Extra Fysisch Colloquium

Location: University of Leiden, C.J. Gorterzaal, Oortgebouw. Time: 14:00 hrs

Crystal Growth, Neutron Scattering and Spin Correlations: A Tale of Two Complex Oxides

M. Greven, Departments of Applied Physics and Photon Science,Stanford, CA 94305, USA

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 model 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 (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 lessstudied electron-doped materials, the AF phase extends much further with doping and it appears to overlap with the SC 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 new inelastic magnetic neutron scattering measurements point to the distinct possibility that superconductivity does not co-exist with genuine long-range antiferromagnetism. We furthermore show evidence that the so-called pseudogap phenomenon in the electron-doped materials arises from a build-up of AF spin correlations [2].

[1] O.P Vajk et al., Science 295, 1691 (2002).

[2] E.M. Motoyama et al., Nature 455, 186 (2007).