arXiv subjects

P. -A. Hervieux

Publications and source records attributed to P. -A. Hervieux.

17 recordsLinked to original sources

arXiv2026

First measurement of the antihydrogen production cross section through the charge-exchange reaction of low-energy antiprotons with orthopositronium

The GBAR experiment has measured the formation rate of antihydrogen from antiproton impact on a positronium cloud for antiproton kinetic energies of 4 and 6 keV. This is the first charge-exchange cross section measurement performed using antiproton beams, which are provided by the AD-ELENA facility at CERN. The measured cross section values are $(14.1 \pm 1.3 \mathrm{(stat)} ^{+2.2}_{-1.4} \mathrm{(sys)}) \times 10^{-16}$~cm$^2$ at 6.2 keV energy and $(8.7 \pm 2.4 \mathrm{(stat)} ^{+1.5}_{-0.09} \mathrm{(sys)}) \times 10^{-16}$~cm$^2$ at 4.15 keV energy, and agree with recent theoretical three-body calculations for antihydrogen formation. These measurements are an important input for experiments with antimatter, e.g the planned measurements of antihydrogen gravitational acceleration in the GBAR collaboration.

hep-ex↗

arXiv2026

Monitoring antiproton numbers with a CMOS detector in a dense-track environment

The production of antihydrogen by the GBAR experiment at AD/ELENA requires good knowledge of the number of incident keV antiprotons, which can be problematic. We have used a commercial CMOS digital camera mounted around the experimental vacuum chamber to determine antiproton numbers from ionising particles created in the annihilation process on the surface of microchannel plate detectors which are used for beam imaging. We show that the multiplicity of emerging charged particles is as expected for individual annihilations of antiprotons with nucleons at rest, taking into account the surrounding material budget. Most of those particles are in the minimal ionising regime, but can be detected with nearly 100% efficiency in the CMOS pixel detector, while due to the thin depletion layer the device is insensitive to background gammas. Thanks to the high granularity and small pixel size millions of antiproton annihilations can be reconstructed in a dense tracking environment over a large dynamic range with good resolution. From cluster length studies of non perpendicular tracks the thickness of the depletion zone and effective detection area was estimated. The cluster length also allows for a monitoring of track angles. Antiproton numbers are determined from the number of reconstructed clusters in the CMOS sensor by means of the covered solid angle relative to a calibration measurements with well known beam intensities at the most upstream location of the GBAR apparatus. Material effects on the emerging annihilation products were estimated by Monte Carlo (Geant4) calculations, while annihilation artefacts on the complex surface of a microchannel plate are cancelled out in this approach. This method minimises largely systematic uncertainties, leading to a final error of roughly 10% for the reconstruction of absolute antiproton numbers.

physics.ins-det↗

arXiv2026

A determination of the backscattering probability of low-energy antiprotons

It is commonly assumed that antiprotons impinging on a material surface annihilate promptly with the nuclei of the material. However, at kinetic energies of a few keV, this assumption may not hold. As with low-energy protons, electrons or positrons that can be reflected from a target, they may undergo large-angle Coulomb scattering before annihilation occurs, thereby appearing to be "backscattered" from the material surface. This backscattering fraction, largely unknown, is a crucial ingredient to the determination of the production cross-section of antihydrogen atoms in the GBAR experiment. This paper presents a determination of the probability that 4 and 6 keV antiprotons backscatter on the surface of a Micro-Channel Plate detector used for beam imaging at GBAR. No evidence for backscattering has been found and an upper limit of 14% at 68% confidence level has been set on this probability. The impact of backscattering on the determination of the number of antiprotons that participate in antihydrogen production in GBAR is also addressed.

hep-ex↗

arXiv2024

Influence of atomic modeling on electron capture and shaking processes

Ongoing experimental efforts to measure with unprecedented precision electron-capture probabilities challenges the current theoretical models. The short range of the weak interaction necessitates an accurate description of the atomic structure down to the nucleus region. A recent electron-capture modeling has been modified in order to test the influence of three different atomic descriptions on the decay and shaking probabilities. To this end, a specific atomic modeling was developed in the framework of the relativistic density-functional theory, exploring several exchange-correlation functionals and self-interaction-corrected models. It was found that the probabilities of total shaking, tested on both photoionization and electron-capture processes, depend strongly on the accuracy of the atomic modeling. Predictions of capture probabilities have been compared with experimental values evaluated from available published data for different radionuclides from $^{7}$Be to $^{138}$La. New high-precision measurements are strongly encouraged because the accuracy of the current experimental values is insufficient to test the models beyond the inner shells.

physics.atom-ph↗

arXiv2022

Pseudo-PT symmetric Dirac equation : effect of a new mean spin angular momentum operator on Gilbert damping

