arXiv subjects

David Barbalas

Publications and source records attributed to David Barbalas.

8 recordsLinked to original sources

arXiv2025

Decoupling momentum and energy relaxation rates in cuprate strange metals via giant THz nonlinearities

Understanding the $T$-linear normal-state resistivity of cuprates remains a central physics challenge. The associated momentum relaxation rate, $\Gamma_M$, saturates near the conjectured ``Planckian" bound $\Gamma_M\sim kT/\hbar$, but the mechanism underlying the anomalous scattering remains unresolved. Here we employ nonlinear terahertz spectroscopy to systematically study La$_{2-x}$Sr$_x$CuO$_4$ across a broad temperature and doping range. We measure the normal-state third-order susceptibility, $|\chi^{(3)}|\approx 6\times10^{-9}$ m$^2$/V$^2$, among the largest in the THz regime, enabling direct access to the rarely measured electronic energy relaxation rate, $\Gamma_E$. Strikingly, $\Gamma_E$ is 10-40 times smaller than $\Gamma_M$, revealing that the scatterings responsible for momentum loss and $T$-linear resistivity do not remove appreciable energy from the electrons. While $\Gamma_M (T)$ is consistent with quasi-elastic scattering from bosonic modes above their characteristic energy scale, this is incompatible with the increasing temperature dependence of $\Gamma_E(T)$. Our results exclude phonons as the source of $T$-linear resistivity and impose strong constraints on possible mechanisms.

cond-mat.supr-con↗

arXiv2025

Absence of two-phonon quasi-elastic scattering in the normal state of doped--SrTiO$_3$ by THz pump-probe spectroscopy

Multi-pulse nonlinear THz spectroscopies enable a new understanding of interacting metallic systems via their sensitivity to novel correlation functions. Here, we investigated the THz nonlinear properties of the dilute metallic phase of doped-SrTiO$_3$ thin films using nonlinear terahertz 2D coherent spectroscopy. We observed a large $\chi^{(3)}$ response in the low temperature region where the dc electrical resistivity follows a T$^2$-dependence. This is largely a pump-probe response, which we find is governed by a single energy relaxation rate that is much smaller at all temperatures than the momentum relaxation rates obtained from the optical conductivity. This indicates that the processes that dominate the resistive scattering are not the same as those that remove energy from the electronic system. Moreover the fact that the energy relaxation rate is an increasing function of temperature indicates that the excitations that do carry away energy from the electronic system cannot be considered as quasi-elastic and as such soft two-phonon electron scattering does not play a major role in the physics as proposed. This indicates that these materials' resistive T$^2$ scattering likely originates in electron-electron interactions despite the very small Fermi wave vectors at the lowest dopings.

cond-mat.str-el↗

arXiv2024

Anomalous electronic energy relaxation and soft phonons in the Dirac semimetal Cd$_3$As$_2$

We have used a combination of linear response time-domain THz spectroscopy (TDTS) and high-field non-linear THz spectroscopy to separately probe the electronic momentum and energy relaxation rates respectively of the Dirac semimetal Cd$_3$As$_2$. We find, consistent with prior measurements, that Cd$_3$As$_2$ has an enormous nonlinearities in the THz frequency range. We extract the momentum relaxation rate of Cd$_3$As$_2$ using Drude fits to the optical conductivity. We also conduct THz range 2D coherent spectroscopy. The dominant response is a pump-probe signal, which allow us to separately extract the energy relaxation rate. We find that the rate of energy relaxation decreases down to the lowest measured temperatures. We connect this to Cd$_3$As$_2$ anomalous lattice dynamics, evidence for which is found in its low thermal conductivity and soft phonons in Raman scattering. The lack of a peak in the energy relaxation rate as a function of T can be connected to the linear in T dependence of the current relaxation e.g. the phonon scattering is elastic down to the lowest measured temperatures approximately 120 K.

cond-mat.str-el↗

arXiv2023

Energy Relaxation and dynamics in the correlated metal Sr$_2$RuO$_4$ via THz two-dimensional coherent spectroscopy

Separating out the contributions of different scattering channels in strongly interacting metals is crucial in identifying the mechanisms that govern their properties. While momentum or current relaxation rates can be readily probed via \textit{dc} resistivity or optical/THz spectroscopy, distinguishing different kinds of inelastic scattering can be more challenging. Using nonlinear THz 2D coherent spectroscopy, we measure the rates of energy relaxation after THz excitation in the strongly interacting Fermi liquid, Sr$_2$RuO$_4$. Energy relaxation is a bound on the total scattering and specifically a measure of contributions to the electron self-energy that arise from {\it inelastic} coupling to a bath. We observe two distinct energy relaxation channels: a fast process that we interpret as energy loss to the phonon system and a much slower relaxation that we interpret as arising from a non-equilibrium phonon effects and subsequent heat loss through diffusion. Interestingly, even the faster energy relaxation rate is at least an order of magnitude slower than the overall momentum relaxation rate, consistent with strong electron interactions and the dominance of energy-conserving umklapp or interband electron-electron scattering in momentum relaxation. The slowest energy relaxation rate decays on a sub-GHz scale, consistent with the relaxation dynamics of non-equilibrium phonons. Our observations reveal the versatility of nonlinear THz spectroscopy to measure the energy relaxation dynamics in correlated metals. Our work also highlights the need for improved theoretical understanding of such processes in interacting metals.

