Unusual Quantum Coherence of Atomic Vibrations Reported in Boron Arsenide
Researchers have reported an unusual quantum coherence of atomic vibrations, known as phonons, in cubic boron arsenide. This semiconductor material exhibits promising electronic and thermal properties.
Understanding Phonons
All solid materials contain atoms that vibrate within their lattices. These vibrations, or phonons, manifest in two primary forms:
Acoustic Phonons
These are collective vibrations where atoms move in the same direction, resembling a low hum. They are crucial for heat conduction, particularly in electronics.
Optical Phonons
These involve atoms moving in opposite directions, creating a higher-energy vibration resembling a squeak. Optical phonons govern infrared thermal radiation and can transmit information.
Typically, they have a shorter lifetime than acoustic phonons due to energy transfer, often through a process called three-phonon scattering, where one optical phonon transfers energy to two acoustic phonons.
Unique Properties of Boron Arsenide
In boron arsenide, the conventional three-phonon scattering process does not occur. An optical phonon in this material possesses more energy than any possible combination of two outgoing acoustic phonons, preventing the transfer of energy through this pathway.
Consequently, optical phonons in boron arsenide are notably long-lived, primarily transferring energy via the less probable four-phonon scattering, a process involving splitting into three particles.
Research Findings
The research team, including groups from Rice University, University of Houston, and Texas Tech University, produced high-quality crystals using only boron-11 isotopes. They employed high-resolution Raman and infrared spectroscopy to investigate phonon scattering pathways at various temperatures.
Key findings include:
- Record-high coherence for phonons at low temperatures, where vibrations completed nearly a thousand cycles before fading, significantly more than in typical materials.
- Analysis of coherence temperature dependence confirmed that four-phonon scattering is dominant over three-phonon scattering in boron arsenide.
- The presence of the boron-10 isotope was identified as the primary factor contributing to coherence loss at the quantum ground state. Structural defects in the samples did not appear to affect the coherence of optical phonons.
Future Implications
The findings suggest that removing isotope impurities could extend the lifetime of these phonons by a factor of ten.
This research supports further isotope engineering efforts in boron arsenide, positioning it as a potential semiconductor platform for quantum phononics.