The pseudo-PT symmetric Dirac equation is proposed and analyzed by using a non-unitary Foldy-Wouthuysen transformations. A new spin operator PT symmetric expectation value (called the mean spin operator) for an electron interacting with a time-dependent electromagnetic field is obtained. We show that spin magnetization - which is the quantity usually measured experimentally - is not described by the standard spin operator but by this new mean spin operator to properly describe magnetization dynamics in ferromagnetic materials and the corresponding equation of motion is compatible with the phenomenological model of the Landau-Lifshitz-Gilbert equation (LLG).

quant-ph↗

arXiv2020

Dzyaloshinskii-Moriya interaction induced by an ultrashort electromagnetic pulse: Application to coherent (anti)ferromagnetic skyrmion nucleation

We show how a Dzyaloshinskii-Moriya interaction can be generated in an ultrathin metal film from the induced internal electric field created by an ultrashort electromagnetic pulse. This interaction does not require structural inversion-symmetry breaking, and its amplitude can be tuned depending on the amplitude of the field. We perform first-principles calculations to estimate the strength of the field-induced magnetoelectric coupling for ferromagnetic Fe, Co, and Ni, and antiferromagnetic Mn, as well as FePt alloys. Last, using atomistic simulations, we demonstrate how an isolated antiferromagnetic skyrmion can be coherently nucleated from the collinear background by an ultrashort pulse in electric field on a 100-fs timescale.

cond-mat.mes-hall↗

arXiv2016

Study of the ultrafast dynamics of ferromagnetic materials with a Quantum Monte Carlo atomistic model

We study of the ultrafast dynamics of the atomic angular momentum in ferrimagnets irradiated by laser pulses. My apply a quantum atomistic spin approach based on the Monte Carlo technique. Our model describes the coherent transfer of angular momentum between the spin and the orbital momentum as well as the quenching of the orbital momentum induced by the lattice field. The Elliott-Yafet collision mechanism is also included. We focus on elementary mechanisms that lead to the dissipation of the total angular momentum in a rare earth-transition metal (RE-TM) alloy in which the two sublattices have opposite spin orientation. Our model shows that the observed ultrafast quenching of the magnetization can be explained microscopically by the transfer of spin between the sublattices and by the quenching of the localized orbital angular momentum.

cond-mat.mes-hall↗

arXiv2016

Bose-Einstein condensation of positronium: modification of the s-wave scattering length below the critical temperature

The production of a Bose-Einstein condensate made of positronium may be feasible in the near future. Below the condensation temperature, the positronium collision process is modified by the presence of the condensate. This makes the theoretical description of the positronium kinetics at low temperature challenging. Based on the quasi-particle Bogoliubov theory, we describe the many-body particle-particle collision in a simple manner. We find that, in a good approximation, the full positronium-positronium interaction can be described by an effective scattering length. Our results are general and apply to different species of bosons. The correction to the bare scattering length is expressed in terms of a single dimensionless parameter that completely characterizes the condensate.

quant-ph↗

arXiv2016

Coherent spin-light-induced mechanisms in the semi-relativistic limit of the self-consistent Dirac-Maxwell equations

We present a self-consistent mean-field model based on a two-component Pauli-like equation that incorporates quantum and relativistic effects (up to second-order in 1/c) for both external and internal electromagnetic fields. By taking the semi-relativistic limit of the Dirac-Maxwell equations in the presence of an external electromagnetic field we obtain an analytical expression of a coherent light-induced mean-field Hamiltonian. The latter exhibits several mechanisms that involve the internal mean fields created by all the electrons and the external electromagnetic field (laser). The role played by the light-induced current density and the light-induced second-order charge density acting as sources in Maxwell's equations are clarified. In particular, we identify clearly four different mechanisms involving the spins that may play an important role in coherent ultrafast spin dynamics.

quant-ph↗

arXiv2014

$β$-electron spectrum: static screened Coulomb field and exchange effects

We consider the energy spectrum of emitted electrons in $β$-decay. Exact Coulomb Dirac wave functions describing the $β$-electron in the Coulomb field of the daughter nucleus are used. Further, the improved wave functions which include the screening of the Coulomb field due to the atomic electron cloud are also used. Thus, the interaction between the $β$-electron and the field due to the daughter atom is treated within a nonperturbative approach. Are shown the modifications due to the screening on the $β$ spectra and shown that those effects are very important. In addition, are addressed the contributions to the $β$ spectra due to the exchange terms and shown that the corresponding effects can be substantial. Higher orders arising from the multipole expansion are considered. A comparison of the theoretical results obtained in this work has been made with recent experimental data and a very good agreement was observed.

