Tuning a quantum phase boundary with microwaves

The ferromagnet LiHoF₄ is a realization of the dipole-coupled transverse field Ising model with a Curie temperature of 1.53 K and a transverse-field-driven quantum phase transition (QPT) at 5 T. The strong hyperfine coupling of the Ho³⁺ ion leads to an effective spin rescaling to I+J below 400 mK, increasing the transverse field required to drive the QPT. This spin rescaling can be suppressed by exciting the nuclear moments into their highest spin states using a strong RF field in the microwave band as a pump. We employ a high-Q loop-gap resonator tuned to approximately 3 GHz to drive the nuclear spins and simultaneously perform dc magnetometry to probe the bulk magnetization. We tune the location of the phase transition as a function of microwave power, and explore the further possibility of studying quantum quench dynamics in a system with a thermodynamic number of spins.