arXiv subjects

Nicolo Defenu

Publications and source records attributed to Nicolo Defenu.

12 recordsLinked to original sources

arXiv2025

Large-$n$ $O(n)$ with long-range interactions: integrability and resonance dynamics

We study the large-$n$ dynamics of the long-range quantum $O(n)$ model, focusing on the strong long-range regime $\alpha<d$. The dynamics of the model exhibits non-trivial features on mesoscopic timescales $t\sim\ln N$, due to the activation of parametric resonances of the nearly degenerate quantum modes. By using recent results establishing the integrability of the large-$n$ limit, we derive the exact resonance conditions and the associated phase diagram, construct the reduced Hamiltonian governing the nonlinear evolution of the resonant sector, and determine the quasiperiodic spectrum of the resulting mesoscopic dynamics. This framework offers a quantitative understanding on how quantum deviation can drive the system away from the mean-field limit. In particular, the presence of multiple resonant modes enhances the logarithmic growth of entanglement and leads to spatially modulated correlations.

cond-mat.stat-mech↗

arXiv2025

Breakdown of adiabaticity in topological quantum liquids

We study the temporal behavior of topological quantum fluids with strong long-range couplings under slow external perturbations, whose rate $\delta$ approaches the quasi-static limit $\delta\to 0$. As expected, due to strong long-range interactions, the system lies in the mean-field universality and the density of defects for drives across the quantum critical point is adiabatic $n_{\rm exc}\propto \delta^{2}$. However, if the drive is instead terminated precisely at the edge of the topological non-trivial phase, the number of generated excitations becomes extensive $n_{\rm exc}\propto O(1)$. This result fundamentally breaks the established universal behavior observed in local topological quantum fluids and demonstrates a novel mechanism for the breakdown of adiabaticity in fermionic systems with strong long-range interactions.

cond-mat.str-el↗

arXiv2025

Ensemble Inequivalence in Long-Range Quantum Spin Systems

Ensemble inequivalence occurs when a systems thermodynamic properties vary depending on the statistical ensemble used to describe it. This phenomenon is known to happen in systems with long-range interactions and has been observed in many classical systems. In this study, we provide a detailed analysis of a long-range quantum ferromagnet spin model that exhibits ensemble inequivalence. At zero temperature ($T = 0$), the microcanonical phase diagram matches that of the canonical ensemble. However, the two ensembles yield different phase diagrams at finite temperatures. This behavior contrasts with the conventional understanding in statistical mechanics of systems with short-range interactions, where thermodynamic properties are expected to align across different ensembles in the thermodynamic limit. We discuss the implications of these findings for synthetic quantum long-range platforms, such as atomic, molecular, and optical (AMO) systems.

cond-mat.stat-mech↗

arXiv2022

Tunable tachyon mass in the PT-broken massive Thirring model

We study the full phase diagram of a non-Hermitian PT-symmetric generalization of the paradigmatic two-dimensional massive Thirring model. Employing the non-perturbative functional renormalization-group, we find that the model hosts a regime where PT symmetry is spontaneously broken. This new phase is characterized by a relevant imaginary mass, corresponding to monstronic excitations displaying exponentially growing amplitudes for time-like intervals and tachyonic (Lieb-Robison-bound breaking, oscillatory) excitations for space-like intervals. Furthermore, since the phase manifests itself as an unconventional attractive spinodal fixed point, which is typically unreachable in finite real-life systems, we find that the effective renormalized mass reached can be tuned through the microscopic parameters of the model. Our results further predict that the new phase is robust to external gauge fields, contrary to the celebrated BKT phase in the PT unbroken sector. The gauge field then provides an effective and easy means to tune the renormalized imaginary mass through a wide range of values, and therefore the amplitude growth/oscillation rate of the corresponding excitations.

