Temperature Dependence Of Critical Current Research Articles

Multiple andreev reflections in topological insulator nanoribbons

Superconducting proximity junctions made of topological insulator (TI) nanoribbons (NRs) provide a useful platform for studying topological superconductivity. We report on the fabrication and measurement of Josephson junctions (JJs) using Sb-doped Bi2Se3 NRs in contact with Al electrodes. Aharonov–Bohm and Altshuler–Aronov–Spivak oscillations of the axial magneto-conductance of TI NR were observed, indicating the existence of metallic surface states along the circumference of the TI NR. We observed the supercurrent in the TI NR JJ and subharmonic gap structures of the differential conductance due to multiple Andreev reflections. The interface transparency of the TI NR JJs estimated based on the excess current reaches τ = 0.83, which is among the highest values reported for TI JJs. The temperature dependence of critical current is consistent with the short and ballistic junction model confirming the formation of highly transparent superconducting contacts on the TI NR. Our observations would be useful for exploring topological Josephson effects in TI NRs.

A robust nitridation technique for fabrication of disordered superconducting TiN thin films featuring phase slip events

Disorder induced phase slip (PS) events appearing in the current voltage characteristics (IVCs) are reported for two-dimensional TiN thin films produced by a robust substrate mediated nitridation technique. Here, high temperature annealing of Ti/Si3N4 based metal/substrate assembly is the key to produce majority phase TiN accompanied by TiSi2 & elemental Si as minority phases. The method itself introduces different level of disorder intrinsically by tuning the amount of the non-superconducting minority phases that are controlled by annealing temperature (Ta) and the film thickness. The superconducting critical temperature (Tc) strongly depends on Ta and the maximum Tc obtained from the demonstrated technique is about 4.8 K for the thickness range ~ 12 nm and above. Besides, the dynamics of IVCs get modulated by the appearance of intermediated resistive steps for decreased Ta and the steps get more prominent for reduced thickness. Further, the deviation in the temperature dependent critical current (Ic) from the Ginzburg–Landau theoretical limit varies strongly with the thickness. Finally, the Tc, intermediate resistive steps in the IVCs and the depairing current are observed to alter in a similar fashion with Ta and the thickness indicating the robustness of the synthesis process to fabricate disordered nitride-based superconductor.

Photoresponse of current-biased superconductor/normal metal strip with large ratio of resistivities

We study the perspective of thin bilayer strips made of highly resistive (in the normal state) superconductor (S) and low resistive normal metal (N) as a detector of electromagnetic radiation. A small minigap in the quasiparticle spectra, induced in the N layer due to proximity induced superconductivity from the S layer and the ability of low resistive N layer to carry a larger superconducting current than the host dirty superconductor leads to steep temperature dependence of critical current of SN strip in wide temperature ranges. It results in higher sensitivity of current-biased SN strip to the electromagnetic radiation in comparison with S strip. We present experimental results on several microns wide MoN/Cu and MoN strips, which confirm this statement in the bolometric regime. We also discuss possible advantages of single photon counting by narrow SN strips.

Temperature dependence of critical currents in REBCO thin films with artificial pinning centers

Conventionally, δTc type (order parameter modulation) and δl type (mean free path modulation) pinning mechanisms have been proposed to explain the temperature dependence of the flux pinning of superconducting materials. According to previous studies, it is assumed that the temperature dependence of Jc of REBa2Cu3O7 (REBCO, RE = Y, Gd, Sm, etc) films without artificial pinning centers (APCs) is δl type, but it is unidentified when APCs are introduced into the films. In this paper, GdBCO thin films doped with BaHfO3 (BHO) deposited on LaAlO3 substrates by pulsed laser deposition were studied. A target exchange method was used to alternately ablate two targets of pure GdBCO and BHO for introducing nanorods as APCs into GdBCO films. Since the insulative BHO acts as a strong pinning center, the δTc pinning mechanism is expected for the temperature dependence of Jc of these thin films. However, the experimental results showed that the Jc of the films with BHO nanorods was determined by the δl pinning mechanism over a wide temperature range. In order to explain these unexpected results, we examined the pinning mechanism by nanorods based on a resultant pinning force model.

