The a.c. Josephson effect in a resistive state of narrow superconducting channels

Current steps in the current-voltage characteristic of a narrow superconducting channel in the resistive state under microwave irradiation are calculated. The current steps appear when the frequency of microwave irradiation Ω satisfies the condition $\text{Ω = mω}$, where ω is the Josephson frequency of the phase-slippage and m is an integer.     

Influence of self-heating on the step structures in the current-voltage characteristic of superconducting films

The current-induced transition between the normal and the superconducting state of Sn, In, and Pb films is investigated by varying the helium bath temperature, film thickness, mean free path, and an external magnetic field oriented parallel to the film surface. The experiments show that, farther fromT _c , heating effects play an important role. The heating of the films relative to the helium bath by the liberation of Joule heat in the resistive state and the influence of hotspot formation on the step structures in the current-voltage characteristics of the samples are established with a graphite thermometer. the temperature and the size of the hotspots are determined by the power dissipation, the surface heat transfer, and the heat conduction within the film.     

Dynamical properties of superconducting nanowires

The dynamical properties of thin superconducting wires (nanowires) are studied using numerical simulations based on a one-dimensional time-dependent Ginzburg-Landau equation, which is modified by introducing an order parameter u characterizing the “purity” of the superconductor material. It is established that relatively long nanowires (with lengths much greater than the coherence length) made of a “pure” superconductor (u > 1) are characterized by two critical current density values: j _c1 and j _c2. For j < j _c1, the total current is entirely superconducting, whereas for j > j _c2, the current is purely normal. In the intermediate region of current densities, j _c1 < j < j _c2, the total current contains both superconducting and normal components (mixed state) and the nanowire exhibits the generation of high-frequency electromagnetic waves. The current-voltage characteristics are constructed and the radiation spectrum is obtained. The properties of short superconducting nanowires (with lengths on the order of the coherence length) coincide with those of the Josephson junction. In the case of an “impure” superconductor (u < 1), the nanowire is characterized by a single critical current density.     

Correlation between structural and superconducting properties of nano-granular disordered Nb thin films

We report on low temperature transport measurements on nano-granular Nb thin films deposited on Si (1 0 0) substrates using DC magnetron sputtering. The superconducting transition temperature (T_c) is found to decrease monotonically with the increase of the lattice parameter (a) irrespective of its thickness and grain size. The superconducting transition temperature is found to depend only on the lattice parameter whereas the normal state resistivity depends both on lattice parameter and the details of the sample morphology. We have modeled this T_c variation with lattice expansion in terms of Debye temperature reduction using Morse potential as the interatomic potential in Nb.     

Periodic microwave absorption in superconducting whiskers

The microwave absorption and structural studies of BiSrCaCuO (2212) whiskers have been performed. The low-angle boundaries in the (a, b) and (a, c) planes have been observed. The periodic microwave absorption signal indicated the flux quantization in a system of weak links and superconducting loops formed on the boundaries. Regular character of this signal suggests possible applications in SQUID magnetometry. Lower Josephson critical fieldH _clJ=1.6 Oe and the critical temperatureT _c=89 K have been determined. The Josephson penetration depth was estimated from the Ginzgurg-Landau, and Clem models.     

Nonlinearity of the current-voltage characteristics for YBa2Cu3O7–x single crystals and the Berezinskii-Kosterlitz-Thouless transition

The nonlinearity of the current-voltage characteristics for YBa₂Cu₃O_{7 ? x} single crystals is investigated in the vicinity of the superconducting transition temperature T _c , and the mechanisms responsible for the nonlinearity of the current-voltage characteristics are considered. The current-voltage characteristics and the amplitudes of the higher voltage harmonics for the YBa₂Cu₃O_{7 ? x} single crystals are directly measured using modulation Fourier analysis. It is shown that the positions of the maxima in the amplitudes of the higher voltage harmonics coincide with the Berezinskii-Kosterlitz-Thouless transition temperature T _BKT. This coincidence can provide a means for determining the transition temperature T _BKT. A number of parameters characterizing the YBa₂Cu₃O_{7 ? x} single crystals in the superconducting state are estimated using the experimental results and the model of a Berezinskii-Kosterlitz-Thouless transition. The parameters thus estimated are in good agreement with the data available in the literature.     

Phase transitions in hybrid SFS structures with thin superconducting layers

Features of a phase transition between 0 and π states in superconductor/ferromagnet/superconductor (SFS) Josephson structures with thin superconducting layers and a ferromagnetic barrier are studied experimentally and theoretically. The dependence of the critical temperature T_c of a transition of the hybrid structure to a superconducting state on the thickness of superconducting layers d_s is analyzed by a local method involving measurements of the nonlinear microwave response of the system by a near-field probe. An anomalous increase in the measured temperature T_c at the reduction of the thickness d_s is detected and is attributed to the 0-π transition.     

