Possible Bose-Einstein condensate of magnons in single-crystalline Pb2V3O9

We report the growth and the characterization of single crystals of the S=1/2 spin-dimer compound Pb2V3O9. Magnetic-susceptibility, torque magnetometry, heat-capacity, and muon-spin-relaxation measurements provide evidence for a field-induced Bose-Einstein condensate of magnons in this system. At low temperatures, the field-dependent phase boundary between the dimerized and the condensed state is well described by the expression T*proportional to (H-H-c1)(1/phi), with phi = (2.00+/-0.04).