Transport Supercurrent Research Articles

Impact of the transport supercurrent on the density of states in the weak link

The impact of the transport supercurrent on the density of states in the superconducting thin film with the weak link is investigated. At the weak link the order parameter is locally suppressed due to the order parameter phase difference. This results in the appearance of the zero-energy states, which are strongly influenced by the supercurrent especially at the phase difference close to Pi. The observability of these effects with the scanning tunneling spectroscopy is discussed.

Vortex-like Current States in Josephson Ballistic Point Contacts

The stationary Josephson effect is studied theoretically in the situation when there is externally injected transport supercurrent which flows in the banks parallel to the contact interface. Coexistence of this supercurrent with the order parameter phase difference between the banks of the contact results in the appearance of quasiparticles in the weak link, which create the current, localized in the vicinity of the contact and counter-flowing to the transport supercurrent. We review the results of our previous study of weak links between current-carrying conventional and unconventional (d-wave) superconductors. And further we study the weak link in the form of double point contact between current-carrying superconductors. Current distribution patterns containing vortex-like states are obtained.

Vortex-like current states in Josephson ballistic point contacts

The stationary Josephson effect is studied theoretically in the situation when there is externally injected transport supercurrent which flows in the banks parallel to the contact interface. Coexistence of this supercurrent with the order parameter phase difference between the banks of the contact results in the appearance of quasiparticles in the weak link, which create the current, localized in the vicinity of the contact and counter-flowing to the transport supercurrent. We review the results of our previous study of weak links between current-carrying conventional and unconventional (d-wave) superconductors. And further we study the weak link in the form of double point contact between current-carrying superconductors. The current distribution patterns, containing the vortex-like states, are obtained.

Evaluation of supercurrent distribution in high- Tc superconductor by scanning SQUID microscopy

Scanning SQUID microscope (SSM) is a powerful tool for observing small magnetic flux. It measures the z-component signal of flux density, but it is also available for observation of current distribution by inverting the Biot–Savart's law. We report the two-dimensional vector mapping of current distributions in high- T c superconducting YBa 2Cu 3O 7− y (YBCO) thin films obtained by converting magnetic-field data measured by SSM. The current distribution contains those from both transport supercurrent and vortex current. The transport supercurrent is found to flow mainly along the edge of a stripline. The results are in good qualitative agreement with the calculated results based on simple London model. The YBCO films with an artificial grain boundary, i.e. Josephson boundary are also observed. The transport current seems to flows with avoiding the vortices, and an irregular vortex is observed at a point of grain boundary.

Supercurrent distribution in high-TC superconducting YBa2Cu3O7−y thin films by scanning superconducting quantum interference device microscopy

The two-dimensional vector mapping of current distributions in high-TC superconducting YBa2Cu3O7−y thin films obtained by converting magnetic-field data measured by scanning superconducting quantum interference device (SQUID) microscopy is reported. The current distribution contains the contributions from both transport supercurrent and vortex current. The transport supercurrent is found to flow mainly along the edge of a stripline, and the numerical calculation based on the simple London model by taking the specific sample-detector geometry into account is given. The agreement between the experimental data and the calculated results is good, demonstrating that the scanning SQUID microscope provides a useful tool for studying the current distribution in a superconductor.

High field behavior of artificially engineered boundaries in melt-processed YBa2Cu3O7−δ

Artificial bulk “zero-angle” boundaries parallel to the c axis have been engineered between large melt-processed YBa2Cu3O7−δ (YBCO) grains and observed to carry a transport supercurrent at fields up to at least 5 T at 77 K. The temperature and angular dependencies of the boundary resistance have exactly the same form as those of the grains, which is evidence that the grains are intimately coupled. The limiting mechanism for current transfer across these boundaries is, therefore, not a simple weak link or Josephson effect. This joining technique is extremely promising for production of macroscopic engineering artifacts.

Spectral Flow, Magnus Force and Mutual Friction Via the Geometric Optics Limit of Andreev Reflection

The notion of spectral flow has given new insight into the motion of vortices in superfluids and superconductors. For a BCS superconductor the spectrum of low energy vortex core states is largely determined by the geometric optics limit of Andreev reflection. We use this to follow the evolution of the states when a stationary vortex is immersed in a transport supercurrent. If the core spectrum were continuous, spectral flow would convert the momentum flowing into the core via the Magnus effect into unbound quasi-particles — thus allowing the vortex to remain stationary without a pinning potential or other sink for the inflowing momentum. The discrete nature of the states, however, leads to Bloch oscillation which thwart the spectral flow. The momentum can escape only via relaxation processes. Taking these into account permits a physically transparent derivation of the mutual friction coefficients.

