loris ferrari

Followers

Following

Public Views

Matteo Piraccini

Research Fellow, Alma Mater Studiorum Bologna. Associate Researcher Equipe SAPRAT, EPHE, Paris.

Enrico Occhiuto

Jonathan Frassineti

Università di Bologna

Pier Franco Nali

Independent Researcher

MAURIZIO Fanni

Università degli Studi di Trieste

Pietro Micaletti

Andrea Bocchieri

Michele Cini

Istituto Nazionale di Fisica Nucleare

michele cini

Michele Nardelli

Interests

Uploads

Papers by loris ferrari

Elementi DI Termodinamica Statistica

Download

On the Relaxation Time of Certain Two-Level Systems I N Amorphous Structures

Le Journal De Physique Colloques, 1981

The origin and dynamics of 2-level systems in amorphous structures have been recently interpreted... more The origin and dynamics of 2-level systems in amorphous structures have been recently interpreted by the authors in terms of D+ D - intimate pairs interactions. Since the "tunnelling" particles are electrons (and not atoms) and the quantum picture of the model is quite different from that first introduced by Anderson et Al. in 1972, the problem concerned with the relaxation time τ of 2-level systems has been reviewed and τ re-calculated. At given temperature, τ is shown to be much shorter than predicted by previous theories. It is suggested that the corresponding 2-level systems represent the "anomalous" (or fast) ones introduced by Black and Halperin for interpretating some time-dependent experimental data obtained at low temperature.

Download

$Research paper thumbnail of Parametric Resonance in a dissipative system \'a la Kronig-Penney$

Parametric Resonance in a dissipative system \'a la Kronig-Penney

Cornell University - arXiv, Dec 1, 2018

The competition between parametric resonance (PR) and dissipation is studied in the damped Krönig... more The competition between parametric resonance (PR) and dissipation is studied in the damped Krönig-Penney model, with time-dependent dissipation rate γ(t). In the classical case, it is shown that dissipation leaves just a finite number of PR-bands at most, suppressing those at higher frequencies. An analysis of the Lewis-Reisenfeld invariant I(q, p, ρ) is performed, showing that, in the PR regime, the auxiliary function ρ(t) can be chosen bounded or unbounded, depending on the initial conditions.

Download

Exact diagonalization of the truncated Bogoliubov Hamiltonian

arXiv: Quantum Gases, 2016

The present short note is simply intended to communicate that I have analytically diagonalized th... more The present short note is simply intended to communicate that I have analytically diagonalized the Bogoliubov truncated Hamiltonian $H_c$~\cite{Bogo1,Bogo2}, in an interacting bosonic gas. This is the natural prosecution of my work~\cite{MS}, now denoted as (I), where the diagonalization was performed only in the subspace corresponding to zero momentum collective excitations (CE).

Download

Why pushing a bell does not produce a sound

American Journal of Physics, 2019

The difference between 'beating' and 'pushing' results in the perception that a push just makes t... more The difference between 'beating' and 'pushing' results in the perception that a push just makes the object move as a whole, while a beat produces also a sound. Through a detailed analysis of the physics underlying such everyday experiences, we identify the strength, the duration and the softness of the applied contact force, as the main (measurable) characteristics that mark such difference. The strength determines the final velocity ∆v achieved by the body. The duration 2τ compares to the time τ the sound takes to cross the body. The softness γ (a positive exponent) results from the shape in time of the contact force. Those three elements enter the formula for the intensity of the sound produced. The relevant role of the softness is stressed and specific values are calculated for a thin metallic bar, chosen as the simplest possible model system.

Download

Quantum parametric resonance of a dissipative oscillator: fading and persistent memory in the long-time evolution

The European Physical Journal Plus, 2019

The evolution of a quantum oscillator, with periodically varying frequency and damping, is studie... more The evolution of a quantum oscillator, with periodically varying frequency and damping, is studied in the two cases of parametric resonance (PR) producing a limited, or unlimited stretching of the wave function. The different asymptotic behaviors of the energy distribution, show the non trivial interplay between PR and the initial quantum state. In the first case, the oscillator's mean energy tends asymptotically to a fully classical value, independent of the initial state, with vanishing relative quantum fluctuations. In the second case, the memory of the initial state persists over arbitrary long time scales, both in the mean value and in the large quantum fluctuations of the energy.

