arXiv subjects

Lara Benfatto

Publications and source records attributed to Lara Benfatto.

At least 19 recordsLinked to original sources

arXiv2026

Measuring momentum-resolved dissipation of phonon-polaritons in LiNbO$_3$ with terahertz driving

Mapping the dispersion of polaritons, hybrid quasiparticles arising from light-matter coupling, can provide key insights into the material dielectric response, coupling strength, and energy transfer pathways with other excitations. In this work, we present THz pump-Raman probe (TP-RP) as a versatile method for mapping the polariton dispersion in polar non-centrosymmetric materials, demonstrated here for the case of phonon-polaritons in LiNbO$_3$. By resonantly driving polaritonic modes with a broadband THz pump and probing them with a tunable NIR Raman pulse, TP-RP allows for the extraction of the momentum-dependence of both their frequency and damping rate with high accuracy. The spectral features observed in the pump-probe signal, including the polaritonic response as well as pulse artifacts, are reproduced within a many-body theoretical approach. Applying the technique to study the E(TO$_1$) phonon of LiNbO$_3$ enables the combined analysis of theory and experiments to uncover a nontrivial frequency dependence of the phonon intrinsic damping rate, revealing possible anharmonic couplings to other modes.

cond-mat.mtrl-sci↗

arXiv2026

Coherent driving of displacive Higgs fluctuations in superconductors

Intense phase-stable terahertz (THz) laser pulses can drive collective modes coherently via multi-photon excitation pathways in a manner different than the standard resonant mechanism operative in linear response. Here we show that in superconductors the nonlinear optical response can be enhanced when excited quasiparticles activate a (non-resonant) static displacement of the superconducting order parameter, in full analogy with the displacive excitation of coherent phonons in opaque materials. By combining numerical simulations with analytical results we demonstrate that the displacive mechanism to excite the Higgs mode is operative in both $s$-wave and $d$-wave superconductors. We validate this prediction experimentally by the temperature dependence of the phase of the nonlinear first harmonic in superconducting $s$-wave NbN. We also discuss how the order-parameter relaxation at large times, which can be experimentally accessed via pump-probe protocols is connected to energy-dissipative processes. Our results offer a novel perspective on the ability of intense THz fields to measure, and eventually control, the parametric dependence of the optical response on collective degrees of freedom.

cond-mat.supr-con↗

arXiv2026

Light-induced Faraday effect from dynamical breakdown of Kleinman symmetry

The observation of anomalously large polarization rotations in pump-probe experiments with circularly polarized light has recently challenged the conventional understanding of the inverse Faraday effect. The striking magnitude of these responses implies the generation of effective magnetic fields orders of magnitude larger than theoretical expectations, raising fundamental questions about the nature of light-induced time-reversal symmetry breaking. In this work we demonstrate that a static polarization rotation can originate entirely from the antisymmetric component of the third-order optical susceptibility, without generating a macroscopic magnetization of the material. We show that this light-induced Faraday effect is inherently dynamical, emerging when Kleinman symmetry breaks down. Using a minimal sp tight-binding model on a square lattice, we demonstrate that the light-induced Faraday response can be sizable even far from dissipative resonances. While the effect emerges at a purely electronic level, we show that resonant coupling with phonons can significantly enhance the pump-probe response.

cond-mat.mtrl-sci↗

arXiv2026

Ultrafast Two-Dimensional Spectroscopy Uncovers Ubiquitous Electron-Paramagnon Coupling in Cuprate Superconductors

The coupling between electronic excitations and collective bosonic modes is fundamental to the emergence of high-temperature superconductivity in cuprates. Despite extensive effort, conventional equilibrium and pump-probe optical spectroscopies still struggle to disentangle couplings to different bosonic modes when their energy scales overlap. Here we overcome this limitation using ultrafast two-dimensional electronic spectroscopy (2DES), which correlates coherent excitation and detection photon energies with femtosecond time resolution. Applied to optimally doped Bi$_2$Sr$_2$Ca$_{0.92}$Y$_{0.08}$Cu$_2$O$_{8+\delta}$, 2DES reveals a pronounced off-diagonal resonance arising from the ultrafast generation of non-thermal bosons with energy $\hbar\Omega_\mathbf{q}\simeq200$ meV. By comparing the measured spectra with a theoretical framework that explicitly includes the interaction between charge-transfer and magnetic excitations, we identify these bosons as paramagnons with momenta centered near $(\pi/2,\pi/2)$ and extending toward $(0,\pi)$ and $(\pi,0)$. The resonance persists across a large range of temperatures and doping concentrations, demonstrating that high-energy paramagnons are ubiquitously and strongly coupled to electronic excitations throughout the cuprate phase diagram. Time-domain analysis constrains the build-up of the paramagnon population to $\lesssim 10$ fs, placing a lower bound $\lambda \gtrsim 0.7$ on the coupling strength. More broadly, our results establish 2DES as a powerful approach for disentangling mode-selective electron-boson interactions and addressing decoherence dynamics, thereby establishing a new avenue for investigating strongly correlated quantum materials. These findings also provide a direct framework for future time-resolved resonant inelastic X-ray scattering experiments aimed at tracking the ultrafast dynamics of magnetic excitations.

