Pressure-Induced Magnetic Quantum Phase Transitions from Gapped Ground   State in TlCuCl3
  Magnetization maesurements under hydrostatic pressure were performed on an S=1/2 coupled spin system TlCuCl3 with a gapped ground state under magnetic field H parallel to the [2,0,1] direction. With increasing applied pressure P, the gap decreases and closes completely at Pc=0.42 kbar. For P>Pc, TlCuCl3 undergoes antiferromagnetic ordering. A spin-flop transition was observed at Hsf=0.7T. The spin-flop field is approximately independent of pressure, although the sublattice magnetization increases with pressure. The gap and Neel temperature are presented as function is attributed to to the relative enhancement of the interdimer exchange interactions compared with the intradimer exchange interaction. 
Kenji Goto, Masashi Fujisawa, Toshio Ono, Hidekazu Tanaka and Yoshiya   Uwatoko