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Numerous classes of dopant have been added to MgB 2 in order to raise the upper critical field, B c 2 , and hence to increase the field range over which the pinned superconductor has the possibility of supporting supercurrent. Thus dopant additions to grain-boundary-pinned MgB 2 , for example, have the effect of raising the high field critical current density, J c ( B ). However, at low fields, when B is relatively small compared to B c 2 , J c ( B ) decreases as B c 2 increases. This leads to a low field convergence, or even the intersection, of the J c ( B ) curves of a family of variously doped MgB 2 strands. Two important conclusions derive from this “crossover effect”: (i) Doping-induced increases of B c 2 should be applied only if improved high field properties are required. For low field applications of MgB 2 such as: low field nuclear magnetic resonance imaging (MRI), synchrotron insertion devices, and current leads, doping should be avoided since not only would the increased B c 2 degrade J c , but the possible chemical byproducts of doping may reduce connectivity; (ii) If an across-the-board increase in J c ( B ) is desired there is no substitute for increased connectivity, in regard to which densification of the MgB 2 layer (the subject of a separate report) is recommended.

The Josephson current passing through a S / I / S gapped graphene-based junction, where superconductivity in the S region is induced by depositing unconventional d -wave superconductor is investigated. The energy levels of massive Dirac fermions are exactly found for Andreev bound states. We illustrate the effect of characteristic of d -wave pairing symmetry on the Andreev bound states and the Josephson current. It is shown that the Josephson current vanishes for special range of superconductivity phase, φ = φ 1 − φ 2 and the position of the maximum current depends on the mass gap of graphene. The critical supercurrent varies in an oscillatory manner as function of the barrier strength, so that the period of oscillations does not change by increasing the effective mass of quasiparticles.

The effect of DC current injection on the AC current carrying characteristics of superconducting thin film rings was analyzed because of the possibility for using similar ferromagnetic/high temperature superconducting (FM/HTS) structures for contactless investigation of the spin-injection process. It was shown, that the injection leads to decreasing of the critical current I C1 , determined by using AC magnetic field. A superposition of AC and DC currents in the HTS ring and a breaking of Cooper pairs may cause this effect. The effect was investigated experimentally at 77 K in HTS Y 1 Ba 2 Cu 3 O 7− x (YBCO) thin film samples with YBCO or La 0.7 Sr 0.3 MnO 3 (LSMO) current injecting electrodes: I C1 of the last sample was found to be more sensitive to the injection process. The results allow concluding, that a change of the I C1 caused by both mechanisms can be evaluated separately and the contactless method can be successfully used for the investigation of the spin-injection effect.

To a high- T c superconducting (HTS) maglev system which needs large levitation force density, the magnetized bulk high- T c superconductor (HTSC) magnet is a good candidate because it can supply additional repulsive or attractive force above a permanent magnet guideway (PMG). Because the induced supercurrent within a magnetized bulk HTSC is the key parameter for the levitation performance, and it is sensitive to the magnetizing process and field, so the magnetized bulk HTSC magnets with different magnetizing processes had various levitation performances, not only the force magnitude, but also its force relaxation characteristics. Furthermore, the distribution and configuration of the induced supercurrent are also important factor to decide the levitation performance, especially the force relaxation characteristics. This article experimentally investigates the influences of different magnetizing processes and trapped fields on the levitation performance of a magnetized bulk HTSC magnet with smaller size than the magnetic inter-pole distance of PMG, and the obtained results are qualitatively analyzed by the Critical State Model. The test results and analyses of this article are useful for the suitable choice and optimal design of magnetized bulk HTSC magnets.

We report an experimental study on the effect of supercurrent injection on vortex states in mesoscopic superconductors. The sample consists of an Al square with side of 1.1 μm, which is directly connected to Al leads for current injection. The vortex penetration/expulsion is detected by the voltage change across a small tunnel junction attached to the square. We find that the vortex penetration/expulsion fields are significantly influenced by the external current of the order of 10 μA. The shift of the vortex penetration/expulsion fields is interpreted in terms of the forces exerted by the external current.

