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UCLA researchers have made an unexpected discovery about the thermal conductivity of boron arsenide, a newly found semiconductor with potential for heat management and electronics applications. Normally, the thermal conductivity of materials increases under high pressure, but the researchers found that this was not the case for boron arsenide. They applied pressure to samples of the material and found that its thermal conductivity actually decreased under high pressure conditions, which are hundreds of times greater than at the bottom of the ocean.  The team led by Yongjie Hu used ultrafast optics measurements to study the atomic vibrations of the boron arsenide crystal lattice and found that the decrease in thermal conductivity was due to competition between two different heat transfer processes in the crystal as pressure increased.  This discovery challenges our understanding of heat transport in extreme conditions and could lead to new breakthroughs in the field.