Stanford University
Title: Recent Neutron Scattering Studies of Complex Oxides
Abstract:
Quantum phase transitions in the presence of quenched disorder are at the forefront of research in the field of strongly-correlated electron systems, yet there have been relatively few experimental model systems. Complementary neutron scattering and numerical experiments demonstrate that the randomly-diluted square-lattice Heisenberg antiferromagnet La2Cu1-p(Zn,Mg)pO4 (Zn/Mg-LCO) is an excellent model magnet for the study of site percolation in the quantum-spin limit S = 1/2 [1]. Quantum Monte Carlo results for the bilayer Heisenberg antiferromagnet indicate that the finite-temperature properties of Zn/Mg-LCO near the percolation threshold are controlled by the effective proximity to a new quantum critical point [2].
Since the superconductivity in the high-Tc cuprates occurs in close proximity to antiferromagnetic (AFM) phases, it is essential to understand the nature of nearby magnetic ground states. A percolation picture has been used to qualitatively describe the extended nature of the AFM phase in the electron-doped material Nd2-xCexCuO4. New measurements of the spin correlations and ordered moment in non-superconducting samples allow a quantitative comparison with numerics for the disorder problem and with recent theory for the one-band Hubbard model [3]. Finally, it is shown that magnetic-field effects, previously ascribed to a quantum phase transition from the superconducting to an AFM state, are spurious paramagnetism due to a secondary chemical phase [4].
[1] O.P. Vajk et al., Science 295, 1691 (2002).
[2] O.P. Vajk and M. Greven, Phys. Rev. Lett. 89, 177202 (2002).
[3] P.K. Mang et al., cond-mat/0307093.
[4] P.K. Mang, S. Larochelle, and M. Greven, Nature 426, 139 (2003).