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Beginning with high-$T_c$ cuprate materials, it has been observed that many superconductors exhibit so-called "Homes scaling", in which the zero-temperature superfluid density, $\rho_{s0}$, is proportional to the product of the normal-state dc conductivity and the superconducting transition temperature, $\sigma_\mathrm{dc} T_c$. For conventional, s-wave superconductors, such scaling has been shown to be a natural consequence of elastic-scattering disorder, not only in the extreme dirty limit but across a broad range of scattering parameters. Here we show that when an analogous calculation is carried out for elastic scattering in d-wave superconductors, a stark contrast emerges, with $\rho_{s0} \propto \left(\sigma_\mathrm{dc} T_c \right)^2$ in the dirty limit, in apparent violation of Homes scaling. Within a simple approximate Migdal--Eliashberg treatment of inelastic scattering, we show how Homes scaling is recovered. The normal-state behavior of near optimally doped cuprates is dominated by inelastic scattering, but significant deviations from Homes scaling occur for disorder-dominated cuprate systems, such as underdoped YBCO and overdoped LSCO, and in very clean materials with little inelastic scattering, such as Sr$_2$RuO$_4$. We present a revised analysis where both axes of the original Homes scaling plot are normalized by the Drude plasma weight, $\omega_{p,D}^2$, and show that new universal scaling emerges, in which the superfluid fractions of dirty s-wave and dirty d-wave superconductors coalesce to a single point at which normal-state scattering is occurring at the Planckian bound. The combined result is a new tool for classifying superconductors in terms of order parameter symmetry, as well as scattering strength and character. Although our model starts from a Fermi-liquid assumption it describes underdoped cuprates surprisingly well., Comment: 15 pages, 5 figures

We analyze the optical pump-probe reflection and transmission coefficients when the photoinduced response depends nonlinearly on the incident pump intensity. Under these conditions, we expect the photoconductivity depth profile to change shape as a function of the incident fluence, unlike the case when the photoinduced response is linear in the incident intensity. We consider common optical nonlinearities, including photoconductivity saturation and two-photon absorption, and we derive analytic expressions for the photoconductivity depth profile when one or more is present. We review the theory of the electromagnetic transmission and reflection coefficients in a stratified medium, and we derive general expressions for these coefficients for a medium with an arbitrary photoconductivity depth profile. For several photoconductivity profiles of importance in pump-probe spectroscopy, we show that the wave equation can be transformed into one of three standard differential equations$\unicode{x2014}$the Bessel equation, the hypergeometric equation, and the Heun equation$\unicode{x2014}$with analytic solutions in terms of their associated special functions. From these solutions, we derive exact analytic expressions for the optical coefficients in terms of the photoconductivity at the optical interface, and we discuss their limiting forms in various physical limits. Our results provide a systematic guide for analyzing pump-probe measurements over a wide range of pump intensities, and establishes a framework for constraining the systematic uncertainty associated with nonlinear photoconductivity profile distortion.

$\alpha$-MnTe is an antiferromagnetic semiconductor with above room temperature $T_N$ = 310 K, which is promising for spintronic applications. Recently, it was reported to be an altermagnet, containing bands with momentum-dependent spin splitting; time-resolved experimental probes of MnTe are therefore important both for understanding novel magnetic properties and potential device applications. We investigate ultrafast spin dynamics in epitaxial MnTe(001)/InP(111) thin films using pump-probe magneto-optical measurements in the Kerr configuration. At room temperature, we observe an oscillation mode at 55 GHz that does not appear at zero magnetic field. Combining field and polarization dependence, we identify this mode as a magnon, likely originating from inverse stimulated Raman scattering. Magnetic field-dependent oscillations persist up to at least 335 K, which could reflect coupling to known short-range magnetic order in MnTe above $T_N$. Additionally, we observe two optical phonons at 3.6 THz and 4.2 THz, which broaden and redshift with increasing temperature., Comment: 27 pages including supplemental

After more than a decade of research on photoinduced superconductivity, the experimental evidence for its existence remains controversial. Recently, we identified a fundamental flaw in the analysis of several influential results on K$_3$C$_{60}$ and showed that similar measurements on other compounds suffer from the same problem. We described how to account for this systematic error, and reanalyzed evidence that had previously been advanced for both photoinduced superconductivity and Higgs-mediated terahertz amplification. We found that both phenomena may be understood instead as a photoenhancement of the carrier mobility that saturates with fluence, with no need to appeal to a photoinduced phase transition to a superconducting state. We summarize this reinterpretation and describe how subsequent work on K$_3$C$_{60}$ provides quantitative support for it., Comment: Abstract accepted for presentation at the IRMMW-THz 2023 Conference in Montreal, Canada. This work has been submitted to the IEEE for possible publication

