bluuebunny/crossref_metadata_2025 huggingface dataset:
Highly scalable readout electronics for large multi-channel dc-SQUID systems
Authors: Bechstein S. , Drung D. , Scheiner M. , Petsche F. , Ali Valiollahi S. , Körber R. , Barthelmess H. J.
Published: June 2020

SQUID instruments for biomagnetic diagnostics, radiometry and astrophysics are often equipped with a large number of devices. For systems with more than a few tens of sensors, read-out with an equivalent number of independent components is not an appropriate solution to meet both, technical and financial requirements. In such cases, specifically tailored readout electronics are typically used. In this presentation, the general concept of a new highly scalable and flexible electronics is presented which is intended for operation of up to several 1000 SQUID channels in the linearizing flux locked loop mode. A prototype version of the electronics for the readout of a low-noise biomagnetic system involving 72 SQUID sensors is demonstrated. Measurement results regarding gain, bandwidth, noise, and power consumption will be presented. Finally, we discuss other potential applications in magnetometry and radiation detection with transition edge sensors or metallic magnetic calorimeters.
Increasing integration scale of superconductor electronics beyond one million Josephson junctions
Authors: Tolpygo Sergey K , Semenov Vasili K
Published: June 2020

We review the existing fabrication processes for superconductor digital electronics and describe approaches to increasing the scale of integration of superconducting digital circuits from the current level of about one million Josephson junctions (JJs) on a 1-cm2 chip toward ten million JJs per chip. We present designs of ac-clocked Single Flux Quantum (SFQ) shift registers, convenient benchmarking circuits, in a 250-nm-linewidth superconductor electronics fabrication process developed recently at MIT Lincoln Laboratory (MIT LL). For shift registers using resistively shunted JJs with Josephson critical current density, J
c of 100 μA/μm2, we achieved a record-high circuit density of 4.2·106 JJs per cm2, a factor of three higher than the previous record obtained in the MIT LL 350-nm-linewidth SFQ5ee process. Using self-shunted JJs with J
c of 600 μA/μm2, we increased this record circuit density to 7.4·106 JJs per cm2.
Static Timing Analysis (STA) with Timing Bleed: Certifying Much Higher Performance for Rapid Single Flux Quantum (RSFQ) Logic
Authors: Wang Fangzhou , Zhang Bo , Pedram Massoud , Gupta Sandeep
Published: June 2020

Josephson-junction based technologies, such as RSFQ, are receiving growing investments due to their high speeds and low power.

As the time interval between the data input and the clock becomes smaller, clock-to-Q delay of a flip-flop increases. In conventional timing analysis, setup time is defined as the interval where clock-to-Q delay increases by 10%. This allows the pipeline to be divided into separate stages and STA to be performed independently for each stage.

However, since RSFQ is pipelined at gate-level, setup time is a significant portion of the clock period and makes this timing constraint extremely conservative. Instead, we define setup time based on the probability of causing a logic error and develop a new STA method that allows larger increases in clock-to-Q delay, i.e., timing bleed, whenever the data input arrives late. We present results of simulations for benchmark circuits with process variations to demonstrate that our new method certifies much higher speeds for RSFQ logic.
Multi Fluxon Storage and its Implications for Microprocessor Design
Authors: Katam Naveen K , Zha Haipeng , Pedram M , Annavaram M
Published: June 2020

One of the main challenges for single flux quantum (SFQ) technology to successfully implement a microprocessor is to have a compact and robust on-chip memory that can be used for register files and cache memory. In this context, we designed high capacity destructive readout (HC-DRO) cells. The design is based on our insight that each cell can store more than one SFQ pulse with the same area footprint of a regular DRO cell. In our current design, we demonstrate a single HC-DRO cell that can store up to three SFQ pulses thereby enabling us to store the equivalent of two bits of memory in a single cell. We have designed a register file (RF) architecture for a microprocessor and used the HC-DRO cells to reduce the Josephson junction count required for its implementation. The RF architecture presented in this work can also be used for implementing cache memories. We also demonstrate the benefits of doubling the storage density with HC-DRO cells by designing a 2-bit branch predictor circuit block for a 5-stage pipeline microprocessor that dramatically improves the cycles per instruction metric of an SFQ based microprocessor. All the design details are presented in the paper by verifying the design concepts through JSIM simulations and Verilog simulations.
Possible dynamics of Josephson junction arrays connected to high-Q tank circuit
Authors: Kornev Victor K. , Kolotinskiy Nikolay V.
Published: June 2020

Serial Josephson junction arrays connected to high-Q tank circuit are analysed and discussed. Numerical simulation of the systems shows two possible oscillation modes above resonance frequency when the tank impedance is capacitive. These are the inphase oscillation mode and the collectively antiphase oscillation mode. This fact is responsible for complicate switching dynamics, which can block parametric resonance excitation. Stability domains of the modes depend on McCumber parameter of the junctions. Increase in this parameter is favourable to the inphase mode. When the inphased oscillations are feasible, the resonance peak-like peculiarities on IV curve and Shapiro steps can be described with the analytic theory derived earlier for one-junction case and extended over the array systems.
A compact, low-power <1K cooling platform for superconducting nanowire detectors
Authors: Ronson Emily , Chase Simon , Kenny Lee
Published: June 2020

