Quantum Magnets under Pressure: Controlling Elementary Excitations in   TlCuCl3
  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. 
Ch. R\"uegg, B. Normand, M. Matsumoto, A. Furrer, D. McMorrow, K.   Kr\"amer, H.-U. G\"udel, S. Gvasaliya, H. Mutka, M. Boehm