Evgueny Kochetov

SU(2) coherent‐state path integral

The SU(2) coherent‐state path integral is used to represent the matrix element of a propagator in the SU(2) coherent‐state basis. It is argued that the continuum representation of this integral is correct provided the necessary boundary term is taken into account. In the case of the SU(2) dynamical symmetry the path integral is explicitly computed by means of a change of variables, the SU(2) motion of the underlying phase space. The correct stationary‐phase expansion for the propagator in terms of the total action including boundary term and classical trajectories is obtained.

Spin coherent-state path integrals and the instanton calculus

We use an instanton approximation to the continuous-time spin coherent-state path integral to obtain the tunnel splitting of classically degenerate ground states. We show that provided the fluctuation determinant is carefully evaluated, the path integral expression is accurate to order O(1/j). We apply the method to the LMG model and to the molecular magnet Fe8 in a transverse field.

Path integral over the generalized coherent states

A path integral written in terms of the group theoretic coherent states by using the Kahler structure of the coherent state manifold with the particular emphasis on the boundary‐fixing term derivation is considered herein. The path integral for a propagator of the system with Hamiltonian linear in the SU(2)/SU(1,1) generators is shown to be diagonalized by an appropriate motion in the phase space.

Electronic properties of disclinated flexible membrane beyond the inextensional limit: application to graphene

A gauge-theory approach to describe Dirac fermions on a disclinated flexible membrane beyond the inextensional limit is formulated. The elastic membrane is considered as an embedding of a 2D surface into R(3). The disclination is incorporated through an SO(2) gauge vortex located at the origin, which results in a metric with a conical singularity. A smoothing of the conical singularity is accounted for by replacing a disclinated rigid plane membrane with a hyperboloid of near-zero curvature pierced at the tip by the SO(2) vortex. The embedding parameters are chosen to match the solution to the von Karman equations. A homogeneous part of that solution is shown to stabilize the theory. The modification of the Landau states and density of electronic states of the graphene membrane due to elasticity is discussed.

Resonating valence-bond theory of superconductivity for dopant carriers: Application to the cobaltates

Within the

Breakdown of the mean-field description of the Nagaoka phase

t\text{\ensuremath{-}}J

Gauge invariance and spinon-dopon confinement in the t-J model: implications for Fermi surface reconstruction in the cuprates

model Hamiltonian we present a resonating valence-bond mean-field theory directly in terms of dopant particles. We apply this theory to

Ising t–J model close to half filling: a Monte Carlo study

{\mathrm{Na}}_{x}\mathrm{Co}{\mathrm{O}}_{2}∙y{\mathrm{H}}_{2}0

Doped carrier formulation of the t-J model : Projection constraint and the effective Kondo-Heisenberg lattice representation

and show that the resulting phase diagram

BCS-type mean-field theory for thet−Jmodel in the SU(2|1) superalgebra representation

{T}_{c}