Surface Of High-temperature Superconductor Research Articles

A nonlinear relaxation formulation of the p-curl problem modelling high-temperature superconductors: A modified Yee's scheme

We propose a new tool for the study of the parabolic p-curl problem modelling the magnetic field in a high-temperature superconductor (HTS), a problem involving an operator ∇×(|∇×H|p−2∇×H) analogous to the p-Laplacian. This so-called nonlinear p-curl problem is computationally expensive in three space dimensions because sharp fronts in the magnetic field develop near the surface of HTS. The proposed technique is a relaxation model of the p-curl problem which is shown to provide a consistent and stable approximation according to first, an inner/outer layer analysis and secondly, according to a time scale expansion. The relaxation model leads to monotone approximations of the front because of the presence of anisotropic diffusion. Most importantly, it can be used to develop new schemes that inherit the properties of the relaxation model.In order to demonstrate the relevance of the relaxation model, this paper applies the approximation to the construction of a second-order nonlinear finite-difference time-domain (FDTD) method, similar to Yee's scheme. It is shown that the natural discretization of the p-curl inspired by Yee's scheme leads to an unstable scheme, while a discretization based on the relaxation model is stable. We propose two numerical discretizations that are respectively first and second order in time, but both second-order in space. The linear and nonlinear stability of the schemes is studied and it is demonstrated that the schemes are both discrete divergence free. We verify the numerical schemes using manufactured solutions and Mayergoyz's moving-front solution in both one and two space dimensions, as well as on a magnetization of a typical HTS.

What can Andreev bound states tell us about superconductors?

Zero-energy Andreev bound states, which manifest themselves in the tunnelling spectra as zero-bias conductance peaks (ZBCPs), are abundant at interfaces between superconductors and other materials and on the nodal surface of high-temperature superconductors. In this review, we focus on the information such excitations can provide on the properties of superconductor systems. First, a general introduction to the physics of Andreev bound states in superconductor/normal metal interfaces is given with a particular emphasis on why they appear at zero energy in d-wave superconductors. Then, specific spectroscopic tunnelling studies of thin films, bilayers and junctions are described, focusing on the corresponding ZBCP features. Scanning tunnelling spectroscopy (STS) studies show that the ZBCPs on the c-axis YBa2Cu3O7-δ (YBCO) films are correlated with the surface morphology and appear only in proximity to (110) facets. STS on c-axis La1.88Sr0.12CuO4 (LSCO) films exhibiting the 1/8 anomaly shows spatially modulated peaks near zero bias associated with the anti-phase ordering of the d-wave order parameter predicted at this doping level. ZBCPs were also found in micrometre-size edge junctions of YBCO/SrRuO3/YBCO, where SrRuO3 is ferromagnetic. Here, the results are consistent with a crossed Andreev reflection effect (CARE) at the narrow domain walls of the SrRuO3 ZBCPs measured in STS studies of manganite/cuprate bilayers could not be attributed to CARE because the manganite's domain wall is much larger than the coherence length in YBCO, and instead are attributed to proximity-induced triplet-pairing superconductivity with non-conventional symmetry. And finally, ZBCPs found in junctions of non-intentionally doped topological insulator films of Bi2Se3 and the s-wave superconductor NbN are attributed to proximity-induced px + ipy triplet order parameter in the topological material.This article is part of the theme issue 'Andreev bound states'.

Thermal properties of a cylindrical YBa2Cu3Ox superconductor in a levitation system: triggered by nonlinear dynamics

The vibration of a permanent magnet (PM) levitated upon a high temperature superconductor (HTS) shows anomalous motion under external disturbance. In this paper we construct a cantilevered beam experimental setup composed of a bulk PM and a thermally insulated cylindrical YBa2Cu3Ox superconductor. When the levitation system is disturbed by vertical excitation, the thermal character of the superconductor surface could be measured directly. Our experiments on a clean and large single-domain superconductor show that a giant temperature spike appears once the levitated PM experiences period doubling oscillation. We develop a numerical simulation for the analysis of the nonlinear vibration of the levitated PM coupled with the nonlinear electromagnetic force between the PM and HTS, taking into account heat diffusion. Using this procedure, we explore the electromagnetic and thermal properties at the thermally insulated HTS surface when the levitated PM shows a period doubling vibration. We find a remarkable difference between the experimental results and simulation. In order to interpret this temperature difference, we suggest a type of flux motion triggered by the electromagnetic force when it is far larger than the pinning force of the superconductor. The quantitative approach is based on the analysis process of the partial flux jump as a result of the flux creep. Finally, the calculated result is shown to be very close to the experimental result.

