Normal Metal Research Articles

Thermoelectric properties of a quantum dot attached to normal metal and topological superconductor

The thermoelectric properties of hybrid systems based on a single-level quantum dot coupled to a normal-metal/half-metallic lead and attached to a topological superconductor wire are investigated. The topological superconductor wire is modeled by a spinless p-wave superconductor which hosts both a Majorana bound state at its extremity and above gap quasiparticle excitations. The main interest of our investigation is to study the interplay of sub-gap and single-particle tunneling processes and their contributions to the thermoelectric response of the considered system. The above gap tunneling driven by a temperature gradient is responsible for relatively large thermopower, whereas sub-gap processes only indirectly influence the thermoelectric response. The thermoelectric coefficients, including electric conductance, Seebeck coefficient (thermopower), heat conductance, and figure of merit, are calculated by means of the non-equilibrium Green’s function technique and the temperature dependence of the superconducting gap is considered within the BCS theory. We also consider the system out of equilibrium working as a heat engine. The output power and the corresponding efficiency are presented. Interestingly, under certain conditions, it is possible to extract more power in the superconducting phase than in the normal phase, with comparable efficiency.

Influence of the interfacial topological effect on the behavior of transport current in MgB2 material in the Meissner state and mixed state

Our research showed that a physical phenomenon appears in MgB2 wires, which has not been reported in previous studies. We have found that the flow of transport current in the current penetration depth of normal metal areas leads to the creation of voltage in magnetic fields from 0 to Bc1. Scanning electron microscope (SEM) images showed that normal areas can be formed at the junction between the superconducting material and the diffusion barrier. SEM photos indicate that this surface is very non-homogeneous and porous. This new physical phenomenon, that is, the voltage created by these normal areas at the current penetration depth is called, in this paper, the interfacial topological effect. Further measurements showed that above Bc1, the voltage disappears and becomes unmeasurable for magnetic fields above 120 mT. This is due to the appearance of a mixed state and current flow at a deeper depth of the superconducting material. From the interfacial topological effect, transport current flows on the outside surface of the superconducting MgB2 wires not only in magnetic fields from 0 to Bc1 but also in the mixed state and disappears in magnetic fields above 120 mT. This indicates that it is related to the superconducting state. The measurements performed by using a physical property measurement system (PPMS) for the low current (100 mA) indicate that the interfacial topological effect is on the boundary of two superconducting materials: the Nb diffusion barrier and the MgB2 material. In addition, the PPMS measurement results point out that the magnetic field eliminates the interfacial topological effect. Further results indicate that the appearance of voltage (the interfacial topological effect) in a MgB2 coil does not mean that the coil is not superconducting and cannot conduct the transport current without loss. Our results indicate that test procedures for MgB2 coils should assume exceeding the magnetic field of 120 mT because in this field, the voltage can disappear to zero and the transport current flows without loss. This is a very important factor for the future production of superconducting coils made of MgB2 wires with the Nb barrier on an industrial scale.

Compressive Mechanical Properties of a New High Density NiW Alloy

Abstract In order to apply a new high density NiW alloy to hollow kinetic energy penetrators, the mechanical properties of the alloy at high temperature and high strain rate are studied by Hopkinson pressure bar. The results show that the strength of the alloy decreases significantly with the increase of temperature, and the decreasing rate between 900°C to 1200°C is about 4 times that between 25°C to 900°C; the alloy has a strain rate strengthening effect, and the strain rate sensitivity increases with the increase of temperature under dynamic loading; unlike normal metals, the tangent modulus of the alloy does not decrease monotonously with the increase of temperature due to the precipitation of Ni4W phase, the solid solution strengthening mechanism and the dynamic recovery mechanism jointly determine the trend of the tangent modulus.

Nonreciprocal superconductivity.

We introduce the notion of nonreciprocal superconductors where inversion and time-reversal symmetries are broken, giving rise to an asymmetric energy dispersion. We demonstrate that nonreciprocal superconductivity can be detected by Andreev reflection. In particular, a transparent junction between a normal metal and a nonreciprocal superconductor generally exhibits an asymmetric current-voltage characteristic, which serves as a defining feature of nonreciprocal superconductivity. Unlike the superconducting diode effects, our detection scheme has the advantage of avoiding large critical currents that turn the superconducting state to normal. Last, we discuss candidates for nonreciprocal superconductivity, including graphene, UTe2, as well as engineered platforms.

