Transverse voltage in superconductors at zero applied magnetic field

A systematic study of the transverse voltage at zero magnetic field in the superconducting state is reported. The effects of warming rate, temperature, applied magnetic field, and electrical current on the transversal resistance (R_XY) of polycrystalline superconducting sample are taken into account. At zero magnetic field two peaks are observed in R_XY(T) curves which are related to the double superconducting transition in the R_XX(T) component. In the superconducting (R_XX = zero) and normal states no transverse voltage has been detected at zero magnetic field as expected. The results are discussed within the framework of the motion of Abrikosov and Josephson vortices and anti-vortices. A new scaling relation between transverse and longitudinal components given by R_XY ～ dR_XX/dT has been confirmed.     

AC response of YBa2Cu3O7 thin film superconductor

Low-field ac measurements of magnetic susceptibility of YBa₂Cu₃O₇ high-temperature superconducting thin film were carried out over a wide range of temperatures and ac magnetic field amplitudes. A strong field dependence of the real χ′ and imaginary χ″ components was observed. The field dependence of the imaginary component is used to extract the temperature dependence of the critical current density in the sample using the critical state model. The exponent β of the power law relation J_c ～ (1 ? T/T_c)^β was determined from ac-susceptibility data and different values were found. Experimental results are compared with predictions of some existing theoretical models describing the ac response of superconducting thin film in perpendicular ac field.     

Deposition of Gd2Zr2O7 single buffer layers with different thickness for YBa2Cu3O7−δ coated conductors on metallic substrates

A series of Gd₂Zr₂O₇ (GZO) single buffer layers with different thicknesses were epitaxially grown on highly textured Ni–5 at.% W tapes using pulsed laser deposition. These allow the subsequent growth of high-quality superconducting YBa₂Cu₃O_7?δ layers. The superconducting transition temperature T_c reaches a maximum value of 92.4 K as well as a narrow transition width of 0.8 K for the optimized GZO layer thickness. The inductive measurements show the critical current density as high as 1.2 MA/cm² at 77 K in self-field, indicating that a GZO single buffer layer is a suitable alternative for simplifying the second generation high T_c superconducting coated conductors architecture.     

The evolution of electromagnetic response at low frequencies with the development of superconducting condensation in Tl2Ba2CaCu2O8+δ

We report the in-plane temperature dependent ac impedance spectrum of Tl₂Ba₂CaCu₂O_8+δ (Tl-2212) single crystal film with thickness 3000 Å and T_c = 96 K. In process of the superconducting condensation, the impedance spectrum z(ω,T₀) changed considerably and was very similar with the evolution of the spectrum under various dc bias voltages in the shallow superconducting state. The spectrum weight transfers from high frequency regions to lower ones and eventually diverges towards the dc limit. From the analysis, it seems that the superconducting ‘islands’ form first and then grow larger, till the formation of a ‘continent’. The impedance spectrum z(ω,B_I) at various dc bias B_I have also been studied.     

Increase of critical current density with doping carbon nano-tubes in YBa2Cu3O7−δ

The effects of carbon nano-tubes (CNTs) on the crystal structure and superconducting properties of YBa₂Cu₃O_7?δ (Y-123) compound were studied. Samples were synthesized using standard solid-state reaction technique by adding CNT up to 1 wt% and X-ray diffraction data confirm the single phase orthorhombic structure for all the samples. Current–voltage measurements in magnetic fields up to 9 T were used to study the pinning energy U_J and critical current density J_c as a function of magnetic field at fixed temperature. We find that while T_c does not change much with the CNT doping (91–92 K), both U_J and J_c increase systematically up to 0.7 wt% CNT doping in a broad magnetic field ranges between 0.1 and 9 T and J_c in the 0.7 wt% CNT doped sample is at least 10 times larger than that of the pure Y-123. The scanning electron microscope image shows that CNTs are forming an electrical-network between grains. These observations suggest that the CNT addition to the Y-123-compounds improve the electrical connection between superconducting grains to result in the J_c increase.     

Effects of growth rate and buffer-layer on Bi2Sr2CaCu2Oy thin film fabrication

The effect of the growth rate on the Bi₂Sr₂CaCu₂O_y (Bi2212) thin film quality on MgO substrate is investigated at several growth rates from 0.175 to 3 nm/min. The maximal step height on the film surface is improved from about 100 to 6 nm by the reduction of growth rate to 0.5 nm/min and simultaneously the superconducting critical temperature attaining to a zero resistance T_c(R=0), is also improved from 50 to 63 K. The surface morphologies of the upmost Bi-superconducting thin films with the intermediate layers on MgO substrate is also studied in contrast to that deposited directly on the MgO substrate.     

