References of "Vanderheyden, Benoît"
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See detailMeasurement of the magnetic field inside the holes of a drilled bulk high-Tc superconductor
Lousberg, Grégory ULg; Fagnard, Jean-François ULg; Noudem, Jacques G et al

in Superconductor Science and Technology (2009), 22

We use macroscopic holes drilled in a bulk YBCO superconductor to probe its magnetic properties in the volume of the sample. The sample is subjected to an AC magnetic flux with a density ranging from 30 ... [more ▼]

We use macroscopic holes drilled in a bulk YBCO superconductor to probe its magnetic properties in the volume of the sample. The sample is subjected to an AC magnetic flux with a density ranging from 30 to 130 mT and the flux in the superconductor is probed by miniature coils inserted in the holes. In a given hole, three different penetration regimes can be observed: (i) the shielded regime, where no magnetic flux threads the hole; (ii) the gradual penetration regime, where the waveform of the magnetic field has a clipped sine shape whose fundamental component scales with the applied field; and (iii) the flux concentration regime, where the waveform of the magnetic field is nearly a sine wave, with an amplitude exceeding that of the applied field by up to a factor of two. The distribution of the penetration regimes in the holes is compared with that of the magnetic flux density at the top and bottom surfaces of the sample, and is interpreted with the help of optical polarized light micrographs of these surfaces. We show that the measurement of the magnetic field inside the holes can be used as a local characterization of the bulk magnetic properties of the sample. [less ▲]

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See detailRandom matrix model for antiferromagnetism and superconductivity on a two-dimensional lattice
Vanderheyden, Benoît ULg; Jackson, A. D.

in Physical Review B (Condensed Matter and Materials Physics) (2009), 79(14), 144502

We suggest a new mean-field method for studying the thermodynamic competition between magnetic and superconducting phases in a two-dimensional square lattice. A partition function is constructed by ... [more ▼]

We suggest a new mean-field method for studying the thermodynamic competition between magnetic and superconducting phases in a two-dimensional square lattice. A partition function is constructed by writing microscopic interactions that describe the exchange of density and spin fluctuations. A block structure dictated by spin, time-reversal, and bipartite symmetries is imposed on the single-particle Hamiltonian. The detailed dynamics of the interactions are neglected and replaced by a normal distribution of random matrix elements. The resulting partition function can be calculated exactly. The thermodynamic potential has a structure which depends only on the spectrum of quasiparticles propagating in fixed condensation fields, with coupling constants that can be related directly to the variances of the microscopic processes. The resulting phase diagram reveals a fixed number of phase topologies whose realizations depend on a single coupling parameter ratio, alpha. Most phase topologies are realized for a broad range of values of alpha and can thus be considered robust with respect to moderate variations in the detailed description of the underlying interactions. [less ▲]

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See detailBulk high-Tc superconductors with drilled holes: how to arrange the holes to maximize the trapped magnetic flux?
Lousberg, Grégory ULg; Ausloos, Marcel ULg; Vanderbemden, Philippe ULg et al

in Superconductor Science and Technology (2008), 21

Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing and the heat exchange with the cooling liquid. However, drilling holes also reduces the amount of magnetic flux that can be ... [more ▼]

Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing and the heat exchange with the cooling liquid. However, drilling holes also reduces the amount of magnetic flux that can be trapped in the sample. In this paper, we use the Bean model to study the magnetization and the current line distribution in drilled samples, as a function of the hole positions. A single hole perturbs the critical current flow over an extended region that is bounded by a discontinuity line, where the direction of the current density changes abruptly. We demonstrate that the trapped magnetic flux is maximized if the centre of each hole is positioned on one of the discontinuity lines produced by the neighbouring holes. For a cylindrical sample, we construct a polar triangular hole pattern that exploits this principle; in such a lattice, the trapped field is ∼20% higher than in a squared lattice, for which the holes do not lie on discontinuity lines. This result indicates that one can simultaneously enhance the oxygen annealing, the heat transfer and maximize the trapped field. [less ▲]

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See detailAn ac susceptometer for the characterization of large, bulk superconducting samples
Laurent, Philippe ULg; Fagnard, Jean-François ULg; Vanderheyden, Benoît ULg et al

in Measurement Science & Technology (2008), 19(8), 085705

The main purpose of this work was to design, develop and construct a simple, low-cost ac susceptometer to measure large, bulk superconducting samples (up to 32 mm in diameter) in the temperature range ... [more ▼]