Download

Collective excitations in an interacting boson gas beyond Bogoliubov theory

Physica B: Condensed Matter, 2017

In a gas of N interacting bosons, the Hamiltonian Hc, obtained by dropping all the interaction te... more In a gas of N interacting bosons, the Hamiltonian Hc, obtained by dropping all the interaction terms between free bosons with momenthk = 0, is diagonalized exactly. The resulting eigenstates | S, k, η depend on two discrete indices S, η = 0, 1,. .. , where η numerates the quasiphonons carrying a momenthk, responsible for transport or dissipation processes. S, in turn, numerates a ladder of 'vacua'|S, k, 0 , with increasing equispaced energies, formed by boson pairs with opposite moment. Passing from one vacuum to another (S → S ± 1), results from creation/annihilation of new momentless collective excitations, that we call vacuons. Exact quasiphonons originate from one of the vacua by 'creating' an asymmetry in the number of opposite moment bosons. The well known Bogoliubov collective excitations (CEs) are shown to coincide with the exact eigenstates | 0, k, η , i.e. with the quasiphonons created from the lowest-level vacuum (S = 0). All this is discussed, in view of existing or future experimental observations of the vacuons (PBs), a sort of bosonic Cooper pairs, which are the main factor of novelty beyond Bogoliubov theory.

Download

Corrigendum to “Exact canonic eigenstates of the truncated Bogoliubov Hamiltonian in an interacting bosons gas [Physica B 496 (2016) 38–44]”

Physica B: Condensed Matter, 2017

Download

Coherent production of phonons from time-dependent strains

Physical Review B, 1997

A simple model of a classical oscillator, with time-dependent frequency, is applied to the normal... more A simple model of a classical oscillator, with time-dependent frequency, is applied to the normal modes of condensed systems. Taking advantage of the analogy with the stationary Schrödinger equation in one dimension, the existence of special modes ͑s modes͒, with exponentially increasing amplitude, is proven, both for disordered and for periodic time-dependent strains. Anharmonic effects are then accounted for in terms of absorption processes for the excess phonons ͑s phonons͒, coherently produced by the strain. A nonmetallic continuum lattice is used as a reference system for applications. Possible macroscopic effects, such as the resonant activation of optical modes and the production of time-controlled shock waves, are outlined. ͓S0163-1829͑97͒02826-9͔

Download

Erratum: Quantum oscillator with fluctuating time-dependent frequency [Phys. Rev. A 57 , 2347 (1998)]

Physical Review A, 1998

A small numerical error was made in the calculation of the nuclear charge distribution correction... more A small numerical error was made in the calculation of the nuclear charge distribution corrections to the 2s hyperfine splitting that affects the last digit of ␦ and ␦ B. Removing this error almost does not affect the hyperfine splitting values ⌬E and given in Table IV ͑the effect is much smaller than the error bars indicated in the table͒, but changes the values and the high precision predictions slightly for the hyperfine splitting of 209 Bi 80ϩ and 165 Ho 64ϩ given in Table V. At the experimentally interesting cases of Ho, Re, Pb, and Bi, the correct values of ␦ and ␦ B are 0.0480 and 0.050, 0.0749 and 0.079, 0.1120 and 0.119, and 0.1187 and 0.126, respectively. The corresponding values of are 1.1274͑3͒, 1.1850͑4͒, 1.2417͑5͒, and 1.2542͑5͒. In Table V, the hyperfine splitting values should be replaced by 0.7969͑2͒ eV for 209 Bi 80ϩ , and 0.3051͑1͒ eV for 165 Ho 64ϩ. The corresponding transition probabilities are 12.03͑2͒ s Ϫ1 and 0.674͑1͒ s Ϫ1. A version of the paper with this correction made is available from the e-print archive, physics/9707006.

Download

Anelastic collapse of small cavities in metals

The Physics of Metals and Metallography, 2015

The stability of a small cavity in a virtually infinite solid is studied in the context of anelas... more The stability of a small cavity in a virtually infinite solid is studied in the context of anelasticity for two metals (Al and Cu). It is shown that the collapse occurs due to the dependence of the shear modulus on the relative volume change. It is shown that above a minimum external pressure P min the system cannot self sustain, unless the internal pressure does exceed a critical value, compensating the collapsing effect. The dif ferent models of anelasticity adopted are shown to have a scarce influence on the main results. In particular, the value of P min turns out to range about the 10% of the bulk modulus at zero pressure, for both metals and almost independently from the anelastic model.