Performance Test of an 8 kA @ 10-T 4.2-K ReBCO-CORC Cable

CERN is developing high-current ReBCO conductor on round core (CORC) cables for application in future detector and accelerator magnets. A characterization test on a ReBCO-CORC cable sample and its joints is performed in the 10-T FRESCA cable test facility at CERN. The sample is taken from the first 12-m-long CORC production. Key is the characterization of the field- and temperature-dependent critical currents of the CORC cable at 1.9 K and 4.2 K. Secondary objectives include evaluating the response of the CORC cable to quenches and the performance of cylindrical low resistive cable terminals especially designed and manufactured for use on CORC cables. The 7.6-mm CORC cable features 8 kA at 4.2 K and 10 T, and the joint terminals show a $25\pm 5-{n}\Omega$ resistance for 20-cm length.

Double disordered YBCO coated conductors of industrial scale: high currents in high magnetic field

A significant increase of critical current in high magnetic field, up to 31 T, was recorded in long tapes manufactured by employing a double-disorder route. In a double-disordered high-temperature superconductor (HTS), a superimposing of intrinsic and extrinsic disorder takes place in a way that (i) the intrinsic disorder is caused by local stoichiometry deviations that lead to defects of crystallinity that serve as pining centers in the YBa2Cu3Ox−δ matrix and (ii) the extrinsic disorder is introduced via embedded atoms or particles of foreign material (e.g. barium zirconate), which create a set of lattice defects. We analyzed possible technological reasons for this current gain. The properties of these tapes over a wider field-temperature range as well as field anisotropy were also studied. Record values of critical current as high as 309 A at 31 T, 500 A at 18 Tm and 1200 A at 5 T were found in 4 mm wide tape at 4.2 K and B perpendicular to tape surface. HTS layers were processed in medium-scale equipment that allows a maximum batch length of 250 m while 22 m long batches were provided for investigation. Abnormally high ratios (up to 10) of critical current density measured at 4.2 K, 19 T to critical current density measured at 77 K, self-field were observed in tapes with the highest in-field critical current. Anisotropy of the critical current as well as angular dependences of n and α values were investigated. The temperature dependence of critical current is presented for temperatures between 4.2 and 40 K. Prospects for the suppression of the dog-bone effect by Cu plating and upscale of processing chain to >500 m piece length are discussed.

A Nanoscale-Localized Ion Damage Josephson Junction Using Focused Ion Beam and Ion Implanter.

High-T(c) Josephson junctions were fabricated by nanolithography using focused ion beam (FIB) milling and ion implantation. The junctions were formed in a YBa2Cu3O7-x, thin film in regions defined using a gold-film mask with 50-nm-wide (top) slits, engraved by FIB. The focused ion beam system parameters for dwell time and passes were set to remove gold up to a precise depth. 150 keV oxygen ions were implanted at a nominal dose of up to 5 x 10(13) ions/cm2 into YBa2Cu3O7-x microbridges through the nanoscale slits. The current-voltage curves of the ion implantation junctions exhibit resistive-shunted-junction-like behavior at 77 K. The junction had an approximately linear temperature dependence of critical current. Shapiro steps were observed under microwave irradiation. A 50-nm-wide slit and 0-20-nm-thick buffer layers were chosen in order to make Josephson junctions due to the V-shape of the FIB-milled trench.

Superconducting transition properties of grain boundaries in MgB2 films

The intergrain interface of unconventional superconductors suppresses the order parameter and, thus, serves as a bottleneck for the supercurrent. Such a characteristic of the grain interface is a major limiting factor for the lossless-current-carrying capacity of superconducting transmission lines on the one hand, but can also provide a clue to the development of Josephson elements on the other. We investigated the critical transition properties of the MgB2 intergrain nanobridges patterned by using a focused ion beam. The nanobridge containing a single intergrain interface had nominal dimensions of 200 ȁ 300 nm in width and length. We studied the current-voltage characteristics, the temperature-dependent critical current, and the normal state resistance. The measured MgB2 interface properties showed a wide spectrum of the coupling strength, ranging from a weak Josephson coupling to a very strong coupling comparable to an intragranular one. The origins of such a variety of intergrain coupling strengths were analyzed in conjunction with the lattice structure at the grain interface.

Electromigration kinetics and critical current of Pb-free interconnects

Electromigration kinetics of Pb-free solder bump interconnects have been studied using a single bump parameter sweep technique. By removing bump to bump variations in structure, texture, and composition, the single bump sweep technique has provided both activation energy and power exponents that reflect atomic migration and interface reactions with fewer samples, shorter stress time, and better statistics than standard failure testing procedures. Contact metallurgies based on Cu and Ni have been studied. Critical current, which corresponds to the Blech limit, was found to exist in the Ni metallurgy, but not in the Cu metallurgy. A temperature dependence of critical current was also observed.

