Modeling and simulations of many body physics and phase separation in nanoclusters and nanomaterials

A.N. Kocharian, K. Fang, G.W. Fernando, A. Balatsky, K. Palandage
California, US

Keywords: high Tc superconductors, graphene, topological insulators, phase transition

Summary:

The Variational Cluster Approximation (VCA) in the two-dimensional (2d) Hubbard model with repulsion of electrons is used to rigorously calculate the intrinsic cooperative effects in strongly correlated materials with bipartite square and honeycomb lattice geometries. The Mott-Hubbard gap, manifested as a smooth metal-insulator transition, both in square and honeycomb lattices at half filling (n=1), is in agreement with the generic 2d phase diagram. However, a density variation with the chemical potential shows their distinct structural differences away from half filling. For example, at doping in equilibrium we found discontinuous transition and density anomaly in square lattices signaling a phase separation instability and discontinuous transition into inhomogeneous state with hole rich (metallic) and hole poor (insulating) regions. In contrast, honeycomb lattice does not have such anomaly but instead the density displays a smooth transition and describes a continuous evolution of homogenous (metallic) state. The implication of VCA results to HTSCs, layered graphene, topological insulators as well as comparison to other studies are discussed. The VCA provides strong support for spontaneous phase separation instability found in our quantum cluster calculations [1]. [1] A. N. Kocharian, G. W. Fernando, K. Palandage and J. W. Davenport, Phys. Rev. B 74 024511 (2006).