The Josephson current in an asymmetric graphene-based SG1/FB/SG2 junction where SG1,2 are graphene layers which have been induced into two superconducting states having order parameters Δ1 and Δ2 (Δ1 ≠ Δ2) on the left and right sides of a ferromagnetic barrier FB of thickness d, respectively is studied. The presence of the exchange energy Eex and the gate potential VG in the barrier FB are taken into account. For the case of k///0, we find that the Josephson current depends on the exchange energy but is independent of the gate voltage. We find that increasing Δ2 can induce the junction to become a π-junction. This does not occur in the case of similar junctions having conventional superconductors in them. The critical current at zero temperature for Δ2 → ∞ has the form I C ( Δ 2 → ∞ ) = 2 e Δ 1 / ℏ = 2 I C ( Δ 2 = Δ 1 ) . This behavior of the Josephson current in a graphene junction is quite different from that of the supercurrent in conventional asymmetric junctions. For those junctions, it is predicted that I C ∝ Δ 1 Δ 2 , leading to IC → ∞ as Δ2 → ∞. A transition from a 0-junction to a π-junction is observed as χex ( χ ex ∼ 2 E ex d / ℏ v F ) is increased. In a 0-junction, the spin supercurrents arising from the spin dependent Andreev energy levels do not vanish. This leads to IS = I↑ + I↓ = 0 but with I↑ = −I↓ ≠ 0.

We investigate the properties of Andreev multiple reflection and supercurrent in a normal-metal junction with a quantum dot embedded in a mesoscopic superconducting ring. The quantum dot is in the Coulomb blockade regime and contains several energy levels. The supercurrent through the junction is created by an external magnetic flux threaded through the ring. The supercurrent is related to the multiple Andreev reflection at interfaces between the superconductor and the normal metal, which is influenced by the Coulomb blockade on the quantum dot. Using a tight-binding model with Bogoliubov-de Gennes pairing potential, the supercurrent versus the external flux is calculated. By switching on the gate voltage applied on the dot, the supercurrent shows a series of peaks and valleys for various values of the flux through the ring. The results suggest the strong effect of the quantum dot on the supercurrent in such a structure, and the possible methods of controlling the supercurrent.

We report on new techniques for the investigation of the radial and the axial homogeneity of top seeded melt-textured RE–Ba–Cu–O (REBCO) monoliths. Whereas the traditional Hall probe scanning technique of trapped fields at the surface of the superconductor provides only global information on the patterns of supercurrent flow, a modified scanning technique employing sequentially increasing activation fields leads to more detailed information on the flux penetration process and, therefore, on possible radial inhomogeneities of the defect structure for flux pinning. Particularly valuable information on this subject can be deduced from a new technique, the magnetoscan, that allows the flow of the local supercurrent in large bulk monoliths on the scale of less than 1 mm to be investigated. Combination of the magnetoscan with optical or electron beam imaging of the sample surface results in full characterization of the bulk microstructure, developed during the growth process, and its effective flux pinning capability. We show that these techniques can also be used to investigate axial inhomogeneities, i.e. those developing along the c -axis (growth direction), by removing layers of the monolith step by step and by observing the microstructure and the local supercurrent flow in sequential layers. It turns out that the c -growth sector is always characterized by a lower current carrying capability and that characteristic features of the growth morphology appear at the a -growth sector boundaries. Finally, initial results on the current flow in multi-seeded bulks will be presented.

A theory is given to explain the physics behind the flow of low-frequency ac transport current around a closed superconducting circuit, where the circuit consists of two long, straight, parallel, uniform conductors, connected to each other at one end and to an applied emf at the other end. Thus one conductor is the return path for the other. A question of interest is what drives the current at any given point in the circuit. The answer given here is a surface charge, where the purpose of the surface charge is to spread the local emf around the circuit, so that at each point in the conductor it produces, together with the electric field of the vector potential, the electric field necessary for the current to flow. But it is then necessary to explain how the surface charge gets there, which is the central problem of the present analysis. The conclusion is that the total current density consists of the superposition of a large transport current and a very much smaller current system of a different symmetry. The transport current density is defined as a two-dimensional current density with no divergence. It flows uniformly along the conductor length, but can vary over the cross-section. The small additional current density has a much different symmetry, being three-dimensional and diverging at the surface of the conductor. Based on a slightly modified Bean model the transport current is treated as supercurrent having the value ± J c , while the small additional system of current is like normal current, with a density given by the electric field divided by a resistivity. The electric field is computed from the sum of the negative time derivative of the vector potential and the negative gradient of the scalar potential due to the surface charge. It has components parallel and perpendicular to the long axis of the conductor. Thus the small normal current density has a perpendicular component which flows into or out of the surface thereby creating the surface charge. Since the circuit has charge neutrality one can picture the small system of normal current density at a given point along the conductor length, during a given half-cycle, as flowing in a perpendicular direction out of the surface and then turning to flow around the circuit to a similar point in the return path, where it turns and flows into the surface. The perpendicular component varies in strength along the length of the conductor, producing a surface charge density which varies approximately linearly along the length. Such a surface charge produces a uniform electric field inside the conductor which adds to the electric field of the vector potential and determines the flow of transport current. It is shown that one can arrive at the above results by a careful solution of the Maxwell equations, where the solution also satisfies the demands of the Bean model.

