5,959 research outputs found
Fermionic Coset Models as Topological Models
By considering the fermionic realization of coset models, we show that
the partition function for the model defines a Topological Quantum
Field Theory and coincides with that for a 2-dimensional Abelian BF system. In
the non-Abelian case, we prove the topological character of coset models
by explicit computation, also finding a natural extension of 2-dimensional BF
systems with non-Abelian symmetry.Comment: 14p
Diagnosing order by disorder in quantum spin systems
In this paper we study the frustrated J1-J2 quantum Heisenberg model on the
square lattice for J2 > 2J1, in a magnetic field. In this regime the classical
system is known to have a degenerate manifold of lowest energy configurations,
where standard thermal order by disorder occurs. In order to study its quantum
version we use a path integral formulation in terms of coherent states. We show
that the classical degeneracy in the plane transverse to the magnetic field is
lifted by quantum fluctuations. Collinear states are then selected, in a
similar pattern to that set by thermal order by disorder, leaving a Z2
degeneracy. A careful analysis reveals a purely quantum mechanical effect given
by the tunneling between the two minima selected by fluctuations. The effective
description contains two planar (XY -like) fields conjugate to the total
magnetization and the difference of the two sublattice magnetizations. Disorder
in either or both of these fields produces the locking of their conjugate
observables. Furthermore, within this scenario we argue that the quantum state
is close to a product state.Comment: 8 pages, 3 figure
- âŠ