Transition of Cs3C60 crystals with Rb impurity to the superconducting state

The statistical theory of phase transition in fullerides of alkali metals to the superconducting state has been elaborated. The calculation of the free energy of crystals has been performed on the basis of molecular-kinetics and the equilibrium equations have been examined. The critical temperature of superconductivity has been estimated, and its concentration dependence has been ascertained. The calculation results have been compared with experimental data. The numerical estimation for energetic parameters of pairing of Cooper pairs electrons has been performed.     

Improvement of flux pinning in GdBa2Cu3O7−δ thin film by nanoscale ferromagnetic La0.67Sr0.33MnO3 pretreatment of substrate surface

The present paper presents the effects of ferromagnetic La_0.67Sr_0.33MnO₃ (LSMO) nanoparticles on the pinning characteristics of an epitaxial GdBa₂Cu₃O_7?δ ((Gd) BCO) film deposited on top. LSMO nanoparticles with the size between 10 and 20 nm were obtained on a (001) STO substrate by RF sputtering method. The analyses of magnetic measurements revealed that the presence of a complex vortex pinning mechanism within the (Gd) BCO film deposited on the undecorated substrate. With respect to a reference (Gd) BCO film, two additional pinning effects in LSMO decorated (Gd) BCO film were observed. One is the effect of the threading dislocations due to LSMO nanoparticles, and the other one is the LSMO nanoparticles itself. Like other nanosized particles, the LSMO nanoparticles on substrate will induce threading dislocations passing through the (Gd) BCO film. These threading dislocations serve as correlated pinning centers along the c-axis, which lead to the improvement of flux pinning properties in LSMO decorated (Gd) BCO film. According to the ferromagnetic property of LSMO material, a magnetic pinning mechanism was proposed to explain the significant increase of Jc and Fp values. The flux pinning abilities of LSMO nanoparticles were evaluated. The enhanced pinning characteristics in the high field region and high temperature together with the movement of H _Fpmax (the corresponding field of F_pmax) also suggest a magnetic pinning mechanism for LSMO nanoparticles existed in LSMO decorated (Gd) BCO film.     

Effect of polysilane addition on spark plasma sintering and superconducting properties of MgB2 bulks

Polycrystalline MgB₂ bulk samples were produced by ex-situ spark plasma sintering (SPS) using the oxygen-free preceramic polymer additive poly(dimethylsilane) as a source for carbon doping and as a sintering aid. Two major effects were identified. One is the significant enhancement of the densification kinetics during sintering for all tested compositions. The second one is the improvement of the high field critical current density for a certain level of the poly(dimethylsilane) addition.     

Superconductivity in polycrystalline diamond thin films

Superconductivity was discovered in heavily boron-doped diamond thin films deposited by the microwave plasma assisted chemical vapor deposition (MPCVD) method. Advantages of the MPCVD deposited diamond are the controllability of boron concentration in a wide range, and a high boron concentration, especially in (111) oriented films, compared to that of the high-pressure high-temperature method. The superconducting transition temperatures are determined to be 8.7 K for T_c onset and 5.0 K for zero resistance by transport measurements. And the upper critical field is estimated to be around 7 T.     

Microstructure and superconducting properties of MgB2 films prepared by solid state reaction of multilayer precursors of the elements

Surface morphology and superconducting properties of MgB₂ superconducting thin films prepared by ex-situ annealing of multilayer Mg/B precursors in Mg vapor are studied.Depending on the precursor structure different physical and microstructural properties of the superconductor evolve. Structure and composition of the films are analyzed by scanning electron microscopy and wavelength dispersive x-ray spectroscopy. It is found that certain precursor structures can lead to high quality superconducting films, however, in specific precursor structures mechanical stress leads to the formation of wrinkles strongly affecting the superconducting homogeneity of the films. A correlation between microstructure and superconducting properties, such as pinning or critical current density, can be provided via magneto-optical Faraday microscopy.     

Synthesis of LaRhAsO and superconductivity within the LaFe1−xRhxAsO system

The compound LaRhAsO is reported for the first time. We show that an entire LaFe_1−xRh_xAsO solid solution is possible. Powder X-ray diffraction, resistivity and magnetic measurements were carried out on polycrystalline samples. Superconductivity onset is observed for x = 0.05, 0.10, and 0.15 with a maximum resistivity of ∼16 K. The fluorine doped samples of the new compound LaRhAsO remain metallic down to 5 K.     

Point contact investigation on Bi2Sr2CaCu2O8+y thin films

We have measured point contacts with a gold tip on Bi₂Sr₂CaCu₂O_8+y thin films and Bi₂Sr₂CaCu₂O_8+y/SrTiO₃ double layers. The results show tunneling or direct conductivity behaviour depending on the junction parameters and can be fitted by corresponding theoretical models. From fitting procedure of differential characteristics by modified Blonder-Tinkham-Klapwijk (BTK) theory the Fermi velocity v_F=5×10⁵ m/s, ratio 2(4.2K)/kT_c=7.9 and coherence length _ab=3.5 nm were obtained. The changing of the interface transmission by additional layers of different thickness corresponding to metallic or insulating behaviour is shown. In the frame of the inelastic scattering of quasiparticles the linear background of differential conductance is discussed.This work was supported by German BMFT under Contract No.l3N5924A and Slovak Grant Agency for Sciences (Grants Nos. 2/990125/93 and 2/999185/92).     

Normal metal, ohmic contacts to high-temperature superconducting YBa2Cu3O7-δ thin films suitable for multichip modules

Normal metal, ohmic contacts to high-temperature superconductor (HTSC) materials will be used to form via structures between HTSC interconnect levels, and also, substrate bonding pads in a superconducting multichip module (SMCM). Specific contact resistivities below 10⁻⁸ Ω cm² will be required for such contacts to control signal attenuation and local contact heating of the LN₂cooled SMCM. Previous work on normal metal/superconducting contacts has not focused on metallization schemes which will be stable during subsequent high-temperature processing. Metal contacts of gold, silver, and palladium were formed on superconducting thin films of YBa₂Cu₃O_7-δ via evaporation and sputtering through a shadow mask followed by annealing in various ambients and at several temperatures. Palladium contacts oxidized readily during anneal, and sputtered gold contacts required additional processing and exhibited higher specific contact resistivities. The best contacts were obtained by a controlled-cooling oxygen anneal of evaporated gold or silver, as indicated by normal-state specific contact resistivities of 3 × 10⁻⁵ Ω cm² and 4 × 10⁻⁵ Ω cm², respectively. This work differs from previously published results by describing contacts which required no extensive preparation of the HTSC surface and were stable to 700 °C, indicating these contacts would be compatible with subsequent high-temperature processing of the additional HTSC layers required in a multi-level SMCM.     

Quantum Transition to Supersolid Phase

We study a possible coexistence of superconducting state and charge density waves which, in a broad sense, might be called a supersolid phase. We investigate the infinite dimensional (d=) attractive Hubbard model by applying a sublattice dependent Gutzwiller wave function |BCS as a variational wave function describing the ground state. One may naively expect that the BCS superconducting state evolves continuously to the Bose–Einstein condensed state of bipolarons as the attractive interaction increases, as far as the system is dilute. However, we show that our variational wave function has lower energy than the simple BCS wave function for all electron densities and the interaction strengths. Our variational parameters increase (g _A,B) as we increase the interaction strength (U). The energy gap turns out to be a mixture of s and extended-s waves. In the vicinity of half-filling, we find a quantum transition from a simple superconducting phase to a supersolid phase with increase of the electron density and/or the interaction strength.