A number of YBa 2 Cu 3 O 7 thin superconducting films have been grown on different single crystal substrates under different conditions using pulsed-laser deposition and magnetron sputtering techniques. Employing magnetization measurements, critical current density ( J c ) dependences on the applied magnetic field ( B a ) have been obtained for the films. The analysis of the J c ( B a ) behaviour suggests that the range of the B a -independent J c plateau depends on the interplay between the vortex pinning of certain, particularly effective defects and thermally activated depinning of individual vortices. The latter process is found to be the main factor responsible for the temperature dependence of the size of the J c -plateau. This temperature behaviour is independent of the properties of the films and their orientation with respect to the field. The absolute J c0 value at B a →0 T is, in contrast to the plateau, more sensitive to pinning media transparency to the supercurrent flow.

We investigated the paramagnetic supercurrent and the transition points between different vortex states in mesoscopic superconducting disks. We observed paramagnetic supercurrent in four disks: two 33 nm thick and two 50 nm thick, all 1.50 μm in diameter. Analyses of the supercurrent symmetry and the intervals between transition points showed that defects did not play a crucial role in our samples or in the samples first reported to show the paramagnetic Meissner effect (PME) [Nature 396 (1998) 144]. This supports our previous conclusion that the origin of the PME is the paramagnetic supercurrent.

We have fabricated Nb square superconducting networks and measured critical current density with current flowing along the direction of 0° and 45° tilted from the lattice of the networks. The temperature and magnetic field dependences of the critical current density for both directions are measured and compared with the flux penetration pattern in those samples. It is found that the anisotropy of the critical current density reflects the flux penetration pattern in the superconducting network samples. Near T c at zero field, anomalous enhancements of voltage for 0° are observed.

Operation of the superconducting fault current limiter of the inductive type is analyzed together with the magnetic diffusion process that occurs in the high-Tc superconducting tube. Due to the highly non-linear voltage–current relation of the superconductor, a critical state model based on the power law is used to compute resistivity changes during transition from the superconducting state to the resistive state. The influence of the penetration depth, for various operating modes, on the magnetic flux density distribution across the SC tube wall is investigated using the slab geometry and a linear approximation of the magnetic diffusion process.

The present work deals with the study of the supercurrent density as a function of temperature and various microscopic interactions that exist in bilayer high temperature cuprate superconducting materials. For this purpose a tight binding bilayer Hubbard Hamiltonian has been considered that includes the in-plane (within CuO2 plane) and out-ofplane interactions. The in-plane component of the model Hamiltonian is just in the BCS form with in-plane hopping energy and effective attractive interaction that comprises on-site and intersite interactions and is responsible to form pairing of the charge carriers within the plane. The out-of-plane part of the Hamiltonian includes single particle tunneling along with intrinsic Josephson like coupling between the planes within the unit cell which is responsible for transfer of single particle as well as Cooper pair from one layer to another and vice-versa, in the superconducting state. The situation for bilayered cuprates considered here is equivalent to a Josephson’s coupled SIS junction. To derive the expressions of the superconducting order parameter, carrier density and supercurrent density, we rely on the Green’s function equations of motion approach within the simple BCS formalism. The numerical analysis shows that in bilayer cuprates the supercurrent density depend on various in-plane and out-of-plane contributions as well as on temperature in an essential way. Finally, we have compared our theoretical results on supercurrent density with that of recent experimental results on temperature and pressure dependence of supercurrent density in bilayer superconducting cuprate materials and found to be in qualitative agreement. 2002 Elsevier Science B.V. All rights reserved.

