arXiv subjects

Andrea Cavalleri

Publications and source records attributed to Andrea Cavalleri.

At least 19 recordsLinked to original sources

arXiv2026

Dynamically suppressed lattice rotations in SrTiO$_3$ as a basis for photo-induced ferroelectricity

Photo-induced ferroelectricity in the quantum paraelectric SrTiO$_3$ involves the dynamical interplay between a coherently driven Ti-O stretching vibration and multiple structural degrees of freedom, including antiferrodistortive rotations, strain, and the polar mode instability. In the high-temperature cubic phase, in the absence of average antiferrodistortion, time-resolved X-ray diffuse scattering has evidenced a correlation between a photo-induced reduction in antiferrodistortive fluctuations and the emergence of ferroelectric order. Here, we complement these measurements with time-resolved elastic X-ray diffraction in the low-temperature tetragonal phase, in which antiferrodistortive fluctuations are small but a finite average rotation has set in. In this phase, we observe a long-lived reduction of the equilibrium antiferrodistortive rotation angle. A unified theory of the nonlinear lattice dynamics based on first-principles calculations describes the dynamics in both high-temperature cubic and low-temperature tetragonal phases, providing a basis for light-induced ferroelectricity in SrTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2026

Giant Resonant Enhancement of Photoinduced Dynamical Cooper Pairing, far above $T_c$

Pump-probe experiments performed on $\mathrm{K}_3\mathrm{C}_{60}$ have unveiled both optical and transport signatures of metastable light-induced superconductivity up to room temperature, far above $T_c$. Recent experiments have uncovered that excitation in the vicinity of $50 ~\textrm{meV}$ enables the observation of high temperature light-induced superconductivity at significantly lower fluences. Inspired by these experiments we develop a mechanism which can explain such a giant resonant enhancement of light-induced superconductivity. Within a minimal non-linear Holstein model, we show that resonantly driving optical Raman modes leads to a time-dependent electron-phonon coupling. Such a coupling then modulates the effective electron-electron attraction, with the strongest modulations occurring when the drive is resonant with the phonon frequency. These dynamical modulations of the pairing interactions lead to Floquet-BCS instabilities at temperatures far exceeding equilibrium $T_c$, as observed in experiments. We conclude by discussing the implications of our general analysis on the $\mathrm{K}_3\mathrm{C}_{60}$ experiments specifically and suggesting experimental signatures of our mechanism.

cond-mat.supr-con↗

arXiv2025

Re-entrant unconventional superconductivity induced by rare-earth substitution in Nd1-xEuxNiO2 thin films

High temperature superconductivity is typically associated with strong coupling and a large superconducting gap, yet these characteristics have not been demonstrated in the nickelates. Here, we provide experimental evidence that Eu substitution in the spacer layer of Nd1-xEuxNiO2 (NENO) thin films enhances the superconducting gap, driving the system toward a strong-coupling regime. This is accompanied by a magnetic-exchange-driven magnetic-field-enhanced superconductivity. We investigate the upper critical magnetic field, Hc2, and superconducting gap of superconducting NENO thin films with x=0.2 to 0.35. Magnetoresistance measurements reveal magnetic-field-enhanced superconductivity in NENO films. We interpret this phenomenon as a result of interaction between magnetic Eu ions and superconducting states in the Ni dx2-y2 orbital. The upper critical magnetic field strongly violates the weak-coupling Pauli limit. Infrared spectroscopy confirms a large gap-to-Tc ratio $2 Δk_B T_c \approx 5 - 6$, indicating a stronger coupling pairing mechanism in NENO relative to the Sr-doped NdNiO2. The substitution of Eu in the rare-earth layer provides a method to modify the superconducting gap in Nd-based nickelates, an essential factor in engineering high-Tc superconductivity in infinite-layer nickelates.

