Transport Current Research Articles

Flux-pinning mediated superconducting diode effect in NbSe2/CrGeTe3 heterostructure

In ferromagnet/superconductor bilayer systems, dipolar fields from the ferromagnet can create asymmetric energy barriers for the formation and dynamics of vortices through flux pinning. Conversely, the flux emanating from vortices can pin the domain walls of the ferromagnet, thereby creating asymmetric critical currents. Here, we report the observation of a superconducting diode effect (SDE) in a NbSe2/CrGeTe3 van der Waals heterostructure in which the magnetic domains of CrGeTe3 control the Abrikosov vortex dynamics in NbSe2. In addition to extrinsic vortex pinning mechanisms at the edges of NbSe2, flux-pinning-induced bulk pinning of vortices can alter the critical current. This asymmetry can thus be explained by considering the combined effect of this bulk pinning mechanism along with the vortex tilting induced by the Lorentz force from the transport current in the NbSe2/CrGeTe3 heterostructure. We also provide evidence of critical current modulation by flux pinning depending on the history of the field setting procedure. Our results suggest a method of controlling the efficiency of the SDE in magnetically coupled van der Waals superconductors, where dipolar fields generated by the magnetic layer can be used to modulate the dynamics of the superconducting vortices in the superconductors.

Probing Quantum Phases in Ultra-High-Mobility Two-Dimensional Electron Systems Using Surface Acoustic Waves.

Transport measurement, which applies an electric field and studies the migration of charged particles, i.e., the current, is the most widely used technique in condensed matter studies. It is generally assumed that the quantum phase remains unchanged when it hosts a sufficiently small probing current, which is, surprisingly, rarely examined experimentally. In this Letter, we study the ultra-high-mobility two-dimensional electron system using a propagating surface acoustic wave, whose traveling speed is affected by the electrons' compressibility. The acoustic power used in our Letter is several orders of magnitude lower than previous reports, and its induced perturbation to the system is smaller than the transport current. Therefore we are able to observe the quantum phases become more incompressible when hosting a perturbative current.

Effects of Soliton Creation on Transient Transport through a Polymer Chain.

By using a nonadiabatic molecular dynamics method combined with the hierarchical equations of motion, we have investigated the nonequilibrium transient transport through a conjugated polymer chain. The polymer chain is described by the Su-Schrieffer-Heeger model, and its two ends are coupled with metal electrodes of different chemical potentials. In order to present the evolutions of the electronic injection and transport in the real-time domain, we have mainly discussed the dynamic relaxation processes of the excited states and transient transport currents. It is found that due to the existence of electron-phonon couplings in the conjugated polymers, creation of solitons not only affects the time of the system achieving the steady state but also leads to periodical oscillations of the steady-state transport currents with time in our simulations. Furthermore, with increasing applied bias voltage, the steady-state transport electronic current increases, which proved that the creation of the solitons can assist the electronic transport. These results have shown that the creation of the excited states is important in understanding the transport properties in organic nanostructures.

What mechanistic information is contained in solid-supported membrane electrophysiology measurements of average transporter currents? A Bayesian analysis

What mechanistic information is contained in solid-supported membrane electrophysiology measurements of average transporter currents? A Bayesian analysis

Inter-discharge optimization for fast, reliable access to ASDEX Upgrade advanced tokamak scenario

Rapid inter-discharge simulation and optimization using the RAPTOR code have allowed the development of a reliable and reproducible early heating strategy for an advanced tokamak (AT) scenario on ASDEX Upgrade. Solving for electron heat and current diffusion in RAPTOR with ad-hoc formulas for heat transport and electron cyclotron current drive (ECCD) efficiency is found to robustly recover the coupled dynamics of Te and q, while maintaining model parameters fixed for all discharges. The pedestal top boundary condition in pre-shot simulations is set by a newly derived scaling law for the electron pressure at ρ = 0.8, using a data set of previous AT discharges. RAPTOR simulations have allowed to develop an understanding of the onset of 3/2 tearing modes, which were observed to have a detrimental impact on confinement when low magnetic shear conditions are present at the rational surface during the high-β phase. Delaying the NBI heating, by a specific time interval found via simulations, has led to avoiding these modes. A non-linear optimization scheme has been applied to optimize the ECCD deposition radii to reach a stationary state with qmin>1 at the beginning of the flat-top phase, while ensuring a non-zero magnetic shear at q = 1.5 throughout the high-β phase, and has been successfully tested in experiment. However, further experiments, aiming for qmin>1.5 , have highlighted limitations of the present feedforward control approach in the presence of shot-to-shot variations that are not included in the applied model. Application of real-time model-based control is proposed to overcome model-reality mismatches in future work.

