- Email: [email protected]
Effect of structural symmetry on the magnetic and superconducting properties in La1.875Ba0.125−xSrxCuO4
Physica C 357±360 (2001) 256±259
www.elsevier.com/locate/physc
Eect of structural symmetry on the magnetic and superconducting properties in La1:875Ba0:125 xSrxCuO4 H. Goka *, M. Fujita, Y. Ikeda, K. Yamada Institute for Chemical Research, Kyoto University, Gokasho, Uji 611-0011, Japan Received 16 October 2000; accepted 29 January 2001
Abstract We investigate interplay among magnetism, superconductivity and crystal structure of 1=8-doped La1:875 Ba0:125 x Srx CuO4 using single crystals. A step-like change in the superconducting transition temperature Tc takes place at a speci®c Sr concentration around x 0:08. In the lower concentration region x 6 0:075, Tc , is remarkably suppressed to around 10 K and weakly depends on x. Furthermore, a structural phase transition from low temperature orthorhombic (LTO) to low temperature tetragonal or Pccn phase occurs. At the transition an anomalous upturn of magnetic susceptibility is observed upon cooling under a magnetic ®eld perpendicular to CuO2 plane. On the other hand, in the LTO phase with higher Sr concentration x P 0:085, Tc is nearly constant around 30 K. Ó 2001 Published by Elsevier Science B.V. PACS: 74.25.DW; 74.25.Ha; 74.62.Bf; 74.72.Dn Keywords: LBSCO; 1=8 problem; Single crystal; Magnetic susceptibility; LTO-LTT/Pccn transition
1. Introduction In La2 x Bax CuO4 , the superconductivity is strongly suppressed at the hole concentration p per Cu around 1/8 [1] where the low temperature tetragonal (LTT) phase appears below the transition temperature Td2 [2]. On the other hand, the superconductivity is weakly suppressed for p 0:115 in the low temperature orthorhombic (LTO) phase of La2 x Srx CuO4 (LSCO). Therefore, we expect the LTT phase enhances the suppression of superconductivity. In fact, in Nd doped LSCO, Tranquada et al. [3] proposed a stripe order of
*
Corresponding author. Fax: +81-774-38-3118. E-mail address: [email protected] (H. Goka).
doped holes in the LTT phase. According to their model, the ¯uctuated charge stripes are pinned more tightly in the LTT compared to the LTO phase and hence the suppression of superconductivity is induced by the static stripe order. Previously, several groups studied a relation between the suppression of superconductivity and crystal structure using La1:875 Ba0:125 x Srx CuO4 (LBSCO) [4,5]. In this system, one can control the structural transition from the LTO to LTT or Pccn-orthorhombic phase by changing Ba/Sr ratio without the change of hole concentration. Two dierent results on the x-dependence of Tc are so far reported using the powder samples; a linear dependence on x [4] or a rather discontinuous increase of Tc in accordance with the disappearance of LTT phase [5]. In this paper, we report an experimental
0921-4534/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 2 2 3 - 4
H. Goka et al. / Physica C 357±360 (2001) 256±259
257
reinvestigation of this system using single crystals to clarify the details of structural eect on the suppression of superconductivity. 2. Experimental We grew a series of single crystals of LBSCO with x 0:05, 0.06, 0.075, 0.085 and 0.10 using a travelling solvent ¯oating zone method. The lattice constants a and c at room temperature were determined using X-ray diraction on the powder samples obtained by grinding the single crystals. We performed neutron diraction measurements to study the structural transition using the threeaxis spectrometer TOPAN and the double-axis spectrometer KSD installed in the JRR-3M at the JAERI in Tokai, Japan. Tc of each sample was determined using a SQUID magnetometer (Quantum Design, MPMS series) under a magnetic ®eld of 10 Oe on the zero-®eld-cooled state. The anisotropy of magnetic susceptibility was investigated by applying a magnetic ®eld of 5 T either parallel or perpendicular to the CuO2 plane. In the following chapters we use a reciprocal lattice unit for the high temperature tetragonal phase.
Fig. 1. Lattice constants a and c of LBSCO at room temperature as a function of Sr concentration x. Data for x 0 and 0.125 are referred from Ref. [6].
