Magnon pairing in quantum spin nematic

¹ Service de Physique Statistique, Magnétisme et Supraconductivité, UMR-E9001 CEA-INAC/UJF 17 rue des Martyrs, F-38054 Grenoble cedex 9, France
² Institute for Solid State Physics, University of Tokyo - Kashiwanoha, 5-1-5, Chiba 277-8581 Japan
³ Max-Planck-Institut für Physik Komplexer Systeme - Nöthnitzer str. 38, D-01187 Dresden, Germany

^a mike.zhitomirsky@cea.fr

Received: 20 October 2010
Accepted: 25 October 2010

Abstract

Competing ferro- and antiferromagnetic exchange interactions may lead to the formation of bound magnon pairs in the high-field phase of a frustrated quantum magnet. With decreasing field, magnon pairs undergo a Bose-condensation prior to the onset of a conventional one-magnon instability. We develop an analytical approach to study the zero-temperature properties of the magnon-pair condensate, which is a bosonic analog of the BCS superconductors. The representation of the condensate wave function in terms of the coherent bosonic states reveals the spin-nematic symmetry of the ground state and allows one to calculate various static properties. Sharp quasiparticle excitations are found in the nematic state with a small finite gap. We also predict the existence of a long-range–ordered spin-nematic phase in the frustrated chain material LiCuVO₄ at high fields.