Superconducting nanowire single photon detectors (SNSPD) with high detection efficiency, low dark count rate, small timing jitter and short recovery times require cryogenic cooling. Detector performance is strongly dependent on temperature and though many detectors will operate at ~4K, lower temperatures offer significant performance gains. Fortunately, the technology for sub-Kelvin cooling is now mature and products are available that offer simple operation, with reliable and repeatable performance at relatively low cost. In this paper we review performance data from tests on more than 45 individual sub-Kelvin modules manufactured by Chase Research Cryogenics. We compare modules of different sizes and discuss how modules can be scaled to achieve a range of technical specifications for specialised end-user applications.
Hybrid cooling system with cryocooler and liquid-nitrogen for HTS-SQUID system
Authors: Hato Tsunehiro , Tsukamoto Akira , Adachi Seiji , Tanabe Keiichi
Published: June 2020

We developed a hybrid cooling system consisting of a liquid-nitrogen Dewar and a cryocooler with the aim of cooling HTS-SQUIDs fast at operation sites, extending cooling period time, and avoiding noise increase caused by the cryocooler. Liquid nitrogen is evaporated mainly by thermal inflow into the Dewar. Thus, we tried reducing the thermal inflow into a glass Dewar by cooling its inner surface using a small Stirling cryocooler with a cooling capacity of 16 W at 77 K, and examined the cooling period and the operation procedure. We successfully kept 0.6 liter of liquid nitrogen for one week with 0.2 liter reduction. It was also indicated that long time and low noise operation of HTS-SQUIDs would be possible in the hybrid cooling system by temporarily stopping the cryocooler during measurements.
Progress towards innovative and energy efficient logic circuits
Authors: Pagano Sergio , Salina Gaetano , Napoli Anna , Attanasio Carmine , Barone Carlo , Bobba Fabrizio , Carapella Giovanni , Leo Antonio , Nigro Angela , Cristiano Roberto , Ejrnaes Mikkel , Lisitskiy Mikhail , Martucciello Nadia , Pepe Giovanni Piero , Parlato Loredana , Cirillo Matteo , Lucci Massimiliano , Merlo Vittorio , Messina Antonino , Militello Benedetto
Published: June 2020

The integration of superconductive nanowire logic memories and energy efficient computing Josephson logic is explored. Nanowire memories are based on the integration of switchable superconducting nanowires with a suitable magnetic material. These memories exploit the electro-thermal operation of the nanowires to efficiently store and read a magnetic state. In order to achieve proper memory operation a careful design of the nanowire assembly is necessary, as well as a proper choice of the magnetic material to be employed. At present several new superconducting logic families have been proposed, all tending to minimize the effect of losses in the digital Josephson circuits replacing resistors by adequate combinations of inductors in the logic gates. Among those, the nSQUID concept relies on the propagation of vortices along underdamped Josephson transmission lines. While, in principle, this logic family provides very low dissipation, several issues, such as speed and stability, have yet to be addressed. The current status of design and testing of n-SQUID logic gates and circuits as well as of the design and implementation of the nanowire memories is reported
Measurement of Magnetic Particles by Hexagonal Pseudo Seven-Channel HTS SQUID Array
Authors: Tanaka S , Kabasawa M , Hayashi K , Ohtani T
Published: June 2020

We developed a high-Tc superconducting (HTS) three-channel Superconducting Quantum Interference Device (SQUID) array and applied it to the measurement of a flow of magnetic nano-particles (MNPs).Three magnetometers were positioned in-line with a separation of 2.3 mm. The pick-up loop consisted of a 1 mm × 1 mm square loop; the size of a SQUID washer slit was 5 μm × 60 μm. Using this array, we arranged a hexagonal pseudo seven-channel SQUID array. The signal from the water-diluted MNPs, Resovist, in motion was measured by the array in a DC bias magnetic field and an AC modulation magnetic field. The dependence of the SQUID signal on the angle of the MNP flow to the baseline of the array was measured and discussed. The results suggest that the signal peak values are highly dependent on the flow path angle and that most of the flow path can be identified by the shape of the waveforms. This method is a promising candidate for brain activity investigation of small animals.
High-Resolution Digital SQUID Magnetometer using Sub-Flux Quantum Feedback
Authors: Myoren Hiroaki , Okabe Kosuke , Matsunawa Ryo , Itagaki Kohki , Naruse Masato , Taino Tohru
Published: June 2020

Digital SQUIDs with the single flux quantum (SFQ) feedback have attracted much attention because of the feasibility of realizing a wide dynamic range and high slew rate for digital magnetometers. In order to realize higher resolution, we have studied a digital SQUID with sub-flux quantum feedback. Combining the sub-flux quantum feedback to a feedback loop and the direct SFQ feedback to a main loop triggered by carry SFQ pulses from an up/down counter. In this study, we discuss the method of multi-flux quantum generation and compact design of the SFQ up/down counter for high-speed operation of the digital SQUID magnetometer.
Fine optimization of Josephson critical current in SQUID devices by thermal annealing
Authors: Vettoliere Antonio , Talamo Oliviero , Silvestrini Paolo , Valentino Massimo , Granata Carmine
Published: June 2020

In the present article an experimental study regarding optimization and fine-tuning and of Superconducting QUantum Interference Device (SQUID) parameters by a thermal annealing is presented. It allows to modify the parameters in order to meet a specific application or to adjust the device parameters to reduce the magnetic field noise preventing the work instability conditions due to a critical current value that exceed the expected one. In particular, we carried out the measurements of the critical current and the voltage swing as a functions of the annealing temperatures or time showing the effect of such procedures on the magnetic field spectral density of the SQUID magnetometers The experimental results demonstrate that it is possible to optimize the main device parameters reducing the SQUID noise to a suitable value improving the stability of working operations.
Investigation of NanoSQUIDs Fabricated with a Range of Focused Ion Beam Sources
Authors: Polychroniou E , Gallop J , Godfrey T , Cox D , Long G , Chen J , Romans E , Hao L
Published: June 2020

