Biomedical subjects

F M Peeters

Publications and source records attributed to F M Peeters.

At least 19 recordsLinked to original sources

PubMed2006

Symmetric and asymmetric vortex-antivortex molecules in a fourfold superconducting geometry.

In submicron superconducting squares in a homogeneous magnetic field, Ginzburg-Landau theory may admit solutions of the vortex-antivortex type, conforming to the symmetry of the sample [L. F. Chibotaru, Nature (London) 408, 833 (2000)10.1038/35048521]. Here we show that these fascinating, but never experimentally observed states, can be enforced by artificial fourfold pinning, with their diagnostic features enhanced by orders of magnitude. The second-order nucleation of vortex-antivortex molecules can be driven by either temperature or an applied magnetic field, with stable asymmetric vortex-antivortex equilibria found on its path.

Journal Article↗

PubMed2006

Inhomogeneous melting in anisotropically confined two-dimensional clusters.

Molecular dynamic simulations are performed to investigate the melting process of two-dimensional clusters of classical charged particles trapped in an anisotropic parabolic potential. The confined particles interact through a repulsive potential. We find that the eccentricity of the confinement potential strongly affects the melting pattern of such clusters. Increasing the eccentricity of the confinement potential drives the system through three different melting regimes. Inhomogeneous melting is the typical melting process for anisotropically confined clusters and its appearance in small systems occurs in a distinct form called here internal intershell melting. The latter involves only particles in the center of the cluster while particles on the far left and right of the cluster are still ordered having a much higher melting temperature. Using the Lindemann's criterion the melting temperatures are determined as a function of the different parameters. The internal intershell melting process is found for both long-range (i.e., logarithmic) and short-range (i.e., screened Coulomb) interparticle interaction. Decreasing the range of the interparticle interaction increases the eccentricity of the confinement potential for which internal intershell melting can occur.

Journal Article↗

PubMed2006

Novel commensurability effects in superconducting films with antidot arrays.

Vortex configurations in superconducting films with regular arrays of antidots (holes) are calculated within the nonlinear Ginzburg-Landau theory. In addition to the well-established matching phenomena, we predict (i) the nucleation of giant-vortex states between the antidots, (ii) the combination of giant- and multivortices at rational matching fields, and (iii) for particular values of the vorticity, symmetry imposed creation of vortex-antivortex configurations.

Journal Article↗

PubMed2006

Spectrum of classical two-dimensional Coulomb clusters.

The frequency spectrum of a system of classical charged particles interacting through a Coulomb repulsive potential and which are confined in a two-dimensional parabolic trap is studied. It is shown that, apart from the well-known center-of-mass and breathing modes, which are independent of the number of particles in the cluster, there are more "universal" modes whose frequencies depend only slightly on the number of particles. To understand these modes the spectrum of excitations as a function of the number of particles is compared with the spectrum obtained in the hydrodynamic approach. The modes are classified according to their averaged vorticity and it is shown that these "universal" modes have the smallest vorticity and follow the hydrodynamic behavior.

Journal Article↗

PubMed2005

Structure and spectrum of anisotropically confined two-dimensional clusters with logarithmic interaction.

We studied the structural and spectral properties of a classical system consisting of a finite number of particles, moving in two dimensions, and interacting through a repulsive logarithmic potential and held together by an anisotropic harmonic potential. Increasing the anisotropy of the confinement potential can drive the system from a two-dimensional (2D) to a one-dimensional (1D) configuration. This change occurs through a sequence of structural transitions of first and second order which are reflected in the normal mode frequencies. Our results of the ground state configurations are compared with recent experiments and we obtained a satisfactory agreement. The transition from the 1D line structure to the 2D structure occurs through a zigzag transition which is of second order. We found analytical expressions for the eigenfrequencies before the zigzag transition, which allowed us to obtain an analytical expression for the anisotropy parameter at which the zigzag transition occurs as a function of the number of particles in the system.

Journal Article↗

PubMed2005

Melting and evaporation in classical two-dimensional clusters confined by a Coulomb potential.

