Ultra-thin TaN Films for Superconducting Nanowire Single-Photon Detectors

Ultra-thin films of superconducting tantalum nitride are deposited by reactive magnetron sputtering on heated sapphire substrates. The critical temperature T _C=10.25 K is reached for films thicker than 10 nm. A superconducting nanowire single-photon detector in the form of a meander line with a width of 110 nm was made from 5 nm thick TaN film. The detector had a transition temperature of 8.3 K and a critical current density of 4 MA/cm² at 4.2 K. A photon detection efficiency of 20% has been obtained for the detector with a filling factor of 0.55 at wavelengths up to 700 nm.     

Proton-induced reduction ofR s,Jc, andT c in YBa2Cu3O7−δ thin films

We have explored the effect of 2-MeVH⁺ irradiation on the superconducting transport properties of thin films of YBa₂Cu₃O_7?δ [T _c, J_c(B=0; 77 K, 4.2 K), andR _s(36 GHz;T)]. The inductively measured critical temperatureT _c changed slowly and uniformly (～2 K per 10¹⁶/cm²) for fluences less than ～3×10¹⁶/cm². Beginning at ～3–4×10¹⁶/cm², the superconducting transition broadened and dropped more quickly with fluence. The critical current density measured at 77 and 4.2 K changed roughly linearly with fluence. The microwaveT _c (as defined by the sharp transition inR _s as a function of temperature) resembled the low-frequency inductiveT _c measurement at low fluences but was depressed more strongly for large fluences. The residual surface resistance (～6–10 mΩ) was not affected for fluences up to 5×10¹⁶/cm². We have interpreted the sudden and reproducible reduction in the microwaveT _c transition as a sensitive indicator of disruption in the copper-oxygen chain sublattice and compared the proton-induced change to that observed in oxygen gettering studies of bulk materials.     

Preparation of superconducting LiTi2O4 thin films

Superconducting LiTi₂O₄ thin films were prepared for the first time by the r.f. magnetron sputtering method and subsequent heat treatment. Their superconducting transition temperatures are 10.9–11.5 K, close to the transition temperatures of samples prepared by sintering or metling. Uniform films shows metallic conduction and have a resistivity of $\text{(4.3?8.8) × 10}{}^{-4}\text{Ω}\text{cm at}\text{15}\text{K}$ and a superconducting critical current density of 1.2 × 10⁴ A cm^-2 at 4.2 K. It was also found that the films are stable in an argon gas atmosphere.     

The superconducting transition temperature and its high-pressure behavior of tetragonal Nb3Sn

The superconducting transition temperatureT _C of the tetragonal phase of a Nb₃Sn single crystal was determined calorimetrically to be 17.78±0.02 K. It is suppressed by the application of hydrostatic pressure at a rate of ?(1.40±0.05)×10^?5 K bar^?1. The previous observations of double superconducting transitions in Nb₃Sn and other A15 compounds are discussed.     

Magnetoresistance and Irreversibility Fields of Bismuth-Based 1G Tape

The magnetoresistance and irreversibility fields of commercial bismuth-based 1G tape were studied on the basis of the temperature dependencies of the magnetoresistance at the two relative orientations of magnetic field and superconductor plane. The critical temperatures of this tape are the following: T _{c50 %} = 110.3 K and T _c0 = 109.9 K, and the width of superconducting transition is ΔT = 0.5 K. The widths of the transition to the superconducting state versus applied magnetic fields were derived for both orientations. The experimental data were fitted using the formula ΔT = C H ^m + ΔT ₀. The irreversibility field values were obtained and successfully fitted as a function of temperature. At 77 K, they were found to amount to H _{i r r} = 72.8 kOe and H _{i r r} = 5.5 kOe for the parallel and perpendicular directions, respectively.     

The nature of the transition atH c2 in low-κ type II lead-indium superconducting alloys

The anomalous magnetic and calorimetric properties atH _c2, which we reported for a lead-indium sample, are in agreement with the prediction of a bulk first-order phase transition from recent theories for critical κ superconductors \(\kappa \approx 1/\sqrt 2 \) . Other experimentalists, however, explain this behavior as an effect of the superconducting surface sheath. In an attempt to clarify the issue, magnetization and specific heat measurements are reported here on two samples with nominal concentrations of 1.9 and 1.4 at % indium in lead. The latter is type I above 6 K (T _c =7.12 K). It is shown that surface effects are relevant for the effect reported previously. However, after empirical surface treatments have been applied to eliminate or at least minimize such effects, most of the results for the sample, which fulfills the theoretical requirements, are in agreement with theoretical predictions of Fischer, Usadel, and Jacobs. A bulk first-order transition atH _c2 is thus, nevertheless, likely to occur for strong-coupling type II superconductors with critical κ values.     