The main purpose of this work was to design, develop and construct a simple, low-cost ac susceptometer to measure large, bulk superconducting samples (up to 32 mm in diameter) in the temperature range 78–120 K. The design incorporates a double heating system that enables a high heating rate (25 K h−1) while maintaining a small temperature gradient (<0.2 K) across the sample. The apparatus can be calibrated precisely using a copper coil connected in series with the primary coil. The system has been used successfully to measure the temperature dependence of the ac magnetic properties of entire RE–Ba–Cu–O [(RE)BCO] bulk superconducting domains. A typical ac susceptibility measurement run from 78 K to 95 K takes about 2 h, with excellent temperature resolution (temperature step ∼ 4 mK) around the critical temperature, in particular. [less ▲]

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See detailSimulation of the highly non linear properties of bulk superconductors: finite element approach with a backwardmethod and a single time step
Lousberg, G.; Ausloos, Marcel ULg; Geuzaine, Christophe ULg et al

in Proceedings of the fourth international conference on advanced computational methods in engineering, ACOMEN 2008 (2008)

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See detailMagnetic shielding properties of high-temperature superconducting tubes subjected to axial fields
Denis, Samuel ULg; Dusoulier, Laurent; Dirickx, Michel ULg et al

in Superconductor Science and Technology (2007), 20(3), 192-201

We have experimentally studied the magnetic shielding properties of a cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic fields. The magnetic response has been investigated as a ... [more ▼]

We have experimentally studied the magnetic shielding properties of a cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic fields. The magnetic response has been investigated as a function of the dimensions of the tube, the magnitude of the applied field and the frequency. These results are explained quantitatively by employing the method of Brandt ( 1998 Phys. Rev. B 58 6506) with a Jc( B) law appropriate for a polycrystalline material. Specifically, we observe that the applied field can sweep into the central region either through the thickness of the shield or through the opening ends, the latter mechanism being suppressed for long tubes. For the first time, we systematically detail the spatial variation of the shielding factor ( the ratio of the applied field over the internal magnetic field) along the axis of a high-temperature superconducting tube. The shielding factor is shown to be constant in a region around the centre of the tube, and to decrease as an exponential in the vicinity of the ends. This spatial dependence comes from the competition between two mechanisms of field penetration. The frequency dependence of the shielding factor is also discussed and shown to follow a power law arising from the finite creep exponent n. [less ▲]

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See detailField penetration into hard type-II superconducting tubes: effects of a cap, a non-superconducting joint, and non-uniform superconducting properties
Denis, Samuel ULg; Dirickx, Michel ULg; Vanderbemden, Philippe ULg et al

in Superconductor Science and Technology (2007), 20(5), 418-427

Using the numerical method of Brandt (1998 Phys. Rev. B 58 6506), we study the penetration of a uniform magnetic field that is gradually applied parallel to the axis of finite type-II superconducting ... [more ▼]

Using the numerical method of Brandt (1998 Phys. Rev. B 58 6506), we study the penetration of a uniform magnetic field that is gradually applied parallel to the axis of finite type-II superconducting tubular samples with strong pinning. This study is carried out in view of designing low-frequency magnetic shields by exploiting the diamagnetic properties of type-II superconductors. First, we compare the field penetration into open and closed tubes. For long tubes (length larger than three times the outer diameter), we show that a cap weakly affects the maximum magnetic induction that can be shielded, but greatly increases the region over which the field is nearly uniform. When the length of the tube is shorter, both the maximum shieldable magnetic induction and the uniformity of the field attenuation are enhanced by closing the tube. We also show that making a hole in the cap, which is often necessary for applications, does not greatly affect the shielding properties provided the diameter of the hole is small compared to that of the tube (hole diameter smaller than a quarter of the outer tube diameter). In view of designing large size magnetic shields, superconducting tubes of finite size need to be joined together. We therefore examine in a second part how the presence of a non-superconducting joint between the tubes affects the shielding efficiency. It is shown that the effect of a joint depends upon its position along the tube axis and strongly increases with its size. Third, we study how non-uniform superconducting properties affect the shielding capabilities. [less ▲]

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See detailCharacterisation of the magnetic shielding properties of YBaCuO thick films prepared by electrophoretic deposition on silver substrates
Denis, Samuel; Grenci, G.; Dusoulier, Laurent et al

in Journal of Physics: Conference Series (2006), 43

This communication reports experimental results on the superconducting properties of YBaCuO thick films prepared by electrophoretic deposition on silver substrates. The magnetic shielding properties of ... [more ▼]

