Short Wire Research Articles

Experimental Test and Analysis of AC Losses in Multifilamentary MgB2 Wire

AC losses in superconductors are essential for the design of cooling system for large scale power applications. Magnesium diboride (MgB2) superconducting wires have been investigated and manufactured over the last decade due to cheap raw materials and flexibility for coil design. In addition, multifilamentary MgB2 wires have been manufactured to reduce ac losses. In this paper, self-field ac losses of multifilamentary MgB2 wires with magnetic barrier are investigated using both experimental and numerical methods. A short straight wire sample and a coil sample are tested under various temperatures and frequencies between 16- and 128-Hz. The test results show that the transportation loss is independent of the operating temperature. On basis of both theoretical and numerical study, it is found that hysteresis loss in superconductor accounts only for a small fraction of the transportation losses, ferromagnetic hysteresis loss in the magnetic barrier dominates when the transport current is low, whereas eddy current loss dominates when the transport current is close to the critical current.

Flexible broadband polarization converter based on metasurface at microwave band**Project supported by the Fundamental Research Funds for the Central Universities, China (Grant No. kfjj20180401), the National Natural Science Foundation of China (Grant No. 61471368), the Aeronautical Science Foundation of China (Grant No. 20161852016), the China Postdoctoral Science Foundation (Grant No. 2016M601802), and Jiangsu Planned Projects for Postdoctoral Research Funds, China

A flexible broadband linear polarization converter is proposed based on the metasurface operating at microwave band. To achieve bandwidth extension property, long and short metallic arc wires, as well as the metallic disks placed over a ground plane, are combined into the polarizer, which can generate three neighboring resonances. Due to the combination of the first two resonances and the optimized size and thickness of the unit cell, the polarization converter can have a weak incident angle dependence. Both simulated and measured results confirm that the average polarization conversion ratio is over 85% from 11.3 GHz to 20.2 GHz within a broad range of incident angle from 0° to 45°. Moreover, the proposed polarization converter based on flexible substrates can be applied for conformal design. The simulation and experiment results demonstrate that our designed polarizer still keeps high polarization conversion efficiency, even when it adheres to convex cylindrical surfaces. The periodic metallic structure of the designed polarizer has great potential application values in the microwave, terahertz, and optic regimes.

Design and analysis of multi‐band polarisation selective metasurface

In this article, based on the idea of the metamaterial designing, the authors present a design and analysis of a polarisation selective metasurface (PSM), providing a multi-band response. The proposed PSM is composed three metallic layers. The middle- and lower-level layers are formed of combinations of short and long metal wire with periodic arrangement and structural dimensions, while the upper layer is consisting of gridded square loops arrays. Among the three polarisation selective operating bands generated by PSM, the 3-db transmission is used as the threshold of the operating band. TE polarised wave passband bandwidth is 1.5 and 1.83 GHz, respectively, while TM polarised wave passband bandwidth is 1.13 GHz. In each passband, the insertion loss is <;0.5 dB. Through simulation and measurement, the authors can find that the PSM structure designed in this article has excellent reconfigurable characteristics in the range of 0° to 60° and is insensitive to incident angle. The excellent polarisation selection characteristics can not only be applied to satellite communication systems as polarisation wave generators or polarisation separation devices but also have important significance in improving the anti-interference performance and detection capability of satellite antennas.

A 3GS/s 12-bit Current-Steering Digital-to-Analog Converter (DAC) in 55 nm CMOS Technology

A 3GS/s 12-bit current-steering digital-to-analog converter (DAC) fabricated in 55 nm complementary metal–oxide–semiconductor (CMOS) technology has been presented. A partial randomization dynamic element matching (PRDEM) method based on switching sequence optimization is proposed to mitigate the mismatch effect and suppress the harmonic distortion with low hardware complexity. In the switching current cell, the cascode structure together with “always-ON” small current sources are used to keep the output impedance high and uniform. A compact layout of the switching current array is carefully designed, featuring short wires routing and small parasitic capacitance. According to the measured results at 3GS/s, this DAC demonstrates a spurious-free dynamic range (SFDR) of 74.64 dBc at low frequency and 50 dBc at 1.5 GHz output. The chip occupies an active area of 0.2 × 0.48 mm2 and consumes a total power of 495 mW.