A novel selenization technique for fabrication of superconducting FeSex thin film

A combinative method with reactive sputtering deposition and selenization technique was applied to prepare superconducting FeSe_x films on LaAlO₃ substrates successfully. The influence of selenizing temperature on film components was studied. FeSe_0.96 and FeSe films had similar and good performances in transport measurement but little difference in magnetic property. The critical onset temperature got to 11.2 K and T_c,0 got to 4 K approximately. X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy were used to analyze the ratio of constituents and morphology of several selenized films. FeSe_x film had a porous structure on surface and no well preferred orientation, which were confirmed to have little influence on superconducting properties.     

Determination of superconducting gap of SmFeAsFxO1−x superconductors by Andreev reflection spectroscopy

The superconducting gap in FeAs-based superconductor SmFeAs(O_1?xF_x) (x = 0.15 and 0.30) and the temperature dependence of the sample with x = 0.15 have been measured by Andreev reflection spectroscopy. The intrinsic superconducting gap is independent of contacts while many other “gap-like” features vary appreciably for different contacts. The determined gap value of 2Δ = 13.34 ± 0.47 meV for SmFeAs(O_0.85F_0.15) gives 2Δ/k_BT_C = 3.68, close to the BCS prediction of 3.53. The superconducting gap decreases with temperature and vanishes at T_C, in a manner similar to the BCS behavior but dramatically different from that of the nodal pseudogap behavior in cuprate superconductors.     

Influence of a magnetic field on the electronic Raman response in high-Tc cuprates

The effect of a magnetic field parallel to the CuO₂ plane on the Raman spectra is investigated based on the Slave–Boson approach to the t − t′ − J model with a Zeeman field and the random-phase approximation. We find that the Raman spectra intensities in the superconducting (SC) state are suppressed for both the B_1g and the B_2g channels with a slight shift of the peaks position toward lower frequency in the B_1g channel and a negligible shift in the B_2g channel due to the Zeeman splitting. There is a field-induced peak and dip structure at low energy response in the B_2g channel at very low temperature. While rising temperature has a similar effect in reducing the Raman response peak in the superconducting (SC) state, it smears out the field-induced peak and dip structure in the B_2g channel. We compare these results with experiments and give explanations based on the field-induced changes of the density of the superconducting condensate, the momentum distribution of the quasiparticle energy and the scattering rate.     

Superconducting properties of the pyrochlore oxide Cd2Re2O7

We report the superconducting properties of the pyrochlore oxide Cd₂Re₂O₇. The bulk superconducting transition temperature T_c is about 1.0 K, and the upper critical field H_c2 determined by the measurement of specific heat under magnetic fields is 0.29 T. The superconducting coherence length is estimated to be 34 nm. Specific heat data measured on single crystals suggest that the superconducting gap of Cd₂Re₂O₇ is nodeless.     

Above-gap and subgap differential conductance anomaly in concentrated magnetic semiconductor Zn0.32Co0.68O1 − v/Pb superconductor hybrid junctions

The Zn_0.32Co_0.68O_1 − v/Pb hybrid junctions were prepared, where the concentrated magnetic semiconductor Zn_0.32Co_0.68O_1 − v is in the region of variable range hopping transport instead of the ballistic or diffusive transport. The high differential conductance peak at gap voltage and two above-gap peaks were observed below the superconducting critical temperature. Moreover, both the zero bias conductance peak and the finite bias conductance peak were observed below the gap voltage. All these differential conductance peaks systematically evolve and finally disappear as the temperature or the magnetic field increases. These transport phenomena were explained by phase coherent Andreev reflection in the presence of strong disorder, magnetic impurity scattering, and spin polarization.     