cond-mat.supr-con↗

arXiv2025

Enhanced coherence in the periodically driven two-dimensional XY model

Strong optical drives have been shown to induce transient superconducting-like response in materials above their equilibrium $T_c$. Many of these materials already exhibit short-range superconducting correlations in equilibrium. This motivates the question: can external driving enhance coherence in systems with superconducting correlations but no long-range order? We explore this scenario in the two-dimensional XY model with a periodically modulated stiffness using overdamped Langevin dynamics. We find that, even though the modulation leaves the average coupling unchanged, the drive can markedly increase long-range, time-averaged correlations in systems well above the equilibrium Berezinskii-Kosterlitz-Thouless temperature. The outcome depends on the ratio of the drive frequency to the intrinsic relaxation rate: faster drives primarily heat the system, suppressing correlations and conductivity. For slower drives, the optical conductivity is modified so that the real part exhibits a prolonged effective Drude scattering time, while the imaginary part has a strengthened low-frequency $1/ω$ behavior. We map out these regimes across temperature, frequency, and amplitude, and rationalize them via simple analytics and vortex-thermalization arguments. Overall, we identify a generic nonequilibrium route to enhance coherence in XY-like systems, with potential relevance to experiments reporting light-induced superconductivity.

cond-mat.supr-con↗

arXiv2025

Optically-Induced Faraday-Goldstone Waves

Faraday waves, typically observed in driven fluids, result from the confluence of nonlinearity and parametric amplification. Here we show that optical pulses can generate analogous phenomena that persist much longer than the pump time-scales in ordered quantum solids. We present a theory of ultrafast light-matter interactions within a symmetry-broken state; dynamical nonlinear coupling between the Higgs (amplitude) and the Goldstone (phase) modes drives an emergent phason texture that oscillates in space and in time: Faraday-Goldstone waves. Calculated signatures of this spatiotemporal order compare well with measurements on K$_{0.3}$MnO$_{3}$; Higgs-Goldstone beating, associated with coherent energy exchange between these two modes, is also predicted. We show this light-generated crystalline state is robust to thermal noise, even when the original Goldstone mode is not. Our results offer a new pathway for the design of periodic structures in quantum materials with ultrafast light pulses.

cond-mat.mes-hall↗

arXiv2025

Cavity-altered superconductivity

Is it feasible to alter the ground state properties of a material by engineering its electromagnetic environment? Inspired by theoretical predictions, experimental realizations of such cavity-controlled properties without optical excitation are beginning to emerge. Here, we devised and implemented a novel platform to realize cavity-altered materials. Single crystals of hyperbolic van der Waals (vdW) compounds provide a resonant electromagnetic environment with enhanced density of photonic states and prominent mode confinement. We interfaced hexagonal boron nitride (hBN) with the molecular superconductor $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br ($κ$-ET). The frequencies of infrared (IR) hyperbolic modes of hBN match the IR-active carbon-carbon stretching molecular resonance of ($κ$-ET) implicated in superconductivity. Nano-optical data supported by first-principles molecular Langevin dynamics simulations confirm the presence of resonant coupling between the hBN hyperbolic cavity modes and the carbon-carbon stretching mode in ($κ$-ET). Meissner effect measurements via magnetic force microscopy demonstrate a strong suppression of superfluid density near the hBN/($κ$-ET) interface. Non-resonant control heterostructures, including RuCl$_3$/($κ$-ET) and hBN/$\text{Bi}_2\text{Sr}_2\text{CaCu}_2\text{O}_{8+x}$, do not display the superfluid suppression. These observations suggest that hBN/($κ$-ET) realizes a cavity-altered superconducting ground state. Our work highlights the potential of dark cavities devoid of external photons for engineering electronic ground state properties of complex quantum materials.

cond-mat.supr-con↗

arXiv2025

Gap Inhomogeneity in Cuprates: a view from Two-Dimensional Josephson Echo Spectroscopy

Novel theoretical developments have allowed to connect microscopic disorder in bosonic collective excitations to the signatures in two-dimensional terahertz spectroscopy (Gómez Salvador et al. 2025). Here, we employ this framework to analyze the recently measured Josepshon echoes in optimally doped La$_{2-x}$Sr$_x$CuO$_4$ (Liu et al. 2024). We consider the spatial gap inhomogeneities -- observed in scanning tunneling microscopy -- as input for the disorder in the superfluid density, and compute the resulting echo peaks. The excellent agreement supports the interpretation that the gap inhomogeneity arises solely from pairing gap fluctuations, with no evidence for non-superconducting competing local orders. Finally, we study the microscopic origin of the inelastic processes, contributing to the damping of the Josephson plasmon at low temperatures, and conclude that it can be attributed to nodal quasiparticles.