Effect of nuclear irradiation on the superconducting transition temperatures of A-15 materials

A mechanism for the degradation of superconducting transition temperatures T_c by high energy nuclear irradiation in A-15 materials is proposed. The radiation is supposed to produce small “disordered” regions having a very low T_c, in a matrix having the original T_c of the material. The composite system has a well defined but lower T_c due to the proximity effect of the “disordered” regions.     

Reentrant resistance via spin-polarized quasiparticle self injection in CMR/HTSC heterostructures

Epitaxial superlattices of half-metal, colossal magnetoresistive La_2/3Ca_1/3MnO₃ (HM-CMR) and high-T_c superconducting YBa₂Cu₃O_7-δ (HTSC) are grown with thick and thin modulation lengths (Λ) of YBCO/LCMO, with Λ = 280 nm and 12.5 nm; respectively, on SrTiO₃ (0 0 1) single-crystalline substrates by pulsed laser deposition. Transport measurements R(T) show a resistive state below T = 35 K although the superconducting transition temperature is found to be T_c = 60 K and 63 K for both different superlattices, respectively. The onset of the resistive state coincides with a magnetic transition of the samples. This can be explained by a diffusion of spin-polarized quasiparticles into the superconducting film. Which can be considered as evidence for inverse-proximity effects over a wide temperature range in HM-CMR/HTSC heterostructures.     

Josephson vortex lattice melting in Bi-2212 probed by commensurate oscillations of Josephson flux-flow

We studied the commensurate semifluxon oscillations of Josephson flux-flow in Bi-2212 stacked structures near T_c as a probe of melting of a Josephson vortex lattice. We found that oscillations exist above 0.5 T. The amplitude of the oscillations is found to decrease gradually with the temperature and to turn to zero without any jump at T = T₀ (3.5 K below the resistive transition temperature T_c), thus, indicating a phase transition of the second order. This characteristic temperature T₀ is identified as the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature, T_BKT, in the elementary superconducting layers of Bi-2212 at zero magnetic field. On the basis of these facts, we infer that melting of a triangular Josephson vortex lattice occurs via the BKT phase with formation of characteristic flux loops containing pancake vortices and antivortices. The B-T phase diagram of the BKT phase found from our experiment is consistent with theoretical predictions.     

μSR studies of heavy fermion systems

We have performed both zero field and high transverse field measurements at dilution refrigerator temperatures on a number of heavy electron systems, examining the superconducting and magnetic properties of these interesting materials. Among the materials studied to date are UBe₁₃, URu₂Si₂ and U₆Fe. The magnetic field penetration depth in the superconducting state of UBe₁₃ is greater than 10000 Å, as no increase in the transverse field relaxation rate is observed belowT _c . A sharp increase in the precession frequency is seen, starting atT _c . This frequency shift shows little temperature dependence at low temperature; we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we have observed relaxation in high transverse field due to the formation of a flux lattice in U₆Fe, a material where the electron effective mass is rather lighter than in other heavy fermion systems. The relaxation exhibits a sharp onset atT _c=3.9 K, and is flat at low temperatures as expected for a conventional superconductor.     

Superconducting properties and structural phase transformation of Nb2N induced by C+ - and N+-implantation

Thin films of h.c.p. Nb₂N implanted uniformly wilh 25 at.% C⁺ or N⁺ ions were found to transform into the f.c.c. Bl structure; film surface temperatures did not exceed 200°C. The resistive superconducting transition sharpened markedly and T_c was found to increase after implantation, presumably due to the structural change. X-Ray diffraction measurements were used to determine crystal structure and lattice parameters. A possible mechanism for the transformation is discussed.     

Fluctuations in superconductors belowT c

Thermal fluctuations of the order parameter in the superconducting state are investigated, particularly near the transition temperature, using the time dependent Ginzburg-Landau theory. These fluctuations give rise to a contribution of the dynamical conductivity, which strongly increases as the temperature is raised towardsT _c , in contrast to the temperature dependence of the electromagnetic response due to the static order parameter. At the transition temperature this contribution joins continuously to the extra response (conductivity, susceptibility) which has been calculated and observed in the temperature region aboveT _c , where it represents the onset of superconductivity in the normal state. Particularly the dynamical conductivity due to fluctuations is calculated and discussed for bulk material, thin films and thin wires belowT _c . The temperature and frequency dependence should be observable in microwave experiments.     

