arXiv subjects

F. M. Peeters

Publications and source records attributed to F. M. Peeters.

At least 415 records · Page 23Linked to original sources

arXiv2003

Double exchange mechanisms for Mn doped III-V ferromagnetic semiconductors

A microscopic model of indirect exchange interaction between transition metal impurities in dilute magnetic semiconductors (DMS) is proposed. The hybridization of the impurity d-electrons with the heavy hole band states is largely responsible for the transfer of electrons between the impurities, whereas Hund rule for the electron occupation of the impurity d-shells makes the transfer spin selective. The model is applied to such systems as $n-$type GaN:Mn and $p-$type (Ga,Mn)As, $p-$type (Ga,Mn)P. In $n-$type DMS with Mn$^{2+/3+}$ impurities the exchange mechanisms is rather close to the kinematic exchange proposed by Zener for mixed-valence Mn ions. In $p-$type DMS ferromagnetism is governed by the kinematic mechanism involving the kinetic energy gain of heavy hole carriers caused by their hybridization with 3d electrons of Mn$^{2+}$ impurities. Using the molecular field approximation the Curie temperatures $T_C$ are calculated for several systems as functions of the impurity and hole concentrations. Comparison with the available experimental data shows a good agreement.

cond-mat.str-el↗

arXiv2003

Accuracy of the Hartree-Fock method for Wigner molecules at high magnetic fields

Few-electron systems confined in two-dimensional parabolic quantum dots at high magnetic fields are studied by the Hartree-Fock (HF) and exact diagonalization methods. A generalized multicenter Gaussian basis is proposed in the HF method. A comparison of the HF and exact results allows us to discuss the relevance of the symmetry of the charge density distribution for the accuracy of the HF method. It is shown that the energy estimates obtained with the broken-symmetry HF wave functions become exact in the infinite magnetic-field limit. In this limit the charge density of the broken-symmetry solution can be identified with the classical charge distribution.

cond-mat.mes-hall↗

arXiv2003

Dynamics of the superconducting condensate in the presence of a magnetic field. Channelling of vortices in superconducting strips at high currents

On the basis of the time-dependent Ginzburg-Landau equation we studied the dynamics of the superconducting condensate in a wide two-dimensional sample in the presence of a perpendicular magnetic field and applied current. We could identify two critical currents: the current at which the pure superconducting state becomes unstable ($J_{c2}$ \cite{self1}) and the current at which the system transits from the resistive state to the superconducting state ($J_{c1}<J_{c2}$). The current $J_{c2}$ decreases monotonically with external magnetic field, while $J_{c1}$ exhibits a maximum at $H^*$. For sufficient large magnetic fields the hysteresis disappears and $J_{c1}=J_{c2}=J_c$. In this high magnetic field region and for currents close to $J_c$ the voltage appears as a result of the motion of separate vortices. With increasing current the moving vortices form 'channels' with suppressed order parameter along which the vortices can move very fast. This leads to a sharp increase of the voltage. These 'channels' resemble in some respect the phase slip lines which occur at zero magnetic field.

cond-mat.supr-con↗

arXiv2003

Current-voltage characteristics of quasi-one-dimensional superconductors: An S-curve in the constant voltage regime

Applying a constant voltage to superconducting nanowires we find that its IV-characteristic exhibits an unusual S-behavior. This behavior is the direct consequence of the dynamics of the superconducting condensate and of the existence of two different critical currents: j_{c2} at which the pure superconducting state becomes unstable and j_{c1}<j_{c2} at which the phase slip state is realized in the system.

cond-mat.supr-con↗

arXiv2003

Condition of the occurrence of phase slip centers in superconducting nanowires under applied current or voltage

Experimental results on the phase slip process in superconducting lead nanowires are presented under two different experimental conditions: constant applied current or constant voltage. Based on these experiments we established a simple model which gives us the condition of the appearance of phase slip centers in a quasi-one-dimensional wire. It turns out that the competition between two relaxations times (relaxation time of the absolute value of the order parameter $τ_{|ψ|}$ and relaxation time of the phase of the order parameter in the phase slip center $τ_ϕ$) governs the phase slip process. Phase slip phenomena, as periodic oscillations in time of the order parameter, is possible only if the gradient of the phase grows faster than the value of the order parameter in the phase slip center, or equivalently if $τ_ϕ<τ_{|ψ|}$.

cond-mat.supr-con↗

arXiv2003

Generic properties of a quasi-one dimensional classical Wigner crystal

We studied the structural, dynamical properties and melting of a quasi-one-dimensional system of charged particles, interacting through a screened Coulomb potential. The ground state energy was calculated and, depending on the density and the screening length, the system crystallizes in a number of chains. As a function of the density (or the confining potential), the ground state configurations and the structural transitions between them were analyzed both by analytical and Monte Carlo calculations. The system exhibits a rich phase diagram at zero temperature with continuous and discontinuous structural transitions. We calculated the normal modes of the Wigner crystal and the magneto-phonons when an external constant magnetic field $B$ is applied. At finite temperature the melting of the system was studied via Monte Carlo simulations using the $modified$ $Lindemann$ $criterion$ (MLC). The melting temperature as a function of the density was obtained for different screening parameters. Reentrant melting as a function of the density was found as well as evidence of directional dependent melting. The single chain regime exhibits anomalous melting temperatures according to the MLC and as a check we study the pair correlation function at different densities and different temperatures, formulating a different criterion. Possible connection with recent theoretical and experimental results are discussed and experiments are proposed.

