are presented here along with a thorough exposition of the theoretical methods, and a discussion of broader experimental consequences to [[Cs$_2$CuCl$_4$]{}]{} and other materials.'
author:
- 'Oleg A. Starykh'
- Hosho Katsura
- Leon Balents
bibliography:
- 'sensitivity.bib'
title: 'Extreme sensitivity of a frustrated quantum magnet: [[Cs$_2$CuCl$_4$]{}]{}'
---

Introduction {#sec:introduction}
============

The spin-$1/2$ nearest-neighbor Heisenberg antiferromagnet on the two dimensional triangular (hexagonal) lattice is the simplest theoretical model for frustrated quantum magnetism.[@Anderson_RVB] Without additional perturbations, the model is believed to order at zero temperature into a 3-sublattice coplanar ground state.[@Huse_Elser_1988_PRL; @Bernu1992PRL; @Sorella1999PRL] However, one may expect a strong sensitivity to additional perturbations to the isotropic triangular lattice Hamiltonian. The material [[Cs$_2$CuCl$_4$]{}]{} provides an interesting example of a spatially [*anisotropic*]{} spin-$1/2$ triangular antiferromagnet.[@Coldea2001PRL]  For several years, neutron scattering,[@Coldea2003PRB] magnetization, and specific heat measurements[@RaduPRL2005; @TokiwaPRB2006] on [[Cs$_2$CuCl$_4$]{}]{} have intrigued the community with unexpected behavior. These experimental properties have been suggested by a variety of authors, including the experimentalists themselves, to indicate exotic physics such as a spin-liquid ground state, unconventional “spinon” excitations at higher energies, and quantum criticality. Theoretical work on this material has been intense. [@Chung2003PRB; @IsakovPRB2005PRB; @AliceaPRL2005; @kohno2009dps; @kohno07:_spinon_and_tripl_in_spatial; @Starykh2007; @PhysRevLett.103.197203]

An advantage of [[Cs$_2$CuCl$_4$]{}]{} is that the small exchange 