Vapor–liquid–solid silicon nanowires growth catalyzed by indium: study of indium oxide effect

In this work, we report on vapor–liquid–solid growth of silicon nanowires (SiNWs) catalyzed by indium, a low-eutectic post-transition metal. The indium catalyst is synthesized ex situ by annealing indium-coated substrates using two different annealing processes: rapid thermal annealing (RTA) and conventional process. The effect of annealing parameters on indium catalyst properties is studied. We show that after conventional annealing at 600 °C during 45 min, the indium layer is cracked into elongated and inhomogeneous islands of different sizes. X-ray diffraction (XRD) analysis depicts in addition to pure indium planes the presence of new peaks attributed to indium oxide planes formed during annealing. While by using RTA process, oxide-free indium particles were successfully grown in one step by during short time (5 min) at 400 and 450 °C. Quasi-spherical and homogeneously distributed indium particles were obtained at 450 °C. A comparative study between SiNWs catalyzed by indium catalyst prepared following the two different processes was carried out. The indium oxide presence negatively affected the catalytic property of indium, resulting in a lower density of the grown SiNWs. An improvement of the SiNWs density was achieved with RTA-annealed catalyst, as more indium particles were present to act as active catalyst to the growth. The catalyst annealing conditions also affected the size of the SiNWs, with shorter wires with RTA process. However, the shape of the SiNWs was similar in both studied cases, where the wires were bent and kinked. The morphology investigation of the SiNWs also shows that the SiNWs have a core–shell structures consisting of amorphous and crystalline silicon.

An original setup to measure grounding resistances using fast impulse currents and very short leads

This paper presents a method for measuring grounding resistances, which is based on the use of fast impulse current and short leads on the auxiliary measurement circuits. Usually, the conductor length of the voltage and current circuits used in grounding resistance measurements may be quite long depending on the grounding characteristics. Using the technique proposed in this paper, measurements are performed using short cable in the potential circuit and short wires, cable or LIA (Portuguese acronym to Artificial Infinite Line), in the current circuit. The proposed technique is evaluated by comparisons with measurement results using classical techniques and also through numerical simulations.

Photon-Mediated Thermoelectric and Heat Currents through a Resonant Quantum Wire-Cavity System

We theoretically consider a short quantum wire, which on both ends is connected to leads that have different temperatures. The quantum wire is assumed to be coupled to a cavity with a single-photon mode. We calculate the heat and thermoelectric currents in the quantum wire under the effect of the photon field. In the absence of the photon field, a plateau in the thermoelectric current is observed due to the thermal smearing at a high temperature gradient. In the presence of the resonance photon field, when the energy spacing between the lowest states of the quantum wire is approximately equal to the photon energy, a suppression in thermoelectric current and negativity in the heat current are seen due to the dressed electron-photon states. It is also found that the cavity with high photon energy has more influence on the thermoelectric current at a high temperature gradient.

Development of a method to measure the thermal conductivity of pressurised solutions containing dense gases using 11000 g/mol polydimethylsiloxane and carbon dioxide as example fluid

The development of a method to measure the thermal conductivity of mixtures containing pressurised gases is presented. As example fluid, we use carbon dioxide mixed with a linear polydimethylsiloxane (PDMS) with a molecular weight of 11000 g/mol. Experiments were carried out at 25 °C, 40 °C and 60 °C in a pressure range of up to 16 MPa. Thermal conductivity was measured in a high-pressure view cell using two different sensors: a cylindrical needle sensor and a short hot wire. Both sensors are based on the principle of a transient linear heat source. Their applicability was compared and evaluated. Rather low molecular weight polydimethylsiloxane was chosen as model substance to close the data gap regarding the thermal conductivity of gas saturated solutions, pressurised with carbon dioxide. All experiments were carried out under isothermal conditions. It was the aim of the present work to develop and to test an adequate measuring instrument; this involves the selection and implementation of appropriate auxiliary equipment, too.

Properties of the Majorana-state tunneling Josephson junction mediated by an interacting quantum dot

We consider a model of a Josephson junction of two topological superconducting wires mediated by an interacting quantum dot. An additional normal electrode coupled to the dot from the top allows to probe its density of states. The Majorana states adjacent to the dot hybridize across the junction and from a bound state in the dot. The dot is subjected to the effective magnetic field arising from the superposition of the fields driving each wire into topological states, which, dependent on the angle between the fields, introduces variable Zeeman splitting of the dot active level. We show that electron interactions in the dot diminish the characteristic for Majoranas zero bias peak arising in the transverse conductance through the dot and introduce an overall asymmetry of the conductance. They also renormalize the hybridization between the end-state Majoranas in shorter wires. The Majorana spin polarization is determined by the effective magnetic field in the dot. Phase-biased Josephson current exhibits spin polarization in thermal equilibrium, which possesses characteristic periodicity, and its sign can be switched when an unpaired Majorana state is present in the junction. We also observe spin-dependent Majorana state ‘leaking’, which can be controlled by the position of the dot level in energy scale.