Current-carrying states in superconductor/insulator and superconductor/semiconductor superlattices in the mesoscopic regime

We discuss some of the basic theoretical aspects of current-carrying states in superconducting superlattices with tunnel barriers in the mesoscopic regime, when p₀ ^− 1 ⪡ a ⪡ ξ₀(a is the superconducting layer thickness, p₀is the Fermi momentum, ξ₀is the BCS coherence length and ℏ =  1). We establish the necessary conditions for the observation of the classical Josephson effect (with sinusoidal current–phase dependence) and derive self-consistent analytical expressions for the critical Josephson current. These expressions are proportional to the small factor a / ξ₀and have unusual temperature dependence as compared with the single-junction case. For certain parameter values, the superconducting gap exhibits an exponential decrease due to pair-breaking effect of the supercurrent. The supercurrent can completely destroy the superconductivity of the system above a certain characteristic temperature T ^* . In this paper, we also study the effect of intrabarrier exchange interactions. We show that this effect is strongly enhanced compared with the single-junction case and can manifest itself in an exponential decrease of the critical temperature.     

Yttrium doped La1−xYxO0.9F0.1FeAs superconductors: Hall and thermopower studies

The effect of yttrium substitution at the lanthanum site on the superconducting properties of La_1?xY_xO_0.9F_0.1FeAs (‘x’ = 0, 0.10, 0.20, 0.30, 0.50 and 0.60) oxypnictides has been studied. Powder X-ray diffraction studies confirm single phases till x = 0.1 beyond which minor amount of Y₂O₃ is observed. The temperature dependence of resistivity measurements confirm the superconducting transition temperature (T_c) of 34.8 (±0.05) K and corresponding Meissner transition at 34.3 K in the ‘x’ = 0.3 composition which is higher than that reported for the parent phase (LaO_0.9F_0.1FeAs (T_c = 28 K)). Further increase in the concentration of yttrium leads to broadening and suppression of the superconducting transition. The value of H_c2 at zero temperature is estimated to be about 60.5 T. The Seebeck coefficient (S) shows a negative sign indicating that the major contribution to the conductivity is by electrons. The Hall coefficient (R_H) also remains negative throughout the temperature range supporting the thermopower results. The lattice parameters (a and c) decreases and the charge-carrier density increases with yttrium doping.     

Preparation of YBCO film for microwave filter using a hybrid route

Pure (0 0 l)-textured CeO₂ buffer layers were deposited on single crystal r-plane Al₂O₃ (1–102) substrate by a hybrid process which was combined with magnetron sputtering for the seed layer and metal–organic deposition for the subsequent layer. Strongly c-axis oriented YBCO films were deposited on the CeO₂ buffered r-cut Al₂O₃ (1–102) substrates. Atomic force microscope and scanning electronic microscopy results show that the prepared buffers and YBCO films are relatively dense and smooth. The critical current of the YBCO films exceeds 1.5 MA/cm² at 77 K with the superconducting transition temperature of 90 K. The surface resistivity is as below as 14 μΩ at 1 GHz frequency. The results demonstrate that the hybrid route is a very promising method to prepare YBCO films for microwave application, which can combine the sputtering advantage for preparing of highly c-axis oriented CeO₂ buffer layers and the advantages of metal–organic deposition with rapid processing, low cost and easy preparation of large-area YBCO films.     

STM spectroscopy of an organic superconductor

Electron tunneling spectroscopy of the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂using low temperature scanning tunneling microscope (STM) is reported. The tunneling differential conductance in the superconducting phase was obtained in theb–cplane of a single crystal, by varying the tip position on the sample surface. The differential conductance is reduced near zero bias voltage and enhanced at the gap edge, associated with the superconducting gap structure below[formula] K. The gap width differs slightly from sample to sample, while the overall functional shape of the conductance is sample-independent. The tunneling conductance is reduced to almost zero near zero bias voltage, while it is finite inside the gap edge. The curve obtained cannot be fit to the BCS density of states withs-wave pairing symmetry, even if the life-time broadening of one-electron levels is taken into account. Finite conductance inside the gap edge suggests anisotropy of the gap. However, the conductance curve obtained is not explained by a simpled-wave symmetry for Δ(k). The reduced conductance near zero bias voltage suggests a finite gap. An anisotropic model with a finite gap, in which Δ(k) varies depending on the direction ink-space, is examined. The tunneling conductance in the low-energy region is almost fit by the model with Δ_min = 2 meV and Δ_max = 6 meV. The finite conductance is explained by introducing a small effect of life time broadening. We conclude that the gap is anisotropic and is finite (at least Δ_min = 2 meV) on the entire Fermi surface.     

