probe microanalysis did not detect any Cs, K, or Cl flux components in the crystals. As-grown single crystals of F-substituted LaOBiS2 and CeOBiS2 exhibited superconductivity at about 3 K, whereas F-substituted NdOBiS2 was superconductive at approximately 5 K. The superconducting anisotropy of single crystal F-substituted LaOBiS2 and NdOBiS2 was estimated to be 30–45 according to the effective mass model, whereas the anisotropy for F-substituted CeOBiS2 single crystals was 13–21. The F-substituted CeOBiS2 single crystals exhibited a magnetic order around 7 K that apparently coexisted with superconductivity below approximately 3 K. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. BiS2-based superconductor B. Flux growth D. Superconducting anisotropy
1. Introduction The discovery of new BiS2-based superconductors, Bi4O4S3 [1,2] and RO1 zFzBiS2 (R¼ La, Ce, Pr, Nd, Yb) [3–11], has attracted a great deal of interest. The substitution of O by F in these materials is instrumental in initiating superconductivity by inducing carriers in superconducting layers. Investigation of these superconductors has so far proven problematic owing to a lack of single crystals; however, F-substituted NdOBiS2 single crystals were recently grown using an alkali metal chloride flux in vacuum [12,13]. The growth of the single crystals of various compositions is highly desirable for investigating the effect of different rare earth elements and F content on the intrinsic properties of F-substituted ROBiS2. In this paper, we grew F-substituted ROBiS2 (R¼ La, Ce, Nd) single crystals with various F contents using a CsCl/KCl flux. The composition and transport properties were then examined to understand the intrinsic properties of these materials, with subsequent discussion on the effect of rare earth elements and F substitution.
2. Experimental Single crystals of F-substituted ROBiS2 (R¼La, Ce, Nd) were grown by a high-temperature flux method in a vacuumed quartz tube using R2S3, Bi, Bi2S3, Bi2O3, BiF3, CsCl, and KCl as raw materials. The synthesis n Correspondence to: University of Yamanashi, Center for Crystal Science and Technology, Miyamae 7-32, Kofu, Yamanashi 400-8511, Japan. Tel.: þ 81 55 220 8610; fax: þ81 55 254 3035. E-mail address: [email protected] (M. Nagao).
0038-1098/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2013.10.019
and characterization procedures followed earlier studies in the literature, except for the amount of CsCl/KCl flux used [12]. The raw materials were weighed to obtain a nominal composition of RO1 xFxBiS2 (x¼ 0–1.0). A mixture of raw materials (0.8 g) and CsCl/KCl flux (5.0 g) with a molar ratio of 5:3 was combined using a mortar, and then sealed in a quartz tube under vacuum. This mixed powder was heated at 800 1C for 10 h, cooled slowly to 600 1C at a rate of 1 1C/h, and then furnace-cooled to room temperature. The quartz tube was opened in air, and the flux was dissolved in the quartz tube using distilled water. The remaining product was then filtered, and washed with distilled water. The crystal structure and composition of the resultant single crystals was evaluated by X-ray diffraction (XRD) using CuKα radiation, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The transport properties of the single crystals were also measured by a standard four-probe method with constant current mode using a Physical Property Measurement System (Quantum Design; PPMS DynaCool). The electrical terminals were fabricated from silver paste. The angular (θ) dependence of resistivity (ρ) in the flux liquid state was measured under various magnetic fields (H), with the superconducting anisotropy (γs) calculated using the effective mass model [14–16]. The temperature dependence of magnetization (M) under zero-field cooling (ZFC) and field cooling (FC) was measured by a superconducting quantum interface device (SQUID) magnetometer, with an applied magnetic field of 10 Oe parallel to the ab-plane. 3. Results and discussion Fig. 1 shows a typical SEM image for an F-substituted CeOBiS2 single crystal, demonstrating a plate-like shape, which is 1.0–2.0 mm
34
M. Nagao et al. / Solid State Communications 178 (2014) 33–36
in size and 10–20 μm in thickness. The in-plane orientation of the crystals was not measured. Fig. 2 shows the XRD pattern of a welldeveloped plane in a single crystal grown from a starting powder with the nominal composition of CeO0.7F0.3BiS2. The peak positions agree with those of the 00l diffraction peaks of F-substituted CeOBiS2 described in the literature [5]. The presence of only 00l diffraction peaks of the CeOBiS2 structure indicates a well-developed ab-plane.