cond-mat.mes-hall↗

arXiv2003

Current distribution in a two dimensional electron gas exposed to a perpendicular nonhomogeneous magnetic field of a chess configuration

We have studied a finite two-dimensional electron system exposed to a normal nonhomogeneous magnetic field of a chess configuration. Using the conformal mapping method we obtain an exact analytical solution for the electric field distribution in terms of the Jacobi functions. The obtained formula is exploited to calculate the physical quantities of interest: the current density distribution, the linear density of charges accumulated along the magnetic interfaces, the magneto-resistance, and the Hall resistance.

cond-mat.mes-hall↗

arXiv2003

Effect of strain on the magneto-exciton groundstate in InP/GaInP quantum disks

The groundstate properties of an exciton in a self-assembled quantum disk are calculated in the presence of a perpendicular magnetic field. For sufficient wide and thin dots, the strain field leads to a confinement of the heavy hole within the dot and the system is type I, while the light hole is confined outside the dot and the system is type II. However, with increasing disk thickness, the strain induces a transition of the heavy hole from inside the disk towards the radial boundary outside the disk. For the exciton, we predict a heavy-hole to light-hole transition as a function of the disk thickness, i.e. forming a "ring-like" hole wavefunction. There is a range of parameters (radius and height of the disk) for which a magnetic field can induce such a heavy to light hole transition. The diamagnetic shift was compared with results from magneto-photoluminescence experiments, where we found an appreciable discrepancy. The origin of this discrepancy was investigated by varying the disk parameters, the valence band offset, and the effective masses.

cond-mat.mes-hall↗

arXiv2003

The interaction between a superconducting vortex and an out-of-plane magnetized ferromagnetic disk: influence of the magnet geometry

The interaction between a superconducting vortex in a type II superconducting film (SC) and a ferromagnet (FM) with out-of-plane magnetization is investigated theoretically within the London approximation. The dependence of the interaction energy on the FM-vortex distance, film thickness and different geometries of the magnetic structures: disk, annulus(ring), square and triangle are calculated. Analytic expressions and vectorplots of the current induced in the SC due to the presence of the FM are presented. For a FM disk with a cavity, we show that different local minima for the vortex position are possible, enabling the system to be suitable to act as a qubit. For FMs with sharp edges, like e.g. for squares and triangles, the vortex prefers to enter its equilibrium position along the corners of the magnet.

cond-mat.supr-con↗

arXiv2003

MgB2: superconductivity and pressure effects

We present a Ginzburg-Landau theory for a two-band superconductor with emphasis on MgB$_{2}$. Experiments are proposed which lead to identification of the possible scenarios: whether both $σ$- and $π$-bands superconduct or $σ$% -alone. According to the second scenario a microscopic theory of superconducting MgB$_{2}$ is proposed based on the strongly interacting $% σ$-electrons and non-correlated $π$-electrons of boron ions. The kinematic and Coulomb interactions of $σ$% -electrons provide the superconducting state with an anisotropic gap of $s$% *-wave symmetry. The critical temperature $T_{c}$ has a non-monotonic dependence on the distance $r$ between the centers of gravity of $σ$- and $π$-bands. The position of MgB$_{2}$ on a bell-shaped curve $T_{c}$ ($% r)$ is identified in the overdoped region. The derived superconducting density of electronic states is in agreement with available experimental and theoretical data. It is argued that the effects of pressure are crucial to identify the microscopic origin of superconductivity in MgB$_{2}$. Possibilities for $T_{c}$ increase are discussed.

cond-mat.supr-con↗

arXiv2003

Ginzburg-Landau theory and effects of pressure on a two-band superconductor : application to MgB2

We present a model of pressure effects of a two-band superconductor based on a Ginzburg-Landau free energy with two order parameters. The parameters of the theory are pressure as well as temperature dependent. New pressure effects emerge as a result of the competition between the two bands. The theory then is applied to MgB2. We identify two possible scenaria regarding the fate of the two $σ$ subbands under pressure, depending on whether or not both subbands are above the Fermi energy at ambient pressure. The splitting of the two subbands is probably caused by the E2g distortion. If only one subband is above the Fermi energy at ambient pressure (scenario I), application of pressure diminishes the splitting and it is possible that the lower subband participates in the superconductivity. The corresponding crossover pressure and Gruneisen parameter are estimated. In the second scenario both bands start above the Fermi energy and they move below it, either by pressure or via the substitution of Mg by Al. In both scenaria, the possibility of electronical topological transition is emphasized. Experimental signatures of both scenaria are presented and existing experiments are discussed in the light of the different physical pictures.

