Session Y7: Superconductivity: Devices, Vortex Dynamics, and Conductors

Silicon superconducting quantum interference device

We have studied a Superconducting Quantum Interference SQUID device made from a single layer thin film of superconducting silicon. The superconducting layer is obtained by heavily doping a silicon wafer with boron atoms using the Gas Immersion Laser Doping (GILD) technique. The SQUID device is composed of two nano-bridges (Dayem bridges) in a loop and shows magnetic flux modulation at low temperature and low magnetic field. The overall behavior shows very good agreement with numerical simulations based on the Ginzburg-Landau equations.   

Waveguide-integrated NbTiN superconducting nanowire single-photon detector with ultralow jitter

We demonstrate NbTiN superconducting nanowire single-photon detectors (SNSPDs) integrated with Si$_{\mathrm{3}}$N$_{\mathrm{4}}$ waveguides for counting visible and infrared photons. The nanowires with different width (30-90 nm) and length (40-80 um) are patterned into U-shapes on 200nm-thick Si$_{\mathrm{3}}$N$_{\mathrm{4}}$ waveguides, and the photons travelling along the waveguides could be efficiently absorbed by the nanowires via evanescent coupling. With the use of high-speed SiGe cryogenic amplifier, which operates together with the detector chip at the temperature of 1.7K, the jitter of the detection system is measured to be only 19 ps due to the improved signal-to-noise ratio (SNR), compared to 48 ps measured with room-temperature amplifiers. By investigating the background noise level and the pulse shape of the output signal from the detector, we determine the contribution of the noise to the final system jitter is less than 3ps, indicating that our results are very close to the intrinsic jitter of the detector. \newline   

A Superconducting Ion Detection Scheme for Atom Probe Tomography

Superconducting detectors are a promising avenue for improving the performance of Atom Probe Microscopes. Many types of superconducting detectors have been developed within the past several decades, each with its own strengths and weaknesses. Many of these detectors are inherently slow, bulky, require complex multiplexing schemes to attain position sensitivity, or require complex read-out electronics. In response to the rigorous demands of atom probe technology, and with the goal of developing an elegant, simple to use solution, we have developed a novel superconducting delay line detector. The principal of detection is to use the kinetic energy of incoming ions to generate excess quasiparticles in a superconducting stripline. These quasiparticles generate a measurable signal which propagates along the delay line. By measuring the timing of the output signals from this delay line, we are able to measure the time of flight for the ion and the position of its impact on the detector. We will be presenting on the performance of this detector as measured in a Field Ion Microscope.   

Highly flexible, mechanically robust superconducting wire consisting of NbN-carbon-nanotube nanofibril composites

A flexible superconducting fiber is prepared by twisting carbon nanotube (CNT) sheets coated with sputter-deposited niobium nitride (NbN) layer to form the shape of yarn. Twisted CNT yarn, which has been extensively studied due to its high flexibility as well as excellent mechanical properties, and NbN, which is a superconducting material with high transition temperature (Tc) and critical magnetic field (Hc), are combined together by the deposition of NbN layer on free-standing CNT-sheet substrate followed by the biscrolling process. We tried many experimental conditions to investigate the superconducting properties of NbN-CNT yarn as a function of NbN thickness and number of CNT-sheet layers, and found out that the superconducting property of NbN on CNT-sheet can be comparable to that of NbN thin film on the normal solid substrate. In addition, the superconducting property survived even under the condition of severe mechanical deformation such as knotting. These results show the potential application of this technology as a large-scale fabrication method of flexible, mechanically robust, high performance superconducting wire. This work is supported by the Institute for Basic Science (IBS-R011-D1), and by the National Research Foundation (BSR-2013R1A1A1076063) funded by the Ministry of Science, ICT {\&} Future Planning, Republic of Korea.   

Development of semi-rigid cables for low temperature superconducting detectors

We are developing semi-rigid cables for accurate readout of superconducting radiation/particle detectors and other low temperature experiments. The center conductor with a diameter of 0.86 mm is separated with seamless metal outer conductor by dielectric material, polytetrafluoroethylene. We used various metal materials with low thermal conductivity for the electrical conductors such as stainless-steel, cupro-nickel, brass, beryllium-copper, phosphor-bronze, niobium-titanium, and niobium. In addition to the conventional semi-rigid cables, low-pass-filter type cables were manufactured and evaluated to cut the high frequency noise into superconducting detectors. We measured their low thermal conductance and attenuation property up to 10 GHz below the liquid helium temperature.   