This communication reports experimental results on the superconducting properties of YBaCuO thick films prepared by electrophoretic deposition on silver substrates. The magnetic shielding properties of the coatings were characterised by various methods. First, the electrical resistance and the transport critical current density, Jc, were determined. Our coatings exhibit a superconducting transition at a temperature of 90 K. Next, shielding characterisations were carried out at 77 K for samples having either a slab or a cylindrical geometry. In both cases, the frequency of the applied magnetic field was 103 Hz; the field behind the shielding wall was measured by a pick-up coil connected to a lock-in amplifier. In the case of cylindrical samples and for an applied induction lower than 1 G, the field inside the shielding enclosure is reduced by a factor greater than 106 (i.e. 120 dB) with respect to the applied field. [less ▲]

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See detailYBa2Cu3O(7-delta) thick films on Ag prepared by the electrophoretic deposition technique
Dusoulier, Laurent; Denis, Samuel; Fagnard, Jean-François ULg et al

in Journal of Physics: Conference Series (2006), 43

YBa2Cu3O7-δ thick films have been deposited onto Ag substrates by the Electrophoretic Deposition (EPD) technique. Different microstructures and electrical behaviours were observed depending on the ... [more ▼]

YBa2Cu3O7-δ thick films have been deposited onto Ag substrates by the Electrophoretic Deposition (EPD) technique. Different microstructures and electrical behaviours were observed depending on the starting powder. Coatings prepared from commercial powder displayed significant porosity and the superconducting transition width was found to be magnetic-field dependent. Films produced from home-made coprecipitated powder are denser but contain some secondary phases. No dependence of the resistive transition as a function of magnetic field (H ≤ 20 Oe) was observed in that case. [less ▲]

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See detailTexturation of YBa2Cu3O7-delta thick films by electrophoretic deposition under magnetic field
Dusoulier, Laurent; Denis, Samuel ULg; Nutal, Nicolas ULg et al

in Key Engineering Materials [=KEM] (2006), 314

YBa2Cu3O7-delta coatings were deposited by electrophoretic deposition (EPD) onto Ni substrates. Particles of different sizes and shapes were used in order to study the influence of the powder ... [more ▼]

YBa2Cu3O7-delta coatings were deposited by electrophoretic deposition (EPD) onto Ni substrates. Particles of different sizes and shapes were used in order to study the influence of the powder microstructure on the film density. Texturation of the thick films was induced by application of a magnetic field during the electrophoretic deposition. X-ray diffraction analysis has clearly shown preferred c-axis alignment of the YBa2Cu3O7-delta films along the direction normal to the substrate surface. Scanning electron microscopy and optical polarised light microscopy were used to characterise the microstructure of the coatings, revealing a nonrandom platelets organisation. [less ▲]

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See detailTemperature dependence in random matrix models with pairing condensates
Vanderheyden, Benoît ULg; Jackson, A. D.

in Physical Review D (2005), 72(1), 016003

We address a number of issues raised by a manuscript of Klein, Toublan, and Verbaarschot [hep-ph/0405180] in which the authors introduce a random matrix model for QCD with two colors, two flavors, and ... [more ▼]

We address a number of issues raised by a manuscript of Klein, Toublan, and Verbaarschot [hep-ph/0405180] in which the authors introduce a random matrix model for QCD with two colors, two flavors, and fermions in the fundamental representation. Their inclusion of temperature terms differs from the approach adopted in previous work on this problem [Phys. Rev. D 64, 074016 (2001)]. We demonstrate that the two approaches are related by a transformation that leaves the thermodynamic potential invariant and which therefore has no effect on physical observables. [less ▲]

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See detailRandom matrix models for chiral and diquark condensation
Vanderheyden, Benoît ULg; Jackson, A. D.; Lansberg, Jean-Philippe ULg et al

in AIP Conference Proceedings (2005), 775

We consider random matrix models for the thermodynamic competition between chiral symmetry breaking and diquark condensation in QCD at finite temperature and finite baryon density. The models produce mean ... [more ▼]

We consider random matrix models for the thermodynamic competition between chiral symmetry breaking and diquark condensation in QCD at finite temperature and finite baryon density. The models produce mean field phase diagrams whose topology depends solely on the global symmetries of the theory. We discuss the block structure of the interactions that is imposed by chiral, spin, and color degrees of freedom and comment on the treatment of density and temperature effects. Extension of the coupling parameters to a larger class of theories allows us to investigate the robustness of the phase topology with respect to variations in the dynamics of the interactions. We briefly study the phase structure as a function of coupling parameters and the number of colors. [less ▲]

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See detailInversion of the Biot-Savart Law: an approach based on discrete sine and cosine transforms
Dirickx, Michel; Denis, Samuel ULg; Vanderheyden, Benoît ULg et al

in Johansen, T. H.; Shantsev, D. V. (Eds.) Magneto-Optical Imaging, Proceedings of the NATO Advanced Research Workshop, held in Øystese, Norway, 28-30 August 2003 (2004)