Doping Dependent Quasiparticle Relaxation Dynamics in?SmFeAsO1?x F x Single Crystals: Comparison of?Spin-Density Wave and Superconducting States

We use optical femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in undoped and near-optimally doped SmFeAsO_1?x F _x iron-pnictide superconductor single crystals (SC). In the undoped SC, a single relaxation process with a divergent-like relaxation time at the spin density wave (SDW) transition is observed. From the relaxation time in the normal state, significantly above T _SDW, the second moment of the Eliashberg function is determined to be ?????? ²????135?meV². Below T _SDW, the temperature dependence of the photoexcited reflectivity transients indicates the appearance of a bottleneck due to opening of a SDW gap with a BCS-like temperature dependence and the characteristic magnitude, 2??_SDW/k _B T _SDW=7??3, at?4.2?K. In the superconducting SC, multiple relaxation processes are present. In addition to the relaxation processes observed in the normal state, a distinct superconducting state relaxation component is observed consistent with the presence of BCS T-dependent gaps.     

Crystallization-induced superconductivity in amorphous Ti-Ta-Si alloys

Amorphous alloys containing 0 to 40 at% Ta and 15 to 20 at% Si have been produced in the ternary Ti-Ta-Si system by rapidly quenching the melts using a melt-spinning technique. The amorphous alloys did not show any superconducting transition down to liquid helium temperature (4.2 K). However, a transition was detected above 4.2 K after inducing crystallization in these alloys by annealing at appropriate temperatures. The superconducting transition temperature, T _c, increased with increasing tantalum content and showed the highest value of 7.6 K for the Ti₄₅Ta₄₀Si₁₅ alloy annealed for 1 h at 1073 K. An upper critical magnetic field, H _c2 of 4.7×10⁶ Am^–1 at 4.2 K and a critical current density, J _c, of 1.5×10⁴ A cm^–2 at zero applied field and 4.2 K were recorded for this alloy. Detailed electron microscopic studies of the crystallization behaviour of the amorphous alloys established that a supersaturated solid solution of tantalum in -Ti with a bcc structure forms first, followed by the precipitation of the bc tetragonal Ta₃Si compound. Since Ta₃Si is not superconducting above 4.2 K, it has been concluded that superconductivity in the crystallized alloys is due to the precipitation of -Ti(Ta) solid solution.     

Hall effect in detwinned single crystals of YBa2(Cu1−x Zn x )3O7 −δin the mixed state

The effect of zinc doping on the mixed-state Hall effect in detwinned single crystals of YBa₂Cu₃O_{7 ?δ} has been studied. We observe a sign reveral of the Hall effect in crystals with a superconducting transition temperature (T _c) as low as 71 K. The Hall conductivity in the mixed state follows theC ₁/H+C ₂ H field dependence. The absolute value of the coefficientC ₁ increases faster than (1–T/T _c) with decreasing temperature.C ₂ increases rapidly asT falls belowT _c. The temperature dependence of the ratioC ₁/C ₂ scales approximately asT/T _c for different levels of zinc doping.     

Longitudinal ultrasonic attenuation in normal and superconducting lead at low temperatures

We have measured longitudinal ultrasonic attenuation along the [110] direction in normal and superconducting states in two single crystals of lead, one made from high-purity lead and the other made with high-purity lead doped with 0.1 at % gold. In both specimens an amplitude-dependent effect in the superconducting state has been observed. The data have been taken in the frequency range from 12 to 108 MHz. In high-purity lead the amplitude-independent ratio α_s/α_n shows the frequency dependence observed by Randorff and Marshall, whereas in the doped specimen this ratio shows a very small spread with frequency. In both specimens deformation does not change the α_s/α_n ratio appreciably.     

Effect of Te Doping on the Structure and Superconductivity of K x Fe2−y Se2−z Te z Single Crystals

We report the structure, transport, and magnetic properties of K _x Fe_2?y Se_2?z Te _z single crystals grown by optical floating zone technique. The phase separation phenomena were observed in the Te-doped samples. With increasing Te doping level, the c-axis lattice parameter expands for both insulating/semiconducting and superconducting phases while the superconducting transition temperature (T _c) decreases and eventually vanishes at z = 0.51. The critical current density was estimated to be 10³–10⁴ A/cm² for the all doped samples. The upper critical field and anisotropic superconducting ratio increase with Te doping. We compared the results of critical current density J _c, upper critical field μ ₀ H _c2 and apparent thermally activated energy U ₀ for the samples with z = 0, 0.09, and 0.16. The influence of Te doping on the vortex pinning and the implication of U ₀ versus μ ₀ H is discussed.     