Spin-Resolved Magneto-Tunneling and Giant Anisotropic g-Factor in Broken Gap InAs-GaSb Core-Shell Nanowires.

We experimentally and computationally investigate the magneto-conductance across the radial heterojunction of InAs-GaSb core-shell nanowires under a magnetic field, B, up to 30 T and at temperatures in the range 4.2-200 K. The observed double-peak negative differential conductance markedly blue-shifts with increasing B. The doublet accounts for spin-polarized currents through the Zeeman split channels of the InAs (GaSb) conduction (valence) band and exhibits strong anisotropy with respect to B orientation and marked temperature dependence. Envelope function approximation and a semiclassical (WKB) approach allow to compute the magnetic quantum states of InAs and GaSb sections of the nanowire and to estimate the B-dependent tunneling current across the broken-gap interface. Disentangling different magneto-transport channels and a thermally activated valence-to-valence band transport current, we extract the g-factor from the spin-up and spin-down dI/dV branch dispersion, revealing a giant, strongly anisotropic g-factor in excess of 60 (100) for the radial (tilted) field configurations.

A flux pump driven non-soldering closed-loop HTS magnet and its electromagnetic-thermal semi-analytical modelling method

We propose a new flux pump driven non-soldering closed-loop (NSCL) high temperature superconducting (HTS) magnet free of any soldering joints throughout the magnet for persistent current mode (PCM) operation. All the superconducting parts of the magnet are wound directly with HTS closed-loop ring split from an 8 mm REBCO tape using our new cutting scheme free of any soldering requirements. The magnet contains two single-pancake coils connected in series forming a closed circuit through two parallel bridge branches. Two thermal switches set on the two bridge branches control the on–off of the two bridges. A copper coil with iron core installed around one of the bridges is employed as the flux pump to drive the HTS magnet. An electromagnetic-thermal semi-analytical modelling method is proposed to analyse the pumping process by which the transport current in the magnet is calculated. The theoretical limit equation of the saturation current is improved as well. The proposed method can predict the current of the NSCL HTS magnet during the pumping process and provide results that are close to experiments. Experiments verify both the feasibility of the proposed flux pump driven NSCL HTS magnet and the modelling method. The results show that the NSCL HTS magnet works well in the PCM, which provides inspiration to the design of PCM operation of high field HTS magnets. The proposed modelling method also helps guide the design of different forms of HTS magnets and the flux pumps driving them.

Non-Markovian master equation for quantum transport of fermionic carriers.

We propose a simple, yet feasible, model for quantum transport of fermionic carriers across tight-binding chain connecting two reservoirs maintained at arbitrary temperatures and chemical potentials. The model allows for elementary derivation of the master equation for the reduced single particle density matrix in a closed form in both Markov and Born approximations. In the Markov approximation the master equation is solved analytically, whereas in the Born approximation the problem is reduced to an algebraic equation for the single particle density matrix in the Redfield form. The non-Markovian equation is shown to lead to resonant transport similar to Landauer's conductance. It is shown that in the deep non-Markovian regime the transport current can be matched with that obtained by the non-equilibrium Green's function method.

Enhancing electron transport of 7-bis(diphenylphosphoryl)-9,9′-spirobifluorene through polarity reduction of polar covalent bonds using organic molecular additives

In the electron-transporting organic semiconductor, polar covalent bonds are a necessary design to deepen the Lowest Occupied Molecular Orbitals for effective charge injection. However, these bonds can increase molecular dipole moment, which can impede charge transport. In this study, molecular additives including Tetrathiafulvalene (TTF), 4-(2,3-Dihydro-1,3-dimethyl-1H-benzimidazol-2-yl)-N,N-dimethylbenzenamine (N-DMBI), and tetrakis(dimethylamno)ethylene (TDAE) were investigated. Despite being electron donors, no charge transfer occurred between the molecular additives and the π conjugated core of 7-bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13). However, the addition of 2% TTF resulted in a higher current density, while N-DMBI and TDAE led to a decrease. X-ray photoelectron spectroscopy, density functional theory, and reduced density gradient analysis indicated that a marginal charge transfer occurred from phosphorus atoms to TTF, forming a strong attractive bond between P = O and TTF, which was absent in N-DMBI and TDAE. The low molecular polarity and lack of polar covalent bonds are prerequisites for polarity reduction of the P = O bond in SPPO13, as demonstrated by the 2% addition of TTF resulting in higher electron transport current.