hep-th↗

arXiv2022

Villain model with long-range couplings

The nearest-neighbor Villain, or periodic Gaussian, model is a useful tool to understand the physics of the topological defects of the two-dimensional nearest-neighbor $XY$ model, as the two models share the same symmetries and are in the same universality class. The long-range counterpart of the two-dimensional $XY$ model has been recently shown to exhibit a non-trivial critical behavior, with a complex phase diagram including a range of values of the power-law exponent of the couplings decay, $\sigma$, in which there are a magnetized, a disordered and a critical phase (arXiv:2104.13217). Here we address the issue of whether the critical behavior of the two-dimensional $XY$ model with long-range couplings can be described by the Villain counterpart of the model. After introducing a suitable generalization of the Villain model with long-range couplings, we derive a set of renormalization-group equations for the vortex-vortex potential, which differs from the one of the long-range $XY$ model, signaling that the decoupling of spin-waves and topological defects is no longer justified in this regime. The main results are that for $\sigma<2$ the two models no longer share the same universality class. Remarkably, within a large region of its phase diagram, the Villain model is found to behave similarly to the one-dimensional Ising model with $1/r^2$ interactions.

cond-mat.stat-mech↗

arXiv2021

Entanglement propagation and dynamics in non-additive quantum systems

The prominent collective character of long-range interacting quantum systems makes them promising candidates for quantum technological applications. Yet, lack of additivity overthrows the traditional picture for entanglement scaling and transport, due to the breakdown of the common mechanism based on excitations propagation and confinement. Here, we describe the dynamics of the entanglement entropy in many-body quantum systems with a diverging contribution of the long-range two body potential to the internal energy. While in the strict thermodynamic limit entanglement dynamics is shown to be suppressed, a rich mosaic of novel scaling regimes is observed at intermediate system sizes, due to the possibility to trigger multiple resonant modes in the global dynamics. Quantitative predictions on the shape and timescales of entanglement propagation are made, paving the way to the observation of these phases in current quantum simulators. This picture is connected and contrasted with the case of local many body systems subject to Floquet driving.

quant-ph↗

arXiv2021

Berezinskii-Kosterlitz-Thouless phase transitions with long-range couplings

The Berezinskii-Kostelitz-Thouless (BKT) transition is the paradigmatic example of a topological phase transition without symmetry-breaking, where a quasi-ordered phase, characterized by a power law scaling of the correlation functions at low temperature, is disrupted by the proliferation of topological excitations above the critical temperature $T_{\rm BKT}$. In this letter, we consider the effect of long-range decaying couplings $\sim r^{-2-\sigma}$ on this phenomenon. After pointing out the relevance of this non trivial problem, we discuss the phase diagram, which is far richer than the corresponding short-range one. It features -- for $7/4<\sigma<2$ -- a quasi ordered phase in a finite temperature range $T_c < T < T_{\rm BKT}$, which occurs between a symmetry broken phase for $T T_{\rm BKT}$. The transition temperature $T_c$ displays unique universal features quite different from those of the traditional, short-range XY model. Given the universal nature of our findings, they may be observed in current experimental realizations in $2D$ atomic, molecular and optical quantum systems.

cond-mat.stat-mech↗

arXiv2020

Self-consistent harmonic approximation with non-local couplings

We derive the self-consistent harmonic approximation for the $2D$ XY model with non-local interactions. The resulting equation for the variational couplings holds for any form of the spin-spin coupling as well as for any dimension. Our analysis is then specialized to power-law couplings decaying with the distance $r$ as $\propto 1/r^{2+\sigma}$ in order to investigate the robustness, at finite $\sigma$, of the Berezinskii-Kosterlitz-Thouless (BKT) transition, which occurs in the short-range limit $\sigma \to \infty$. We propose an ansatz for the functional form of the variational couplings and show that for any $\sigma>2$ the BKT mechanism occurs. The present investigation provides an upper bound for the lower critical threshold $\sigma^\ast=2$, above which the traditional BKT transition persists in spite of the LR couplings.