We discuss a systematic uncertainty in time-resolved optical conductivity measurements that becomes important at high pump intensities. We show that common optical nonlinearities can distort the photoconductivity depth profile, and by extension distort the photoconductivity spectrum. We show evidence that this distortion is present in existing measurements on K$_{3}$C$_{60}$, and describe how it may create the appearance of photoinduced superconductivity where none exists. Similar errors may emerge in other pump-probe spectroscopy measurements, and we discuss how to correct for them., Comment: 5 pages, 5 figures, supplemental material. v2: Some acknowledgments removed upon request. v3: Corrected an error in Eq. (5) and revised the affected figures and discussion accordingly. Added sections to supplemental material. v4: Updated title; other minor changes suggested by journal editors

We present a maximum-likelihood method for parameter estimation in terahertz time-domain spectroscopy. We derive the likelihood function for a parameterized frequency response function, given a pair of time-domain waveforms with known time-dependent noise amplitudes. The method provides parameter estimates that are superior to other commonly-used methods, and provides a reliable measure of the goodness of fit. We also develop a simple noise model that is parameterized by three dominant sources, and derive the likelihood function for their amplitudes in terms of a set of repeated waveform measurements. We demonstrate the method with applications to material characterization., Comment: 15 pages, 6 figures; minor formatting updates

We study the pump-probe response of three insulating cuprates and develop a model for its recombination kinetics. The dependence on time, fluence, and both pump and probe photon energies imply many-body recombination on femtosecond timescales, characterized by anomalously large trapping and Auger coefficients. The fluence dependence follows a universal form that includes a characteristic volume scale, which we associate with the holon-doublon excitation efficiency. This volume varies strongly with pump photon energy and peaks near twice the charge-transfer energy, suggesting that the variation is caused by carrier multiplication through impact ionization.

Moller scattering is one of the most fundamental processes in QED. Understanding it to high precision is necessary for a variety of modern nuclear and particle physics experiments. In a recent calculation, existing soft-photon radiative corrections were combined with new hard-photon bremsstrahlung calculations to take into account the effect of photon emission at any photon energy, where the electron mass was included at all steps. To test the calculation, an experiment was carried out using the 3 MV Van de Graaff electrostatic accelerator at the MIT High Voltage Research Laboratory. Momentum spectra at three scattering angles at an incident electron energy of 2.5 MeV are reported here, and compared to the simulated radiative Moller spectra, based on our previous calculation. Good agreement between the measurements and our calculation is observed in the momentum spectrum at the three angles.

We calculate superfluid density for a dirty d-wave superconductor. The effects of impurity scattering are treated within the self-consistent t-matrix approximation, in weak-coupling BCS theory. Working from a realistic tight-binding parameterization of the Fermi surface, we find a superfluid density that is both correlated with T_c and linear in temperature, in good correspondence with recent experiments on overdoped La2-xSrxCuO4., Comment: 10 pages, 6 figures, including appended erratum

We establish a detailed phenomenology of photocarrier transport in the copper oxide plane by studying the transient terahertz photoconductivity of Sr$_2$CuO$_2$Cl$_2$ and YBa$_2$Cu$_3$O$_6$. The peak photoconductivity saturates with fluence, decays on multiple picosecond timescales, and evolves into a state characterized by activated transport. The time dependence shows little change with fluence, indicating that the decay is governed by first-order recombination kinetics. We find that most photocarriers make a negligible contribution to the dc photoconductivity, and we estimate the intrinsic photocarrier mobility to be 0.6-0.7 cm$^2$/V s at early times, comparable to the mobility in chemically doped materials.

