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Landau’s work was crucial for the development of the modern theory of phase transitions. He showed that such transitions can be classified by an order parameter, which in the low-temperature phase becomes nonzero. Together with Ginzburg he made this order parameter a spacetime-dependent order field and introduced a local energy functional whose extrema yield field equations and whose fluctuations determine the universal critical behavior of second-oder transitions. In the same spirit, but from a dual point of view, I have developed in the last twenty years a disorder field theory that describes phase transitions via the statistical mechanics of grand-canonical ensembles of vortex lines in superfluids and superconductors, or of defect lines in crystals. The Feynman diagrams of the disorder fields are pictures of the vortex or defect lines. A nonzero ground state expectation value of the disorder field at high temperature signalizes the proliferation of line like excitations in the ordered phase. It was this description of the superconductor that led in 1982 to a first understanding of the order of the superconducting phase transition. Recent experimental progress in the critical regime of high-Tc superconductors will be able to verify the predicted tricritical point of the Ginzburg parameter κ≈0.8/2 where the second-order transition becomes first-order. [ABSTRACT FROM AUTHOR]

We study the Josephson effect and pairing correlations in SFFS junctions that consist of conventional superconductors (S) connected through two metallic monodomain ferromagnets (F) with transparent and spin-inactive interfaces. We solve the Eilenberger equations for arbitrary relative orientation of magnetizations of the two F layers in the clean limit, and calculate spatial variation of pair amplitudes, singlet fs, and odd-frequency triplet ft0 and ft1, with 0 and ±1 spin projections, as well as the Josephson current. We find that for the angle 0<α<π between in-plane magnetizations, both spin singlet and triplet pair amplitudes in F layers decay in the same oscillatory manner with distance from the FS interfaces. The computed triplet amplitude ft1 penetrates into the superconductors and monotonically decays over the superconducting coherence length, which is the same distance as the singlet amplitudes fs saturates to the bulk value. [ABSTRACT FROM AUTHOR]

These lecture notes are an extension of my previous notes [1] presented in this lecture series and are concerned with the recently emerging research field of unconventional superconductivity in non-centrosymmetric metals. Inversion symmetry together with time reversal symmetry represent key symmetries for the formation of Cooper pairs in superconductors and allows to distinguish between even-parity spin-singlet and odd-parity spin-triplet pairing. The absence of at least one of two symmetries leads to the spin-splitting of the electronic states, through Zeeman fields (loss of time reversal symmetry) and through antisymmetric spin-orbit coupling (loss of inversion symmetry), which has a strong influence on the Cooper pairing states possible. Anderson’s theorems show the basic symmetry requirements for the Cooper pair formation. The meaning of these theorems can be demonstrated in a perturbative analysis of the superconducting instability. The structure of the pairing states are derived for systems without inversion and time reversal symmetry, and are shown to be non-unitary. In the case of non-centrosymmetric materials the pairing interaction displays interesting spin-orbit coupling-induced features which are analyzed within a toy model for the superconductivity in CePt3Si, one of the non-centrosymmetric heavy Fermion superconductors, in order to give a catalogue of possible pairing states in this material. A further important point is the essentially universal behavior of the spin susceptibility in the superconducting phase of a non-centrosymmetric materials. This behavior is spectacularly manifested in the upper critical field of CeRhSi3 and CeIrSi3. Magneto-electric effects represent one of the most extraordinary parts in the phenomenology of non-centrosymmetric superconductors. Two examples of magneto-electric behaviors are discussed: (1) the helical phase in the mixed superconducting state and (2) relation between supercurrent and the spin magnetization. Eventually also the possibility of surface Andreev bound states is discussed and it is shown that such states can carry spin currents. [ABSTRACT FROM AUTHOR]

The wave function was proposed for description of quantum phenomena on the atomic level. But now it is well known that quantum phenomena are observed not only on atomic level and the wave function is used for description of macroscopic quantum phenomena, such as superconductivity. The essence of the wave function on level elementary particles was and is the subject of heated argument among founders of quantum mechanics and other physicists. This essence seems more clear in superconductor. But impossibility of probabilistic interpretation of wave function in this case results to obvious contradiction of quantum principles with some fundamental principles of physics. [ABSTRACT FROM AUTHOR]