Fabrication of BaKFeAs intergrain nanobridges by using a focused ion beam

We have studied the fabrication and the transport properties of Ba0.6K0.4Fe2As2 intergrain nanobridges patterned by using a focused ion beam (FIB). Prior to FIB etching, 8-µm-wide bridges were prepatterned from Ba0.6K0.4Fe2As2 films by using argon ion milling with a photoresist mask. The lowest-possible beam current of 1.5 pA was used for the FIB nanobridge pattern to minimize the etching damage to the bridge. The nanobridge contained a single grain boundary, and the nominal dimensions were 200 nm in width and 100 nm in length. We have also studied current-voltage (I-V) characteristics, the temperature-dependent critical current (Ic), and the normal-state resistance (RN). The transport measurements of the nanobridge showed a strong-coupling nature with a high critical current density of 1.25 × 106 A/cm2 at 4.2 K, which is comparable to the intragrain value. The nanobridge showed the onset of a resistive transition at 37 K and zero resistance at 27 K. Measured I-V curves were dominated mainly by Josephson coupling, showing resistivelyshunted-junction (RSJ) behaviors with multiple transitions at low temperatures. The temperature dependence of the critical current was Ic ∼ (1 − T/Tc)1.0.

A predictive model of the temperature dependence of AC transport losses in(Bi, Pb)2Sr2Ca2Cu3Ox tapes

Critical currents and AC losses of(Bi, Pb)2Sr2Ca2Cu3Ox superconducting tapes were measured in self-field as a function of temperature. Theexperimental data of the temperature dependence of critical current were compared withcalculated results. An approach to calculating AC losses as a function of temperature wasdeveloped and the calculated AC losses were compared with the measured data. The studyshows that AC losses at any temperature can be estimated using the model from thecritical parameters or from the measured AC loss factor at a certain temperature, such as77 K.

Influence of the ferromagnetic layer on the pair breaking and low alternating current field magnetic response in superconductor/ferromagnet bilayer

We investigate the influence of the ferromagnetic layer on the magnetic and transport properties of YBa2Cu3O7−δ in YBa2Cu3O7−δ (YBCO)/La0.7Sr0.3MnO3 (LSMO) bilayers. The temperature dependent dc magnetization study reveals the presence of magnetic anisotropy in YBCO/LSMO bilayer as compared to the pure YBCO layer. The ac susceptibility study on YBCO/LSMO bilayers reveals stronger pinning and the temperature dependent critical current is found to be less prone to temperature. Besides, the current (I) dependent electrical transport studies on YBCO/LSMO exhibit a significant reduction in the superconducting Tc with increase in I and it follows I2/3 dependence in accord with the pair breaking effect. The higher reduction of superconducting Tc in YBCO/LSMO is believed to be due to the enhanced pair-breaking induced by the spin polarized carriers being injected into the superconductor.

Properties of strong-coupled MgB2 nanobridges fabricated by focused ion beam

We have studied extremely strong-coupled MgB2 inter-grain nanobridges prepared by using focused ion beam. Nominal width and length of the bridges were 200nm. The superconducting transition temperature and the critical current density of the bridges were the same as those of unpatterned film. Current–voltage (I–V) curves showed large excess currents, which is believed to be due to the flux-flow effect. Nevertheless, I–V curves showed substantial amount of Josephson coupling effect near Tc. The temperature-dependent critical current was Ic∼(Tc−T)1.5, similar to that of a thin superconducting film in Ginzburg–Landau theory.

MgB 2 grain boundary nanobridges prepared by focused ion beam

We have fabricated MgB2 grain boundary nanobridges by focused-ion-beam etch and studied their transport properties. Nanobridges with a nominal width and length of 100 nm were patterned across naturally formed single grain boundaries in the microbridges, which were prepatterned by a standard argon ion milling technique. We have studied current-voltage (I-V) characteristics, the temperature-dependent critical current, and the normal-state resistance. The measured properties were interpreted based on a flux flow model. In the I-V curves, a typical resistively shunted-junction characteristic was observed near Tc, however, as temperature decreases, flux-flow behavior became dominant, in accordance with the crossover of the ratio of the bridge length to the coherence length from the single-phased regime to the flux-flow regime. The temperature-dependent critical current was Ic(T)∼(1−T/Tc)1–1.5, similar to that of a superconducting film. The normal-state resistance increased steeply as temperature approaches Tc, in agreement with the flux-flow theory.