Excess currents larger than the point contact limit in normal-metal/superconducting junctions

In a point contact NS junction, perfect Andreev reflection occurs over a range of voltages equal to the superconducting energy gap, producing an excess current of I_exc =  (4 / 3)(2 eΔ / h). If the superconductor has a finite width, rather than the infinite width of the point contact, one cannot neglect superfluid flow inside the superconducting contact. The energy range available for perfect Andreev reflections then becomes larger than the superconducting gap, since superfluid flow alters the dispersion relation inside the finite width superconductor. We find a maximum excess current of approximately (7 / 3)(2 eΔ / h) when the width of the superconductor is approximately 7 / 3 times the width of the normal metal.     

Epitaxial growth of Ce2Y2O7 buffer layers for YBa2Cu3O7−δ coated conductors using reel-to-reel DC reactive sputtering

Biaxially textured Ce₂Y₂O₇ (CYO) films were deposited on Ni–5at.%W (Ni–5W) tapes by a DC reactive sputtering technique in a reel-to-reel system. Subsequent YBa₂Cu₃O_7?δ (YBCO) films were prepared using pulsed laser deposition leading to a simplified coated conductor architecture of YBCO/CYO/Ni–5W. X-ray diffraction measurements revealed an epitaxial growth of the CYO buffer layer with a texture spread down to 2.2° and 4.7° for the out-of-plane and in-plane alignment, respectively. Microstructural investigations showed a dense, smooth and crack-free surface morphology for CYO film up to a thickness of 350 nm, implying an effective suppression of cracks due to the incorporation of Y in CeO₂. The superconducting transition temperature T_c of about 90 K with a narrow transition of 0.8 K and the inductively measured critical current density J_c of about 0.7 MA/cm² indicate the potential of the single CYO buffer layer.     

Transparent CdO/n-GaN(0001) heterojunction for optoelectronic applications

Undoped CdO films were prepared by sol–gel method. Transparent heterojunction diodes were fabricated by depositing n-type CdO films on the n-type GaN (0001) substrate. Current–voltage (I–V) measurements of the device were evaluated, and the results indicated a non-ideal rectifying characteristic with I_F/I_R value as high as 1.17×10³ at 2 V, low leakage current of 4.88×10⁻⁶ A and a turn-on voltage of about 0.7 V. From the optical data, the optical band gaps for the CdO film and GaN were calculated to be 2.30 eV and 3.309 eV, respectively. It is evaluated that interband transition in the film is provided by the direct allowed transition. The n-GaN (0001)/CdO heterojunction device has an optical transmission of 50–70% from 500 nm to 800 nm wavelength range.     

Large transport critical currents of powder-in-tube Sr0.6K0.4Fe2As2/Ag superconducting wires and tapes

We report the achievement of transport critical currents in Sr_0.6K_0.4Fe₂As₂ wires and tapes with a T_c = 34 K. The wires and tapes were fabricated through an in situ powder-in-tube process. Silver was used as a chemical addition as well as a sheath material. All the wire and tape samples have shown the ability to transport superconducting current. Critical current density J_c was enhanced upon silver addition, and at 4.2 K, a largest J_c of ～1200 A/cm² (I_c = 9 A) was achieved for 20% silver added tapes, which is the highest in iron-based wires and tapes so far. The J_c is almost field independent between 1 T and 10 T, exhibiting a strong vortex pinning. Such a high transport critical current density is attributed to the weak reaction between the silver sheath and the superconducting core, as well as an improved connectivity between grains. We also identify a weak-link behavior from the apparent drop of J_c at low fields and a hysteretic phenomenon. Finally, we found that compared to Fe, Ta and Nb tubes, Ag was the best sheath material for the fabrication of high-performance 122 type pnictide wires and tapes.     

High pressure studies on Fe-pnictide superconductors

A review of high pressure studies on Fe-pnictide superconductors is given. The pressure effects on the magnetic and superconducting transitions are discussed for different classes of doped and undoped FeAs-compounds: ROFeAs (R = rare-earth), AeFe₂As₂ (Ae = Ca, Sr, Ba), and AFeAs (A = Li, Na). Pressure tends to decrease the magnetic transition temperature in the undoped or only slightly doped compounds. The superconducting T_c increases with low pressure for underdoped FeAs-pnictides, remains approximately constant for optimal doping, and decreases linearly in the overdoped range. The undoped LaOFeAs and AeFe₂As₂ become superconducting under pressure although non-hydrostatic pressure condition seems to play a role in CaFe₂As₂. The superconductivity in the (undoped) AFeAs is explained as a chemical pressure effect due to the volume contraction caused by the small ionic size of the A-elements. The binary FeSe shows the largest pressure coefficient of T_c in the Se-deficient superconducting phase.