arXiv subjects

Michele Pini

Publications and source records attributed to Michele Pini.

9 recordsLinked to original sources

arXiv2025

Non-thermal pairing glue of electrons in the steady state

The study of mechanisms for enhancing superconductivity has been a central topic in condensed matter physics due to the combination of fundamental and technological interests. One promising route is to exploit non-equilibrium effects in the steady state. Efforts in this direction have so far focused on enhancing the pairing mechanism known from thermal equilibrium through modified distributions for the electrons or the bosons mediating the electron-electron interaction. In this work, we identify an additional pairing mechanism that is active only outside thermal equilibrium. By generalizing Eliashberg theory to non-equilibrium steady states using the Keldysh formalism, we derive a set of Eliashberg equations that capture the effect of this genuinely non-thermal pairing glue even in the weak-coupling regime. We discuss two examples where this mechanism has a major impact. First, in a temperature-bias setup, we find that superconductivity is enhanced when the boson mediator is colder than the electrons. Second, we find that an incoherent drive of the boson mediator at energies much greater than the temperature pushes the system far from thermal equilibrium but leaves the critical coupling essentially unchanged, owing to a competition between electron heating and the enhancement of pairing by the non-thermal glue.

cond-mat.supr-con↗

arXiv2025

Formation of Cavity-Polaritons via High-Order Van Hove Singularities

We consider polaritons formed by hybridizing particle-hole excitations of an insulating phase with a cavity photon at sub-gap frequencies, where absorption is suppressed. The strength of the hybridization is driven by the Van Hove singularity in the joint density of states (JDOS) at the band gap: the stronger the singularity, the more a photon is hybridized with the interband transitions. In order to increase the singularity and thus the polariton hybridization without absorption, we propose to engineer a non-parabolic momentum dispersion of the bands around the gap in order to implement a high-order Van Hove singularity (HOVHS) in the JDOS. Ultracold atoms in tunable optical lattices are an ideal platform to engineer two-dimensional gapped phases with non-trivial band dispersions at the gap. Moreover, the intrinsic non-interacting nature of polarized fermionic atoms prevents the emergence of sub-gap excitations, which are common in solid-state systems and could otherwise spoil the absence of absorption below the gap. Our findings identify band-engineering at the gap edge as a promising route for polariton control with applications in quantum-nonlinear optics.

cond-mat.quant-gas↗

arXiv2025

Cavity-induced Eliashberg effect: superconductivity vs charge density wave

Recent experiments have shown that non-equilibrium effects can play a key role in cavity-based control of material phases, notably in systems with charge-density-wave order. Motivated by this, we extend the theory of the Eliashberg effect, originally developed for superconducting phases, to charge-density-wave phases. Starting from a minimal electronic model where superconductivity and charge-density-wave order are equivalent at equilibrium, we introduce coupling to cavity photons, which are in turn coupled to an environment at a temperature different from the one of the electronic environment. This drives the system into a non-thermal steady state, which breaks the equivalence between superconductivity and charge-density-wave order. In the superconducting case, we recover the known behavior: a shift from continuous to discontinuous phase transitions with bistability. In contrast, the charge-density-wave case displays richer behavior: tuning the cavity frequency induces both continuous and discontinuous transitions, two distinct ordered phases, and a bistable regime ending at a critical point. These findings demonstrate that the scope of cavity-based non-thermal control of quantum materials is broader than at thermal equilibrium, and strongly depends on the targeted phases.

cond-mat.str-el↗

arXiv2025

The fate of the Fermi surface coupled to a single-wave-vector cavity mode

The electromagnetic field of standing-wave or ring cavities induces a spatially modulated, infinite-range interaction between atoms in an ultracold Fermi gas, with a single wavelength comparable to the Fermi length. This interaction has no analog in other systems of itinerant particles and has so far been studied only in the regime where it is attractive at zero distance. Here, we fully solve the problem of competing instabilities of the Fermi surface induced by single-wavelength interactions. We find that while the density-wave (superradiant) instability dominates on the attractive side, it is absent for repulsive interactions, where the competition is instead won by non-superradiant superfluid phases at low temperatures, with Fermion pairs forming at both vanishing and finite center-of-mass momentum. Moreover, even in the absence of such symmetry-breaking instabilities, we find the Fermi surface to be always nontrivially deformed from an isotropic shape. We estimate this full phenomenology to be within reach of dedicated state-of-the-art experimental setups.

cond-mat.quant-gas↗

arXiv2025

Mutual friction and vortex Hall angle in a strongly interacting Fermi superfluid

The motion of a quantized vortex is intimately connected with its microscopic structure and the elementary excitations of the surrounding fluid. In this work, we investigate the two-dimensional motion of a single vortex orbiting a pinned anti-vortex in a unitary Fermi superfluid at varying temperature. By analyzing its trajectory, we measure the yet-unknown longitudinal and transverse mutual friction coefficients, which quantify the vortex-mediated coupling between the normal and superfluid components. Both coefficients increase while approaching the superfluid transition. They provide access to the vortex Hall angle, which is linked to the relaxation time of the localized quasiparticles occupying Andreev bound states within the vortex core, as well as the intrinsic superfluid parameter associated with the transition from laminar to quantum turbulent flows. We compare our results with numerical simulations and an analytic model originally formulated for superfluid $^3$He in the low-temperature limit, finding good agreement. Our work highlights the interplay between vortex-bound quasiparticles and delocalized thermal excitations in shaping vortex dynamics in unitary Fermi superfluids. Further, it provides a novel testbed for studying out-of-equilibrium vortex matter at finite temperatures.

