High-Tc cuprate strange metals are characterized by a DC resistivity that scales linearly with T from the onset of superconductivity to the crystal melting temperature, characterized by a current... Show moreHigh-Tc cuprate strange metals are characterized by a DC resistivity that scales linearly with T from the onset of superconductivity to the crystal melting temperature, characterized by a current life time τℏ≃ℏ/(kBT), the “Planckian dissipation”. At the same time, the optical conductivity ceases to be of the Drude form at high temperatures, suggesting a change of the underlying dynamics that surprisingly leaves the T-linear DC resistivity unaffected. We use the AdS/CFT correspondence that describes strongly coupled, densely many-body entangled metallic states of matter to study the DC thermoelectrical transport properties and the optical conductivities of the local quantum critical Gubser-Rocha holographic strange metal in 2+1 dimensions in the presence of a lattice potential, a prime candidate to compare with experiment. We find that the electrical DC resistivity is linear in T at low temperatures for a large range of potential strengths and wave vectors, even as it transitions between different dissipative regimes. At weak lattice potential the optical conductivity evolves as a function of increasing 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 and notably its temperature evolution can be fully 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 instead where momentum conservation no longer plays a role in the transport. We confirm that in this regime the thermal diffusivity appears to be insensitive to the breaking of translations and can be explained by Planckian dissipation originating in universal microscopic chaos. A similar behavior has been found for holographic metals with strong homogeneous momentum relaxation. The charge diffusivity does not submit to this chaos explanation, even though the continuing linear-in-T DC resistivity saturates to an apparent universal slope, numerically equal to a Planckian rate. Show less
We consider the propagation of electrons in a lattice with an anisotropic dispersion in the x -y plane (lattice constant a), such that it supports open orbits along the x axis in an out-of-plane... Show moreWe consider the propagation of electrons in a lattice with an anisotropic dispersion in the x -y plane (lattice constant a), such that it supports open orbits along the x axis in an out-of-plane magnetic field B. We show that a point source excites a "breathing mode," a state that periodically spreads out and refocuses after having propagated over a distance . pound = (eaB/h)-1 in the x direction. Unlike known magnetic focusing effects, governed by the classical cyclotron radius, this is an intrinsically quantum mechanical effect with a focal length oc h over bar. Show less
We present a method to calculate the optical conductivity of semi-Dirac and pseudospin models based on the evaluation of quasiparticle velocity correlators which also describe the phenomenon of... Show moreWe present a method to calculate the optical conductivity of semi-Dirac and pseudospin models based on the evaluation of quasiparticle velocity correlators which also describe the phenomenon of Zitterbewegung. Applying this method to the semi-Dirac model with merging Dirac cones and gapped dice and Lieb lattice models we find exact analytical expressions for optical longitudinal and Hall conductivities. For the semi-Dirac model the obtained expressions allow us to analyze the role of spectrum anisotropy, Van Hove singularities, and Dirac cones in longitudinal conductivity. In addition, we predict signatures of topological phase transition with changing gap parameter in such a system that are manifested in dc transport at low temperatures. For the dice and Lieb lattices we emphasize the role of the spectral gap, which defines frequency thresholds related to transitions to and from a flat band. Show less
Heumen, E. van; Feng, X.B.; Cassanelli, S.; Neubrand, L.; Jager, L. de; Berben, M.; ... ; Zaanen, J. 2022
Unlocking the mystery of the strange metal state has become the focal point of high-Tcresearch, not because of its importance for superconductivity, but because it appears to represent a truly... Show moreUnlocking the mystery of the strange metal state has become the focal point of high-Tcresearch, not because of its importance for superconductivity, but because it appears to represent a truly novel phase of matter dubbed "quantum supreme matter. " Detected originally through high magnetic field, transport experiments, signatures of this phase have now been uncovered with a variety of probes. Our high resolution optical data of the low-Tccuprate superconductor, Bi2-xPbxSr2-yLayCuO6+delta allows us to probe this phase over a large energy and temperature window. We demonstrate that the optical signatures of the strange metal phase persist throughout the phase diagram. The strange metal signatures in the optical conductivity are twofold: (i) a low energy Drude response with Drude width on the order of temperature and (ii) a high energy conformal tail with a doping dependent power-law exponent. While the Drude weight evolves monotonically throughout the entire doping range studied, the spectral weight contained in the high energy conformal tail appears to be doping and temperature independent. Our analysis further shows that the temperature dependence of the optical conductivity is completely determined by the Drude parameters. Our results indicate that there is no critical doping level inside the superconducting dome where the carrier density starts to change drastically and that the previously observed "return to normalcy " is a consequence of the increasing importance of the Drude component relative to the conformal tail with doping. Importantly, both the doping and temperature dependence of the resistivity are largely determined by the Drude width. Show less
