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

O. Breunig, M. Garst, E. Sela, B. Buldmann, P. Becker, L. Bohatý, R. Müller, T. Lorenz
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 $\mu_0 H_b^{\rm cr} \approx 2$ T before around 3.5 T the description in terms of an effective spin-$\frac{1}{2}$ chain becomes inapplicable.
View original: http://arxiv.org/abs/1307.2870