Table 1 shows the effect of varying the starting material composition on the F content, c-axis lattice parameter, and superconducting anisotropy (γs) of single crystals. The F composition is normalized by the total F and O content, and is defined in the starting material and resultant single crystals as x and y, respectively. Single crystals were obtained only when the value of x was between 0.3 and 0.9. The chemical ratio of R:Bi:S in the crystals, determined by EPMA was 1.017 0.05:0.99 7 0.05:2.00; this ratio is in agreement agreed with stoichiometry. The obtained values were normalized using S ¼2.00, with Nd and Bi measured to a precision of two decimal places. No Cs, K, and Cl were detected in the crystals by EPMA with a minimum sensitivity limit of 0.1 wt%. On increasing the value of x, the value of y also increases and reaches saturation at 0.46 (R ¼La), 0.66 (R ¼Ce), and 0.38 (R¼Nd) when xZ0.7. Although the analytical values of y, when x¼0.7 and 0.9 are consistent within analytical error, the physical properties
Fig. 1. Typical SEM image of F-substituted CeOBiS2 single crystal.
Fig. 2. XRD pattern of well-developed plane of F-substituted CeOBiS2 single crystal grown from the starting powder with nominal composition of CeO0.7F0.3BiS2.
Fig. 3. (Color online) XRD pattern of 004 diffraction peaks of the F-substituted ROBiS2 (R ¼La, Ce, Nd) single crystals.
Table 1 Nominal F compositions in the starting materials (x) dependence on the analytical F compositions in the single crystals (y), c-axis lattice parameter and superconducting anisotropy (γs) in the grown single crystals. Nominal F compositions in the starting materials (x) 0.3
0.7
0.9
R: La
Analytical F compositions in the single crystals (y) c-Axis lattice parameter (Å) Superconducting anisotropy (γs)
0.23 13.57 —
0.46 13.37 35–37
0.46 13.39 36–45
R: Ce
Analytical F compositions in the single crystals (y) c-Axis lattice parameter (Å) Superconducting anisotropy (γs)
0.53 13.54 —
0.65 13.39 13–21
0.66 13.40 14
R: Nd
Analytical F compositions in the single crystals (y) c-Axis lattice parameter (Å) Superconducting anisotropy (γs)
0.26 13.56 30–34
0.37 13.43 30–31
0.38 13.41 37–40
—: Unmeasurable at our system.
M. Nagao et al. / Solid State Communications 178 (2014) 33–36
35
Fig. 4. (Color online) Temperature dependence of resistivities along the ab-plane for F-substituted (a) LaOBiS2, (b) CeOBiS2, (c) NdOBiS2 single crystals at 2–10 K.
Fig. 6. Data in Fig. 5 after scaling of angular θ dependence of resistivity ρ at a reduced magnetic field of Hred ¼H(sin2 θþ γs 2cos2 θ)1/2. (a) F-substituted LaOBiS2 of x¼ 0.9, y¼ 0.46. (b) F-substituted CeOBiS2 of x ¼0.7, y¼0.65. Fig. 5. Angular θ dependence of resistivity ρ in flux liquid state at various magnetic fields (bottom to top, (a) 0.1, 0.3, 0.5, 0.7, 0.9, 1.0, 1.5, 2.0, 2.5, 3.0, 5.0, 7.0, and 9.0 T, (b) 0.1, 0.3, 0.5, 0.7, 0.9, 1.0, 3.0, and 5.0 T) for the single crystals of (a) F-substituted LaOBiS2 grown from the starting materials of x ¼0.9, y¼ 0.46 and (b) F-substituted CeOBiS2 of x ¼0.7, y¼0.65.
