Eur. Phys. J. B (2015) 88: 176
https://doi.org/10.1140/epjb/e2015-60211-6

Regular Article

Path integral approach to order by disorder selection in partially polarized quantum spin systems

¹ IFLP-CONICET, Departamento de Física, Universidad Nacional de La Plata, C.C. 67, 1900 La Plata, Argentina
² Laboratoire de Physique Théorique, IRSAMC, CNRS and Université de Toulouse, UPS, 31062 Toulouse, France

^a e-mail: lamas@fisica.unlp.edu.ar

Received: 16 March 2015
Received in final form: 22 May 2015
Published online: 8 July 2015

Abstract

In this paper we study the frustrated J₁−J₂ quantum Heisenberg model on the square lattice for J₂>J₁/2, 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.