arXiv subjects

F. Brosens

Publications and source records attributed to F. Brosens.

At least 19 recordsLinked to original sources

arXiv2015

Optical properties of inhomogeneous metallic hydrogen plasmas

We investigate the optical properties of hydrogen as it undergoes a transition from the insulating molecular to the metallic atomic phase, when heated by a pulsed laser at megabar pressures in a diamond anvil cell. Most current experiments attempt to observe this transition by detecting a change in the optical reflectance and/or transmittance. Theoretical models for this change are based on the dielectric function calculated for bulk, homogeneous slabs of material. Experimentally, one expects a hydrogen plasma density that varies on a length scale not substantially smaller than the wave length of the probing light. We show that taking this inhomogeneity into account can lead to significant corrections in the reflectance and transmittance. We present a technique to calculate the optical properties of systems with a smoothly varying density of charge carriers, determine the optical response for metallic hydrogen in the diamond anvil cell experiment and contrast this with the standard results. Analyzing recent experimental results we obtain $σ^{Drude}_{DC}=(2.1 \pm 1.3) \times 10^3$ ($Ω$ cm)$^{-1}$ for the conductivity of metallic hydrogen at 170 GPa and 1250 K.

cond-mat.mtrl-sci↗

arXiv2006

On the ground state energy of a gas of interacting polarons in a magnetic field

The ground-state energy of a three-dimensional polaron gas in a magnetic field is investigated. An upper bound for the ground-state energy is derived within a variational approach which is based on a many-body generalization of the Lee-Low-Pines transformation. The basic contributing ingredients found are the ground-state energy and the static structure factor of the homogeneous electron gas in a magnetic field. Both these quantities are derived in the Hartree-Fock approximation. The resulting ground-state energy of the polaron gas is analyzed as a function of the electron density and of the magnetic field strength.

cond-mat.str-el↗

arXiv2006

Variational path-integral treatment of a translation invariant many-polaron system

A translation invariant N-polaron system is investigated at arbitrary electron-phonon coupling strength, using a variational principle for path integrals for identical particles. An upper bound for the ground state energy is found as a function of the number of spin up and spin down polarons, taking the electron-electron interaction and the Fermi statistics into account. The resulting addition energies and the criteria for multipolaron formation are discussed.

cond-mat.str-el↗

arXiv2004

Ground state and optical conductivity of interacting polarons in a quantum dot

The ground-state energy, the addition energies and the optical absorption spectra are derived for interacting polarons in parabolic quantum dots in three and two dimensions. A path integral formalism for identical particles is used in order to take into account the fermion statistics. The approach is applied to both closed-shell and open-shell systems of interacting polarons. Using a generalization of the Jensen-Feynman variational principle, the ground-state energy of a confined N-polaron system is analyzed as a function of N and of the electron-phonon coupling constant. As distinct from the few-electron systems without the electron-phonon interaction, three types of spin polarization are possible for the ground state of the few-polaron systems: (i) a spin-polarized state, (ii) a state where the spin is determined by Hund's rule, (iii) a state with the minimal possible spin. A transition from a state fulfilling Hund's rule, to a spin-polarized state occurs when decreasing the electron density. In the strong-coupling limit, the system of interacting polarons turns into a state with the minimal possible spin. These transitions should be experimentally observable in the optical absorption spectra of quantum dots.

cond-mat.mes-hall↗

arXiv2004

Dynamical Exchange Effects in a Two-Dimensional Many-Polaron Gas

We calculate the influence of dynamical exchange effects on the response properties and the static properties of a two-dimensional many-polaron gas. These effects are not manifested in the random-phase approximation which is widely used in the analysis of the many-polaron system. Here they are taken into account by using a dielectric function derived in the time-dependent Hartree-Fock formalism. At weak electron-phonon coupling, we find that dynamical exchange effects lead to substantial corrections to the random-phase approximation results for the ground state energy, the effective mass, and the optical conductivity of the polaron system. Furthermore, we show that the reduction of the spectral weight of the optical absorption spectrum at frequencies above the longitudinal optical phonon frequency, due to many-body effects, is overestimated by the random-phase approximation.