Fractional Josephson effect with and without Majorana zero modes

It is known that the low-energy physics of the Josephson effect in the presence of Majorana zero modes exhibits a $4\pi$ periodicity as the Aharonov-Bohm flux varies in contrast to the $2\pi$ Josephson periodicity in usual superconducting junctions. We study this fractional Josephson effect in 1D topological superconductors in Majorana nanowire systems by focusing on the features of the phase-energy relations in a superconducting semiconductor nanowire with spin-orbital coupling by including different factors operational in experimental systems, such as short wire length, suppression of superconducting gap, and the presence of an Andreev bound state. We show that even in the absence of Majorana zero modes, some non-topological physical effects can manifest a $4\pi$ periodicity of the phase-energy relation in the Josephson junction, thus providing a false positive signal for fractional Josephson effect with no underlying Majorana zero modes. Furthermore, we consider several scenarios of inhomogeneous chemical potential distributions in the superconducting nanowire leading to four Majorana bound states and construct the effective four Majorana model to correctly describe the low-energy theory of the Josephson effect. In this setup, multiple Majorana zero modes can also have the $4\pi$ fractional Josephson effect, although the underlying physics arises from Andreev bound states since two close by Majorana bound states effectively form Andreev bound states. Our work demonstrates that the mere observation of a fractional Josephson effect simulating $4\pi$ periodicity cannot, by itself, be taken as the definitive evidence for topological superconductivity. This finding has important implications for the ongoing search for non-Abelian Majorana zero modes and efforts for developing topological qubits.

Meta-Surface Antenna Array Decoupling Designs for Two Linear Polarized Antennas Coupled in H-Plane and E-Plane

In this paper, meta-surface superstrates are used to decouple two linear polarized antennas coupled in H-plane and E-plane, respectively. By properly designing the geometry of the double layer short wires as the unit cell of the meta-surface, as well as the height of the meta-superstrate, two linearly polarized antennas can be decoupled in H-plane and E-plane, respectively. Both the antenna pairs coupled in E- and H-planes with and without meta-surfaces are fabricated and measured. The results demonstrate that the antennas with the meta-surface superstrate are able to operate in the band of 3.3–3.7 GHz with reflection better than −15 dB and the isolation can be improved from 10 to 25 dB in the H-plane case and can be improved from 15 to 30 dB in the E-plane case. Such decoupling method can be applied extensively in 5G base stations where size constraints are becoming stringent.

A Meta-Surface Decoupling Method for Two Linear Polarized Antenna Array in Sub-6 GHz Base Station Applications

In this paper, an extremely compact two-element linear polarized multiple-input-multiple-output (MIMO) antenna array with a metasurface superstrate is proposed. Instead of using periodic square split-ring resonators which occupy larger space and which are incident angle variant, double-layer short wire is utilized as the unit cell of the metasurface. The metasurface is compact in size and effective in decoupling two nearby Bowtie antennas strongly coupled in the H-plane with the spacing of only 0.27 wavelength. After decoupling, the isolation between the two antennas has been improved from around 10 dB to more than 25 dB within the band of 2300 to 2690 MHz while their reflection remained below −15 dB. Moreover, the radiation pattern after adding the metasurface superstrate is well maintained with total efficiency improvement by about 10%, and the envelope correlation coefficient between the two antennas is reduced from 0.35 to below 0.12 within the whole band of interest. The proposed method can find plenty of applications in MIMO and 5G communication systems.

Spin-orbit coupling and resonances in the conductance of quantum wires

We investigate a possibility of pair electron-electron ($e\ensuremath{-}e$) collisions in a ballistic wire with spin-orbit coupling and only one populated mode. Unlike in a spin-degenerate system, a combination of spin splitting in momentum space with a momentum-dependent spin precession opens up a finite phase space for pair $e\ensuremath{-}e$ collisions around three distinct positions of the wire's chemical potential. For a short wire, we calculate the corresponding resonant contributions to the conductance, which have different power-law temperature dependencies, and, in some cases, vanish if the wire's transverse confinement potential is symmetric. Our results may explain the recently observed feature at the lower conductance plateau in InAs wires.

X-pinch X-ray emission on a portable low-current, fast rise-time generator

We report on experiments exploring X-ray emission from an X-pinch driven by a small Marx-waterline generator supplying 50 kA with a risetime of 50 ns and a peak voltage of ∼250 kV. Both standard crossed wire loads and hybrid loads utilizing conical metal electrodes with a single short wire in between them were studied, and in both cases reliable modes of operation were obtained for X-ray radiography. Soft (few keV) and Hard (&amp;gt;5 keV) X-ray emission characteristics were observed. With standard X-pinches, soft radiation emanated from a small hot spot about 3 μm in size, along with hard radiation from a ∼200 μm region close to this hot spot. With hybrid X-pinches, the hot spot was &amp;lt;7 μm in size. There was a clear correlation between the soft and hard X-ray emission—pinches that produced intense soft X-ray emission from a small hot spot also produced the most intense, localized hard X-ray emission.

