Quantum Phase Transition in Pr2CuO4 to Collinear Spin State in Inclined   Magnetic Field: A Neutron Diffraction Observation
  In the external field slightly inclined to the $x$- or y-axis of the frustrated tetragonal atiferromagnet Pr2CuO4, a transition is discovered from the phase with orthogonal antiferromagnetic spin subsystems along [1,0,0] and [0,1,0] to the phase with the collinear spins. This phase is shown to be due to the pseudodipolar interaction, and transforms into the spin-flop phase S perp H asymptotically at very high field. The discovered phase transition holds at T=0 and is a quantum one, with the transition field being the critical point and the angle between two subsystems being the order parameter. 
V.P. Plakhty (Petersburg Nuclear Physics Institute), S.V. Maleyev   (Petersburg Nuclear Physics Institute), S.V. Gavrilov (P...