Single-Particle Spectral Density of a Bose Gas in the Two-Fluid Hydrodynamic Regime

TL;DR

This paper derives an explicit expression for the single-particle spectral density in a Bose superfluid's two-fluid hydrodynamic regime, analyzing the contributions of first and second sound modes and their experimental detectability.

Contribution

It provides a simplified derivation of Hohenberg and Martin's 1965 results and explores the temperature-dependent spectral contributions of sound modes in a Bose gas.

Findings

Second sound mode dominates the spectral density at higher temperatures.

The spectral density's relative weights of sound modes vary with temperature.

Experimental observation of second sound via photoemission spectroscopy is discussed.

Abstract

In Bose supefluids, the single-particle Green's function can be directly related to the superfluid velocity-velocity correlation function in the hydrodynamic regime. An explicit expression for the single-particle spectral density was originally written down by Hohenberg and Martin in 1965, starting from the two-fluid equations for a superfluid. We give a simple derivation of their results. Using these results, we calculate the relative weights of first and second sound modes in the single-particle spectral density as a function of temperature in a uniform Bose gas. We show that the second sound mode makes a dominant contribution to the single-particle spectrum in relatively high temperature region. We also discuss the possibility of experimental observation of the second sound mode in a Bose gas by photoemission spectroscopy.