3. Result As shown in Fig. 1, the obtained lattice constants a and c monotonically decrease with increasing x and are smoothly connected to the values of powder samples at x 0 and 0.125 [6]. We, therefore, con®rm that the systematic Sr doping is carried out in the present samples. The magnetic susceptibility shown in Fig. 2 clearly reveals a discontinuous change in Tc between x 0:075 and 0.085. Td2 was determined by monitoring the intensity of (1,0,0) superlattice peak which develops in the LTT or Pccn phase. Since the peak splitting of (1,1,0) from twinned domains disappears for x 0:05 but remains for x 0:06 and 0.075 samples upon cooling down to T 8 K, the LTT phase for x 0:05 and the orthorhombic Pccn phase for x 0:06 and 0.075 appear below the structural transition. As shown in Figs. 3 and 4, we observed a sharp change in the magnetic
Fig. 2. The dependence of superconducting shielding eect on temperature of LBSCO under a magnetic ®eld 10 Oe.
susceptibility at Tdm similar to the previous reports for LBSCO [7,8]. By using single crystals, we newly revealed the anisotropy of the anomaly that is only observable under a magnetic ®eld perpendicular to the CuO2 plane. We furthermore, con®rm the coincidence of Tdm and Td2 for samples which show the LTO±LTT/Pccn transition x 0:05±0:075. Therefore, the upturn of magnetic susceptibility is closely related to the LTT/Pccn transition. For x 0:085 sample near the boundary, we observed both the anomaly of the magnetic susceptibility at
258
H. Goka et al. / Physica C 357±360 (2001) 256±259
15 K and the superlattice (1,0,0) peak at around 8.5 K though the temperature dependence of the peak intensity is not measured at present. On the other hand, no well-de®ned the susceptibility anomaly exists for x 0:10 as shown in Fig. 4.
4. Discussion
Fig. 3. The dependence of magnetic susceptibility on temperature of LBSCO with x 0:05 under a magnetic ®eld perpendicular (solid circles) or parallel (open squares) to CuO2 plane. Inset shows the dependence of scattered neutron intensity on temperature at a reciprocal position (1,0,0) in the LTT structure.
Fig. 4. The magnetic susceptibilities of LBSCO for x 0:05, 0.06, 0.075 and 0.10 under a magnetic ®eld perpendicular to CuO2 plane. For convenience, vertical axes for the data of x 0:05 and 0.06 are shifted.
In Fig. 5, we present the phase diagram of LBSCO obtained using single crystals. A step-like change in Tc nearly corresponds to the boundary between the LTO and LTT/Pccn phase around T 0 K. As mentioned above, the static stripe order of holes in the LTT is a strong candidate for the origin of the suppression of superconductivity. Indeed, similar to the case of Nd doped LSCO, we observed superlattice peaks re¯ecting the charge order as observed in both the LTT and Pccn phases [9]. The upturn in the magnetic susceptibility suggests a change in the spin structure at Td2 . Detailed
Fig. 5. The phase diagram of LBSCO obtained by single crystals. In the ®gure, circles and open triangles represent Tc and Td2 , respectively. Solid and broken lines are guides to the eye. Thin broken line schematically shows x-dependence of Td2 reported by Maeno et al. [5]. Shaded region denotes the LTT or Pccn phase. Closed triangle at x 0:085 represents the temperature at which the susceptibility anomaly appears (see text for detail). Data point for x 0 is referred from Ref. [1].
H. Goka et al. / Physica C 357±360 (2001) 256±259
study by neutron scattering is needed to clarify the spin structure in both the LTO and LTT/Pccn phases and the relation of spin structure with hole stripe order. It should be noted that previous values of Td2 determined by X-ray powder diraction (shown by a thin broken line in Fig. 5) are substantially lower than those obtained by the present single crystal neutron scattering measurement [5]. We remark that in the latter measurement both (1,0,0) superlattice peak intensity and (1,1,0) peak splitting are monitored while for the former only (2,2,0) peak splitting was studied. Then for the former it is very dicult to detect the appearance of Pccn phase due to the tiny dierence in the peak splitting between the LTO and Pccn phases. Therefore, Td2 determined from (2,2,0) peak splitting may correspond to the transition temperature below which the LTT phase develops. Strictly speaking, in the present phase diagram of Fig. 5 we need to draw a boundary line between the LTT and Pccn phases which is expected to exist between x 0:05 and 0.06. 5. Conclusion We studied an eect of crystal structure on the superconductivity using 1/8-doped single crystals of La1:875 Ba0:125 x Srx CuO4 (LBSCO). A step-like change in Tc takes place at a speci®c Sr concentration around x 0:08. In the lower concentration region x 6 0:075, a structural phase transition from LTO to LTT or Pccn phase occurs and Tc is remarkably suppressed to around 10 K in the latter phase. At the structural transition an anomalous upturn of magnetic susceptibility is
259
observed upon cooling under a magnetic ®eld perpendicular to CuO2 plane. These results suggest that the LTT/Pccn structure enhances the suppression of superconductivity probably due to the static hole stripes stabilized in this structure.
