arXiv subjects

Marios H. Michael

Publications and source records attributed to Marios H. Michael.

At least 19 recordsLinked to original sources

arXiv2026

Optical and magnetic signatures of drive-enhanced coherence in phase-disordered superconducting bilayers

Recent experiments demonstrating superconducting-like characteristics in materials above their equilibrium transition temperatures following strong sub-picosecond optical excitation have stimulated theoretical interest in phase-disordered superconductors subject to a periodically modulated superfluid stiffness. Previous analyses of the single-layer optical conductivity have shown that such a drive can enhance the low-frequency $1/ω$ response, even when the period-averaged stiffness remains unchanged. We extend these studies to bilayer systems, such as the YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) cuprate, by considering drives with different symmetries. Specifically, we investigate a layer-symmetric drive, in which the two planes stiffen and soften simultaneously, and a layer-antisymmetric drive, in which one plane stiffens while the other softens. Both drives enhance correlations of the layer-averaged phase and generate comparable superconducting-like optical conductivities. Only the symmetric drive, however, strengthens relative-phase locking and the associated counterflow response to a magnetic field parallel to the planes. Thus, an enhanced $1/ω$ optical conductivity does not by itself imply enhanced magnetic screening. In view of applications to driven YBCO, where superconducting-like in-plane optics and transient diamagnetism have both been reported, our result provides a symmetry diagnostic: within a phase-synchronization mechanism, the experimentally active low-frequency stiffness modulation must contain a substantial layer-symmetric component.

cond-mat.supr-con↗

arXiv2026

Cavity Control of Strongly Correlated Electrons Beyond Resonant Coupling

Interfacing materials with electromagnetic cavities offers a route to modify equilibrium properties through structured vacuum fluctuations. The coupling between light and correlated electrons lacks a characteristic energy scale, making vacuum induced ground state modifications of such systems inherently off-resonant and sensitive to the full photon mode structure. We develop a consistent cavity-QED formalism for dispersive electromagnetic environments in the Coulomb gauge, capturing both dynamical dressing via the vector potential and static screening by the dielectric. With this formalism, we perform a non-perturbative study of the cavity-induced modification of magnetism in the Hubbard model close to half-filling, including all cavity modes and with parameters determined from first principles. At half-filling, we show that the modification of the magnetic exchange interaction $J$ is controlled by a generalized Purcell factor, proportional to the frequency integrated photonic density of states relative to free space. This result identifies polaritonic surface cavities as promising platforms to modify correlated systems. For the surface cavity a competition between static screening and dynamical dressing via the vector potential leads to a net enhancement of $J$, directly observable in two-magnon Raman spectroscopy. The inclusion of screening is essential to obtain even qualitatively correct results. At weak doping, a variational exact diagonalization of the cavity-coupled $t$-$J$ polaron reveals that a low-frequency surface cavity can reverse the nodal-antinodal dichotomy of the bare polaron dispersion. This effect lies beyond mean field theory and is observable via ARPES measurements. Our framework establishes a concrete design principle linking cavity geometry to material response in the off-resonant regime, which will guide future experimental and theoretical explorations.

quant-ph↗

arXiv2026

Flux-Floquet instability in fluctuating superconductors

In the past decade, photo-induced superconducting-like behaviors have been reported in a number of materials driven by intense pump fields. Of particular interest is the high-Tc cuprate YBCO, where such effect has been reported up to the so-called pseudogap temperature T*~300-400 K. In a recent experiment, a transient magnetic field which is proportional to and in the same direction of an applied field has been observed outside the sample, suggestive of flux exclusion due to the Meissner effect. In this paper, we present an alternative interpretation of these experiments based on a mechanism that we term the flux-Floquet instability of the sine-Gordon (SG) model. We take as our premise the model of preformed Cooper pairs in the pseudogap phase. Starting from the local superconducting order parameter in equilibrium, we introduce an extended SG model to describe the dynamics of the relative phase between the layers of a bilayer. We demonstrate that a combination of external magnetic field and strong terahertz drive used in experiments by Fava et al. results in a novel Floquet instability in the model. This instability leads to currents at the edges of the bilayer formed by defects or grain boundaries, with the current flowing in the opposite direction of the equilibrium screening current, producing a giant paramagnetic magnetization in the same direction as the applied field. We show how this scenario can fit most of the available data. To the extent that this model can account for the data, we conclude that the experiments have the important consequence of revealing the presence of local pairing in the pseudogap phase. More broadly our results reveal a new instability in the SG equation that is of fundamental interest, with potential applications such as providing a mechanism for generating large magnetic fields at ultrafast time scales in Josephson devices.

