arXiv subjects

Claudio Castellani

Publications and source records attributed to Claudio Castellani.

At least 19 recordsLinked to original sources

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↗

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

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

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

Vortex supersolid in the XY model with tunable vortex fugacity

In this paper, we investigate the XY model in the presence of an additional potential term that independently tunes the vortex fugacity favouring their nucleation. By increasing the strength of this term and thereby the vortex chemical potential {\mu}, we observe significant changes in the phase diagram with the emergence of a normal vortex-antivortex lattice as well as a superconducting vortex-antivortex crystal (lattice supersolid) phase. We examine the transition lines between these two phases and the conventional non-crystalline one as a function of both the temperature and the chemical potential. Our findings suggest the possibility of a peculiar tricritical point where second-order, first-order, and infinite-order transition lines meet. We discuss the differences between the present phase diagram and previous results for two-dimensional Coulomb gas models. Our study provides important insights into the behaviour of the modified XY model and opens up new possibilities for investigating the underlying physics of unconventional phase transitions.

cond-mat.str-el↗

arXiv2022

THz non-linear optical response in cuprates: predominance of the BCS response over the Higgs mode

Recent experiments with strong THz fields in unconventional cuprates superconductors have clearly evidenced an increase of the non-linear optical response below the superconducting critical temperature Tc. As in the case of conventional superconductors, a theoretical estimate of the various effects contributing to the non-linear response is needed in order to interpret the experimental findings. Here we report a detailed quantitative analysis of the non-linear THz optical kernel in cuprates within a realistic model, accounting for the band structure and disorder level appropriate for these systems. We show that the BCS quasiparticle response is the dominant contribution for cuprates, and its polarization dependence accounts very well for the third-harmonic generation measurements. On the other hand, the polarization dependence of the THz Kerr effect is only partly captured by our calculations, suggesting the presence of additional effects when the system is probed using light pulses with different central frequencies.

cond-mat.supr-con↗

arXiv2021

Transport signatures of fragile-glass dynamics in the melting of the two-dimensional vortex lattice

In two-dimensional (2D) systems, the melting from a solid to an isotropic liquid can occur via an intermediate phase that retains orientational order. However, in 2D superconducting vortex lattices, the effect of orientational correlations on transport, and their interplay with disorder remain open questions. Here we study a 2D weakly pinned vortex system in amorphous MoGe films over an extensive range of temperatures ($\bm{T}$) and perpendicular magnetic fields ($\bm{H}$) using linear and nonlinear transport measurements. We find that, at low fields, the resistivity obeys the Vogel-Fulcher-Tamman (VFT) form, $\bm{ρ(T)\propto\exp[-{W}(H)/(T-T_0(H))]}$, characteristic of fragile glasses. As $\bm{H}$ increases, $\bm{T_0(H)}$ is suppressed to zero, and a standard vortex liquid behavior consistent with a $\bm{T=0}$ superconducting transition is observed. Our findings, supported also by simulations, suggest that the presence of orientational correlations gives rise to a heterogeneous dynamics responsible for the VFT behavior. The effects of quenched disorder become dominant at high $\bm{H}$, where a crossover to a strong-glass behavior is observed. This is a new insight into the dynamics of melting in 2D systems with competing orders.

cond-mat.supr-con↗

arXiv2021

Uniformly frustrated XY model: strengthening of the vortex lattice by intrinsic disorder

In superconducting films, the role of intrinsic disorder is typically to compete with superconductivity by fragmenting the global phase coherence and lowering the superfluid density. Nonetheless, when a transverse magnetic field is applied to the system and an Abrikosov vortex lattice forms, the presence of disorder can actually strengthen the superconducting state against thermal fluctuations. By means of Monte Carlo simulations on the uniformly frustrated XY model in two dimensions, we show that while for weak pinning the superconducting critical temperature $T_c$ increases with the applied field $H$, for strong enough pinning the experimental decreasing dependence between $T_c$ and $H$ is recovered with a resulting more robust vortex lattice.

