Structural and transport properties of detwinned crystals of La2−xSrxCuO4

Physica C 235-240 (1994) 1285-1286 North-Holland

PHYSICA®

Structural a n d T r a n s p o r t Properties of D e t w i n n e d Crystals of La2_xSrxCuO 4 F. Nakamura, M. Kodama, S. Nishizaki, Y. Tanaka, Y. Maeno and T. Fujita

Department of Physics, Faculty of Science, Hiroshima University, Higashi-Hiroshima 724, Japan We studied a process of detwinning of La2_xSr2CuO 4 (x=0.14) by X-ray diffraction measurements between 5 and 300K. Detwinned crystals are obtained by cooling under a small uniaxial pressure of about 2MPa along the tetragonal <110> T direction. We also report on the effect of twin formation on the inplane transport properties. 1. INTRODUCTION In L a 2 _ x S r x C u O 4 (LSCO), a twinned structure is formed spontaneously below the structural transition temperature Td from a tetragonal to an orthorhombic phase [1]. So far the presence of twins has prevented measurements of bulk properties in the orthorhombic phase concerning the anisotropy within the CuO 2 plane. Recently, we have found that the crystals are readily detwinned when they are cooled under a uniaxial pressure [2, 3]. In this paper, we report a detwinning process and the effect of the twins on transport properties.

intensities of these two orthorhombic peaks. Pressure dependence of the detwinning process was examined by cooling the crystal from room temperature to 60K, each time for a different uniaxial-pressure setting. The fraction of domains with <100> O along the pressure rapidly increases with increasing applied pressure up t,, 0.5MPa. A major part of the crystal becomes oriented with a small pressure of 2MPa. The temperature dependence of the fraction of domains with <100> o and <010> o was obtained between 5 and 300K under uniaxial pressure of 1.8MPa along <110> T, as shown in Fig. 1.

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In this study, we used LSCO crystals grown by a traveling-solvent floating-zone method [4]. The Sr concentration was determined by electronprobe microanalysis to be x=0.138(3). This value is consistent with that expected from Td~195K. In order to examine the detailed detwinning process, X-ray diffraction measurements at low tll'kv ,emt,,.,r,,t-,,.,~, ,,,,~,.e 1,,,.,.,u...,.,,.= on a ,..l.yO,-. wlth ,r,,~ dimension L ] 10 xL]T0 xL001 =3.48 ×3.59 x2.90mm 3 (tetragonal notation) under a uniaxial pressure along T [2]. The domain with <100> o is preferably oriented along the uniaxial pressure. A diffraction peak (220) in the tetragonal phase splits into two peaks (400) and (040) in the orthorhombic phase on cooling below Td. The volume fractions of dot ins with <100> o and <010> o are evaluated from the ratio of integrated 4-

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Figure 1. The intensity ratios of the X-ray diffraction peaks which reflect the volume fractions of domains with <100> o and <010> o along the applied uniaxial pressure.

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E Nakamura et al. / Physica C 235-240 (1994) 1285-1286

1286

Upon cooling (e, A), the domains are gradually oriented below Td. The domains abruptly became highly <100> 0 oriented along the pressure below 160K. Although this particular behavior might be sample dependent, we also observed similar behavior in the crystal used for the transport measurements. After a terminal fraction of the domain polarization is reached at low temperatures, the fraction of domains with <100> 0 and <010> 0 does not change in the following warming process (©, A) unless the temperature exceeds Td.

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In order to clarify the effect of the twin formation on transport properties, we have also performed simultaneous measurements of the dcresistivity and thermal expansivity using the crystal with the dimension Lll0XL1T0XL0o1 = (14)3ram 3 (tetragonal notation). Figure 2 shows the temperature dependence of the temperaturederivative of the resistivity dPab/dT and of a coefficient of linear expansion Crab obtained by a strain-gauge method on cooling without pressure. The fraction of domains with <010> o is estimated to be 64% along the current direction below 100K. This indicates a substantial effect of stress caused by the attached electrodes. Below Td, d pab/dT changes corresponding to the changes in the domain structure with temperature, which are reflected in aat, as unstable behavior down to ab,)ut 120K. On warming, the temperature dependence of d Pab/dT does ,tot show such an unslable change, in accord with a smooth change in the domain structure. Thus, the effect of scattering by the twin boundaries appears to be substantial in the in-plane resistivity. The measurements were also performed under the applied pressure of 1.8MPa, in order to obtain the in.o~rmatio,, of the ~isotropy in ,,e ,,,-w",,,~ conduction. The fraction of domains with <010> 0 is "6% along the current direction. However, the behavior of Pab w a s essentially unchanged from that without pressure. The anisotmpy is likely too

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Figure 2. The temperature dependence of d Pab/dT and Crab on cooling without applied pressure. Here, Pab is measured along <110> T, perpendicular to the direction of the measurement of aab. small to be observed within the precision of our measurements. 4. CONCLUSION Detwinned crystals of LSCO have been obtained by applying such a small uniaxial pressure as about 2MPa. Because the in-plane transport measurements are affected by scattering at the twin boundaries, it is important to orient the domains for such measurements. However, the anisotropy within the CuO 2 plane appears to be negligibly small in the resistivity. REFERENCES 1. A.Sawada et aL, JprLJ.Appl.Phys. 28, (1989) L1787. 2. Y.Maeno et aL, in Advances in Superconductivity 1,7 (Springer-Verlag,Tokyo,1994). 3. S.Nakayama et aL, in this issue. 4. l.Tanaka and H.Kojima, Nature 337(1989)21.