Self-consistent evaluation of proximity and inverse proximity effects with pair-breaking in diffusive superconducting–normal metal junctions

Self-consistent evaluation of proximity and inverse proximity effects with pair-breaking in diffusive superconducting–normal metal junctions

Болометры с подвешенными абсорберами

A technology for manufacturing suspended bridges to improve the performance of superconducting micro- and nanodevices, sensors and electronic coolers is described. Samples of bolometers and electron coolers of the superconductor-insulator-normal metal-insulator-superconductor (SINIS) structure have been fabricated, in which the strip of normal metal does not touch the substrate, but hangs between the banks of superconducting aluminum. The fabrication technology involves isotropic chemical etching of aluminum and critical point drying, which allows the superconductor layer under the normal bridge to be removed and prevents the bridge from sagging and sticking to the substrate. The current-voltage characteristics of such devices were measured at a temperature of 0.3 K.

Influence of the Spin Hall Effect on the Resonance Frequency and Magnetic Susceptibility of a Magnonic Waveguide

The effect of the variation of the spin current on the magnetic susceptibility of a magnonic waveguide in the form of a “ferromagnet–normal metal” heterostructure is investigated. Based on the Landau–Lifshitz–Gilbert model with the current term in the Slonczewski–Berger form, which describes the magnetization dynamics including the spin moment transfer, expressions are obtained for the real and imaginary parts of the magnetic susceptibility in the geometry of surface spin waves in the damping mode. The resulting model correctly approximates experimental data demonstrating an increase in the amplitude of spin waves propagating in a YIG/Pt heterostructure. It is shown that an increase in the spin current leads to an increase in the resonance frequency of spin waves and in the magnetic susceptibility tensor components in resonance. The results of this study can be used to design waveguides for spin waves with controllable losses and high-sensitivity magnetic field sensors.

Transport of non-equilibrium quasiparticle excitations in superconducting aluminum

The electron transport of non-equilibrium quasiparticles injected into superconducting aluminum from a normal metal has been experimentally studied at ultralow temperatures. We studied hybrid nanostructures in the form of a T-shaped normal metal electrode (copper) – a dielectric tunnel layer (aluminum oxide) – a superconducting fork (aluminum), which can be considered as a solid-state analogues of a two-beam optical interferometer. At fixed bias voltages larger than the superconducting gap, a non-monotonic dependence of the tunnel current on perpendicular magnetic field is observed. The effect is interpreted as the presence of a coherent component of the quasiparticle current.

Universality of the Spectra of Multiterminal Josephson Junction

Andreev bound states are formed in multiterminal structures based on normal metals and superconductors. Their spectrum is determined by the system parameters, in particular the scattering phases and transmission coefficients at the nodes. The article found conditions under which Andreev bound states are universal: they don’t change with any change in the reflection phases. As a consequence, the spectrum is completely determined by the transport characteristics of the system. The result was obtained for a structure in the form of a normal metal M-finite star, each of the rays (terminals) Nk of which is in contact with its superconductor Sk, 1 ≤ k ≤ M. Together they form a multiterminal Josephson junction. At the center of the structure there is a non-magnetic impurity with its some scattering matrix.

Viscous terahertz photoconductivity of hydrodynamic electrons in graphene.

Light incident upon materials can induce changes in their electrical conductivity, a phenomenon referred to as photoresistance. In semiconductors, the photoresistance is negative, as light-induced promotion of electrons across the bandgap enhances the number of charge carriers participating in transport. In superconductors and normal metals, the photoresistance is positive because of the destruction of the superconducting state and enhanced momentum-relaxing scattering, respectively. Here we report a qualitative deviation from the standard behaviour in doped metallic graphene. We show that Dirac electrons exposed to continuous-wave terahertz (THz) radiation can be thermally decoupled from the lattice, which activates hydrodynamic electron transport. In this regime, the resistance of graphene constrictions experiences a decrease caused by the THz-driven superballistic flow of correlated electrons. We analyse the dependencies of the negative photoresistance on the carrier density, and the radiation power, and show that our superballistic devices operate as sensitive phonon-cooled bolometers and can thus offer, in principle, a picosecond-scale response time. Beyond their fundamental implications, our findings underscore the practicality of electron hydrodynamics in designing ultra-fast THz sensors and electron thermometers.

