Anomalous magnetoconductivity and relaxation times in holography

TL;DR

This paper investigates how axial anomaly-induced magnetoconductivity behaves in strongly coupled holographic models with explicit axial charge breaking, revealing universal features and the persistence of positive magnetoconductivity.

Contribution

It introduces holographic models with explicit axial charge breaking and derives analytical expressions for DC magnetoconductivity, highlighting universal behavior and the impact of symmetry breaking.

Findings

DC magnetoconductivities have a universal form depending on gauge field mass and magnetic field.

Axial charge relaxation time increases linearly with magnetic field at large B.

Positive magnetoconductivity persists even with short relaxation times and explicit symmetry breaking.

Abstract

We study the magnetoconductivity induced by the axial anomaly via the chiral magnetic effect in strongly coupled holographic models. An important ingredient in our models is that the axial charge is non-conserved beyond the axial anomaly. We achieve this either by explicit symmetry breaking via a non-vanishing non-normalisable mode of an axially charged scalar or using a Stuckelberg field to make the AdS-bulk gauge field massive. The DC magnetoconductivites can be calculated analytically. They take a universal form in terms of gauge field mass at the horizon and quadratic dependence on the magnetic field. The axial charge relaxation time grows linearly with magnetic field in the large $B$ regime. Most strikingly positive magnetoconductivity is still present even when the relaxation times are short $\tau_5 \approx 1/(\pi T)$ and the axial charge can not be thought of as an approximate symmetry. In the $U(1)_A$ explicit breaking model, we also observe that the axial magnetic conductivity in the limit of strong symmetry breaking approaches the same universal value as for anomalous holographic superconductors in the zero temperature limit.