# Relative-Zeta and Casimir energy for a semitransparent hyperplane selecting transverse modes

## TL;DR

This paper analyzes the relative zeta function and thermal Casimir energy for a semitransparent hyperplane with mode-dependent interactions, providing explicit formulas especially when the interaction is mode-restricted.

## Contribution

It introduces a novel approach to studying the relative zeta function and Casimir energy for hyperplanes with mode-dependent delta interactions, extending previous models.

## Key findings

- Explicit expression for the relative zeta function in this setting
- Derivation of the thermal Casimir energy for the hyperplane model
- Closed-form solutions when the interaction is mode-restricted

## Abstract

We study the relative zeta function for the couple of operators $A_0$ and $A_\alpha$, where $A_0$ is the free unconstrained Laplacian in $L^2(\mathbf{R}^d)$ ($d \geq 2$) and $A_\alpha$ is the singular perturbation of $A_0$ associated to the presence of a delta interaction supported by a hyperplane. In our setting the operatorial parameter $\alpha$, which is related to the strength of the perturbation, is of the kind $\alpha=\alpha(-\Delta_{\parallel})$, where $-\Delta_{\parallel}$ is the free Laplacian in $L^2(\mathbf{R}^{d-1})$. Thus $\alpha$ may depend on the components of the wave vector parallel to hyperplane; in this sense $A_\alpha$ describes a semitransparent hyperplane selecting transverse modes. As an application we give an expression for the associated thermal Casimir energy. Whenever $\alpha=\chi_{I}(-\Delta_{\parallel})$, where $\chi_{I}$ is the characteristic function of an interval $I$, the thermal Casimir energy can be explicitly computed.