arXiv subjects

Fabrizio Palumbo

Publications and source records attributed to Fabrizio Palumbo.

At least 19 recordsLinked to original sources

arXiv2023

Harnessing Attention Mechanisms: Efficient Sequence Reduction using Attention-based Autoencoders

Many machine learning models use the manipulation of dimensions as a driving force to enable models to identify and learn important features in data. In the case of sequential data this manipulation usually happens on the token dimension level. Despite the fact that many tasks require a change in sequence length itself, the step of sequence length reduction usually happens out of necessity and in a single step. As far as we are aware, no model uses the sequence length reduction step as an additional opportunity to tune the models performance. In fact, sequence length manipulation as a whole seems to be an overlooked direction. In this study we introduce a novel attention-based method that allows for the direct manipulation of sequence lengths. To explore the method's capabilities, we employ it in an autoencoder model. The autoencoder reduces the input sequence to a smaller sequence in latent space. It then aims to reproduce the original sequence from this reduced form. In this setting, we explore the methods reduction performance for different input and latent sequence lengths. We are able to show that the autoencoder retains all the significant information when reducing the original sequence to half its original size. When reducing down to as low as a quarter of its original size, the autoencoder is still able to reproduce the original sequence with an accuracy of around 90%.

cs.LG↗

arXiv2021

Negative R-parity scissors modes in the Two-Rotors Model

In a previous work I constructed R-negative scissors states in the Two-Rotors Model in which the rotors are two-sided classical bodies. Such states have |K| = 1; 0 components and negative space parity. Here I extend this work including reflections in a plane through the rotors symmetry axes obtaining also R-negative states of positive parity. I then associate reflections and R-operations with corresponding operations on internal noncollective variables of the rotors. I evaluate the em transition strengths of these states and compare them with the corresponding strengths of R-positive scissors modes.

nucl-th↗

arXiv2019

Positive and negative parity states of the Two-Rotor Model and scissors modes

In previous investigations of the Two-Rotor Model with axially symmetric rotors the wave functions were assumed to be invariant under inversion of the axes of the rotors, which restricted the spectrum to positive parity states. We relax this condition requiring only that the wave functions be invariant under the square of the inversion operators. As a result we get positive as well as negative parity states that are all split by tunneling effect.

nucl-th↗

arXiv2016

From the pion cloud of Tomonaga to the electron pairs of Schrieffer: many body wave functions from nuclear physics to condensed matter physics

It is well known that diverse pieces of models and physical ideas coming from different areas of physics converged in the BCS theory of superconductivity. On the contrary it is little known that the formalism developed in the Tomonaga quantum field theory of the pion-nucleon system was an important ingredient for the development of BCS theory. We discuss the evolution of these ideas in quantum field theory providing an unconventional historical perspective.

cond-mat.supr-con↗

arXiv2016

Entanglement in the states of the Two-Rotor-Model

The eigenfunctions of the Two-Rotors Model are superpositions of states corresponding to precessions of the rotors around two orthogonal axes. In Nuclear Physics such an entanglement has not been directly confirmed. We discuss how it might be observed and compare with the application of the TRM to single domain nanoparticles. In the latter case, in addition to the present indirect evidence there is the possibility of direct observation in experiments at temperatures of the order of 1 K.

nucl-th↗

arXiv2015

Observation of Scissors Modes in solid state systems with a SQUID

The occurrence of scissors modes in crystals that have deformed ions in their cells has been predicted some time ago. The theoretical value of their energy is rather uncertain, however, ranging between 10 and a few tenths of eV, with the corresponding widths of 10^-7, 10^-6 eV. Their observation by resonance fluorescence experiments therefore requires a photon spectrometer covering a wide energy range with a very high resolving power. We propose and discuss a new experiment in which such difficulties are overcome by measuring with a SQUID the variation of the magnetic field associated with the excitation of scissors modes.

cond-mat.mtrl-sci↗

arXiv2014

A Two Rotor Model with spin for magnetic Nanoparticles

We argue that for some species of magnetic nanoparticles the macrospin can have a nonvanishing moment of inertia and then an orbital angular momentum. We represent such nanoparticles by two interacting rigid rotors one of which has a large spin attached to the body, namely by a Two Rotor Model with spin. By this model we can describe in a unified way the cases of nanoparticles free and stuck in an elastic or rigid matrix. We evaluate the magnetic susceptibilities for the latter case and under some realistic assumptions we get results in closed form. A crossover between thermal and purely quantum hopping occurs at a temperature much larger than that at which purely quantum tunneling becomes important. A comparison with some experimental data is outlined.

cond-mat.mes-hall↗

arXiv2013

Scissors Modes: The elusive breathing overtone

The Two-Rotor Model predicts two levels above the Scissors Modes with degenerate intrinsic energy. They have $J^π= 0^+,2^+$ and are referred to as overtones. Their energy is below threshold for nucleon emission, which should make them observable. The $J^π=0^+$ overtone, that has the structure of an isovector breathing mode, has vanishing $E0$ amplitude so that cannot be directly excited, but it could be reached in the decay of the $J=2^+$ overtone. We discuss such a process and evaluate the $B(E2)$ strength, which, however, turns out to be very small.

nucl-th↗

arXiv2012

Transfer matrix for Kogut-Susskind fermions in the spin basis

In the absence of interaction it is well known that the Kogut-Susskind regularizations of fermions in the spin and flavor basis are equivalent to each other. In this paper we clarify the difference between the two formulations in the presence of interaction with gauge fields. We then derive an explicit expression of the transfer matrix in the spin basis by a unitary transformation on that one in the flavor basis which is known. The essential key ingredient is the explicit construction of the fermion Fock space for variables which live on blocks. Therefore the transfer matrix generates time translations of two lattice units.