SQUIDs (Superconducting Quantum Interference Devices) are macroscopic quantum devices capable of detecting and measuring a wide variety of physical parameters with unprecedented sensitivity. SQUIDs based on nanobridge weak links have shown increasing promise for quantum information and quantum sensing applications such as single spin detection. Focussed ion beam etched nanobridges have properties which can enhance nanoSQUID device performance but are often limited in terms of their non-hysteretic operating temperature range. Here we describe measurements of FIB-milled nanobridges, as single weak links or in nanoSQUIDs, made using either Ga, Xe or Ne ion beam sources. Their properties as a function of temperature, bias current, magnetic field and microwave power are measured and modelled according to a range of superconductivity models, as a means for improved understanding of the associated nanobridge parameters. We further propose techniques to extend the non-hysteretic operating temperature range of the devices.
Dark counts double switching rates in NbTiN Superconducting Nanowire Single Photon Detectors
Authors: Salvoni D , Ejrnaes M , Parlato L , Yang X Y , You L X , Wang Z , Pepe G P , Cristiano R
Published: June 2020

In this work we present a study of the dark counts rate in a NbTiN Superconducting Nanowire Single Photon Detectors (SNSPD). We measure the distribution of the time intervals elapsed between two consecutive dark pulses at the fixed temperature of 4,2K. Due to the stochastic nature of the dark counts, the distribution is expected to have a Poisson shape but what we observe is a combination of two Poisson-like processes. A further analysis of the distributions dependence on the bias current highlights that the weight of the two process is not constant. In the scenario presented by Ejrnaes et al. [1], this result can confirm that, in this temperature regime, dark counts are generated mainly by multiple consecutive fluctuation events and the contribution to the dark counts rate coming from single fluctuations increases in the high bias current region.
Spectral filtering with diffractive reflection gratings for fast superconducting detectors in the THz frequency range
Authors: Schmid A , Kuzmin A , Wuensch S , Ilin K , Bruendermann E , Scherhaufer D , Mueller A-S , Siegel M
Published: June 2020

In this work we describe the design process of a grating spectrometer with fast superconducting THz detectors based on Niobiumnitride. The main application is the use as a diagnostic tool for accelerator-based sources of THz radiation like synchrotrons, where interactions of the electron bunches in storage ring lead to fluctuations in spectrum and intensity of the emitted radiation.

Guidelines for the design of the two main parts, grating and detector design, are given. Here, a blazed reflection grating for a frequency range of 1 THz to 5 THz has been developed and successfully fabricated in a high-precision milling process out of brass. Also three different planar antenna designs (double-slot, annular slot and log-per spiral) in combination with quasioptical bandpass filters for the THz range have been simulated and evaluated for best spectral resolution and power coupling.

Finally, we have verified grating performance for pulsed THz radiation of 1 THz in an experiment conducted at the synchrotron light source of the KIT. Excellent agreement of the grating performance to the simulations has been found.
Hard X-ray Fluorescence measurements with TESs at the Advanced Photon Source
Authors: Guruswamy Tejas , Gades Lisa M , Miceli Antonino , Patel Umeshkumar M , Weizeorick John T , Quaranta Orlando
Published: June 2020

Transition Edge Sensor (TES) spectrometers for hard X-ray beamline science will enable improved X-ray emission and absorption spectroscopy in the information-rich 2 to 20 keV energy range. We are building a TES-based instrument for the Advanced Photon Source (APS) synchrotron, to be made available to beamline users. 24-pixel prototype arrays have recently been fabricated and tested. The first spectroscopy measurements using these arrays are promising, with a best single-pixel energy resolution of 11.2 eV and saturation energy > 20 keV. We present a series of recent X-ray Fluorescence measurements involving transition metal elements and multi-element samples with closely spaced emission lines, in particular a Cu-Ni-Co thin film and a foil of Cu and Hf. The TES-measured spectra are directly compared to spectra measured with silicon drift detectors at an APS beamline, demonstrating the improved X-ray science made possible by TES spectrometers.
Multiphysics simulation of a superconducting bolometer working in a portable cryostat
Authors: Napolitano Andrea , Ferracin Samuele , Gerbaldo Roberto , Ghigo Gianluca , Gozzelino Laura , Torsello Daniele , Laviano Francesco
Published: June 2020

We present the characterization and the numerical simulation of a high-temperature superconducting bolometer based on an YBa2Cu3O7-x (YBCO) film, which is patterned in serpentine elements and selectively modified by means of micro-collimated high-energy heavy-ion irradiation. The local irradiation of YBCO yields to a controlled reduction of the critical temperature and of the critical current, but the resistance versus temperature slope is preserved near Tc. In this way, only the irradiated part of the YBCO films is used for detecting infrared radiation, while the pristine one serves for electro-thermal feedback. The bolometer has been engineered in a portable cryostat, which is equipped with an optical window for suitably filtering the spectrum of the incoming electromagnetic radiation. The optimization of the cryostat has been performed with modelling by finite element method of the whole apparatus, along with simulations of the bolometer response.
Development of a Josephson junction based single photon microwave detector for axion detection experiments
Authors: Alesini D , Babusci D , Barone C , Buonomo B , Beretta M M , Bianchini L , Castellano G , Chiarello F , Di Gioacchino D , Falferi P , Felici G , Filatrella G , Foggetta L G , Gallo A , Gatti C , Giazotto F , Lamanna G , Ligabue F , Ligato N , Ligi C , Maccarrone G , Margesin B , Mattioli F , Monticone E , Oberto L , Pagano S , Paolucci F , Rajteri M , Rettaroli A , Rolandi L , Spagnolo P , Toncelli A , Torrioli G
Published: June 2020

