Berlin 2008 – scientific programme

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SYPT: Symposium Quantum Phase Transitions

SYPT 1: SYPT

SYPT 1.3: Invited Talk

Monday, February 25, 2008, 15:10–15:45, H 0105

Elementary Excitations in Quantum Critical Antiferromagnets — •Christian Ruegg — London Centre for Nanotechnology, University College London, London, UK

Spin singlet formation is ubiquitous in condensed matter physics, and occurs in systems as seemingly disparate as the Kondo insulators, structurally dimerized quantum antiferromagnets, and potentially as resonating valence bonds in magnets with Kagome, square-lattice, and ladder exchange geometry. In dimerized antiferromagnets, e.g. TlCuCl₃, the ground state is quantum disordered at low temperature and a product of such singlets (S=0), with triplet excited states (S=1 quasi-particles). In a magnetic field or as a function of the ratio between exchange interactions, quantum phase transitions (QPT) to phases with magnetic order can be realized if the spin gap Δ(p,H) to the excited states vanishes. Recent inelastic neutron scattering results are presented about the spin dynamics in the proximity of such QPTs. In this region of the phase diagram, where magnetic order is continuously suppressed, novel elementary excitations appear.