We introduce Coulomb interactions in the holographic description of strongly interacting systems by performing a (current-current) double-trace deformation of the boundary theory. In the theory dual to a Reissner-Nordström background, this deformation leads to gapped plasmon modes in the density-density response, as expected from conventional RPA calculations. We further show that by introducing a (d + 1)-dimensional Coulomb interaction in a boundary theory in d spacetime dimensions, we recover plasmon modes whose dispersion is proportional to |k|, as observed for example in graphene layers. Moreover, motivated by recent experimental results in layered cuprate high-temperature superconductors, we present a toy model for a layered system consisting of an infinite stack of (spatially) two-dimensional layers that are coupled only by the long-range Coulomb interaction. This leads to low-energy 'acoustic plasmons'. Finally, we compute the optical conductivity of the deformed theory in d = 3 + 1, where a logarithmic correction is present, and we show how this can be related to the conductivity measured in Dirac and Weyl semimetals.

We study various thermodynamic and transport properties of a holographic model of a nodal line semimetal (NLSM) at finite temperature, including the quantum phase transition to a topologically trivial phase, with Dirac semimetal-like conductivity. At zero temperature, composite fermion spectral functions obtained from holography are known to exhibit multiple Fermi surfaces. Similarly, for the holographic NLSM we observe multiple nodal lines instead of just one. We show, however, that as the temperature is raised these nodal lines broaden and disappear into the continuum one by one, so there is a finite range of temperatures for which there is only a single nodal line visible in the spectrum. We compute several transport coefficients in the holographic NLSM as a function of temperature, namely the charge and thermal conductivities, and the shear viscosities. By adding a new non-linear coupling to the model we are able to control the low frequency limit of the electrical conductivity in the direction orthogonal to the plane of the nodal line, allowing us to better match the conductivity of real NLSMs. The boundary quantum field theory is anisotropic and therefore has explicitly broken Lorentz invariance, which leads to a stress tensor that is not symmetric. This has important consequences for the energy and momentum transport: the thermal conductivity at vanishing charge density is not simply fixed by a Ward identity, and there are a much larger number of independent shear viscosities than in a Lorentz-invariant system.

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We study the Coulomb drag between two strange-metal layers using the Einstein-Maxwell-Dilaton model from holography. We show that the low-temperature dependence of the drag resistivity is ρD ∝ T 4 , which strongly deviates from the quadratic dependence of Fermi liquids. We also present numerical results at room temperature, using typical parameters of the cuprates, to provide an estimate of the magnitude of this effect for future experiments. We find that the drag resistivity is enhanced by the plasmons characteristic of the two-layer system.