We report the imaging of supercurrent distribution in high- T c superconductor YBa 2 Cu 3 O 7− δ (YBCO) thin film based on the principle of the measurement of THz radiation emission from the current biased YBCO thin film when excited by the femtosecond fiber laser pulses. It is observed that the image obtained by Ti:sapphire laser has better resolution as it shows the clear and sharp supercurrent distribution at the edges of YBCO thin film compared to the image obtained using femtosecond fiber laser source. We attribute this difference to the larger beam spot size in the case of fiber laser source compared to that obtained in the case of Ti:sapphire laser source.

Using an RF-SQUID configuration we have measured the temperature dependence of the current–phase relation (CPR) of Nb/InAs(2DEG)/Nb (two-dimensional electron gas, 2DEG) Josephson junctions with high interface transparency. At low temperatures, significant deviations from a sinusoidal CPR are observed. The experimental results are fitted by the scattering matrix theory of Brouwer and Beenakker, additionally taking into account the proximity effect between the Nb and the 2DEG in the inversion layer on p-type bulk InAs. We have studied also the influence of an external gate voltage applied to the InAs(2DEG) channel. It is shown that the amplitude of RF-SQUID oscillations can be controlled by gate voltage with only modest deterioration of the SQUID signal.

Current distributions in a thin, flat superconductor ring located between two coaxial cylindrical soft magnets of high permeability are studied. Such a heterostructure protects the flux-free state of the ring even in the presence of strong total supercurrents, preventing entry of magnetic flux from the edges of the ring. A Fredholm integral equation of the first kind governing the current distribution in the Meissner state of the ring is derived and solved numerically for different relative permeabilities of the magnets and various distances between the magnets and the ring. This reveals that the current distribution tends to become homogeneous when the relative permeabilities are increased, the effect already saturating at values of these quantites of a few hundred. When the distances between the magnets and the ring are reduced, the current peaks near the circumferences of the ring decrease, thereby contributing to the large total supercurrent and, concomitantly, to the strong longitudinal component of the magnetic field.

We used dc magnetization and transport measurement to estimate the superconducting critical current densities ( J c ) of polycrystalline MgB 2 sintered under high temperature and high pressure. We measured the current–voltage ( I–V ) characteristics and found the existence of a vortex-glass phase in the field–temperature ( H–T ) plane. This is notable in that the vortex-glass phase can be observed even in a polycrystalline specimen, which suggests that the supercurrent is not sensitive to the grain boundaries. Moreover, the transport (intergrain) J c seems to be comparable to a magnetic (intragrain) J c .

We measure the magnetic flux dependence of the supercurrent in diffusive Nb/n-InAs/Nb mesoscopic SNS Josephson junctions with junction length ranging from 220 to 720 nm. In the longer junctions, the modulation periodicity of the maximum supercurrent by magnetic flux is a quantum flux, Φ 0 = h /2 e as usual. However, we find that the magnetic flux periodicity becomes twice or 2 Φ 0 = h / e when the junction lengths are shorter than 400 nm.

Supercurrent distributions in vortex-penetrated YBa 2 Cu 3 O 7− δ thin-film strip lines under various conditions are observed using THz radiation imaging system by scanning femtosecond laser. The experimentally observed supercurrent distribution is compared with distributions calculated by the critical state model for thin films. The results indicate that the critical state model is suitable to describe the vortex penetration in superconductive thin films as long as the amount of the penetrated vortices is relatively small. In the near-complete Meissner state, it is found that the supercurrent distributes as if the critical current was much larger than the general values.

The optical control of magnetic flux in superconducting thin-film loops with a single shot femtosecond optical pulse has been demonstrated. The mechanism is attributed to ultrafast optical supercurrent modulation without phase transition. The optically generated flux is semi-qualitatively evaluated by estimating the amount of the persistent supercurrent circulating along the loop.

