Quantum Magnets under Pressure: Controlling Elementary Excitations inTlCuCl3

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.Although quantum fluctuations of both spin and charge degrees of freedom are the key to the essential physics of many challenging problems in condensed matter systems, the microscopic control of zero-point fluctuations has to date remained largely a theoretical abstraction. However, full control over the interaction parameters can now be effected in cold atomic condensates through the standing-wave amplitudes of the optical lattice. Similarly, in quantum magnets the exchange interactions can be controlled by the application of pressure, altering the effect of spin fluctuations. We follow this approach to investigate the physics of a quantum system whose fluctuations are "tuned" in a continuous way.The most dramatic manifestation of such control is the driving of a quantum phase transition [QPT, Fig. 1(a)] between two different ground states [1]. Structurally dimerized S = 1/2 spin systems offer a particularly clean realization both of the magnetic field-induced QPT, which has been studied extensively in a number of materials [2], and of the qualitatively different magnetic QPT driven by hydrostatic pressure [3]. The Hamiltonian

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Higgs mode and its decay in a two-dimensional antiferromagnet

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Crystals for neutron scattering studies of quantum magnetism

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