Quenching of the nonlinear susceptibility at a T=0 spin glass transition

LiHo_0.167Y_0.833F₄ is a dilute dipolar-coupled Ising magnet with a spin glass transition which can be crossed with temperature T (T_g=0.13 K) or with an effective transverse field Γ(Γ_g=1 K at T=0). The nonlinear susceptibility contains a diverging component which dominates at T=98 mK, but disappears by 25 mK. At the same time, the onset of spin glass behavior in the dissipative linear susceptibility becomes sharper. We conclude that, contrary to theoretical expectations, quantum transitions can be qualitatively different from thermally driven transitions in real spin glasses.