Acknowledgements This work was supported by a grant-in-aid for Scienti®c Research from Ministry of Education, Science, Culture and Sports of Japan and by the Core Research for Evolutional Science and Technology (CREST) Project sponsored by the Japan Science and Technology Corporation.
References [1] A.R. Moodenbaugh, Y. Xu, M. Suenaga, T.J. Folkerts, R.N. Shelton, Phys. Rev. B 38 (1988) 4596. [2] J.D. Axe, A.H. Moudden, D. Hohlwein, D.E. Cox, K.M. Mohanty, A.R. Moodenbaugh, Y. Xu, Phys. Rev. Lett. 62 (1989) 2751. [3] J.M. Tranquada, B.J. Sternlieb, J.D. Axe, Y. Nakamura, S. Uchida, Nature (London) 375 (1995) 561. [4] A. Lappas, L. Cristofolini, K. Prassides, K. Vavekis, A. Amato, F.N. Gygax, M. Pinkpank, A. Schenck, Hyper®ne Interactions 105 (1997) 101. [5] Y. Maeno, A. Odagawa, N. Kakehi, T. Suzuki, T. Fujita, Physica C 173 (1991) 322. [6] E. Takayama-Muromachi, D.E. Rice, Physica C 177 (1991) 195. [7] M. Sera, Y. Ando, S. Kondoh, K. Fukuda, M. Sato, I. Watanabe, S. Nakashima, K. Kumagai, Solid State Commun. 69 (1989) 851. [8] Y. Maeno, A. Odagawa, N. Kakehi, T. Suzuki, T. Fujita, Research Report on Mechanism of Superconductivity, vol. 1, 1991, p. 95. [9] M. Fujita, H. Goka, K. Yamada, unpublished data.
www.elsevier.com/locate/physc
Eect of structural symmetry on the magnetic and superconducting properties in La1:875Ba0:125 xSrxCuO4 H. Goka *, M. Fujita, Y. Ikeda, K. Yamada Institute for Chemical Research, Kyoto University, Gokasho, Uji 611-0011, Japan Received 16 October 2000; accepted 29 January 2001
Abstract We investigate interplay among magnetism, superconductivity and crystal structure of 1=8-doped La1:875 Ba0:125 x Srx CuO4 using single crystals. A step-like change in the superconducting transition temperature Tc takes place at a speci®c Sr concentration around x 0:08. In the lower concentration region x 6 0:075, Tc , is remarkably suppressed to around 10 K and weakly depends on x. Furthermore, a structural phase transition from low temperature orthorhombic (LTO) to low temperature tetragonal or Pccn phase occurs. At the transition an anomalous upturn of magnetic susceptibility is observed upon cooling under a magnetic ®eld perpendicular to CuO2 plane. On the other hand, in the LTO phase with higher Sr concentration x P 0:085, Tc is nearly constant around 30 K. Ó 2001 Published by Elsevier Science B.V. PACS: 74.25.DW; 74.25.Ha; 74.62.Bf; 74.72.Dn Keywords: LBSCO; 1=8 problem; Single crystal; Magnetic susceptibility; LTO-LTT/Pccn transition
1. Introduction In La2 x Bax CuO4 , the superconductivity is strongly suppressed at the hole concentration p per Cu around 1/8 [1] where the low temperature tetragonal (LTT) phase appears below the transition temperature Td2 [2]. On the other hand, the superconductivity is weakly suppressed for p 0:115 in the low temperature orthorhombic (LTO) phase of La2 x Srx CuO4 (LSCO). Therefore, we expect the LTT phase enhances the suppression of superconductivity. In fact, in Nd doped LSCO, Tranquada et al. [3] proposed a stripe order of
*
Corresponding author. Fax: +81-774-38-3118. E-mail address: [email protected] (H. Goka).
