I will review Bose-Einstein condensation in quantum magnets. In these
materials a field-induced quantum phase transition to XY antiferromagnetic
order can be mapped onto Bose-Einstein Condensation. This gives us the
opportunity to study BEC in the thermodynamic limit, as well as an
interesting case where magnetic field rather than temperature can be the
tuning parameter for BEC transition. I will focus in particular on the
metal-organic quantum magnet NiCl2-4SC(NH2)2. Here the S = 1 Ni system
orders antiferromagnetically between Hc1 = 2.1 and Hc2 = 12.6 T and a wide
range of experimental techniques suggest the BEC universality class. We also
find that that mass of the bosons that condense can be strongly suppressed
by quantum fluctuations, resulting in a remarkable asymmetry between the
properties at Hc1 and Hc2. I will present magnetization, thermal
conductivity and specific heat data to probe BEC. I will also mention
follow-on topics like the BEC to Bose-Glass phase transition, and spin
supersolidity.
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