Density profiles in a quantum Coulomb fluid near a hard wall

TL;DR

This paper analyzes quantum Coulomb fluids near a hard wall, deriving analytical density profiles and electrostatic potentials considering quantum effects, screening, and mass differences, revealing quantum surface charges and their decay behaviors.

Contribution

It introduces a systematic method to compute density profiles and potentials in quantum Coulomb fluids near walls, incorporating quantum effects and screening.

Findings

Density profiles vanish Gaussianly near the wall over de Broglie wavelengths.

Quantum surface charge appears proportional to Planck's constant .

Electrostatic potential decays exponentially over the classical Debye length.

Abstract

Equilibrium particle densities near a hard wall are studied for a quantum fluid made of point charges which interact via Coulomb potential without any regularization. In the framework of the grand-canonical ensemble, we use an equivalence with a classical system of loops with random shapes, based on the Feynman-Kac path-integral representation of the quantum Gibbs factor. After systematic resummations of Coulomb divergences in the Mayer fugacity expansions of loop densities, there appears a screened potential $\phi$. It obeys an inhomogeneous Debye-H\"uckel equation with an effective screening length which depends on the distance from the wall. The formal solution for $\phi$ can be expanded in powers of the ratios of the de Broglie thermal wavelengths $\laa$'s of each species $\alpha$ and the limit of the screening length far away from the wall. In a regime of low degeneracy and weak coupling, exact analytical density profiles are calculated at first order in two independent parameters. Because of the vanishing of wave-functions close to the wall, density profiles vanish gaussianly fast in the vicinity of the wall over distances $\laa$'s, with an essential singularity in Planck constant $\hbar$. When species have different masses, this effect is equivalent to the appearance of a quantum surface charge localized on the wall and proportional to $\hbar$ at leading order. Then, density profiles, as well as the electrostatic potential drop created by the charge-density profile, also involve a term linear in $\hbar$ andwhich decays exponentially fast over the classical Debye screening length $\xid$. The corresponding contribution to the global surface charge exactly compensates the charge in the very vicinity of the surface, so that the net electric field vanishes in the bulk, as it should.