arXiv subjects

Loris Di Cairano

Publications and source records attributed to Loris Di Cairano.

16 recordsLinked to original sources

arXiv2026

A microcanonical approach to criticality in the mean-field $ϕ^4$ model: evidence of intrinsic microcanonical structure before the thermodynamic limit

Collective critical behavior is often identified with thermodynamic nonanalyticities and divergences emerging only in the infinite-size limit. Here we adopt a complementary viewpoint: criticality is a structural property due to the rearrangement of the interactions among system's constituents that already exists at finite size and becomes singular only asymptotically. We show that the microcanonical entropy derivatives provide a natural finite-$N$ arena where such structure is encoded in intrinsic extremal/inflection morphologies, and that microcanonical inflection-point analysis (MIPA) turns these morphologies into a unique finite-size critical marker and a well-defined critical trajectory. Using the mean-field $ϕ^4$ model as a stringent benchmark, we reconstruct $β_N(\varepsilon)$ and $γ_N(\varepsilon)$ from microcanonical simulations, validate them against analytic results, and demonstrate that the MIPA trajectory converges to the exact thermodynamic critical point while simultaneously organizing the approach of other observables to their asymptotic behavior. Our results elevate finite-size criticality from a rounded remnant of the thermodynamic limit to a measurable and predictive object in its own right, with direct relevance to modern finite-system platforms and numerical studies.

cond-mat.stat-mech↗

arXiv2026

The geometric origin of criticality: a universal mechanism in mean-field rotor Hamiltonians

We introduce a universal criterion for criticality in mean-field rotor Hamiltonians based on the geometric structure of the constant-energy shell. Rather than characterizing the onset of a phase transition through the conventional thermodynamic singularities alone, we show that the relevant information is already encoded in the way the geometry of the shell reorganizes along distinguished collective directions. For a broad class of finite-dimensional trigonometric mean-field interactions, the trace of the Weingarten operator (representing the principal curvatures) admits a universal collective expansion in terms of the order-parameter amplitudes. This expansion defines an energy-dependent quadratic form whose eigenmodes identify the geometrically unstable channels of the system. Criticality is then associated with the vanishing of the corresponding curvature coefficients, yielding a direct geometric selection principle for the modes that become unstable at the transition. In this way, the phase transition in mean-field systems (usually of first- or second-order) is reformulated as a geometric instability phenomenon intrinsic to the microcanonical energy shell. The resulting framework is geometrically universal within the class considered, independent of model-specific details except for a finite set of collective couplings. Moreover, our approach recovers the known critical channels in standard mean-field rotor models while extending naturally to multimode and spectrally coupled cases. These results support a view in which critical behavior can be understood as reorganizations of energy-shell geometry triggered by a collective restructuring of the underlying energy-shell geometry.

cond-mat.stat-mech↗

arXiv2026

The Constrained Origin of Canonical and Microcanonical Ensembles in Quantum Theory

In quantum theory, equilibrium statistical mechanics is usually formulated through the canonical ensemble, whose privileged status is tied to the Euclidean continuation of time evolution. The microcanonical ensemble, by contrast, is commonly introduced as a separate spectral construction. In this work we show that this asymmetry is representational rather than structural. We formulate the system in an extended Hilbert space in which time is promoted to an auxiliary clock degree of freedom and physical states are selected by a reparametrization-invariant constraint operator $\hat C = \hat P_T + \hat H$. The corresponding projector $δ(\hat C)$ provides a single unified object from which both canonical and microcanonical ensembles emerge as complementary projections in the clock sector. In the clock-time representation, a purely imaginary clock separation yields the Euclidean kernel and the canonical partition function. In the conjugate clock-energy representation, the same projector reduces to the spectral operator $δ(\hat H-E)$ and hence to the microcanonical density of states. The main consequence is structural: canonical and microcanonical statistics need not be introduced as independent constructions, since both are already encoded in the same constrained quantum dynamics.

quant-ph↗

arXiv2026

Criticality Beyond Nonanalyticity: Intrinsic Microcanonical Signatures of Phase Transitions

Phase transitions are conventionally defined by nonanalyticities of thermodynamic potentials in the thermodynamic limit. In this Letter, we show that the singularity is not the definition of criticality but its asymptotic outcome: criticality is already written in the microcanonical entropy derivatives at any finite size as intrinsic morphological structures -- inflection points and extrema. The singularity is then the endpoint of a sharpening process that evolves with increasing system size. Combining microcanonical inflection-point analysis (MIPA) with the Berlin-Kac spherical model -- for which the microcanonical density of states is known in closed form at every finite $N$ -- we systematically identify these structures in the energy profiles of entropy derivatives that encode the transition. An inflection point in the inverse temperature $β_N(ε)=\partial_εS_N$ and a pronounced peak in its derivative $γ_N(ε)=\partial^2_εS_N$ define a well-controlled pseudocritical trajectory whose controlled sharpening and drift culminate in the macroscopic cusp at the critical energy $ε_c$ in the thermodynamic limit. This establishes an intrinsic, order-parameter-free notion of criticality that precedes its singular asymptotic representation.

