As a type of energy carrier, polaritons can play a dominant role in the thermal conductivity of nano/microstructures. Here, we report an asymmetric thermal conductivity mediated by polaritons that break the Lorentz reciprocity. In contrast to existing approaches that rely on nonlinearity or time modulation, we leverage nonreciprocal polaritons induced by magnetic effects. We show that nonreciprocal surface plasmon polaritons in time-reversal symmetry-breaking systems, including magneto-optical materials and magnetic Weyl semimetals, can alter the symmetry of thermal conductivity in systems like thin films, resulting in direction-dependent thermal conductivity. Thermal conductivities of reciprocal material systems can also be made asymmetric through near-field coupling with these material systems. In accompaniment with the surging interest in nonreciprocal thermal radiation by polaritons, we extend the role of nonreciprocal polaritons from radiative heat transfer systems to conduction systems, paving the way for next-generation thermal devices and efficient energy management solutions.

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• Heat transfer
• Non-reciprocal propagation
• Photonics
• Plasmonics
• Surface plasmon polariton
• Thermal transport
• Semiconductors
• Weyl semimetal
• Boltzmann theory