cond-mat.supr-con↗

arXiv2025

Broadband Single-Shot THz Sampling Using Reflection Gratings

Single-shot electro-optic sampling (EOS) is a powerful method enabling the measurement of weak terahertz signals that would otherwise require prohibitively long acquisition times. This is generally achieved by encoding the EOS time delay into a spatial, angular, or frequency coordinate. In general, this technique operates well up to 3 THz but becomes more challenging for larger bandwidths, due to dispersion and imaging imperfections. Here, we demonstrate a reliable angular-encoding single shot EOS implementation that reaches frequencies beyond 6 THz. Diffraction simulations are used to design the experimental setup and adapt this technique to commercial reflection gratings, removing the need for custom-built echelon mirrors. Furthermore, we show that, contrary to earlier reports, group delay dispersion from angular dispersion does not reduce the bandwidth of single-shot EOS.

physics.optics↗

arXiv2025

Two-dimensional spectroscopy of bosonic collective excitations in disordered many-body systems

We present a novel theoretical approach for computing and analyzing two-dimensional spectroscopy of bosonic collective excitations in disordered many-body systems. Specifically, we employ the Keldysh formalism to derive, within a non-pertubative treatment of disorder effects, the third-order nonlinear response and obtain two-dimensional spectroscopy maps. In the weak nonlinear regime of our formalism, we demonstrate the ability of the echo peak to distinguish between elastic and inelastic scattering processes, in perfect agreement with the intuition developed in isolated two-level systems. Furthermore, we discuss unique many-body effects on the echo peak signature arising from interaction induced quantum fluctuations. In particular, we show that these quantum fluctuations induce a finite nonrephasable broadening and examine how the echo peak is influenced by the attractive or repulsive nature of the collective excitations.

cond-mat.str-el↗

arXiv2025

Space-time duality in polariton dynamics

The spatial and temporal dynamics of wave propagation are intertwined. A common manifestation of this duality emerges in the spatial and temporal decay of waves as they propagate through a lossy medium. A complete description of the non-Hermitian wave dynamics in such a lossy system, capturing temporal and spatial decays, necessitates the use of complex-valued frequency and/or wavenumber Eigen-values. Here, we demonstrate that the propagation of polaritons - hybrid light-matter quasiparticles - can be broadly controlled in space and time by temporally shaping their photonic excitation. Using time-domain terahertz near-field nanoscopy, we study plasmon polaritons in bilayer graphene at sub-picosecond time scales. Suppressed spatial decay of polaritons is implemented by temporally engineering the excitation waveform. Polaritonic space-time metrology data agree with our dynamic model. Through the experimental realization and visualization of polaritonic space-time duality, we uncover the effects of the spatio-temporal engineering of wave dynamics; these are applicable to acoustic, photonic, plasmonic, and electronic systems.

physics.optics↗

arXiv2024

Metastable Photo-Induced Superconductivity far above $T_{\textrm{c}}$

Inspired by the striking discovery of metastable superconductivity in $\mathrm{K}_3\mathrm{C}_{60}$ at 100K, far above $T_{\textrm{c}}=20K$, we discuss possible mechanisms for long-lived, photo-induced superconductivity. Starting from a model of optically-driven Raman phonons coupled to inter-band electronic transitions, we develop a microscopic mechanism for photo-controlling the pairing interaction. Leveraging this mechanism, we first investigate long-lived superconductivity arising from the thermodynamic metastable trapping of the driven phonon. We then propose an alternative route, where the superconducting gap created by an optical drive leads to a dynamical bottleneck in the equilibration of quasi-particles. We conclude by discussing implications of both scenarios for experiments that can be used to discriminate between them. Our work provides falsifiable explanations for the nanosecond-scale photo-induced superconductivity found in $\mathrm{K}_3\mathrm{C}_{60}$, while simultaneously offering a theoretical basis for exploring metastable superconductivity in other quantum materials.