cond-mat.supr-con↗

arXiv2020

Interplay of spin waves and vortices in the 2D XY model at small vortex-core energy

The Berezinskii-Kosterlitz-Thouless (BKT) mechanism describes universal vortex unbinding in many two-dimensional systems, including the paradigmatic XY model. However, most of these systems present a complex interplay between excitations at different length scales that complicates theoretical calculations of nonuniversal thermodynamic quantities. These difficulties may be overcome by suitably modifying the initial conditions of the BKT flow equations to account for noncritical fluctuations at small length scales. In this work, we perform a systematic study of the validity and limits of this two-step approach by constructing optimised initial conditions for the BKT flow. We find that the two-step approach can accurately reproduce the results of Monte-Carlo simulations of the traditional XY model. In order to systematically study the interplay between vortices and spin-wave excitations, we introduce a modified XY model with increased vortex fugacity. We present large-scale Monte-Carlo simulations of the spin stiffness and vortex density for this modified XY model and show that even at large vortex fugacity, vortex unbinding is accurately described by the nonperturbative functional renormalisation group.

cond-mat.str-el↗

arXiv2018

Kane-Fisher weak link physics in the clean scratched-XY model

The nature of the superfluid-insulator transition in 1D has been much debated recently. In particular, to describe the strong disorder regime characterized by weak link proliferation, a scratched-XY model has been proposed [New J. Phys. \textbf{18}, 045018 (2016)], where the transport is dominated by a single anomalously weak link and is governed by Kane-Fisher weak link physics. In this article, we consider the simplest problem to which the scratched-XY model relates: a single weak link in an otherwise \textit{clean} system, with an intensity $J_W$ which decreases algebraically with the size of the system $J_W\sim L^{-α}$. Using a renormalization group approach and a vortex energy argument, we describe the Kane-Fisher physics in this model and show that it leads to a transition from a transparent regime for $K>K_c$ to a perfect cut for $K<K_c$, with an adjustable $K_c=1/(1-α)$ depending on $α$. We check our theoretical predictions with Monte Carlo numerical simulations complemented by finite-size scaling. Our results clarify two important assumptions at the basis of the scratched-XY scenario, the behaviors of the crossover length scale from weak link physics to transparency and of the superfluid stiffness.

cond-mat.stat-mech↗

arXiv2018

Disordered XY model: effective medium theory and beyond

We study the effect of uncorrelated random disorder on the temperature dependence of the superfluid stiffness in the two-dimensional classical XY model. By means of a perturbative expansion in the disorder potential, equivalent to the T-matrix approximation, we provide an extension of the effective-medium-theory result able to describe the low-temperature stiffness, and its separate diamagnetic and paramagnetic contributions. These analytical results provide an excellent description of the Monte Carlo simulations for two prototype examples of uncorrelated disorder. Our findings offer an interesting perspective on the effects of quenched disorder on longitudinal phase fluctuations in two-dimensional superfluid systems.

cond-mat.supr-con↗

arXiv2018

Melting of the vortex lattice through intermediate hexatic fluid in a-MoGe thin film

The hexatic fluid refers to a phase in between a solid and a liquid which has short range positional order but quasi-long range orientational order. In the celebrated theory of Berezinskii, Kosterlitz and Thouless and subsequently refined by Halperin, Nelson and Young, it was predicted that a 2-dimensional hexagonal solid can melt in two steps: first, through a transformation from a solid to a hexatic fluid which retains quasi long range orientational order and then from a hexatic fluid to an isotropic liquid. In this paper, using a combination of real space imaging and transport measurements we show that the 2-dimensional vortex lattice in a-MoGe thin film follows this sequence of melting as the magnetic field is increased. Identifying the signatures of various transitions on the bulk transport properties of the superconductor, we construct a vortex phase diagram for a two dimensional superconductor.