The thermal properties of a two-dimensional classical cluster of negatively charged particles bound by a punctual positive charge are presented. The melting phenomenon is analyzed and the features which characterize such a solid-liquid transition are highlighted. We found that the presence of metastable states strongly modifies the melting scenario, and that the melting temperature of the system is determined by the height of the saddle point energy separating the ground state and the metastable state. Due to the particular type of confinement potential considered in this paper, we also found that, at sufficiently large temperature, the cluster can become thermally ionized.

Journal Article↗

PubMed2005

Mesoscopic field and current compensator based on a hybrid superconductor-ferromagnet structure.

A rather general enhancement of superconductivity is demonstrated in a hybrid structure consisting of a submicron superconducting (SC) sample combined with an in-plane ferromagnet (FM). The superconducting state resists much higher applied magnetic fields for both perpendicular polarities, as the applied field is screened by the FM. In addition, FM induces (in the perpendicular direction to its moment) two opposite currents in the SC plane, under and aside the magnet, respectively. Because of the compensation effects, superconductivity persists up to higher applied currents. With increasing current, the sample undergoes SC-"resistive"-normal state transitions through a mixture of vortex-antivortex and phase-slip phenomena.

Journal Article↗

PubMed2005

Structure, normal mode spectra, and mixing of a binary system of charged particles confined in a parabolic trap.

We study the mixing of two different kinds of particles, having different charge and/or mass, interacting through a pure Coulomb potential, and confined in a parabolic trap. The structure of the cluster and its normal mode spectrum are analyzed as a function of the ratio of the charges (mass ratio) of the two types of particles. We show that particles are not always arranged in a shell structure. Mixing of the particles goes hand in hand with a large number of metastable states. The normal modes of the system are obtained, and we find that some of the special modes can be tuned by varying the ratio between the charges (masses) of the two species. The degree of mixing of the two type of particles is summarized in a phase diagram, and an order parameter that describes quantitatively the mixing between particles is defined.

Journal Article↗

PubMed2005

Multistep radial melting in small two-dimensional classical clusters.

We report on a molecular dynamics study of small classical two-dimensional clusters with ringlike configurations. We focus on the particles motion at low temperatures before the radial and angular melting sets in. It is shown that in magic number configurations a local radial melting of subshells occur, which is related to the intershell rotation.

Journal Article↗

PubMed2005

Bubble, stripe, and ring phases in a two-dimensional cluster with competing interactions.

A system of classical charged particles confined in a two-dimensional trap and interacting through a competing short-range attraction and long-range repulsion potential is studied. The structure of the system strongly depends on the interaction range of the short-range attraction potential and the total number of particles. Depending on the appropriate choice of parameters for the attractive potential, the particles organize themselves in bubbles, stripe phases, and ringlike configurations, or combinations of both of them. Detailed phase diagrams are presented for systems consisting of N=2 up to N=6 particles. General rules are derived for the different subsequent transitions between those configurations and how the ground state configuration of a large system can be deduced from the one of a small number of particles.

Journal Article↗

PubMed2005

Vortex-antivortex nucleation in magnetically nanotextured superconductors: magnetic-field-driven and thermal scenarios.

Within the Ginzburg-Landau formalism, we predict two novel mechanisms of vortex-antivortex nucleation in a magnetically nanostructured superconductor. Although counterintuitive, nucleation of vortex-antivortex pairs can be activated in a superconducting (SC) film covered by arrays of submicron ferromagnets (FMs) when exposed to an external homogeneous magnetic field. In another scenario, we predict the thermal induction of vortex-antivortex configurations in SC-FM samples. This phenomenon leads to a new type of Little-Parks oscillations of the FM magnetization-temperature phase boundary of the SC film.

Journal Article↗

PubMed2005

Structural phase transitions and unusual melting behavior in a classical two-dimensional Coulomb bound cluster.

The melting properties of a cluster with N equally charged particles confined by a Coulomb potential are studied. The system exhibits a structural phase transition before it melts. The melting process is not dominated by the usual thermal hops between stable states. We also show that the symmetry of the ground state configuration is a dominant factor in determining the melting temperature and that more confined particles in the cluster do not necessarily have a higher melting temperature.