cond-mat.supr-con↗

arXiv2025

Raman fingerprint of high-temperature superconductivity in compressed hydrides

The discovery of high-temperature superconductivity in hydrogen-rich compounds under extreme pressures has prompted great excitement, intense research, but also debate over the past decade. Electrical transport has been the primary diagnostic tool for identifying superconductivity in these systems, whereas complementary probes, including magnetic, spectroscopic, tunnelling and ultrafast methods, remain mostly qualitative due to experimental constraints and sample heterogeneity. Recent concerns over their reliability have fuelled controversy, leading to scepticism and pointing out the need for alternative, quantitative approaches. In this study, we acquired unprecedented high-quality Raman spectra of hexagonal LaH10 at approximately 145 GPa and low temperatures, in conjunction with electrical transport measurements. Upon cooling, we observe a drop of resistivity and simultaneous remarkable variations of phonon frequencies and linewidths. These effects are interpreted and perfectly reproduced by the Migdal-Eliashberg theory, providing a definitive proof of phonon-mediated superconductivity and enabling a quantitative determination of the superconducting energy gap. Our results establish Raman spectroscopy as a robust, contact-free probe with micrometric resolution for studying high temperature superconductivity, opening a powerful route to its discovery and characterization.

cond-mat.supr-con↗

arXiv2025

Two-dimensional THz spectroscopy in electronic systems: a many-body diagrammatic approach

The term two-dimensional coherent spectroscopy (2DCS) usually refers to experimental setups where a coherently generated electric field in a sample is recorded over many runs as a function of two time variables: the delay $\tau$ between two consequent excitation pulses and the time $t$ over which the signal is emitted. While its implementation in the femtosecond time domain for studying vibrational molecular states has been developed for over two decades, its experimental application in the THz domain to interacting electronic systems remains in its infancy. This work provides a general theoretical framework for describing and interpreting 2DCS using a many-body language based on a perturbative diagrammatic expansion, as widely applied in linear spectroscopy. Focusing on centrosymmetric systems, we show that interpreting the 2D maps can be recast into two complementary problems. The first is the evaluation of a third-order response function to the gauge field. In the velocity gauge, this leads to semi-analytical expressions that both reduce computational complexity and assist in assigning spectral features to microscopic processes, as shown using a toy model of electrons undergoing a charge-density wave transition. The second is a careful treatment of multi-wave propagation effects, which, in bulk systems, can obscure the intrinsic nonlinear response, demonstrated here for soft superconducting Josephson plasmons. Our results provide a solid foundation for extending 2DCS to complex interacting systems and offer a flexible method to realistically model nonlinear responses across arbitrary spectral widths.

cond-mat.supr-con↗

arXiv2024

Ghost Josephson plasmon in bilayer superconductors

The experimental measurement of collective charge fluctuations in metals and superconductors is a preferential tool to benchmark fundamental interactions in solids. Recent experiments in multicomponent systems, from superconducting layered cuprates to multiband metals, highlighted striking effects due to the interplay between different degrees of freedom. In this paper we provide a physical explanation for the existence of a "ghost" Josephson plasmon in bilayer superconductors, layered systems with two layers per unit cells that interact with two different Josephson couplings. We show that one of the two plasmons that emerge after the breaking of the translational symmetry along the out-of-plane direction is connected to counterflowing current fluctuations polarized perpendicularly to the planes. This effect makes it a staggered mode that is virtually transverse at small out-of-plane momenta qc, explaining why it is hidden in the density response at small qc. Our work offers an additional perspective on the understanding of collective excitations in systems with multiple intertwined degrees of freedom.

cond-mat.supr-con↗

arXiv2024

Phonon-Polaritons in Non-Centrosymmetric Systems: Theory of Terahertz Pump-Optical Probe Spectroscopy

