Single domain YBCO/Ag bulk superconductors fabricated by seeded infiltration and growth

We have applied the seeded infiltration and growth (IG) technique to the processing of samples containing Ag in an attempt to fabricate Ag-doped Y-Ba-Cu-O (YBCO) bulk superconductors with enhanced mechanical properties. The IG technique has been used successfully to grow bulk Ag-doped YBCO superconductors of up to 25 mm in diameter in the form of single grains. The distribution of Ag in the parent Y-123 matrix fabricated by the IG technique is observed to be at least as uniform as that in samples grown by conventional top seeded melt growth (TSMG). Fine Y-211 particles were observed to be embedded within the Y-123 matrix for the IG processed samples, leading to a high critical current density, Jc, of over 70 kA/cm2 at 77.3 K in self-field. The distribution of Y-211 in the IG sample microstructure, however, is inhomogeneous, which leads to a variation in the spatial distribution of Jc throughout the bulk matrix. A maximum-trapped field of around 0.43 T at 1.2 mm above the sample surface (i.e. including 0.7 mm for the sensor mould thickness) is observed at liquid nitrogen temperature, despite the relatively small grain size of the sample (20 mm diameter × 7 mm thickness)

Backflow and dissipation during the quantum decay of a metastable Fermi liquid

The particle current in a metastable Fermi liquid against a first-order phase transition is calculated at zero temperature. During fluctuations of a droplet of the stable phase, in accordance with the conservation law, not only does an unperturbed current arise from the continuity at the boundary, but a backflow is induced by the density response. Quasiparticles carrying these currents are scattered by the boundary, yielding a dissipative backflow around the droplet. An energy of the hydrodynamic mass flow of the liquid and a friction force exerted on the droplet by the quasiparticles have been obtained in terms of a potential of their interaction with the droplet.Comment: 5 pages (REVTeX), to be published in Phys. Rev.

Growth rate and superconducting properties of Gd-Ba-Cu-O bulk superconductors melt processed in air

A generic Mg-doped Nd-Ba-Cu-O seed crystal has been developed recently for the fabrication of any type of rare earth (RE) based (RE)-Ba-Cu-O single grain bulk superconductor in air. The new generic seed simplifies significantly the top seeded melt growth (TSMG) process for light rare earth based (Nd, Sm, Gd, or mixed rare earth elements) bulk superconductors, in particular. GdBCO single grains have been fabricated successfully in air using the new seed in a cold-seeding process. In this study, precursor powders were enriched with different amounts of BaO2 to investigate the extent of substitution of Gd for Ba in the Gd1+xBa2-xCu3O7-delta solid solution phase. The growth process of large single grains in air was investigated at various growth temperatures under isothermal processing conditions. Crystal growth rate as a function of under-cooling and BaO2 content has been determined from these experiments. The spatial variation of Tc and transition temperature width for applied field aligned along the a/b and c-axis of grains fabricated with different BaO2 content has also been investigated in order to understand the extent of the formation of Gd/Ba solid solution with varying growth temperature and precursor composition. These results have been used to establish the optimum conditions for fabricating solid solution-free, large single grains of GdBCO in air

Strongly coupled artificial bulk HTS grain boundaries with high critical current densities

A multi-seeding process has been developed to fabricate single Y-Ba-Cu-O (YBCO) grains containing strong artificial grain boundaries. Multi-seeding of heterogeneous YBCO grains with controlled orientation was achieved using large Sm-Ba-Cu-O (SmBCO) single crystal seeds of rod-like geometry with slots of various widths (up to 13 mm) cut into their bottom surface (i.e. parallel to the c-axis of the seed) to produce a bridge-like structure. Several YBCO grains with artificial grain boundaries were fabricated from these seed crystals and used to investigate the effect of varying the distance between the individual grain nucleation sites and the grain orientation (in-plane and out of plane) on the nature of grain boundaries. The measured local magnetic critical current density (Jc) and the magnitude of the trapped field across these artificial grain boundaries indicate that seed alignment is a key parameter in achieving strongly-coupled grain boundaries in multi-seeded grains

Survival of charmonia above Tc in anisotropic lattice QCD

We find a strong evidence for the survival of $J/\Psi$ and $\eta_c$ as spatially-localized $c\bar c$ (quasi-)bound states above the QCD critical temperature $T_c$, by investigating the boundary-condition dependence of their energies and spectral functions. In a finite-volume box, there arises a boundary-condition dependence for spatially spread states, while no such dependence appears for spatially compact states. In lattice QCD, we find almost {\it no} spatial boundary-condition dependence for the energy of the $c\bar c$ system in $J/\Psi$ and $\eta_c$ channels for $T\simeq(1.11-2.07)T_c$. We also investigate the spectral function of charmonia above $T_c$ in lattice QCD using the maximum entropy method (MEM) in terms of the boundary-condition dependence. There is {\it no} spatial boundary-condition dependence for the low-lying peaks corresponding to $J/\Psi$ and $\eta_c$ around 3GeV at $1.62T_c$. These facts indicate the survival of $J/\Psi$ and $\eta_c$ as compact $c\bar c$ (quasi-)bound states for $T_c < T < 2T_c$.Comment: 4 pages, 1 figur

