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Quantum dimer magnet

In condensed matter physics, the quantum dimer magnet state is one in which quantum spins in a magnetic structure entangle to form a singlet state. These entangled spins act as bosons and their excited states (triplons) can undergo Bose-Einstein condensation (BEC). The quantum dimer system was originally proposed by Matsubara and Matsuda as a mapping of the lattice Bose gas to the quantum antiferromagnet. Quantum dimer magnets are often confused as valence bond solids; however, a valence bond solid requires the breaking of translational symmetry and the dimerizing of spins. In contrast, quantum dimer magnets exist in crystal structures where the translational symmetry is inherently broken. There are two types of quantum dimer models: the XXZ model and the weakly-coupled dimer model. The main difference is the regime in which BEC can occur. For the XXZ model, the BEC occurs upon cooling without a magnetic field and manifests itself as a symmetric dome in the field versus temperature phase diagram centered about H = 0. The weakly-coupled dimer model does not magnetically order in zero magnetic field, but instead orders upon the closing of the spin gap, where the BEC regime begins and is a dome centered at non-zero field.

Bose-Einstein condensation in the weakly-coupled dimer model
The Bose-Einstein condensation in quantum dimer systems is, at its essence, a field-induced magnetically ordered state that comes about from the Zeeman splitting of the triplet states. The bosons of the Bose-Einstein condensate can be thought of as the component of the spin parallel to the applied magnetic field, reaching a maximum when the spins become polarized by the field. The difference between the Bose-Einstein condensation and a typical ordered state is the spontaneous breaking of the spin's U(1) symmetry (i.e. the circular symmetry transverse to an applied magnetic field). This spontaneous symmetry breaking gives rise to a Goldstone boson that is measureable via a inelastic neutron scattering (amongst other techniques). == References ==
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