Superconducting Transition Curves Research Articles

The effect of annealing on the transverse electrotransport in YBa2Cu3O7–δ single crystals irradiated with high-energy electrons

The effects of annealing on the transverse resistivity of YBa2Cu3O7–δ single crystals irradiated with high-energy electrons are considered. It is found that annealing restores the resistivity to a certain intermediate level, including the positions of the steps along the superconducting transition curve that are typical of that level.

Superconducting switching due to a triplet component in the Pb/Cu/Ni/Cu/Co2Cr1 -x Fe x Al y spin-valve structure.

We report the superconducting properties of the Co2Cr1−xFexAly/Cu/Ni/Cu/Pb spin-valve structure the magnetic part of which comprises the Heusler alloy layer HA = Co2Cr1−xFexAly with a high degree of spin polarization (DSP) of the conduction band and a Ni layer of variable thickness. The separation between the superconducting transition curves measured for the parallel (α = 0°) and perpendicular (α = 90°) orientation of the magnetization of the HA and the Ni layers reaches up to 0.5 K (α is the angle between the magnetization of two ferromagnetic layers). For all studied samples the dependence of the superconducting transition temperature Tc on α demonstrates a deep minimum in the vicinity of the perpendicular configuration of the magnetizations. This suggests that the observed minimum and the corresponding full switching effect of the spin valve is caused by the long-range triplet component of the superconducting condensate in the multilayer. Such a large effect can be attributed to a half-metallic nature of the HA layer, which in the orthogonal configuration efficiently draws off the spin-polarized Cooper pairs from the space between the HA and Ni layers. Our results indicate a significant potential of the concept of a superconducting spin-valve multilayer comprising a half-metallic ferromagnet, recently proposed by A. Singh et al., Phys. Rev. X 2015, 5, 021019, in achieving large values of the switching effect.

Fabrication and Characterization of Superconducting Bilayer (Al/Ti) Transition-Edge Sensor Bolometer Array

A four-by-four transition-edge sensor (TES) bolometer array based on an Al/Ti bilayer on an Si3N4 substrate for thermal connection, showing an unbiased transition temperature of 1.2 K, was fabricated and characterized. Formation of the Al/Ti bilayer with tetragonal structure was confirmed by x-ray diffraction analysis. The surface roughness of 3.11 nm calculated by atomic force microscopy was found to be negligible compared with the particle size (19.66 nm), indicating a smooth surface. The smoothness of the resistance–temperature (R–T) curve corresponds to the smoothness of the surface of the bilayer sensor, and the superconducting transition curve of the suspended bilayer TES was designed with a biased Tc of 0.45 K and ΔTc = 2 mK. Such TES bolometer arrays are suitable for astrophysical observations and are extensively used in cosmic microwave background polarization detection.

The effect of resolidification on preform optimized infiltration growth processed (Y, Nd, Sm, Gd)BCO, multi-grain bulk superconductor

Controlling the microstructure of superconductors by incorporating the flux pinning centers and reducing the macro-defects to improve high field performance is the topic of recent research. In continuation, the preform optimized infiltration growth (POIG) processed YBa2Cu3O7−δ (YBCO) system, Y-site substituted with three mixed RE (Nd, Sm, Gd) elements is investigated. 20wt.% of (Nd, Sm, Gd)2BaCuO5 were mixed with Y2BaCuO5 and POIG processed in reduced oxygen atmosphere to obtain YNSG superconductor. No seed is employed for crystal growth; hence the processed samples are multi-grained. Microstructural and compositional investigations on YNSG revealed the presence of different phases in the matrix as well as in precipitates which are of the order of submicron to 4μm. A large fraction of macro-defects (∼6% of porosity) was observed in the YNSG sample. For reducing the unwanted macro-defects and refine the non-superconducting precipitates, processed YNSG sample is pressed and resolidified (by infiltrating the liquid phases once again) in an argon atmosphere and the structural, microstructural, elemental and superconducting properties are compared with YNSG and undoped samples. Due to spatial scatter in superconducting critical temperatures, caused by the distribution of different REBCO unit cells in YBCO, superconducting transition curve is sharp in YNSG, whereas the resolidified sample showed the broad transition due to solidified liquid phases.

