A stack of thin plates made of different materials lies on a table. If you heat it from below, heat will rise, but will it spread sideways just as quickly? In layered semiconductors, the answer is no: heat runs faster along the layers than across them. This is like water in a stack of paper: it spreads easily along the sheets but seeps through the stack with difficulty.
Measuring both directions of heat flow accurately is no easy task. Conventional methods see the cross-plane heat movement well but are nearly blind to the in-plane direction. The researchers proposed a new approach: they direct two laser beams at the sample (one heats, the other reads the temperature) and they move these beams apart at different distances. By analyzing how the amplitude and phase of the thermal response change at different offsets, both thermal conductivities can be computed.
For data processing, they used a smart algorithm that automatically finds the best match between the model and the experiment. On a sample of alternating layers of aluminum arsenide and gallium arsenide (period thickness 52 nm), they obtained: across the layers—14.7 W/(m·K), and along—37.4 W/(m·K). The 2.5-fold difference shows how strongly anisotropy affects heat transfer. This method will be useful in developing new thermoelectrics and microchip cooling.
The difference in thermal conductivity along and across the layers in the studied material is greater than in wood: in pine, heat travels about 2 times faster along the grain than across it.