arXiv subjects

Sergio Ciuchi

Publications and source records attributed to Sergio Ciuchi.

At least 19 recordsLinked to original sources

arXiv2026

Majorana fermions at self-generated interfaces

The Kitaev model describing a one-dimensional topological superconducting chain is known to support two Majorana fermions localized at the systems endpoints when the parameters are tuned to the topological phase. In this work, we investigate the possibility that Majorana fermions may also emerge away from the physical boundaries of the chain. To this purpose, we generalize the Kitaev model by incorporating a local coupling between the electronic density and a classical elastic (lattice) field. This electron-lattice interaction can induce phase separation between superconducting regions characterized by distinct topological invariants, thereby generating internal interfaces that host Majorana bound states. Under these conditions, a dilute gas of Majorana fermions can be realized in the bulk of the system.

cond-mat.supr-con↗

arXiv2026

Quantum Thermalization beyond Non-Integrability and Quantum Scars in a Multispecies Bose-Josephson Junction

The modern framework for quantum thermalization is grounded in the Eigenstate Thermalization Hypothesis (ETH), in which non-integrability and chaos are historically assumed as prerequisites. This work investigates this relationship in a three-species Bose-Josephson Junction (BJJ) with mutual interactions, experimentally achievable in current ultracold-atom platforms. After a thorough characterization of quantum chaos in this system, we examine the occurrence of thermal behavior expected when ETH holds. We identify three distinct regimes: chaotic, integrable, and separable. Remarkably, quantum thermalization occurs in both the chaotic and integrable regimes, while it breaks down in and near the separable limit - supporting that non-integrability is not a necessary condition for thermalization. Furthermore, since the system exhibits collective phenomena in the semiclassical limit, we identify ergodicity breaking phenomena such as athermal states in the chaotic regime classifiable as quantum scars, which show no signs of thermalization, consistently with a weak form of ETH.

cond-mat.stat-mech↗

arXiv2025

Inelastic scattering and transient localization from coupling to two-level systems

We consider an electron interacting locally with two-level systems (TLSs) as an archetypal model for charge transport in the presence of inelastic scatterers. To assess the importance of quantum effects in the optical and d.c. conductivity we solve the model numerically without approximations using the finite temperature Lanczos method (FTLM), and compare the results with Dynamical Mean Field Theory (DMFT). In the slow fluctuation limit, the coupling to the TLSs causes transient localization of the carriers analogous to the one recently found in the electron-boson scattering problem, featuring enhanced resistivities and displaced Drude peaks. Fast inelastic scatterers suppress localization, restoring a more conventional regime where transport and optical properties are governed by independent scattering events.

cond-mat.str-el↗

arXiv2025

Fragility of local moments against hybridization with flat bands

The Kondo screening of a localized magnetic moment crucially depends on the spectral properties of the electronic bath to which it is coupled. Unlike textbook examples, realistic systems as well as dynamical mean-field theory of correlated lattice models force us to consider sharp features in the hybridization function near the Fermi energy. Divergencies of this kind can play a relevant role in twisted bilayer graphene, for which local-moment formation and isospin entropy at finite temperature are currently under the spotlight. We clarify how a low-frequency singularity impacts the screening mechanisms by means of a toy model with a tunable $\delta$-peak in the hybridization function, superimposed to a regular part. Our analysis unveils an unexpectedly big impact on the local-moment physics already for a parametrically small weight of the flat band in the bath.

cond-mat.str-el↗

arXiv2025

Effective enhancement of the electron-phonon coupling driven by nonperturbative electronic density fluctuations

We present a dynamical mean-field study of the nonperturbative electronic mechanisms, which may lead to significant enhancements of the electron-phonon coupling in correlated electron systems. Analyzing the effects of electronic correlations on the lowest-order electron-phonon processes, we show that in the proximity of the Mott metal-to-insulator transition of the doped square lattice Hubbard model, where the isothermal charge response becomes particularly large at small momenta, the coupling of electrons to the lattice is strongly increased. This, in turn, induces significant corrections to both the electronic self-energy and phonon-mediated pairing interaction, indicating the possible onset of a strong interplay between lattice and electronic degrees of freedom even for small values of the bare electron-phonon coupling.

