The spin-$\frac{1}{2}$ XXZ chain system Cs${}_2$CoCl${}_4$ in a transverse magnetic field

Abstract

Comparing high-resolution specific heat and thermal expansion measurements to exact finite-size diagonalization, we demonstrate that Cs${}_2$CoCl${}_4$ for a magnetic field along the crystallographic b axis realizes the spin-$\frac{1}{2}$ XXZ chain in a transverse field. Exploiting both thermal as well as virtual excitations of higher crystal field states, we find that the spin chain is in the XY-limit with an anisotropy $J_z/J_{\perp }\approx 0.12$ substantially smaller than previously believed. A spin-flop Ising quantum phase transition occurs at a critical field of $\hat{I}{¼}_0{H}_b^{\mathrm{cr}}\approx 2$ T before around 3.5 T the description in terms of an effective spin-$\frac{1}{2}$ chain becomes inapplicable.

5 pages, 3 figures