Progress in manufacturing 40m long YBCO coated stainless steel tapes by ion beam assisted deposition and by high-rate pulsed laser deposition is reported. A critical current of 84.5A (77K, self field) was observed for end-to-end measurements of the 40m long tape based on a 1.05μm thick and 3.6mm wide YBCO film. This implies that a critical current of 235A per cm-width is achieved by just a 1 μm thick YBCO layer. Analysis of the influence of local defects onto the integral U(I) curves allowed to formulate a general criterion for the homogeneity in terms of reduced number of tape parameters. A deterioration of tapes induced by mechanical torsion starts at a very high deformation degree: irreversibility threshold corresponds to 3000 angular degrees per meter for 4mm wide coated tape loaded with a 40N tensile axial force. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Powder-in-tube PIT MgB2 tapes have been prepared through an in-situ process using a steel sheath. Magnesium hydride MgH2 and amorphous boron powders were mixed to synthesize MgB2 superconductor. The mixed powder was encased in a steel tube and rolled into a monocore tape of about 5mm in width and 0.5mm in thickness. Different kinds of ceramic powder, i.e., SiC, TiC, WC and Si3N4 were added to the core. The addition of nanopowder, e.g. SiC and TiC, has been found to enhance appreciably the Jc in MgB2 tapes. The Ic for the 5 wt% SiC or 10 wt% TiC addition tapes reaches 100 A at 4.2 K and 7 T, which corresponds to the core Jc of higher than 10 kA/cm2. The improvement of the core Jc in higher magnetic field may results from the increase of upper critical field Hc2 due to an incorporation of ceramic nanopowder such as SiC and TiC. Furthermore, the core Jc for the hot-pressed MgB2 tapes with SiC addition reached 40 kA/cm2 at 7 T, which is enhanced by a factor of 4 compared to that without pressing during the heat treatment. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Achieving high critical currents in YBa2Cu3O7 (YBCO) films on metallic polycrystalline substrates (coated conductors) is an essential step to make coated conductors a technological reality. Therefore it is extremely important to identify the vortex pinning centers responsible for the high critical current density (Jc), to find ways to further improve the vortex pinning in these superconducting films. We present a comparative study of the angular and field dependences of Jc for two successful approaches used to improve vortex pinning, one that introduces correlated defects and another one that increases the random pinning contribution. The correlated defects are obtained by the introduction of BaZrO3 nano-particles into the superconducting film matrix, which in turn induce c-axis correlated defects. The enhanced random pinning contribution is achieved by mixing different rare earth. The presence of additional correlated defects results in a slower Jc field decay together with a Jc enhancement around the c-axis region. The rare earth substitutions yield Jc enhancements at self-field and at low fields. We show that even though at 75.5 K and intermediate field range both methods produce comparable results, at lower temperatures the correlated defects become more effective pinning the vortices, inducing higher enhancements of Jc. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Studies of symmetries and the consequences of breaking them have led to deeper understanding in many areas of science. The high-temperature superconductors, discovered in 1986, motivated an unprecedented worldwide flurry of research, not only because applications are promising, but because they also represent a new state of matter that breaks certain fundamental symmetries. In this narrative for nonspecialists, we provide a general background on broken symmetries and superconductivity. Then we show how planar tunneling spectroscopy can detect the broken symmetries of gauge (superconductivity), reflection (d-wave superconducting order parameter), and time-reversal (ferromagnetism). © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