Non-uniform current distribution in parallel-wound no-insulation high-temperature superconductor coil during ramping and fast discharging operations

A parallel-wound no-insulation (PWNI) high-temperature superconductor (HTS) coil is a kind of pancake-shaped no-insulation (NI) coil wound with parallel-stacked HTS tapes, which combines the characteristics of a NI coil and non-twisted stacked-tape cable. It shows a significant advantage in accelerating the ramping response compared with traditional NI HTS coils wound by a single tape, and is a promising alternative for large-scale high-field magnets. The stacked cable approach can lead to current redistribution between parallel tapes during ramping operations. It couples with the turn-to-turn current redistribution and leads to a much more complicated current redistribution inside the PWNI coil, the mechanism of which remains unclear so far. The aim of this work is to investigate electromagnetic behavior of a PWNI HTS coil in ramping and fast discharging process. A simulation model was developed by integrating an equivalent circuit network model and an improved T–A model. A three-tape PWNI coil and its insulated counterpart were wound and tested, and this model was validated by charging and discharging tests. Results show that there is a significant non-uniform current distribution on parallel tapes in the same turn during ramping operations and the maximum azimuthal current (transport current) can be 2.26 times the minimum one in the three-tape PWNI coil in this study. Meanwhile, the radial current shows a considerable accumulation in the tape near turn-to-turn contacts and the radial current through the turn-to-turn contacts can be 4.16 times of that the flow through tape-to-tape contacts (parallel tapes) in the same turn. During the fast discharging process, a significant coupling current is generated in the PWNI coil, leading to a large opposite transport current in local areas; the amplitude of variation of this can be 4.66 times the initial operating current. The radial current shows a similar distribution but opposite direction to that during ramping, and its amplitude is two orders of magnitude higher. These results provide practical guidelines for the design of large-scale high-field HTS magnets.

Time-dependent magnetotransport in a hybrid graphene/single magnetic molecule structure

Graphene is potentially attractive for spintronics due to its weak intrinsic spin–orbit effect leading to a long spin relaxation length. This study examines molecular couplings on a planar graphene sheet, where one molecule is attached to the graphene sheet connected to ferromagnetic leads. The effects of a strong magnetic field, gate voltage, and time-dependent bias voltage on spin and charge tunneling currents are investigated. It is indicated that the studied model system could be controlled by applying a gate voltage and a time-dependent bias voltage. Regarding the interaction between graphene, magnetic molecule, and magnetic field, it is observed that the mutual interaction coefficients between them have the most considerable effect on the transport current. Applying the magnetic field shows that the response time of the current is very short, and the current is not zero in the steady state. In this situation, the current passes through the system continuously.

Synthesis and Research of Critical Parameters of Bi-HTSC Ceramics Based on Glass Phase Obtained by IR Heating

In this paper influence of the excess Ca and Cu cations on the critical temperature (Tc) and critical transport current density (Jc) of high-temperature superconducting ceramics of the compositions (HTSC) Bi1.6Pb0.4Sr2Ca2.1Cu3.1Oy, Bi1.6Pb0.4Sr2Ca2.25Cu3.25Oy and Bi1.6Pb0.4Sr2Ca3Cu4Oy synthesized by the glass-ceramic method has been studied. The synthesis of superconducting ceramics was carried out on the basis of the glass phase, obtained by ultra-fast quenching of the melt. Melting of the mixture of starting components was carried out without the use of a crucible under the influence of IR radiant heating. Analysis of the elemental composition of the samples of the initial precursors showed a significant deviation from stoichiometry in oxygen (increase), as well as a decrease in calcium content. The synthesis of HTSC ceramics was carried out at a temperature of 849–850 °C for 96 h with intermediate grinding every 24 h. Studies of the phase composition of ceramic samples by X-ray diffraction have shown that HTSC ceramics consist only of a superconducting high-temperature phase Bi-2223. Studies of current-carrying characteristics by the four-point probe method according to the criterion of 1 µV/cm2 have shown that high-temperature superconducting ceramics of the compositions Bi1.6Pb0.4Sr2Ca2.1Cu3.1Oy, Bi1.6Pb0.4Sr2Ca2.25Cu3.25Oy and Bi1.6Pb0.4Sr2Ca3Cu4Oy have an increased density of critical transport current of 9.12 A/cm2, 7.62 A/cm2 and 7.26 A/cm2, respectively. At the same time, it was found that with a decrease in the content of Ca and Cu cations in HTSC ceramics, an increase in the critical current density is observed.

