Collective modes of polarizable holographic media in magnetic fields

TL;DR

This paper investigates collective electromagnetic modes in a neutral holographic plasma under magnetic fields, revealing new plasmon-like modes and analyzing the limits of hydrodynamic descriptions at strong magnetic fields.

Contribution

It introduces mixed boundary conditions for Maxwell fields in holography to study collective modes at finite wave-vector and magnetic fields, uncovering novel plasmon-like modes and mode collisions.

Findings

Discovery of two plasmon-like modes with magnetic field-dependent frequencies

Identification of a mode collision affecting hydrodynamic expansion convergence

The radius of convergence increases quadratically with magnetic field strength

Abstract

We consider a neutral holographic plasma with dynamical electromagnetic interactions in a finite external magnetic field. The Coulomb interactions are introduced via mixed boundary conditions for the Maxwell gauge field. The collective modes at finite wave-vector are analyzed in detail and compared to the magneto-hydrodynamics results valid only at small magnetic fields. Surprisingly, at large magnetic field, we observe the appearance of two plasmon-like modes whose corresponding effective plasma frequency grows with the magnetic field and is not supported by any background charge density. Finally, we identify a mode collision which allows us to study the radius of convergence of the linearized hydrodynamics expansion as a function of the external magnetic field. We find that the radius of convergence in momentum space, related to the diffusive transverse electromagnetic mode, increases quadratically with the strength of the magnetic field.