In a boron arsenide crystal, the atoms dance a leisurely dance. After a short light pulse, the lattice begins to vibrate, and these vibrations, known as phonons, redistribute energy so unhurriedly that every step seems measured out in advance. Phonons here don't just carry heat: they keep their motions in sync for tens of picoseconds, and that sets the material sharply apart from familiar semiconductors.

To watch the dance, the physicists aimed X-ray pulses at the crystal from different angles. Each angle is sensitive to its own type of vibration: transverse waves, where atoms shift perpendicular to the direction of travel, and longitudinal ones, where they compress and stretch along that direction. It turned out that transverse vibrations thermalize in 29.2 picoseconds, while longitudinal ones take 13.9. For comparison, in silicon this process takes about 2 picoseconds, and in diamond, 4.4. Boron arsenide behaves like a dancer who doesn't want the performance to end.

But the most surprising part is coherence. Two types of vibration, at frequencies of 0.19 and 0.27 terahertz, kept oscillating in sync throughout the entire 50-picosecond observation, with no noticeable decay. In lead telluride, such coherence almost completely vanishes after 50 picoseconds; in silicon, less than half remains; in boron arsenide, practically all of it does. This dance never misses a beat.

It used to be thought that after excitation, a lattice quickly settles into overall equilibrium and all vibrations blend together. Now it's clear that in boron arsenide, individual modes lead their own lives for much longer. This is direct evidence of weak phonon scattering, the very reason the material conducts heat better than most crystals. The slow dance of the atoms is the secret of its exceptional thermal conductivity.