physics.atom-ph↗

arXiv2012

Breather mode in the many-electron dynamics of semiconductor quantum wells

We demonstrate the existence of a novel breather mode in the self-consistent electron dynamics of a semiconductor quantum well. A non-perturbative variational method based on quantum hydrodynamics is used to determine the salient features of the electron breather mode. Numerical simulations of the time-dependent Wigner-Poisson or Hartree equations are shown to be in excellent agreement with our analytical results. For asymmetric quantum wells, a signature of the breather mode is observed in the dipole response, which can be detected by standard optical means.

cond-mat.mes-hall↗

arXiv2012

Magnetization reversal in isolated and interacting single-domain nanoparticles

Computational and experimental results on the thermally-induced magnetization reversal in single-domain magnetic nanoparticles are reported. The simulations are based on the direct integration of the Fokker-Planck equation that governs the dynamics of the magnetic moment associated with the nanoparticles. A mean field approximation is used to account for the influence of the dipolar interaction between nanoparticles. It is shown that the interactions can either speed up or slow down the reversal process, depending on the angle between the external magnetic field and the axis of easy magnetization. The numerical results are in good agreement with experimental measurements on cobalt-platinum nanoparticles.

cond-mat.mes-hall↗

arXiv2009

Ultrafast Magnetization Dynamics in Diluted Magnetic Semiconductors

We present a dynamical model that successfully explains the observed time evolution of the magnetization in diluted magnetic semiconductor quantum wells after weak laser excitation. Based on the pseudo-fermion formalism and a second order many-particle expansion of the exact p-d exchange interaction, our approach goes beyond the usual mean-field approximation. It includes both the sub-picosecond demagnetization dynamics and the slower relaxation processes which restore the initial ferromagnetic order in a nanosecond time scale. In agreement with experimental results, our numerical simulations show that, depending on the value of the initial lattice temperature, a subsequent enhancement of the total magnetization may be observed within a time scale of few hundreds of picoseconds.

cond-mat.mes-hall↗

arXiv2008

Collective Electron Dynamics in Metallic and Semiconductor Nanostructures

Understanding the electron dynamics and transport in metallic and semiconductor nanostructures -- such as metallic nanoparticles, thin films, quantum wells and quantum dots -- represents a considerable challenge for today's condensed matter physics, both fundamental and applied. In this review article, we will describe the collective electron dynamics in metallic and semiconductor nanostructures using different, but complementary, approaches. For small excitations (linear regime), the spectral properties can be investigated via quantum mean-field models of the TDLDA type (time-dependent local density approximation), generalized to account for a finite electron temperature. In order to explore the nonlinear regime (strong excitations), we will adopt a phase-space approach that relies on the resolution of kinetic equations in the classical phase space (Vlasov and Wigner equations). The phase-space approach provides a useful link between the classical and quantum dynamics and is well suited to model effects beyond the mean field approximation (electron-electron and electron-phonon collisions). We will also develop a quantum hydrodynamic model, based on velocity moments of the corresponding Wigner distribution function: this approach should lead to considerable gains in computing time in comparison with simulations based on conventional methods, such as density functional theory (DFT). Finally, the magnetization (spin) dynamics will also be addressed.

cond-mat.str-el↗

arXiv2008

Fidelity decay in trapped Bose-Einstein condensates

The quantum coherence of a Bose-Einstein condensate is studied using the concept of quantum fidelity (Loschmidt echo). The condensate is confined in an elongated anharmonic trap and subjected to a small random potential such as that created by a laser speckle. Numerical experiments show that the quantum fidelity stays constant until a critical time, after which it drops abruptly over a single trap oscillation period. The critical time depends logarithmically on the number of condensed atoms and on the perturbation amplitude. This behavior may be observable by measuring the interference fringes of two condensates evolving in slightly different potentials.

quant-ph↗

arXiv2007

Autoresonant control of the many-electron dynamics in nonparabolic quantum wells

The optical response of nonparabolic quantum wells is dominated by a strong peak at the plasmon frequency. When the electrons reach the anharmonic regions, resonant absorption becomes inefficient. This limitation is overcome by using a chirped laser pulse in the autoresonant regime. By direct simulations using the Wigner phase-space approach, the authors prove that, with a sequence of just a few pulses, electrons can be efficiently detrapped from a nonparabolic well. For an array of multiple quantum wells, they can create and control an electronic current by suitably applying an autoresonant laser pulse and a slowly varying dc electric field.

cond-mat.mes-hall↗

arXiv2006

Loschmidt echo in a system of interacting electrons

We study the Loschmidt echo for a system of electrons interacting through mean-field Coulomb forces. The electron gas is modeled by a self-consistent set of hydrodynamic equations. It is observed that the quantum fidelity drops abruptly after a time that is proportional to the logarithm of the perturbation amplitude. The fidelity drop is related to the breakdown of the symmetry properties of the wave function.

quant-ph↗