Bose-Einstein condensation in spin dimer compounds

Magnetic materials and ultracold atomic gases are extremely disparate physical systems, yet universal principles of organization cause them to exhibit the identical phenomenon of 'Bose-Einstein condensation' (BEC) at low temperatures. This unique correspondence is established by an experimental study of magnetic spin-dimer materials in high magnetic fields, presented in this thesis. These model systems are especially attractive for experimentation, since the external magnetic field provides an experimental handle to tune the system in the vicinity of the quantum critical region.; Spin dimer materials consist of a weakly coupled network of spin pairs (dimers) such that the ground state at low magnetic fields is a direct product of singlets (i.e.) a quantum paramagnet. Novel magnetic order is observed above a critical magnetic field Hc1 where the triplet and singlet levels become degenerate. Experimental results are presented here on two families of new materials in the rapidly developing class of spin dimer compounds.; Results from magnetisation, magneto-caloric effect, specific heat, magnetic torque, and inelastic neutron scattering measurements are presented on single crystals of A2Cu(BO3)2 (A = Sr, Ba) and BaCuSi 2O6. Critical scaling analysis was performed on the measured phase boundary separating the magnetically ordered phase from the paramagnetic phase in BaCuSi2O6, which has a relatively accessible critical magnetic field. Critical scaling analysis of the measured phase boundary reveals power law behaviour consistent with the three dimensional (3 d) BEC universality class down to T ∼ 0.6 K, unambiguously identifying the ordered state as a 3d BEC of triplets, corresponding to a canted XY antiferromagnet.; Extremely unusual critical scaling behaviour is observed in BaCuSi 2O6 nearer the QCP, close to T ∼ 0.03 K, characteristic of the two dimensional (2d) BEC universality class. We identify inter-layer decoupling due to geometrical frustration as the mechanism responsible for this unique manifestation of a lower dimensional QCP in the 3 d BaCuSi2O6 spin system. This is the first experimental realisation of dimensionally reduced criticality, and constitutes a proof of principle that dimensionality can become an emergent property of a QCP.