Riemann-Cartan holography and conductivity

TL;DR

This paper explores how a Riemann-Cartan bulk with torsion influences boundary conductivity in holographic duality, suggesting non-minimal torsion couplings better explain experimental conductivity data.

Contribution

It introduces a holographic model with torsion in the bulk and analyzes its impact on boundary conductivity, highlighting the significance of non-minimal torsion couplings.

Findings

Bulk torsion induces spin current at the boundary.

Non-minimal torsion couplings better match experimental conductivity.

Torsion effects modify boundary electromagnetic responses.

Abstract

Generalizing the usual setup for holographic duality, where bulk spacetime is described by pseudo-Riemannian geometry, we consider a Riemann-Cartan bulk with non-trivial torsion as a background for an Abelian bulk gauge field dual to $U(1)$ boundary current. Working in the probe limit, we explore how the bulk torsion, which induces spin current at the boundary, affects the conductivity of the boundary theory. We consider standard types of non-minimal couplings between torsion and the electromagnetic field found in the literature, and the results seem to suggest that these torsion couplings are more suitable candidates, compared to the common minimal coupling regime, for a holographic explanation of the existing experimental findings regarding conductivity.