Enhanced Superconductivity in Double-Doping Cu0.15TaSe2−x S x

Superconducting Cu x TaSe2(x=0.05, 0.15) and Cu0.15TaSe2−x S x (x=0, 0.5, 1, 1.5) single crystals have been systematically fabricated by a chemical vapor transport method. It is found that the double doping in TaSe2, i.e., the simultaneous intercalation of Cu and substitution of Se by S, can substantially enhance the superconducting transition temperature. Transport property measurements give evidence of the coexistence and competition of charge density wave state and superconductivity in Cu x TaSe2 which provide meaningful information to understand the complex electronic states in this system. The parallel shift and the fan-shape broadening behaviors are observed in the superconducting transition curves under magnetic fields of Cu0.15TaSeS and TaSeS, respectively, indicating an increase of coherence length and suppression of superconducting fluctuation induced by copper intercalation.

Transition Edge Sensor (TES) X-Ray Detecting System with Sensitivity Correction to Stabilize the Spectrum Peak Center

Transition Edge Sensor (TES) is an energy dispersive X-ray detecting system with high energy resolution. The energy resolution of this system depends on the steepness of superconducting transition curve from normal to superconducting state, heat capacitance and the operating temperature. The TES is based on the dilution refrigerator cooled by about 100 mK. The energy resolution is calculated about 1-2 eV for the detector with maximum detecting energy as 10 eV. The energy resolution also depends on the superconducting current flowing through the TES device because the superconducting current is affected by the temperature stability of the refrigerator. The fluctuation of the superconducting current means the fluctuation of the X-ray spectrum peak center. We have developed the sensitivity correction system to stabilize the peak center of the X-ray spectrum. The peak center of X-ray spectrum correlates with heater power to keep the base temperature of TES device at a constant temperature. The peak center of X-ray spectrum is calibrated by monitoring the heater power at constant time interval using the correlation curve between the peak center of X-ray spectrum and heater power.

Preparation and Characterization of TiN Seed Layer in All-Conductive Multilayer Structure for Coated Conductors

TiN films were successfully prepared on biaxially textured Ni-5 at.%W substrates by pulsed laser deposition (PLD), serving as a seed layer of an all-conductive architecture, i.e., SrRuO3(SRO)/TiN, for coated conductors. The structure and surface morphology of the TiN films were noticeably affected by the substrate temperature and pulse repetition rate. The subsequent conductive SRO and superconducting YBCO layer were deposited on the best sample of TiN buffered Ni-5 at.%W substrates. X-ray diffraction analysis confirmed that the biaxial textures were transferred from the TiN seed layer to the SRO cap layer and YBCO film with excellent out-of-plane and in-plane textures. The superconducting transition curves and the temperature-dependent resistivity of YBCO films on all-conductive buffer and on the traditional insulated CeO2/YSZ/Y2O3 were also investigated.

Inhomogeneity of Superconductivity and Stripe Correlations at x ∼ 0.21 in La2-xSrxCuO4

With the aim at investigating the relationship between the anomalous decrease in Tc, the development of the Cu-spin correlation and the inhomogeneity of superconductivity at x ∼ 0.21 in the overdoped regime of La2-xSrxCuO4, we have performed the in-plane electrical-resistivity, ρab and magnetization measurements. Measurements of the temperature dependence of ρab in magnetic fields have revealed that the superconducting (SC) transition curve shifts to the low-temperature side almost in parallel with increasing field above ∼ 9 T at x ∼ 0.21, which is similar to that observed for stripe-ordered La-214 systems with the hole concentration of ∼ 1/8 per Cu. On the other hand, it has been found that the magnetization curve shows the so-called second peak in the SC state at x ∼ 0.21, which is due to the strong vortex pinning in normal-state regions in the nano-scale inhomogeneous SC state. Accordingly, it is likely that the stripe correlations are developed in the nano-scale inhomogeneous SC state at x ∼ 0.21.

Mixed bulk-filament nature in superconductivity of the charge-density-wave conductor ZrTe3

We have shown that the superconducting transitions in ZrTe${}_{3}$ are successive from filamentary to bulk, based on experimental results obtained for a superconducting specific-heat anomaly and the anisotropic superconducting transition curve of resistivity. A small jumplike superconducting specific-heat anomaly with a large and widely extended tail was observed to closely follow the anisotropic zero-resistance transition. We concluded that the jumplike anomaly signifies the onset of the long-range order of Cooper pairs with the opening of a superconducting gap at ${T}_{C}$, while the large and widely extended tail indicates behavior induced by Cooper pairs with a very short coherence length. As a limit for a very short coherence length, we propose local pairs with Bose characteristics. As a result, we can understand that the filamentary superconductivity is caused by the local pairs, and the large and widely extended tail is in a crossover region between superconductivity induced by local pairs and that induced by Cooper pairs. Based on a discussion about the origin of the change in the pair coupling from the starting of local pairing to Cooper pairing in ZrTe${}_{3}$, we conclude that ``mixed bulk-filament superconductivity'' results from unique electronic structural changes in the quasi-1D+3D (D, dimensional) Fermi surfaces after the CDW transition.