We have fabricated ramp-edge type junctions with different high- T c superconductor electrodes; YBa 2 Cu 3 O 7− y (YBCO) and Bi 2 Sr 2 CaCu 2 O 8+ y (BSCCO). Thin films were prepared on SrTiO 3 (1 0 0) substrates by a pulsed laser deposition method. YBCO was selected as a base electrode and BSCCO as a counter electrode, by considering the growth conditions for these thin films. The samples were etched by Ar ion-beam milling to pattern the shape of the ramp-edge type junction. We have prepared several junctions with and without a PrBa 2 Cu 3 O 7− y barrier. In case of the junction without a barrier material, T c 's of YBCO and BSCCO electrodes were about 90 and 60–70 K, respectively, and it was found that the maximum supercurrent between the ab planes of YBCO (RE123 superconductor) and BSCCO (Bi-based superconductor) was about 1 mA at 4.2 K. Shapiro steps were also observable.

We fabricated Cu-sheathed MgB 2 tapes by a powder-in-tube method without heat treatment using commercially available two different MgB 2 powders. One powder composed of fine MgB 2 grains which were fully covered with thick MgO layers caused transport critical current density ( J c ) values of zero at 4.2 K and 0 T, while another powder composed of relatively large MgB 2 grains which were partially covered with thin MgO layers did J c values of about 10 kA/cm 2 . These results indicate that the quality of powder is very important to obtain good grain connectivity for supercurrent flow. Neither microstructural change of the large MgB 2 grains nor J c decrease of the corresponding tapes was appreciable after an exposure in air for one month, suggesting that the large MgB 2 grains are unlikely to receive damages by moisture and oxygen rapidly. Heat treatment of these Cu-sheathed tapes resulted in a deterioration of J c due to the reaction of Cu and Mg.

We report a pump and probe terahertz (THz) excitation technique to investigate the ultrafast carrier dynamics in YBa2Cu3O7−x (YBCO). THz pulses generated by laser-induced supercurrent modulation are used to probe the time resolved evaluation of the THz emission. The technique is revealed to allow us to study the carrier dynamics, such as fast THz field screening effect inside the YBCO film and changes in the electrodynamics due to Cooper pair breaking and hot carrier excitation, and so on.

The microwave response of intrinsic Josephson junctions (IJJs) in mesa structures formed on HgI 2 -intercalated Bi 2 Sr 2 CaCu 2 O 8+ δ single crystals was studied in a range of microwave frequencies above the junction plasma frequency. Under 73–76 GHz microwave irradiation, the supercurrent branch becomes resistive above a certain onset microwave power. At a low current bias, the current–voltage characteristics show linear behavior, while at a high current bias, the resistive branch splits into multiple sub-branches. The voltage spacing between neighboring sub-branches increases with the microwave power and the total number of sub-branches is almost identical to the number of IJJs in the mesa. All the experimental results suggest that each sub-branch represents a specific mode of collective motion of Josephson vortices generated by the microwave irradiation in a stack of IJJs.

We study vortex penetration into YBCO thin films by terahertz radiation imaging. The supercurrent distributions and corresponding magnetic field ones in the thin film strips with and without the ordered array of small antidots are quantitatively evaluated in a remanent state and their temperature dependence is discussed. The results indicate that the distributions and their temperature dependence in the strips with the antidots remarkably differ from those without the antidots.

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 2 Cu 3 O 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.

We have observed the supercurrent distribution in YBa 2 Cu 3 O 7− δ (YBCO) strip lines under bias current and magnetic fields using terahertz (THz) radiation imaging. By mapping the amplitude of the THz pulses, which are emitted from the YBCO strip lines excited by ultrashort optical pulses, the supercurrent distribution is visualized. It is found that the spatial resolution of the obtained image depends on the excitation laser spot size. Irreversibility of the current distribution between increasing and decreasing the bias current is observed, which is semiquantitatively explained by the theoretical calculation based on the Bean critical state model for thin films. This technique can be a unique and powerful tool for the noncontact evaluation of the supercurrent distribution in high- T c superconducting thin films.

We investigated superconductor/normal metal/superconductor (SNS) junctions made of short Au or Cu wires between Nb or Al banks. The Nb based junctions display inherent electron heating effects induced by the high thermal resistance of the NS boundaries. The Al based junctions show in addition subharmonic gap structures in the differential conductance d I /d V and a pronounced peak in the excess noise at very low voltages V . We suggest that the noise peak is caused by fluctuations of the supercurrent at the onset of Josephson coupling between the superconducting banks. At intermediate temperatures where the supercurrent is suppressed a noise contribution ∝1/ V remains, which reflects the strong nonequilibrium of quasiparticles even at very small voltages.

