# Holographic response from higher derivatives with homogeneous disorder

## TL;DR

This paper investigates the charge response in a holographic system with higher derivatives and homogeneous disorder, revealing phase crossover behaviors and effects of disorder on conductivity spectra.

## Contribution

It analyzes bounds on higher derivatives coupling, explores optical conductivity behavior, and uncovers disorder-induced phase transitions and spectral features in holographic models.

## Key findings

- No tighter bounds on coupling than Schwarzschild-AdS background.
- Crossover from coherent to incoherent metallic phase with increasing disorder.
- Disorder can induce a soft gap and suppress peaks in conductivity spectra.

## Abstract

In this letter, we study the charge response from higher derivatives over the background with homogeneous disorder introduced by axions. We first explore the bounds on the higher derivatives coupling from DC conductivity and the anomalies of causality and instabilities. Our results indicate no tighter constraints on the coupling than that over Schwarzschild-AdS (SS-AdS) background. And then we study the optical conductivity of our holographic system. We find that for the case with $\gamma_1<0$ and the disorder strength $\hat{\alpha}<2/\sqrt{3}$, there is a crossover from a coherent to incoherent metallic phase as $\hat{\alpha}$ increases. When $\hat{\alpha}$ is beyond $\hat{\alpha}=2/\sqrt{3}$ and further amplified, a peak exhibits again at low frequency. But it cannot be well fitted by the standard Drude formula and new formula for describing this behavior shall be called for. While for the holographic system with the limit of $\gamma_1\rightarrow 1/48$, the disorder effect drives the hard-gap-like at low frequency into the soft gap and suppresses the pronounced peak at medium frequency.