Spin-12XXZChain SystemCs2CoCl4in a Transverse Magnetic Field

Abstract

Comparing high-resolution specific heat and thermal expansion measurements to exact finite-size diagonalization, we demonstrate that ${\mathrm{Cs}}_{2}{\mathrm{CoCl}}_{4}$ 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 ${J}_{z}/{J}_{\ensuremath{\perp}}\ensuremath{\approx}0.12$, substantially smaller than previously believed. A spin-flop Ising quantum phase transition occurs at a critical field of ${\ensuremath{\mu}}_{0}{H}_{b}^{\mathrm{cr}}\ensuremath{\approx}2\text{ }\text{ }\mathrm{T}$ before around 3.5 T the description in terms of an effective spin-$\frac{1}{2}$ chain becomes inapplicable.