of these crystals are different each other as described later. The increase in x shortens the lattice parameters, but gives almost the same value as when xZ0.7. The minimum c-axis lattice parameter is about 13.4 Å regardless of the rare earth metals used. Fig. 3
shows the 004 diffraction peaks of F-substituted ROBiS2 (R ¼La, Ce, Nd) single crystals. The peak position shifted towards a higher angle with an increasing x value, which suggests an increase in the c-axis lattice parameter as shown in Table 1. Fig. 4 shows the temperature dependence of resistivity for F-substituted ROBiS2 (R¼ La, Ce, Nd) single crystals at 2–10 K; similar trends were also observed with F-substituted LaOBiS2 and CeOBiS2. When x ¼0.3, LaOBiS2 and CeOBiS2 crystals did not
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M. Nagao et al. / Solid State Communications 178 (2014) 33–36
Fig. 7. Temperature dependence of magnetization under zero-field cooling (ZFC) and field cooling (FC) with an applied field of 10 Oe parallel to the ab-plane for the obtained single crystal using F-substituted CeOBiS2 of (a) x ¼ 0.3, y¼0.53, (b) x ¼0.7, y ¼0.65 and (c) x ¼0.9, y¼ 0.66 in the starting materials.
exhibit zero resistivity even down to 2 K; when x ¼0.7 and 0.9 superconducting transition temperature (Tc) was observed at around 3 K. The transport properties when x¼ 0.3, 0.7, and 0.9 exhibit semiconducting behavior, but LaOBiS2 only becomes metallic when x¼0.9. Only in single crystals of CeOBiS2, the increase in x from 0.7 to 0.9 significantly suppressed Tc although the value of y remained almost the same. The origin of this difference however is unclear; therefore, further examination is warranted. On the other hand, F-substituted NdOBiS2 crystals where x ¼0.3, 0.7, and 0.9 exhibited superconducting behavior with a Tc of approximately 5 K and all crystals demonstrated metallic behavior. The resistivity of the normal state seems independent of the x and y values, which may be attributed to the increase in surface roughness. The rough surface of these crystals could be observed by eye when compared with the surface of F-substituted LaOBiS2 and CeOBiS2 single crystals. The broad transition when x¼ 0.7 may be due to the low crystallinity. The angular (θ) dependence of resistivity (ρ) was measured under different magnetic fields (H) in the flux liquid state to estimate the superconducting anisotropy (γs) of F-substituted ROBiS2 single crystals, as reported in Refs. [14,15]. The reduced field (Hred) is calculated using the following equation for an effective mass model: H red ¼ Hð sin 2 θ þ γ s 2 cos 2 θÞ1=2
ð1Þ
where θ is the angle between the ab-plane and the magnetic field [16]; and Hred is calculated from H and θ. The superconducting anisotropy (γs) was estimated from a best scaling of the ρ–Hred relationship. Fig. 5 displays the angular (θ) dependence of resistivity (ρ) for different magnetic fields (H¼ 0.1–5.0 or 0.1–9.0 T) in the flux liquid state for (a) F-substituted LaOBiS2 grown under conditions of x ¼0.9, y¼0.46 and (b) F-substituted CeOBiS2 where x ¼0.7, y¼0.65. The ρ–θ curve exhibited a twofold symmetry. Fig. 6 shows the ρ–Hred scaling obtained from the ρ–θ curves in Fig. 5 using Eq. (1). The scaling was performed by taking γs ¼45 and γs ¼16, as shown in Fig. 6(a) and (b), respectively. Table 1 shows a summary of the superconducting anisotropy of F-substituted ROBiS2 single crystals with various x and y