May 30: “T-linear resistivity, optical conductivity and Planckian transport for a holographic local quantum critical metal in a periodic potential” by Koenraad Schalm

Abstract: High Tc cuprate strange metals are noted for a DC-resistivity that scales linearly with T from the onset of superconductivity to the crystal melting temperature, indicative of a Planckian dissipation life time τℏ≃ℏ/(kBT). At the same time, the optical conductivity ceases to be of the Drude form at high temperatures, suggesting a change in dynamics that surprisingly leaves the T-linear DC-resistivity unaffected. We use the AdS/CFT correspondence that describes strongly coupled, densely entangled states of matter to study the DC and optical conductivities of the local quantum critical Gubser-Rocha holographic strange metal in 2+1D in the presence of a lattice potential, a prime candidate to compare with experiment. We find that the DC-resistivity is linear in T at low temperatures for a range of potential strengths and wavevectors, even as it transitions between different dissipative regimes. At weak lattice potential the optical conductivity evolves with temperature from a Drude form to a bad metal characterized by a mid-IR resonance without changing the DC transport, similar to that seen in cuprate strange metals. This mid-IR peak can be understood as a consequence of Umklapp hydrodynamics: i.e. hydrodynamic perturbations are Bloch modes in the presence of a lattice. At strong lattice potential an incoherent metal is realized where momentum conservation no longer plays a role in transport. In this regime the thermal diffusivity appears insensitive to the breaking of translations and can be explained by Planckian dissipation originating in universal microscopic chaos. The charge diffusivity cannot be explained this way, though the continuing linear-in-T DC resistivity saturates to an apparent universal slope, numerically equal to a Planckian rate. We conjecture that this may originate in chaos properties that differ between charged and neutral operators.