Thermoelectric transport in holographic quantum matter under shear strain

Flat band electronic systems exhibit a rich landscape of correlation-driven phases, both at the charge neutrality and finite electronic density, featuring exotic electromagnetic and thermodynamic responses. Motivated by these developments, in this paper, we explicitly include the effects of the chemical potential in a holographic model featuring approximately flat bands. In particular, we explore the phase diagram of this holographic flat band system as a function of the chemical potential. We find that at low temperatures and densities, the system features a nematic phase, transitioning into the Lifshitz phase as the chemical potential or temperature increases. To further characterize the ensuing phases, we investigate the optical conductivity and find that this observable shows strong anisotropies in the nematic phase.

Probing holographic flat bands at finite density

Grandi,

Juričić,

Landea

et al. 2024

Mechanical stability of homogeneous holographic solids under finite shear strain

Baggioli,

Li,

et al. 2024

We study the linear stability of holographic homogeneous solids (HHS) at finite temperature and in presence of a background shear strain by means of a large scale quasi-normal mode analysis which extends beyond the hydrodynamic limit. We find that mechanical instability can arise either as a result of a complex speed of sound — gradient instability — or of a negative diffusion constant. Surprisingly, the simplest HHS models are linearly stable for arbitrarily large values of the background strain. For more complex HHS, the onset of the diffusive instability always precedes that of the gradient instability, which becomes the dominant destabilizing process only above a critical value of the background shear strain. Finally, we observe that the critical strains for the two instabilities approach each other at low temperatures. We conclude by presenting a phase diagram for HHS as a function of temperature and background shear strain which shows interesting similarities with the physics of superfluids in presence of background superfluid velocity.

Breaking rotations without violating the KSS viscosity bound

Baggioli

Cremonini

Early

et al. 2023

We revisit the computation of the shear viscosity to entropy ratio in a holographic p-wave superfluid model, focusing on the role of rotational symmetry breaking. We study the interplay between explicit and spontaneous symmetry breaking and derive a simple horizon formula for η/s, which is valid also in the presence of explicit breaking of rotations and is in perfect agreement with the numerical data. We observe that a source which explicitly breaks rotational invariance suppresses the value of η/s in the broken phase, competing against the effects of spontaneous symmetry breaking. However, η/s always reaches a constant value in the limit of zero temperature, which is never smaller than the Kovtun-Son-Starinets (KSS) bound, 1/4π. This behavior appears to be in contrast with previous holographic anisotropic models which found a power-law vanishing of η/s at small temperature. This difference is shown to arise from the properties of the near-horizon geometry in the extremal limit. Thus, our construction shows that the breaking of rotations itself does not necessarily imply a violation of the KSS bound.