We follow the evolution of the elementary excitations of the quantum antiferromagnet ${\mathrm{TlCuCl}}_3$ 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.

• Received 20 March 2008

DOI:https://doi.org/10.1103/PhysRevLett.100.205701