cond-mat.supr-con↗

arXiv2018

BKT universality class in the presence of correlated disorder

The correct detection of the Berezinskii-Kosterlitz-Thouless (BKT) transition in quasi-two-dimensional superconductors still remains a controversial issue. Its main signatures, indeed, are often at odds with the theoretical expectations. In a recent work\cite{me} we have shown that the presence of spatially correlated disorder plays a key role in this sense being it the reason underlying the experimentally-observed smearing of the universal superfluid-density jump. In the present paper, we closely investigate the effects of correlated disorder on the BKT transition, addressing specifically the issue whether it changes or not the BKT universality class.

cond-mat.supr-con↗

arXiv2017

Optical signatures of the superconducting Goldstone mode in granular aluminum: experiments and theory

Recent advances in the experimental growth and control of disordered thin films, heterostructures, and interfaces provide a fertile ground for the observation and characterisation of the collective superconducting excitations emerging below $T_c$ after breaking the $U(1)$ gauge symmetry. Here we combine THz experiments in a nano-structured granular Al thin film and theoretical calculations to demonstrate the existence of optically-active phase modes, which represent the Goldstone excitations of the broken gauge symmetry. By measuring the complex transmission trough the sample we identify a sizeable and temperature-dependent optical sub-gap absorption, which cannot be ascribed to quasiparticle excitations. A quantitative modelling of this material as a disordered Josephson array of nano-grains allows us to determine, with no free parameters, the structure of the spatial inhomogeneities induced by shell effects. Besides being responsible for the enhancement of the critical temperature with respect to bulk Al, already observed in the past, this spatial inhomogeneity provides a mechanism for the optical visibility of the Goldstone mode. By computing explicitly the optical spectrum of the superconducting phase fluctuations we obtain a good quantitative description of the experimental data. Our results demonstrate that nanograins arrays are a promising setting to study and control the collective superconducting excitations via optical means.

cond-mat.supr-con↗

arXiv2008

Strongly Correlated Superconductivity rising from a Pseudo-gap Metal

We solve by Dynamical Mean Field Theory a toy-model which has a phase diagram strikingly similar to that of high $T_c$ superconductors: a bell-shaped superconducting region adjacent the Mott insulator and a normal phase that evolves from a conventional Fermi liquid to a pseudogapped semi-metal as the Mott transition is approached. Guided by the physics of the impurity model that is self-consistently solved within Dynamical Mean Field Theory, we introduce an analytical ansatz to model the dynamical behavior across the various phases which fits very accurately the numerical data. The ansatz is based on the assumption that the wave-function renormalization, that is very severe especially in the pseudogap phase close to the Mott transition, is perfectly canceled by the vertex corrections in the Cooper pairing channel.A remarkable outcome is that a superconducting state can develop even from a pseudogapped normal state, in which there are no low-energy quasiparticles. The overall physical scenario that emerges, although unraveled in a specific model and in an infinite-coordination Bethe lattice, can be interpreted in terms of so general arguments to suggest that it can be realized in other correlated systems.

cond-mat.supr-con↗

arXiv2007

Disordered loops in the two-dimensional antiferromagnetic spin-fermion model

The spin-fermion model has long been used to describe the quantum-critical behavior of 2d electron systems near an antiferromagnetic (AFM) instability. Recently, the standard procedure to integrate out the fermions to obtain an effective action for spin waves has been questioned in the clean case. We show that in the presence of disorder, the single fermion loops display two crossover scales: upon lowering the energy, the singularities of the clean fermionic loops are first cut off, but below a second scale new singularities arise that lead again to marginal scaling. In addition, impurity lines between different fermion loops generate new relevant couplings which dominate at low energies. We outline a non-linear sigma model formulation of the single-loop problem, which allows to control the higher singularities and provides an effective model in terms of low-energy diffusive as well as spin modes.

cond-mat.str-el↗