Condensed Matter Sack Lunch Seminar

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 transition-temperature (Tc) superconductivity develops near AF phases, and it is possible that magnetic excitations contribute to the superconducting (SC) 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 it appears to overlap with the SC phase: the archetypical compound Nd(2-x)CexCuO{4\pm\delta} shows bulk superconductivity above , while evidence for AF order has been found up to . However, our new inelastic magnetic neutron scattering measurements point to the distinct possibility that genuine long-range antiferromagnetism and superconductivity do not co-exist. Our measurements furthermore demonstrate that the pseudogap phenomenon in the electron-doped materials arises from a build-up of spin correlations 2.

1 O.P. Vajk et al., Science 296, 1691 (2002).
2 E.M. Motoyama et al., Nature 455, 186 (2007).