Josephson junctions, in appropriate configurations, can be excellent candidates for detection of single photons in the microwave frequency band. Such possibility has been recently addressed in the framework of galactic axion detection. Here are reported recent developments in the modelling and simulation of dynamic behaviour of a Josephson junction single microwave photon detector. For a Josephson junction to be enough sensitive, small critical currents and operating temperatures of the order of ten of mK are necessary. Thermal and quantum tunnelling out of the zero-voltage state can also mask the detection process. Axion detection would require dark count rates in the order of 0.001 Hz. It is, therefore, is of paramount importance to identify proper device fabrication parameters and junction operation point.
Superconducting heterostructure with barrier with strong spin-orbit interaction
Authors: Constantinian K Y , Petrzhik A M , Ovsyannikov G A , Shadrin A V , Kislinskii Yu V , Cristiani G , Logvenov G
Published: June 2020

We report on observation of superconducting current and AC Josephson effect in mesa-heterostructures with spin-singlet superconducting electrodes, interlayered by a barrier material with strong spin-orbit interaction. The mesa-heterostructures were Nb/Au/Sr2IrO4/YBa2Cu3Ox with d=5 nm and 7 nm thick interlayer of strontium iridate Sr2IrO4, which is known as a canted antiferromagnetic insulator with a weak magnetic moment from Ir atoms and with spin-orbit interaction energy ESO
~0.4 eV. The superconducting critical current density was jC
≈0.3 A/cm2 at T=4.2 K for mesa-heterostructures with d=7 nm and a few times higher for d=5 nm. The zero-bias conductance peak has been observed due to low energy states originated at Sr2IrO4/YBa2Cu3Ox interface. Under influence of weak magnetic field the critical current IC
(H) dependences showed Fraunhofer-like pattern indicating absence of pinholes, supported also by oscillating with microwave power Shapiro steps. Fiske resonance steps with voltage positions deviated from the ordinary ones were registered for mesas with d=5 nm.
Occurrence of Flux Jumps in MgB2 Bulk Magnets during Pulse-Field Magnetization
Authors: Sakai N , Oka T , Yamanaka K , Dadiel L , Oki H , Ogawa J , Fukui S , Scheiter J , Häßler W , Yokoyama K , Noudem J , Miryala M , Murakami M
Published: June 2020

The magnetic flux capturing of MgB2 bulk magnets made by spark plasma sintering process has been precisely investigated to clarify the mechanism of flux motions during the pulse-field magnetization processes. The field trapping ratio B
T/B
P was evaluated as a key parameter of field trapping ability which strongly relates to the heat generation due to the rapid flux motion in the samples. The time dependence of magnetic flux density revealed the actual flux motion which penetrated the samples. The trapped fields B
T and field trapping ratios B
T/B
P of various samples were classified into three regions of ‘no flux flow’, ‘fast flux flow’ and ‘flux jump’ according to the generation of heat and its propagation. A flux jump was observed late at 280 ms from the beginning of PFM process, while the field penetration B
P showed its peak at 10 ms. Considering the heat propagation speed, the long-delayed flux jump should be attributed to the macroscopic barriers against the heat propagation to the surface centre of bulk magnet.
Performance and Microstructure of Bi-2212 Wire under Different OverPressures Heat-treatment
Authors: Yang Dongsheng , Yu Min , Jin Huan , Liu Fang , Qin Jinggang
Published: June 2020

Bi2Sr2CaCu2Ox is a potential material for the superconducting magnets of the next generation of Fusion-reactors. A R & D activity based on Bi-2212 wire is running at Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) for the feasibility demonstration of Cable In Conduit Conductor (CICC). The Bi-2212 round wires were heat-treated by the tube furnace at a pressure of 30 atm and 50 atm. Wire diameter, critical current performance, and n value were studied. The surface morphology and grain size distribution of Bi-2212 superconductor after heat treatment analyzed by Electron Backscattering Diffraction (EBSD). The performance difference of Bi-2212 at 30atm and 50atm is small, and the grain size and distribution are similar. The performance difference of Bi-2212 samples with 30atm and 50atm over-pressure heat-treatment is small. 30atm requires less equipment and can obtain a longer constant temperature zone, which is suitable for exploring the heat treatment process of a long conductor.
Simulation of mechanical stresses in reinforced REBaCuO ring bulks during pulsed-field magnetization
Authors: Hirano T. , Fujishiro H. , Naito T. , Ainslie M. D.
Published: June 2020

We have performed numerical simulations of the electromagnetic, thermal and mechanical properties of a REBaCuO ring-shaped bulk with various reinforcement structures during pulsed-field magnetization (PFM). Compressive and tensile electromagnetic stresses,