\textbf{Understanding and eliminating the fast creep problem in Fe-based superconductors}

One surprising characteristic of Fe-based superconductors is that they exhibit flux creep rates ($S)$ as large as, or larger than, those found in oxide high temperature superconductors (HTS). This very fast vortex dynamics appears to be inconsistent with the estimate of the influence of the thermal fluctuations as quantified by the Ginzburg number (\textit{Gi}), which measures the ratio of the thermal energy to the condensation energy in an elemental superconducting volume. In particular, compounds of the $A$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ family (``122'') have \textit{Gi} $\sim $10$^{\mathrm{-5}}$ to 10$^{\mathrm{-4}}$, so $S$ could be expected to lie between that of low $T_{\mathrm{c}}$ materials (where typically \textit{Gi }$\sim $ 10$^{\mathrm{-8}})$ and HTS such as YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{7}}$ (\textit{Gi }$\sim $ 10$^{\mathrm{-2}})$, as indeed occurs in other superconductors with intermediate fluctuations, such as MgB$_{\mathrm{2}}$ (\textit{Gi} $\sim $10$^{\mathrm{-6}}$ to 10$^{\mathrm{-4}})$. We have found the solution to this puzzle: the fast creep rates in 122 compounds are due to non-optimized pinning landscapes. Initial evidence comes from our previous studies showing that the introduction of additional disorder by irradiation decreases creep significantly in 122 single crystals, although still remaining well above the ideal limit. We now have new evidence from 122 thin films demonstrating that S can be reduced to the lower limit set by \textit{Gi} by appropriate engineering of the pinning landscape.   

Tuning Vortex Creep in Irradiated YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{7-\delta }}$ Coated Conductors

YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{7-\delta }}$ coated conductors (CCs) show non-monotonic changes in the temperature-dependent creep rate, $S(T)$, due to mixed pinning landscapes comprised of twin boundaries, planar defects, point defects, and nanoparticle precipitates. Notably, in low fields, there is a conspicuous dip in $S$ as $T$ increases from $\sim $20K to $\sim $65K. The source of this dip is poorly understood. Moreover, pinning landscapes that are favorable for high critical currents, $J_{\mathrm{c}}$, are not necessarily optimal for low $S$. We have found that, though oxygen irradiation introduces few-nm-sized defects that result in significant increases in $J_{\mathrm{c}}$, it is detrimental to creep, increasing $S$ (reducing the dip depth) for $T>$20K. Understanding the source of this dip is crucial to engineering pinning landscapes that concurrently promote high $J_{\mathrm{c}}$ and low $S$. To this end, we study changes in $S(T)$ as we tune the ratio of smaller (point to few-nm-sized) defects to larger nanoparticles in an oxygen-irradiated CC by annealing in O$_{\mathrm{2}}$ at 250\textbf{\textdegree }C to 600\textbf{\textdegree }C. We observe a steady decrease in $S(T>$20K) with increasing annealing temperature. This suggests that pre-existing nanoparticle precipitates are likely responsible for the dip in $S(T)$, and underlines the fact that the effects of defects are not additive, but rather can be competitive.   

Enhancement of critical current through compound defect with proton irradiation and heavy ion irradiation in YBCO coated conductors and FeSe$_{\mathrm{x}}$Te$_{\mathrm{1-x}}$ crystals

We investigate the enhancement of vortex pinning by both point and columnar defects and compare the results in 2G YBCO coated conductors (CC), with T$_{\mathrm{c}}$ 90K, and in FeSe$_{\mathrm{x}}$Te$_{\mathrm{1-x}}$ single crystals with T$_{\mathrm{c}}$ 14K. Both samples were irradiated with 250 MeV Au ions to a dose-matching field of 1T. The samples were then irradiated with 4 MeV protons to a dose of 4x10$^{\mathrm{16}}$ p/cm$^{\mathrm{2}}$ and 8x10$^{\mathrm{16}}$ p/cm$^{\mathrm{2}}$ in the CC and single crystal, respectively. The major effect of compound particle irradiation in both samples resulted in a synergetic enhancement of the critical current across a wide field range, beyond the enhancement from either individual irradiation type.   

