Evolution of the upper critical field and superconducting vortex phase with thickness in PLD-grown Ta fllms

High quality superconducting thin films are the basis for the application of superconducting devices. Here we report the fllm growth and superconducting properties of the Ta films. The films were grown by the pulsed laser deposition technique on the α-Al2O3 substrates. It is found that, with the increase of the fllm thickness from 20 nm to 61 nm, both the superconducting transition temperature Tc and residual resistance ratio RRR display an upward trend, while the upper critical field decreases monotonously in a wide temperature region. A clear anisotropic behavior is revealed by comparing the upper critical fields with two difierent orientations (H ⊥ film and H // film). The anisotropy parameter Γ is found to be as high as 20 for the sample with the thickness of 20 nm. The systematical evolution from two- to three-dimensional features for the superconductivity with the increase of fllm thickness is observed in the temperature dependent upper critical fleld data. Moreover, the vortex liquid region tends to expand with the increase of the fllm thickness.

Using materials for quasiparticle engineering

The fundamental excitations in superconductors – Bogoliubov quasiparticles – can be either a resource or a liability in superconducting devices: they are what enables photon detection in microwave kinetic inductance detectors, but they are a source of errors in qubits and electron pumps. To improve operation of the latter devices, ways to mitigate quasiparticle effects have been devised; in particular, combining different materials quasiparticles can be trapped where they do no harm and their generation can be impeded. We review recent developments in these mitigation efforts and discuss open questions.

Superconductivity in the two-dimensional nonbenzenoid biphenylene sheet with Dirac cone

During the past decade, two-dimensional materials have attracted much attention in superconductivity due to their feasible physical properties and easy chemical modifications. Herein, we use a recently literature reported novel biphenylene sheet (BP sheet) for investigating superconductivity-related physical properties. The electronic states of BP sheet that appeared near the Fermi level are composed of pz orbital of carbon due to sp2 hybridization. Also, an anisotropic Dirac cone is formed just above the Fermi level by crossing two bands comprised of different carbon atoms. One of the two bands is quasi-flat thus leading to a peak of electronic density of states above the Fermi level. In addition, the rotational-vibration phonon mode of the six-membered carbon ring is strongly coupled with electrons. The electron-phonon coupling induces the superconductivity of 6.2 K in BP sheet. Furthermore, both small uniaxial strains and electronic doping can take the Dirac cone and high electronic density of state close to the Fermi level and further raise the superconducting critical temperature to 27.4 K and 21.5 K, respectively. The obtained result suggests that BP sheet with Dirac fermions and superconductivity can be a potential material for the development of future superconducting devices.

Hermite-Chebyshev pseudospectral method for inhomogeneous superconducting strip problems and magnetic flux pump modeling

Numerical simulation of superconducting devices is a powerful tool for understanding the principles of their work and improving their design. Usually, such simulations are based on a finite element method but, recently, a different approach, based on the spectral technique, has been presented for very efficient solution of several applied superconductivity problems described by one-dimensional integro-differential equations or a system of such equations. Here we propose a new pseudospectral method for two-dimensional magnetization and transport current superconducting strip problems with an arbitrary current-voltage relation, spatially inhomogeneous strips, and strips in a nonuniform applied field. The method is based on the bivariate expansions in Chebyshev polynomials and Hermite functions. It can be used for numerical modeling magnetic flux pumps of different types and investigating AC losses in coated conductors with local defects. Using a realistic two-dimensional version of the superconducting dynamo benchmark problem as an example, we showed that our new method is a competitive alternative to finite element methods.

Controllable rectification on the irreversible strain limit of 2G HTS coated conductors

The second generation high-temperature superconducting coated conductors (CCs) have excellent electrical and mechanical properties， and are extensively used in superconducting devices such as fault current limiters, magnets and motors. During the operation of these superconducting devices, superconducting CCs inevitably bear the combination of electromagnetic force and thermal mismatch stress, resulting in straining of YBCO layer along the tape length. It is well known that the strains of superconducting CCs cause degradation of critical current (Ic). Generally, the irreversible strain limit ( ) is used to characterize the phenomenon that Ic of superconducting CCs degrades with axial strain. When the axial strain of superconducting CCs is less than , Ic can be reversibly recovered by over 99% after being unloaded. Therefore, is a key parameter for the design and application of superconducting CC devices. For this reason, to carry out a practical engineering method for improving of superconducting CCs has become a challenge and aroused interests among researchers. This study is based on the idea of precompression. A 316LN stainless steel tape was pretensioned at 77K to improve its elastic strain limit. Then, two superconducting CCs were soldered onto both surfaces of pretensioned stainless steel tape respectively. As a result, of the superconducting CCs can be controlled manually with different precompressions. Taking YBa2Cu3O7-δ (YBCO) CCs produced by SuperPower Inc. as an example, the measurement results show that the of the YBCO CCs increased from 0.39% to 0.73%. Meanwhile, the thickness of the sample did not increase more than once.

