Extreme sensitivity of a frustrated quantum magnet: Cs_2CuCl_4

TL;DR

This study reveals the extreme sensitivity of Cs_2CuCl_4's magnetic phases to tiny interactions, uncovering surprising correlations and phase transitions in a frustrated quantum magnet through theoretical analysis aligned with experiments.

Contribution

The paper provides a detailed theoretical analysis of Cs_2CuCl_4's phase diagram, highlighting the impact of small neglected interactions and revealing new phases and transitions.

Findings

Spins are most correlated perpendicular to triangular layers despite weak inter-layer coupling.

Tiny interactions induce new phases and a commensurate-incommensurate transition.

The phase diagram is highly sensitive to small perturbations, affecting experimental signatures.

Abstract

We report a thorough theoretical study of the low temperature phase diagram of Cs_2CuCl_4, a spatially anisotropic spin S=1/2 triangular lattice antiferromagnet, in a magnetic field. Our results, obtained in a quasi-one-dimensional limit in which the system is regarded as a set of weakly coupled Heisenberg chains, are in excellent agreement with experiment. The analysis reveals some surprising physics. First, we find that, when the magnetic field is oriented within the triangular layer, spins are actually most strongly correlated within planes perpendicular to the triangular layers. This is despite the fact that the inter-layer exchange coupling in Cs_2CuCl_4 is about an order of magnitude smaller than the weakest (diagonal) exchange in the triangular planes themselves. Second, the phase diagram in such orientations is exquisitely sensitive to tiny interactions, heretofore neglected, of order a few percent or less of the largest exchange couplings. These interactions, which we describe in detail, induce entirely new phases, and a novel commensurate-incommensurate transition, the signatures of which are identified in NMR experiments. We discuss the differences between the behavior of Cs_2CuCl_4 and an ideal two-dimensional triangular model, and in particular the occurrence of magnetization plateaux in the latter. These and other related results are presented here along with a thorough exposition of the theoretical methods, and a discussion of broader experimental consequences to Cs_2CuCl_4 and other materials.