Researchers Challenge Kirchhoff’s Law: Heat Flow Becomes Directional

Thermal radiation typically follows a strict symmetry: according to Kirchhoff's law of thermal radiation, an object's ability to absorb heat from a specific direction is identical to its ability to emit heat in that same direction. Breaking this reciprocity is a long-standing challenge in physics, as it would allow engineers to "program" heat flow, effectively creating a one-way valve for thermal energy. A new theoretical model proposes a way to bypass this law by combining two specific materials to create an asymmetrical thermal path. The foundation of the system is a layer of indium arsenide, which handles the absorption and emission of infrared light. By applying a magnetic field, the researchers broke the natural symmetry of this layer, forcing thermal radiation moving in one direction to behave differently than radiation moving in the opposite direction. To make this directionality permanent, the researchers added a grating layer made of germanium-antimony-tellurium (GST). GST is a phase-change material, meaning it can switch between amorphous and crystalline states and maintain that state without a continuous power supply. Once the GST is set to a specific phase, it "locks" the direction of the thermal emission.