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Nonreciprocal thermal transport in a multiferroic helimagnet

Breaking of spatial inversion symmetry (SIS) induces unique phenomena in condensed matter. Besides the classic examples such as natural optical activity and piezoelectricity, the spin-orbit interaction further enriches the effect of SIS breaking, as exemplified by the Rashba effect. In particular, by combining this symmetry with magnetic fields or another type of time-reversal symmetry (TRS) breaking, noncentrosymmetric materials can be made to exhibit nonreciprocal responses, which are responses that differ for rightward and leftward stimuli. For example, resistivity becomes directionally dependent; that is to say, rectification appears in noncentrosymmetric materials in a magnetic field. However, the effect of SIS breaking on thermal transport remains to be elucidated. Here we show nonreciprocal thermal transport in the multiferroic helimagnet TbMnO3. The longitudinal thermal conductivity depends on whether the thermal current is parallel or antiparallel to the vector product of the electric polarization and magnetization. This phenomenon is thermal rectification that is controllable with external fields in a uniform crystal. This discovery may pave the way to thermal diodes with controllability and scalability.