σ
θ

mag

, are developed in the ring-shaped bulk during the ascending and descending stages of PFM, respectively. These stresses increase at lower operating temperatures and for higher applied pulsed fields. In order to reduce these stresses, the ring-shaped bulk was fully encapsulated by outer and inner ring with upper and lower plates made by Al alloy. In particular, this reinforcement structure can achieve the lowest electromagnetic compressive stress, which corresponds to about 54% of that for a conventional ring reinforcement structure, and the electromagnetic tensile stress was also reduced. We also compared the simulation results of the electromagnetic stresses for the ring-shaped bulk to those for a disk-shaped bulk.
Simulation of mechanical stresses in reinforced REBaCuO disk bulks during pulsed-field magnetization
Authors: Shimoyashiki F. , Fujishiro H. , Hirano T. , Naito T. , Ainslie M. D.
Published: June 2020

We have performed numerical simulations of the electromagnetic hoop stress, σθ
, in a REBaCuO disk bulk reinforced by a metal ring during pulsed-field magnetization (PFM) using a solenoid coil, in which the superconducting characteristics of the bulk material were assumed to have realistic J
c-B-T ones. The compressive and tensile σθ
stresses were applied in the bulk during the ascending and descending stages of PFM, respectively. The time and position dependences of the mechanical stresses were estimated. The possibility of mechanical fracture due to these hoop stresses and the effect of the metal ring reinforcement were discussed.
Numerical optimisation of mechanical ring reinforcement for bulk high-temperature superconductors
Authors: Barthlott D , Huang K Y , Durrell J H , Cardwell D A , Holzapfel B , Ainslie M D
Published: June 2020

The finite element method has been used extensively in recent years to solve various problems related to applied superconductivity and provides a useful tool for analysing and predicting experimental results. Based on a recently-developed modelling framework, implemented in the finite element software package COMSOL Multiphysics, investigations on the minimum ring reinforcement required to prevent mechanical failure in bulk high-temperature superconducting magnets have been carried out. Assuming homogeneous Jc
(B,T) across the bulk sample irrespective of its dimensions, the maximum magnetic stresses experienced, and the minimum ring thickness required to prevent the hoop and radial stresses from exceeding the tensile strength of the bulk superconductor have been determined for varying values of the Young’s modulus, radius, height and temperature of a representative single-grain Ag-containing Gd-Ba-Cu-O bulk sample. This comprehensive analysis details the influence each of these key parameters has on the magnetic stress and hence their impact on the necessary ring thickness to prevent mechanical failure in any given system, i.e., for any combination of material properties and sample dimensions.
Microstructure of coated conductors with La- or Nb-doped SrTiO3 conductive buffer
Authors: Ichinose A. , Ota K. , Hamada T. , Inoue S. , Horii S. , Doi T.
Published: June 2020

A novel configuration for coated conductors (CCs) was proposed to reduce material cost and consisted of YBa2Cu3Oy (YBCO) / conductive buffer(s) / Ni-electroplated {100} <001> textured Cu, and SUS316 lamination tape. Conductive buffers are a key subject to reduce material cost of CCs. In the CC using the conductive buffer of Nb-doped SrTiO3 (Nb-STO), a high-J
C YBCO (2.5 MA/cm2 at 77 K and self-field) was successfully obtained. Moreover, a clean interface between the buffer and the metal tape without oxides was confirmed using TEM analysis. However, the resistivity of the Nb-STO buffer increased to be several Ω·cm after the YBCO deposition, which was beyond expectations. Therefore, the La-doped STO (La-STO) conductive buffer was incorporated into CCs as a novel conductive buffer. Two CCs with the different conductive buffers were compared in terms of microstructure analysis using transmission electron microscopy.
Towards a reliable bridge joint between REBCO coated conductors
Authors: Kirchner A. , Nielsch K. , Hühne R.
Published: June 2020

REBa2Cu3O7-x
(REBCO; RE = Y, Gd) coated conductor wires are commercially available up to a length of about 1 km. However, for large-scale devices like superconducting coils for high-field magnets several kilometres of a coated conductors are required. Therefore, it is desirable to use joints, which exhibit similar superconducting properties as the coated conductor itself. In this study, we jointed commercial REBCO coated conductors by a two-step procedure. At first, a superconducting soldering solution was developed and deposited on unstabilized coated conductors via chemical solution deposition. The soldering precursor is based on a Cu-rich solution with a metal cation ratio Y:Ba:Cu of 1:2:4. Secondly, a piece of the coated conductor was delaminated between the superconducting and the buffer layer and used as bridge between two other conductors covered with the soldering layer. Annealing the resulting bridge joint under pressure results in a mechanical stable configuration.
In-depth resistance analysis of REBCO tape joints with indium insert and solders
Authors: Hayasaka R , Ito S , Kato T , Yokoe D , Hashizume H
Published: June 2020

Joints between REBCO (rare-earth barium copper oxide) tapes with low joint resistance are crucial for many superconducting applications. Joining REBCO tapes with indium insert (In-joint) is a promising joining method to fabricate low resistive joints at low temperatures (20–120°C). This study investigated the joining conditions of In-joints such as pickling, surface roughness, joining time, and temperature. The joint resistivity (product of joint resistance and joint area) was successfully reduced to 22–30 nΩcm2 at 77 K in self-field. The constitutive factors of the joint resistivity were analysed separately along with the crosssectional observations. In this study, the interface resistivity of the REBCO tape was measured as 8.5 nΩcm2 for one REBCO tape by the previously proposed method. The resistivity of the joining interface Cu/In was calculated as <3 nΩcm2 by subtracting the other resistivities from the entire joint resistivity. This result reveals the lower limit of the joint resistivity: the sum of the resistivity (nΩcm2) of indium (measurable by thickness), the resistivity of Cu/In (<3nΩcm2), and the interface resistivity of the REBCO tape (measurable beforehand). Furthermore, we demonstrated a lower and less varied joint resistivity of In-joints than those of the soldered joints.
Parameter boundaries for the heteroepitaxial growth of REBCO films by e-beam quantitative evaporation on inclined substrate deposited MgO buffered Hastelloy tapes
Authors: Troshyn Oleksiy , Hoffmann Christian , Große Veit , Hänisch Jens , Holzapfel Bernhard
Published: June 2020