cond-mat.supr-con↗

arXiv2026

Parametrically amplified Josephson plasma waves in YBa_2Cu_3O_(6+x): evidence for local superconducting fluctuations up to the pseudogap temperature $T^*$

Experiments that subject underdoped $\rm{YBa_2Cu_3O_{6+x}}$ (YBCO) to intense terahertz pulses at temperatures between the transition temperature $T_c$ and the pseudogap scale $T^*$ have revealed a reflectivity edge that resembles that of the superconducting state, together with second harmonic generation of a probe pulse modulated at a similar frequency. These have been interpreted in terms of parametric amplification of the lower Josephson plasmon mode. Since this mode is often associated with coherent oscillations between bilayers in the YBCO structure, these experiments have led to the suggestion that the intense pump has created (or revealed) in-plane pair coherence up to $T^* \approx 400K$. In this paper we propose an alternative explanation by assuming the existence of local pair amplitude and phase at equilibrium for $T_c < T < T^*$. The phase correlation spans only a few lattice constants and we do not assume any pump-induced enhancement of this correlation, either in-plane or between bilayers. Instead, the coherent drive, via a parametric amplification process, induces coherence in the Josephson currents between members of bilayers. When combined with a Floquet framework, the reflectivity data can be explained. The key point is that in the lower Josephson plasmon, the coupling between bilayers is mainly capacitive; the Josephson current between bilayers can be set to zero without strongly affecting the parametric amplification process. Importantly, while superconducting coherence may not be created by the pump, the pseudogap phase must possess a local pairing amplitude at equilibrium. Consequently, these experiments have strong implications for the understanding of the pseudogap phase.

cond-mat.supr-con↗

arXiv2026

Finite-momentum Cooper plasmons in superconducting terahertz microcavities

The phase mode of a superconductor's order parameter encodes fundamental information about pairing and dissipation, but is typically inaccessible at low frequencies due to the Anderson-Higgs mechanism. Superconducting samples thinner than the London penetration depth, however, support a gapless phase mode whose dispersion can be reshaped by a proximal screening layer. Here, we theoretically and experimentally show that this screened phase mode in a superconducting thin film integrated into on-chip terahertz circuitry naturally forms a superconducting microcavity that hosts resonant finite-momentum standing waves of supercurrent density, which we term Cooper plasmons. We measure two Cooper plasmons in a superconducting NbN microcavity and demonstrate that their resonance frequencies and linewidths independently report the density of participating carriers and plasmon's dissipation at finite momenta. Our results reveal an emergent collective mode of an integrated superconductor-circuit system and establish design principles for engineering or suppressing Cooper plasmons in superconducting terahertz devices and circuits.

cond-mat.supr-con↗

arXiv2026

Response regimes in on-chip THz spectroscopy

On-chip THz spectroscopy enables quantitative measurements of the optical conductivity of sub-wavelength 2D materials by tightly confining THz fields in metallic transmission line structures interfaced to the material. However, because the probed structures are smaller than the THz wavelength, finite-size and environmental effects can strongly influence the measured response. Here, we identify the conditions under which a metallic sample exhibits a genuine Drude response and when finite-size and environmental effects must be considered. We further introduce and characterize an additional regime, the Phantom-Drude response, which mimics Drude behavior but instead originates from the superposition of multiple finite-momentum plasmonic resonances. If unrecognized, this regime can lead to misinterpretation of intrinsic material properties. We systematically show how the Phantom-Drude response can emerge and demonstrate its sensitivity to sample dimensions, transmission line geometry, material shape, and gate properties, providing practical guidelines to avoid this regime in future on-chip THz measurements.

