Vacancy excitation spectrum in solid 4He and longitudinal phonons

Area of Science:

• Quantum physics
• Condensed matter physics
• Materials science

Background:

• Solid helium-4 (4He) exhibits unique quantum phenomena due to its low mass and weak interatomic interactions.
• Understanding point defects, such as vacancies, is crucial for comprehending the thermodynamic and dynamic properties of solid 4He.
• Previous studies have often relied on macroscopic models or less computationally intensive methods.

Purpose of the Study:

• To conduct the first microscopic ab initio calculation of the excitation spectrum of a vacancy in solid 4He.
• To determine the energy-wave vector dispersion relation for vacancies at melting density.
• To compute the effective mass of vacancies and compare calculated phonon spectra with experimental data.

Main Methods:

• Microscopic ab initio calculations were employed, representing a first-principles approach.
• The shadow wave function variational technique was developed and utilized for the computations.
• Calculations were performed at the melting density of solid 4He.

Main Results:

• The excitation spectrum of a vacancy was determined, yielding a bandwidth of 6–10 K in hexagonal close-packed (hcp) solid 4He.
• The effective mass of the vacancy was found to be approximately 0.35 times the mass of a 4He atom.
• The computed spectrum of longitudinal phonons showed good agreement with recent experimental findings.

Conclusions:

• The study provides a detailed microscopic understanding of vacancy excitations in solid 4He.
• The calculated effective mass and excitation bandwidth offer valuable parameters for theoretical models of quantum solids.
• The agreement with experimental phonon data validates the employed ab initio methodology.