Comparative study on the impact of different E-J relations on the performance of resistive superconducting fault current limiters under high and low impedance faults in cryo-electric aircraft

To achieve the goal of zero emission in aviation, it is critical to advance electric aircraft technologies, especially in short to medium range flights. Superconducting fault current limiters (SFCLs) have shown promising potential in electric aircraft system as a protection solution to limit the peak of fault current. Many research works focused on the SFCL performance under low impedance faults, where the fault current is much higher than the nominal current and the SFCL will transit to normal (metal) state quickly. However, there are limited reports on investigating the SFCL reacting to high impedance faults for electric aircraft system, where the SFCL does not abruptly transit to normal state. In this paper, we built an electric-thermal coupled model in MATLAB which simulates the behaviour of a resistive type SFCL both under low and high impedance faults. We investigated three mathematical relations of local electric field E and local current density J representing the transition from superconducting state to normal state, both under high and low impedance fault. The three mathematical relations are classic E-J power-law, modified E-J power-law and two- stage E-J power law. The electric-thermal model of the SFCL incorporating all these three types of mathematical relations, respectively, have been used to calculate the SFCL responses under high and low impedance faults, and compared with each other, as well as testing results. The limited fault current, voltage drop, resistance and temperature variations in the SFCL are observed and analysed. The results have shown that the proposed SFCL model is able to accurately characterise a resistive type SFCL under both high impedance and low impedance fault.

Role of the Pseudomonas koreensis BB2.A.1 and Serratia liquefaciens BB2.1.1 Bacterial Strains in Maize Trace Metal Stress Management.

Plant-growth-promoting rhizobacteria (PGPR), in addition to their well-known direct effects on plant growth and development, have been reported to be effective in plant abiotic (trace metal, drought, etc.) and biotic (phytopathogens, insects, etc.) stress management. PGPRs are involved in shaping the fate of trace metals in the rhizosphere and plants and thus may also reduce trace metal stress in plants. The aims of our study were to isolate and select indigenous trace-metal-resistant PGP strains and investigate their effects on maize germination and early development. The roles of the two selected strains, Pseudomonas koreensis and Serratia liquefaciens isolated from trace-metal-contaminated soil were investigated to mitigate trace metal stress in 21-day-old Zea mays seedlings. In the present study, 13 bacterial strains were isolated and screened for PGP traits under normal and trace metal stress conditions. The effect of two selected strains was further studied on plant experiments. The germination process, plant growth parameters (length, weight, dry matter content), photosynthetic activity, GPOX activity, trace metal accumulation, and translocation in microbes inoculated Cd (0.5 mM), Zn (1 mM), and Cd + Zn (0.1 + 0.5 mM) treated maize plants was studied. Our results revealed that trace metal toxicity, in terms germination and growth parameters and antioxidant enzyme activity, was enhanced upon inoculation with Pseudomonas koreensis BB2.A.1. Chlorophyll content and accumulation studies showed enhanced results following inoculation with Serratia liquefaciens BB2.1.1. Therefore, both bacterial strains possessed beneficial traits that enabled them to reduce metal toxicity in maize.

Boosting spatial and energy resolution in STM with a double-functionalized probe.

The scattering of superconducting pairs by magnetic impurities on a superconducting surface leads to pairs of sharp in-gap resonances known as Yu-Shiba-Rusinov (YSR) bound states. Similar to the interference of itinerant electrons scattered by defects in normal metals, these resonances reveal a periodic texture around the magnetic impurity. The wavelength of these resonances is, however, often too short to be resolved even by methods capable of atomic resolution, i.e., scanning tunneling microscopy (STM). We combine a CO molecule with a superconducting cluster pre-attached to an STM tip to maximize both spatial and energy resolution, thus demonstrating the superior properties of such double-functionalized probes by imaging the spatial distribution of YSR states. Our approach reveals rich interference patterns of the hybridized YSR states of two Fe atoms on Nb(110), previously inaccessible with conventional STM probes. This advancement extends the capabilities of STM techniques, providing insights into superconducting phenomena at the atomic scale.

