Torbjörn Einarsson

FRUSTRATED HONEYCOMB HEISENBERG ANTIFERROMAGNET : A SCHWINGER-BOSON APPROACH

We analyze the frustrated Heisenberg antiferromagnet defined on a honeycomb lattice using a Schwinger-boson mean-field theory. The spin-wave velocity and the susceptibility are presented as functions of the strength of the frustrating interaction for spin S=1/2, and the dynamic structure factor is calculated for various temperatures and frustrations. For large S, we find an increased Neel stability with respect to the classical case.

Symmetries and mean-field phases of the extended Hubbard model

The two-dimensional extended Hubbard model that includes a nearest-neighbor Heisenberg interaction is studied using a mean-field theory where quasiparticles are defined by an U(8) group of canonical transformations permitting both broken gauge, spin, and sublattice symmetry. The theory is further extended to incorporate a possible twist in the spin-quantization axis, so that the competition between superconductivity, charge density waves, and Neel and spiral antiferromagnetic order can be monitored within one single theory. Our results for positive Hubbard {ital U} and Heisenberg exchange {ital J} suggest that antiferromagnetic ordering dominates close to half-filling, while spiral states and {ital d}-wave superconductivity compete when doping is introduced. For moderate values of {ital J}, we find a phase diagram where a phase transition occurs from an antiferromagnet to a {ital d}-wave superconductor as doping is increased. A narrow region of ({ital s}+{ital id})-wave superconductor is found for some values of {ital J} and {ital U}.

Circle maps with symmetry-breaking perturbations

Abstract Renormalization group methods are used to describe the transition to chaos that occurs in dissipative quasiperiodic systems in the presence of a small subharmonic perturbation. This is accomplished by studying the circle map with the symmetry θ → θ + 2 π / n , and looking at crossover exponents for perturbations obeying the usual 2π symmetry. In order to interpret these exponents, the standard renormalization group analysis must be extended to include extra functions. This has been worked out in detail for the case n = 2.