Spin-1/2 XXZ Chain System Cs2CoCl4 in a Transverse Magnetic Field

Comparing high-resolution specific heat and thermal expansion measurements to exact finite-size diagonalization, we demonstrate that Cs2CoCl4 for a magnetic field along the crystallographic b axis realizes the spin-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(perpendicular to) approximate to 0.12, substantially smaller than previously believed. A spin-flop Ising quantum phase transition occurs at a critical field of mu H-0(b)cr approximate to 2 T before around 3.5 T the description in terms of an effective spin-1/2 chain becomes inapplicable.