Beyond minimal coupling for charged scalars? Modified electrodynamics and London-penetration tests

TL;DR

This paper proposes a modified electrodynamics framework for charged scalar fields that predicts a rescaled magnetic penetration depth in superconductors, and tests this prediction against experimental data from various materials.

Contribution

It introduces a non-gauge-invariant coupling approach for scalar electrodynamics and analyzes its implications for superconducting properties and experimental measurements.

Findings

Rescaled magnetic penetration depth $ o rac{ ext{original}}{ oot 2 ext{}}$ in the modified framework.

Experimental data for Nb, YBCO, and Ba(Fe,Co)$_2$As$_2$ support the hypothesis $ ext{optical} > ext{magnetic}$ penetration depth.

Standard theory predicts $ ext{optical} o ext{magnetic}$ depth equality, contrasting with the modified model.

Abstract

While standard minimal coupling works well for Dirac fermions, its application to scalar fields features a known ``peculiarity'': the term linear in $A_\mu$ does not coincide with the conserved Noether current of the interacting theory. We recently proposed choosing a different principle for electromagnetic interactions, namely a linear coupling $A_\mu J^\mu$ with $J^\mu$ a (globally) conserved current, accepting the consequence that one must abandon full local gauge invariance in the electromagnetic sector and adopt an extended electrodynamics (of Aharonov--Bohm type) that can couple consistently to non-locally-conserved currents. We present the physical motivations offered for proposing the modified coupling and discuss general consequences of reducing gauge invariance. We then focus on the central condensed-matter claim: for bosonic charged condensates, the modified framework predicts a rescaled magnetic penetration depth $\lambda \to \lambda/\sqrt{2}$, while leaving other key qualitative features of superconducting electrodynamics and the type-I/type-II distinction unchanged (up to an equivalent rescaling of the GL parameter). Finally, we analyze experimental data for a London-length consistency check based on independent measurements of the ratio $n_s/m^\ast$ between carrier density and effective mass. We compare for five materials an ``optical'' penetration depth $\lambda_{\mathrm{opt}}$ inferred from IR/THz superfluid spectral weight with a ``magnetic'' depth $\lambda_{\mathrm{mag}}$ obtained independently (LE-$\mu$SR, TF-$\mu$SR, microwave methods, etc.). Data for Nb, YBCO and Ba(Fe,Co)$_2$As$_2$ confirm the hypothesis $\lambda_{\mathrm{opt}}>\lambda_{\mathrm{mag}}$, with a ratio not far from 1.4; data for Pb are inconclusive while data for MgB$_2$ indicate $\lambda_{\mathrm{opt}}\simeq\lambda_{\mathrm{mag}}$ as predicted by the standard theory.