"Experimental evidence for Bose Einstein condensation of triplets in spin dimer compounds"

Spin dimer compounds are based on strongly coupled pairs of spins (dimers), which by virtue of the crystal structure only weakly interact with each other. For spin ½ the ground state is a product of singlets and there is a gap to excited triplet states. A field-tuned quantum critical point (QCP) separates the low-field quantum paramagnetic state from a state characterized by long-range magnetic order. In the absence of terms in the spin Hamiltonian that explicitly break U(1) symmetry, this phase transition can be interpreted as a Bose-Einstein condensation (BEC) of delocalized triplets. Despite considerable theoretical interest in this class of material, there are very few examples available for experimental study. In this talk I will describe the high field behavior of two new spin dimer compounds that we have recently been able to synthesize; A₂Cu(BO₃)₂ (where A = Sr & Ba) and BaCuSi₂O₆ (also known as "Han purple").

The spin gap in A₂Cu(BO₃)₂ is rather large (7.4 and 7.9 meV for A = Sr and Ba respectively), all but precluding direct measurements of the ordered state. On the other hand, BaCuSi₂O₆ has a smaller spin gap of just 3.1 meV, corresponding to a critical field H_c1 of approximately 23 T. Consequently, we have been able to probe the ordering transition, determining the critical exponent governing the approach of the phase boundary towards the QCP at H_c1. Our results show that the phase transition is indeed in the BEC universality class. Moreover, we have found evidence for a dramatic cross-over to 2-dimensional behavior close to the QCP; a consequence of the unique geometry of the BaCuSi₂O₆ body-centered tetragonal lattice.