Magnetic-field effects on the in-plane electrical resistivity in single-crystalLa2−xBaxCuO4andLa1.6−xNd0.4SrxCuO4aroundx=18: Implication for the field-induced stripe order

Temperature dependence of the in-plane electrical resistivity, ${\ensuremath{\rho}}_{\mathrm{ab}}$, in various magnetic fields has been measured in the single-crystal ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{CuO}}_{4}$ with $x=0.08$, 0.10, 0.11 and ${\mathrm{La}}_{1.6\ensuremath{-}x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ with $x=0.12$. It has been found that the superconducting transition curve shows a so-called fan-shape broadening in magnetic fields for $x=0.08$, while it shifts toward the low-temperature side in parallel with increasing field for $x=0.11$ and 0.12 where the charge-spin stripe order is formed at low temperatures. As for $x=0.10$, the broadening is observed in low fields and it changes to the parallel shift in high fields above $9\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Moreover, the normal-state value of ${\ensuremath{\rho}}_{\mathrm{ab}}$ at low temperatures markedly increases with increasing field up to $15\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. It is possible that these pronounced features of $x=0.10$ are understood in terms of the magnetic-field-induced stabilization of the stripe order suggested from the neutron-scattering measurements in the $\mathrm{La}\text{\ensuremath{-}}214$ system. The ${\ensuremath{\rho}}_{\mathrm{ab}}$ in the normal state at low temperatures has been found to be proportional to $\mathrm{ln}(1∕T)$ for $x=0.10$, 0.11, and 0.12. The $\mathrm{ln}(1∕T)$ dependence of ${\ensuremath{\rho}}_{\mathrm{ab}}$ is robust even in the stripe-ordered state.

H–T phase diagram of high-Tc Ba–K–Bi–O

The magnetic behavior of Ba–K–Bi–O single crystals below Tc was investigated. The magnetic moment hysteresis loops were examined with the help of a small Hall detector. The complex analysis of the data on the temperature dependencies of the residual magnetic moment and Hc1, obtained from the magnetic experiments, and the resistive superconducting transition curves give rise to the following picture: at Tc=30 K the transition to the continuous superconducting phase takes place, while at T*=17 K one more transition happens––to the spatially inhomogeneous superconductor–insulator state, that also could be destroyed either by temperature or by magnetic field.

The interplay of magnetic order and superconductivity in Gd xY 1− xNi 2B 2C

Resistivity, ac susceptibility and magnetization measurements are reported for polycrystalline samples of the Gd x Y 1− x Ni 2B 2C series as a function of temperature and magnetic field. The magnetic Gd impurities cause an almost linear decrease of the superconducting transition temperature T c with increasing Gd content in the range of x<0.24. For Gd concentrations between 0.24 and 0.3, an increasing broadening of the superconducting transition curves and an almost constant superconducting transition temperature T c have been observed. The effect of the 4f local moments manifests in a complete suppression of superconductivity for x⩾0.3 and in antiferromagnetic ordering for x>0.3. In zero applied magnetic field, a distinct concentration region around x∼0.3 has been revealed separating superconductivity and antiferromagneting ordering. A metamagnetic transition has been observed in the compound with x=0.5 at a magnetic field of 0.8 T.

Revisit the electronic phase diagram of high temperature superconductors: macroscopic phase separation in heavily overdoped regime

Superconducting diamagnetic properties have been investigated on a series of La 2− x Sr x CuO 4 single crystals. The superconducting transition curve M( T) evolves systematically from a one-step transition in the underdoped region to a two-step transition in the heavily overdoped regime. The unexpected second transition in the heavily overdoped region can be well explained within the framework of an electronic macroscopic phase separation. The resulting phase due to the phase separation contains many tiny superconducting islands surrounded by non-superconducting good metallic regions. Therefore in the overdoped region, the criticality of superconductivity becomes the same as that in the underdoped region although there are very different normal state properties between these two regions. This gives a strong indication that the superconductivity in the underdoped region and overdoped region may have the same underlying mechanism (pairing and phase coherence).

