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At least 1,099 records · Page 61
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Calculation of AC Magnetizing Loss of ReBCO Superconducting Tapes Subjected to Applied Distorted Magnetic Fields

In recent years, there are fast-increasing concerns on the utilizations of superconducting rotating electrical machines in different application areas, such as ship propulsion systems, aircraft drivers, and wind turbine generators, since these machines exhibit the merits of high current density, compact design, high power density, light weight, high torque density as well as high efficiency. One of the main limitations in front of the vast use of superconducting tapes in the fabrication of electrical machineries is AC magnetizing loss when tapes are exposed to an external magnetic field, which can decrease the critical current density of wires, as well. In the literature, most of the research works have been done on calculation of the AC magnetizing loss under a pure external magnetic field, while in reality, magnetic flux lines in AC electrical machines are usually distorted with harmonics because of different reasons such as distorted leakage flux, distributed coils of a winding in several slots, cogging fields, mechanical faults, etc. Since these distorted fields contain harmonics, then in this paper, the AC magnetizing loss of superconducting tapes has been electromagnetically modeled and calculated when they are subjected to nonsinusoidally distorted external magnetic fields. The magnetic field dependency of critical current density has been considered in a proposed finite element model. The results have shown that the AC magnetizing loss increases significantly under a distorted applied field compared with a sinusoidal one. In addition, the loss increase depends on the harmonic content which would increases drastically with total harmonic distortion of the applied magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Magnetic and Thermal Design of HTS Quadrupole Magnet for Newly Developed Superconducting Proton Cyclotron Beam Line

A magnetic and thermal design of high-temperature superconducting (HTS) quadrupole magnet for newly developed superconducting proton cyclotron was presented in this research. With superconducting technology, the design can reduce the magnet size and remove the heat loads more efficiently. Calculations are conducted with finite element method (FEM) to study the quadrupole magnet’s magnetic and thermal properties. For the magnet cold-mass and cryostat system, the heat load is mainly generated by conduction and radiation. Multilayer thermal insulation and G10 supports are used to restrict them. The magnetic field distribution in the HTS coils is calculated to consider the critical current degradation of the YBCO tapes. The effects on the field gradient quality in relation to the pole tip profile and end chamfer are analyzed. The sixth multipole component in the integral field can be improved with end chamfer. Finally, a 20 T/m HTS quadrupole magnet with integral field uniformity less than 0.05% out to 75% of the inscribed radius is proposed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Superconductivity in Oxides Generated by Percolating Electron or Hole Bipolarons

It is remarkable that in over three decades since the discovery of high-temperature superconductivity (HTS) in cuprates, no other class of materials has been found to exhibit this property above the liquid nitrogen boiling point at ambient pressure. Here, we describe the conformation of hole and electron bipolarons in doped cuprates and tungstates. Important similarities and differences between them are discussed. Both hole and electron bipolarons can percolate forming filaments or clusters in the bulk or on the surfaces as well. Upon cooling, these electronic objects can show superconducting properties. It is proposed that oxygen-reduced tungsten oxide WO{sub 3−x} in bulk or thin film form is a promising material to explore the HTS. Such material would be another example of bipolaronic superconductivity similar to cuprates. However, there is also an important difference: in the cuprates, holes predominantly enter the oxygen orbitals and hole bipolarons are composed of three-spin polarons, whereas in the tungstates, doped electrons are located on tungsten ions and W{sup 5+}–O{sup 2−}–W{sup 5+}electron bipolarons form from 5d{sup 1} W{sup 5+} polarons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Pair Breaking, Pseudogap, and Superconducting T{sub c} of Hole-Doped Cuprates: Interrelations and Implications

Irrespective of the class they belong to, all the hole-doped high-transition temperature (T{sub c}) cuprate superconductors show an anticorrelation between the superconducting transition temperature and the characteristic pseudogap energy in the underdoped region. The doping-dependent pseudogap in the quasiparticle spectral density is believed to remove low-energy electronic states and thereby reduce the superconducting condensate. Impurities within the CuO{sub 2} plane, on the other hand, break Cooper pairs in the unitarity limit and diminish superfluid density. Both pseudogap in pure cuprates and impurity scattering in disordered cuprates reduces T{sub c} very effectively. In this study, we have compared and contrasted the mechanisms of T{sub c} degradation due to pseudogap and impurity scattering in hole-doped cuprates. We have suggested a framework where both these factors can be treated on somewhat equal footing. Beside impurity and pseudogap-dependent superconducting transition temperature, the proposed scenario has been employed to investigate the disorder and hole content-dependent isotope exponent in high-T{sub c} cuprates in this work.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Superconducting Coplanar Waveguide Resonators Capable of Cofabrication with Josephson Junctions