cond-mat.str-el↗

arXiv2025

Stabilization of Fermi-liquid behavior by interactions in disordered metals

We study the interplay of electron-electron and electron-disorder scattering in correlated Fermi liquids by the disordered Hubbard model using dynamical mean-field theory with an IPT-CPA solver. We find significant violations of Matthiessen's rule (additivity of scattering mechanisms) which we explain in terms of the screening of the disorder potential by interactions, leading to a protection of the electron-electron inelastic scattering rate against disorder. We also show that large disorder can lead to a surprising enhancement of the electron-electron scattering that contrasts with the competition seen in the elastic channel. Our results compare positively with available resistivity data in the disordered Fermi liquid phase of the correlated organic metals $\kappa$-(ET)$_{2}$X, and rationalize the strong sample dependence of the $T^2$ coefficients observed in the resistivity of correlated perovskite oxides such as SrVO$_3$.

cond-mat.str-el↗

arXiv2024

Strange metal transport from coupling to fluctuating spins

Metals hosting strong electronic interactions, including high-temperature superconductors, behave in ways that do not conform to normal Fermi liquid theory. To pinpoint the microscopic origin of this strange metal behavior, here we reexamine the d.c. and frequency-dependent conductivity of the two-dimensional t-J model taking advantage of recent improvements made on the finite temperature Lanczos method, enabling numerically exact calculations at unprecedentedly low temperatures and high spectral resolution. We find that strange metallicity is pervasive in the temperature-doping phase diagram whenever anti-ferromagnetic order is suppressed, and advocate that key insights on Planckian relaxation can be gained by extending the study to the frequency and time domain. Our results indicate that Planckian behavior does not originate from the scattering properties of the current carriers, being instead rooted in the quantum statistical nature of the charge response.

cond-mat.str-el↗

arXiv2023

Interacting nodal semimetals with non-linear bands

We investigate the quasi-particle and transport properties of a model describing interacting Dirac and Weyl semimetals in the presence of local Hubbard repulsion $U$, where we explicitly include a deviation from the linearity of the energy-momentum dispersion through an intermediate-energy scale $\Lambda$. Our focus lies on the correlated phase of the semimetal. At the nodal point, the renormalization of spectral weight at a fixed temperature $T$ exhibits a weak dependence on $\Lambda$ but is sensitive to the proximity to the Mott transition. Conversely, the scattering rate of quasi-particles and the resistivity display high-temperature exponents that crucially rely on $\Lambda$, leading to a crossover towards a conventional Fermi-liquid behaviour at finite T. Finally, by employing the Nernst-Einstein relation for conductivity, we identify a corresponding density crossover as a function of the chemical potential.

cond-mat.str-el↗

arXiv2023

Universal scaling near band-tuned metal-insulator phase transitions

We present a theory for band-tuned metal-insulator transitions based on the Kubo formalism. Such a transition exhibits scaling of the resistivity curves, in the regime where $Tτ>1$ or $μτ>1$, where $τ$ is the scattering time and $μ$ the chemical potential. At the critical value of the chemical potential, the resistivity diverges as a power law, $R_c \sim 1/T$. Consequently, on the metallic side there is a regime with negative $dR/dT$, which is often misinterpreted as insulating. We show that scaling and this `fake insulator' regime is observed in a wide range of experimental systems. In particular, we show that Mooij correlations in high-temperature metals with negative $dR/dT$ can be quantitatively understood with our scaling theory in the presence of $T$-linear scattering.

