Ch. Rüegg, B. Normand, M. Matsumoto, A. Furrer, D. F. McMorrow, K. W. Krämer, H.-U. Güdel, S. N. Gvasaliya, H. Mutka and M. Boehm
Quantum Magnets under Pressure: Controlling Elementary Excitations in TlCuCl₃
Phys. Rev. Lett. 100, 205701/1-4 (2008)                 

Abstract: We follow the evolution of the elementary excitations of the quantum antiferromagnet TlCuCl₃ through the pressure-induced quantum critical point, which separates a dimer-based quantum disordered phase from a phase of long-ranged magnetic order. We demonstrate by neutron spectroscopy the continuous emergence in the weakly ordered state of a low-lying but massive excitation corresponding to longitudinal fluctuations of the magnetic moment. This mode is not present in a classical description of ordered magnets, but is a direct consequence of the quantum critical point.