An Efficient and Accurate Method for HTS Surface Resistance Measurement

A new way to design resonator for efficiently measuring surface resistance (RS) of high-temperature superconductor (HTS) thin films with low uncertainty based on improved image dielectric method is developed in this paper. Simple formula is given to evaluate measurement error caused by the variation of tan δ of sapphire rod. According to the formula, this error could be decreased by choosing appropriate size of sapphire rod. Three resonators using improved image dielectric method were designed based on this new way. Interlaboratory round robin test was performed with the aim of investigating the measurement uncertainty. The round robin test shows that the relative uncertainties are no more than 12.6 % which is less than 20 % the relative uncertainty value specified in international standard on HTS thin film RS measurement. As far as the authors know, these are the most efficient devices to measure the RS of HTS thin film with equal uncertainty.

Effect of orientation of the heat-release surface on heat transfer to liquid nitrogen

The heat transfer from the surface of high-temperature superconductors to liquid nitrogen is studied. The effect of the heat flux density and heat-release surface orientation in the gravitational field on heat exchange characteristics is studied. It is shown that the heat transfer coefficient in the region of developed bubble boiling on a double-side cooled tape has a minimum at tilt angles close to 45°. The relations describing the dependence of the heat transfer coefficient on the heat-release surface orientation are derived.

Quantum oscillations and the Fermi surface of high-temperature cuprate superconductors

Abstract Over 20 years since the discovery of high temperature superconductivity in cuprates (Bednorz and Muller, 1986 [1] ), the first convincing observation of quantum oscillations in underdoped YBa2Cu3O6.5 (Doiron-Leyraud et al., 2007 [2] ) has deeply changed the theoretical landscape relevant to these materials. The Fermi surface is a basic concept of solid state physics, which underpins most physical properties (electrical, thermal, optical, etc.) of a metal. Even in the presence of interactions, this fundamental concept remains robust. While there was little doubt about the existence of a Fermi surface on the overdoped side of the phase diagram of the cuprates, the discovery of quantum oscillations in the underdoped regime was a surprise. The small pockets inferred from the measurements in underdoped YBa2Cu3Oy contrast with the large orbit found in overdoped Tl2Ba2CuO6 + δ. A central issue in understanding the phase diagram of high temperature superconductors is the origin of this difference at opposite sides of the superconducting dome. This review aims to shed light on this issue by bringing together recent results of quantum oscillation and transport measurements under high magnetic fields in hole-doped cuprates.

Study of HTS Bulk Current Lead With Metal Alloy Impregnation Under Vacuum

The high-temperature superconductor (HTS) bulk current lead (BCL) is a crucial device for a large number of current applications such as the propulsion motor with HTS winding. The coexistence of low thermal conductivity and superconductivity enables us to shrink the dimension of the power system. To use the BCL effectively, we have to optimize the temperature gradient across the BCL along its length. For this purpose, it is essential to obtain a better physical contact between the bulk HTS surface and Cu terminal leads. There are, however, many pores at the surface of the bulk which cause difficulties for joining. In order to solve this problem, we focused on an impregnation technique which has been known to improve strength and thermal characteristics of the bulk HTS itself by using epoxy. We succeeded to fill in the pores of the bulk HTS surface with a metal alloy impregnation. Following this method, we made the BCL of Gd-Ba-Cu-O by ourselves. By adopting a suitable cooling with a GM cryocooler, the resistance of the BCL was 0.8 muOmega across the BCL under a current flow of 300 A, considering one end was at 40 K and the other one was at 70 K. The properties of the present BCL are satisfactory for connecting a coil in large current power systems such as superconducting motors.

Studies of physico-chemical properties and fractal dimensions of selected high-temperature superconductor surfaces

Properties relating to porosity of solids (fractal dimensions, surface roughness parameters) were evaluated from atomic force microscopy (AFM) and nitrogen adsorption-desorption isotherms measured at 77 K for selected high-temperature [(RE) Ba2Cu3O7−x, RE=Y, Sm] superconductors. Adsorption capacity, specific surface area, fractal dimensions were determined from adsorption-desorption isotherms. The adsorption isotherms of all samples were S-shaped and belong to type II according to the IUPAC classification. A linear relationship was demonstrated between the fractal coefficients calculated by using the two methods and values of adsorption capacity of monolayer.

In situ doping control of the surface of high-temperature superconductors

In copper-oxide superconductors, charge carriers must be added to the insulating ‘parent’ compound before superconductivity appears. Exactly how the dopants affect the crystalline surface and evolving Fermi surface is now clear.