Crtical Current by Design Through Large-scale Simulations

Understanding the dynamic behavior of vortex matter in complicated pinning landscapes is a major challenge for both fundamental science and energy applications. In particular, optimizing type, size and density of pinning centers can significantly enhance the critical current. Based on the time-dependent Ginzburg-Landau equation, we developed a numerical approach towards finding these optimal pinning configurations. I will give an overview of this new paradigm, called Critical Current by Design.\\ {\it References:}\\ $\left[1\right]$ I. A. Sadovskyy, A. E. Koshelev, C. L. Phillips, D. A. Karpeev, A. Glatz, J. of Comp. Phys. {\bf 294}, 639 (2015).\\ $\left[2\right]$ A. E. Koshelev, I. A. Sadovskyy, C. L. Phillips, A. Glatz, arXiv:1509.04212 (2015).\\ $\left[3\right]$ Ivan A. Sadovskyy, Ying Jia, Maxime Leroux, Jihwan Kwon, Hefei Hu, Lei Fang, Carlos Chaparro, Shaofei Zhu, Ulrich Welp, Jianmin Zuo, Venkat Selvamanickam, George W. Crabtree, Alexei E. Koshelev, Andreas Glatz, and Wai-Kwong Kwok, arXiv:1509.06446 (2015).   

Simulation of the vortex dynamics in a real pinning landscape of YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{7-\delta }}$ coated conductors

We present a critical current analysis of a real high-temperature superconducting (HTS) sample in a magnetic field by combining state-of-the-art large-scale Ginzburg-Landau simulations with reconstructive three-dimensional scanning transmission electron microscopy tomography of the pinning landscape in Dy-doped YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{7-\delta }}$. This methodology provides a unique look at the vortex dynamics in the presence of a complex pinning landscape, responsible for the high current-carrying capacity characteristic of commercial HTS wires. Our method demonstrates very good functional and quantitative agreement of the critical current between simulation and experiment, providing a new predictive tool for HTS wires design.   

Rapid Doubling of the Critical Current of YBa$_{\mathrm{2}}$Cu$_{\mathrm{3}}$O$_{\mathrm{7-\delta }}$ Coated Conductors.

We demonstrate the doubling of the critical current density of production-line REBCO coated conductors (CCs) in fields of 6 T \textbar \textbar c at 27 K by irradiation with a 3.5-MeV oxygen ion beam. This doubling of Jc is achieved within one second or less opening an industrially viable approach to address a persisting challenge, namely the greatly reduced performance of CCs in even modest applied magnetic fields. TEM images reveal that the enhanced critical current is due to finely dispersed small clusters approximately 5 nm in diameter. The major effect of the irradiation-induced defects is the reduction of the field-dependence of Jc, which we attribute to the mixed pinning landscape composed of strong pre-existing pin sites and the finely dispersed irradiation-induced defects. Work supported by the Center for Emergent Superconductivity, an EFRC funded by the U.S. Department of Energy, Office of Basic Energy Sciences. Patterning and microstructural characterization were performed at the Center for Nanoscale Materials, an Office of Science user facility, supported by the Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.   

Numerical optimization methods for critical currents in superconductors

In this work, I present optimization methods for maximizing the critical current in high-temperature superconductors for energy applications. The critical current in the presence of an external magnetic field is mostly defined by the pinning landscape (pinscape) within the superconductor, which prevents magnetic vortices from moving and, therefore, increases its critical current. Our approach is to generate different pinscapes and obtain the resulting critical current by large-scale time-dependent Ginzburg-Landau equations [J. Comp. Phys. \textbf{294}, 639 (2015)]. Pinning centers could be any combination of defects, including spherical and columnar defects. The parameters controlling the pinscape are adaptively adjusted in order to find the optimal parameter set, which maximizes the critical current. Here, we compare different optimization methods and discuss their performance.   

Probing Topological Matter with Sound

We introduce a microscopic formulation to identify the stress in a quantum fluid to compute the stress viscosity with sound waves. The viscosity stress tensor is used to determine, e.g. the ultrasound attenuation in superconductors. When an Abrikosov lattice is formed on the surface of a Topological Insulator in a external magnetic field, Majorana modes form dispersive bands. We show that the ultrasound attenuation is modified by the Majorana modes offering a novel method to identify Topological Superconductors. Moreover we compute the stress tunneling which uses Majorana modes and represent the sound analogue of the Andreev crossed reflection. We check the violation of the sound momentum conservation of systems which only exists on the boundary of a higher dimensional system,e.g. a $1D$ chiral fermion which can exist at the boundary of a $2D$ Quantum Hall system.