cond-mat.supr-con↗

arXiv2024

Magnetic field expulsion in optically driven YBa$_2$Cu$_3$O$_{6.48}$

Coherent optical driving in quantum solids is emerging as a new research frontier, with many demonstrations of exotic non-equilibrium quantum phases. These are based on engineered band structures, and on stimulated nonlinear interactions between driven modes. Enhanced functionalities like ferroelectricity, magnetism and superconductivity have been reported in these non-equilibrium settings. In high-Tc cuprates, coherent driving of certain phonon modes induces a transient state with superconducting-like optical properties, observed far above T$_c$ and throughout the pseudogap phase. Questions remain not only on the microscopic nature of this phenomenon, but also on the macroscopic properties of these transient states, beyond the documented optical conductivities. Crucially, it is not clear if driven cuprates exhibit Meissner-like diamagnetism. Here, the time-dependent magnetic-field amplitude surrounding a driven YBa$_2$Cu$_3$O$_{6.48}$ sample is probed by measuring Faraday rotation in a GaP layer adjacent to the superconductor. For the same driving conditions that result in superconducting-like optical properties, an enhancement of magnetic field at the edge of the sample is detected, indicative of induced diamagnetism. The dynamical field expulsion measured after pumping is comparable in size to the one expected in an equilibrium type II superconductor of similar shape and size with a volume susceptibility $χ_v$ of order -0.3. Crucially, this value is incompatible with a photo-induced increase in mobility without superconductivity. Rather, it underscores the notion of a pseudogap phase in which incipient superconducting correlations are enhanced or synchronized by the optical drive.

cond-mat.supr-con↗

arXiv2024

Observation of polarization density waves in SrTiO3

The nature of the "failed" ferroelectric transition in SrTiO3 has been a long-standing puzzle in condensed matter physics. A compelling explanation is the competition between ferroelectricity and an instability with a mesoscopic modulation of the polarization. These polarization density waves, which should become especially strong near the quantum critical point, break local inversion symmetry and are difficult to probe with conventional x-ray scattering methods. Here we combine a femtosecond x-ray free electron laser (XFEL) with THz coherent control methods to probe inversion symmetry breaking at finite momenta and visualize the instability of the polarization on nanometer lengthscales in SrTiO3. We find polar-acoustic collective modes that are soft particularly at the tens of nanometer lengthscale. These precursor collective excitations provide evidence for the conjectured mesoscopic modulated phase in SrTiO3.

cond-mat.mtrl-sci↗

arXiv2024

Principles of 2D terahertz spectroscopy of collective excitations: the case of Josephson plasmons in layered superconductors

Two-dimensional terahertz spectroscopy (2DTS), a terahertz analogue of nuclear magnetic resonance, is a new technique poised to address many open questions in complex condensed matter systems. The conventional theoretical framework used ubiquitously for interpreting multidimensional spectra of discrete quantum level systems is, however, insufficient for the continua of collective excitations in strongly correlated materials. Here, we develop a theory for 2DTS of a model collective excitation, the Josephson plasma resonance in layered superconductors. Starting from a mean-field approach at temperatures well below the superconducting phase transition, we obtain expressions for the multidimensional nonlinear responses that are amenable to intuition derived from the conventional single-mode scenario. We then consider temperatures near the superconducting critical temperature $T_c$, where dynamics beyond mean-field become important and conventional intuition fails. As fluctuations proliferate near $T_c$, the dominant contribution to nonlinear response comes from an optical parametric drive of counter-propagating Josephson plasmons, which gives rise to 2D spectra that are qualitatively different from the mean-field predictions. As such, and in contrast to one-dimensional spectroscopy techniques, such as third harmonic generation, 2DTS can be used to directly probe thermally excited finite-momentum plasmons and their interactions. Our theory provides a clear interpretation of recent 2DTS measurements on cuprates, and we discuss implications beyond the present context of Josephson plasmons.

cond-mat.supr-con↗

arXiv2023

Probing photo-induced granular superconductivity in K$_{3}$C$_{60}$ thin films with an ultrafast on-chip voltmeter