hep-lat↗

arXiv2011

Diquarks in the nilpotency expansion of QCD and their role at finite chemical potential

We assume that the most important quark correlations are pairwise at all baryon densities. We introduce correlated pairs by means of Bogoliubov transformations which are functions of time and spatial gauge fields, in the formalism of the transfer matrix with lattice regularization. The dependence on time and gauge fields allows us to enforce gauge invariance and other symmetries term by term in the transformed quantities. The resulting action should be suitable for the description of multiquark mesons and baryons as states of a quark and a diquark. We derive the quark contribution to the free energy at finite chemical potential in a certain approximation. Its expression cannot be evaluated analytically, but it has a definite sign.

hep-lat↗

arXiv2011

Scissors Mode and dichroism in an anisotropic crystal

We suggest that in an anisotropic crystal there should be a new mechanism of dichroism related to a scissors mode, a kind of excitation observed in several other many-body systems. Such an effect should be found in crystals, amorphous systems and also metallo-proteins. Its signature is a strong magnetic dipole transition amplitude, which is a function of the angle between the momentum of the photon and the anisotropy axis of the cell.

cond-mat.other↗

arXiv2011

Scissors Modes:The first overtone

Scissors modes were predicted in the framework of the Two-Rotor Model. This model has an intrinsic harmonic spectrum, so that the level above the Scissors Mode, the first overtone, has excitation energy twice that of the Scissors Mode. Since the latter is of the order of 3 MeV in the rare earth region, the energy of the overtone is below threshold for nucleon emission, and its width should remain small enough for the overtone to be observable. We find that $B(E2)\uparrow_{overtone} = {3 /over 64 θ_0^{2}}B(E2)\uparrow_{scissors}$, where $θ_0$ is the zero-point oscillation amplitude, which in the rare earth region is of order $ 10^{-1}$.

nucl-th↗

arXiv2011

Scissors modes in crystals with cubic symmetry

We recently suggested that the Scissors Mode (a collective excitation in which one system rotates with respect another one conserving its shape) can occur in crystals with axially symmetric atoms as a precession of these atoms around the anisotropy axis of their cells, giving rise to a form of dichroism. In the present paper we investigate how the Scissors Mode can be realized in crystals with cubic symmetry and evaluate its photo-absorption cross-section. This turns out to be of the same order of magnitude as that for crystals with axially symmetric atoms, but does not exhibit any correlation between the direction of the photon and the axes of the cell.

cond-mat.other↗

arXiv2011

Chiral symmetry breaking and quark confinement in the nilpotency expansion of QCD

We apply to lattice QCD a bosonization method previously developed in which dynamical bosons are generated by time-dependent Bogoliubov transformations. The transformed action can be studied by an expansion in the inverse of the nilpotency index, which is the number of fermionic states in the structure function of composite bosons. When this number diverges the model is solved by the saddle point method which has a variational interpretation. We give a stationary covariant solution for a background matter field whose fluctuations describe mesons. In the saddle point approximations live fermionic quasiparticles with quark quantum numbers which are confined, in the sense that they propagate only in pointlike color singlets. Conditions for chiral symmetry breaking are determined, to be studied numerically, and a derivation of mesons-nucleons action is outlined.

hep-lat↗

arXiv2011

Spin-Orbit Locking and Scissors Modes in rare earth crystals with uniaxial symmetry

A recent experiment has questioned the standard relative value of spin-orbit and crystal-field strengths in rare-earth $4f$ electron systems, according to which the first should be one order of magnitude larger that the second. We find it difficult to reconcile the standard values of crystal field strength with the Single Ion Model of magnetic anisotropy. If in rare-earth systems the spin-orbit force is much larger than the crystal field, however, spin and orbit of $4f$ electrons should be locked to each other. For rare earths with non-vanishing spin, an applied magnetic field should rotate both spin and charge density profile. We suggest experiments to investigate the possible occurrence of such Spin-Orbit Locking, thus making a test of the standard picture, by studying the Scissors Modes in such systems.

cond-mat.other↗

arXiv2010

Absence of sign problem in the (saddle point approximation of the) nilpotency expansion of QCD at finite chemical potential

We have developed a method to derive the (approximate) quark contribution to the fermion free energy of QCD on a lattice, at finite temperature and chemical potential, with Kogut-Susskind fermions in the flavor basis. We show here the expression at zero temperature. This result has been obtained at the lowest order of the nilpotency expansion. At this order the well known "sign problem" does not arise and the quark contribution to the action can be used as a statistical weight in the Monte Carlo simulations.

hep-lat↗

arXiv2010

Exciting the Scissors Mode in crystals with strong spin-orbit coupling as in Bose-Einstein Condensates

In a recent study of the magnetic properties of rare-earth systems the two extreme situations have been considered in which the crystalline electrostatic field is large or small with respect to the spin-orbit interaction. In the first case the orbitals of localized electrons are firmly coupled to the lattice so that while an applied magnetic field rotates the spin, the charge profile remains fixed to the lattice. In the second case an applied magnetic field rotates both spin and density profile, even though through a very small angle. We show that in this second case we have, in addition to the use of photons or electrons, the possibility of exciting and observing the Scissors Mode in crystals in a way analogous to that used in Bose-Einstein Condensates.

cond-mat.str-el↗

arXiv2008

Bogoliubov transformations and fermion condensates in lattice field theories

We apply generalized Bogoliubov transformations to the transfer matrix of relativistic field theories regularized on a lattice. We derive the conditions these transformations must satisfy to factorize the transfer matrix into two terms which propagate fermions and antifermions separately, and we solve the relative equations under some conditions. We relate these equations to the saddle point approximation of a recent bosonization method and to the Foldy-Wouthuysen transformations which separate positive from negative energy states in the Dirac Hamiltonian.

hep-lat↗