The Disordered Ising Ferromagnet in Transverse Field

The phase diagram and critical behavior of the model quantum magnet LiHo${}_x$Y${}_{1-x}$F${}_4$ were studied for x=0.44, 0.65, and 1.0. Measurements of the phase boundary and critical exponents show mean-field behavior in the classical variable, temperature, but significant deviations from mean-field character emerge in the presence of the quantum variable, transverse magnetic field. Rounding and hysteresis of the magnetic susceptibility in the highly disordered and low temperature limit point to an amplified role for non-equilibrium physics in this regime.