The technology of electron beam (e-beam) quantitative evaporation of oxide powders at THEVA is quite unique among the many standard in-situ growth methods of high-temperature superconducting (HTS) films. As well known for YBa2Cu3O7-x (YBCO) film growth the temperature and oxygen pressure should be in proximity of the YBCO stability line in the Bormann diagram. In case of e-beam quantitative evaporation, we observe a shift in the DyBa2Cu3O7-x (DyBCO) stability line towards lower pressures and higher temperatures compared to YBCO. We investigated the temperature and oxygen partial pressure boundaries of epitaxial DyBCO growth on inclined substrate deposited MgO (ISD-MgO) buffered Hastelloy tapes. For completeness, the influence of powder stoichiometry and evaporation rate on HTS epitaxial growth is shown. For the first time high-quality HTS films have been grown by e-beam quantitative evaporation without employing an oxygen shuttle and with oxygen in the chamber background instead.
Angular J
c measurements at 77 K in-field, on an ISD REBCO coated conductor using a straightforward mechanical scribing technique to reduce tape width
Authors: Gurnham C W A , Große V , Hampshire D P
Published: June 2020

E-J traces have been measured on an inclined substrate deposition coated conductor up to 0.7 T as a function of angle at 77K. A large change in J
c is seen on reversal of the direction of the magnetic field, associated with different surface barriers to flux nucleation. The effect of angular hysteresis and field hysteresis on J
c has been found to be much smaller. Narrow bridges have been successfully fabricated by a simple scribing technique using a diamond-tipped scribe and a template. These bridges have been made from 1.4 mm to 4 mm width on the 12 mm width tape, which decrease I
c linearly with width. Measurements before and after the heat treatment for strain gauge glue curing at 170 °C for 120 min have been made and confirm there is no systematic decrease in J
c.
Effect of oxygenation process on flux pinning in pristine and BaHfO3 nanocomposite GdBa2Cu3O7 superconducting thin films
Authors: Popov Ruslan , Ackermann Kai , Rijckaert H. , Hänisch Jens , Van Driessche I. , Holzapfel Bernhard
Published: June 2020

Pristine and BaHfO3 (BHO) nanocomposite GdBa2Cu3O7 (GdBCO) superconducting thin films were deposited at 800 °C, 0.4 mbar O2 partial pressure using a Nd:YAG laser (λ=355 nm) with 10 Hz frequency on MgO substrate. The influence of the oxygenation process on flux pinning and electrical transport properties was investigated by changing the annealing temperature (T
ann) between 450 °C and 780 °C and the holding time (t
hold) between 10 and 30 min. The irreversibility field shows the largest shift toward higher fields for T
ann = 450 °C and thus the largest in-field J
c. Angular dependence of J
c for most of the pristine GdBCO films exhibits a large peak for B‖c, however the sample with T
ann = 550 °C has a large peak in the 90°-120° region, i.e. near B ‖ ab. For BHO nanocomposites, a decrease in T
ann increases in-field J
c by a factor of 2.
Scaling analyses on the critical current density in MgB2/NbN/Si thin film
Authors: Nishida Akihiko , Taka Chihiro , Chromik Stefan
Published: June 2020

Scaling analyses are performed on the critical current density J
c in MgB2/NbN/Si thin film. In our previous work on MgB2/SiC/Si film [Mat. Sci. Eng. 502 (2019) 012184], we have shown that J
c data well scale on a single line with the reduced scaling formula more than 10 orders of magnitude. We extend these studies onto MgB2 film with NbN buffer layer in comparison with SiC buffer. Experimental J
c data under perpendicular magnetic field to the film are reduced against transition temperature and critical field, and applicability of the comprehensive scaling formula is examined over 16 orders of magnitude. Our scaling formula is revealed to be well applicable to the NbN-buffered film as well.
Critical current anisotropy in Fe(Se,Te) films irradiated by 3.5 MeV protons
Authors: Leo A , Grimaldi G , Nigro A , Ghigo G , Gozzelino L , Torsello D , Braccini V , Sylva G , Ferdeghini C , Putti M
Published: June 2020

Irradiation effects are often used to improve the effective pinning in superconductors, but sometimes they can reveal detrimental for superconducting properties. Fe(Se,Te) has been proven to be a very robust material against irradiation, in particular proton irradiation, thus configuring as an ideal material to work in harsh environments such as particle accelerators or fusion reactors. Anyway, the study of the pinning activation energy in Fe(Se,Te) thin film irradiated by 3.5 MeV protons suggests that this treatment can modify the anisotropy of the films pinning. Thus here we present the result of further investigation analyzing the effect of proton irradiation on the critical current and the pinning force both for the magnetic field applied parallel and perpendicular to the sample surface. We find that, although a slight effect on the critical current anisotropy is observed, the pinning landscape is not affected by the irradiation process. This confirms that Fe(Se,Te) can be considered for devices working in harsh environments.
High frequency vortex dynamics in YBa2Cu3O7−x
with Ba2YTaO6-Ba2YNbO6 nanodefects
Authors: Torokhtii K. , Alimenti A. , Rizzo F. , Augieri A. , Celentano G. , Frolova A. , Silva E. , Pompeo N.
Published: June 2020