values. The γs of the crystal was estimated to be 30–45 when R ¼La and Nd, whereas the value was lower than expected (13–21) when R ¼ Ce. Fig. 7 demonstrates the temperature dependence of magnetization for F-substituted CeOBiS2 single crystals where (a) x¼ 0.3, y ¼0.53, (b) x ¼0.7, y¼0.65 and (c) x ¼0.9, y¼ 0.66. Magnetic transitions (Tm) were only observed in superconducting crystals when x ¼0.7 and 0.9, and their temperatures are above Tc. The appearance of this magnetic transition in single crystals agrees with that of polycrystals with similar compositions [5,9], and provides evidence that this magnetic transition is an intrinsic property. Further
characterization of these single crystals is therefore required in order to investigate the origin of this phenomenon. 4. Conclusion Successful growth of F-substituted ROBiS2 (R¼La, Ce, Nd) single crystals was achieved by using CsCl/KCl flux containing varying rare earth metals and F contents. An increase in the F content was found to enhance the values of substituted F, but an increase above 70% failed to produce further substitution. While the superconducting anisotropies (γs) of single crystals of RO1 yFyBiS2 (R¼ La, Nd, y¼0.23–0.46) were estimated to be 30–45, R¼ Ce (y¼0.53–0.66) produced a lower value of 13–21. The F-substituted CeOBiS2 single crystals exhibited a magnetic order at about 7 K in addition to the superconducting transition at approximately 3 K. Acknowledgments The authors would like to thank Dr. M. Fujioka and Dr. H. Okazaki of the National Institute for Materials Science for their valuable advice. References [1] Y. Mizuguchi, H. Fujihisa, Y. Gotoh, K. Suzuki, H. Usui, K. Kuroki, S. Demura, Y. Takano, H. Izawa, O. Miura, Phys. Rev. B 86 (2012) 220510(R). [2] S. Kumar Singh, A. Kumar, B. Gahtori, G. Sharma, S. Patnaik, V.P.S. Awana, J. Am. Chem. Soc. 134 (2012) 16504. [3] Y. Mizuguchi, S. Demura, K. Deguchi, Y. Takano, H. Fujihisa, Y. Gotoh, H. Izawa, O. Miura, J. Phys. Soc. Jpn. 81 (2012) 114725. [4] V.P.S. Awana, A. Kumar, R. Jha, S. Kumar Singh, A. Pal, Shruti, J. Saha, S. Patnaik, Solid State Commun. 157 (2013) 21. [5] J. Xing, S. Li, X. Ding, H. Yang, H.-H. Wen, Phys. Rev. B 86 (2012) 214518. [6] R. Jha, A. Kumar, S. Kumar Singh, V.P.S. Awana, J. Supercond. Novel Magn. 26 (2013)499 26 (2013). [7] S. Demura, Y. Mizuguchi, K. Deguchi, H. Okazaki, H. Hara, T. Watanabe, S.J. Denholme, M. Fujioka, T. Ozaki, H. Fujihisa, Y. Gotoh, O. Miura, T. Yamaguchi, H. Takeya, Y. Takano, J. Phys. Soc. Jpn. 82 (2013) 033708. [8] R. Jha, A. Kumar, S. Kumar Singh, V.P.S. Awana, J. Appl. Phys. 113 (2013) 056102. [9] D. Yazici, K. Huang, B.D. White, A.H. Chang, A.J. Friedman, M.B. Maple, Philos. Mag. 93 (2013) 673. [10] G. Kalai Selvan, M. Kanagaraj, S. Esakki Muthu, R. Jha, V.P.S. Awana, S. Arumugam, Phys. Status Solidi Rapid Res. Lett. 7 (2013) 510. [11] C.T. Wolowiec, D. Yazici, B.D. White, K. Huang, M.B. Maple, Phys. Rev. B 88 (2013) 064503. [12] M. Nagao, S. Demura, K. Deguchi, A. Miura, S. Watauchi, T. Takei, Y. Takano, N. Kumada, I. Tanaka, J. Phys. Soc. Jpn. 82 (2013) 113701. [13] J. Liu, D. Fang, Z. Wang, J. Xing, Z. Du, X. Zhu, H. Yang, H.-H. Wen, arXiv:1310.0377. [14] Y. Iye, I. Oguro, T. Tamegai, W.R. Datars, N. Motohira, K. Kitazawa, Physica C 199 (1992) 154. [15] H. Iwasaki, O. Taniguchi, S. Kenmochi, N. Kobayashi, Physica C 244 (1995) 71. [16] G. Blatter, V.B. Geshkenbein, A.I. Larkin, Phys. Rev. Lett. 68 (1992) 875.