Field and temperature dependence of critical currents in industrially manufactured high-Tc wires and tapes

In the frame of the European Fusion Technology Programme, CRPP Villigen evaluated the performance of currently available commercial High-Tc wires and tapes in view of their suitability for magnet coils in future fusion reactors. We measured the critical current of Bi-2212 round wire and Bi-2223 tapes from two different manufacturers at temperatures up to 77 K. Magnetic fields up to 12 T were applied parallel as well as normal to the plane of the tapes (always normal to the current direction) to investigate the materials' anisotropy. At temperatures up to about 50 K and in sufficiently high fields we observed that the critical current decreases exponentially with the magnetic field in the 2223 tapes. In the same regime the Ic values were significantly different for increasing and decreasing field (hysteresis), most likely related to the different magnetisation states of the samples and therefore the local magnetic fields at the grain boundaries. High-Tc tapes and particularly round wires are promising candidates for conductors in fusion magnets because of their excellent high-field properties. They also show potential for the use at temperatures above 4.2 K in the 12 to 15 T range if cooling between 20 and 50 K became an attractive alternative to helium-based refrigeration. Our measurements show that the required current densities are already available industrially.

Superconducting nanobridge made from YBa 2Cu 3O 7 film by using focused ion beam

We have fabricated a superconducting nanobridge from an YBa 2Cu 3O 7 thin film by a focused ion beam method. The bridge was patterned from a 3-μm-wide YBa 2Cu 3O 7 line which was predefined by argon ion milling with a photoresist mask. A 300-nm-wide and 200 nm-thick bridge was patterned by milling nanotrenches in the superconducting bridge. Current–voltage characteristics showed typical resistively-shunted-junction properties. The temperature-dependent critical current was I c( T) ∼ (1 − T/ T c) 1 and the normal state resistance R N( T) increased with decreasing temperature, both of which are typical to a superconductor–insulator–superconductor junction. Those characteristics are believed to be due to naturally formed grain boundaries crossing the bridges.

Temperature dependence of critical currents of two-gap superconductors

In this paper, we consider the two-gap Ginzburg-Landau (G-L) free energy functional including the interactions, and obtain a very simple and explicit formula which presents the relation between the critical current density and temperature. The result shows that the temperature dependence of critical current density of a two-gap superconductor is of the form Jc ∝ (Tc* − T)1/2 at low temperatures and Jc ∝ (Tc* − T)3/2 near the critical temperature. Our critical current density expression is in accord with the previous theoretical work and experimental data.

Single-layer 2nd-order SQUID gradiometer and fabrication of YBa2Cu3O7 nanobridges as the Josephson elements

We have studied fabrication of second-order SQUID gradiometers from single-layer of high-Tc. film. The gradiometer contains three parallel-connected pickup coils which are directly coupled to a step-edge junction SQUID. In the study of short-baseline devices, we achieved an unshielded noise of 0.84 pT/cm2/Hz½ at 1 Hz for a well-balanced gradiometer. As Josephson elements of lone-baseline devices we made submicron YBCO bridges by using a focused ion beam method. In temperature-dependent critical currents, Ic(T), and normal state resistances, RN(T), the bridge showed an SIS-type behavior, which is believed to be due to naturally formed grain boundaries crossing the bridges.

Plasticity in current-driven vortex lattices

We present a theoretical analysis of recent experiments on current-driven vortex dynamics in the Corbino disk geometry. This geometry introduces controlled spatial gradients in the driving force and allows the study of the onset of plasticity and tearing in clean vortex lattices. We describe plastic slip in terms of the stress-driven unbinding of dislocation pairs, which in turn contribute to the relaxation of the shear, yielding a nonlinear response. The steady state density of free dislocations induced by the applied stress is calculated as a function of the applied current and temperature. A criterion for the onset of plasticity at a radial location $r$ in the disk yields a temperature-dependent critical current that is in qualitative agreement with experiments.

Properties of SNS Josephson junctions with HfTi interlayers

The fabrication process in superconductor–normal metal–superconductor (SNS) technology has been improved to raise the characteristic voltage of SNS active elements. Josephson junctions with normal metal interlayers made from HfTi were realized in ramp-type design with dimensions down to the deep sub-micron range. To investigate the influence of the interfaces, junctions with different sequences of interim layers were fabricated, according to Nb/HfTi/Nb trilayer and Nb/HfTi–Nb–HfTi/Nb multilayer structures. Typical values of the characteristic voltage and the critical current density are V c =107 μ V and j c=550 kA/cm 2 for Nb/HfTi/Nb junctions and V c =460 μ V and j c=210 kA/cm 2 for Nb/HfTi–Nb–HfTi/Nb junctions, each for areas of A=0.2 μ m 2. The junctions were experimentally investigated for the temperature dependence of critical currents, the influence of externally applied magnetic fields, and the application of microwave power.