Junction Model Research Articles

Nanoscale Thermal Transport Model of Magnetic Tunnel Junction (MTJ) Device for STT-MRAM

Spin-transfer-torque magnetic random access memory (STT-MRAM) based on magnetic tunnel junction (MTJ) device has attracted significant attention from academics and industries. Nevertheless, the thermal effect under bias operation affects the overall performance and stability of a device. To evaluate the thermal phenomenon in MTJ devices accurately, the non-equilibrium effect between the electron and the phonon near the electrode-barrier interface cannot be ignored. The existence of the interface leads to additional thermal resistance hindering the heat conduction of tunnel junction. Therefore, an effective equivalent model concerning the interface effect is necessary to represent the heat transportation of the device. In this article, a nanoscale thermal transport model of the MTJ device is proposed. The influence of the thermal conductivity of the nano-oxide layer on the temperature distribution is discussed. The interface energy balance transport model is used to clarify the non-equilibrium relationship between the phonon and the electron at the CoFeB/MgO/CoFeB interfaces. A parameterization study is conducted to illustrate the temperature distribution in the nanoscale thermal transport model. Furthermore, the phonon-electron coupling distance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\delta$ </tex-math></inline-formula> ) based on the thermal conductivity of the phonon ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\kappa _{p}$ </tex-math></inline-formula> ) and the electron ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\kappa _{e}$ </tex-math></inline-formula> ) of the device is implemented to construct the effective equivalent model using finite element modeling (FEM), which could realistically reflect the thermal transport under working conditions. The established equivalent model has a guiding role in exploring the thermal transportation in the nanoscale MTJ device.

Dynamic analytical model for shaft-tunnel junction under P-SV waves

Shaft-tunnel junction is a typical case of critical underground joint structures. The abrupt structural change between the shaft and the tunnel makes it particularly susceptible to seismic damages. The focal point of this paper is the dynamic responses of shaft-tunnel junction under P-SV waves, for which a dynamic analytical model is developed. The shaft is assumed to be conditionally rigid, and the tunnel is modeled as a Euler-Bernoulli beam. The ground motion is calculated using the stiffness matrix method for layered media. The soil-structure interactions are mediated by a series of supporting springs and dashpots. Within the plane of wave propagation, the shaft has three generalized coordinates, and the tunnel is described by two displacement functions. Their analytical solutions are therefore derived in the frequency domain. Validity of the solutions is examined by comparison with the numerical results of two validation models. According to the solutions, responses of the shaft are controlled by the impedance of the surrounding ground, its own inertia properties, and the shaft-tunnel interactions. Among them, the impedance of the surrounding ground is by far the predominant factor. Responses of the tunnel are the superpositions of the shaft displacements and the free field displacements. The influence of the shaft attenuates exponentially with increasing distance from it. If the distance is sufficiently large, responses of the tunnel are dominated by the propagation of the P-SV waves.

Spin Polarized Electronic Transport and Photocurrent in Chiral Methionine Molecule via Magnetic Tunnel Junction Model from First Principles

AbstractThe spin‐dependent transport properties through chiral amino acids and their modulations by light and magnetic field are important for understanding the intrinsic magnetism and photomagnetic coupling of chiral molecules. Here, the spin‐dependent electronic transport through chiral methionine (R,S‐MET) molecules is studied by theoretical calculations through a magnetic tunnel junction (MTJ) model. R‐MET molecule outputs fully spin polarized photocurrents with different spin channels, while S‐MET is insensitive under polarized light, revealing the spin polarized photocurrent selectivity of chiral molecules. Photocurrent increases greatly in Fe4N/S‐MET/Au MTJ, where S‐MET shows unidirectional spin filtering effect. Fully spin polarized photocurrent appears in Fe4N/R,S‐MET/Au MTJs, whose spin channel can be switched by adjusting magnetization directions. Particularly, chiral‐induced spin selectivity effect appears in Fe4N/R,S‐MET/Au MTJs under specific polarized light and magnetization direction. Understanding the spin‐dependent electronic transport of chiral amino acids can provide a theoretical foundation for life science and bioelectronic devices.

Static and dynamic transport properties of multi-terminal, multi-junction microSQUIDs realized with Nb/HfTi/Nb Josephson junctions

The progressive miniaturization of superconducting quantum interference devices (SQUIDs) used, e.g. for magnetic imaging on the nanoscale or for the detection of the magnetic states of individual magnetic nanoparticles causes increasing problems in realizing a proper flux-bias scheme for reading out the device. To overcome the problem, a multi-terminal, multi-junction layout has been proposed and realized recently for the SQUID-on-tip configuration, which uses constriction-type Josephson junctions (JJ). This geometry is also interesting for SQUIDs based on overdamped superconductor—normal metal—superconductor (SNS) JJ. We fabricated four-terminal, four-junction SQUIDs based on a trilayer Nb/HfTi/Nb process and study their static and dynamic transport properties in close comparison with numerical simulations based on the resistively and capacitively shunted junction model. Simulations and measurements are in very good agreement. However, there are large differences to the transport properties of conventional two-junction SQUIDs, including unusual phase-locked and chaotic dynamic states which we describe in detail. We further extract the current-phase relation of our SNS junctions, which turns out to be purely sinusoidal within the experimental error bars.

Single-junction quantum-circuit refrigerator

We propose a quantum-circuit refrigerator (QCR) based on photon-assisted quasiparticle tunneling through a single normal-metal–insulator–superconductor (NIS) junction. In contrast to previous studies with multiple junctions and an additional charge island for the QCR, we directly connect the NIS junction to an inductively shunted electrode of a superconducting microwave resonator making the device immune to low-frequency charge noise. At low characteristic impedance of the resonator and parameters relevant to a recent experiment, we observe that a semiclassical impedance model of the NIS junction reproduces the bias voltage dependence of the QCR-induced damping rate and frequency shift. For high characteristic impedances, we derive a Born–Markov master equation and use it to observe significant non-linearities in the QCR-induced dissipation and frequency shift. We further demonstrate that, in this regime, the QCR can be used to initialize the linear resonator into a non-thermal state even in the absence of any microwave drive.