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Growth and superconducting properties of F-substituted ROBiS2 (R¼ La, Ce, Nd) single crystals Masanori Nagao a,b,n, Akira Miura a, Satoshi Demura b, Keita Deguchi b, Satoshi Watauchi a, Takahiro Takei a, Yoshihiko Takano b, Nobuhiro Kumada a, Isao Tanaka a a b
University of Yamanashi, 7-32 Miyamae, Kofu, Yamanashi 400-8511, Japan National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 7 October 2013 Received in revised form 17 October 2013 Accepted 21 October 2013 by P. Chaddah Available online 29 October 2013
F-substituted ROBiS2 (R¼ La, Ce, Nd) superconducting single crystals with different F concentrations were grown successfully using a CsCl/KCl flux. All crystals produced had a plate-like shape, with a welldeveloped ab-plane 1–2 mm in size. Electron probe microanalysis did not detect any Cs, K, or Cl flux components in the crystals. As-grown single crystals of F-substituted LaOBiS2 and CeOBiS2 exhibited superconductivity at about 3 K, whereas F-substituted NdOBiS2 was superconductive at approximately 5 K. The superconducting anisotropy of single crystal F-substituted LaOBiS2 and NdOBiS2 was estimated to be 30–45 according to the effective mass model, whereas the anisotropy for F-substituted CeOBiS2 single crystals was 13–21. The F-substituted CeOBiS2 single crystals exhibited a magnetic order around 7 K that apparently coexisted with superconductivity below approximately 3 K. & 2013 Elsevier Ltd. All rights reserved.
Keywords: A. BiS2-based superconductor B. Flux growth D. Superconducting anisotropy
1. Introduction The discovery of new BiS2-based superconductors, Bi4O4S3 [1,2] and RO1 zFzBiS2 (R¼ La, Ce, Pr, Nd, Yb) [3–11], has attracted a great deal of interest. The substitution of O by F in these materials is instrumental in initiating superconductivity by inducing carriers in superconducting layers. Investigation of these superconductors has so far proven problematic owing to a lack of single crystals; however, F-substituted NdOBiS2 single crystals were recently grown using an alkali metal chloride flux in vacuum [12,13]. The growth of the single crystals of various compositions is highly desirable for investigating the effect of different rare earth elements and F content on the intrinsic properties of F-substituted ROBiS2. In this paper, we grew F-substituted ROBiS2 (R¼ La, Ce, Nd) single crystals with various F contents using a CsCl/KCl flux. The composition and transport properties were then examined to understand the intrinsic properties of these materials, with subsequent discussion on the effect of rare earth elements and F substitution.
2. Experimental Single crystals of F-substituted ROBiS2 (R¼La, Ce, Nd) were grown by a high-temperature flux method in a vacuumed quartz tube using R2S3, Bi, Bi2S3, Bi2O3, BiF3, CsCl, and KCl as raw materials. The synthesis n Correspondence to: University of Yamanashi, Center for Crystal Science and Technology, Miyamae 7-32, Kofu, Yamanashi 400-8511, Japan. Tel.: þ 81 55 220 8610; fax: þ81 55 254 3035. E-mail address: [email protected] (M. Nagao).
0038-1098/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2013.10.019
and characterization procedures followed earlier studies in the literature, except for the amount of CsCl/KCl flux used [12]. The raw materials were weighed to obtain a nominal composition of RO1 xFxBiS2 (x¼ 0–1.0). A mixture of raw materials (0.8 g) and CsCl/KCl flux (5.0 g) with a molar ratio of 5:3 was combined using a mortar, and then sealed in a quartz tube under vacuum. This mixed powder was heated at 800 1C for 10 h, cooled slowly to 600 1C at a rate of 1 1C/h, and then furnace-cooled to room temperature. The quartz tube was opened in air, and the flux was dissolved in the quartz tube using distilled water. The remaining product was then filtered, and washed with distilled water. The crystal structure and composition of the resultant single crystals was evaluated by X-ray diffraction (XRD) using CuKα radiation, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). The transport properties of the single crystals were also measured by a standard four-probe method with constant current mode using a Physical Property Measurement System (Quantum Design; PPMS DynaCool). The electrical terminals were fabricated from silver paste. The angular (θ) dependence of resistivity (ρ) in the flux liquid state was measured under various magnetic fields (H), with the superconducting anisotropy (γs) calculated using the effective mass model [14–16]. The temperature dependence of magnetization (M) under zero-field cooling (ZFC) and field cooling (FC) was measured by a superconducting quantum interface device (SQUID) magnetometer, with an applied magnetic field of 10 Oe parallel to the ab-plane. 3. Results and discussion Fig. 1 shows a typical SEM image for an F-substituted CeOBiS2 single crystal, demonstrating a plate-like shape, which is 1.0–2.0 mm
34
M. Nagao et al. / Solid State Communications 178 (2014) 33–36
in size and 10–20 μm in thickness. The in-plane orientation of the crystals was not measured. Fig. 2 shows the XRD pattern of a welldeveloped plane in a single crystal grown from a starting powder with the nominal composition of CeO0.7F0.3BiS2. The peak positions agree with those of the 00l diffraction peaks of F-substituted CeOBiS2 described in the literature [5]. The presence of only 00l diffraction peaks of the CeOBiS2 structure indicates a well-developed ab-plane.