Two separate research topics are discussed. We discuss noise measurements of single electron transistors and the gain dependence of the noise. We will also describe the fabrication and preliminary measurements on several different superconductor/two-dimensional electron gas/superconductor structures.

We describe a novel quantum detection mechanism in the superconducting film carrying supercurrent. The mechanism incorporates growing normal domain and breaking of superconductivity by the bias current. A single photon absorbed in the film creates transient normal spot that causes redistribution of the current and, consequently, increase of the current density in superconducting areas. When the current density exceeds the critical value, the film switches into resistive state and generates the voltage pulse. Analysis shows that a submicron-wide film of conventional low temperature superconductor operated in liquid helium may detect single far-infrared photon. The amplitude and duration of the voltage pulse are in the millivolt and picosecond range, respectively. The quantitative model is presented that allows simulation of the detector utilizing this detection mechanism.

The critical current in a superconductor/two-dimensional electron gas junction/superconductor junction is controlled by an injection current. In case of a carrier injection from one contact to the semiconductor through the two-dimensional electron gas region of the junction to an ohmic contact at the opposite side a complete suppression of the supercurrent is obtained. The experimental results are compared to a theoretical model based in the solution of the Gor'kov equations, which takes the finite width of the sample into account.

We report measurements on three-terminal superconductor–semiconductor–superconductor injection devices demonstrating enhancement of the supercurrent by injection from a superconducting injector electrode. Two other electrodes were used to form the detector junction. Applying a small voltage to the injector electrode reduced the maximum supercurrent (the critical current) between the detector electrodes. When the injector voltage was increased further (upto the gap voltage Δ/e), the critical current between the detector electrodes increased and approached a new maximum. This is evidence of enhancement of the supercurrent by non-equilibrium injection into bound supercurrent-carrying Andreev states. The effect persists to temperatures where the equilibrium supercurrent has vanished.

Metal oxide heterostructures consisting of the ferromagnetic La1−xSrxMnO3 (x≈0.3) underlayer, thin (d≅5–10 nm) SrTiO3 barrier layer and YBa2Cu3O7 cap layer were grown on single crystal LaAlO3 (1 0 0) substrates at 825°C under oxygen pressure of about 200 Pa by single source pulsed injection MOCVD method. The technological conditions were optimized in order to improve crystallinity and surface quality of the constituent layers. Wet etching was applied to form tape-like superconducting films for investigation of both longitudinal and transverse electrical transport in the heterostructures. Significant suppression of supercurrent has been found by injecting tunneling spin-polarized current from the underlying FM layer.

The critical current densities in bulk melt-textured YBa2Cu3Ox and across superconducting “weld” joints are measured using scanning Hall probe measurements of the trapped magnetic field in ring samples. With this method, critical current densities are obtained without the use of electrical contacts. Large persistent currents are induced in ring samples at 77 K, after cooling in a 3 kG field. These currents can be determined from the magnetic field they produce. At 77 K a supercurrent exceeding 2000 A (about 104 A/cm2) was induced in a 2 cm diameter ring; this current produces a magnetic field exceeding 1.5 kG in the bore of the ring. We demonstrate that when a ring is cut, and the cut is repaired by a superconducting weld, the weld joint can transmit the same high supercurrent as the bulk.

Double-barrier highly asymmetric Nb–Al oxide–Al–Nb oxide–Nb structures with reproducible characteristics were fabricated. The heterocontacts with the middle Al layer thickness ranging from 4 to 6 nm exhibited a well-defined d.c. Josephson supercurrent I c at 4.2 K and characteristic voltages V c = I c R N ( R N is the normal resistance, V c defines the response time of the junction) from 0.3 to 0.4 mV. Two prominent features in the quasiparticle current–voltage curves have been observed: a so-called `knee' in the energy-gap region and an additional (to the linear voltage dependence) current at higher biases. They are discussed within a simple Landauer–Buttiker scattering approach to the phase-coherent quasiparticle transport in a quasiballistic S–I 1 –N–I 2 –S heterostructure with an extremely great difference between the barrier transparencies.

The advantages of a radio-frequency (r.f.) method in comparison with conventional transport measurements for characterization of superconducting weak links are discussed. In particular it is pointed out, that (i) the current–phase relationship (CPR) can be deduced from r.f. measurements of the phase-biased weak link without any fitting parameters and (ii) the supercurrent is measurable even if the Josephson coupling energy is considerably smaller than the thermal energy. Examples of the CPR for several configurations of Josephson junctions are shown.

