Ch. Rüegg, N. Cavadini, A. Furrer, K. Krämer, H. U. Güdel, P. Vorderwisch and H. Mutka
Spin dynamics in the high-field phase of quantum-critical S = 1/2 TlCuCl₃
Appl. Phys. A 74, S840-S842 (2002)                 

Abstract: An external magnetic field suppresses the spin-energy gap in singlet ground state S=1/2 TlCuCl₃. The system becomes quantum-critical at H_c 5.7 T, where the energy of the lowest Zeeman-split triplet excitation crosses the nonmagnetic ground state. Antiferromagnetic ordering is reported above H_c, which underlines the three-dimensional nature of the observed quantum phase transition. The intrinsic parameters of S=1/2 TlCuCl₃ allow us to access the critical region microscopically by neutron scattering. A substantial study of the spin dynamics in the high-field phase of TlCuCl₃ at T=1.5 K up to H=12 T was performed for the first time. The results possibly indicate two dynamical regimes, which can be understood within characteristically renormalized triplet modes and a low-lying dynamics of potentially collective origin.