Table 1 shows the effect of varying the starting material composition on the F content, c-axis lattice parameter, and superconducting anisotropy (γs) of single crystals. The F composition is normalized by the total F and O content, and is defined in the starting material and resultant single crystals as x and y, respectively. Single crystals were obtained only when the value of x was between 0.3 and 0.9. The chemical ratio of R:Bi:S in the crystals, determined by EPMA was 1.017 0.05:0.99 7 0.05:2.00; this ratio is in agreement agreed with stoichiometry. The obtained values were normalized using S ¼2.00, with Nd and Bi measured to a precision of two decimal places. No Cs, K, and Cl were detected in the crystals by EPMA with a minimum sensitivity limit of 0.1 wt%. On increasing the value of x, the value of y also increases and reaches saturation at 0.46 (R ¼La), 0.66 (R ¼Ce), and 0.38 (R¼Nd) when xZ0.7. Although the analytical values of y, when x¼0.7 and 0.9 are consistent within analytical error, the physical properties
Fig. 1. Typical SEM image of F-substituted CeOBiS2 single crystal.
Fig. 2. XRD pattern of well-developed plane of F-substituted CeOBiS2 single crystal grown from the starting powder with nominal composition of CeO0.7F0.3BiS2.
Fig. 3. (Color online) XRD pattern of 004 diffraction peaks of the F-substituted ROBiS2 (R ¼La, Ce, Nd) single crystals.
Table 1 Nominal F compositions in the starting materials (x) dependence on the analytical F compositions in the single crystals (y), c-axis lattice parameter and superconducting anisotropy (γs) in the grown single crystals. Nominal F compositions in the starting materials (x) 0.3
0.7
0.9
R: La
Analytical F compositions in the single crystals (y) c-Axis lattice parameter (Å) Superconducting anisotropy (γs)
0.23 13.57 —
0.46 13.37 35–37
0.46 13.39 36–45
R: Ce
Analytical F compositions in the single crystals (y) c-Axis lattice parameter (Å) Superconducting anisotropy (γs)
0.53 13.54 —
0.65 13.39 13–21
0.66 13.40 14
R: Nd
Analytical F compositions in the single crystals (y) c-Axis lattice parameter (Å) Superconducting anisotropy (γs)
0.26 13.56 30–34
0.37 13.43 30–31
0.38 13.41 37–40
—: Unmeasurable at our system.
M. Nagao et al. / Solid State Communications 178 (2014) 33–36
35
Fig. 4. (Color online) Temperature dependence of resistivities along the ab-plane for F-substituted (a) LaOBiS2, (b) CeOBiS2, (c) NdOBiS2 single crystals at 2–10 K.
Fig. 6. Data in Fig. 5 after scaling of angular θ dependence of resistivity ρ at a reduced magnetic field of Hred ¼H(sin2 θþ γs 2cos2 θ)1/2. (a) F-substituted LaOBiS2 of x¼ 0.9, y¼ 0.46. (b) F-substituted CeOBiS2 of x ¼0.7, y¼0.65. Fig. 5. Angular θ dependence of resistivity ρ in flux liquid state at various magnetic fields (bottom to top, (a) 0.1, 0.3, 0.5, 0.7, 0.9, 1.0, 1.5, 2.0, 2.5, 3.0, 5.0, 7.0, and 9.0 T, (b) 0.1, 0.3, 0.5, 0.7, 0.9, 1.0, 3.0, and 5.0 T) for the single crystals of (a) F-substituted LaOBiS2 grown from the starting materials of x ¼0.9, y¼ 0.46 and (b) F-substituted CeOBiS2 of x ¼0.7, y¼0.65.
