Occurrence of superconductivity - relevance of magnetic interactions

For superconducting transition metals and their alloys it is shown that there is a strong possibility of having antiferromagnetic interaction in the conduction band. A-15 superconductors noted for their high T_c values are ideally placed to have such an interaction. Electron spin resonance studies carried out by the authors on high T_c films of Nb₃Ge strongly coroborate it.     

Review of Thermal Boundary Resistance of High-Temperature Superconductors

Solid-solid thermal boundary resistance plays an important role in the thermal stability of many electronic circuits, microdevices, and superconducting devices. The thermal boundary resistance (R _b ) at any interface causes a temperature discontinuity, which can result in heat accumulation on one side of the boundary and raise the temperature much above the stable region, causing device failure. With the advent of high-critical-temperature (high-T _c ) superconductors, it is possible to make superconducting devices at practically achievable temperatures. As the current trend goes toward the development of more and more high-T_c superconducting devices, the need for a better understanding of the thermal boundary resistance of high-T_c superconductors becomes mandatory. This paper compiles all the theoretical and experimental work to date onR _b in high-T_c superconductors, both in thin-film and bulk forms, and provides a critical review of the cited works. This paper also describes the possible effect of the superconducting state onR _b for high-T _c superconductors, based on the experiments for both high-T_c and low-T_c bulk superconductors, and a possible explanation for these data based on the existing theory for low-T _c superconductors.     

Structure and properties of Nb<Subscript>3</Subscript>Sn coatings produced by unsteady-current electrocodeposition of Nb and Sn

We have studied the effect of electric-current mode on the structure and characteristics of niobium stannide coatings produced by electrochemical coreduction of niobium and tin ions at the cathode in molten salts. The results demonstrate that single-phase Nb₃Sn coatings with a superconducting transition temperature T _c = 17.3–17.9 K can be obtained using unsteady-current deposition. The coatings produced in galvanostatic mode and by ac deposition at a frequency of 50 Hz have a columnar grain structure. Current-reversal deposition with pulse ratios above 7–9 results in a layered microstructure with layers parallel to the substrate surface, instead of the columnar microstructure, and ensures a considerably higher critical current.     

Thermal conductivity of Ta-Nb alloys in superconducting,mixed, and normal states

The thermal conductivity of the type II superconductors Ta₉₅Nb₅, Ta₈₀Nb₂₀, Ta₆₀Nb₄₀, and Ta₂₀Nb₈₀ has been measured as a function of magnetic field up to 14 kOe and of temperature between 0.5 and 4.5 K. The temperature dependence of the thermal conductivity in the superconducting state is well accounted for on the basis of the Bardeen, Rickayzen, and Tewordt theory above 0.4T _c . The lattice thermal conductivity limited by dislocation scattering is observed below 0.3T _c . The thermal conductivity near the upper critical fieldH _c2 shows a linear dependence on magnetic field as predicted theoretically by Caroli and Cyrot. After the correction of the phonon contribution, the experimental results for the dirtiest sample, Ta₆₀Nb₄₀, are found in good quantitative agreement with the theory. Deviations from the theory for less dirty alloys depend on the electron mean free path.     

Effect of carbon on the structure and superconducting properties of Nb<Subscript>3</Subscript>Sn coatings produced by electrocodeposition

The effect of carbon doping on the structure and superconducting properties of Nb₃Sn coatings produced by electrocodeposition of Nb and Sn from molten salts was investigated. The results demonstrate that the addition of fine-particle graphite powder to the melt has an insignificant effect on the microstructure, critical current, and superconducting transition temperature T_c of the coatings but reduces the superconducting transition width (T_c0.05). Doping of Nb₃Sn with carbon from the gas phase (Ar + CCl₄) ensures the formation of a fine-grained microstructure and increases the critical current of the material by a factor of 3–3.5 compared to coatings grown in pure argon atmosphere.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1467–1475.Original Russian Text Copyright © 2004 by Kolosov.Presented in part at the II International Conference on Key Issues in Science, Materials Research, and Technology of Carbon, Moscow, 2003.     

CESR in superconductors: Relevance of highT c copper oxide systems

CESR is a highly effective tool to study the interaction among conduction electrons in normal metals. In superconductors belowT _c it can reveal vital information concerning the pairing interactions. A comparative study of highT _c and conventional superconductors is presented and it is shown that the disappearance of CESR in the superconducting state is a feature common to both conventional and highT _c superconductors and establishes the importance of exchange interactions in pairing. This is supported by experimental observations.     

The superconducting properties and structure of Nb3Ga

The influence of composition and heat-treatment on the superconducting transition temperature, T _c, of Nb₃Ga has been studied. A complementary structural investigation of the alloys was carried out using X-ray diffraction to determine the degree of long-range order and the proportions of the phases present. It was found that the greater the deviation from stoichiometry, and the more disordered the Nb₃Ga, the lower were the T _c values. The superconducting transition temperature was maximized by producing a non equilibrium structure of essentially fully ordered, near-stoichiometric Nb₃Ga.     

