1. Exotic magnetic states in two-dimensional organic superconductors
R. T. Clay, Department of Physics & Astronomy, Mississippi State University
The organic charge-transfer salts (CTS) are quasi-one and quasi-two dimensional crystalline materials that display a large number of unusual electronic phases, including superconductivity. In the CTS as in other strongly-correlated superconductors, superconductivity is often found adjacent in the phase diagram to an insulating state.
We performed large-scale quantum Monte Carlo calculations on the Hubbard-Holstein (HH) model. The HH model includes two important interactions, which are both present in molecular materials such as the CTS: (i) electron-electron interactions, and (ii) electron-phonon interactions. Previous work and mean-field calculations suggest that only insulating phases are possible at half filling (average electron density of 1 per site). Our results show:
A metallic phase exists between the two possible insulating states.
In this phase, electrons are bound in pairs, and pair motion creates conductivity. While our calculation was in one dimension here, this state would become superconducting in two or three dimensions.
 Fig. 1: QMC results for up to 128 sites. Top (a): at the transition, vanishing of logarithmic size corrections indicates a Kosterlitz-Thouless type transition. Bottom (b): staggered charge susceptibility, an order parameter for the Peierls phase.
Phase diagram