Quantum Critical Behavior for a Model Magnet.

The classical, thermally driven transition in the dipolar-coupled Ising ferromagnet LiHoF 4 sTc ­ 1.53 Kd can be converted into a quantum transition driven by a transverse magnetic field Ht at T ­ 0. The transverse field, applied perpendicular to the Ising axis, introduces channels for quantum relaxation, thereby depressing Tc . We have determined the phase diagram in the Ht-T plane via magnetic susceptibility measurements. The critical exponent, g ­ 1, has a mean-field value in both the classical and quantum limits. A solution of the full mean-field Hamiltonian using the known LiHoF4 crystal-field wave functions, including nuclear hyperfine terms, accurately matches experiment. [S0031-9007(96)00753-3]