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See detailMicroscopic spectral density in random matrix models for chiral and diquark condensation
Vanderheyden, Benoît ULg; Jackson, A. D.

in Physical Review D (2003), 67(8), 085016

We examine random matrix models of QCD which are capable of supporting both chiral and diquark condensation. A numerical study of the spectral densities near zero virtuality shows that the introduction of ... [more ▼]

We examine random matrix models of QCD which are capable of supporting both chiral and diquark condensation. A numerical study of the spectral densities near zero virtuality shows that the introduction of color in the interactions does not alter the one-body results imposed by chiral symmetry. A model with three colors has the spectral density predicted for the chiral ensemble with a Dyson index beta=2; a pseudoreal model with two colors exhibits the spectral density of the chiral ensemble with beta=1. [less ▲]

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See detailÉtude de la paradiaphonie (NEXT) au sein de câbles composés de paires torsadées
Vanderheyden, Benoît ULg; Van Droogenbroeck, Marc ULg

Report (2002)

Le réseau téléphonique est constitué d’une succession de câbles mis bout à bout et interconnectés au droit des répartiteurs. Cette hétérogénéité de câblage n’est guère favorable pour la réduction des ... [more ▼]

Le réseau téléphonique est constitué d’une succession de câbles mis bout à bout et interconnectés au droit des répartiteurs. Cette hétérogénéité de câblage n’est guère favorable pour la réduction des interférences entre signaux véhiculés sur les différentes paires. Un opérateur de télécommunications doit également tenir compte de la longue période définie pour l’amortissement comptable d’un réseau câblé, ce qui fait que les câbles risquent fort de n’être remplacés qu’en cas de panne ou d’insuffisance manifeste des performances. Le dégroupage de la boucle locale intervenu récemment complique encore la donne, car un opérateur historique comme Belgacom doit autoriser l’accès direct à la paire d’un abonné au bénéfice d’un opérateur alternatif. Ce dernier aura pour objectif d’offrir des services à haute valeur ajoutée et donc, techniquement, consommateur d’une plus large bande passante et générateur de plus d’interférence. Face à ces difficultés opérationnelles, les opérateurs et les organismes de normalisation ont défini des gabarits régissant le niveau de puissance pour tout signal émis sur une paire. À partir de ces gabarits et en fonction de son réseau, un opérateur pourra déterminer les caractéristiques des câbles à acquérir de manière à permettre la co-existence soit de signaux de même densité spectrale mais avec un haut taux de remplissage du câble, soit de signaux fort différents. Cette étude se concentre sur plusieurs points relatifs à la paradiaphonie au sein de câbles composés de paires torsadées. Dans un premier temps, nous aborderons la délicate question de la modélisation des phénomènes de paradiaphonie entre paires composant un même câble. Cette étude fournira le point de départ des développements relatifs à la fabrication de câbles de type S-Z et des effets de diaphonie dus à la distribution des paires dans le plan transversal. [less ▲]

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See detailRandom matrix study of the phase structure of QCD with two colors
Vanderheyden, Benoît ULg; Jackson, A. D.

in Physical Review. D : Particles and Fields (2001), 64(7), 074016

We apply a random matrix model to the study of the phase diagram of QCD with two colors, two flavors, and a small quark mass. Although the effects of temperature are only included schematically, this ... [more ▼]

We apply a random matrix model to the study of the phase diagram of QCD with two colors, two flavors, and a small quark mass. Although the effects of temperature are only included schematically, this model reproduces most of the ground state predictions of chiral perturbation theory and also gives a qualitative picture of the phase diagram at all temperatures. It leads, however, to an unphysical behavior of the chiral order parameter and the baryon density in vacuum and does not support diquark condensation at arbitrarily high densities. A better treatment of temperature dependence leads to correct vacuum and small temperature properties. We compare our results at both high and low densities with the results of microscopic calculations using the Nambu-Jona-Lasinio (NJL) model and discuss the effects of large momentum scales on the variations of condensation fields with chemical potential. [less ▲]

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See detailA random matrix model for color superconductivity at zero chemical potential
Vanderheyden, Benoît ULg; Jackson, A. D.

in Physical Review. D : Particles and Fields (2000), 61(7), 076004

We discuss random matrix models for the spontaneous breaking of both chiral and color symmetries at zero chemical potential and finite temperature. Exploring different Lorentz and gauge symmetric color ... [more ▼]

