Sr concentration dependence of incommensurate elastic magnetic peaks in La2−xSrxCuO4

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Abstract

Neutron scattering measurements were carried out to study the static magnetic structures of superconducting La2−xSrxCuO4 with x=0.10 and 0.13. We found that there exist incommensurate magnetic peaks around (π, π) which start appearing below T*. Combined with previous reports for x=0.10, 0.12 and 0.15, we noticed that T* becomes maximum at x=0.12. The static magnetic correlation length ξ was estimated to be 88 Å for x=0.13, which is considerably shorter than ξ>200 Å reported for x=0.12.

Introduction

It is now well recognized that the high temperature superconductivity (HTSC) in cuprates is realized near the boundary of antiferromagnetic insulator, which is called the Mott insulator. Upon increasing x in La2−xSrxCuO4, various important phase transition temperatures consecutively appear such as TN (Néel ordering: 0.0≤x<0.02), TSG (spin glass transition: 0.02<x<0.05) and Tc (superconducting transition: x>0.05).

Recently, however, Suzuki et al. [1] and Kimura et al. [2] reported experimental evidences that antiferromagnetic spin-density wave (SDW) and superconductivity coexist below Tc for x=0.10 and 0.12. The SDW state with the incommensurate modulation wave vector, ϵ, is established in the superconducting state, which indicates a coupling between SDW and HTSC. They found that the ϵ value increases as x increases and appears linearly related to Tc as observed in the inelastic magnetic scattering [3]. As the SDW state is enhanced in the vicinity of x=0.12, there have been controversial discussions on the physical origin of SDW with respect to the suppression of HTSC around x=1/8 (1/8 problem).

In order to elucidate the coexistence of SDW and superconductivity around x=0.12, we have carried out systematic investigations of SDW at x=0.10 and 0.13, which are complementary to previous studies for x=0.10, 0.12 and 0.15. In this report, we present the first experimental result showing that the SDW state is extended to x=0.13.

Section snippets

Experimental

Single crystals of La2−xSrxCuO4with x=0.10 and 0.13 have been grown using a double-ellipsoidal lamp-image furnace by a traveling solvent floating zone (TSFZ) method. Details of crystal growth were described in Ref. [4]. Most part of the crystals (approximately 0.7 cm3) were used for neutron-scattering measurements. All the crystals were annealed under O2 flowing at 900°C for 50 h and subsequently at 500°C for 50 h, followed by gradual cooling in the furnace. Thus prepared samples were

x=0.13

Fig. 1(a) shows scans along [010] through an incommensurate elastic peak for x=0.13 around (0.5+ϵ,0.5,0) at 1.4 and 31.3 K. As discovered by Lee et al. [8] for La2CuO4+δ (LCO), we confirmed that the incommensurate peaks for x=0.13 are shifted from k=0.5 by Δq=0.03 r.l.u., which we define as the Y-shift. A horizontal bar at the center of the peak indicates the instrumental resolution. Obviously the FWHM of the peak at 1.4 K is larger than the instrumental resolution. To determine an inverse

Conclusions

At this point, we can point out the following important results, although quantitative studies on the coexistence of SDW and HTSC state is warranted.

  • 1.

    There exist incommensurate magnetic peaks around (π, π), which start appearing below T*. Combined with previous reports for x=0.10, 0.12 and 0.15, we noticed that T* becomes maximum at x=0.12.

  • 2.

    The line width of the SDW elastic peak is presumably sharpest at x=0.12 and considerably broadens at x=0.13.

  • 3.

    Compared with the results from x=0.12, we deduced

Acknowledgements

We thank R.J. Birgeneau, Y.S. Lee and G. Shirane for the stimulating discussions. This work is supported by a Grant-In-Aid for Scientific Research of Monbusyou and Core Research for Evolutional Science and Technology (CREST) project sponsored by the Japan Science & Technology Cooperation.

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