A one–dimensional model for superconductivity in a thin wire of slowly varying cross–section

Using formal asymptotics, a one–dimensional Ginzburg–Landau model describing superconductivity in a thin wire of arbitrary shape and slowly varying crossndash;section is derived. The model is valid for all magnetic fields and for temperatures T , such that the thickness of the wire is much less than the coherence length ξ( T ). The model is used to calculate the normal–superconducting transition curves for closed wire loops of different cross-sections, as functions of temperature and the magnetic flux cutting the loop. This shows a periodic dependence on flux, superimposed on a parabolic background.

Refrigeration load calculation of a HTSC current lead under AC conditions

A 5 kA/sub rms//50 kV HTSC hybrid AC current lead is developed within a BRITE/EURAM cooperation between Alcatel Alsthom, Hoechst and Siemens. For the optimisation of the conductor geometry computer codes have been realised. For the evaluation of AC losses in the superconducting melt cast processed Bi-2212 part, the Bean model has not turned out to be practicable especially when the current exceeds the I/sub c/(T) curve, This is of course related to the low n-value (n/spl les/10) of the superconducting transition curve in this material. A new calculation based on experimental E(J,B,T) curves allows the evaluation of AC losses in the temperature range of interest. These results are used for the calculation of the maximum stable length and the low temperature heat load which is more than three times smaller than in an all metal DC lead. Together with the 77 K heat load of the normal part of the AC lead, the overall 300 K refrigeration load is calculated.

Determination of transition curves of high‐Tc superconductors by phase contrast microscopy

AbstractA new method which allows the detection of the superconducting phase transition of high‐Tc superconductors (HTSC) on a microscopic scale is reported. Micro‐size holes in thin foils of superconducting material are examined in a transmission electron microscope at varying temperatures. The superconducting transition induces small changes in the image intensity within the holes, which can be detected by using electronic image analysis. Superconducting transition curves are then obtained for various types of high‐Tc superconductors and for given values of the applied magnetic field.

Formation of thick YBa2Cu3O7- delta and YBa2Cu3O7- delta -Ag films on Y2BaCuO5 and BaZrO3 substrates

Resistivity and superconducting transition curves, critical current and morphology studies, using a scanning electron microscope, are reported for a series of YBa2Cu3O7- delta and YBa2Cu3O7- delta +5, 10 and 20 wt.% Ag thick films. The above are studied for various maximum processing temperatures on supposedly inert substrates made from both Y2BaCuO5 and BaZrO3. The width of the best transition curves is about 10 K for the Y2BaCuO5 and 5 K for the BaZrO3 substrates. For BaZrO3 substrates the onset temperature is severely reduced. The addition of Ag is shown to improve the transition curves and the critical current densities. In all cases the critical current densities are much lower than the best values previously reported.

Structural instability of the 2212-like structure with strontium substituted by calcium for Bi2Sr3-xCaxCu2Oy thin films

Superconducting Bi2Sr3-xCaxCu2Oy films with x ranging from 1.05 to 2.20 have been prepared using molecular beam epitaxy (MBE) with four metal sources and post-annealing of the evaporated metal mixture films in oxygen and air atmospheres. Annealing in air at 800 degrees C for 1-3 h yields a single-phase state with 2212-like structure for films with Ca content up to x=1.5. A slight decrease of the zero resistance superconducting transition temperature from 85 to 80 K and a lattice constant c from 30.8 to 30.6 AA were measured upon substituting Sr with Ca. Annealing times up to 20 h showed, for Ca-rich films, changes in the X-ray diffraction spectra as well as in superconducting transition curves. The changes are interpreted in terms of 2202- and 2223-like interlayers and intergrowth formation due to a structural instability of the 2212-like structure with Sr partially substituted by Ca.

Laser patterning of microbridges and the DC squid structure on Bi-Sr-Ca-Cu-O film

Patterning of BiSrCaCuO film by laser ablation to make microbridges and DC SQUID structures is reported. No degradation in the superconducting transition curve of the film is observed after patterning. The transport critical current density Jc for the microbridge is 2.4*103 A cm-2 at 77 K.

SUPERCONDUCTING TRANSITION CURVE OF HIGH TEMPERATURE OXIDE CERAMICS

The superconducting transition curve of high temperature oxide ceramics is investigated on the basis of a developed statistical approach. Two-step transition curve is interpreted according to the model, including proximity effect appearance.