Direct Observation of a Persistent Thermal Current

Thermal currents arise in the presence of temperature gradients, analogous to electrical currents driven by voltage differentials. Beyond such normal currents, persistent electrical currents flowing without a voltage differential arise in superconductors. However, persistent thermal currents in the absence of temperature differentials have never been experimentally detected. Here, we report a direct experimental detection of a persistent thermal current via calorimetric measurements performed using a custom-fabricated platform consisting of three magneto-optical Indium Gallium Arsenide discs positioned symmetrically in the near-field of each other. When reciprocity is broken by applying an orthogonal magnetic field, an asymmetric thermal conductance appears between the discs, providing an unambiguous signature of a persistent heat current. The direction of this current is reversed with the magnetic field orientation and the magnitude scales with the magnetic field, confirming its origin in broken reciprocity. This work uncovers a completely novel transport phenomenon that can enable the control of energy flow in non-reciprocal devices created from magneto-optical and topological materials and unlocks transformative energy storage and thermal management applications.