Material Design and Discovery in Full-Heusler Compounds: A Comprehensive First-Principles Analysis of XMg2Hg, XMgHg2, and X2MgHg (X = Sc, Li)

This study conducts a comprehensive first-principles analysis of the structural, mechanical, phonon dispersion, and electronic properties of XMg2Hg, XMgHg2, and X2MgHg (X = Sc and Li) compounds. Using energy-volume curves, cohesive and formation energy, and phonon dispersion analyses, we confirm the stability of these compounds. Our calculations reveal that Li2MgHg and ScMg2Hg are more stable in the cubic structure with space group F4¯3m (216), whereas other compounds are stable in the Fm3¯m (225) structure. Phonon dispersion calculations indicate dynamical stability for all compounds except Li2MgHg in the Fm3¯m structure and Sc2MgHg and LiMg2Hg in the cubic structure with space group F4¯3m (216). Mechanical stability is confirmed through the calculation of elastic constants, with Sc-based compounds showing higher bulk modulus, shear modulus, and Young's modulus compared to Li-based compounds. Electronic properties, analyzed through density of states and band structure calculations, confirm the metallic nature of these compounds, with significant contributions from Mg atoms at the Fermi energy. The study also identifies distinct electronic features such as flat electron bands and a Dirac point at the Gamma point for ScMgHg2​. Pressure-dependent studies indicate these materials are normal metals without topological phase transitions.

(Invited) Is the [5,5] C120-D5d Fullertube a Metal?

Single-walled nanotubes (SWNTs) with a roll-up vector (n = m) have long been predicted to exhibit metallic behavior at infinite nanotube lengths.1 Moreover, in contrast with normal metals, armchair nanotubes e.g., [5,5] should result in exceptional ballistic transport properties.2 However, the question remains at what finite length of a SWNT or fullertube does metallic character begin, if at all. From a computational viewpoint, Cioslowski has shown that the HOMO-LUMO bandgap decreases to ~1 eV in an oscillatory fashion in approaching [5,5] C200 fullertubes.3 Although the HOMO-LUMO gap has been extensively used to predict the kinetic stability of fullerenes and metallofullerenes with less than 100 carbon atoms, Fowler has argued that the HOMO-LUMO gap can not be used to predict the kinetic stability of large fullerenes.4 More recently, Lilijeroth and coworkers have measured the experimental and DFT HOMO-LUMO gap for graphene nanoribbons (GNR) and find that GNRs with lengths of 5 nm have bandgaps of ~0.1 eV.5 These workers also point out that DFT calculated HOMO-LUMO bandgaps are always overestimated relative to experimental values. In this presentation, I will present the evidence in support of metallic character for [5,5] C120-D5d.6,7

Large Superconducting Diode Effect in Chiral Carbon Nanotubes

Superconducting electronics are ubiquitous in devices ranging from single-photon detectors to quantum bits. One of the most common superconducting circuit elements is a Josephson junction (JJ)—a junction of two superconductors coupled through either a small normal metal or an insulator. Recently, non-reciprocal switching currents in JJs based on traditional superconductors have been reported, demonstrating a robust diode effect. This so-called Josephson diode effects (JDEs) has since attracted interest to shed light on material and device properties, but also to enable new applications. In this talk, we will present theoretical results on the JDE in chiral carbon nanotubes (CNTs). We find that these CNT-JJs can exhibit large diode efficiencies when an external magnetic field is applied along the nanotube. Additionally, our numerical simulations show that the polarity of the diode can be tuned by electrostatically gating the CNT-JJ. We will discuss the microscopic details that give rise to large diode efficiencies and gate-tunability in chiral CNT-JJs.This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award DE-SC0022245. The work at Sandia is supported by a LDRD project. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Observation and characterization of titanium-like nano-filament in TiO2 memristor using superconducting electrode(s) and Andreev spectroscopy