Superconducting coplanar waveguide resonators and Josephson junctions are crucial components of superconducting quantum information processing chips. Resonators are usually fabricated through lift-off or etching, while Josephson junctions are made by trilayer process or double-angle evaporation. To simplify the fabrication process, we propose a scheme that uses the same process steps for both Josephson junctions and resonators. Simulations and experiments are conducted to confirm that resonators fabricated via the proposed process have quality factor comparable to those via traditional process. This work paves the way for quantum information processing on superconducting large-scale integrated circuits.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Evaluation of Superconducting Properties and Diffusion Behavior of Ex Situ and In Situ Bulk MgB{sub 2} Materials with Ni Coating

Ex situ and in situ bulk MgB{sub 2} superconducting materials have successfully been produced by the simple amorphous boron and nano-amorphous boron powders with the processes of ball milling, pressing, and annealing. The superconducting properties and diffusion behavior of MgB{sub 2} samples after nickel (Ni) coating process have been characterized and compared to the microstructure and performance of uncoated (bare) MgB{sub 2} bulk sample. One surface of MgB{sub 2} superconductor sample was coated with a thin Ni layer of about 50 − 60μm thickness using vapor deposition techniques in versatile high vacuum coater, then every sample was annealed at temperatures between 923 and 1123 K for 1 h. The role of annealing temperature on physical, electrical, superconducting, and structural characterizations of bare and Ni-coated ex situ/in situ MgB{sub 2} bulk superconductors has been studied using X-ray diffraction and dc electrical resistivity versus temperature measurements. Finally, the Ni diffusion coefficients (D{sub Ni} = D{sub 0}exp(E/k{sub B}T)) are calculated in the temperature range to calculate the required minimum activation energy value for the Ni atoms/ions into the ex situ MgB{sub 2} crystal structure for the first time.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Effect of Zinc on the Structural, Electrical, and Mechanical Properties of YBCO-123 Superconducting Nanoparticles Prepared by an Acetate-Based Sol-Gel Process

In this study, zinc (Zn)-doped YBaCuO (YBCO)-123-based high-temperature superconducting samples were produced using the sol-gel method, which is commonly used for preparing nanosize materials. Zn ions were substituted by Cu ions (YBa{sub 2}Cu{sub 3-x}Zn{sub x}O), and Zn doping effects on structural, electrical, and mechanical properties of the YBCO-123 superconductors were examined in detail. Undoped sample was prepared under same conditions. X-ray diffraction analysis (XRD) was used to determine phase analysis and lattice parameters of the superconducting samples. To analyze the microstructure properties, scanning electron microscope (SEM) measurements were performed. Resistivity and microhardness measurements were also carried out for superconducting and mechanic properties, respectively. Vickers microhardness, young modulus, fracture toughness, and yield strength values of the samples were calculated. Microhardness measurements were analyzed using the Kick’s law, proportional sample resistance (PSR) model, EPD model, the Hays-Kendall (HK) approach, and indentation-induced cracking (IIC) model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Correlating aluminum layer deposition rates, Josephson junction microstructure, and superconducting qubits’ performance

An ultra-thin aluminum oxide layer is a key component for Josephson junctions (JJ) in superconducting quantum bits (qubits). This layer serves as a barrier layer for Cooper pairs tunneling between the superconducting electrodes and significantly influences the overall performance of the junction. In this study, we investigate the impact of aluminum deposition rates on the microstructure and chemical variation of the aluminum oxide layer, as well as the device's yields and qubits’ lifetimes. Surprisingly, although the oxide layer roughness and thickness variation improve noticeably with an increasing Al deposition rate from 0.5 Å/s to 5 Å/s, the qubit's coherence time is almost unchanged. We attribute this to the fact that Cooper pairs tunnel only through the thinnest region of the barrier. Additionally, we revealed a stress-induced grain boundary sliding short-circuit failure mechanism in the JJ. Our discovery provides a vital understanding of oxide microstructure and JJ functionality, which are critical for improving the coherence time of superconducting qubits.

