P6: Quantum phase transitions in frustrated magnetic systems

P. Wölfle, P. Schmitteckert

Quantum fluctuations in strongly interacting systems have a dual role: on the one hand they tend to suppress long-range-order, on the other hand they may lead to novel ground states characterized by slowly decaying spatial correlations. We are proposing to investigate the magnetic correlations near quantum phase transitions in antiferromagnetic spin systems with competing interactions and/or geometric frustration. Three lines of approach will be used to determine the phase diagram, the excitation spectrum and dynamical response functions.

First we will use a diagrammatic pseudofermion approximation that we have shown to give excellent results for the pure AF Heisenberg model to explore the nature of the intermediate phase found in numerical studies to be interlaced between the AF and Collinear phases of the J1-J2 model and to study quasi one-dimensional model systems relevant for certain spin frustrated compounds (see below).

Second, we will construct spin liquid trial states with chirality correlations and fractionalized excitations for the anisotropic triangular Heisenberg model thought to describe systems like Cs2CuCl4, and for the J1-J2 model.

Third, we will use the Density-Matrix-Renormalization-Group method as a numerical tool to provide specific benchmarks for analytical methods and as a tool to calculate the properties of more complicated models like a helical six-leg spin 2 ladder expected to describe Ca3Co2O6. This requires the development of a new diagonalization algorithm designed to cope with highly degenerate systems.