We discuss random matrix models for the spontaneous breaking of both chiral and color symmetries at zero chemical potential and finite temperature. Exploring different Lorentz and gauge symmetric color structures of the random matrix interactions, we find that spontaneous chiral symmetry breaking is always thermodynamically preferred over diquark condensation. Stable diquark condensates appear only as SU(2) rotated chiral condensates, which do not represent an independent thermodynamic phase. Our analysis is based on general symmetry arguments and hence suggests that no stable and independent diquark phase can form in QCD with two flavors at zero quark chemical potential. [less ▲]

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See detailRandom matrix model for chiral symmetry breaking and color superconductivity in QCD at finite density
Vanderheyden, Benoît ULg; Jackson, A. D.

in Physical Review. D : Particles and Fields (2000), 62(9), 094010

We consider a random matrix model which describes the competition between chiral symmetry breaking and the formation of quark Cooper pairs in QCD at finite density. We study the evolution of the phase ... [more ▼]

We consider a random matrix model which describes the competition between chiral symmetry breaking and the formation of quark Cooper pairs in QCD at finite density. We study the evolution of the phase structure in temperature and chemical potential with variations of the strength of the interaction in the quark-quark channel and demonstrate that the phase diagram can realize a total of six different topologies. A vector interaction representing single-gluon exchange reproduces a topology commonly encountered in previous QCD models, in which a low-density chiral broken phase is separated from a high-density diquark phase by a first-order line. The other five topologies either do not possess a diquark phase or display a new phase and new critical points. Since these five cases require large variations of the coupling constants away from the values expected for a vector interaction, we conclude that the phase diagram of finite density QCD has the topology suggested by single-gluon exchange and that this topology is robust. [less ▲]

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See detailSelf-consistent approximations: application to a quasiparticle description of the thermodynamic properties of relativistic plasmas
Vanderheyden, Benoît ULg; Baym, Gordon

in Michael, Bonitz (Ed.) PROGRESS IN NONEQUILIBRIUM GREEN'S FUNCTIONS (2000)

We generalize the concept of conserving, Phi-derivable, approximations to relativistic field theories. Treating the interaction field as a dynamical degree of freedom, we derive the thermodynamical ... [more ▼]

We generalize the concept of conserving, Phi-derivable, approximations to relativistic field theories. Treating the interaction field as a dynamical degree of freedom, we derive the thermodynamical potential in terms of fully dressed propagators, an approach which allows us to resolve the entropy of a relativistic plasma into contributions from its interacting elementary excitations. We illustrate the derivation for a hot QED plasma of massless particles. We also discuss how the self-consistency of the treatment manifests itself into relationships between the contributions from interaction and matter fields. [less ▲]

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See detailInteracting elementary excitations in ultrarelativistic plasmas: Damping mechanisms and plasma thermodynamics
Vanderheyden, Benoît ULg

Doctoral thesis (1998)

This dissertation examines the properties of the elementary excitations in quark-gluon and electromagnetic plasmas. Due to Debye screening and Landau damping mechanisms, the interaction fields fall off at ... [more ▼]

This dissertation examines the properties of the elementary excitations in quark-gluon and electromagnetic plasmas. Due to Debye screening and Landau damping mechanisms, the interaction fields fall off at large distances, except for static magnetic and color-magnetic fields. The lack of screening of the magnetic interaction leads to a singularity at small momentum transfer in the matrix element for fermion-fermion scattering via the exchange of a single gluon, or a single photon. We analyze the effects of this singularity on the quasiparticle lifetimes in plasmas at zero and at finite temperature T. First, at T = 0, we find that the fermion excitations above the ground state are long-lived, as the Pauli exclusion principle limits the collision phase space. Because the magnetic interaction is long-ranged, the damping rates of the excitations near the edge of the Fermi sea vary linearly with the excitation energy. The quasiparticles are therefore not as well- defined as those encountered in non-relativistic plasmas, with short-ranged interactions, where the variation of the damping rates away from the Fermi surface is quadratic, thus slower. Second, we find that in plasmas at finite temperatures, the magnetic interaction introduces correlations between the successive scatterings of the fermion quasiparticles on the charges in the system. This effect leads to a decay law that decreases in time more rapidly than an exponential; however the quasiparticles are long-lived. We also derive a framework for ascertaining the effects of the long-ranged magnetic interaction on the thermodynamic properties of relativistic plasmas. Our method generalizes the concept of conserving, Φ-derivable, approximations to relativistic field theories. By treating the interaction field as a dynamical degree of freedom, we are able to derive the thermodynamical potential in terms of full propagators. This approach allows us to resolve the entropy into contributions from its interacting elementary excitations. [less ▲]

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