Coherence time↗
DOE OSTI2020

Moiré superconductivity

Recently, superconductivity was discovered at very low densities in slightly misaligned graphene multilayers. Surprisingly, despite extremely low electronic density (about 10 –4 electrons per unit cell), these systems realize strong-coupling superconductivity, with the transition temperature being a large fraction of the Fermi energy $(T_C ~ 0.1_{ϵ_{F}})$. In this work, we propose a qualitative explanation for this remarkable phenomenon, highlighting similarities and qualitative differences with the conventional uniform high-density superconductivity. Most importantly, we find that periodic superimposed potential generically enhances local interactions relative to nonlocal (for instance, Coulomb) interactions. In addition, the density of states is enhanced as well, exponentially in modulation strength for low lying bands in some cases. Combination of these two effects makes moiré systems natural intermediate or strong-coupled superconductors, with potential for very high transition temperatures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Noncontact Andreev Reflection as a Direct Probe of Superconductivity on the Atomic Scale

Direct detection of superconductivity has long been a key strength of point-contact Andreev reflection. However, its applicability to atomic-scale imaging is limited by the mechanical contact of the Andreev probe. To this end, we present a new method to probe Andreev reflection in a tunnel junction, leveraging tunneling spectroscopy and junction tunability to achieve quantitative detection of Andreev scattering. This method enables unambiguous assignment of superconducting origins of current-carrying excitations, as well as detection of higher order Andreev processes in atomic-scale junctions. Herein, we furthermore revealed distinct sensitivity of Andreev reflection to natural defects, such as step edges, even in classical superconductors. The methodology opens a new path to nano- and atomic-scale imaging of superconducting properties, including disordered superconductors and proximity to phase transitions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Unraveling Enhanced Superconductivity in Single-Layer FeSe through Substrate Surface Terminations

Single-layer FeSe on SrTiO 3 (001) substrates shows a superconducting transition temperature much higher than that of bulk FeSe, which has been attributed to factors such as electron doping, interfacial electron–phonon coupling, and electron correlations. To pinpoint the primary driver, we grew single-layer FeSe films on SrTiO 3 (001) substrates with coexisting TiO 2 and SrO surface terminations. Scanning tunneling spectroscopy revealed a larger superconducting gap (17.0 meV) on the TiO 2 -termination than on the SrO-termination (10.5 meV). Tunneling spectroscopy also showed a larger work function on the SrO surface, resulting in reduced charge transfer to FeSe, as confirmed by angle-resolved photoemission spectroscopy. Scanning transmission electron microscopy further revealed distinctive interfacial atomic-scale structures, with the Se–Fe–Se tetrahedral angle changing from 109.5° on the SrO-termination to 104.9° on the TiO 2 -termination. Compared to dynamical mean field theory calculations, our results indicate that enhanced superconductivity in single-layer FeSe/TiO 2 arises from optimal electron correlations, in addition to sufficient charge transfer from the substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Superconductivity in the Narrow-Gap Semiconductor CsBi 4 Te 6

Superconductivity was discovered in the narrow-gap semiconductor CsBi4Te6. A superconducting transition around 4.4 K was observed for p-type samples in temperature-dependent resistivity and magnetic susceptibility data Stoichiometric CsBi4Te6 is not a superconductor. A remarkably high critical field of similar to 10 T was estimated from the field dependent resistivity data The strongly anisotropic CsBi4Te6 system is monoclinic and the first member of a larger homologous series Cs-4[Bi2n+4Te3n+6] that exhibits unconventional superconductivity, suggesting that proper doping of the homologous series may create a novel class of superconductors from semiconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Giant phonon anomaly associated with superconducting fluctuations in the pseudogap phase of cuprates

The pseudogap in underdoped cuprates leads to significant changes in the electronic structure, and was later found to be accompanied by anomalous fluctuations of superconductivity and certain lattice phonons. Here we propose that the Fermi surface breakup due to the pseudogap, leads to a breakup of the pairing order into two weakly coupled sub-band amplitudes, and a concomitant low energy Leggett mode due to phase fluctuations between them. This increases the temperature range of superconducting fluctuations containing an overdamped Leggett mode. In this range inter-sub-band phonons show strong damping due to resonant scattering into an intermediate state with a pair of overdamped Leggett modes. In the ordered state, the Leggett mode develops a finite energy, changing the anomalous phonon damping into an anomaly in the dispersion. Finally, this proposal explains the intrinsic connection between the anomalous pseudogap phase, enhanced superconducting fluctuations and giant anomalies in the phonon spectra.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures

Search for Majorana fermions renewed interest in semiconductor–superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime. Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor–superconductor ohmic contacts. Supercurrent with characteristic temperature dependence of a ballistic junction has been observed across 0.6 μm, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Direct Spectroscopic Evidence for Phase Competition between the Pseudogap and Superconductivity in Bi 2 Sr 2 CaCu 2 O 8+δ

In the high-temperature (T c ) cuprate superconductors, increasing evidence suggests that the pseudogap, existing below the pseudogap temperature T*, has a distinct broken electronic symmetry from that of superconductivity. Particularly, recent scattering experiments on the underdoped cuprates have suggested that a charge ordering competes with superconductivity. However, no direct link of this physics and the important low-energy excitations has been identified. We report an antagonistic singularity at Tc in the spectral weight of Bi 2 Sr 2 CaCu 2 O 8+δ as a compelling evidence for phase competition, which persists up to a high hole concentration p ~ 0.22. Comparison with a theoretical calculation confirms that the singularity is a signature of competition between the order parameters for the pseudogap and superconductivity. Our observation of the spectroscopic singularity at finite temperatures over a wide doping range provides new insights into the nature of the competitive interplay between the two intertwined phases and the complex phase diagram near the pseudogap critical point.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Tunable spin polarization and superconductivity in engineered oxide interfaces

Advances in growth technology of oxide materials allow single atomic layer control of heterostructures. In particular delta doping, a key materials’ engineering tool in today’s semiconductor technology, is now also available for oxides. Here we show that a fully electric-field-tunable spin-polarized and superconducting quasi-2D electron system (q2DES) can be artificially created by inserting a few unit cells of delta doping EuTiO 3 at the interface between LaAlO 3 and SrTiO 3 oxides. Spin polarization emerges below the ferromagnetic transition temperature of the EuTiO 3 layer (T FM = 6–8 K) and is due to the exchange interaction between the magnetic moments of Eu-4 f and of Ti-3 d electrons. Moreover, in a large region of the phase diagram, superconductivity sets in from a ferromagnetic normal state. As a result, the occurrence of magnetic interactions, superconductivity and spin–orbit coupling in the same q2DES makes the LaAlO 3 /EuTiO 3 /SrTiO 3 system an intriguing platform for the emergence of novel quantum phases in low-dimensional materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Visualizing heavy fermion confinement and Pauli-limited superconductivity in layered CeCoIn 5

Layered material structures play a key role in enhancing electron–electron interactions to create correlated metallic phases that can transform into unconventional superconducting states. The quasi-two-dimensional electronic properties of such compounds are often inferred indirectly through examination of bulk properties. Here we use scanning tunneling microscopy to directly probe in cross-section the quasi-two-dimensional electronic states of the heavy fermion superconductor CeCoIn 5 . Our measurements reveal the strong confined nature of quasiparticles, anisotropy of tunneling characteristics, and layer-by-layer modulated behavior of the precursor pseudogap gap phase. In the interlayer coupled superconducting state, the orientation of line defects relative to the d-wave order parameter determines whether in-gap states form due to scattering. Spectroscopic imaging of the anisotropic magnetic vortex cores directly characterizes the short interlayer superconducting coherence length and shows an electronic phase separation near the upper critical in-plane magnetic field, consistent with a Pauli-limited first-order phase transition into a pseudogap phase.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Directly visualizing the sign change of d-wave superconducting gap in Bi 2 Sr 2 CaCu 2 O 8+δ by phase-referenced quasiparticle interference

The superconducting state is formed by the condensation of Cooper pairs and protected by the superconducting gap. The pairing interaction between the two electrons of a Cooper pair determines the gap function. Thus, it is pivotal to detect the gap structure for understanding the mechanism of superconductivity. In cuprate superconductors, it has been well established that the gap may have a d-wave function. This gap function has an alternative sign change in the momentum space. It is however hard to visualize this sign change. Here we report the measurements of scanning tunneling spectroscopy in Bi 2 Sr 2 CaCu 2 O 8+δ and conduct the analysis of phase-referenced quasiparticle interference (QPI). We see the seven basic scattering vectors that connect the octet ends of the banana-shaped contour of Fermi surface. The phase-referenced QPI clearly visualizes the sign change of the d -wave gap. Lastly, our results illustrate an effective way for determining the sign change of unconventional superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