An ac susceptometer for the characterization of large, bulk superconducting samples

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 ▲]

Magnetic shielding properties of high-temperature superconducting tubes subjected to axial fields

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 ▲]

Characterisation of the magnetic shielding properties of YBaCuO thick films prepared by electrophoretic deposition on silver substrates

in Journal of Physics: Conference Series [=JPCS] (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 ▲]

Texturation of YBa2Cu3O7-delta thick films by electrophoretic deposition under magnetic field

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 ▲]

Random matrix models for chiral and diquark condensation

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 ▲]

Microscopic spectral density in random matrix models for chiral and diquark condensation

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 ▲]

Random matrix study of the phase structure of QCD with two colors

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 ▲]

Random matrix model for chiral symmetry breaking and color superconductivity in QCD at finite density

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 ▲]

Interacting elementary excitations in ultrarelativistic plasmas: Damping mechanisms and plasma thermodynamics

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 ▲]

Damping rates of hard momentum particles in a cold ultrarelativistic plasma

in Physical Review. D : Particles and Fields (1997), 56(8), 5108-5122

We compute the damping rates of one-particle excitations in a cold ultrarelativistic plasma to leading order in the coupling constant e for three types of interaction: Yukawa coupling to a massless scalar ... [more ▼]

We compute the damping rates of one-particle excitations in a cold ultrarelativistic plasma to leading order in the coupling constant e for three types of interaction: Yukawa coupling to a massless scalar boson, QED, and QCD. Damping rates of charged particles in QED and QCD are of order e(3) mu, while the damping rates of other particles are of order e(4) mu or e(4) mu log(1/e). We find that the damping rate of an electron or of a quark is constant far from the Fermi surface, and decreases linearly with the excitation energy close to the Fermi surface. This unusual behavior is attributed to the long-range magnetic interactions. [less ▲]