cond-mat.stat-mech↗

arXiv2025

The Geometric Foundations of Microcanonical Thermodynamics: Entropy Flow Equation and Thermodynamic Equivalence

We develop a geometric foundation of microcanonical thermodynamics in which entropy and its derivatives are determined from the geometry of phase space, rather than being introduced through an a priori ensemble postulate. Once the minimal structure needed to measure constant -- energy manifolds is made explicit, the microcanonical measure emerges as the natural hypersurface measure on each energy shell. Thermodynamics becomes the study of how these shells deform with energy: the entropy is the logarithm of a geometric area, and its derivatives satisfy a deterministic hierarchy of entropy flow equations driven by microcanonical averages of curvature invariants (built from the shape/Weingarten operator and related geometric data). Within this framework, phase transitions correspond to qualitative reorganizations of the geometry of energy manifolds, leaving systematic signatures in the derivatives of the entropy. Two general structural consequences follow. First, we reveal a thermodynamic covariance: the reconstructed thermodynamics is invariant under arbitrary descriptive choices such as reparametrizations and equivalent representations of the same conserved dynamics. Second, a geometric microcanonical equivalence is found: microscopic realizations that share the same geometric content of their energy manifolds (in the sense of entering the curvature sources of the flow) necessarily yield the same microcanonical thermodynamics. We demonstrate the full practical power of the formalism by reconstructing microcanonical response and identifying criticality across paradigmatic systems, from exactly solvable mean-field models to genuinely nontrivial short-range lattice field theories and the 1D long-range XY model with $1/r^α$ interactions.

cond-mat.stat-mech↗

arXiv2025

Non-equilibrium quantum field theory of the free-electron laser in Keldysh formalism

We develop a non-equilibrium quantum field theory of the free-electron laser based on the Preparata model, using the real-time Keldysh formalism. Starting from a microscopic Lagrangian for a relativistic electron beam coupled to a single radiation mode, we construct a Keldysh functional integral, perform the large-N rescaling, and integrate out the electronic degrees of freedom. This yields an effective action for the FEL mode in which dispersion, gain, and noise are all generated by a single electronic self-energy built from the current correlations of the beam. For a stationary Gaussian beam, we obtain closed analytic expressions for the retarded and Keldysh components of the self-energy, which directly encode frequency pulling, gain reduction due to energy spread, and the noise spectrum experienced by the field. At low frequency, the theory reduces to a Landau-Ginzburg-Keldysh description of a single complex mode with a mass, growth rate, nonlinearity, and noise strength fully determined by beam current, energy spread, and detuning. In this framework, the FEL threshold appears as a continuous non-equilibrium phase transition in the laser universality class: the coherent field amplitude plays the role of an order parameter, while the amplitude of critical fluctuations is fixed by the microscopic noise kernel. The result is a minimal open quantum field theory analog of Vlasov-Maxwell FEL theory, in which gain, dispersion, and noise arise from a unified self-energy framework rather than from separate phenomenological ingredients.

quant-ph↗

arXiv2025

Phase Transitions as Emergent Geometric Phenomena: A Deterministic Entropy Evolution Law

We show that thermodynamics can be formulated naturally from the intrinsic geometry of phase space alone-without postulating an ensemble, which instead emerges from the geometric structure itself. Within this formulation, phase transitions are encoded in the geometry of constant-energy manifold: entropy and its derivatives follow from a deterministic equation whose source is built from curvature invariants. As energy increases, geometric transformations in energy-manifold structure drive thermodynamic responses and characterize criticality. We validate this framework through explicit analysis of paradigmatic systems-the 1D XY mean-field model and 2D $ϕ^4$ theory-showing that geometric transformations in energy-manifold structure characterize criticality quantitatively. The framework applies universally to long-range interacting systems and in ensemble-inequivalence regimes.

cond-mat.stat-mech↗

arXiv2025

Secret Entanglement, Public Geometry. Quantum Cryptography from a Geometric Perspective

Can a secret be hidden not in which quantum state is prepared, but in the way that state \emph{moves} through its space of possibilities? Motivated by this question, we propose an essential geometric perspective on quantum cryptography in which projective Hilbert space and its entanglement foliations play a central role. The basic ingredients are: (a) the Fubini-Study metric on the manifold of pure states, (b) a family of entanglement measures viewed as scalar functions on this manifold, and (c) controlled trajectories generated by unitary operations. The geometric structure -- state manifold, metric, and allowed moves -- is fully public, as is the functional form of the entanglement family. What remains secret is the choice of parameter $θ$ that selects a specific entanglement functional $E_θ$ and the corresponding foliation into constant-entanglement hypersurfaces. In this setting, classical messages are encoded not only in the sequence of states but also in the pattern of upward, downward, or tangential steps with respect to the hidden foliation. We formalize this idea in terms of geometric entanglement codes and illustrate it with two toy constructions in which incompatible foliations play the role of mutually unbiased bases.

