Non-Unitary Fermionic Quasinormal Modes at Zero Frequency

TL;DR

This paper investigates fermionic quasinormal modes at zero frequency in a novel gravity background dual to non-commutative Yang-Mills theory, revealing complex operator dimensions and the absence of zero frequency quasinormal modes, impacting holographic Fermi liquid models.

Contribution

It introduces a new supergravity solution dual to non-commutative Yang-Mills theory and analyzes fermionic dynamics, showing complex operator dimensions and implications for Fermi surface properties.

Findings

No zero frequency quasinormal modes due to complex operator dimensions.

Operator dimensions in AdS2 are complex conjugates, affecting unitarity.

Fermi energy may not be uniquely determined by chemical potential.

Abstract

We consider the dynamics of a probe fermion charged under a U(1) Maxwell field and a two form potential $B_{(2)}$ in a five dimensional gravity background. The gravity background is constructed from a new solution we find of type IIB supergravity. This new solution is expected to be dual to non-commutative Yang-Mills theory in the 't Hooft limit with global U(1) currents. We study the zero frequency, near horizon behavior of the fermion, where the equations of motion reduce to that of two interacting fermions in AdS$_2$ with an electric field. We show that the operator dimensions in the AdS$_2$ space are complex, leading to the two components of the retarded Green's function in the dual theory to be complex conjugates of each other. In order to preserve unitarity, this result implies there are no zero frequency quasinormal modes in our system. This has important implications for generalizations of recent holographic Fermi liquid setups with AdS$_2$ regions, as it suggests that infinite lifetime excitations can have energies above/below the chemical potential. Therefore, the Fermi energy may not be uniquely set by the chemical potential. Furthermore, since the gravity background breaks rotational symmetry along the spatial directions of the dual Yang-Mills theory, we do not expect the Fermi surface to be spherical in shape in momentum space.