cond-mat.other↗

arXiv2003

Characterization of shell filling of interacting polarons in a quantum dot through their optical absorption

The method for calculating the ground-state energy and the optical conductivity spectra is developed for a system of a finite number of interacting arbitrary-coupling polarons in a spherical quantum dot with a parabolic confinement potential. The path-integral formalism for identical particles is used in order to take into account the fermion statistics. Using a generalization of the Jensen-Feynman variational principle, the ground-state energy of a confined N-polaron system is analyzed as a function of N and of the electron-phonon coupling strength. The calculated optical conductivity spectra of the N-polaron system in a quantum dot manifest features related to ground-state transitions between states with different total spin.

cond-mat.str-el↗

arXiv2003

Use of the chemical potential for a limited number of fermions with a degenerate groundstate

For fermions with degenerate single-particle energy levels, the usual relation between the total number of particles and the chemical potential $μ$ is only satisfied for a specific number of particles, i.e. those leading to closed shells. The treatment of an arbitrary number of fermions requires a modification of the chemical potential, similar to the one proposed by Landsberg for Bose-condensed systems. We study the implications of the required modification for fermions in a potential, by calculating the ground state energy, the free energy, the density, the partition function and the dynamic two-point correlation function. It turns out that the modified relation between the fugacity and the number of particles leads to the correct ground state energy and density. But for other quantities like the entropy and the two-point correlation functions, an additional correction is required and derived. These calculations indicate that many-body perturbation theories based on $H-μN$ with Lagrange multiplier $μ$, are not applicable in unmodified form for a fixed number of fermions at low temperature.

cond-mat.stat-mech↗

arXiv2000

Density of a gas of spin polarized fermions in a magnetic field

For a fermion gas with equally spaced energy levels that is subjected to a magnetic field, the particle density is calculated. The derivation is based on the path integral approach for identical particles, in combination with the inversion techniques for the generating function of the static response functions. Explicit results are presented for the ground state density as a function of the magnetic field with a number of particles ranging from 1 to 45.

cond-mat.stat-mech↗

arXiv2000

Many-polaron system confined to a quantum dot: ground-state energy and optical absorption

We find for the first time the ground state energy and the optical absorption spectra for N electrons (holes) interacting with each other and with the longitudinal optical (LO) phonons at an arbitrary electron-phonon coupling strength $α$ in a parabolic confinement potential. A recently developed path integral formalism for identical particles is used in order to account for the fermion statistics. The approach is applicable to closed and open shells. Using an extension of the Jensen-Feynman variational principle, the ground state energy of the N-polarons system in a parabolic confinement potential is analyzed as a function of N and $α$. A ferromagnetic-to-nonmagnetic transition is shown to occur between states with different total spin of the system in the case of strong electron-phonon interaction. This transition is manifested through the optical absorption spectra and should be experimentally observable. Strong mixing between zero-phonon and one-phonon states is revealed in the optical absorption spectrum, when the confinement frequency parameter is in resonance with the LO phonon frequency ("confinement-phonon resonance"). Moments of the optical absorption spectra are calculated for a N-polaron system in a parabolic quantum dot.

cond-mat.str-el↗

arXiv2000

Addendum on rotational properties of confined bosons: time-inversion referencing

In this addendum we introduce the concept of time-inversion referencing. This is an extension of hypertext allowing authors to cite papers that where not yet published (or even not yet written) when they publish a manuscript. We are convinced that this concept will prove very useful especially when adjustments to the so-called intellectual property rights have to be made. We apply the concept to our paper on rotational properties of trapped bosons.

cond-mat.stat-mech↗

arXiv2000

Comment on: rotational properties of trapped bosons

Based on the Hellman-Feynman theorem it is shown that the average square radius of a cloud of interacting bosons in a parabolic well can be derived from their free energy. As an application, the temperature dependence of the moment of inertia of non-interacting bosons in a parabolic trap is determined as a function of the number of bosons. Well below the critical condensation temperature, the Bose-Einstein statistics are found to substantially reduce the moment of inertia of this system, as compared to a gas of ``distinguishable'' particles in a parabolic well.