cond-mat.quant-gas↗

arXiv2023

Nonthermal electron-photon steady states in open cavity quantum materials

Coupling a system to two different baths can lead to novel phenomena escaping the constraints of thermal equilibrium. In quantum materials inside optical cavities, this feature can be exploited as electrons and cavity-photons are easily pulled away from their mutual equilibrium, even in the steady state. This offers new routes for a non-invasive control of material properties and functionalities. We show that the absence of thermal equilibrium between electrons and photons leads to reduced symmetries of the steady-state electronic distribution function. Moreover, by defining an effective temperature from the on-shell distribution function, we find a non-monotonic behaviour as a function of cavity frequency, consistent with recent experimental findings. Finally, we show that, the non-thermal behaviour leads to qualitative modifications of the material's properties, as the standard Sommerfeld expansion for observables is modified by a leading-order correction linearly proportional to the temperature difference between the two baths and to the frequency-derivative of the electron damping.

cond-mat.str-el↗

arXiv2023

Evolution of an attractive polarized Fermi gas: From a Fermi liquid of polarons to a non-Fermi liquid at the Fulde-Ferrell-Larkin-Ovchinnikov quantum critical point

The evolution of an attractive polarized two-component Fermi gas at zero temperature is analyzed as its polarization is progressively decreased, from full polarization (corresponding to the polaronic limit) down to a critical polarization when superfluidity sets in. This critical polarization and the nature of the associated superfluid instability are determined within a fully self-consistent $t$-matrix approach implemented exactly at zero temperature. In this way, the polarization-vs-coupling phase diagram at zero temperature is constructed throughout the whole BCS-BEC crossover. Depending on the coupling strength of the inter-particle interaction between the two components, the superfluid instability can be either toward a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) phase or toward a standard polarized BCS phase. The evolution with polarization of the quasi-particle parameters in the normal Fermi gas turns out to be notably different in the two cases. When the instability is toward a polarized BCS superfluid, quasi-particles in the proximity of the two Fermi surfaces remain well defined for all polarizations. When the instability is instead toward an FFLO superfluid, precursor effects become apparent upon approaching the FFLO quantum critical point (QCP), where the quasi-particle residues vanish and the effective masses diverge. This behavior leads to a complete breakdown of the quasi-particle picture characteristic of a Fermi liquid, similarly to what occurs in heavy-fermion materials at an antiferromagnetic QCP. At unitarity, the system is further investigated at finite temperature, making it possible to identify a non-Fermi liquid region in the temperature-vs-polarization phase diagram associated with the underlying FFLO QCP.

cond-mat.quant-gas↗

arXiv2022

Controlling Collective Phenomena Via the Quantum State of Interaction-Mediators: Changing the Criticality of Photon-Mediated Superconductivity Via Fock States of Light

How are two-body scattering and the resulting collective phenomena affected by preparing the mediator of interactions in different quantum states? This question has recently become experimentally relevant in a specific non-relativistic version of QED implemented within materials, where standard techniques of quantum optics are available for the preparation of desired quantum states of the photon mediating interactions between matter's constituents. We develop the necessary non-equilibrium approach for computing the vertex function and find that, in addition to the energy and momentum structure of the scattering, a further structure emerges which reflects the Hilbert-space distribution of the mediator's quantum state. This emergent structure becomes non-trivial for non-Gaussian quantum states of the mediator, and can dramatically affect scattering and collective phenomena. As a first application, we show that by preparing photons in pure Fock states one can enhance pair correlations, and even modify the criticality of the superconducting phase transition. Our results also reveal that the thermal mixture of Fock states regularises the strong pair correlations present in each of its components, yielding the standard Bardeen-Cooper-Schrieffer criticality. Besides the above QED platform, ultracold atomic mixtures are among the most promising candidates for the experimental implementation of these ideas.

quant-ph↗

arXiv2019

Pair fraction in a finite temperature Fermi gas on the BEC side of the BCS-BEC crossover

We investigate pairing in a strongly interacting two-component Fermi gas with positive scattering length. In this regime, pairing occurs at temperatures above the superfluid critical temperature; unbound fermions and pairs coexist in thermal equilibrium. Measuring the total number of these fermion pairs in the gas we systematically investigate the phases in the sectors of pseudogap and preformed-pair. Our measurements quantitatively test predictions from two theoretical models. Interestingly, we find that already a model based on classical atom-molecule equilibrium describes our data quite well.

cond-mat.quant-gas↗