arXiv subjects

Ralph Romero III

Publications and source records attributed to Ralph Romero III.

9 recordsLinked to original sources

arXiv2026

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, $Γ_M$, saturates near the conjectured ``Planckian" bound $Γ_M\sim kT/\hbar$, 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, $|χ^{(3)}|\approx 6\times10^{-9}$ m$^2$/V$^2$, among the largest in the THz regime, enabling direct access to the rarely measured electronic energy relaxation rate, $Γ_E$. Strikingly, $Γ_E$ is 10-40 times smaller than $Γ_M$, revealing that the scatterings responsible for momentum loss and $T$-linear resistivity do not remove appreciable energy from the electrons. While $Γ_M (T)$ is consistent with quasi-elastic scattering from bosonic modes above their characteristic energy scale, this is incompatible with the increasing temperature dependence of $Γ_E(T)$. Our results exclude phonons as the source of $T$-linear resistivity and impose strong constraints on possible mechanisms.

cond-mat.supr-con↗

arXiv2025

Terahertz range polarization rotation in the candidate time-reversal symmetry breaking superconductor BiNi

Here we report the observation of time-reversal symmetry (TRS) breaking superconductivity in a BiNi bilayer using terahertz (THz) polarimetry. Leveraging a novel high-precision THz polarimetry technique, we detect, in the superconducting state and at zero magnetic field, the smallest polarization rotation of THz light measured to date. By using the MgO substrate itself as an optical resonator, we can reference the Faraday and Kerr rotations to each other. We observe a low-frequency Kerr rotation on the order of several hundred microradians in the superconducting phase, a clear signature consistent with TRS-breaking superconductivity. Our measurements enable direct access to the THz-range Hall conductivity. Through a Kramers-Kronig analysis, we link these low-energy measurements to prior high-frequency magneto-optic Kerr effect (MOKE) data. This connection provides critical insight into the nature of the TRS-breaking state, supporting a multiband superconducting scenario over a disordered single-band interpretation for the origin of the Kerr effect.

cond-mat.supr-con↗

arXiv2025

Amplitude mode in a multi-gap superconductor MgB$_2$ investigated by terahertz two-dimensional coherent spectroscopy

We have investigated the terahertz (THz) nonlinear response of the multigap superconductor MgB$_2$, using THz two-dimensional coherent spectroscopy (THz 2DCS). With broadband THz drive fields, we identified a nonlinear response at twice the lower superconducting gap energy $2Δ_π$ at the lowest temperatures. Using narrow-band THz driving pulses, we observed first (FH) and third harmonic responses. The FH intensity shows a monotonic increase with decreasing temperature when properly normalized by the driving field strength. This is distinct from the single-gap superconductor NbN, where the FH signal exhibited a resonant enhancement at temperatures when twice the gap energy $2Δ$ was resonant with the driving photon energy, which was interpreted to originate from the superconducting amplitude mode. Our results in MgB$_2$ are consistent with a well-defined amplitude mode only at the lowest temperatures and indicate strong damping as temperature increases. This likely indicates the importance of interband coupling in MgB$_2$ and its influence on the nature of the amplitude mode and its damping.

cond-mat.supr-con↗

arXiv2025

Planckian scattering and parallel conduction channels in the iron chalcogenide superconductors FeTe$_{1-x}$Se$_x$

The remarkable linear in temperature resistivity of the cuprate superconductors, which extends in some samples from $T_c$ to the melting temperature, remains unexplained. Although seemingly simple, this temperature dependence is incompatible with the conventional theory of metals that dictates that the scattering rate, $1/τ$, should be quadratic in temperature if electron-electron scattering dominates. Understanding the origin of this temperature dependence and its connection to superconductivity may provide the key to pick the lock of high-temperature superconductivity. Using time-domain terahertz spectroscopy (TDTS) we elucidate the low temperature conducting behavior of two FeTe$_{1-x}$Se$_x$ (FTS) samples, one with almost equal amounts of Se and Te that is believed to be a topological superconductor, and one that is more overdoped. Constrained with DC resistivity, we find two conduction channels that add in parallel, a broad one in frequency with weak temperature dependence and a sharper one whose scattering rate goes as the Planckian limited rate, $\sim kT/h$. Through analysis of its spectral weight we show the superconducting condensate is mainly drawn from the channel that undergoes this Planckian scattering.

cond-mat.supr-con↗

arXiv2024

Energy and momentum relaxation through the Curie temperature in an itinerant ferromagnet

In this work, we combine conventional linear response time-domain THz spectroscopy with non-linear THz-pump THz-probe techniques to study metallic strained thin films of $\mathrm{Ca}_2\mathrm{RuO}_4$, which undergo a transition into a ferromagnetic state at 10 K. Such measurements allowing us to independently measure momentum and energy relaxation rates. We find that while the momentum relaxation rate decreases significantly at the ferromagnetic transition, the energy relaxation rate remains unaffected by the emergence of magnetic order. This shows that the dominant changes to scattering across the transition correspond to scatterings that relax momentum without relaxing energy. It is consistent with a scenario where energy is not carried off by coupling to collective magnetic degrees of freedom. Instead, the principal channel for energy relaxation remains the conventional one e.g. coupling to acoustic phonons. This observation validates the approximation used in the conventional understanding of resistive anomalies of ferromagnets across the Curie temperature, which due to critical slowing down, spin fluctuations can be treated as effectively static and scattering off of them elastic. This scenario can likely be extended to resistive anomalies at other phase transitions to charge- and spin-density wave states in kagome metals or pnictide system

cond-mat.str-el↗

arXiv2024

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 $2Δ$. With narrow-band THz pulses, a THz nonlinear signal was identified at the driving frequency $Ω$ and exhibited a resonant enhancement at temperature when $Ω= 2Δ$. 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 $Ω=2Δ$ 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↗

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

High-precision measurements of terahertz polarization states with a fiber coupled time-domain THz spectrometer

We present a new method for high precision measurements of polarization rotation in the frequency range from 0.2 to 2.2 THz using a fiber coupled time-domain THz spectrometer. A free standing wire-grid polarizer splits THz light into orthogonal components which are then measured by two separate detectors simultaneously. We theoretically model the uncertainties introduced by optical component non-idealities and predict that we may expect to achieve precisions of order 2.3 $μ$rad (0.13 mdeg) and accuracies of 0.8$\%$ when anti-symmetrizing the response with respect to an applied field. Anti-symmetrization improves precision by more than four orders of magnitude. We demonstrate this method on a 2D electron gas in magnetic field and show that we achieve a precision of 20 $μ$rad (1.1 mdeg) for small polarization rotation angles. A detailed description of the technique and data analysis procedure is provided, demonstrating its capability to precisely measure polarization states in the 0.2 to 2.2 THz range.

physics.optics↗

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↗