Electrodeposition of Silver Gold Alloys on ${\rm Bi}_{2}{\rm Sr}_{2}{\rm CaCu}_{2}{\rm O}_{8+\delta}$ Ceramics

A metallic coating on the surface of bulk high temperature superconductors (HTS) is a promising alternative in order to increase the HTS stability for large scale applications. The coating provides an evacuation path for the heat generated by hot spots within the HTS, which improves their thermal stability. This is even more evident in thick films geometries, which are frequently used in fault current limiters. In this paper, silver-gold alloys have been deposited on 1.8 mm in diameter Bi-2212 textured bars using an electrodeposition technique in non-aqueous solvents. The deposition parameters have been adjusted to obtain a coating of different alloys with gold contents in the range 0-5 at % and with a thickness up to 45 mum. Electrical and thermal measurements have been performed. The results show the possibility of controlling the thermal and electrical conductivities of the composite material by adjusting the layer thickness and composition of the metallic coating.

Studies of heterogeneity properties of selected high-temperature superconductor surfaces

Abstract Nitrogen adsorption measured at 77 K was used to characterize the surface heterogeneity of high-temperature superconductor surfaces. Properties relating to adsorption and porosity of the solids (adsorption capacity, specific surface area, radii and volume of the pores, pore-size distribution function) were determined from nitrogen adsorption–desorption isotherms and atomic force microscopy (AFM) for a series of oxide superconductors. It is shown that the adsorption isotherms of all samples are S-shaped and belong to type II according to the IUPAC classification. On the basis of the nitrogen adsorption isotherms and AFM data, fractal dimensions were determined and correlations found with adsorption and porosity parameters.

Self-assembled monolayer cleaning methods: Towards fabrication of clean high-temperature superconductor nanostructures

Although extensive amounts of research have been carried out on superconductor-normal metal-superconductor (SNS) electronic devices, the fabrication of superconductor SNS devices still remains difficult. Surface modification of high-temperature superconductors could be a way to control the interface of SNS electronic device fabrication. Here, we developed a cleaning method for thin films of high-temperature superconductor surface based on self-assembled monolayers. High-quality c-axis orientated YBa2Cu3O7−δ (i.e., YBCO) and Y0.6Ca0.4Ba1.6La0.4Cu3O7−δ (i.e., TX-YBCO) thin films were deposited by standard laser ablation methods. YBCO/Au/YBCO and TX-YBCO/Au/TX-YBCO planar type junctions were fabricated by photolithography, focused-ion-beam milling, and ex situ sputter depositions. A 40–50nm nanotrench was ion milled on the thin film by FIB, and a thin gold layer was deposited by an ex situ method on the nanotrench to connect the two separated high-temperature superconductor electrodes. SEM, AFM, and R vs T resistivity measurements were used to compare the corrosion layer formed in the interface of the SNS junctions with the SAM cleaned SNS junction. Evidence here suggests that the SAM cleaning method can be used to remove the degradation layer on the surface of cuprate superconductors. The obtained contact resistivity value, (10−8Ωcm2) for a SNS junction with SAM treatment is comparable with that of SNS junctions fabricated by the in situ methods.

Performance of Large-Size Superconducting Coil in 0.21T MRI System

A high-temperature superconductor (HTS) was used on magnetic resonance imaging (MRI) receiver coils to improve image quality because of its intrinsic low electrical resistivity. Typical HTS coils are surface coils made of HTS thin-film wafers. Their applications are severely limited by the field of view (FOV) of the surface coil configuration, and the improvement in image quality by HTS coil is also reduced as the ratio of sample noise to coil noise increases. Therefore, previous HTS coils are usually used to image small in vitro samples, small animals, or peripheral human anatomies. We used large-size HTS coils (2.5-, 3.5-, and 5.5-in mean diameter) to enhance the FOV and we evaluated their performance through phantom and human MR images. Comparisons were made among HTS surface coils, copper surface coils, and cool copper surface coils in terms of the signal-to-noise ratio (SNR) and sensitivity profile of the images. A theoretical model prediction was also used to compare against the experimental result. We then selected several human body parts, including the wrist, feet, and head, to illustrate the advantage of HTS coil over copper coil when used in human imaging. The results show an SNR gain of 200% for 5.5-in HTS coil versus same size copper coils, while for 2.5- and 3.5-in coils it is 250%. We also address the various factors that affect the performance of large size HTS coils, including the coil-to-sample spacing due to cryogenic probe and the coil-loading effect.

Corrosion and corrosion protection of epitaxialYB2Cu3O7−δ films quantified by studies of surface resistanceRs(T,Hrf) as a function of temperature and radio-frequency magnetic field

Radio-frequency (rf) superconducting devices are verysensitive to corrosion because of the small penetration depthsλ<300 nm of the rfmagnetic fields, Hrf. Qualification of high temperature superconductor (HTS) filters for launchinginto outer space requires resistance against corrosion for three weeks at40 °C and 93% relative humidity. A protective layer is essential to shieldthe HTS against these severe atmospheric conditions and to avoiddegradation of HTS films. Such degradation has been reduced by a thinBaF2/Y F3 protective layerproduced by soaking the YBa2Cu3O7−δ (YBCO) films in HF solution. Results on the surface resistanceRs(77 K,Hrf) of YBCO films of different quality before and after HF soaking, as well as after exposure toa highly humid atmosphere, are presented. They are explained by weak link corrosion bydeoxygenation, easing the permeation of Josephson fluxons. These nonlinear effects, edgeenhanced by strips and holes, ultimately limit the applications of strip line orTE011 modes, being quantitatively related to Josephson fluxon hysteresis losses causing localheating. These results on YBCO films shed new light on HTS material science andcorrosion, into thermal runaway of HTS and nonlinear surface impedances of HTS films,and their dependences on the rf mode.

