Isoelectronic determination of the thermal Casimir force

TL;DR

This study measures the thermal Casimir force using differential force experiments with nickel and gold coatings, comparing results to theoretical models to determine the most accurate description of the force.

Contribution

It provides the first experimental comparison of the thermal Casimir force with Drude and plasma models, highlighting the importance of magnetic properties in the theoretical description.

Findings

Drude model with magnetic properties ruled out

Plasma model with magnetic properties agrees with data

Magnetic properties significantly affect Casimir force predictions

Abstract

Differential force measurements between spheres coated with either nickel or gold and rotating disks with periodic distributions of nickel and gold are reported. The rotating samples are covered by a thin layer of titanium and a layer of gold. While titanium is used for fabrication purposes, the gold layer (nominal thicknesses of 21, 37, 47 and 87 nm) provides an isoelectronic environment, and is used to nullify the electrostatic contribution but allow the passage of long wavelength Casimir photons. A direct comparison between the experimental results and predictions from Drude and plasma models for the electrical permittivity is carried out. In the models the magnetic permeability of nickel is allowed to change to investigate its effects. Possible sources of errors, both in the experimental and theoretical sides, are taken into account. It is found that a Drude response with magnetic properties of nickel taken into account is unequivocally ruled out. The full analysis of the data indicates that a dielectric plasma response with magnetic properties of Ni included shows good agreement with the data. Neither a Drude nor a plasma dielectric response provide a satisfactory description if the magnetic properties of nickel are disregarded.