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The low-temperature physics of weakly coupled spin-1/2 dimers with an external magnetic field H can be described in a
reduced Hilbert space where only the two lowest energy states are kept for each dimer. Under certain circumstances, such a truncation leads to simple bosonic effective models where the two retained states with n = 0 or 1 are equivalent to hard-core bosons on a 2D lattice. Therefore a very rich variety of bosonic
critical phenomena might be expected when the external magnetic field is tuned, depending on the amount of frustration, and spin/space anisotropies : Bose-Einstein condensation, superfluidity, crystallization, supersolidity. . .
In such a context, this presentation will focus on two important issues related to recent theoretical and experimental questions regarding field-induced bosonic phases and their associated critical behaviors in spin-1/2 bilayer systems : -Occurence, mechanisms, and universality class of a field-induced supersolid phase in the vicinity of a magnetization plateau [1]. -Role and influence of frustrated inter-bilayer couplings on the reduced dimensionnality mechanism and the critical properties of the Han-Purple compound BaCuSi2 O6 [2]. [1] K.-K. Ng, T. K. Lee, PRL 97, 127204 (2006) ; N. Laflorencie, F. Mila, PRL 99, 027202 (2007). [2] S. E. Sebastian et al., Nature 441, 617 (2006) ; Ch. Ruegg et al., PRL 98, 017202 (2007) ; S. Kramer et al., PRB 76, 100406(R) (2007). |
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