T. R. Thurston

Lattice instability in single-crystal La2−xSrxCuO4

Abstract The structural phase transition from the tetragonal to the orthorhombic phase of doped and undoped samples of La2−xSrxCuO4 has been investigated by using inelastic neutron scattering techniques. The rotational nature of the soft mode leads to moderate electron-phonon coupling and the mode is unlikely to enhance significantly conventional phonon mediated superconductivity.

Quasielastic and Inelastic Spin Fluctuations in Superconducting La2-xSrxCuO4

We review the results of recent neutron scattering studies of the spin fluctuations in samples of La1.89Sr0.11CuO4 which are ~ 80% superconducting with Tc = 10 K. The structure factor, S(Q), reflects three dimensional modulated spin correlations with an in-plane correlation length of order 18 ± 6 A. The fluctuations evolve with temperature from being predominantly dynamic at high temperatures to mainly quasielastic (|ΔE| < 0.5 meV) at low temperatures. No significant differences are observed in the normal and superconducting states.

Two Dimensional Quantum Spin Fluid — Progenitor of High Temperature Superconductivity —

Neutron scattering probed novel two dimensional antiferromagnetic spin correlations in pure La2CuO4 which is the mother compound of high temperature superconductors of doped La2-xMxCuO4. Cu2+ moments are ordered instantaneously over very large areas in the CuO2 plane but there is no measurable time averaged staggered moment. Since the dynamical behavior observed is much akin to that in the quantum antigerromagnetic chain, we concluded that spin fluctuations observed at finite temperatures are quantum spin liquid state like that in quantum antiferromagnetic chain at T = 0.

NEUTRON SCATTERING STUDIES OF MAGNETIC EXCITATIONS IN La2−xSrxCuO4

The results of recent neutron scattering studies on La 2−x Sr x CuO 4 are reviewed. For × = 0 the material is an antiferromagnetic insulator, and the spins in the CuO 2 sheets are well-described by the 2D S = 1/2 square lattice Heisenberg model. With increasing × the Neel state is destroyed and is replaced at low temperatures, at least for samples which are nonmetallic or have low superconducting T c s, by a spin glass. The instantaneous correlations as well as those of the glass are incommensurate with the crystal lattice. The spin-spin correlation length in the doped CuO 2 sheets equals the average separation between the holes which ultimately carry the supercurrent.