cond-mat.stat-mech↗

arXiv2000

Many Body Diffusion and Interacting Electrons in a Harmonic Confinement

We present numerically exact energy estimates for two-dimensional electrons in a parabolic confinement. By application of an extension of the recently introduced many-body diffusion algorithm, the ground-state energies are simulated very efficiently. The new algorithm relies on partial antisymmetrization under permutation of particle coordinates. A comparison is made with earlier theoretical results for that system.

cond-mat↗

arXiv2000

The Partition Function of a Spinor Gas

For a spinor gas, i.e., a mixture of identical particles with several internal degrees of freedom, we derive the partition function in terms of the Feynman-Kac functionals of polarized components. As an example we study a spin-1 Bose gas with the spins subjected to an external magnetic field and confined by a parabolic potential. From the analysis of the free energy for a finite number of particles, we find that the specific heat of this ideal spinor gas as a function of temperature has two maxima: one is related to a Schottky anomaly, due to the lifting of the spin degeneracy by the external field, the other maximum is the signature of Bose-Einstein condensation.

cond-mat.stat-mech↗

arXiv1999

Condensation and interaction range in harmonic boson traps: a variational approach

For a gas of N bosons interacting through a two-body Morse potential a variational bound of the free energy of a confined system is obtained. The calculation method is based on the Feynman-Kac functional projected on the symmetric representation. Within the harmonic approximation a variational estimate of the effect of the interaction range on the existence of many-particle bound states, and on the N-T phase diagram is obtained.

cond-mat↗

arXiv1999

The center-of-mass response of confined systems

For confined systems of identical particles, either bosons or fermions, we argue that the parabolic nature of the confinement potential is a prerequisite for the non-dissipative character of the center of mass response to a uniform probe. For an excitation in a parabolic confining potential, the half width of the density response function depends nevertheless quantitatively on properties of the internal degrees of freedom, as is illustrated here for an ideal confined gas of identical particles with harmonic interparticle interactions.

cond-mat.stat-mech↗

arXiv1999

Confined Harmonically Interacting Spin-Polarized Fermions in a Magnetic Field: Thermodynamics

We investigate the combined influence of a magnetic field and a harmonic interparticle interaction on the thermodynamic properties of a finite number of spin polarized fermions in a confiment potential. This study is an extension using our path integral approach of symmetrized density matrices for identical particles. The thermodynamical properties are calculated for a three dimensional model of N harmonically interacting spin polarized fermions in a parabolic potential well in the presence of a magnetic field. The free energy and the internal energy are obtained for a limited number of particles. Deviations from the thermodynamical limit become negligible for about 100 or more particles, but even for a smaller number of fermions present in the well, scaling relations similar to those of the continuum approximation to the density of states are already satisfied.

cond-mat.stat-mech↗

arXiv1998

Momentum distribution of confined bosons: temperature dependence

The momentum distribution function of a parabolically confined gas of bosons with harmonic interparticle interactions is derived. In the Bose-Einstein condensation region, this momentum distribution substantially deviates from a Maxwell-Boltzmann distribution. It is argued that the determination of the temperature of the boson gas from the Bose-Einstein momentum distribution function is more appropriate than the currently used fitting to the high momentum tail of the Maxwell-Boltzmann distribution.

quant-ph↗

arXiv1998

Correlations in a Confined gas of Harmonically Interacting Spin-Polarized Fermions

For a fermion gas with equally spaced energy levels, the density and the pair correlation function are obtained. The derivation is based on the path integral approach for identical particles and the inversion of the generating functions for both static responses. The density and the pair correlation function are evaluated explicitly in the ground state of a confined fermion system with a number of particles ranging from 1 to 220 and filling the Fermi level completely.

cond-mat.stat-mech↗