of these crystals are different each other as described later. The increase in x shortens the lattice parameters, but gives almost the same value as when xZ0.7. The minimum c-axis lattice parameter is about 13.4 Å regardless of the rare earth metals used. Fig. 3
shows the 004 diffraction peaks of F-substituted ROBiS2 (R ¼La, Ce, Nd) single crystals. The peak position shifted towards a higher angle with an increasing x value, which suggests an increase in the c-axis lattice parameter as shown in Table 1. Fig. 4 shows the temperature dependence of resistivity for F-substituted ROBiS2 (R¼ La, Ce, Nd) single crystals at 2–10 K; similar trends were also observed with F-substituted LaOBiS2 and CeOBiS2. When x ¼0.3, LaOBiS2 and CeOBiS2 crystals did not
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M. Nagao et al. / Solid State Communications 178 (2014) 33–36
Fig. 7. Temperature dependence of magnetization under zero-field cooling (ZFC) and field cooling (FC) with an applied field of 10 Oe parallel to the ab-plane for the obtained single crystal using F-substituted CeOBiS2 of (a) x ¼ 0.3, y¼0.53, (b) x ¼0.7, y ¼0.65 and (c) x ¼0.9, y¼ 0.66 in the starting materials.
exhibit zero resistivity even down to 2 K; when x ¼0.7 and 0.9 superconducting transition temperature (Tc) was observed at around 3 K. The transport properties when x¼ 0.3, 0.7, and 0.9 exhibit semiconducting behavior, but LaOBiS2 only becomes metallic when x¼0.9. Only in single crystals of CeOBiS2, the increase in x from 0.7 to 0.9 significantly suppressed Tc although the value of y remained almost the same. The origin of this difference however is unclear; therefore, further examination is warranted. On the other hand, F-substituted NdOBiS2 crystals where x ¼0.3, 0.7, and 0.9 exhibited superconducting behavior with a Tc of approximately 5 K and all crystals demonstrated metallic behavior. The resistivity of the normal state seems independent of the x and y values, which may be attributed to the increase in surface roughness. The rough surface of these crystals could be observed by eye when compared with the surface of F-substituted LaOBiS2 and CeOBiS2 single crystals. The broad transition when x¼ 0.7 may be due to the low crystallinity. The angular (θ) dependence of resistivity (ρ) was measured under different magnetic fields (H) in the flux liquid state to estimate the superconducting anisotropy (γs) of F-substituted ROBiS2 single crystals, as reported in Refs. [14,15]. The reduced field (Hred) is calculated using the following equation for an effective mass model: H red ¼ Hð sin 2 θ þ γ s 2 cos 2 θÞ1=2
ð1Þ
where θ is the angle between the ab-plane and the magnetic field [16]; and Hred is calculated from H and θ. The superconducting anisotropy (γs) was estimated from a best scaling of the ρ–Hred relationship. Fig. 5 displays the angular (θ) dependence of resistivity (ρ) for different magnetic fields (H¼ 0.1–5.0 or 0.1–9.0 T) in the flux liquid state for (a) F-substituted LaOBiS2 grown under conditions of x ¼0.9, y¼0.46 and (b) F-substituted CeOBiS2 where x ¼0.7, y¼0.65. The ρ–θ curve exhibited a twofold symmetry. Fig. 6 shows the ρ–Hred scaling obtained from the ρ–θ curves in Fig. 5 using Eq. (1). The scaling was performed by taking γs ¼45 and γs ¼16, as shown in Fig. 6(a) and (b), respectively. Table 1 shows a summary of the superconducting anisotropy of F-substituted ROBiS2 single crystals with various x and y