We studied pinning characteristics of YBa2Cu3O7−x
(YBCO) with 5 mol.% Ba2YTaO6/Ba2YNbO6 (BYNTO). In contrast to widely used d.c methods, at microwaves very short oscillations of the fluxons are involved giving the information on both the pinning strength, through the pinning constant and on dissipation, related to the vortex core physics, through the viscosity η. We estimate the pinning constant kp
, the vortex viscosity η and also the maximum creep factor χmax. The results were compared not only with the pristine YBCO sample but also with the effect of another type of columnar defects (BZO) in YBCO. All film samples were grown by Pulsed Laser Deposition (PLD) technique at nearly the same conditions to ensure the uniform. We detect an improvement of all pinning characteristics of YBCO-BYNTO sample in the temperature range 62 - 90 K and for field up to 0.75 T. While the average diameter of the defects is similar for YBCO-BYNTO and YBCO-BZO the key of the superior characteristics in YBCO-BYNTO could be both the concentration of columns and an addition of another phase of point-like defects.
Effects of Zn doping on the pinning potential and the glass-liquid transition temperature of YBa2Cu3O6+δ
films
Authors: Ackermann Kai , Mai Vadim , Hänisch Jens , Holzapfel Bernhard
Published: June 2020

A pristine YBa2Cu3O6+δ
film as well as two Zn-doped YBa2(Cu1−x
Zn
x
)3O6+δ
films with doping levels x of 0.2 % and 0.4 %, respectively, grown epitaxially on SrTiO3 single crystalline substrates by pulsed laser deposition, were investigated regarding the effect of the doping on the glass-liquid transition line and the pinning potential distribution. Zn doping did not show any effect on the shape of the transition line but caused a suppression of the transition temperature T
GL of about 16 K%−1. Although the doping did not lead to an additional pinning effect, the increase of the critical current density j
c towards lower temperatures is mainly driven by the variation of the threshold critical current density, j
cm. The width of the critical current density distribution j
0 saturates at low temperatures and only drops above ~ 0.9T
GL.
Strictly application-oriented REBCO bulk fabrication
Authors: Floegel - Delor U. , Riedel T. , Schirrmeister P. , Koenig R. , Kantarbar V. , Liebmann M. , Werfel F. N.
Published: June 2020

The ability to produce large-grain REBCO (RE=Y, Rare Earth) bulk superconductors by melt texturing growth has been improved and refined significantly over the last 20 years. Magnetic bearings, transportation systems, and scientific instrumentation are becoming major innovations of HTS bulk materials. Large grain production is dominated by cold top-seeding melt growth (TSMG). For material optimizing we demonstrate strictly application-oriented fabrication close to the final geometry and thus reducing mechanical machining and assembling work. YBCO hollow cylinder design for journal bearings with radial c axes orientation are obtained by radial-seeding geometry (RSMG). YBCO ring tiles up 120 mm show a perfect cylinder geometry with in-wall a, b orientation and radial c axes. We report improved electric and mechanical bulk properties and demonstrate great production effectiveness. For achieving high field permanent magnets up to 10 T we compare YBCO bulk and ring geometry. Top/bottom double seeded melt growth (DSMG) grain samples provide better magnetic flux trapping capability and show more homogenous properties.
Magnetizing characteristic evaluation of REBCO bulk in PFM using different yokes
Authors: Yokoyama K , Oka T
Published: June 2020

A REBCO bulk magnet is able to generate a several-tesla-class magnetic field by a compact system with low power consumption. Pulsed field magnetization (PFM), which is one of major magnetization method, has the advantage that the bulk is activated using a conventional equipment in a few seconds, and, therefore, it is suitable for practical use. However, a trapped field is lower than the maximum performance of material. To increase the field, improvement of magnetization method and modification of exciting equipment are considered. We aim to improve the trapped field by changing the exciting system simply and inexpensively. In the bulk magnet system, soft-iron yokes are generally used to extend the pulse width. This paper evaluated magnetizing characteristic quantitatively. A single pulsed field was applied to a new GdBCO bulk using ϕ64-mm and ϕ80-mm soft-iron yokes. The pulse width of ϕ80-mm yoke was about 35% longer than that of ϕ64-mm yoke. When comparing the trapped flux density and total magnetic flux, these values of ϕ80-mm yoke were higher than those of ϕ64-mm yoke. Also, the temperature rise of ϕ80-mm yoke was decreased by approximately 5% as compared with ϕ64-mm yoke, implying that the same effect as the long-pulse method was achieved.
Comparison of properties of a bulk HTS and a stack of HTS tapes after FC and ZFC
Authors: Kurbatova E , Kurbatov P , Kuschenko E , Sysoev M , Kulaev Y
Published: June 2020