Color Ultrafast Photography Study of a High-Current Discharge Initiated by Exploding a Short Wire in the Atmosphere

In this paper, we describe the ultrafast slit-scanning photography of light phenomena arising at the stage of the electrical breakdown in the atmosphere and in a channel initiated by a wire explosion. The characteristics and features of the photographic registration of the SFR-1 camera using color and spectrozonal negative aerial films sensitive in the visible and near infrared regions of the optical spectrum are presented. Using photograms in color, we were able to visualize small-scale inhomogeneities, a current-carrying channel and its “shell,” emissions of matter, and shock waves in the plasma. This technique made it possible to record a secondary shock wave in the plasma on the discharge axis during the explosion of short (length ≈15 mm) copper and nichrome wires with a diameter of 100–150 μm at a stored energy of ~100 J. At the moment of cumulation in the plasma, a “hot spot” with a minimum size of ~0.5 mm was formed on the discharge axis. It may be the source of narrow-beam coherent radiation with X-ray photon energies of 10–30 keV. The cumulation mechanism is discussed based on the regularities of the z-pinch phenomenon in a discharge initiated by a wire explosion.

Non-Gaussian Current Fluctuations in a Short Diffusive Conductor.

We report the measurement of the third moment of current fluctuations in a short metallic wire at low temperature. The data are deduced from the statistics of voltage fluctuations across the conductor using a careful determination of environmental contributions. Our results at low bias agree very well with theoretical predictions for coherent transport with no fitting parameter. By increasing the bias voltage we explore the crossover from elastic to inelastic transport.

Application of Ampere’s law to a non-infinite wire and to a moving charge

In this work we demonstrate how to apply Ampere’s law to a non-infinite wire that is a part of a complete circuit with a steady current. We show that this can be done by considering the magnetic field from the whole circuit, without having to directly introduce the displacement current. This example can be used to isolate and clarify students’ confusion about the application of Ampere’s law to a short wire. The second part of this work focuses on the application of Ampere’s law to a non-relativistic moving charge. It exposes the students to the Dirac delta function in a physical example and guides them to finding the magnetic field of a moving charge in a reasonable way.

Current correlations for the transport of interacting electrons through parallel quantum dots in a photon cavity

We calculate the current correlations for the steady-state electron transport through multi-level parallel quantum dots embedded in a short quantum wire, that is placed in a non-perfect photon cavity. We account for the electron–electron Coulomb interaction, and the para- and diamagnetic electron–photon interactions with a stepwise scheme of configuration interactions and truncation of the many-body Fock spaces. In the spectral density of the temporal current–current correlations we identify all the transitions, radiative and non-radiative, active in the system in order to maintain the steady state. We observe strong signs of two types of Rabi oscillations.

Studying the Dynamics of Hybrid X-Pinches

Since the first experiments with hybrid X-pinches (HXP), it has been shown that a short wire between two conical tungsten electrodes produces the same radiation as a standard X-pinch. Since HXP has a simple construction, it is commonly used in many laboratories instead of standard pinches. Although details of the HXP dynamics remain poorly studied, the main factors governing HXP formation are investigated experimentally in this work. It is shown that the process of the electrode plasma formation is one of the main factors that lead to single hot spot bursts. Experimental results allow the fitting of HXP parameters and determination of the size of the source of X-ray radiation. It is also shown the radiation of HXP can be used as a source for point projection radiography in the wavelength range of soft to hard X-rays. Such sources can be also used for absorption spectroscopy and even electron beams.

Backscattered EM-wave manipulation using low cost 1-bit reflective surface at W-band

The design of low cost 1-bit reflective (non-absorptive) surfaces for manipulation of backscattered EM-waves and radar cross section (RCS) reduction at W-band is presented in this article. The presented surface is designed based on the reflection phase cancellation principle. The unit cell used to compose the proposed surface has an obelus (division symbol of short wire and two disks above and below) like shape printed on a grounded dielectric material. Using this unit cell, surfaces that can efficiently manipulate the backscattered RCS pattern by using the proposed obelus-shaped unit cell (as ‘0’ element) and its mirrored unit cell (as ‘1’ element) in one surface with a 180° ± 35° reflection phase difference between their reflection phases are designed. The proposed surfaces can generate various kinds of backscattered RCS patterns, such as single, three, or four lobes or even a low-level (reduced RCS) diffused reflection pattern when those two unit cells are distributed randomly across the surface aperture. For experimental characterization purposes, a 50 × 50 mm2 surface is fabricated and measured.