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Field-induced thermal transport in BEC antiferromagnets

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Recent experiments in BEC
quantum magnets exhibit a dramatic evolution of

the thermal conductivity of these materials in
magnetic field. By considering various relaxation mechanisms of
bosonic excitations in the vicinity of the BEC quantum-critical point at
finite temperature we provide a detailed explanation of several
unusual features of the data. We identify the leading impurity-scattering
interaction and demonstrate that its renormalization due to quantum fluctuations
of the paramagnetic state compensates the related mass renormalization
effect. This explains the enigmatic absence of the asymmetry between the
two critical points in the low-T thermal conductivity data, while such an
asymmetry is prominent in many other physical quantities. The observed
characteristic "migration'' of the peak in thermal conductivity away from
the transition points as a function of temperature is explained as due to a
competition between an increase in the number of heat carriers and an
enhancement of their mutual scattering. An important role of the three-boson
scattering processes within the ordered phase of these systems is
also discussed. Other qualitative and quantitative features of the
experiment are clarified and the future directions are sketched.