values. The γs of the crystal was estimated to be 30–45 when R ¼La and Nd, whereas the value was lower than expected (13–21) when R ¼ Ce. Fig. 7 demonstrates the temperature dependence of magnetization for F-substituted CeOBiS2 single crystals where (a) x¼ 0.3, y ¼0.53, (b) x ¼0.7, y¼0.65 and (c) x ¼0.9, y¼ 0.66. Magnetic transitions (Tm) were only observed in superconducting crystals when x ¼0.7 and 0.9, and their temperatures are above Tc. The appearance of this magnetic transition in single crystals agrees with that of polycrystals with similar compositions [5,9], and provides evidence that this magnetic transition is an intrinsic property. Further
characterization of these single crystals is therefore required in order to investigate the origin of this phenomenon. 4. Conclusion Successful growth of F-substituted ROBiS2 (R¼La, Ce, Nd) single crystals was achieved by using CsCl/KCl flux containing varying rare earth metals and F contents. An increase in the F content was found to enhance the values of substituted F, but an increase above 70% failed to produce further substitution. While the superconducting anisotropies (γs) of single crystals of RO1 yFyBiS2 (R¼ La, Nd, y¼0.23–0.46) were estimated to be 30–45, R¼ Ce (y¼0.53–0.66) produced a lower value of 13–21. The F-substituted CeOBiS2 single crystals exhibited a magnetic order at about 7 K in addition to the superconducting transition at approximately 3 K. Acknowledgments The authors would like to thank Dr. M. Fujioka and Dr. H. Okazaki of the National Institute for Materials Science for their valuable advice. References [1] Y. Mizuguchi, H. Fujihisa, Y. Gotoh, K. Suzuki, H. Usui, K. Kuroki, S. Demura, Y. Takano, H. Izawa, O. Miura, Phys. Rev. B 86 (2012) 220510(R). [2] S. Kumar Singh, A. Kumar, B. Gahtori, G. Sharma, S. Patnaik, V.P.S. Awana, J. Am. Chem. Soc. 134 (2012) 16504. [3] Y. Mizuguchi, S. Demura, K. Deguchi, Y. Takano, H. Fujihisa, Y. Gotoh, H. Izawa, O. Miura, J. Phys. Soc. Jpn. 81 (2012) 114725. [4] V.P.S. Awana, A. Kumar, R. Jha, S. Kumar Singh, A. Pal, Shruti, J. Saha, S. Patnaik, Solid State Commun. 157 (2013) 21. [5] J. Xing, S. Li, X. Ding, H. Yang, H.-H. Wen, Phys. Rev. B 86 (2012) 214518. [6] R. Jha, A. Kumar, S. Kumar Singh, V.P.S. Awana, J. Supercond. Novel Magn. 26 (2013)499 26 (2013). [7] S. Demura, Y. Mizuguchi, K. Deguchi, H. Okazaki, H. Hara, T. Watanabe, S.J. Denholme, M. Fujioka, T. Ozaki, H. Fujihisa, Y. Gotoh, O. Miura, T. Yamaguchi, H. Takeya, Y. Takano, J. Phys. Soc. Jpn. 82 (2013) 033708. [8] R. Jha, A. Kumar, S. Kumar Singh, V.P.S. Awana, J. Appl. Phys. 113 (2013) 056102. [9] D. Yazici, K. Huang, B.D. White, A.H. Chang, A.J. Friedman, M.B. Maple, Philos. Mag. 93 (2013) 673. [10] G. Kalai Selvan, M. Kanagaraj, S. Esakki Muthu, R. Jha, V.P.S. Awana, S. Arumugam, Phys. Status Solidi Rapid Res. Lett. 7 (2013) 510. [11] C.T. Wolowiec, D. Yazici, B.D. White, K. Huang, M.B. Maple, Phys. Rev. B 88 (2013) 064503. [12] M. Nagao, S. Demura, K. Deguchi, A. Miura, S. Watauchi, T. Takei, Y. Takano, N. Kumada, I. Tanaka, J. Phys. Soc. Jpn. 82 (2013) 113701. [13] J. Liu, D. Fang, Z. Wang, J. Xing, Z. Du, X. Zhu, H. Yang, H.-H. Wen, arXiv:1310.0377. [14] Y. Iye, I. Oguro, T. Tamegai, W.R. Datars, N. Motohira, K. Kitazawa, Physica C 199 (1992) 154. [15] H. Iwasaki, O. Taniguchi, S. Kenmochi, N. Kobayashi, Physica C 244 (1995) 71. [16] G. Blatter, V.B. Geshkenbein, A.I. Larkin, Phys. Rev. Lett. 68 (1992) 875.