quant-ph↗

arXiv2025

Geometric Entanglement Entropy on Projective Hilbert Space

Entanglement for pure bipartite states is most commonly quantified in a state-by-state manner to each pure state of a bipartite system a scalar quantity, such as the von Neumann entropy of a reduced density matrix. This provides a precise local characterization of how entangled a given state is. At the same time, this local description naturally invites a set of complementary, more global questions about the structure of the space of pure states: How abundant are the states with a given amount of entanglement within the full state space? Do the manifolds of constant entanglement exhibit distinct geometric regimes? These questions shift the focus from assigning an entanglement value to a single state to understanding the global organization and geometry of entanglement across the entire manifold of pure states. In this work, we develop a geometric framework in which these questions become natural. We regard the projective Hilbert space of pure states, endowed with the Fubini-Study metric, as a Riemannian manifold and promote bipartite entanglement to a macroscopic functional on this manifold. Its level sets stratify the space of pure states into hypersurfaces of constant entanglement, and we define a geometric entanglement entropy as the log-volume of these hypersurfaces, weighted by the Fubini-Study gradient of entanglement. This quantity plays the role of a microcanonical entropy in entanglement space: it measures the degeneracy of a given entanglement value in the natural quantum geometry. The framework is illustrated first in the simplest case of a single spin-1/2 and then for bipartite entanglement of spin systems, including a two-qubit example where explicit calculations can be carried out, along with a sketch of the extension to spin chains.

quant-ph↗

arXiv2025

Repulsive Inverse-Distance Interatomic Interaction from Many-Body Quantum Electrodynamics

Interactions between objects can be classified as fundamental or emergent. Fundamental interactions are either extremely short-range or decay inversely with the separation distance, such as the Coulomb potential between charges or the gravitational attraction between masses. In contrast, emergent quantum van der Waals (vdW) and Casimir interactions decay considerably faster ($R^{-6}$ or $R^{-7}$) with distance $R$. Here we apply perturbative quantum electrodynamics (QED) to a many-body (MB) system of atoms modeled as charged harmonic oscillators, and reveal a persistent inverse-distance MB-QED interaction stemming from the coupling between virtual photons and molecular plasmons in the non-retarded regime. This interaction, scaling with the third power of the fine-structure constant, is reminiscent of the Lamb shift for a single atom. Although weaker than vdW forces, this MB-QED $R^{-1}$ interaction may substantially surpass gravitational attraction in future experiments probing quantum gravity at microscopic scales.

quant-ph↗

arXiv2025

The specific heat anomaly stems from a third-order phase transition in the 2D lattice sine-Gordon model

The specific heat anomaly (SHA) is broadly observed in statistical mechanics, appearing as a smooth, system-size-independent peak in the specific heat, in contrast to the singular behavior typical of second-order phase transitions (PTs). Its origin remains heavily debated: some attribute it to finite-size effects, others to unidentified phase transitions. Here we investigate SHA using the two-dimensional sine-Gordon (2D-sG) model and microcanonical inflection point analysis (MIPA), uncovering two key results. First, we show that the roughening transition in the 2D-sG model is a genuine third-order PT under MIPA, where the standard thermodynamic quantities remain continuous. This clarifies the ambiguity in the literature, where this transition was often, though inconclusively, attributed to a Berezinskii-Kosterlitz-Thouless (BKT) transition. Through the use of MIPA and a comprehensive analysis of standard thermodynamic observables, we provide a coherent thermodynamic characterization that redefines the nature of this transition. Second, we find that the SHA is not itself a PT but rather the thermodynamic fingerprint of this third-order transition. These findings clarify the nature of SHA within the 2D-sG model and suggest that similar anomalies in other systems, such as the XY model, may likewise originate from third-order PTs, rather than mere crossovers. Our results provide a consistent thermodynamic interpretation of the SHA and highlight the broader relevance of third-order transitions in systems previously thought to exhibit only low-order or crossover transition.

cond-mat.stat-mech↗

arXiv2023

Phase Transitions in Abelian Lattice Gauge Theory: Production and Dissolution of Monopoles and Monopole-Antimonopole Pairs