cond-mat.str-el↗

arXiv2023

Revealing novel aspects of light-matter coupling in terahertz two-dimensional coherent spectroscopy: the case of the amplitude mode in superconductors

Recently developed terahertz (THz) two-dimensional coherent spectroscopy (2DCS) is a powerful technique to obtain materials information in a fashion qualitatively different from other spectroscopies. Here, we utilized THz 2DCS to investigate the THz nonlinear response of conventional superconductor NbN. Using broad-band THz pulses as light sources, we observed a third-order nonlinear signal whose spectral components are peaked at twice the superconducting gap energy $2\Delta$. With narrow-band THz pulses, a THz nonlinear signal was identified at the driving frequency $\Omega$ and exhibited a resonant enhancement at temperature when $\Omega = 2\Delta$. General theoretical considerations show that such a resonance can only arise from a disorder-activated paramagnetic coupling between the light and the electronic current. This proves that the nonlinear THz response can access processes distinct from the diamagnetic Raman-like density fluctuations, which are believed to dominate the nonlinear response at optical frequencies in metals. Our numerical simulations reveal that even for a small amount of disorder, the $\Omega=2\Delta$ resonance is dominated by the superconducting amplitude mode over the entire investigated disorder range. This is in contrast to other resonances, whose amplitude-mode contribution depends on disorder. Our findings demonstrate the unique ability of THz 2DCS to explore collective excitations inaccessible in other spectroscopies.

cond-mat.supr-con↗

arXiv2022

Disorder-enhanced effective masses and deviations from Matthiessen's rule in PdCoO$_2$ thin films

The observation of hydrodynamic transport in the metallic delafossite PdCoO$_2$ has increased interest in this family of highly conductive oxides, but experimental studies so far have mostly been confined to bulk crystals. In this work, the development of high-quality thin films of PdCoO$_2$ has enabled a thorough study of the conductivity as a function of film thickness using both dc transport and time-domain THz spectroscopy. With increasing film thickness from 12 nm to 102 nm, the residual resistivity decreases and we observe a large deviation from Matthiessen's rule (DMR) in the temperature dependence of the resistivity. We find that the complex THz conductivity is well fit by a single Drude term. We fit the data to extract the spectral weight and scattering rate simultaneously. The temperature dependence of the Drude scattering rate is found to be nearly independent of the residual resistivity and cannot be the primary mechanism for the observed DMR. Rather, we observe large changes in the spectral weight as a function of disorder, changing by a factor of 1.5 from the most disordered to least disordered films. We believe this corresponds to a mass enhancement of $\geq 2$ times the value of the bulk effective mass which increases with residual disorder. This suggests that the mechanism behind the DMR observed in dc resistivity is primarily driven by changes in the electron mass. We discuss the possible origins of this behavior including the possibility of disorder-enhanced electron-phonon scattering, which can be systematically tuned by film thickness.

cond-mat.str-el↗

arXiv2022

Anomalous high-temperature THz nonlinearity in superconductors near the metal-insulator transition

The interplay of strong disorder and superconductivity is a topic of long-term interest in condensed matter physics. Here we explore the nonlinear THz response of superconducting NbN films close to the 3D metal-insulator transition. For the least disordered samples, the magnitude of the nonlinear $χ^{(3)}$ response follows the temperature dependence of the superfluid density as expected. In contrast, for high disorder samples near the metal-insulator transition the $χ^{(3)}$ nonlinearity persists to temperatures as high as even 4 times the $T_c$ of the cleanest sample. We discuss the possible origins of this remarkably large nonlinearity, including the possibility that it arises in an enhancement of the temperature scales of superconductivity close to localization. Our work highlights the importance of finite frequency nonlinear THz experiments in detecting superconducting correlations even into regions where long-range ordered superconductivity does not persist.

cond-mat.supr-con↗

arXiv2022

An optical investigation of the heavy fermion normal state in superconducting UTe$_2$

The recently discovered superconductor, UTe$_2$, has attracted immense scientific interest due to the experimental observations that suggest odd-parity superconductivity. It is believed that the material becomes a heavy-fermion metal at low temperatures although details of the normal state are unclear. Using Fourier transform infrared spectroscopy (FTIR), the normal state electronic structure of UTe$_2$ was investigated at zero applied magnetic field. Combining the measured reflectivity with the dc resistivity, the complex optical conductivity was obtained over a large frequency range. The frequency dependence of the real part of the optical conductivity exhibits a MIR peak around 4000 cm$^{-1}$ and a narrow Drude peak that develops below 40 K. A combination of density functional and dynamic mean field theory (DFT + DMFT) gives spectra in close correspondence to the experiment. Via this comparison we attribute the prominent MIR peak to inter-band transitions involving a narrow U 5$f$ feature that develops near the Fermi level. In this regard, our data gives spectroscopic evidence for the existence of a low energy Kondo resonance at temperatures just above the onset of superconductivity and implicates heavy electrons in the formation of the superconducting state. We find that the coherent Kondo resonance is primarily associated with a collapse of scattering and less with a transfer of spectral weight.

cond-mat.supr-con↗