Spin dynamics in the high-field phase of quantum-critical S =1/2 TlCuCl3

An external magnetic field suppresses the spin-energy gap in singlet ground state S=1/2 TlCuCl3. The system becomes quantum-critical at Hc≈5.7 T, where the energy of the lowest Zeeman-split triplet excitation crosses the nonmagnetic ground state. Antiferromagnetic ordering is reported above Hc, 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.