Charge density response and fake plasmons in holographic models with strong translation symmetry breaking

Abstract

We study the charge density response in holographic models with explicit translation symmetry breaking which is relevant in the IR; in particular, we focus on Q-lattices and the Bianchi VII helix. We show that the hydrodynamic sound mode is removed from the spectrum due to the strong momentum relaxation and therefore, the usual treatment of the plasmon as Coulomb-dressed zero sound does not apply. Furthermore, the dominant coherent modes in the longitudinal channel, which control the neutral density-density correlator, are the diffusive modes. We show these modes are strongly suppressed when the boundary Coulomb interaction is turned on. This renders the low frequency charge density response spectrum completely incoherent and featureless.At intermediate frequencies, we observe a broad feature -the fake plasmonin the dressed correlator, which could be confused with an overdamped plasmon. However, its gap is set by the scale of translation symmetry breaking instead of the plasma frequency. This broad feature originates from the non-hydrodynamic sector of the holographic spectrum, and therefore, its behaviour, typical of strongly correlated quantum critical systems with holographic duals, deviates from the standard Fermiliquid paradigm.

On the hydrodynamics of (2 + 1)-dimensional strongly coupled relativistic theories in an external magnetic field

“…In the earliest approaches this was achieved through relaxing the equation of motion for the ideal charged relativistic fluid by means of an explicit effective mechanism of momentum dissipation [22,24,25,68,69]. These results have been compared against many holographic realisations with explicit translation symmetry breaking, spanning: massive gravity models [28,[70][71][72][73][74][75], axion models [38,[76][77][78][79][80][81][82][83][84][85][86], and striped theories [87][88][89][90][91][92]. More recently the holographic-hydrodynamics community has been pursuing methods to make the momentum relaxation mechanism either spontaneous or pseudo-spontaneous [93], opening the path to the construction of hydrodynamic theories of phonons, pseudo-phonons and charge density waves [14,21,29,[94][95][96][97][98].…”

“…More recently the holographic-hydrodynamics community has been pursuing methods to make the momentum relaxation mechanism either spontaneous or pseudo-spontaneous [93], opening the path to the construction of hydrodynamic theories of phonons, pseudo-phonons and charge density waves [14,21,29,[94][95][96][97][98]. These hydrodynamic theories have been successfully compared to various holographic models [31,88,94,96,, and applied to interpret experimental the transport results of some strongly coupled condensed matter systems such as bad metals and cuprates.…”

Transverse collective modes in interacting holographic plasmas

“…In more specific terms, the quasi-particle nature of plasmons, and the idea of the Lindhard continuum, are not manifest in these cases. These new exciting experimental results stimulated a lot of activity in the holographic community [6][7][8][9][10][11][12][13][14], driven by the hope of describing these new phenomena within the framework of strongly coupled theories and their gravitational duals. A first positive outcome of this program has been the observation of an anomalous attenuation, or damping, of the plasmon mode at k = 0, in contrast to the result from Fermi-liquid theory where the mode is protected from decay by the absence of any available decay channel.…”

Density response of holographic metallic IR fixed points with translational pseudo-spontaneous symmetry breaking

“…Moreover, the existence of a non-Drude-like optical conductivity in these materials is currently been revisited by various experimental groups. In order to connect with experiments, it would be interesting to find holographic models where the AC-conductivity is Drude-like, but the dressed density response does not display big and narrow peaks [51]. It would also be interesting to explore holographically the mismatch between the longitudinal and transverse density responses in the low momentum regime observed in cuprates [52].…”