Strongly coupled quantum phonon fluid in a solvable model

We study a model of a large number of strongly coupled phonons that can be viewed as a bosonic variant of the Sachdev-Ye-Kitaev model. We determine the phase diagram of the model which consists of a glass phase and a disordered phase, with a first-order phase transition separating them. We compute the specific heat of the disordered phase, with which we diagnose the high-temperature crossover to the classical limit. We further study the real-time dynamics of the disordered phase, where we identify three dynamical regimes as a function of temperature. Low temperatures are associated with a semiclassical regime, where the phonons can be described as long-lived normal modes. High temperatures are associated with the classical limit of the model. For a large region in parameter space, we identify an intermediate-temperatures regime, where the phonon lifetime is of the order of the Planckian time scale $\hbar/k_B T$.Comment: Typos corrected, references added, discussion improve

A Criterion for Strange Metallicity in the Lorenz Ratio

The Wiedemann-Franz (WF) law, stating that the Lorenz ratio $L = \kappa/(T\sigma)$ between the thermal and electrical conductivities in a metal approaches a universal constant $L_0=\pi^2 k_B^2/ (3 e^2)$ at low temperatures, is often interpreted as a signature of fermionic Landau quasi-particles. In contrast, we show that various models of weakly disordered non-Fermi liquids also obey the WF law at $T \to 0$. Instead, we propose using the leading low-temperature correction to the WF law, $L(T)-L_0$ (proportional to the inelastic scattering rate), to distinguish different types of strange metals. As an example, we demonstrate that in a solvable model of a marginal Fermi liquid, $L(T)-L_0\propto -T$. Using the quantum Boltzmann equation approach, we find analogous behavior in a class of marginal- and non-Fermi liquids with a weakly momentum-dependent inelastic scattering. In contrast, in a Fermi liquid, $L(T)-L_0$ is proportional to $-T^2$. This holds even when the resistivity grows linearly with $T$, due to $T-$linear quasi-elastic scattering (as in the case of electron-phonon scattering at temperatures above the Debye frequency).Comment: Main text 4.5 pages (2 Figures) + Supplementary material 7.5 pages (1 Figure) + Reference

T−T-linear resistivity from magneto-elastic scattering: application to PdCrO2_2

An electronic solid with itinerant carriers and localized magnetic moments represents a paradigmatic strongly correlated system. The electrical transport properties associated with the itinerant carriers, as they scatter off these local moments, has been scrutinized across a number of materials. Here we analyze the transport characteristics associated with ultra-clean PdCrO$_2$ -- a quasi two-dimensional material consisting of alternating layers of itinerant Pd-electrons and Mott-insulating CrO$_2$ layers -- which shows a pronounced regime of $T-$linear resistivity over a wide-range of intermediate temperatures. By contrasting these observations to the transport properties in a closely related material PdCoO$_2$, where the CoO$_2$ layers are band-insulators, we can rule out the traditional electron-phonon interactions as being responsible for this interesting regime. We propose a previously ignored electron-magnetoelastic interaction between the Pd-electrons, the Cr local-moments and an out-of-plane phonon as the main scattering mechanism that leads to the significant enhancement of resistivity and a $T-$linear regime in PdCrO$_2$ at temperatures far in excess of the magnetic ordering temperature. We suggest a number of future experiments to confirm this picture in PdCrO$_2$, as well as other layered metallic/Mott-insulating materials.Comment: 7 pages, 3 figures. Supplementary material: 10 pages, 3 figure

Investigation of Planckian behavior in a high-conductivity oxide : PdCrO2

Funding: JFMV and DC are supported by faculty startup grants at Cornell University. ET and EB were supported by the European Research Council (ERC) under grant HQMAT (Grant Agreement No. 817799), the Israel-US Binational Science Foundation (BSF), and the Minerva Foundation.The layered delafossite metal PdCrO2 is a natural heterostructure ofhighly conductive Pd layers Kondo coupled to localized spins in the adjacentMott insulating CrO2 layers. At high temperatures T it has a T-linearresistivity which is not seen in the isostructural but non-magnetic PdCoO2.The strength of the Kondo coupling is known, as-grown crystals are extremelyhigh purity and the Fermi surface is both very simple and experimentally known.It is therefore an ideal material platform in which to investigate 'Planckianmetal' physics. We do this by means of controlled introduction of pointdisorder, measurement of the thermal conductivity and Lorenz ratio and studyingthe sources of its high temperature entropy. The T-linear resistivity is seento be due mainly to elastic scattering and to arise from a sum of severalscattering mechanisms. Remarkably, this sum leads to a scattering rate within10% of the Planckian value of kBT/ℏ.Publisher PDFPeer reviewe

Investigation of Planckian behavior in a high-conductivity oxide:PdCrO₂

The layered delafossite metal PdCrO2 is a natural heterostructure ofhighly conductive Pd layers Kondo coupled to localized spins in the adjacent Mott insulating CrO2 layers. At high temperatures T it has a T-linearresistivity which is not seen in the isostructural but non-magnetic PdCoO2.The strength of the Kondo coupling is known, as-grown crystals are extremelyhigh purity and the Fermi surface is both very simple and experimentally known. It is therefore an ideal material platform in which to investigate 'Planckianmetal' physics. We do this by means of controlled introduction of point disorder, measurement of the thermal conductivity and Lorenz ratio and studying the sources of its high temperature entropy. The T-linear resistivity is seen to be due mainly to elastic scattering and to arise from a sum of several scattering mechanisms. Remarkably, this sum leads to a scattering rate within 10% of the Planckian value of kBT/ℏ