December 21, 2021
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Perceiving Quantum Pathways

PNNL researchers elaborate phonon energy transport in quantum materials

Artist's rendering of phonons

A new simulation from Pacific Northwest National Laboratory demonstrates that grain boundaries hinder phonon transport by channeling phonons into colliding with each other.

The Science

To create the most efficient quantum computers and better detect dark matter, researchers need to understand how carriers of energy, called phonons, transport information through ultra-low temperatures. However, it is difficult to characterize a single phonon because its behavior is buried within a material’s atomic structure. Materials used in quantum computers and dark matter detectors have structures made of complex crystal lattices arranged from atoms of different elements. In these crystal lattices, phonon behaviors are often changed by tiny defects, interfaces, and boundaries. Understanding this behavior becomes more important as temperatures drop and individual phonon behavior has a larger impact. However, little is known about phonon behavior at ultra-low temperatures. A team of researchers at Pacific Northwest National Laboratory led by Dr. Anne M. Chaka, PhD, developed a simulation approach to identify how atomic structures of a heterojunction can affect the phonon transport of energy and information in quantum systems near absolute zero temperatures.

The Impact

The team developed a simulation approach that can efficiently model phonon movement in crystal lattices at ultra-low temperatures in large systems with millions of atoms. The simulation contributes to filling the gap in understanding phonon and other quasi-particle physics in the milli-Kelvin range at the atomic level. It could be used to guide material manufacture and engineering through predicting phonon energy and information transport for quantum computing and other ultrasensitive detectors.

Summary

While atomistic modeling grants researchers access to detailed illustrations of single-particle behaviors, a new challenge emerges at ultra-low temperatures. Very large simulation systems are required to faithfully reproduce the physics in the milli-Kelvin range where quasi-particles like phonons have long wavelengths on the order of microns. Inspired by the wave packet formalism, the researchers applied the phonon wave packet approach in a molecular dynamics simulation framework where single phonons are modeled in the form of short envelopes of atomic movements. This allowed the researchers to avoid an impractically large system size by extending the system only along the phonon propagation directions. By keeping record of the atomic movements during the simulation, the researchers could directly identify and illustrate phonon propagation and scattering, including as phonons interacted with defects, interfaces, and heterojunctions. To illustrate the effect of atomic structure differences on the phonon transport, the researchers applied their model to a realistic silicon and aluminum heterojunction as fabricated by cutting-edge crystal growth techniques widely used in quantum materials. The researchers found that detailed atomic structures like surface reconstructions, defects, and grain boundaries prevent certain types of phonons from crossing an interface but are more permissive to others. For example, certain structures enhance phonon transport by enabling more phonon modes, but grain boundaries hinder phonon transport by channeling phonons into colliding with each other. This finding suggests avenues for achieving selective energy and information transport through atomically precise fabrication techniques that can control the interfacial structures of materials to improve cryogenic device engineering. The model itself demonstrates a powerful method to enhance understanding of phonon transport at the atomic scale in ultra-low temperatures.

PNNL Contact

Zexi Lu, Pacific Northwest National Laboratory, zexi.lu@pnnl.gov

Funding

This research was supported by the Laboratory Directed Research and Development program and the Chemical Dynamics Initiative at Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated by Battelle for the Department of Energy under contract No. DE-AC05-76RL01830. The computational resources were provided by PNNL Institutional Computing.

Published: December 21, 2021

Z. Lu, N.P. Smith, M.P. Prange, R.A. Bunker, J.L. Orrell, and A.M. Chaka, 2021. “Effect of interfacial structures on phonon transport across atomically precise Si/Al heterojunctions”, Physical Review Materials, 5, 086002, PNNL-SA-163751.