Collisionless and hydrodynamic excitations of trapped boson-fermion mixtures

TL;DR

This paper analyzes collective excitations in trapped boson-fermion mixtures, revealing mode behaviors in different regimes and deriving analytic solutions for weak to medium interactions.

Contribution

It introduces a combined scaling ansatz and Thomas-Fermi approach to study monopole and quadrupole modes in various trapping geometries and interaction regimes.

Findings

Monopole mode frequency is consistent in collisionless and hydrodynamic regimes.

Quadrupole mode frequency varies significantly between regimes.

Hydrodynamic quadrupole oscillations with equal amplitudes form an exact eigenstate.

Abstract

Within a scaling ansatz formalism plus Thomas-Fermi approximation, we investigate the collective excitations of a harmonically trapped boson-fermion mixture in the collisionless and hydrodynamic limit at low temperature. Both the monopole and quadrupole modes are considered in the presence of spherical as well as cylindrically symmetric traps. In the spherical traps, the frequency of monopole mode coincides in the collisionless and hydrodynamic regime, suggesting that it might be undamped in all collisional regimes. In contrast, for the quadrupole mode, the frequency differs largely in these two limits. In particular, we find that in the hydrodynamic regime the quadrupole oscillations with equal bosonic and fermionic amplitudes generate an exact eigenstate of the system, regardless of the boson-fermion interaction. This resembles the Kohn mode for the dipole excitation. We discuss in some detail the behavior of monopole and quadrupole modes as a function of boson-fermion coupling at different boson-boson interaction strength. Analytic solutions valid at weak and medium fermion-boson coupling are also derived and discussed.