cond-mat.mes-hall↗

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

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

Visualizing a Terahertz Superfluid Plasmon in a Two-Dimensional Superconductor

The superconducting gap defines the fundamental energy scale for the emergence of dissipationless transport and collective phenomena in a superconductor. In layered high-temperature cuprate superconductors, where the Cooper pairs are confined to weakly coupled two-dimensional copper-oxygen planes, terahertz (THz) spectroscopy at sub-gap millielectronvolt energies has provided crucial insights into the collective superfluid response perpendicular to the superconducting layers. However, within the copper-oxygen planes the collective superfluid response manifests as plasmonic charge oscillations at energies far exceeding the superconducting gap, obscured by strong dissipation. Here, we present spectroscopic evidence of a below-gap, two-dimensional superfluid plasmon in few-layer Bi2Sr2CaCu2O8+x and spatially resolve its deeply sub-diffractive THz electrodynamics. By placing the superconductor in the near-field of a spintronic THz emitter, we reveal this distinct resonance-absent in bulk samples and observed only in the superconducting phase-and determine its plasmonic nature by mapping the geometric anisotropy and dispersion. Crucially, these measurements offer a direct view of the momentum- and frequency dependent superconducting transition in two dimensions. These results establish a new platform for investigating superfluid phenomena at finite momenta and THz frequencies, highlighting the potential to engineer and visualize superconducting devices operating at ultrafast THz rates.

cond-mat.supr-con↗

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

Purcell enhancement of photogalvanic currents in a van der Waals plasmonic self-cavity

Cavities provide a means to manipulate the optical and electronic responses of quantum materials by selectively enhancing light-matter interaction at specific frequencies and momenta. While cavities typically involve external structures, exfoliated flakes of van der Waals (vdW) materials can form intrinsic self-cavities due to their small finite dimensions, confining electromagnetic fields into plasmonic cavity modes, characterized by standing-wave current distributions. While cavity-enhanced phenomena are well-studied at optical frequencies, the impact of self-cavities on nonlinear electronic responses--such as photogalvanic currents--remains largely unexplored, particularly in the terahertz regime, critical for emerging ultrafast optoelectronic technologies. Here, we report a self-cavity-induced Purcell enhancement of photogalvanic currents in the vdW semimetal WTe$_2$. Using ultrafast optoelectronic circuitry, we measured coherent near-field THz emission resulting from nonlinear photocurrents excited at the sample edges. We observed enhanced emission at finite frequencies, tunable via excitation fluence and sample geometry, which we attribute to plasmonic interference effects controlled by the cavity boundaries. We developed an analytical theory that captures the cavity resonance conditions and spectral response across multiple devices. Our findings establish WTe$_2$ as a bias-free, geometry-tunable THz emitter and demonstrate the potential of self-cavity engineering for controlling nonlinear, nonequilibrium dynamics in quantum materials.

cond-mat.mes-hall↗

arXiv2025

Resolving self-cavity effects in two-dimensional quantum materials

Two-dimensional materials and van der Waals (vdW) heterostructures host many strongly correlated and topological quantum phases on the $\sim$ meV energy scale. Direct electrodynamical signatures of such states are thus expected to appear in the terahertz (THz) frequency range (1 THz $\sim$ 4 meV). Because the typical size of vdW heterostructures ($\sim$10 $μm$) is much smaller than the diffraction limit of THz light, probing THz optical conductivities necessitates the use of near-field optical probes. However, interpreting the response of such near-field probes is complicated by finite-size effects, the presence of electrostatic gates, and the influence of the probe itself on material dynamics -- all of which conspire to form polaritonic self-cavities, in which interactions between THz electromagnetic fields and material excitations form discretized standing waves. In this paper, we demonstrate the relevance of self-cavity effects in 2D materials and derive an analytical framework to resolve these effects using the emerging experimental technique of time-domain on-chip THz spectroscopy. We show that by pairing experiments with the analytical theory, it is possible to extract the THz conductivity and resolve collective mode dynamics far outside the light cone, with $\sim μm$ in-plane and $\sim nm$ out-of-plane resolution. This study lays the groundwork for studying quantum phases and cavity effects in vdW heterostructures and 2D quantum materials.