A thin TiO2 semiconductor film embedded between two metal electrodes works as a memristor after being formed by soft breakdown. The forming creates a nano-filament that penetrates through the poorly conducting TiO2 film and connects the electrodes conductively. While previous works characterized the nano-filament properties (shape, composition, and resistivity) by electron microscopy techniques, we present a characterization by electrical measurements. In a typical memristor, both electrodes are made of normal metals. We study the metal/TiO2/metal memristors with a bottom electrode made of a superconducting NbN layer and a top electrode made of a normal (Pt) or superconducting (Nb) metal. The nano-filament connecting the electrodes touches the bottom electrode as a point contact, thus allowing us to perform point-contact Andreev reflection spectroscopy of the NbN superconductor. The spectra, measured below the critical temperature (15 K) of NbN, are analyzed theoretically. The analysis reveals the presence of one nano-filament and determines the nano-filament resistance, Sharvin resistance of the point contact, and Maxwell resistance of the electrodes. Moreover, it shows that the nano-filament is a conical-shaped Ti-like metal point contact with a tip diameter of ∼3–5 nm, Fermi velocity of 2×106m/s, and low-temperature resistivity of ∼10−8–10−7Ωm. Thus, the nano-filament in our device is not the Ti4O7 phase observed in previous works. Remarkably, the point contact spectrum of the superconducting NbN layer shows the Andreev peak typical for ballistic transport. This is because the point contact probes the NbN layer through a thin Al layer that mimics superconductivity of NbN via the proximity effect and eliminates the effects of tunneling and disorder.

Thermal and Coherent Spin Pumping by Noncollinear Antiferromagnets.

Antiferromagnets attract much interest because of their potential for spintronic applications and open fundamental physics questions, but especially noncollinear antiferromagnets remain relatively unexplored. Here, we formulate the thermal and coherent pumping of spins in noncollinear antiferromagnets|normal metal bilayers. We find that the spin current polarization is a vector with components along both the Néel vector and net magnetic moment. The spin mixing conductance for the coherent spin pumping is a tensor with elements depending on the degree of noncollinearity and interface spin configuration. We explain the controversial sign problem of the antiferromagnetic spin Seebeck effect by interface effects and suggest that interface engineering may enhance the spin pumping efficiency.

Mathematical Solution for Vibrational Response of Shear and Normal Deformable Advanced Metal Foam CMBS Treated with Nanocomposite Actuators

This research focuses on examining the vibrational properties of intelligent curved microbeams (CMBs) that incorporate metal foams and are enhanced with carbon nanotubes-reinforced piezoelectric (CNTRP) actuators. An advanced four-variable theory for shear and normal deformation is applied in the polar coordinate framework to investigate the microstructure, thereby negating the need for a shear correction factor. Additionally, the modified couple stress theory (MCST) is utilized to account for scale effects. The structural attributes exhibit thickness-dependent alterations following predefined functions. The governing equations of motion are deduced using Hamilton’s principle. In instances where the structure has simply supported ends, Fourier series functions are utilized to solve these equations. The outcomes are cross-validated in contradiction with previously documented works with more straightforward setups. The inquiry investigates the effects of critical parameters on the vibrational response of the structure. The results are corroborated in simpler scenarios using existing data in the literature to ensure accuracy.

Chirality-Induced Magnet-Free Spin Generation in a Semiconductor.

Electrical generation and transduction of polarized electron spins in semiconductors (SCs) are of central interest in spintronics and quantum information science. While spin generation in SCs is frequently realized via electrical injection from a ferromagnet (FM), there are significant advantages in nonmagnetic pathways of creating spin polarization. One such pathway exploits the interplay of electron spin with chirality in electronic structures or real space. Here, utilizing chirality-induced spin selectivity (CISS), the efficient creation of spin accumulation in n-doped GaAs via electric current injection from a normal metal (Au) electrode through a self-assembled monolayer (SAM) of chiral molecules (α-helix l-polyalanine, AHPA-L), is demonstrated. The resulting spin polarization is detected as a Hanle effect in the n-GaAs, which is found to obey a distinct universal scaling with temperature and bias current consistent with chirality-induced spin accumulation. The experiment constitutes a definitive observation of CISS in a fully nonmagnetic device structure and demonstration of its ability to generate spin accumulation in a conventional SC. The results thus place key constraints on the physical mechanism of CISS and present a new scheme for magnet-free SC spintronics.