Journal Article↗

PubMed2004

Vortex-antivortex lattices in superconducting films with magnetic pinning arrays.

Novel vortex structures are found when a thin superconducting (SC) film is covered with a lattice of out-of-plane magnetized magnetic dots (MDs). The stray magnetic field of the dots confines the vortices to the MD regions, surrounded by antivortices which "crystallize" into regular lattices. First- and second-order transitions are found as the magnetic array is made sparser or MD magnetization larger. For sparse MD arrays fractional vortex-antivortex states are formed, where the crystal symmetry is combined with a nonuniform "charge" distribution. We demonstrate that due to the (anti)vortices and the supercurrents induced by the MDs, the critical current of the sample actually increases if exposed to a homogeneous external magnetic field, contrary to conventional SC behavior.

Journal Article↗

PubMed2004

Experimental evidence for giant vortex states in a mesoscopic superconducting disk.

The response of a mesoscopic superconducting disk to perpendicular magnetic fields is studied by using the multiple-small-tunnel-junction method, in which transport properties of several small tunnel junctions attached to the disk are measured simultaneously. This allows us to make the first experimental distinction between the giant vortex states and multivortex states. Moreover, we experimentally find a magnetic-field induced rearrangement and combination of vortices. The experimental results are well reproduced in numerical results based on the nonlinear Ginzburg-Landau theory.

Journal Article↗

PubMed2004

Nonlinear screening in large two-dimensional Coulomb clusters.

The distortion due to a fixed point impurity with variable charge placed in the center of a classical harmonically confined two-dimensional (2D) large Coulomb cluster is studied. We find that the net topological charge (N(-)-N+ ) of the system is always equal to six independent of the position and charge of the impurity. In comparison with a 2D cluster without impurity charge, only the breathing mode remains unchanged. The screening length is found to be a highly nonlinear function of the impurity charge. For values of the impurity charge smaller than the charge of the other particles, the system has almost the same screening strength. When the impurity charge is larger, the screening length is strongly enhanced. This result can be explained by the competition between the different forces active in the system.

Journal Article↗

PubMed2004

Normal modes of a quasi-one-dimensional multichain complex plasma.

We studied equally charged particles, suspended in a complex plasma, which move in a plane and interact with a screened Coulomb potential (Yukawa type) and with an additional external confining parabolic potential in one direction, which makes the system quasi-one-dimensional (Q1D). The normal modes of the system are studied in the presence of dissipation. We also investigated how a perpendicular magnetic field couples the phonon modes with each other. Two different ways of exciting the normal modes are discussed: (1) a uniform excitation of the Q1D lattice, and (2) a local forced excitation of the system in which one particle is driven by, e.g., a laser. Our results are in very good agreement with recent experimental findings on a finite single chain system [Phys. Rev. Lett. 91, 255003 (2003)]]. Predictions are made for the normal modes of multichain structures in the presence of damping.

Journal Article↗

PubMed2004

Long-range nonlocal flow of vortices in narrow superconducting channels.

We report a new nonlocal effect in vortex matter, where an electric current confined to a small region of a long and sufficiently narrow superconducting wire causes vortex flow at distances hundreds of intervortex separations away. The observed remote traffic of vortices is attributed to a very efficient transfer of a local strain through the one-dimensional vortex lattice (VL), even in the presence of disorder. We also observe mesoscopic fluctuations in the nonlocal vortex flow, which arise due to "traffic jams" when vortex arrangements do not match a local geometry of a superconducting channel.

Journal Article↗

PubMed2004

Influence of a defect particle on the structure of a classical two-dimensional cluster.

A system of classical charged particles interacting through a Coulomb repulsive potential which are confined in a two-dimensional parabolic trap is studied. We allow one or two particles, called defect particles, to have a different mass and/or charge than the other particles. The structure of the whole system depends on the mass and the charge of the defects and the total number of particles in the system. The ground state configurations are investigated and phase diagrams are constructed, which explain the recent experimental results of Phys. Rev. E 64, 11 603 (2001)]]. We found that several of the experimental configurations are metastable and that replacing the Coulomb interparticle potential by an inversely quadratic one has only a minor effect on the results.

Journal Article↗