Condensed-Matter Physics & Materials Science Seminar

Abstract
The study of quantum phase transitions in the presence of disorder is at the forefront of research in the field of correlated electron systems, yet there have been relatively few experimental model materials. We have succeeded in the growth of large single crystals of the randomly-diluted spin-1/2 square-lattice Heisenberg antiferromagnet La2(Cu,Zn,Mg)O4 up to high dilution concentrations. Our neutron scattering measurements of the instantaneous antiferromagnetic (AF) spin correlations, complemented by numerical experiments, demonstrate that this compound is an excellent system for the study of site percolation in the quantum spin-1/2 limit [1].
High-temperature superconductivity develops near AF phases, and it is possible that magnetic excitations contribute to the superconducting pairing mechanism. In order to assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional AF spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the AF phase extends much further with doping and appears to overlap with the superconducting phase: the archetypical compound Nd2-xCexCuO4+d shows bulk superconductivity above x ≈ 0.13, while evidence for AF order has been found up to x ≈ 0.17. However, our recent inelastic magnetic neutron scattering measurements point to the distinct possibility that superconductivity does not coexit with genuine long-range antiferromagnetism [2].
In a third effort, we have succeeded for the first time in growing sizable single crystals of tetragonal HgBa2CuO4+d [3], the single-layer hole-doped compound with the highest superconducting transition temperature. Our initial inelastic neutron scattering results reveal the existence of ‘hidden’ magnetic order below optimal doping [4] and that the magnetic resonance, the most prominent magnetic excitation found in the