Evidence of Bose-Einstein condensation in a quantum magnet formed by free radical tetramers

Several experimental realizations of quantum magnets showing Bose-Einstein condensates (BEC) have been reported in the literature. The typical scenario invokes a ground state described by pairs of localized spins forming singlets. An external magnetic field acts as an effective chemical potential for triplet excitations that can subsequently form the BEC. The T versus H phase diagrams typically display a ``dome'' structure bounded by two critical fields and a field-dependent critical temperature.

In this work, we present experimental and theoretical evidence of Bose-Einstein condensation in an organic crystal formed of weakly interacting S=1/2 tetramers. We show that the physics can be described in terms of a fully rotational invariant system of quantum spins without frustration. Two BEC phases exist: at low fields, only the edge spins of each tetramer contribute to the condensate, while at high-fields, the order is determined by the two central spins. These unusual BECs are separated by a quantum state with half of the spins forming dimerized pairs, and the other half aligned in the direction of the field.