Numerical modelling of HTS tapes under arbitrary external field and transport current via integral method: review and application to electrical machines *

Superconductors in practical use can be subjected to spatially non-uniform and time-varying external magnetic field as well as carrying a transport current, for example, in an electrical machine. This paper verifies that the integral method can be used in situations in which the external field is spatially non-uniform, by providing both theoretical reasoning and simulation results. Variations in the integral method are reviewed, such as how to impose transport current. Further, the integral method is applied to calculate ac loss in superconducting tapes in an air-cored electrical machine in a two-stage process: the external field is calculated in a COMSOL model without superconducting tapes, and exported into the integral method model that consists of the tapes only. The time taken by the integral method is a small fraction of the time taken by the full machine model in COMSOL, which uses the T-A formulation. There are good agreements between the full COMSOL model and two-stage method incorporating the integral method.

Numerical calculations of high temperature superconductors with the J-A formulation

One of the main challenges in superconductivity modeling stems from the strong nonlinearity of the E-J power law relationship. To overcome this difficulty, various numerical models have been developed by the superconductivity community, such as the H formulation and the T-A formulation. These models are implemented based on different state variables in Maxwell’s equations and have the advantage of efficiency and versatility. In this study, a finite element model based on the J-A formulation is further developed to enhance its accuracy and versatility. The discontinuous Lagrange shape function is employed in the J formulation to stabilize the numerical results. Meanwhile, the Lagrange multiplier method is applied to impose the transport current on the superconductors. In terms of applications, the J-A formulation can efficiently simulate the electromagnetic responses not only of superconducting films but also of superconducting bulks. Moreover, homogeneous and multi-scale strategies are introduced to simplify the model and reduce the computation cost, allowing efficient simulation of large-scale HTS systems. Finally, the three-dimensional (3D) J-A formulation is proposed to incorporate the 3D structure of HTS systems, examples including the CORC cables as well as the racetrack coils. These results reveal that the J-A formulation is an efficient and versatile numerical method for calculating the electromagnetic behavior of high temperature superconductors.

Time-dependent development of dynamic resistance voltage of superconducting tape considering heat accumulation

In flux pumps, motors and superconducting magnets, the high temperature superconductor (HTS) coated conductor frequently carries a DC transport current when an oscillating magnetic field is present in the background. Under this circumstance, the interesting effect of dynamic resistance takes place, which can affect the operating performance of superconducting devices: heat accumulation can contribute to the rising temperature of the HTS tape and the dynamic resistance voltage can change accordingly. This article explores the time-dependent development of the dynamic resistance voltage using a numerical modeling considering the thermal effects. After a validation against experimental results, this work investigates the effects of several factors on the structure of the HTS tape on the time-dependent development of the dynamic resistance, thus providing insights toward a better understanding of the time-dependent behavior of HTS tapes under external magnetic fields.

Numerical simulation on AC loss in REBCO tapes carrying non-sinusoidal currents

AC loss is one of the greatest obstacles for high-temperature superconducting (HTS) applications. In some HTS applications, coated conductors carry non-sinusoidal currents. Thus, it is important to investigate the effect of various waveforms on AC loss in coated conductors. In this work, transport AC loss in a 4 mm - wide REBCO coated conductor carrying sinusoidal and non-sinusoidal currents, is numerically investigated. The current amplitudes, the frequency of the transport current, and n-value are varied. Non-sinusoidal transport current waveforms studied include square, five types of trapezoidal, and triangular waveforms. Simulated results show that, for a given current amplitude, AC loss for the square current waveform is the greatest, that for the triangular waveform is the smallest. The sequence of AC loss in the conductor for different current waveforms coincides with the penetration depth, which implies the penetration depth determines the AC loss of the coated conductor. Furthermore, the transport AC loss in the conductor was found to decrease with frequency as f-2/n for non-sinusoidal transport current.

Dynamic resistance loss of the high temperature superconducting coil for superconducting magnetic energy storage

The Superconducting Magnetic Energy Storage (SMES) has excellent performance in energy storage capacity, response speed and service time. Although it's typically unavoidable, SMES systems often have to carry DC transport current while being subjected to the external AC magnetic fields.Under this circumstance, the dynamic resistance loss occurs in conjunction with the magnetization loss. Accordingly, the dissipative heat originating from the dynamic resistance loss will cause thermal overhead and even threaten the stability of the SMES. To ensure the safe operation of the SMES, this study examines dynamic resistance losses of high temperature superconducting (HTS) coils. In addition, this study also compares it with other losses, including magnetization loss, under various conditions such as different DC transport currents, amplitudes, frequencies, and angles of external AC magnetic fields. Furthermore, the analysis of dynamic resistance loss has also taken into account thermal coupling. This research offers valuable insights into the electromagnetic-thermal characteristics of the HTS coil and can be used as a reference for designing SMES systems.