The physics of optically-induced superconductivity remains poorly understood, with questions that range from the underlying microscopic mechanism to the macroscopic electrical response of the non-equilibrium phase. In this paper, we study optically-induced superconductivity in K$_{3}$C$_{60}$ thin films, which display signatures of granularity both in the equilibrium state below T$_{c}$ and in the nonequilibrium photo-induced phase above T$_{c}$. Photo-conductive switches are used to measure the ultrafast voltage drop across a K$_{3}$C$_{60}$ film as a function of time after irradiation, both below and above T$_{c}$. These measurements reveal fast changes associated with the kinetic inductance of in-grain superconductivity, and a slower response attributed to the Josephson dynamics at the weak links. Fits to the data yield estimates of the in-grain photo-induced superfluid density after the drive and the dynamics of phase slips at the weak links. This work underscores the increasing ability to make electrical measurements at ultrafast speeds in optically-driven quantum materials, and demonstrates a striking new platform for optoelectronic device applications.

cond-mat.supr-con↗

arXiv2023

Probing Inhomogeneous Cuprate Superconductivity by Terahertz Josephson Echo Spectroscopy

Inhomogeneities play a crucial role in determining the properties of quantum materials. Yet methods that can measure these inhomogeneities are few, and apply to only a fraction of the relevant microscopic phenomena. For example, the electronic properties of cuprate materials are known to be inhomogeneous over nanometer length scales, although questions remain about how such disorder influences supercurrents and their dynamics. Here, two-dimensional terahertz spectroscopy is used to study interlayer superconducting tunneling in near-optimally-doped La1.83Sr0.17CuO4. We isolate a 2 THz Josephson echo signal with which we disentangle intrinsic lifetime broadening from extrinsic inhomogeneous broadening. We find that the Josephson plasmons are only weakly inhomogeneously broadened, with an inhomogeneous linewidth that is three times smaller than their intrinsic lifetime broadening. This extrinsic broadening remains constant up to 0.7Tc, above which it is overcome by the thermally-increased lifetime broadening. Crucially, the effects of disorder on the Josephson plasma resonance are nearly two orders of magnitude smaller than the in-plane variations in the superconducting gap in this compound, which have been previously documented using Scanning Tunnelling Microscopy (STM) measurements. Hence, even in the presence of significant disorder in the superfluid density, the finite frequency interlayer charge fluctuations exhibit dramatically reduced inhomogeneous broadening. We present a model that relates disorder in the superfluid density to the observed lifetimes.

cond-mat.supr-con↗

arXiv2023

Theory for Anomalous Terahertz Emission in Striped Cuprate Superconductors

Recent experiments in the doped cuprates La$_{2-x}$Ba$_x$CuO$_4$ have revealed the emission of anomalous terahertz radiation after impulsive optical excitation. Here, we theoretically investigate the nonlinear electrodynamics of such striped superconductors and explore the origin of the observed radiation. We argue that photoexcitation is converted into a photocurrent by a second-order optical nonlinearity, which is activated by the breaking of inversion symmetry in certain stripe configurations. We point out the importance of including Umklapp photocurrents modulated at the stripe periodicity itself, which impulsively drive surface Josephson plasmons and lead to a resonant structure of outgoing radiation, consistent with the experiments. We speculate on the utility of the proposed mechanism in the context of generating tunable terahertz radiation.

cond-mat.supr-con↗

arXiv2023

Optically Induced Avoided Crossing in Graphene

Degenerate states in condensed matter are frequently the cause of unwanted fluctuations, which prevent the formation of ordered phases and reduce their functionalities. Removing these degeneracies has been a common theme in materials design, pursued for example by strain engineering at interfaces. Here, we explore a non-equilibrium approach to lift degeneracies in solids. We show that coherent driving of the crystal lattice in bi- and multilayer graphene, boosts the coupling between two doubly-degenerate modes of E1u and E2g symmetry, which are virtually uncoupled at equilibrium. New vibronic states result from anharmonic driving of the E1u mode to large amplitdues, boosting its coupling to the E2g mode. The vibrational structure of the driven state is probed with time-resolved Raman scattering, which reveals laser-field dependent mode splitting and enhanced lifetimes. We expect this phenomenon to be generally observable in many materials systems, affecting the non-equilibrium emergent phases in matter.

cond-mat.mtrl-sci↗