We have performed experiments and simulations of the current distribution process in a CICC with the aim to understand better the coupled thermal, hydraulic and electric process that leads to a stable or unstable transient cable behaviour. The cable, wound from 128 Nb3Sn and pure copper strands, has been tested in the SULTAN facility. A resistive heater, glued on the jacket of the conductor, has been used to start the transient, and the response has been monitored with arrays of Hall plates. In this paper we report the results of simulations, especially the computed Hall signals, and compare them to the experimental data. Based on the experimental results and their interpretation we postulate that large temperature gradients must develop in the helium stream in the cable cross sections during the transient heat pulse. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Knowledge on the deformation state of the strands in International Thermonuclear Experimental Reactor (ITER) type Cable-In-Conduit Conductors (CICC) and its impact on both the transport properties and the superconducting transition is essential for optimal cable and magnet design. To date a relatively large part of the attention is concentrated on the influence of purely axial strain on the performance of Nb3Sn strands and sub-size cables. However, local deformations such as strand-to-strand, strand to conduit contact pressure and strand bending will occur as well in multi strand CICC’s experiencing an electromagnetic load. More basic experimental verification is required on strand level from virgin state towards multi-cyclic loading. Therefore a new test arrangement is developed in which the influence of various principle deformation states that occur in a CICC, i.e. local transverse deformations, transverse homogeneous loads, bending and axial load, can be studied separately or combined. Beside precise displacement and force measurements for stress-strain analysis, the voltage-current and voltage-temperature characteristics are traced. A description of TARSIS is presented and the first preliminary results on periodical strand bending are reported. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Recent progress of the externally and internally stabilized RHQT (rapid-heating, quenching and transformation) processed Nb3Al superconductors are described and compared. The internal stabilization method has been improved by demonstrating that the Ag can be included in the strand in various places, increasing the Ag volume ratio, decreasing an overall residual resistivity of conductor, and enhancing the RRR. An attempt was made to fabricate a coil using such an internally stabilized conductor, and its result has been compared to that of Cu clad conductors. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The dc performance of a recently produced internal tin route Nb3Sn strand with enhanced specification is studied extensively and compared with predecessor wires manufactured by the suppliers for the ITER Model Coils in 1996. The wire has been selected for use in a full size, developmental cable-in-conduit conductor sample, which is being tested in the SULTAN Test Facility. The critical current, Ic, and the index of the current/voltage characteristic, n, are measured over a broad range of field and temperature, using ITER standard sample holders, made of TiAlV grooved cylinders. The behavior of Ic versus applied tensile strain is also investigated at 4.2 K and 12 T, on straight specimens. Scaling law parameters are drawn from the fit of the experimental results. The implications of the test results to the design of the fusion conductors are discussed. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Recent developments in Nb3Al strand for HEP applications are presented and discussed. The present state of available precursor is detailed. Two routes have been shown to lead to successful construction of MF NbAl composite wires. An extrusion method has so far produced the highest filament counts and smallest filament diameters, but requires more processing steps. A direct restack and draw approach has also been demonstrated which minimizes the number of processing steps while still leading to a high performance conductor. This latter route also allows for a lower cost per billet, potentially allowing the exploration of more process parameters. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Supercon has investigated an innovative process for the fabrication of Nb3Al conductor. Monofilament Nb-Al jelly-roll composite wire was fabricated. This wire was then converted to ultra-fine alloy powder by an electro-exploded wire technique, and the powder was used to make conductor by a powder-in-tube process. The development effort demonstrated that alloy powder particle sizes as small as 0.30 micrometers (median by volume) having a composition very near that of the input wire could be achieved using the electro-explosion process. Monofilament powder-in-tube wire was successfully fabricated without wire breakage. The results of the investigation will be discussed, with emphasis on powder characteristics. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

We have sol-gel insulated and characterized short samples of the Cu CTFF in Cu tube MgB2 wire with a view to fabricate the first US MgB2 coil from this conductor. The optimum heat treatment condition for this conductor was found to be half an hour at 700 °C under 4%H2-Ar gas flow. We have sol-gel insulated 25 meters of Cu/MgB2 wire with SnO2-ZrO2, layer wound the coil, impregnated with stycast and then tested it between 4.2 K and 30 K together with a sister sample which was heat treated with the coil. The coil had a Jc of 66190 A/cm2 at 4.2 K and 21428 A/cm2 at 20 K in its self field. This is almost identical to that of the sister sample when the self field effect is considered. This is very significant, as it demonstrate that with CTFF process which is readily scalable, one can produce any lengths of MgB2 conductor which would demonstrate the short sample Jc which is unheard of in the HTS conductors. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The experimental study pointed out that voltage noise power spectrum has two types. One is broad-band noise (BBN), which results from bulk pinning. The other is narrow-band noise (NBN), which is observed near critical current. Therefore, it is expected to occur in the region of vortex creep. NBN is sensitive to even direction of external current, which is supposed to result from surface condition of superconductors. To find out origin of NBN, we perform simulations of the vortex dynamics in the 2D system with pinning array. Simulations show that the periodic pinning potential generates NBN and the first peak frequency is proportional to voltage. We find that the initial vortex arrangement has effect on the first peak frequency. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

We have investigated the magnetostriction of type II superconductors under the viscous flux-flow. The critical state approach was employed to obtain the magnetostriction loops versus applied field. The flux profile equations in the time dependent form have been introduced for both exponential model and Anderson-Kim model. Both shape and scale of the curves are dependent on the frequency of applied field. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]