In the present paper we derive the general expressions for the differential entropy per particle near van Hove singularities (vHs) in the density of states. The dependence of entropy per particle... Show moreIn the present paper we derive the general expressions for the differential entropy per particle near van Hove singularities (vHs) in the density of states. The dependence of entropy per particle on chemical potential and temperature demonstrates different behavior depending on the type of vHs, and distinguishes high-order vHs with different divergence exponents and flat bands. In addition, it allows one to test the ???flatness??? of the band in experiment. We compare the analytic predictions with the numerical calculation of the differential entropy for tight-binding models of graphene, the Lieb lattice, and the square-octagon lattice. Our results show that the obtained analytic expressions capture the main features of the differential entropy, thus serving as a good probe for details of the density of states structure. Show less
We identify a mapping between two-dimensional (2D) electron transport in a minimally twisted graphene bilayer and a one-dimensional (1D) quantum walk, where one spatial dimension plays the role of... Show moreWe identify a mapping between two-dimensional (2D) electron transport in a minimally twisted graphene bilayer and a one-dimensional (1D) quantum walk, where one spatial dimension plays the role of time. In this mapping, a magnetic field B perpendicular to the bilayer maps onto an electric field. Bloch oscillations due to the periodic motion in a 1D Bloch band can then be observed in purely DC transport as magnetoconductance oscillations with periodicity set by the Bloch frequency. Show less
We consider the isotropic spin-21 Heisenberg spin chain weakly perturbed by a local translationally and SU(2)-invariant perturbation. Starting from the local integrals of motion of the unperturbed... Show moreWe consider the isotropic spin-21 Heisenberg spin chain weakly perturbed by a local translationally and SU(2)-invariant perturbation. Starting from the local integrals of motion of the unperturbed model, we modify them in order to obtain quasiconserved integrals of motion (charges) for the perturbed model. Such quasiconserved quantities are believed to be responsible for the existence of the prethermalization phase at intermediate timescales. We find that for a sufficiently local perturbation the quasiconserved quantities indeed exist, and we construct an explicit form for the first few of them. Show less
A hole injected into a Mott insulator will gain an internal structure as recently identified by exact numerics, which is characterized by a nontrivial quantum number whose nature is of central... Show moreA hole injected into a Mott insulator will gain an internal structure as recently identified by exact numerics, which is characterized by a nontrivial quantum number whose nature is of central importance in understanding the Mott physics. In this work, we show that a spin texture associated with such an internal degree of freedom can explicitly manifest after the spin degeneracy is lifted by a weak Rashba spin-orbit coupling (SOC). It is described by an emergent angular momentum J(z) = +/- 3/2 as shown by both exact diagonalization and variational Monte Carlo calculations, which are in good agreement with each other at a finite size. In particular, as the internal structure such a spin texture is generally present in the hole composite even at high excited energies, such that a corresponding texture in momentum space, extending deep inside the Brillouin zone, can be directly probed by the spin-polarized angle-resolved photoemission spectroscopy (ARPES). This is in contrast to a Landau quasiparticle under the SOC, in which the spin texture induced by SOC will not be protected once the excited energy is larger than the weak SOC coupling strength, away from the Fermi energy. We point out that the spin texture due to the SOC should be monotonically enhanced with reducing spin-spin correlation length in the superconducting-pseudogap phase at finite doping. A brief discussion of a recent experiment of the spinpolarized ARPES will be made. Show less
We identify an effect of chirality in the electrical conduction along magnetic vortices in a Weyl superconductor. The conductance depends on whether the magnetic field is parallel or antiparallel... Show moreWe identify an effect of chirality in the electrical conduction along magnetic vortices in a Weyl superconductor. The conductance depends on whether the magnetic field is parallel or antiparallel to the vector in the Brillouin zone that separates Weyl points of opposite chirality. Show less