Is Nb3Si a 25 K superconductor?

Nb₃Si is predicted to have the highest superconducting critical temperature of all known A15 compounds, but it can only be prepared by non-equilibrium methods and resulting T_c values are disappointing. Experimental results on Nb₃Si prepared by a variety of methods are reviewed. By analogy with Nb₃Ge it is argued that T_c for stoichiometric fully ordered A15 Nb₃Si should be 25 K.     

Growth of Nb3Sn in multifilamentary bronze composites

Growth of Nb₃Sn layers in multifilamentary composites has been investigated and their superconducting critical temperatures are measured using both resistive and inductive techniques. The growth parameters are discussed in the light of the analytical models of Reddi et al. Results show that for the composites studied, the rate controlling step for Nb₃Sn growth is diffusion of tin through grain boundaries of Nb₃Sn with the time exponent n determined by both the initial grain size and grain growth. T _c measurements show that for composites with a higher filament number, the width of superconducting transition is broader with no significant change in the onset T _c.     

Nb3Al superconducting tape prepared by CO2 laser beam irradiation

A15 Nb₃AI tape superconductors have been prepared by high power and continuous wave CO₂ laser irradiation onto moving NbAl composite tapes made by the powder method. The irradiated area was melted and then immediately resolidified, which resulted in a dendritic microstructure consisting of Nb dendrites embedded in a Nb₃AI matrix. Irradiated only specimens showed critical temperatures, T_c, >16.0 K and subsequent heat treatment at 750°C enhanced T_c to 18.6 K. For laser irradiated and then heat treated specimens, critical currents, I_c, were little decreased, even at >20 T, compared with specimens which were only heat treated. A maximum critical current density, J_c, for the reacted area of 4.8 × 10⁴ A cm⁻² was obtained at 23 T and 4.2 K.     

Thin-film processing of high-Tc superconductors

This review of high-T _c superconducting thin-film processing focuses on the developments in thin-film deposition technologies since 1987. The common deposition processes are described with reference to their effects on superconductor film performance. A comparative evaluation of the potential of the technologies is also given. The development of multilayers and heterostructures is an important requirement for future device applications and is also described. The latest results of the deposition of novel superconducting materials and deposition on uncommon substrates are discussed. The outlook on some imminent topics of future development in process technologies for high-T _c superconducting thin films is discussed.     

The synthesis of Nb3Ge by RF sputtering

The conditions necessary for the formation of Nb ₃ Ge by low-pressure rf sputtering with a superconducting transition temperatureT _c >21 K have been investigated. Samples have been deposited onto cooled substrates so that the film is first amorphous and then is crystallized by a subsequent annealing, and onto hot substrates, in which case the film is crystalline upon deposition. The highestT _c samples were obtained for a substrate temperature of 735±25° C. The optimum substrate temperature is the same as the optimum annealing temperature for crystallizing films which were first deposited onto cooled substrates. Special conditions are necessary for the formation of single-phase A15 samples of Nb ₃ Ge with an optimumT _c . We have utilized the collisions that the sputtered atoms undergo with the sputtering gas molecules to thermalize the sputtered atoms. We report here on sputtering in both krypton and in argon.     

Role of bounding layers in high-temperature superconductors: Calculation ofT c

A simple model of inter- and intra-layer couplings based on a BCS theory of layered superconductors is used to calculate the transition temperatureT _c of a number of layered systems. The role of the bounding layer in high-T _c oxide superconductors is investigated. Both the mechanisms of direct hopping and pair tunneling between active layers are found to play important roles in governing the behavior ofT _c . We give predictions for the dependence of the superconducting transition temperature on layer parameters.     

The synthesis of the Nb3(Al-Ge) system by sputtering

The variation of the superconducting transition temperatureT _chas been studied in the Nb₃Al _1–x Ge _x system for0x1 for samples produced by rf sputtering onto heated substrates. The effect that different sputtering gas pressures have upon the rate of energy loss of the sputtered atoms due to collisions with neutral sputtering gas atoms is considered. Also considered is how thermalization can be achieved in the fewest number of collisions by matching the mass of the sputtering gas atoms to that of the sputtered atoms. For the case of Nb ₃ (Al-Ge) we show that it is advantageous to use a mixture of sputtering gases so that the light Al atoms can be thermalized as well as the heavier Nb and Ge atoms. It is also thus shown that the same sputtering conditions that are optimal for forming high-T _cNb₃Ge onto heated substrates are not optimal for forming high-T _cNb ₃ Al.Supported by CUNY FRAP and National Science Foundation DMR 74-18138.     