BCS temperature-dependent superconducting energy gap in domain boundaries of melt-textured YBa2Cu3O7

Attenuation and velocity measurements in low magnetic fields in the superconducting state of a melt-textured sample of YBa₂Cu₃O₇ are reported. Changes in both attenuationα and relative velocitydv/v are observed at the penetration fieldH′ _c1 of its domain boundaries. The change indv/v atH′ _c1 appears to be proportional to the repulsive force between vortices. The change inα atH′ _c1 may be produced by a relaxation process involving superconducting quasiparticles in the vicinity of the normal core of the vortices. Analysis of the relaxation time associated with this process yields a temperature-dependent energy gap which follows the BCS dependence. The relaxation time associated with the process is about 10^?11 sec.     

Peculiarities of the current-voltage characteristics of a Josephson medium in a YBCO high-temperature superconductor

The influence of a weak magnetic field (H < 150 Oe) on the current-voltage (I-U) characteristic of a YBa₂Cu₃O_{7 ? x} (YBCO) high-temperature superconductor (HTSC) near the superconducting transition temperature has been studied. It is established that there exist narrow (<0.2 K) temperature regions where the I-U curve exhibits sharp bending for H < 30 Oe and the ohmic behavior changes to a quadratic dependence of the voltage on current in a region of several milliamperes. At higher temperatures, the I-U curve bending exhibits smearing. This behavior is observed at a temperature below that corresponding to a zero critical current. Above a certain current, the temperature and magnetic field exhibit equivalent effects on the I-U curve of YBCO. Experimental results are explained by a sharp decrease in the critical currents of intergranular Josephson junctions under the action of magnetic field and by the current-induced formation of uncoupled (with respect to the order parameter) superconducting grains. Characteristic currents for the transition of the intergranular Josephson medium into an incoherent state are determined and the first critical fields in YBCO are evaluated.     

Magneto-transport and Magnetic Susceptibility of SmFeAsO1−x F x (x=0.0 and 0.20)

We report superconductivity in the SmFeAsO_1?x F _x for the x=0.2 system being synthesized using the single step solid state reaction route. Rietveld analysis of room temperature X-ray diffraction (XRD) data shows the studied samples, SmFeAsO_1?x F _x with x=0.0 and x=0.2, are crystallized in a single phase tetragonal structure with space group P4/nmm. The resistivity measurement shows superconductivity for the x=0.20 sample with T _c (onset) ～51.7 K. The upper critical field, [H _c2(0)] is estimated ～3770 kOe by Ginzburg–Landau (GL) theory. Broadening of superconducting transition in magnetotransport is studied through thermally activated flux flow in an applied field up to 130 kOe. The flux flow activation energy (U/k _B ) is estimated ～1215 K for 1 kOe field. Magnetic measurements exhibited bulk superconductivity with lower critical field (H _c1) of ～1.2 kOe at 2 K. In the normal state, the paramagnetic nature of compound confirms no trace of magnetic impurity, which orders ferromagnetically. AC susceptibility measurements have been carried out for SmFeAsO_0.80F_0.20 sample at various amplitude and frequencies of applied AC drive field. The intergranular critical current density (J _c ) is estimated. Specific heat [C _p (T)] measurement showed an anomaly at around 140 K due to the SDW ordering of Fe, followed by another peak at 5 K corresponding to the antiferromagnetic (AFM) ordering of Sm⁺³ ions in the SmFeAsO compound. Interestingly, the change in entropy (marked by the C _p transition height) at 5 K for Sm⁺³ AFM ordering is heavily reduced in the case of the superconducting SmFeAsO_0.80F_0.20 sample.     