cond-mat.mes-hall↗

arXiv2023

Strange metal behavior from incoherent carriers scattered by local moments

We study metallic transport in an effective model that describes the coupling of electrons to fluctuating magnetic moments with full SU(2) symmetry, exhibiting characteristic behavior of metals at the approach of the Mott transition. We show that scattering by fluctuating local moments causes a fully incoherent regime of electron transport with linear T-dependent resistivities. This strange metal regime is characterized by almost universal, "Planckian" slope and a finite intercept at $T=0$, that we can associate respectively to the fluctuations in orientation and amplitude of the local moments. Our results indicate a route for understanding the microscopic origin of strange metal behavior that is unrelated to quantum criticality and does not rely on the existence of quasiparticles.

cond-mat.str-el↗

arXiv2020

Resistivity Exponents in 3D-Dirac Semimetals From Electron-Electron Interaction

We study the resistivity of three-dimensional semimetals with linear dispersion in the presence of on-site electron-electron interaction. The well-known quadratic temperature dependence of the resistivity of conventional metals is turned into an unusual $T^6$-behavior. An analogous change affects the thermal transport, preserving the linearity in $T$ of the ratio between thermal and electrical conductivities. These results hold from weak coupling up to the non-perturbative region of the Mott transition. Our findings yield a natural explanation for the hitherto not understood large exponents characterizing the temperature-dependence of transport experiments on various topological semimetals.

cond-mat.str-el↗

arXiv2018

Analytical investigation of singularities in two-particle irreducible vertex functions of the Hubbard atom

Two-particle generalized susceptibilities and their irreducible vertex functions play a prominent role in the quantum many-body theory for correlated electron systems. They act as basic building blocks in the parquet formalism which provides a flexible scheme for the calculation of spectral and response functions. The irreducible vertices themselves have recently attracted increased attention as unexpected divergences in these functions have been identified. Remarkably, such singularities appear already for one of the simplest strongly interacting systems: the atomic limit of the half-filled Hubbard model (Hubbard atom). In this paper, we calculate the analytical expressions for all two-particle irreducible vertex functions of the Hubbard atom in all scattering channels as well as the fully irreducible two-particle vertices. We discuss their divergences and classify them by the eigenvalues and eigenvectors of the corresponding generalized susceptibilities. In order to establish a connection to the recently found multivaluedness of the exact self-energy functional $Σ[G]$, we show that already an approximation akin to iterated perturbation theory is sufficient to capture, qualitatively, the divergent structure of the vertex functions. Finally, we show that the localized divergences in the disordered binary mixture model are directly linked to a minimum in the single-particle Matsubara Green's function.

cond-mat.str-el↗

arXiv2018

Inhomogeneous dynamical mean field theory of the small polaron problem

We present an inhomogeneous dynamical mean field theory (I-DMFT) that is suitable to investigate electron-lattice interactions in non-translationally invariant and/or inhomogeneous systems. The presented approach, whose only assumption is that of a local, site-dependent self-energy, recovers both the exact solution of an electron in a generic external potential in the non-interacting limit and the DMFT solution for the small polaron problem in translationally invariant systems. To illustrate its full capabilities, we use I-DMFT to study the effects of defects embedded on a two-dimensional surface. The computed maps of the local density of states reveal Friedel oscillations, whose periodicity is determined by the polaron mass. This can be of direct relevance for the interpretation of scanning-tunneling microscopy (STM) experiments on systems with sizable electron-lattice interactions. Overall, the easy numerical implementation of the method, yet full self-consistency, allows one to study problems in real-space that were previously difficult to access.

