Dynamics of two-dimensional dipole systems

TL;DR

This paper investigates the collective mode behavior and fluctuation spectra of a two-dimensional classical dipole system using analytical and molecular dynamics methods, revealing universal roton-maxon features linked to strong correlations.

Contribution

It provides the first combined analytical and MD analysis of 2D dipole systems, highlighting the emergence of roton-maxon structures in their dispersion relations.

Findings

Deep minima in dispersion curves at high wave numbers

Observation of roton-maxon excitation spectra

Detection of combination frequencies resembling superfluid helium

Abstract

Using a combined analytical/molecular dynamics (MD) approach, we study the current fluctuation spectra and longitudinal and transverse collective mode dispersions of the classical two-dimensional (point) dipole system (2DDS) characterized by the $\phi _{D}(r)=\mu^2/r^3$ repulsive interaction potential; $\mu$ is the electric dipole strength. The interest in such two-dimensional dipole systems (2DDS) is twofold. First, the quasi-long range $1/r^{3}$ interaction makes the system a unique classical many body system, with a remarkable collective mode behavior. Second, the system may be a good model for a closely spaced semiconductor electron-hole bilayer, a system that is in the forefront of current experimental interest. The longitudinal collective excitations, which are of primary interest for the liquid phase, are acoustic at long wavelengths. At higher wave numbers and for sufficiently high coupling strength, we observe the formation of a deep minimum in the dispersion curve preceded by a sharp maximum; this is identical to what has been observed in the dispersion of the zero temperature bosonic dipole system, which in turn emulates so called roton-maxon excitation spectrum of the superfluid $^4$He. The analysis we present gives an insight into the emergence of this apparently universal structure, governed by strong correlations. We study both the liquid and the crystalline solid state. We also observe the excitation of combination frequencies, resembling the roton-roton, roton-maxon, etc. structures in $^4$He.