arXiv subjects

Erik D. Kountz

Publications and source records attributed to Erik D. Kountz.

5 recordsLinked to original sources

arXiv2023

Thermal transport measurements of the charge density wave transition in CsV$_3$Sb$_5$

We study thermalization and thermal transport in single crystals of CsV$_3$Sb$_5$ through the CDW transition by directly measuring thermal diffusivity ($D$), thermal conductivity ($κ$), resistivity ($ρ$), and specific heat ($c$). Commensurate with previous reports, we observe a sharp, narrow anomaly in specific heat associated with a first order transition that results in a CDW state below $\sim94$ K. While a corresponding sharp anomaly in thermal diffusivity is also observed, resistivity and thermal conductivity only exhibit small steps at the transition, where the feature is sharp for resistivity and broader for thermal conductivity. Scrutinizing the thermal Einstein relation $κ=cD$, we find that this relation is satisfied in the entire temperature range, except in a narrow range around the transition. The Wiedemann-Franz law seems to work outside the critical region as well. Below the transition and persisting below the two-phase regime we find strong resemblance between the resistivity anomaly and the specific heat, which may point to a secondary electronic order parameter that emerges continuously below the transition.

cond-mat.str-el↗

arXiv2023

High Resolution Polar Kerr Effect Studies of CsV${}_3$Sb${}_5$: Tests for Time Reversal Symmetry Breaking Below the Charge Order Transition

We report high resolution polar Kerr effect measurements on CsV${}_3$Sb${}_5$ single crystals in search for signatures of spontaneous time reversal symmetry breaking below the charge order transition at $T^* \approx 94$ K. Utilizing two different versions of zero-area loop Sagnac interferometers operating at 1550 nm wavelength, each with the fundamental attribute that without a time reversal symmetry breaking sample at its path, the interferometer is perfectly reciprocal, we find no observable Kerr effect to within the noise floor limit of the apparatus at 30 nanoradians. Simultaneous coherent reflection ratio measurements confirm the sharpness of the charge order transition in the same optical volume as the Kerr measurements. At finite magnetic field we observe a sharp onset of a diamagnetic shift in the Kerr signal at $T^*$, which persists down to the lowest temperature without change in trend. Since 1550 nm is an energy that was shown to capture all features of the optical properties of the material that interact with the charge order transition, we are led to conclude that it is highly unlikely that time reversal symmetry is broken in the charge ordered state in CsV${}_3$Sb${}_5$.

cond-mat.str-el↗

arXiv2020

Anomalous thermal transport and violation of Wiedemann-Franz law in the critical regime of a charge density wave transition

ErTe$_3$ is studied as a model system to explore thermal transport in a layered charge density wave (CDW) material. We present data from thermal diffusivity, resistivity, and specific heat measurements: There is a sharp decrease in thermal conductivity both parallel and perpendicular to the primary CDW at the CDW transition temperature. At the same time, the resistivity changes more gradually. Correspondingly, while well above and below $T_c$, a consistent description of the thermal transport applies with essentially independent electron and phonon contributions (estimated using the Wiedemann Franz law), in the critical regime no such description is possible; the observed behavior corresponds to a strongly coupled electron-phonon critical `soup.'

cond-mat.str-el↗

arXiv2020

Thermalization and Possible Signatures of Quantum Chaos in Complex Crystalline Materials

Analyses of thermal diffusivity data on complex insulators and on strongly correlated electron systems hosted in similar complex crystal structures suggest that quantum chaos is a good description for thermalization processes in these systems, particularly in the high temperature regime where the many phonon bands and their interactions dominate the thermal transport. Here we observe that for these systems diffusive thermal transport is controlled by a universal Planckian time scale $τ\sim \hbar/k_BT$, and a unique velocity $v_E$. Specifically, $v_E \approx v_{ph} $ for complex insulators, and $v_{ph} \lesssim v_E \ll v_{F}$ in the presence of strongly correlated itinerant electrons ($v_{ph}$ and $v_F$ are the phonons and electrons velocities respectively). For the complex correlated electron systems we further show that charge diffusivity, while also reaching the Planckian relaxation bound, is largely dominated by the Fermi velocity of the electrons, hence suggesting that it is only the thermal (energy) diffusivity that describes chaos diffusivity.

cond-mat.str-el↗

arXiv2019

Thermal Diffusivity Above Mott-Ioffe-Regel Limit

We present high-resolution thermal diffusivity measurements on several near optimally doped electron- and hole-doped cuprate systems in a temperature range that passes through the Mott-Ioffe-Regel limit, above which the quasiparticle picture fails. Our primary observations are that the inverse thermal diffusivity is linear in temperature and can be fitted to $D_Q^{-1}=aT+b$. The slope $a$ is interpreted through the Planckian relaxation time $τ\approx\hbar/k_BT$ and a thermal diffusion velocity $v_B$, which is close, but larger than the sound velocity. The intercept $b$ represent a crossover diffusion constant that separates coherent from incoherent quasiparticles. These observations suggest that both phonons and electrons participate in the thermal transport, while reaching the Planckian limit for relaxation time.

cond-mat.str-el↗