We report on I – V characteristics for in situ formed Nb/Au/(1 1 0)YBa 2 Cu 3 O 7− δ (YBCO) Josephson junction, where the homoepitaxial (1 1 0)YBCO film shows ultra-smooth surface morphology. The field dependence of critical supercurrent I c shows anisotropic large junction behavior with normal Fraunhofer patterns expected from BCS model of d x 2 − y 2 wave superconductors. This strongly suggests that the Nb/Au/(1 1 0)YBCO junctions cannot be regarded as atomic scaled corner junctions, in contrast with (0 0 1)/(1 1 0)YBCO grain boundary junctions to show “π-junction” with a pronounced dip near zero fields in field modulation of I c .

We discuss the impact of the proximity effect (PE) on the transport properties of mesoscopic superconducting-normal structures using the quasiclassical Green's function technique. We discuss the properties of dissipationless supercurrents in proximity systems as well as the enhanced diffusion conductivity in nonequilibrium, which shows a nonmontonic, “reentrant” temperature-dependence. Both contributions can be can be controlled by an external phase. In nonequilibrium situations, SNS junctions exhibit novel phenomena like supercurrent reversal or a nonequilibrium-enhanced critical current. The connection to recent experiments is discussed.

The problem of grain boundaries in high temperature superconductors is very important in determining the ability of these materials to carry supercurrent. The success of fabricating polycrystalline bulk YBCO with high-angle grain boundaries carrying high critical currents initiated microstructural studies of these boundaries in order to determine mechanism of current flow across them. TEM studies have been performed on grain boundaries with known Jc, followed by simulation of their atomic and electron structures as well as estimations of their superconducting order parameter. Using a model developed to correlate grain boundary microstructure and superconducting properties, we have shown that coupling behavior of grain boundaries does not depend on misorientation characteristics alone. The grain boundary plane as well as the atomic ordering in the vicinity of grain boundary play an important role in the current carrying capabilities of these grain boundaries.

Supercurrent in the weakly conducting c -axis direction of layered superconductors is studied. It is shown that in the presence of screw dislocations with Burgers vectors parallel to the c -axis, the critical current of small samples is enhanced with respect to the perfect crystal, each dislocation being able to carry a supercurrent I 0 .

We have fabricated well-defined c-axis Bi2Sr2CaCu2O8 (BSCCO)/Au/Nb junctions on cleaved surfaces of BSCCO single crystal and observed typical current–voltage characteristics of Josephson tunnel junctions. A magnetic field dependence of the supercurrent shows that a zero bias current is not a leakage from some pin-holes, but a Josephson current. These results strongly indicate a presence of an s-wave component in BSCCO. The results of the temperature dependence of the Josephson current indicate that the superconducting gap for the s-wave component in BSCCO is about 10−3 of the d-wave component, and that the Tc of the s-wave is about 10 K.

A technique has been developed to “weld” parts made from textured high T c superconductors of the type RBa 2 Cu 3 O x (R=Y or rare earth element). The weld joint supports a supercurrent comparable to that carried by the joined parts. Welding is accomplished by using the textured parts to seed a spacer material which has a lower decomposition temperature than the material comprising the parts. In this paper, we demonstrate the technique by welding YBa 2 Cu 3 O x parts with a TmBa 2 Cu 3 O x spacer layer. The weld joints were evaluated with SEM, optical and magneto-optical imaging techniques.

Papers are reviewed on the resonant tunneling mechanism of the c -axis transport in high-temperature superconducting layered cuprates, proposed by the present author. After an introduction, where the problem is stated and the qualitative principles of the mechanism are described, real physical properties are calculated and compared with experimental data. These include the temperature dependence of the static c -axis conductivity in the normal state, frequency dependence of the optical conductivity and stationary supercurrent along the c -axis. It is demonstrated that for the latter the coherence of resonant tunneling through different centers is of primary importance. In the last section an explanation is proposed of the drastic decrease of T c and increase of the BCS ratio 2 Δ (0)/ T c with underdoping, based on the disruption of interlayer connection and appearance of vortex fluctuations. Two appendices are added illustrating some important features of resonant tunneling on simple models.