This paper presents the results of the analysis of properties of a high-temperature superconducting (HTS) material in two forms - HTS bulks and a stack made of a number of HTS tape layers. Stack of HTS tape is a new type of application of superconductors, which can be used instead of a convention bulk material. In this work, we study the levitation properties of HTS stack and compare it with the same characteristics of HTS bulks to find the effective way of modelling such type of HTS. Experimental measurements of the force interaction between a permanent magnet and HTS material, as well as a "trapped" magnetic field, are performed for both types of a superconductor. The HTS samples are analysed in the case of Field Cooling (FC) and Zero Field Cooling (ZFC). The aim of the research is to analyse experimental results to compose and justify mathematical models for calculation magnetic system with such types of superconductor.
Effect of oxygen contamination on densification of Fe(Se,Te)
Authors: Masi A. , Alvani C. , Angrisani Armenio A. , Bellusci M. , Celentano G. , Fiamozzi Zignani C. , Fabbri F. , La Barbera A. , Padella F. , Pentimalli M. , Silva E. , Vannozzi A. , Varsano F.
Published: June 2020

The optimization of sintering behaviour of iron chalcogenides superconducting materials is mandatory to enhance their critical current density, in order to pursuit their application in the production of superconducting wires. In this context it has been investigated here the effect of oxygen contamination on the material densification, considering the issues related to industrial oxygen-free isolated production lines. Our results show that the densification process is negatively affected by oxygen contamination. However, despite the difference in density, all sintered samples are characterized by similar structural and morphological features, and show comparable electrical and magnetic properties, with low critical current densities (Jc<103 A/cm2). These results suggest that densification is not the key limiting factor in these conditions, and that grain boundary or misorientation factors may play a greater role in limiting the performance of sintered iron chalcogenides superconductors.
Pulsed Laser Deposition of quasi-multilayer superconducting Ba(Fe0.92Co0.08)2As2-BaHfO3 nanocomposite films
Authors: Meyer Sven , Langer Marco , Grünewald Lukas , Hänisch Jens , Gerthsen Dagmar , Holzapfel Bernhard
Published: June 2020

Quasi-multilayer films with perovskite nano-particles have already been investigated and reported in YBCO compounds. Here, we introduce this technique to iron based superconductors by preparing quasi-multilayer nanocomposite films of Co-doped BaFe2As2 with BaZrO3 and BaHfO3 perovskite nanoparticles by a pulsed laser deposition technique using a frequency-tripled Nd:YAG laser (λ = 355 nm) with separate targets of the constituents. We investigated the enhancement of pinning force density and critical current density in relation to critical temperature reduction. Application-relevant properties are discussed in relation to the deposition conditions and the microstructural properties of the films to better understand growth and behavior of strong pinning centers in these materials.
Effect of annealing on structure and superconducting properties in Fe(Se,Te)
Authors: Masi A. , Alvani C. , Angrisani Armenio A. , Augieri A. , Celentano G. , De Marzi G. , Fiamozzi Zignani C. , Fabbri F. , La Barbera A. , Padella F. , Rizzo F. , Silva E. , Vannozzi A.
Published: June 2020

In this paper, the effect of post synthesis annealing treatments on a Fe(Se,Te) polycrystalline material is evaluated and discussed. The samples have been obtained via melting route. The material has been subjected to a high-temperature annealing treatment, carried out for 45 h at 680 °C. The role of the cooling step was investigated comparing samples obtained after a controlled cooling or after quenching in liquid nitrogen. From a morpho-structural point of view, the annealing treatment improves homogeneity, with respect to pristine samples, and influences secondary phase precipitate morphology. Regarding superconducting properties, a key role of the cooling procedure is assessed: controlled cooling leads in fact to a significant improvement of high field behaviour with respect to the melted material, while quenched samples are characterized by a worsening of the superconducting properties. Despite the overall worsening, however, the quenched samples show evidence of the presence of superconducting phases characterized by a remarkably high critical temperature (Tc
> 18 K), observed for these materials only in films or under pressure.
Microwave impedance study of superconducting (Li1-xFex)OHFeSe single crystal
Authors: Barannik A. A. , Cherpak N. T. , Wu Y. , Zhang X. Q. , Wang J. , Dong X. L. , Sun L. , He Y. S.
Published: June 2020

Microwave (MW) surface impedance Zs
=Rs
+iXs
of the recently discovered high-Tc iron-based superconductor (Li0.84Fe0.16)OHFe0.98Se single crystal was measured for the first time. The special X-band “hot finger" resonator allowed carrying out measurements when temperature of the sample under test can be varied from 2K to 90K and in the same time temperature of the resonator remained constant (4.2K). The unusual and important inequality Rs<Xs
at T≥Tc
=42K was found. The possible reason of the observed feature is considered. The temperature dependence of the quasiparticle σ1
and superfluid σ2
conductivities and structure of energy gap are analyzed. Experiments show the temperature dependence of the field penetration depth as Tn
with n=2.8 at T<Tc
/3. The temperature dependence of the superfluid density confirms fully gapped behavior of a superconductor with a complex gap.
Microwave properties of Fe(Se,Te) thin films in a magnetic field: pinning and flux flow
Authors: Pompeo Nicola , Alimenti Andrea , Torokhtii Kostiantyn , Sylva Giulia , Braccini Valeria , Silva Enrico
Published: June 2020

We present here a microwave (16 GHz) investigation of the complex surface impedance in a dc magnetic field (up to 1 T) of Fe(Se,Te) thin ( 300 nm) films. We derive the vortex parameters: the flux-flow resistivity yields information on the dynamics of the quasiparticles, the Labusch parameter yields a measure of the steepness of the pinning potential wells, and the depinning frequency assesses the frequency range where the material is suitable for high-frequency applications in a dc magnetic field. We compare the results to the data obtained in YBCO, Nb and Nb3Sn.