We combine the microcanonical formulation of lattice gauge theories (LGTs) developed by Callaway and the microcanonical inflection point analysis (MIPA) proposed by Bachmann et al. to achieve a systematic characterization of phase transitions (PTs) in U(1) lattice electrodynamics. Besides identifying the well-known deconfinement PT (DPT) due to the neutral pair dissolution, which we classify as a first-order PT, we unequivocally detect three higher-order PTs. According to MIPA, we observe two independent third-order PTs in the confined phase; instead, in the deconfined (Coulomb) phase, we observe a dependent third-order PT. For a deeper understanding of the physical meaning of these PTs, we numerically compute the average number density of monopolar and pair defects as a function of energy. Our analysis reveals that DPT is only one of the major mechanisms observable in LGT. The independent third-order PTs are associated, respectively, to the first occurrence of monopolar topological defects and to the production of pairs.

hep-lat↗

arXiv2022

The Geometric Theory of Phase Transitions

We develop a geometric theory of phase transitions (PTs) for Hamiltonian systems in the microcanonical ensemble. This theory allows to reformulate Bachmann's classification of PTs for finite-size systems in terms of geometric properties of the energy level sets (ELSs) associated to the Hamiltonian function. Specifically, by defining the microcanonical entropy as the logarithm of the ELS's volume equipped with a suitable metric tensor, we obtain an exact equivalence between thermodynamics and geometry. In fact, we show that any derivative of entropy with respect to the energy variable can be associated to a specific combination of geometric curvature structures of the ELSs which, in turn, are precise combinations of the potential function derivatives. In this way, we establish a direct connection between the microscopic description provided by the Hamiltonian and the collective behavior which emerges in a PT. Finally, we also analyze the behavior of the ELSs' geometry in the thermodynamic limit, showing that non-analyticities of the energy-derivatives of the entropy are caused by non-analyticities of certain geometric properties of the ELSs around the transition point. Finally, we validate the theory studying the PTs that occur in the $ϕ^4$ and Ginzburg-Landau-like models.

cond-mat.stat-mech↗

arXiv2021

Subdiffusive-Brownian crossover in membrane proteins: a Generalized Langevin Equation-based approach

In this paper, we propose a Generalized Langevin Equation (GLE)-based model to describe the lateral diffusion of a protein in a lipid bilayer. The memory kernel is represented in terms of a viscous (instantaneous) and an elastic (non instantaneous) component modeled respectively through a Dirac delta function and a three-parameter Mittag-Leffler type function. By imposing a specific relationship between the parameters of the three-parameters Mittag-Leffler function, the different dynamical regimes, namely ballistic, subdiffusive and Brownian, as well as the crossover from one regime to another, are retrieved. Within this approach, the transition time from the ballistic to the subdiffusive regime and the distribution of relaxation times underlying the transition from the subdiffusive to the Brownian regime are given. The reliability of the model is tested by comparing the Mean Squared Displacement (MSD) derived in the framework of this model and the MSD of a protein diffusing in a membrane calculated through molecular dynamics (MD) simulations.

physics.bio-ph↗

arXiv2020

Hamiltonian chaos and differential geometry of configuration space-time

This paper tackles Hamiltonian chaos by means of elementary tools of Riemannian geometry. More precisely, a Hamiltonian flow is identified with a geodesic flow on configuration space-time endowed with a suitable metric due to Eisenhart. Until now, this framework has never been given attention to describe chaotic dynamics. A gap that is filled in the present work. In a Riemannian-geometric context, the stability/instability of the dynamics depends on the curvature properties of the ambient manifold and is investigated by means of the Jacobi--Levi-Civita (JLC) equation for geodesic spread. It is confirmed that the dominant mechanism at the ground of chaotic dynamics is parametric instability due to curvature variations along the geodesics. A comparison is reported of the outcomes of the JLC equation written also for the Jacobi metric on configuration space and for another metric due to Eisenhart on an extended configuration space-time. This has been applied to the Hénon-Heiles model, a two-degrees of freedom system. Then the study has been extended to the 1D classical Heisenberg XY model at a large number of degrees of freedom. Both the advantages and drawbacks of this geometrization of Hamiltonian dynamics are discussed. Finally, a quick hint is put forward concerning the possible extension of the differential-geometric investigation of chaos in generic dynamical systems, including dissipative ones, by resorting to Finsler manifolds.

nlin.CD↗

arXiv2019

Coherent Riemannian-geometric description of Hamiltonian order and chaos with Jacobi metric

By identifying Hamiltonian flows with geodesic flows of suitably chosen Riemannian manifolds, it is possible to explain the origin of chaos in classical Newtonian dynamics and to quantify its strength. There are several possibilities to geometrize Newtonian dynamics under the action of conservative potentials and the hitherto investigated ones provide consistent results. However, it has been recently argued that endowing configuration space with the Jacobi metric is inappropriate to consistently describe the stability/instability properties of Newtonian dynamics because of the non-affine parametrization of the arc length with physical time. To the contrary, in the present paper, it is shown that there is no such inconsistency and that the observed instabilities in the case of integrable systems using the Jacobi metric are artefacts.

cond-mat.stat-mech↗