Thermal diodes
By coupling two materials or “segments” with different resonant frequencies together, a thermal current can be stopped at or let through the interface depending on the temperatures of the segments (top). In this model, one segment (blue) is a chain of particles each of which is connected to its nearest neighbours by elastic springs that satisfy Hooke’s law, while the other (red) is an identical chain but is subject to a sinusoidal potential (green). If the temperature at the red segment is less than that at the blue (bottom left), then the resonant frequencies of the blue segment always concentrate in low frequencies, while those of the red segment concentrate at high frequencies (since the particles are confined in the valleys). As a result, the vibration frequencies of each segment do not match and heat (which is the result of vibrations of the particles) cannot flow very efficiently. However, when the temperature at the red segment is greater than that at the blue (bottom right), the particles can move freely between the barriers and thus their vibration frequencies partly extend to low frequencies, which matches those of the particles in the right segment. This match/mismatch mechanism makes directional thermal conduction, and also negative differential temperature resistance (NDTR), possible. NDTR is essential for making thermal switches and transistors.