cond-mat.str-el↗

arXiv2025

Equilibrium non-linear phononics by electric field fluctuations of terahertz cavities

Selective excitation of vibrational modes using strong laser pulses has emerged as a powerful material engineering paradigm. However, to realize deterministic control over material properties for device applications, it is desirable to have an analogous scheme without a drive, operating in thermal equilibrium. We here propose such an equilibrium analog of the light-driven paradigm, leveraging the strong coupling between lattice degrees of freedom and the quantum fluctuations of the electric field of a THz micro-cavity. We demonstrate this approach by showing, using ab initio data, how electric field fluctuations can induce a sub-dominant ferromagnetic order, on top of the dominant zig-zag antiferromagnet order, in FePS$_3$ close to its Néel temperature.

cond-mat.mtrl-sci↗

arXiv2024

Surface-mediated ultra-strong cavity coupling of two-dimensional itinerant electrons

Engineering phases of matter in cavities requires effective light-matter coupling strengths that are on the same order of magnitude as the bare system energetics, coined the ultra-strong coupling regime. For models of itinerant electron systems, which do not have discrete energy levels, a clear definition of this regime is outstanding to date. Here we argue that a change of the electronic mass exceeding $10\%$ of its bare value may serve as such a definition. We propose a quantitative computational scheme for obtaining the electronic mass in relation to its bare vacuum value and show that coupling to surface polariton modes can induce such mass changes. Our results have important implications for cavity design principles that enable the engineering of electronic properties with quantum light.

cond-mat.str-el↗

arXiv2024

Equilibrium Parametric Amplification in Raman-Cavity Hybrids

Parametric resonances and amplification have led to extraordinary photoinduced phenomena in pump-probe experiments. While these phenomena manifest themselves in out-of-equilibrium settings, here, we present the striking result of parametric amplification in equilibrium. In particular, we demonstrate that quantum and thermal fluctuations of a Raman-active mode amplifies light inside a cavity, at equilibrium, when the Raman mode frequency is twice the cavity mode frequency. This noise-driven amplification leads to the creation of an unusual parametric Raman polariton, intertwining the Raman mode with cavity squeezing fluctuations, with smoking gun signatures in Raman spectroscopy. In the resonant regime, we show the emergence of not only quantum light amplification but also localization and static shift of the Raman mode. Apart from the fundamental interest of equilibrium parametric amplification our study suggests a resonant mechanism for controlling Raman modes and thus matter properties by cavity fluctuations. We conclude by outlining how to compute the Raman-cavity coupling, and suggest possible experimental realization

quant-ph↗

arXiv2024

Nonperturbative Nonlinear Transport in a Floquet-Weyl Semimetal

Periodic laser driving, known as Floquet engineering, is a powerful tool to manipulate the properties of quantum materials. Using circularly polarized light, artificial magnetic fields, called Berry curvature, can be created in the photon-dressed Floquet-Bloch states that form. This mechanism, when applied to 3D Dirac and Weyl systems, is predicted to lead to photon-dressed movement of Weyl nodes which should be detectable in the transport sector. The transport response of such a topological light-matter hybrid, however, remains experimentally unknown. Here, we report on the transport properties of the type-II Weyl semimetal T$\mathrm{_d}$-MoTe$_\mathrm{2}$ illuminated by a femtosecond pulse of circularly polarized light. Using an ultrafast optoelectronic device architecture, we observed injection currents and a helicity-dependent anomalous Hall effect whose scaling with laser field strongly deviate from the perturbative laws of nonlinear optics. We show using Floquet theory that this discovery corresponds to the formation of a magnetic Floquet-Weyl semimetal state. Numerical ab initio simulations support this interpretation, indicating that the light-induced motion of the Weyl nodes contributes substantially to the measured transport signals. This work demonstrates the ability to generate large effective magnetic fields ($>$ 30T) with light, which can be used to manipulate the magnetic and topological properties of a range of quantum materials.

cond-mat.mes-hall↗

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↗