A. J. S. Machado

Two-fluid model for transport properties of granular superconductors

A two-fluid model is proposed to describe the transport properties of granular superconductors. Using the resistively shunted junction model and some aspects of the two-level system theory, a statistical model is developed which takes into account the ratio between the number of normal and superconducting electrons carrying the applied current. The theoretical model reveals excellent agreement when compared to transport properties of four high-Tc superconductors. The results suggest that the two-fluid model is independent of the sample composition, critical temperature and whether the superconducting compound is electron or hole-doped.

Development of YBaCuO bulk superconductor for use as low magnetic field sensor

Polycrystalline samples of nonoriented bulk YBaCuO superconducting specimens display high degree of disorder, semiconducting behavior in the normal state, and broad transition temperature. A regime explaining electrical and magnetic behaviors of high critical temperature superconductors called thermally activated flux flow (TAFF) presents a state in which vortex dynamic exhibits an ohmic behavior. The resistivity in this regime is proportional to the applied magnetic field for small current densities. In this work is presented a systematic study of the magnetoresistance as a function of the applied magnetic field in the range 5 to 100 Oe for polycrystalline samples with various thickness. Results of X-ray diffraction, resistivity as a function of the temperature, I-V characteristic curves and magnetoresistance are shown. The later result was fitted through an ad-hoe model showing good agreement in the 5 to 100 Oe magnetic field range.

Superconductivity and magnetism in the KxMoO2−δ

Coexistence between superconductivity and magnetism is reported for the KxMoO2−δ samples. Photoemission experiments show that the presence of Mo3+ ions is responsible for the weak ferromagnetic ordering observed in the KxMoO2−δ samples. Magnetic ordering temperature and superconducting critical temperature (TC) ratio range from 7 to 18 in this compound. These are the highest ratios reported so far for a magnetic superconductor. TC decreases with increasing potassium composition (x). For the first time, TC near 10 K is reported in the K-Mo-O system.

Interstitial doping induced superconductivity at 15.3 K in Nb5Ge3 compound

It is reported superconductivity in Nb5Ge3C0.3, an interstitial carbide compound. The temperature dependence of the electrical resistivity, ac-susceptibility, and heat capacity (HC) indicate that a bulk type-II superconductivity appears at TC = 15.3 K. Magneto-resistance measurements suggest an upper critical field of BC2 ∼ 10.6 T and a coherence length of ξ ∼ 55 A at zero temperature. Neutron diffraction analyzes locate the carbon atoms at the interstitial 2b site of the Mn5Si3 type-structure. Heat capacity data below TC are well described by BCS theory. The size of the jump at TC is in good agreement with the superconducting volume fraction observed in susceptibility measurements. A Debye temperature and Sommerfeld constant were also extracted from heat capacity data as 343 K and 34 mJ/mol K2, respectively.

Possible electrical-current-driven superconductor-insulator transition

Abstract In this work we have studied the effect of applied electrical current on the resistive behavior of single phase Y 1−X Pr X Ba 2 Cu 3 O 7−Y [Y(Pr)123] polycrystalline samples with x close to the critical Pr concentration x c ≈ 0.57 above which the superconductivity vanishes. The obtained results suggest the occurrence of a superconducting-insulator transition (SIT) driven by the electrical current. In particular, a crossing of current-voltage (I–V) isotherms at magnetic-field-dependent current I C (H) is found, and the scaling behavior of nonlinear resistance R(I,T) = V(T)/I similar to that predicted in the scaling theory of SIT is obtained. We speculate that this possible current-induced SIT can be considered as the dynamical counterpart of the magnetic-field-tuned SIT.

Ta1-xHfxB: A new FeB-prototype superconductor

In this work, we report superconductivity in the new ternary Ta1−xHfxB system. We also show clearly evidence that appropriate amounts of Hf substitution at the Ta site induce the stabilization of the orthorhombic FeB prototype structure in this system. Bulk superconductivity is confirmed by magnetization, electronic transport and specific heat measurements. The Hall signal has temperature dependence and the specific heat data at low temperature deviate from the conventional exponential temperature dependence in Ta0.7Hf0.3B. These results suggest multiband superconductivity in this compound.

Superconducting YBa2Cu3O7−δ films on SrTiO3 by electrodeposition process

Abstract An ac electrodeposition (ED) process to obtain good quality YBa 2 Cu 3 O 7−δ (YBCO) high-temperature superconductor (HTS) films was developed. These films were deposited on SrTiO 3 (100) single crystal substrates previously coated with a thin silver layer (800 A) that was deposited by chemical reduction. A small amount of Ag (0.7% in weight) added to the electrolyte bath was shown to improve the quality of the deposited films. YBCO films after an appropriate heat treatment present single phase material with an onset T c ∼ 90 K, Δ T c ∼ 2 K, homogeneous grain morphology and strong biaxial texture. Transport measurements show a metallic behavior in the normal state and a self-field critical current densities ∼ 10 5 A/cm 2 at 4.2 K and ∼ 10 4 A/cm 2 at 77K.

Synthesis of HgPb2 assisted by high energy ball milling

Abstract The aim of this work was to demonstrate that high energy ball milling process can induce the formation of HgPb2 compound. It has been found mechanical milling is a very effective, low cost and simple technique to produce this material without surfactant and further annealing. The powders after milling for various durations were characterized by X-ray powder diffraction. Milling parameters such as ball to raw material ration (5∶1) and milling time (2 h) were determined to be suitable for synthesising powders of HgPb2. The present investigation also demonstrates that high energy ball milling can produce high quality superconducting samples in the Hg–Pb system.

Critical current density and flux pinning in Zr0.96V0.04B2 superconductor with AlB2 structure

We have investigated the critical current density (Jc) and the flux pinning behavior in Zr0.96V0.04B2 superconductor with an AlB2 structure. V substitutions in Zr sites of non-superconducting ZrB2 system lead to superconductivity, and the 4% V-substituted Zr0.96V0.04B2 compounds show the highest superconducting transition temperature (Tc) of ∼8.7 K. The magnetic hysteresis (M−H) loops for the Zr0.96V0.04B2 demonstrate type-II superconducting behavior in a broad temperature range, and the Jc is estimated from the M−H loops using the Bean model. The analysis of the double-logarithmic Jc(H) plots indicates the dominance of collective pinning in Zr0.96V0.04B2, and that Jc(H) and magnetic field dependences of the flux pinning force density (Fp) are well fitted by the double exponential model which takes into account the existence of two superconducting gaps.

Insights on unconventional superconductivity in HfV

The HfV