Spectral flow, Magnus force, and mutual friction via the geometric optics limit of Andreev reflection.

The notion of spectral flow has given new insight into the motion of vortices in superfluids and superconductors. For a BCS superconductor the spectrum of low energy vortex core states is largely determined by the geometric optics limit of Andreev reflection. We use this to follow the evolution of the states when a stationary vortex is immersed in a transport supercurrent. If the core spectrum were continuous, spectral flow would convert the momentum flowing into the core via the Magnus effect into unbound quasiparticles---thus allowing the vortex to remain stationary without a pinning potential or other sink for the inflowing momentum. The discrete nature of the states, however, leads to Bloch oscillations which thwart the spectral flow. The momentum can escape only via relaxation processes. Taking these into account permits a physically transparent derivation of the mutual friction coefficients. \textcopyright{} 1996 The American Physical Society.

Dc SUSCEPTIBILITY AND CURRENT PATH IN Ag-SHEATHED Bi(Pb)2223 WIRES

dc susceptibility has been used to interpret the percolation behavior of several Ag-sheathed 3-layer Bi(Pb)-compound wires. The wires have been synthesized to obtain relatively inferior transport critical current densities (J c ’s) from 0 to 8×103 A/cm 2 at 77 K and 0 T. Combined with SEM and X-ray data, our observations suggest that the superconductive cores in the wires are highly textured grains with a considerable fraction of voids. The neighboring Bi-grains in the same plane are frequently bridged by grains in the next layer. The transport supercurrent, therefore, flows in a zig-zag pattern across the grain boundaries along the c-direction. The large and relatively clean grain boundaries in the c-planes help to achieve a rather large J c under optimum conditions. However, in such a percolation figure, the intergrowth or layered inclusions become one of the main factors to limit J c .

Characterization of microstructures contributing to degradation of superconductivity in explosively fabricated high Tc superconductors

Techniques of optical metallography, x-ray diffraction, scanning and transmission electron microscopy, and scanning acoustic tomography (microscopy) are used to examine shock-induced microstructures and microcracks in explosively fabricated YBa 2Cu 3O 7 and Bi 7Pb 3Sr 10Ca 10Cu 15O x superconductors. These microstructural features are related to residual degradation of superconductivity as evidenced by alterations in the resistance-temperature (R-T) signatures. Silver additions to YBaCuO superconducting powder is observed to reduce the residual degradation after shock wave loading, and similar manipulations of the R-T signatures are also observed for explosively fabricated and annealed YBaCuO and BiPbSrCaCuO. Transport supercurrent measured as transport critical current density ( J c ) is also observed to depend upon peak shock pressure. The observations suggest that, by manipulating the microstructure prior to shock loading (e.g., by silver additions), the peak shock pressure, and the annealing kinetics following shock loading, it may be possible to develop novel, bulk, superconducting devices by explosive fabrication in composite metal matrices.

Superconducting transport properties of ohmic contacts

The interaction between two-layered thin films SiO2/In (or SiO2/Sn) and n-GaAs substrates after alloying at eleveated temperatures is investigated by using SIMS and electrical measurements. It is shown that under certain conditions an In (or Sn) layer forms low-resistance ohmic contacts to n-GaAs (< 10−5 Ω cm2), preserving the ability to transport supercurrent. The proximity effect in planar Josephson junctions with coupling of ohmic contact material is investigated in order to determine the superconducting transport properties of the ohmic contacts. The coherence length in the ohmic contact material for the system In/n-GaAs (2 × 1018 cm−3) is determined to be (68 ± 5) nm. [Russian Text Ignore]

Critical and supercritical current measurements by a magnetic induction method

Abstract The temperature dependence of the critical current and current-voltage characteristics at supercritical currents have been measured in the low field limit on toroidal samples of the ceramic high Tc superconductor YBa2Cu3O7−δ by a contactless magnetic induction technique that is sensitive to the transport supercurrent but not to any intragrain current loops. The sample constitutes a tertiary winding on a small ferrite transformer core. The secondary voltage provides a very sensitive indication of when the critical current is exceeded, and when it is integrated it yields the supercritical current-voltage characteristic.