Hybrid lattice-light modes, known as phonon-polaritons, represent the backbone of advanced protocols based on THz pumping of infrared modes. Here we provide a theoretical framework able to capture the different roles played by phonon-polaritons in experimental protocols based either on Raman-like pump and probe schemes, typical of four-wave-mixing processes, or on THz pump-visible probe three-wave mixing protocols. By using a many-body description of the nonlinear optical kernel, along with a perturbative solution of nonlinear Maxwell's equations, we highlight the advantages of exploiting broadband THz pumps to enlarge the phase space of the phonon-polariton dispersion accessible in a single experiment. Besides providing a quantitative description of existing and future experiments, our results offer a general framework for the theoretical modeling of the hybridization between light and lattice degrees of freedom in time-resolved experiments.

cond-mat.mtrl-sci↗

arXiv2024

Fragile-to-strong glass transition in two-dimensional vortex liquids

The fragile-to-strong glass transition is a fascinating phenomenon that still presents many theoretical and experimental challenges. A major one is how to tune the fragility of a glass-forming liquid. Here, we study a two-dimensional (2D) system composed of vortices in a superconducting film, which effectively behaves as a 2D glass-forming liquid. We show that the kinetic fragility in this system can be experimentally varied by tuning a single parameter: the external magnetic field $H$ applied transversely to the film. This conclusion is supported by the direct comparison between the analysis of experimental measurements in an amorphous MoGe superconducting film and Monte Carlo simulations in a disordered XY model, that captures the universality class of the two-step melting transition. We show that by increasing disorder strength a fragile-to-strong transition is induced, in close similarity with the experimental findings in a magnetic field. Our numerical results shed light on the evolution of the dynamical heterogeneity from a fragile to strong glass, as due to the subtle interplay between caging effects arising from hexatic order and strong random pinning.

cond-mat.supr-con↗

arXiv2024

Terahertz ionic Kerr effect: Two-phonon contribution to the nonlinear optical response in insulators

The THz Kerr effect measures the birefringence induced in an otherwise isotropic material by a strong THz pulse driving the Raman-active excitations of the systems. Here we provide experimental evidence of a sizable Kerr response in insulating SrTiO3 due to infrared-active lattice vibrations. Such a signal, named ionic Kerr effect, is associated with the simultaneous excitation of multiple phonons. Thanks to a theoretical modeling of the time, polarization and temperature dependence of the birefringence we can disentangle the ionic Kerr effect from the off-resonant electronic excitations, providing an alternative tunable mechanism to modulate the refractive index on ultrashort time-scales via infra-red active phonons.

cond-mat.mtrl-sci↗

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↗

arXiv2024

Optical absorption in tilted geometries as an indirect measurement of longitudinal plasma waves in layered cuprates

Electromagnetic waves propagating in a layered superconductor with arbitrary momentum with respect to the main crystallographic directions display an unavoidable mixing between longitudinal and transverse degrees of freedom. Here we show that this basic physical mechanism explains the emergence of a well-defined absorption peak in the in-plane optical conductivity for light propagating at small tilting angles with respect to the stacking direction in layered cuprates. More specifically, we show that this peak, often interpreted as a spurious leakage of the $c$-axis Josephson plasmon, is instead a signature of the true longitudinal plasma mode occurring at larger momenta. By combining a classical approach based on Maxwell's equations with a full quantum derivation of the plasma modes based on the modelling of the superconducting phase degrees of freedom, we provide an analytical expression for the absorption peak as a function of the tilting angle and light polarization. We suggest that an all-optical measurement in tilted geometry can be used as an alternative way to access plasma-wave dispersion, usually measured by means of large-momenta scattering techniques like resonant inelastic X-ray scattering (RIXS) or electron energy loss spectroscopy (EELS).

cond-mat.supr-con↗

arXiv2023

Unveiling the pairing Symmetry of the superconducting Sn/Si(111) via angle-resolved THz pump spectroscopy

Doping tin surface epitaxially grown on silicon, Sn/Si(111), with boron atoms yields the appearance of a superconducting (SC) phase below $T_c\sim 4-5$K. Even though the pairing mechanism remains unknown, experimental evidence of chiral $d-$wave superconductivity has been recently reported, then ruling out a phonon-mediated pairing. Here we study theoretically the SC phase and symmetries of the doped Sn/Si(111) within a $t-J$ model. We analyze the photo-excitation of the system by intense THz pulses and show that the polarization dependence of the induced current can distinguish between different symmetries of the SC gap, thus providing a novel experimental mean to investigate the spectroscopic features of the Sn/Si(111) across the SC transition.

cond-mat.supr-con↗

arXiv2023

Investigating Josephson plasmons in layered cuprates via nonlinear terahertz spectroscopy