doped holes in the LTT phase. According to their model, the ¯uctuated charge stripes are pinned more tightly in the LTT compared to the LTO phase and hence the suppression of superconductivity is induced by the static stripe order. Previously, several groups studied a relation between the suppression of superconductivity and crystal structure using La1:875 Ba0:125 x Srx CuO4 (LBSCO) [4,5]. In this system, one can control the structural transition from the LTO to LTT or Pccn-orthorhombic phase by changing Ba/Sr ratio without the change of hole concentration. Two dierent results on the x-dependence of Tc are so far reported using the powder samples; a linear dependence on x [4] or a rather discontinuous increase of Tc in accordance with the disappearance of LTT phase [5]. In this paper, we report an experimental
0921-4534/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 2 2 3 - 4
H. Goka et al. / Physica C 357±360 (2001) 256±259
257
reinvestigation of this system using single crystals to clarify the details of structural eect on the suppression of superconductivity. 2. Experimental We grew a series of single crystals of LBSCO with x 0:05, 0.06, 0.075, 0.085 and 0.10 using a travelling solvent ¯oating zone method. The lattice constants a and c at room temperature were determined using X-ray diraction on the powder samples obtained by grinding the single crystals. We performed neutron diraction measurements to study the structural transition using the threeaxis spectrometer TOPAN and the double-axis spectrometer KSD installed in the JRR-3M at the JAERI in Tokai, Japan. Tc of each sample was determined using a SQUID magnetometer (Quantum Design, MPMS series) under a magnetic ®eld of 10 Oe on the zero-®eld-cooled state. The anisotropy of magnetic susceptibility was investigated by applying a magnetic ®eld of 5 T either parallel or perpendicular to the CuO2 plane. In the following chapters we use a reciprocal lattice unit for the high temperature tetragonal phase.
Fig. 1. Lattice constants a and c of LBSCO at room temperature as a function of Sr concentration x. Data for x 0 and 0.125 are referred from Ref. [6].
3. Result As shown in Fig. 1, the obtained lattice constants a and c monotonically decrease with increasing x and are smoothly connected to the values of powder samples at x 0 and 0.125 [6]. We, therefore, con®rm that the systematic Sr doping is carried out in the present samples. The magnetic susceptibility shown in Fig. 2 clearly reveals a discontinuous change in Tc between x 0:075 and 0.085. Td2 was determined by monitoring the intensity of (1,0,0) superlattice peak which develops in the LTT or Pccn phase. Since the peak splitting of (1,1,0) from twinned domains disappears for x 0:05 but remains for x 0:06 and 0.075 samples upon cooling down to T 8 K, the LTT phase for x 0:05 and the orthorhombic Pccn phase for x 0:06 and 0.075 appear below the structural transition. As shown in Figs. 3 and 4, we observed a sharp change in the magnetic
Fig. 2. The dependence of superconducting shielding eect on temperature of LBSCO under a magnetic ®eld 10 Oe.
susceptibility at Tdm similar to the previous reports for LBSCO [7,8]. By using single crystals, we newly revealed the anisotropy of the anomaly that is only observable under a magnetic ®eld perpendicular to the CuO2 plane. We furthermore, con®rm the coincidence of Tdm and Td2 for samples which show the LTO±LTT/Pccn transition x 0:05±0:075. Therefore, the upturn of magnetic susceptibility is closely related to the LTT/Pccn transition. For x 0:085 sample near the boundary, we observed both the anomaly of the magnetic susceptibility at
258
H. Goka et al. / Physica C 357±360 (2001) 256±259
15 K and the superlattice (1,0,0) peak at around 8.5 K though the temperature dependence of the peak intensity is not measured at present. On the other hand, no well-de®ned the susceptibility anomaly exists for x 0:10 as shown in Fig. 4.
4. Discussion
Fig. 3. The dependence of magnetic susceptibility on temperature of LBSCO with x 0:05 under a magnetic ®eld perpendicular (solid circles) or parallel (open squares) to CuO2 plane. Inset shows the dependence of scattered neutron intensity on temperature at a reciprocal position (1,0,0) in the LTT structure.
Fig. 4. The magnetic susceptibilities of LBSCO for x 0:05, 0.06, 0.075 and 0.10 under a magnetic ®eld perpendicular to CuO2 plane. For convenience, vertical axes for the data of x 0:05 and 0.06 are shifted.