Magnetoresistance of superconducting grains in ceramic YBa2Cu3O7 − δ high-temperature superconductors in weak magnetic fields

The current-voltage characteristics $E(j)_{H_{treat} } = const$ of ceramic (granular) YBa₂Cu₃O_6.95 samples preliminarily magnetized in different transverse magnetic fields H _treat have been measured in a zero field (H _ext = 0) at T = 77.3 K for elucidating specific features of dissipation in superconducting grains of high-temperature superconductors (HTSCs). The current-voltage curves measured in the range 0 ≤ H _trap ≤ H _c2J (where H _trap is the magnetic field trapped as a result of the pretreatment in H _treat and H _c2J is the upper critical field of the Josephson weak links) have been used to construct the field dependences of the magnetoresistance ρ_A(H _treat) _{j = const} of superconducting grains. It has been established that the magnetoresistance ρ_A of the superconducting grains is significantly lower than the magnetoresistance ρ_J for the Josephson medium. The dependence of ρ_A on H _treat and on the transport current density j has been investigated. It has been shown that the dependences ρ_A(H _treat) _{j = const} exhibit a clearly pronounced tendency to saturation, ρ_satur, and the value of ρ_satur increases with increasing j. It has been found that the lower critical field H _c1A of the superconducting grains strongly depends on the transport current density.     

Specific heat of a transforming V3Si crystal

The specific heat of a transforming V₃Si crystal in the normal, mixed, and superconducting states has been measured from 6 to 30K in zero and 52.3 kG magnetic fields. An analysis has been carried out in a self-consistent way based on the second-order phase transition from the normal to the superconducting state in zero magnetic field. Various physical parameters characterizing the superconducting and normal states are derived from the thermodynamics and the BCS weak coupling theory. The most important parameter obtained in this analysis is 2Δ(0)/kT_c = 3.46, which indicates a weak coupling in our V₃Si sample.     

Disorder effects in the BCS-BEC crossover region of the attractive Hubbard model

We study the disorder effects upon superconducting transition temperature T _c and the number of local pairs within the attractive Hubbard model in the combined Nozieres-Schmitt-Rink and DMFT + Σ approximations. We analyze the wide range of attractive interaction U, from the weak coupling region, where instability of the normal phase and superconductivity are well described by the BCS model, to the limit of strong coupling, where superconducting transition is determined by Bose-Einstein condensation of compact Cooper pairs, forming at temperatures much higher than superconducting transition temperature. It is shown that disorder can either suppress T _c in the weak coupling limit, or significantly enhance T _c in the case of strong coupling. However, in all cases we actually prove the validity of generalized Anderson theorem, so that all changes in T _c are related to change in the effective bandwidth due to disorder. Similarly, disorder effects on the number of local pairs are only due to these band-broadening effects.     

Threshold perturbations in current-carrying superconducting bridges with a finite length near the critical temperature

Near the critical temperature of a superconducting transition, the energy of the threshold perturbation δF_thr that transfers a superconducting bridge to a resistive state at a current below the critical current I_c has been determined. It has been shown that δF_thr increases with a decrease in the length of a bridge for short bridges with lengths L < ξ (where ξ is the coherence length) and is saturated for long bridges with L ? ξ. At certain geometrical parameters of banks and bridge, the function δF_thr(L) at the current I → 0 has a minimum at L ~ (2–3)ξ. These results indicate that the effect of fluctuations on Josephson junctions made in the form of short superconducting bridges is reduced and that the effect of fluctuations on bridges with lengths ~(2–3)ξ is enhanced.     

Robust superconductivity and transport properties in (Li1-xFex)OHFeSe single crystals

The recently discovered (Li_1-xFe_x)OHFeSe superconductor with T_c about 40 K provides a good platform for investigating the magnetization and electrical transport properties of FeSe-based superconductors. By using a hydrothermal ion-exchange method, we have successfully grown crystals of (Li_1-xFe_x)OHFeSe. X-ray diffraction on the sample shows the single crystalline PbO-type structure with the c-axis preferential orientation. Magnetic susceptibility and resistive measurements show an onset superconducting transition at around T_c=38.3 K. Using the magnetization hysteresis loops and Bean critical state model, a large critical current J_s is observed in low temperature region. The critical current density is suppressed exponentially with increasing magnetic field. Temperature dependencies of resistivity under various currents and fields are measured, revealing a robust superconducting current density and bulk superconductivity.     

Identification and preparation of single phase 90 K oxide superconductor and structural determination by lattice imaging

We have investigated various compositions of Ba-Y-Cu-O, resulting in the preparation of single phase 90 K superconducting material of composition (Ba_0.67Y_0.33)CuO_3−δ and the identification of the structure of this superconducting phase by lattice imaging as a layered perovskite. The single phase material had a resistive transition with a midpoint at 92 K and a 1.3 K width (10%–90%) as well as a sharp low-field (H = 0.01 Oe) susceptibility transition.     

Theory for superconductivity in amorphous transition metals

A theory is presented for superconductivity in amorphous transition metals. It is shown that in contrast to simple metals for transition metals the changes in the phonon spectrum, in the electronic density of states and in the electronic matrix elements which result from strong lattice disorder can enhance as well as decreaseT _c. The numerical results for the superconducting transition temperatureT _c of amorphous 4d-and 5d-transition metals agree well with the experimental results.