Bulk superconducting nano-composites with high critical currents

Flux pinning sites are most effective if their size is comparable to the superconducting coherence length, which is on the nano-meter scale for RE-Ba-Cu-O superconductors [RE = rare earth element]. Introducing nano-phase inclusions directly into the bulk superconducting material has only been partially successful to date, however, due primarily to the absence of chemically stable phases that can co-exist with RE-Ba-Cu-O without suppressing its key superconducting properties. We have identified novel isostructural phases based on (RE)2Ba4CuMOy (where M = W, Zr, Nb, Ag and Bi) and have fabricated successfully superconducting bulk nano-composites with a high current carrying capability. The average size of the nano-inclusions is observed to vary from 20 nm to 300 nm depending on element M. An observed improvement in Jc under low and high external magnetic fields at 77 K correlates directly with an increased density of nano-inclusions in the superconducting matrix

Anisotropic Lattice QCD Studies of Penta-quark Anti-decuplet

Anti-decuplet penta-quark baryon is studied with the quenched anisotropic lattice QCD for accurate measurement of the correlator. Both the positive and negative parity states are studied using a non-NK type interpolating field with I=0 and J=1/2. After the chiral extrapolation, the lowest positive parity state is found at m_{Theta} \simeq 2.25 GeV, which is too massive to be identified with the experimentally observed Theta^+(1540). The lowest negative parity state is found at m_{Theta}\simeq 1.75 GeV, which is rather close to the empirical value. To confirm that this state is a compact 5Q resonance, a new method with ``hybrid boundary condition (HBC)'' is proposed. The HBC analysis shows that the observed state in the negative parity channel is an NK scattering state.Comment: A talk given at International Workshop PENTAQUARK04, July 20-23, 2004 at SPring-8, Japan, 8 pages, 7 figures, 2 table

Properties of Mg-doped Nd-Ba-Cu-O generic seed crystals for the top seeded melt growth of (RE)-Ba-Cu-O bulk superconductors

We have recently developed a new generic seed crystal that has been used successfully to fabricate any oriented, single grain (RE)-Ba-Cu-O bulk superconductor by a cold seeding technique. In this paper we report the chemical, structural and microstructural properties of these seed crystals, including the variation of melting point, crystallographic parameters and volume fraction of Mg-rich inclusions in the Nd1 + xBa 2-x(Cu1-yMgy)3Oz matrix as a function of externally added MgO content. The influence of Mg-doping on the superconducting transition temperatures of YBCO grains fabricated using these seeds is investigated. Finally, an optimum MgO content of the generic seed that effectively controls the orientation of the seeded grain without compromising its superconducting properties is suggested from the many seed crystals fabricated with a wide range of Mg-rich addition

Charmonium properties in deconfinement phase in anisotropic lattice QCD

J/Psi and eta_c above the QCD critical temperature T_c are studied in anisotropic quenched lattice QCD, considering whether the c\bar c systems above T_c are spatially compact (quasi-)bound states or scattering states. We adopt the standard Wilson gauge action and O(a)-improved Wilson quark action with renormalized anisotropy a_s/a_t =4.0 at \beta=6.10 on 16^3\times (14-26) lattices, which correspond to the spatial lattice volume V\equiv L^3\simeq(1.55{\rm fm})^3 and temperatures T\simeq(1.11-2.07)T_c. We investigate the c\bar c system above T_c from the temporal correlators with spatially-extended operators, where the overlap with the ground state is enhanced. To clarify whether compact charmonia survive in the deconfinement phase, we investigate spatial boundary-condition dependence of the energy of c\bar c systems above T_c. In fact, for low-lying S-wave c \bar c scattering states, it is expected that there appears a significant energy difference \Delta E \equiv E{\rm (APBC)}-E{\rm (PBC)}\simeq2\sqrt{m_c^2+3\pi^2/L^2}-2m_c (m_c: charm quark mass) between periodic and anti-periodic boundary conditions on the finite-volume lattice. In contrast, for compact charmonia, there is no significant energy difference between periodic and anti-periodic boundary conditions. As a lattice QCD result, almost no spatial boundary-condition dependence is observed for the energy of the c\bar c system in J/\Psi and \eta_c channels for T\simeq(1.11-2.07)T_c. This fact indicates that J/\Psi and \eta_c would survive as spatially compact c\bar c (quasi-)bound states below 2T_c. We also investigate a $P$-wave channel at high temperature with maximally entropy method (MEM) and find no low-lying peak structure corresponding to \chi_{c1} at 1.62T_c.Comment: 13 pages, 11 figure