Josephson plasmons in layered superconductors represent a natural source of optical non-linearity, thanks to their intrinsically anharmonic nature. Here we derive the selection rules behind non-linear plasmonics showing its dependence on plasmonic branches hidden to other spectroscopies, like RIXS. We benchmark our results for the case of layered cuprates, showing how in a layered system the combined effect of plasmon dispersion and light polarization can move the resonance of the bilayer system away from the plasma edge measured in linear spectroscopy. Our results demonstrate the dependence of the non-linear THz response on the convoluted plasmon dispersion in a momentum region complementary to RIXS, and offer a possible perspective for the generation of THz pulses by artificially designed Josephson heterostructures.

cond-mat.supr-con↗

arXiv2023

Charge density response in layered metals: retardation effects, generalized plasma waves and their spectroscopic signatures

Transverse plasma polaritons and longitudinal plasmons describe the propagation of light-matter modes in an isotropic metal. However, in a layered metal the anisotropy of the bare electromagnetic response mixes the longitudinal and transverse excitations, making the distinction between polariton and plasmon blurred at small wavevectors, where retardation effects of the electromagnetic interactions become quantitatively relevant. In the usual Kubo approach for the linear response, this effect appears as a mixing between the density and the transverse current fluctuations, that requires to revise the standard RPA approach for density correlations where only the instantaneous Coulomb potential is included. In this paper we derive the general expression for the density and current correlation functions at long wavelength in a layered metal, showing that below a crossover scale set by the anisotropy of the plasma frequencies retardation effects make the dispersion of the generalized plasma modes different from the standard RPA result. In addition, the mixed longitudinal and transverse nature of these excitations reflects in a double-peak structure for the density response, that can be eventually accessed by means of high-momentum resolution electron-energy-loss or X-rays spectroscopies.

cond-mat.str-el↗

arXiv2023

Generalized Josephson plasmons in bilayer superconductors

Layered superconductors like High-Tc cuprates display out-of-plane plasma oscillations between layers sustained by the weak Josephson coupling among the superconducting sheets, the so-called Josephson plasmons. Bilayer cuprates hosts two of such modes, but due to the anisotropy of the electronic response their description at generic wavevector cannot be separated from that of the in-plane oscillations. In this paper we provide an analytical theoretical framework able to describe the dispersions and the polarizations of the generalized plasma modes of such systems, that has been only partly addressed by previous work in the literature. We then employ it to explain the peculiar characteristics of their linear optical response, by providing a fully microscopic explanation for the appearance of a finite-frequency peak in the real part of the optical conductivity. On a wider perspective, the complete characterization of the Josephson plasma modes provided by our approach represents a groundwork to address open issues raised by recent experiments with strong THz pulses, able to drive them beyond the linear-response regime.

cond-mat.supr-con↗

arXiv2023

Tracing the dynamics of superconducting order via transient third harmonic generation

Ultrafast optical control of quantum systems is an emerging field of physics. In particular, the possibility of light-driven superconductivity with ultrashort laser pulses has attracted much of attention. To identify non-equilibrium superconductivity, it is necessary to measure fingerprints of superconductivity on ultrafast timescales. Recently non-linear THz third harmonic generation (THG) was shown to directly probe the collective degrees of freedoms of the superconducting condensate including particularly the Higgs mode. Here we extend this idea to light-driven non-equilibrium states in superconducting La2-xSrxCuO4 establishing a protocol to access the transient superconducting (SC) order-parameter fluctuations. We perform an optical pump-THz-THG drive experiment and use a two-dimensional spectroscopy approach to disentangle the driven third-harmonic response of optically excited quasiparticles and the pure condensate response. In this way, 2D spectroscopy separately probes both the ultrafast pair breaking dynamics and transient pairing amplitude of the condensate.

cond-mat.supr-con↗

arXiv2022

Resistivity anisotropy from the multiorbital Boltzmann equation in nematic FeSe

We compute the resistivity anisotropy in the nematic phase of FeSe from the static solution of the multiorbital Boltzmann equation. By introducing disorder at the level of the microscopic multiorbital model we show that even elastic scattering by localized impurities may lead to non-trivial anisotropic renormalization of the electronic velocities, challenging the usual understanding of transport based only on cold- and hot-spots effects. Our model takes into account both the $xz/yz$ and the recently proposed $xy$ nematic ordering. We show that the latter one has a crucial role in order to reproduce the experimentally-measured anisotropy, providing a direct fingerprint of the different nematic scenarios on the bulk transport property of FeSe.

cond-mat.str-el↗