Spin-12 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 Cs ₂CoCl₄ for a magnetic field along the crystallographic b axis realizes the spin-12 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 _⊥≈0.12, substantially smaller than previously believed. A spin-flop Ising quantum phase transition occurs at a critical field of μ₀Hbcr≈2 T before around 3.5 T the description in terms of an effective spin-12 chain becomes inapplicable.