Download

Coherent absorption of energy by electrons in a time-dependent oscillating magnetic field

Physical Review B, 1998

The exponential absorption of energy, characteristic of the harmonic oscillator with time-depende... more The exponential absorption of energy, characteristic of the harmonic oscillator with time-dependent fluctuating frequency ͓L. Ferrari, Phys. Rev. B 56, 593 ͑1997͔͒, is applied to electrons in a large constant magnetic field, with a small oscillating perturbation added. Physical conditions are given, for the model to be implementable in practical cases. These conditions turn out to be accessible to conventional techniques, not too different from those used to detect the de Haas-van Alphen effect. The main effect predicted is an increase of the electronic density of states at the Fermi level, with a parallel increase of the temperature, both saturating at asymptotic values. Some consequences of the effect on the properties of hydrogenated metals are discussed. ͓S0163-1829͑98͒00802-9͔

Download

Boltzmann vs Gibbs: a finite-size match

The long standing contrast between Boltzmann's and Gibbs' approach to statistical thermod... more The long standing contrast between Boltzmann's and Gibbs' approach to statistical thermodynamics has been recently rekindled by Dunkel and Hilbert [1], who criticize the notion of negative absolute temperature (NAT), as a misleading consequence of Boltzmann's definition of entropy. A different definition, due to Gibbs, has been proposed, which forbids NAT and makes the energy equipartition rigorous in arbitrary sized systems. The two approaches, however, are shown to converge to the same results in the thermodynamical limit. A vigorous debate followed ref.[1], with arguments against [2,3] and in favor [4,5,6,7] of Gibbs' entropy. In an attempt to leave the speculative level and give the discussion some deal of concreteness, we analyze the practical consequences of Gibbs' definition in two finite-size systems: a non interacting gas of N atoms with two-level internal spectrum, and an Ising model of N interacting spins. It is shown that for certain measurable quanti...

$the Helmholtz free energy Vg = € —TSg, while the state z = 0 is always a cuspid minimum, ie. Ue(z) — Ve(0) = Tlz|\/2/(Nz) for small |z| (eq.n (22)). However, this makes sense only if t refers to a canonical system, for w he temperature T, defined through eq.n (2), hich the minimization of the Helmholtz free energy is the correct approach to thermal equilibrium. Hence, the interpretation just outlined stems from the assumpti on that Gibbs’ entropy does apply to canonical systems too (or only), which is a pending question. Leaving the alternative between microcanonical or canonical use of Sg to future discussions and investigations, we take the states zweiss (with positive specific heat) as the ones corresponding to the true thermal equilibrium, and stress the most relevant result following from Fig. 2: Figure 3: Gibbs and Boltzmann heat capacities as a function of T ina Weiss ferromagnet (N = 200). A finite discontinuity at T, = 2J (dashed line) vs a divergent discontinuity at Tw < Ty (full line) characterize Boltzmann’s and Gibbs’ heat capacity, respectively.$

Download

A pseudoequilibrium approach to the formation of vacancy-bridge defects in vitreous silica

Lettere al Nuovo Cimento, 1983

ABSTRACT Summary The contribution of intrinsic defects to the residual entropy of vitreous semico... more ABSTRACT Summary The contribution of intrinsic defects to the residual entropy of vitreous semiconductors is assumed as the basic insight for a pseudoequilibrium calculation of the free-energy at the glass-transition temperature. In particular it is shown that the concentration of IVAP-like defects in vitreous silica turns out to be in good agreement with the phenomenological value deduced from low-temperature thermo-acoustical data.

Instability versus nonlinearity in certain nonautonomous oscillators: A critical dynamical t ransition driven by the initial energy

The equation of motion q+Omega(2)(t)q+alpha/q/(gamma-2)q=0 (gamma&gt;2) for the real coordina... more The equation of motion q+Omega(2)(t)q+alpha/q/(gamma-2)q=0 (gamma&gt;2) for the real coordinate q(t) is studied, as an example of the interplay between nonlinearity and instability. Two contrasting mechanisms determine the behavior of q(t), when the time-varying frequency Omega(t) does produce exponential instability in the linear equation q (lin)+Omega(2)(t)q(lin)=0. At low energy, the exponential instability is the dominant effect, while at high energy the bounding effect of the autonomous nonlinear term prevails. Starting from low initial energies, the result of this competition is a time-varying energy characterized by quasiperiodic peaks, with an average recurrence time T(peak). A closed critical curve S(omega) exists in the initial phase space, whose crossing corresponds to a divergence of the recurrence time T(peak). The divergence of T(peak) has a universal character, expressed by a critical exponent a=1. The critical curve S(omega) is the initial locus of the solutions that vanish asymptotically. A close relationship exists between this dynamical transition and the transition from mobile to self-trapped polarons in one spatial dimension. The application to a number of physical problems is addressed, with special attention to the Fermi-Pasta-Ulam problem and to transitions to chaos.