Time-resolved terahertz time-domain spectroscopy (THz-TDS) is an ideal tool for probing photoinduced nonequilibrium metallic and superconducting states. Here, we focus on the interpretation of the two-dimensional response function $\Sigma(\omega;t)$ that it measures, examining whether it provides an accurate snapshot of the instantaneous optical conductivity, $\sigma(\omega;t)$. For the Drude model with a time-dependent carrier density, we show that $\Sigma(\omega;t)$ is not simply related to $\sigma(\omega;t)$. The difference in the two response functions is most pronounced when the momentum relaxation rate of photocarriers is long, as would be the case in a system that becomes superconducting following pulsed photoexcitation. From the analysis of our model, we identify signatures of photoinduced superconductivity that could be seen by time-resolved THz-TDS., Comment: Replaced Fig. 4; minor revisions to captions, text, and bibliography

We report measurements of the polar Kerr effect, proportional to the out-of-plane component of the magnetization, in thin films of the magnetically doped topological insulator $(\text{Cr}_{0.12}\text{Bi}_{0.26}\text{Sb}_{0.62})_2\text{Te}_3$. Measurements of the complex Kerr angle, $\Theta_K$, were performed as a function of photon energy in the range $0.8\text{ eV}<\hbar\omega<3.0\text{ eV}$. We observed a peak in the real part of $\Theta_K(\omega)$ and zero crossing in the imaginary part that we attribute to resonant interaction with a spin-orbit avoided crossing located $\approx$ 1.6 eV above the Fermi energy. The resonant enhancement allows measurement of the temperature and magnetic field dependence of $\Theta_K$ in the ultrathin film limit, $d\geq2$ quintuple layers. We find a sharp transition to zero remanent magnetization at 6 K for $d<8$~QL, consistent with theories of the dependence of impurity spin interactions on film thickness and their location relative to topological insulator surfaces., Comment: 6 pages, 5 figures

Energy- and time-resolved spectroscopy reveals a photoinduced softening of the charge-transfer gap in the insulating copper oxide Sr2CuO2Cl2 that indicates rapid and efficient photoproduction of optical phonons. By relating the pump-probe signal amplitude to the thermal difference spectrum, we estimate that eleven to twenty optical phonons are created for every one 3 eV photon. Assuming relaxation to the optical absorption edge at 1.5 eV, this corresponds to 70-130 meV per boson. While the lower limit is consistent with relaxation exclusively through optical phonons, the upper limit suggests a significant role for magnetic excitations. We observe a photoinduced bleaching of the gap excitation that we associate with phase space filling, and estimate the excluded area of the photoexcited state to be about nine copper oxide plaquettes. The temporal decay of the pump-probe signal is consistent with anharmonic phonon decay.

We present the first study of nonlinear optical third harmonic generation in the strongly correlated charge-transfer insulator Sr2CuO2Cl2. For fundamental excitation in the near-infrared, the THG spectrum reveals a strongly resonant response for photon energies near 0.7 eV. Polarization analysis reveals this novel resonance to be only partially accounted for by three-photon excitation to the optical charge-transfer exciton, and indicates that an even-parity excitation at 2 eV, with a_1g symmetry, participates in the third harmonic susceptibility., Comment: Requires RevTeX v4.0beta5

We present a comparative experimental and theoretical investigation of the two--dimensional charge-transfer gap in the strongly correlated material Sr$_2$CuO$_2$Cl$_2$. We observe an Urbach behaviour in the absorption profile over a surprisingly wide range of energies and temperatures. We present a model that accounts for phonon scattering to infinite order and which allows us to explain the data accurately by assuming coupling of the charge transfer gap exciton to lower energy electronic excitations., Comment: 20 pages, 6 figures, submitted to Phys. Rev. B

We combine the results of terahertz time-domain spectroscopy with far-infrared transmission and reflectivity to obtain the conductivity of SrRuO_3 over an unprecedented continuous range in frequency, allowing us to characterize the approach to zero frequency as a function of temperature. We show that the conductivity follows a simple phenomenological form, with an analytic structure fundamentally different from that predicted by the standard theory of metals., Comment: 4 pages, 3 figures, revtex4b4

We have created a nonequilibrium population of antiferromagnetic spin-waves in Cr2O3, and characterized its dynamics, using frequency- and time-resolved nonlinear optical spectroscopy of the exciton-magnon transition. We observe a time-dependent pump-probe line shape, which results from excitation induced renormalization of the spin-wave band structure. We present a model that reproduces the basic characteristics of the data, in which we postulate the optical nonlinearity to be dominated by interactions with long-wavelength spin-waves, and the dynamics to be due to spin-wave thermalization., Comment: 4 pages, Revtex, with 3 figures embedded in the text