cond-mat.supr-con↗

arXiv2002

Superconducting Wigner Vortex Molecule near a Magnetic Disk

Within the non-linear Ginzburg-Landau (GL) theory, we investigate the vortex structure in a superconducting thin film with a ferromagnetic disk on top of it. Antivortices are stabilized in shells around a central core of vortices (or a giant-vortex) with size-magnetization controlled ``magic numbers''. An equilibrium vortex phase diagram is constructed. The transition between the different vortex phases occurs through the creation of a vortex-antivortex pair under the magnetic disk edge.

cond-mat.supr-con↗

arXiv2002

Structure and correlations in two-dimensional Coulomb confined classical artificial atoms

The ordering of N equally charged particles (-e) moving in two dimensions and confined by a Coulomb potential, resulting from a displaced positive charge Ze is discussed. This is a classical model system for atoms. We obtain the configurations of the charged particles which, depending on the value of N and Z, may result in ring structures, hexagonal-type configurations and for N/Z~1 in an inner structure of particles which is separated by an outer ring of particles. For N/Z<<1 the Hamiltonian of the parabolic confinement case is recovered. For N/Z~1 the configurations are very different from those found in the case of a parabolic confinement potential. A hydrodynamic analysis is presented in order to highlight the correlations effects.

cond-mat.str-el↗

arXiv2002

Multiple flux jumps and irreversible behavior of thin Al superconducting rings

An experimental and theoretical investigation was made of flux jumps and irreversible magnetization curves of mesoscopic Al superconducting rings. In the small magnetic field region the change of vorticity with magnetic field can be larger than unity. This behavior is connected with the existence of several metastable states of different vorticity. The intentional introduction of a defect in the ring has a large effect on the size of the flux jumps. Calculations based on the time-dependent Ginzburg-Landau model allows us to explain the experimental results semi-quantitatively.

cond-mat.supr-con↗

arXiv2002

On the nature of ferromagnetism in diluted magnetic semiconductors: GaAs:Mn, GaP:Mn

A microscopic Hamiltonian for interacting manganese impurities in diluted magnetic semiconductors (DMS) is derived. It is shown that in p -type III-V DMS the indirect exchange between Mn impurities has similarities with the Zener mechanism in transition metal oxides. Here the mobile holes and localized states near the top of the valence band play the role of unoccupied oxygen orbitals which induce ferromagnetism. The Curie temperature estimated from the proposed kinematic exchange agrees with recent experiments on GaAs:Mn. The model is also applicable to the GaP:Mn system.

cond-mat↗

arXiv2002

Topological Defects and Non-homogeneous Melting of Large 2D Coulomb Clusters

The configurational and melting properties of large two-dimensional clusters of charged classical particles interacting with each other via the Coulomb potential are investigated through the Monte Carlo simulation technique. The particles are confined by a harmonic potential. For a large number of particles in the cluster (N>150) the configuration is determined by two competing effects, namely in the center a hexagonal lattice is formed, which is the groundstate for an infinite 2D system, and the confinement which imposes its circular symmetry on the outer edge. As a result a hexagonal Wigner lattice is formed in the central area while at the border of the cluster the particles are arranged in rings. In the transition region defects appear as dislocations and disclinations at the six corners of the hexagonal-shaped inner domain. Many different arrangements and type of defects are possible as metastable configurations with a slightly higher energy. The particles motion is found to be strongly related to the topological structure. Our results clearly show that the melting of the clusters starts near the geometry induced defects, and that three different melting temperatures can be defined corresponding to the melting of different regions in the cluster.

cond-mat.mes-hall↗

arXiv2002

Dynamic transitions between metastable states in a superconducting ring

Applying the time-dependent Ginzburg-Landau equations, transitions between metastable states of a superconducting ring are investigated in the presence of an external magnetic field. It is shown that if the ring exhibits several metastable states at a particular magnetic field, the transition from one metastable state to another one is governed by both the relaxation time of the absolute value of the order parameter tau_{|psi|} and the relaxation time of the phase of the order parameter tau_{phi}. We found that the larger the ratio tau_{|psi|}tau_{phi} the closer the final state will be to the absolute minimum of the free energy, i.e. the thermodynamic equilibrium. The transition to the final state occurs through a subsequent set of single phase slips at a particular point along the ring.

cond-mat.supr-con↗

arXiv2002

Magnetic Pinning of Vortices in a Superconducting Film: The (anti)vortex-magnetic dipole interaction energy in the London approximation

The interaction between a superconducting vortex or antivortex in a superconducting film and a magnetic dipole with in- or out-of-plane magnetization is investigated within the London approximation. The dependence of the interaction energy on the dipole-vortex distance and the film thickness is studied and analytical results are obtained in limiting cases. We show how the short range interaction with the magnetic dipole makes the co-existence of vortices and antivortices possible. Different configurations with vortices and antivortices are investigated.

cond-mat.supr-con↗