Characterization of physicochemical properties of high-temperature superconductor surfaces using nitrogen adsorption

Nitrogen adsorption measured at 77K was used to characterize the energetic and geometrical heterogeneity of high-temperature superconductor surfaces. The porosity of the solids (specific surface area, radii and volume of the pores, as well as the pore-size distribution function) was determined from adsorption data for a series of oxide superconductors ((RE)1.85Ce0.15CuO4−δ, RE=Pr, Nd, Sm; (RE)Ba2Cu3O7−δ, RE=Y, Er; HgBa2Ca2Cu3O8+δ). It is shown that the adsorption isotherms of all samples are S-shaped and belong to group II according to the Brunauer–Emmett–Teller (BET) classification. On the basis of the isotherms of nitrogen adsorption, fractal dimensions were determined.

Influence of shape and thickness on the levitation force of YBaCuO bulk HTS over a NdFeB guideway

Levitation forces of YBaCuO bulk high temperature superconductors (HTS) with different shape and size over a NdFeB guideway were studied. Here, the concentrating magnetic field of the NdFeB guideway was 1.2 T, and the YBaCuO bulk HTSs include three cylindrical samples with different diameter and thickness and one hexagonal sample. The maximum levitation force is as high as 85.3 N at a gap of 5 mm between the bottom surface of YBaCuO bulk HTS and the top surface of the NdFeB guideway, where the applied magnetic field is about 0.8 T. The results show that the shape and the size have large influences on the levitation force of YBaCuO bulk HTSs.

Electric field effects at the surface of high-temperature superconductors

The results of experiments on the influence of electrostatic field (EF) on the electrophysical parameters of high-T C superconductors are presented. It was shown that action of the negative EF leads to an increase of critical temperature for about 6K in ceramic and about 0.6 K in thin film samples. The action of EF on the fluctuation excess conductivity in YBaCuO thin film was studied. It was shown that EF decreases the amplitude of thermodynamic fluctuations of the order parameter in the domain of 3D fluctuations. The negative EF applied to the film surface has been found to increase the activation energy of flux creep in relatively low magnetic fields. The upper critical magnetic field increases in the vicinity of T C under the influence of the EF.

Smoothing of YBa2Cu3O7−δ films by ion cluster beam bombardment

Smoothing high-temperature superconductor (HTS) surfaces, especially HTS thin-film surfaces, is crucial for HTS thin-film device processing. In this letter, we describe a method to planarize the surface of a YBa2Cu3O7−δ HTS film down to a smoothness with a standard deviation of 1 nm or better. The method includes first smoothing the HTS surface by ion cluster beam bombardment, followed by annealing in oxygen ambient to regrow the damaged surface layer. Additional YBCO layers can be grown epitaxially on the treated surface, even without removing the top surface layer, which contained some residual damage after annealing. This method can be integrated into HTS circuit fabrication as a key step of planarization.

Performance of superconducting microwave devices passivated with dielectric materials

We present a set of experiments which show that three dielectric processing variables in particular affect the performance of superconducting microwave devices: processing time and temperature, moisture content of the dielectric material, and surface interactions with the high temperature superconductor (HTS). The changes in microwave performance of a straight-line microstrip resonator before and after passivation were quantified by measurements of the loaded and unloaded quality factors for each resonator. Dielectric materials of varying moisture content were used. The dielectrics were processed at different times and temperatures. This study shows that the degradation of the microwave devices can be minimized by choosing dielectrics which (i) have a low moisture content, (ii) interact as little as possible with the HTS surface, and (iii) can be rapidly processed at relatively low temperatures.

‘Levitation’ of paramagnetic oxygen molecules over the surface of high temperature superconductors

The low temperature adsorption of molecular oxygen on a superconducting YBaCuO ceramic differs dramatically from the case of ordinary physical adsorption since it depends critically upon the electronic state of the adsorbent. When the adsorbent is in the superconducting state at 77 K, the amount of oxygen adsorbed is six times less than one for the adsorbent in the non-superconducting state at 94 K. The phenomenon also depends upon the external magnetic field. The abnormal oxygen adsorption on the superconductor seems to relate to the intrinsic paramagnetism of oxygen molecules, which can repulse from those regions of the adsorbent where the surface is absolutely diamagnetic.