Role of the equilibrium size of Kadanoff blocks in the loop-expansion technique

Physical Review E, 1998

͑1998͔͒ leads to the introduction of the equilibrium size of the Kadanoff blocks as a useful tool... more ͑1998͔͒ leads to the introduction of the equilibrium size of the Kadanoff blocks as a useful tool to approach the critical properties of the 4 model. The present paper aims to elucidate the role of the equilibrium size of the Kadanoff blocks in the loop-expansion technique currently used in the field-theoretic renormalization. While the standard results are readily obtained, aspects emerge that help clarify the true nature of the smallness parameter in the loopexpansion technique. ͓S1063-651X͑98͒00911-8͔

Download

Correlation length and the scaling parameter in the renormalization group

Physical Review E, 1998

The basic procedure of renormalization group theory is used to split the free energy into a Kadan... more The basic procedure of renormalization group theory is used to split the free energy into a Kadanoff block formation part, and a renormalized block-block interaction part. The study of this redistribution as a function of the scaling parameter s shows that there is a stationarity value s* of s, which turns out to have the same critical behavior as the correlation length. It is suggested that s* can be used as an appropriate measure and definition of the correlation length, even for noncritical regions. The calculation of s* is thereby performed explicitly for the Gaussian, and numerically for the S 4 model. A sharp separation between noncorrelated and correlated regimes is also found for the Gaussian model, well above the critical temperature. For the S 4 model, the results suggest that is characterized by a high-temperature Gaussian branch and by a genuine S 4 branch at low temperatures, connected by a ''plateau'' in the intermediate region. ͓S1063-651X͑98͒03303-0͔

Download

Bound states and expansion dynamics of interacting bosons on a one-dimensional lattice

Physical Review A, 2014

The expansion dynamics of bosonic gases in optical lattices has recently been the focus of an inc... more The expansion dynamics of bosonic gases in optical lattices has recently been the focus of an incresing attention, both experimental and theoretical. We consider, by means of numerical Bethe ansatz, the expansion dynamics of initially confined wave packets of two interacting bosons on a lattice. We show that a correspondence between the asymptotic expansion velocities and the projection of the evolved wave function over the bound states of the system exists, clarifying the existing picture for such situations. Moreover, we investigate the role of the lattice in this kind of evolution.

Download

Ab initio analysis of the x-ray absorption spectrum of the myoglobin–carbon monoxide complex: Structure and vibrations

Physical Review B, 2010

We present a comparison between Fe K-edge X-ray absorption spectra of carbonmonoxymyoglobin and i... more We present a comparison between Fe K-edge X-ray absorption spectra of carbonmonoxymyoglobin and its simulation based on density functional theory determination of the structure and vibrations and spectral simulation with multiple scattering theory. An excellent comparison is obtained for the main part of the molecular structure, without any structural fitting parameters. The geometry of the CO ligand is reliably determined using a synergic approach to data analysis. The methodology underlying this approach is expected to be especially useful in similar situations in which high-resolution data for structure and vibrations are available.

Download

Reduction method for the linear quantum or classical oscillator with time-dependent frequency, damping, and driving

Physical Review A, 1999

ABSTRACT We elucidate the relationship between the linear oscillator with time-dependent frequenc... more ABSTRACT We elucidate the relationship between the linear oscillator with time-dependent frequency, damping, and driving, and the autonomous oscillator with unit frequency and no damping or driving. Such a relationship can be derived from a canonical transformation and a redefinition of the time. A study of the scalars of the transformation makes it possible to write down the evolving quantum states for the nonautonomous Hamiltonian given the evolving states for the autonomous harmonic oscillator.

Elementi DI Termodinamica Statistica

Download

On the Relaxation Time of Certain Two-Level Systems I N Amorphous Structures

Le Journal De Physique Colloques, 1981

Download

$Research paper thumbnail of Parametric Resonance in a dissipative system \'a la Kronig-Penney$

Parametric Resonance in a dissipative system \'a la Kronig-Penney

Cornell University - arXiv, Dec 1, 2018

Download

Exact diagonalization of the truncated Bogoliubov Hamiltonian

arXiv: Quantum Gases, 2016

Download