High Temperature Virial Expansion to Universal Quench Dynamics.

Area of Science:

• Statistical mechanics
• Quantum gases
• Non-equilibrium dynamics

Background:

• High temperature virial expansion is established for equilibrium systems.
• Understanding far-from-equilibrium dynamics, especially after quantum quenches, remains a challenge.

Purpose of the Study:

• Generalize the high temperature virial expansion to analyze non-equilibrium quench dynamics.
• Investigate the dynamics of a Bose gas quenched to unitarity.
• Compare theoretical predictions with experimental results.

Main Methods:

• Generalized high temperature virial expansion framework.
• Analysis of Bose gas quench dynamics from noninteracting to unitarity.
• Comparison with experimental data on momentum distribution and relaxation times.

Main Results:

• The momentum distribution shifts: decreasing at low momentum (k < k*) and increasing at high momentum (k > k*).
• A universal value for k*λ (k* times thermal de Broglie wavelength) was determined, matching experimental findings.
• Observed a jump in relaxation time and nonmonotonic energy distribution, consistent with experiments.
• Identified partial thermalization in the long-time steady state: thermalization for low energies (kλ≲1) but not for high momentum tails (kλ≫1).

Conclusions:

• The generalized virial expansion provides a successful theoretical framework for far-from-equilibrium quench dynamics.
• The theory accurately reproduces key experimental observations in a quenched Bose gas.
• The study reveals distinct thermalization behavior in different momentum regimes of the steady state.