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.
@article{arxiv.0803.3720,
title = {Quantum Magnets under Pressure: Controlling Elementary Excitations in TlCuCl3},
author = {Ch. Rüegg and B. Normand and M. Matsumoto and A. Furrer and D. McMorrow and K. Krämer and H. -U. Güdel and S. Gvasaliya and H. Mutka and M. Boehm},
journal= {arXiv preprint arXiv:0803.3720},
year = {2008}
}