cond-mat.mtrl-sci↗

arXiv2018

The origin of Mooij correlations in disordered metals

Sufficiently disordered metals display systematic deviations from the behavior predicted by semi-classical Boltzmann transport theory. Here the scattering events from impurities or thermal excitations can no longer be considered as additive independent processes, as asserted by Matthiessen's rule following from this picture. In the intermediate region between the regime of good conduction and that of insulation, one typically finds a change of sign of the temperature coefficient of resistivity (TCR), even at elevated temperature spanning ambient conditions, a phenomenology that was first identified by Mooij in 1973. Traditional weak coupling approaches to identify relevant corrections to the Boltzmann picture focused on long distance interference effects such as "weak localization", which are especially important in low dimensions (1D, 2D) and close to the zero temperature limit. Here we formulate a strong-coupling approach to tackle the interplay of strong disorder and lattice deformations (phonons) in bulk three-dimensional metals at high temperatures. We identify a polaronic mechanism of strong disorder renormalization, which describes how a lattice locally responds to the relevant impurity potential. This mechanism, which quantitatively captures the Mooij regime, is physically distinct and unrelated to Anderson localization, but realizes early seminal ideas of Anderson himself, concerning the interplay of disorder and lattice deformations.

cond-mat.str-el↗

arXiv2017

Magnetic phases of spin-1 lattice gases with random interactions

A spin-1 atomic gas in an optical lattice, in the unit-filling Mott Insulator (MI) phase and in the presence of disordered spin-dependent interaction, is considered. In this regime, at zero temperature, the system is well described by a disordered rotationally-invariant spin-1 bilinear-biquadratic model. We study, via the density matrix renormalization group algorithm, a bounded disorder model such that the spin interactions can be locally either ferromagnetic or antiferromagnetic. Random interactions induce the appearance of a disordered ferromagnetic phase characterized by a non-vanishing value of spin-glass order parameter across the boundary between a ferromagnetic phase and a dimer phase exhibiting random singlet order. The study of the distribution of the block entanglement entropy reveals that in this region there is no random singlet order.

cond-mat.quant-gas↗

arXiv2017

Disorder-driven metal-insulator transitions in deformable lattices

We show that in presence of a deformable lattice potential, the nature of the disorder-driven metal-insulator transition (MIT) is fundamentally changed with respect to the non-interacting (Anderson) scenario. For strong disorder, even a modest electron-phonon interaction is found to dramatically renormalize the random potential, opening a mobility gap at the Fermi energy. This process, which reflects disorder-enhanced polaron formation, is here given a microscopic basis by treating the lattice deformations and Anderson localization effects on the same footing. We identify an intermediate "bad insulator" transport regime which displays resistivity values exceeding the Mott-Ioffe-Regel limit and with a negative temperature coefficient, as often observed in strongly disordered metals. Our calculations reveal that this behavior originates from significant temperature-induced rearrangements of electronic states due to enhanced interaction effects close to the disorder-driven MIT.

cond-mat.dis-nn↗

arXiv2014

Impact of quantized vibrations on the efficiency of interfacial charge separation in photovoltaic devices

We demonstrate that charge separation at donor-acceptor interfaces is a complex process that is controlled by the combined action of Coulomb binding for electron-hole pairs and partial relaxation due to quantized phonons. A joint electron-vibration quantum dynamical study reveals that high energy vibrations sensitively tune the charge transfer probability as a function of time and injection energy, due to polaron formation. These results have bearings for the optimization of energy transfer both in organic and quantum dot photovoltaics, as well as in biological light harvesting complexes.

cond-mat.mtrl-sci↗

arXiv2014

Robustness against Disorder of Relativistic Spectral Properties in Chalcogenide Alloys

In order to carefully address the interplay between substitutional disorder and spin-orbit-coupling in IV-VI alloys, we propose a novel theoretical approach that integrates the reliability of plane-wave based density-functional theory beyond the local-density approximation with the Coherent Potential Approximation. By applying the proposed method to ternary chalcogenide alloys, we predict a substantial robustness of spectral features close to the Fermi energy against substitutional disorder. Supplementing our first-principles calculations with the analysis of the $k \cdot p$ model for rock-salt chalcogenides, we show that the disorder self-energy is vanishingly small close to the band gap, thus allowing for bulk Rashba-like spin splitting to be observed in ferroelectric alloys, such as PbS$_x$Te$_{1-x}$, and protecting the band-character inversion related to the topological transition in the recently discovered Topological Crystalline Insulator Pb$_{1-x}$Sn$_x$Te.

cond-mat.mtrl-sci↗