cond-mat.stat-mech↗

arXiv2019

Coupled superfluidity of binary Bose mixtures in two dimensions

We consider a two-component Bose gas in two dimensions at low temperature with short-range repulsive interaction. In the coexistence phase where both components are superfluid, inter-species interactions induce a nondissipative drag between the two superfluid flows (Andreev-Bashkin effect). We show that this behavior leads to a modification of the usual Berezinskii-Kosterlitz-Thouless (BKT) transition in two dimensions. We extend the renormalization of the superfluid densities at finite temperature using the renormalization group approach and find that the vortices of one component have a large influence on the superfluid properties of the other, mediated by the nondissipative drag. The extended BKT flow equations indicate that the occurrence of the vortex unbinding transition in one of the components can induce the breakdown of superfluidity also in the other, leading to a locking phenomenon for the critical temperatures of the two gases.

cond-mat.quant-gas↗

arXiv2018

Dynamical critical scaling of long-range interacting quantum magnets

Slow variations (quenches) of the magnetic field across the paramagnetic-ferromagnetic phase transition of spin systems produce heat. In systems with short-range interactions the heat exhibits universal power-law scaling as a function of the quench rate, known as Kibble-Zurek scaling. In this work we analyze slow quenches of the magnetic field in the Lipkin-Meshkov-Glick (LMG) model, which describes fully connected quantum spins. We analytically determine the quantum contribution to the residual heat as a function of the quench rate $δ$ by means of a Holstein-Primakoff expansion about the mean-field value. Unlike in the case of short-range interactions, scaling laws in the LMG model are only found for a ramp ending at the critical point. If instead the ramp is symmetric, as in the typical Kibble-Zurek scenario, after crossing the critical point the system tends to reabsorb the defects formed during the first part of the ramp: the number of excitations exhibits a crossover behavior as a function of $δ$ and tends to a constant in the thermodynamic limit. Previous, and seemingly contradictory, theoretical studies are identified as specific limits of this dynamics. Our results can be tested on several experimental platforms, including quantum gases and trapped ions.

cond-mat.quant-gas↗

arXiv2017

Criticality and Phase Diagram of Quantum Long-Range $\text{O(N)}$ models

Several recent experiments in atomic, molecular and optical systems motivated a huge interest in the study of quantum long-range %spin systems. Our goal in this paper is to present a general description of their critical behavior and phases, devising a treatment valid in $d$ dimensions, with an exponent $d+σ$ for the power-law decay of the couplings in the presence of an $O(N)$ symmetry. By introducing a convenient ansatz for the effective action, we determine the phase diagram for the $N$-component quantum rotor model with long-range interactions, with $N=1$ corresponding to the Ising model. The phase diagram in the $σ-d$ plane shows a non trivial dependence on $σ$. As a consequence of the fact that the model is quantum, the correlation functions are anisotropic in the spatial and time coordinates for $σ$ smaller than a critical value and in this region the isotropy is not restored even at criticality. Results for the correlation length exponent $ν$, the dynamical critical exponent $z$ and a comparison with numerical findings for them are presented.

cond-mat.quant-gas↗

arXiv2014

Fixed Points Structure & Effective Fractional Dimension for O(N) Models with Long-Range Interactions

We study O(N) models with power-law interactions by using functional renormalization group methods: we show that both in Local Potential Approximation (LPA) and in LPA' their critical exponents can be computed from the ones of the corresponding short-range O(N) models at an effective fractional dimension. In LPA such effective dimension is given by $D_{eff}=2d/σ$, where d is the spatial dimension and $d+σ$ is the exponent of the power-law decay of the interactions. In LPA' the prediction by Sak [Phys. Rev. B 8, 1 (1973)] for the critical exponent $η$ is retrieved and an effective fractional dimension $D_{eff}'$ is obtained. Using these results we determine the existence of multicritical universality classes of long-range O(N) models and we present analytical predictions for the critical exponent $ν$ as a function of $σ$ and N: explicit results in 2 and 3 dimensions are given. Finally, we propose an improved LPA" approximation to describe the full theory space of the models where both short-range and long-range interactions are present and competing: a long-range fixed point is found to branch from the short-range fixed point at the critical value $σ_* = 2-η_{SR}$ (where $η_{SR}$ is the anomalous dimension of the short-range model), and to subsequently control the critical behavior of the system for $σ< σ_*$.

cond-mat.stat-mech↗