Influences of Brass Surface Morphology on Leidenfrost Effect during Liquid Nitrogen Cooling

Cooling in liquid nitrogen is a typical service condition of high-temperature superconducting wire, and the variation of boiling stages on the wire protective layers such as the brass layers could be crucial for the quench behavior of superconducting devices. In this study, the influence of brass surface morphology (parameters of surface roughness and fractal dimension) on the Leidenfrost effect (including the wall superheat at critical heat flux and the wall superheat at Leidenfrost point, which are respectively characterized by the temperatures of ΔTCHF and ΔTLP) was studied. The surfaces of brass samples were polished by sandpaper to obtain different morphologies, which were characterized by using white light interferometer images, and the boiling curves were recorded and analyzed by Matlab with lumped parameter method. The experimental results demonstrated that the surface morphology of brass samples could influence the ΔTLP significantly, but had no clear relationship with the ΔTCHF. Moreover, the multi-scaled analysis was carried out to explore the influencing mechanism of surface microstructure, the relationship between ΔTLP and scale was more clear when the scale was small, and the fractal dimension was calculated and discussed together with surface roughness. The findings of this study could be instructive for surface treatment of superconducting wires to suppress quench propagation.

Considerations for Neuromorphic Supercomputing in Semiconducting and Superconducting Optoelectronic Hardware

Any large-scale spiking neuromorphic system striving for complexity at the level of the human brain and beyond will need to be co-optimized for communication and computation. Such reasoning leads to the proposal for optoelectronic neuromorphic platforms that leverage the complementary properties of optics and electronics. Starting from the conjecture that future large-scale neuromorphic systems will utilize integrated photonics and fiber optics for communication in conjunction with analog electronics for computation, we consider two possible paths toward achieving this vision. The first is a semiconductor platform based on analog CMOS circuits and waveguide-integrated photodiodes. The second is a superconducting approach that utilizes Josephson junctions and waveguide-integrated superconducting single-photon detectors. We discuss available devices, assess scaling potential, and provide a list of key metrics and demonstrations for each platform. Both platforms hold potential, but their development will diverge in important respects. Semiconductor systems benefit from a robust fabrication ecosystem and can build on extensive progress made in purely electronic neuromorphic computing but will require III-V light source integration with electronics at an unprecedented scale, further advances in ultra-low capacitance photodiodes, and success from emerging memory technologies. Superconducting systems place near theoretically minimum burdens on light sources (a tremendous boon to one of the most speculative aspects of either platform) and provide new opportunities for integrated, high-endurance synaptic memory. However, superconducting optoelectronic systems will also contend with interfacing low-voltage electronic circuits to semiconductor light sources, the serial biasing of superconducting devices on an unprecedented scale, a less mature fabrication ecosystem, and cryogenic infrastructure.

In situ transport characterization of magnetic states in Nb/Co superconductor/ferromagnet heterostructures

Employment of the non-trivial proximity effect in superconductor/ferromagnet (S/F) heterostructures for the creation of novel superconducting devices requires accurate control of magnetic states in complex thin-film multilayers. In this work, we study experimentally in-plane transport properties of microstructured Nb/Co multilayers. We apply various transport characterization techniques, including magnetoresistance, Hall effect, and the first-order-reversal-curves (FORC) analysis. We demonstrate how FORC can be used for detailed in situ characterization of magnetic states. It reveals that upon reduction of the external field, the magnetization in ferromagnetic layers first rotates in a coherent scissor-like manner, then switches abruptly into the antiparallel state and after that splits into the polydomain state, which gradually turns into the opposite parallel state. The polydomain state is manifested by a profound enhancement of resistance caused by a flux-flow phenomenon, triggered by domain stray fields. The scissor state represents the noncollinear magnetic state in which the unconventional odd-frequency spin-triplet order parameter should appear. The non-hysteretic nature of this state allows for reversible tuning of the magnetic orientation. Thus, we identify the range of parameters and the procedure for in situ control of devices based on S/F heterostructures.