Microstructure and superconductivity in annealed Cu-Nb-(Ti,Zr, Hf) ternary amorphous alloys obtained by liquid quenching

Melt-quenched Cu-Nb-(Ti, Zr, Hf) ternary alloys have been found to be amorphous possessing high strength and good bend ductility. The niobium content in the amorphous alloys was limited to less than 35 at % and the titanium, zirconium or hafnium contents from 25 to 50 at %. The Cu₄₀Nb₃₀(Ti, Hf)₃₀ alloys showed a superconducting transition above the liquid helium temperature (4.2 K) after annealing at appropriate temperatures. The highest transition temperatures attained were 5.6 K for the Cu₄₀Nb₃₀Ti₃₀ alloy annealed for 1 h at 873 K and 8.4 K for the Cu₄₀Nb₃₀Hf₃₀ alloy annealed for 1 h at 1073 K. In addition, these alloys exhibited upper critical magnetic fields of 1.8 to 2.3×10⁶ Am^–1 at 4.2 K and critical current densities of 2×10³ to 1×10⁴ A cm^–2 at zero applied field and 4.2 K. Since the structure of the superconducting samples consisted of ordered phases based on a b c c lattice with a lattice parameter of 0.31 nm, it was concluded that the superconductivity in the Cu₄₀Nb₃₀Ti₃₀ and Cu₄₀Nb₃₀Hf₃₀ alloys was due to the precipitation of the metastable ordered b c c phases.     

Spontaneous Stoichiometry Change in Single Crystals of Superconducting (Ba1−x K x )Fe2As2 Grown by a Rapid-Heating Sn-Flux Method

To prevent the loss of K in growing single crystals of Ba_1−x K _x Fe₂As₂ we developed a rapid-heating Sn-flux method. Large single crystals with the optimal superconducting transition temperature T _C≈38 K were obtained and their structural, chemical and superconducting properties were investigated. Additionally, the effect of post-growth annealing on these crystals at different temperatures was examined. Scanning electron microscopy microprobe studies on a crystal with the composition goal of Ba_0.25K_0.75Fe₂As₂ revealed a well defined separation of two phases with compositions that are suggestive of rational ratios of the K and Ba content.     

Electron density changes at the crystal lattice sites upon superconducting phase transition

It is established that complex copper oxides exhibit a relationship between a change in the electron density at the metal crystal lattice site and the temperature (T _c) of a phase transition to the superconducting state. In crystals containing two structurally inequivalent positions of copper atoms, a change in the electron density upon the transition is different for these lattice sites. In particular, there is a limiting value of the electron density variation upon the superconducting transition, which is different for Cu(1) and Cu(2) lattice sites and corresponds to the two different minimum values of the correlation length. The electron density changes at the sites of a Kronig-Penney crystal lattice upon the superconducting transition are calculated. It is established that the transition from normal to superconducting phase is accompanied by an increase in the charge density at the center of the unit cell. This growth increases with T _c, which agrees with the Mössbauer spectroscopy data.     

Superconducting gap in Pr x Y1−x Ba2Cu4O8 and YBa2−y Sr y Cu4O8 probed by infrared phonon self-energies

We report a reflectivity study of thez-polarized TO-phonons of Pr _x Y_1?x Ba₂Cu₄O₈ and YBa_2?y Sr _y Cu₄O₈ alloys in the temperature range 10–300 K. Anomalies of the frequency and linewidth of the plane-oxygen vibration atω～300 cm^?1 due to the opening of the superconducting gap are found to occur upon crossing the superconducting transition temperatureT _c . Phonon self-energy effects are strongly dependent onT _c , providing evidence for a relative shift of the gap with respect to the energy of phonon.     

Dislocation damping and the ultrasonic attenuation in tin

Measurement of ultrasonic attenuation in single crystals of pure tin has been taken at 300 K for frequencies varying from 3 to 50 MHz and at different dislocation densities. There is a resonant behaviour in attenuation due to dislocation and the resonance frequency v_o increases with an increase in dislocation density A. Experimentally it has been observed that there is a linear variation between lnΛ and $\text{v}{}_{\mathrm{<mtext>o</mtext>}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$. A systematic broadening of the resonance curve takes place with increasing dislocation density. The broadening of the resonance curve has been explained as being due to damping by mutual interaction between the dislocations. An estimate of dislocation damping has been made at different dislocation densities. The total damping due to phonons and electrons has also been estimated at 300K and 77 K.     

Production of single-crystal K3C60

A simplified technique for synthesizing high-quality millimeter-sized single-crystals of K₃C₆₀ is reported. The preparation method consists of a two-step process where pure C₆₀ crystals are first grown and later doped with potassium vapor. The undoped crystals are grown using an open-tube vapor transport with a flowing helium gas carrier. Doping occurs in a highvacuum environment inside a specialized quartz tube which allows phase purity to be monitored via magnetization measurements of the superconducting state. Utilizing the aforementioned procedure, one can attain samples with magnetically determined superconducting transition widths (10–90% diamagnetic shielding) of less than 1 K.