The teleost fish PepT1-type peptide transporters and their relationships with neutral and charged substrates.

In teleosts, two PepT1-type (Slc15a1) transporters, i.e., PepT1a and PepT1b, are expressed at the intestinal level. They translocate charged di/tripeptides with different efficiency, which depends on the position of the charged amino acid in the peptide and the external pH. The relation between the position of the charged amino acid and the capability of transporting the dipeptide was investigated in the zebrafish and Atlantic salmon PepT1-type transporters. Using selected charged (at physiological pH) dipeptides: i.e., the negatively charged Asp-Gly and Gly-Asp, and the positively charged Lys-Gly and Gly-Lys and Lys-Met and Met-Lys, transport currents and kinetic parameters were collected. The neutral dipeptide Gly-Gln was used as a reference substrate. Atlantic salmon PepT1a and PepT1b transport currents were similar in the presence of Asp-Gly and Gly-Asp, while zebrafish PepT1a elicited currents strongly dependent on the position of Asp in the dipeptide and zebrafish PepT1b elicited small transport currents. For Lys- and Met-containing dipeptides smaller currents compared to Gly-Gln were observed in PepT1a-type transporters. In general, for zebrafish PepT1a the currents elicited by all tested substrates slightly increased with membrane potential and pH. For Atlantic salmon PepT1a, the transport current increased with negative potential but only in the presence of Met-containing dipeptides and in a pH-dependent way. Conversely, large currents were shown for PepT1b for all tested substrates but Gly-Lys in Atlantic salmon. This shows that in Atlantic salmon PepT1b for Lys-containing substrates the position of the charged dipeptides carrying the Lys residue defines the current amplitudes, with larger currents observed for Lys in the N-terminal position. Our results add information on the ability of PepT1 to transport charged amino acids and show species-specificity in the kinetic behavior of PepT1-type proteins. They also suggest the importance of the proximity of the substrate binding site of residues such as LysPepT1a/GlnPepT1b for recognition and specificity of the charged dipeptide and point out the role of the comparative approach that exploits the natural protein variants to understand the structure and functions of membrane transporters.

Dominant effect of residual secondary phase of powders on Jc and microstructure of Bi-2212 superconducting wires

Bi2Sr2CaCu2O8+δ (Bi-2212) superconducting round wires exhibited great potential for use in high-field applications. The purity of the precursor powders is critical for the transport current of the wires. However, the role of the residual secondary phase in the precursor powders is not fully understood. Here, the origin of the secondary phase was investigated in precursor powders that were prepared using ultrasonic spray pyrolysis (USP) and calcination processing. The microstructure and phase evolution of the precursor powders during the crystallization process were analyzed. Moreover, the effects that the residual secondary phase has on melting behavior, morphology properties, and the supercurrent flow of Bi-2212 multi-filamentary wires are systematically discussed. The residual secondary phase in the filament caused further crystallization, and this led to the formation of more and larger Bi-2201 grains at the onset of the melting process. The poor microstructure and low critical current of the final Bi-2212 wires can be attributed to the presence of the residual copper-rich phase. Bi-2212 wires that were prepared with fully crystallized powders had a high critical current density (Jc) of 6773 A/mm2 at 4.2 K, self-field. It was revealed that control of the secondary phases in precursor powders is greatly significant for achieving superior values of Jc.

Direct visualization of electronic transport in a quantum anomalous Hall insulator.

A quantum anomalous Hall (QAH) insulator is characterized by quantized Hall and vanishing longitudinal resistances at zero magnetic field that are protected against local perturbations and independent of sample details. This insensitivity makes the microscopic details of the local current distribution inaccessible to global transport measurements. Accordingly, the current distributions that give rise to transport quantization are unknown. Here we use magnetic imaging to directly visualize the transport current in the QAH regime. As we tune through the QAH plateau by electrostatic gating, we clearly identify a regime in which the sample transports current primarily in the bulk rather than along the edges. Furthermore, we image the local response of equilibrium magnetization to electrostatic gating. Combined, these measurements suggest that the current flows through incompressible regions whose spatial structure can change throughout the QAH regime. Identification of the appropriate microscopic picture of electronic transport in QAH insulators and other topologically non-trivial states of matter is a crucial step towards realizing their potential in next-generation quantum devices.