In Fig. 5, we present the phase diagram of LBSCO obtained using single crystals. A step-like change in Tc nearly corresponds to the boundary between the LTO and LTT/Pccn phase around T 0 K. As mentioned above, the static stripe order of holes in the LTT is a strong candidate for the origin of the suppression of superconductivity. Indeed, similar to the case of Nd doped LSCO, we observed superlattice peaks re¯ecting the charge order as observed in both the LTT and Pccn phases [9]. The upturn in the magnetic susceptibility suggests a change in the spin structure at Td2 . Detailed
Fig. 5. The phase diagram of LBSCO obtained by single crystals. In the ®gure, circles and open triangles represent Tc and Td2 , respectively. Solid and broken lines are guides to the eye. Thin broken line schematically shows x-dependence of Td2 reported by Maeno et al. [5]. Shaded region denotes the LTT or Pccn phase. Closed triangle at x 0:085 represents the temperature at which the susceptibility anomaly appears (see text for detail). Data point for x 0 is referred from Ref. [1].
H. Goka et al. / Physica C 357±360 (2001) 256±259
study by neutron scattering is needed to clarify the spin structure in both the LTO and LTT/Pccn phases and the relation of spin structure with hole stripe order. It should be noted that previous values of Td2 determined by X-ray powder diraction (shown by a thin broken line in Fig. 5) are substantially lower than those obtained by the present single crystal neutron scattering measurement [5]. We remark that in the latter measurement both (1,0,0) superlattice peak intensity and (1,1,0) peak splitting are monitored while for the former only (2,2,0) peak splitting was studied. Then for the former it is very dicult to detect the appearance of Pccn phase due to the tiny dierence in the peak splitting between the LTO and Pccn phases. Therefore, Td2 determined from (2,2,0) peak splitting may correspond to the transition temperature below which the LTT phase develops. Strictly speaking, in the present phase diagram of Fig. 5 we need to draw a boundary line between the LTT and Pccn phases which is expected to exist between x 0:05 and 0.06. 5. Conclusion We studied an eect of crystal structure on the superconductivity using 1/8-doped single crystals of La1:875 Ba0:125 x Srx CuO4 (LBSCO). A step-like change in Tc takes place at a speci®c Sr concentration around x 0:08. In the lower concentration region x 6 0:075, a structural phase transition from LTO to LTT or Pccn phase occurs and Tc is remarkably suppressed to around 10 K in the latter phase. At the structural transition an anomalous upturn of magnetic susceptibility is
259
observed upon cooling under a magnetic ®eld perpendicular to CuO2 plane. These results suggest that the LTT/Pccn structure enhances the suppression of superconductivity probably due to the static hole stripes stabilized in this structure.
Acknowledgements This work was supported by a grant-in-aid for Scienti®c Research from Ministry of Education, Science, Culture and Sports of Japan and by the Core Research for Evolutional Science and Technology (CREST) Project sponsored by the Japan Science and Technology Corporation.
References [1] A.R. Moodenbaugh, Y. Xu, M. Suenaga, T.J. Folkerts, R.N. Shelton, Phys. Rev. B 38 (1988) 4596. [2] J.D. Axe, A.H. Moudden, D. Hohlwein, D.E. Cox, K.M. Mohanty, A.R. Moodenbaugh, Y. Xu, Phys. Rev. Lett. 62 (1989) 2751. [3] J.M. Tranquada, B.J. Sternlieb, J.D. Axe, Y. Nakamura, S. Uchida, Nature (London) 375 (1995) 561. [4] A. Lappas, L. Cristofolini, K. Prassides, K. Vavekis, A. Amato, F.N. Gygax, M. Pinkpank, A. Schenck, Hyper®ne Interactions 105 (1997) 101. [5] Y. Maeno, A. Odagawa, N. Kakehi, T. Suzuki, T. Fujita, Physica C 173 (1991) 322. [6] E. Takayama-Muromachi, D.E. Rice, Physica C 177 (1991) 195. [7] M. Sera, Y. Ando, S. Kondoh, K. Fukuda, M. Sato, I. Watanabe, S. Nakashima, K. Kumagai, Solid State Commun. 69 (1989) 851. [8] Y. Maeno, A. Odagawa, N. Kakehi, T. Suzuki, T. Fujita, Research Report on Mechanism of Superconductivity, vol. 1, 1991, p. 95. [9] M. Fujita, H. Goka, K. Yamada, unpublished data.