Irradiation damage of A15-Nb3Si and Nb7Si

The behavior of the superconducting temperatureT _c and the lattice parametera ₀ of polycrystalline Nb₃Si and Nb₇Si thin films as a function of 300-kV -particle fluence was investigated. It was found that theT _c depression of A15 Nb₃Si is of the same size and nature as for other A15 materials. In contrast, the initialT _c degradation of Nb₇Si is a factor of three larger. The observed minimum in theT _c vs fluence dependence of Nb₃Si is discussed and a simple qualitative model is presented.On leave of absence from the Institute of Physics, Czechoslovak Academy of Sciences, Prague, Czechoslovakia.     

Effects of H2/Cl2 ratio on CVD synthesis of superconducting Nb3Ge tapes

This Paper investigates the effects of H₂/Cl₂ ratio, R_g, [H₂/Cl₂(Nb,Ge)] in chemical vapour deposition processes on the synthesis and superconducting properties of Nb₃Ge tapes. Critical temperature, T_c, critical current density, J_c, and grain size of A15 Nb₃Ge vary considerably with the gas ratio, R_g. The lattice parameter of A15 Nb₃Ge and its volume ratio to Ge-rich Nb₅Ge₃ phases are also dependent on R_g. The volume ratio of A15 Nb₃Ge to Nb₅Ge₃ increases with increasing R_g, while the grain size of Nb₃Ge considerably decreases. The highest T_c, 19 K, (mid-point) was obtained for Nb₃Ge tape where the A15 Nb₃Ge compound coexisted with a small amount of tetragonal or hexagonal Nb₅Ge₃ compounds. The largest J_c was ≈ 4.5 × 10⁸ A m⁻² at 16 T and 4.2 K.     

Superconductors of the BPSCCO system obtained from rapid cooling process

Bi-based (BPSCCO) superconductors have been extensively studied due to their interesting superconducting properties, especially those that present high transition temperature (T_c). In this work, superconductors of the BPSCCO system were prepared from rapid cooling process and studied under its structural and magnetic properties. Sample as-prepared shows an amorphous behavior, which is converted progressively into 2223 phase. This process permits the control of Pb or Bi loss and the crystallization of the desired phase using several heat annealing processes. The 2201 and 2212 phases were also observed as intermediate phases, before the crystallization of the 2223 phase. The superconductor obtained in this work presented a T_c around 77-K.     

Research and Prospects of Iron‐Based Superconductors

The discovery of a new superconductor, LaFeAsO_1?xF_x with a superconducting critical temperatureT,_c, of 26 K in 2008, has quickly renewed interest in the exploration of iron‐based superconductors. More than 70 new superconductors have been discovered within several months, with the highest T_c of up to 55 K being observed in the SmFeAsO_1?x compound. High T_cs have previously only been observed in cuprates; these new iron‐based superconductors have been added as second members of the high‐T_c family. The crystal structure of these compounds contains an almost 2D Fe–As layer formed by FeAs₄ tetrahedrons, which can be separated by an oxide or metal layer that provides extra electrons to the Fe–As layer, and the itinerant iron 3d electrons form an antiferromagnetic (AFM) order state in the undoped parent compounds at around 100–200 K. Superconductivity can be induced by carrier doping, which destroys the AFM ground state. In this Review, the most recent findings on and basic experimental facts about this class of high‐T_c materials will be presented, including the various superconducting structures, the synthesis methods, the physical properties of the parent compounds, the doping methods that could produce superconductivity, pressure effects, and the prospects for this new iron‐based high‐T_c family.     

Effect of oxygen deficiency (δ) on transition temperature of yttrium cuprate superconductors

The nature of pairing mechanism as well as transition temperature of yttrium cuprates is discussed using the strong coupling theory. An interaction potential has been developed for the layered structure with two conducting CuO₂(a–b) layers in a unit cell. The interaction potential properly takes care of electron-electron, electron-phonon and electron-plasmon interactions. Furthermore, the electron-phonon coupling parameter (λ), the modified Coulomb repulsive parameter (μ*) and the 2D acoustic phonon (plasmon) energy as a function of oxygen deficiency is worked out. Finally, the superconducting transition temperature (T _c) is then evaluated by using these coupling parameters and obtainedT _c = 95(92)K for Y(Yb)Ba₂Cu₃O_7−δ superconductors withδ = 0·0. The model parameters estimated from the layered structure approach are consistent with the strong coupling theory. The result deduced on the variation ofT _c withδ are in fair agreement with the earlier reported data on yttrium cuprates. The analysis of the above results are discussed.     

Fermionic Superfluidity: From High T c Superconductors to Ultracold Fermi Gases

We present a pairing fluctuation theory which self-consistently incorporates finite momentum pair excitations in the context of BCS—Bose–Einstein condensation (BEC) crossover, and we apply this theory to high T _c superconductors and ultracold Fermi gases. There are strong similarities between Fermi gases in the unitary regime and high T _c superconductors. Here, we address key issues of common interest, especially the pseudogap. In the Fermi gases, we summarize recent experiments including various phase diagrams (with and without population imbalance), as well as evidence for a pseudogap in thermodynamic and other experiments. This work was supported by NSF PHY-0555325 and NSF-MRSEC Grant No. DMR-0213745.