Supercurrents were recently identified as a source of reduced magnet stability which can explain the measured ramp rate limitation in large superconducting magnets. They also explain an unexpected periodic field modulation along the axes of superconducting accelerator dipoles. Supercurrents are extra coupling currents between the strands of a cable which are induced by a variable field sweep rate Ḃ ( x ) along the length of the cable. They flow over the whole cable length and have time constants many orders of magnitudes larger than `normal' interstrand coupling currents. Supercurrents may lead to a highly inhomogeneous current distribution and to additional coupling losses (`supercurrent losses'), even in magnet sections with Ḃ =0. Both effects can drastically reduce the magnet stability during fast ramping up. The complete solution of the space and time dependence is given for a two-strand model cable. The theory of supercurrents can explain typical results of ramp rate limitation in large magnets.

The anomalous phonon mode softening observed in the superconducting phase in inelastic neutron scattering experiments is shown to occur due to the interlayer pair tunnelling mechanism of Wheatley et al. [Phys. Rev. B 37 (1988) 5897]. We show the emergence of a novel supercurrent phonon coupling in Josephson coupled superconducting layers. Difference in phonon frequency shifts for the A g and A u Raman active modes are shown to have a natural explanation within this mechanism. Frequency softening mechanisms for Raman and infrared phonons, in both normal and superconducting state are discussed.

Tunnel junctions have been obtained on Bi 2 Sr 2 CaCu 2 O 8 (Bi 2212) and Bi 2 Sr 2 CuO 6 (Bi 2201) single crystals with T c values of 86 and 5.5 K respectively. A point contact technique is used with an Au tip, but in certain cases, superconductor–insulator–superconductor (SIS) junctions are obtained between separate pieces of the crystal. These SIS junctions are generally quite stable and in the case of Bi 2212 exhibit well-defined, simultaneous Josephson and quasiparticle tunnel currents, with I c R n =2–5 mV. The Josephson supercurrent is observed when the junction conductance exceeds a minimum value, G min , obtained from Ambegaokar–Baratoff theory and the equating of the Josephson coupling energy to k B T . For the Bi 2201 crystals the quasiparticle tunneling conductance resembles that of Bi 2212. In particular, a reproducible dip feature is found at e V ≈3 Δ in both materials. Relatively high conductance Bi 2201 tunnel junctions ( G ≫ G min ) exhibit a large Josephson current with an I c R n product that is close to the gap voltage 2 Δ / e .

By measuring the quality factor of the mechanical oscillator with an attached magnet we investigated the amplitude dependence of the energy loss in a permanent magnet-YBCO superconductor system. When a non-conducting ferrite magnet is used the energy loss is proportional to the amplitude to the 2.79 power. This value is close to the value of 3 expected for the hysteretic motion of flux lines from the Bean model. When a conducting magnet is used the exponent decreased to the value of ∼2.5. This reflects the contribution of eddy current dissipation in a conducting magnet to the overall loss. We identified a novel mechanism of ac loss in the magnet-superconductor system. According to this mechanism the eddy current in a magnet is induced by the ac field generated by the ac supercurrent in response to the motion of a magnet. This mechanism would take place in a rotary bearing if circumferential inhomogeneity of the rotor magnetization occurred.

The paper deals with anomalous phenomena recently observed in operation of models of inductive superconducting current limiters. The anomalies manifest themselves as sharp peaks of voltage or current on the background of a basic AC signal. We demonstrate that these anomalies are observed in AC circuits with models of various current limiters (inductive, hybrid) based on differently processed YBCO elements: sintered ring, melt-textured rings and thin film. It is shown that the nature of these anomalies is self-oscillations appearing in an electric circuit with non-linear superconducting element. The conditions for appearance of these self-oscillations are discussed.

X-ray diffraction rocking curves are used to measure the c axis alignment of TlBa 2 Ca 2 Cu 3 O x films grown on polycrystalline substrates with thickness varying from 3 to 10.5 μm. Films thicker than 3 μm are found to contain two layers: a well aligned (3.5° FWHM) bottom layer, and a poorly aligned (greater than 12° FWHM) top layer. Azimuthal scans show that the component with good long-range out-of-plane alignment retains its characteristic colony microstructure of local in-plane alignment as film thickness increases. The length dependence of the critical current density may be accounted for by assuming that all the supercurrent is carried by the well-aligned component.