Method of quantitative assessment of the shape of the lumbar-thoracic spine

Introduction. The presence of deformation of the lumbar-thoracic spine in the sagittal plane is the main factor determining the health status of adult patients. The studies of the features of human posture with or without spinal deformities in statics and movement have been in the focus of clinician attention for a long time. Recent studies offer a unifi ed approach to assessing the position of the vertebrae. However, no such studies have been conducted to analyze the lumbar-thoracic junction. The purpose of the work — to develop a method of qualitative and quantitative assessment of the vertebra positions in lumbar-thoracic junction. Issues: to develop a schematic model of the lumbar-thoracic spine; to develop a typology of the lumbar-thoracic transition; to develop an objective indicator refl ecting the features of the lumbar-thoracic transition in the patient; to characterize the age characteristics of this area of the spine.Materials and methods. A study of digital radiographs for all spine parts in sagittal projection for 141 patients with dorsopathies, 57 men and 84 women aged from 21 to 88 years, was conducted. The study was performed on a personal computer screen, without patient participation. A single digital X-ray image of the spine in the sagittal projection was obtained for each patient. The occipital vertical and anteroposterior axes of TIX–LV (LVI) vertebrae (r axes) were applied to the combined radiograph. At the intersection points of the axes with the occipital vertical, the perpendiculars to the r axes were restored, and the angles between the perpendiculars and the occipital vertical (angles r) were measured. Statistical analysis was carried out using the Microsoft Offi ce Excel 2007 software package.Results. Schematic models of the lumbar-thoracic junction for all cases were constructed on the basis of the data obtained. The models were used to compare the vertebra positions and describe three form types of lumbar-thoracic junction: normal, straightened and reinforced. An aggregated ArTL indicator is proposed and the boundaries of this indicator were determined for the quantitative assessment of each case. It is demonstrated that the age features for this part of the spine are expressed not in a monotonous change in the average value of ArTL with age, but in an increase in the proportion of patients with straightened and enhanced kyphosis, and it is especially noticeable in the group of people over 75 years old.Conclusion. The proposed technology for assessing the position of the vertebrae of the lumbar-thoracic spine was developed to satisfy the needs of osteopaths and specialists in restorative medicine, and this technology is presented for the fi rst time. In the course of the study, schematic models of the spine of each patient were developed; an ArTL indicator was proposed to quantify the type of the lumbar-thoracic region shape. The boundaries for the diagnosis of each type were determined and a study was conducted to identify the age trend. The study revealed the absence of a linear age trend of changes in this part of the spine. Among people over 75 years of age, patients with straightened or enhanced kyphosis of this zone were more common.

A Tuning Mode of Asymmetric Deformations on the Electric Characteristics of Piezoelectric PN Junctions

Influence of asymmetric deformations and the interface polarized charges (IPCs) on the electric characterization of a piezoelectric PN junction is studied in this paper via a fully-coupled model of piezoelectric PN junctions without the depletion layer assumption. A tuning mode of asymmetric deformations on electric characteristics of piezoelectric PN junctions is put forward and the mechanism is analyzed in detail. It is found that the interfacial potential barrier size in the n/p region can be tuned out of sync under the asymmetric deformations. The improved I–V characteristic of a ZnO PN junction is analyzed in detail by applying asymmetric deformation. It is found that the output current can be obviously enhanced by optimizing the recombination rate not only near the interface but also at the loading point. Particularly, the area with evident recombination rate at the interface can be driven to shift a little deviation from there via IPCs, which can be used to avoid the influence of the interface effects (scattering by interface states and defects there) on the mutual conversion between the majority and minority currents. Obviously, the study possesses referential significance to the mechanical tuning on the performance of piezotronic devices.

Study of the transport properties of cobalt atomic contact under mechanical strain in a nitrogen atmosphere

Achieving an efficient spin-filter effect and large magneto-resistance in molecular junctions is very important in spintronics. By employing the non-equilibrium Green's function formalism combined with the density functional theory, this study investigated the atomic structures and spin-resolved transport properties of molecular junctions consisting of nitrogen (N2) sandwiched between two cobalt (Co) electrodes with different crystal lattices as a function of stretching distance. In the case of face-centred cubic Co electrodes, our calculations showed that the transport properties depend strongly on the stretching process, which changes the contact between the N2 and Co electrodes. Specifically, the maximum spin polarization is due to the configuration of N2 molecules sitting in parallel with the protruding Co atoms. Furthermore, the Co dxz,yz orbitals interact with the N px,y orbitals, which contribute to the high transmission coefficient at the Fermi level for spin-down electrons. Meanwhile, the Co s-dz2 hybridization orbitals, which interact weakly with the N p orbitals, contribute to the small transmission coefficient at the Fermi level. This can also be verified by the molecular junction model adopting simple monatomic Co chain electrodes, which also show a perfect spin-filter effect and achieve an ideal magnetoresistance. In contrast, in the case of hexagonal Co electrodes, the spin-filter effect disappears. However, the effect appears again by adding one more Co atom to the central point contact. Our findings show that the spin-resolved electronic transport of molecular junctions depends on the stretching process and the nature of the electrodes, which can help in the future design of molecular devices based on the spin degree of freedom.

Self-Field Effects in a Josephson Junction Model for ${J_\text{c}}$ in REBCO Tapes

Self-Field Effects in a Josephson Junction Model for ${J_\text{c}}$ in REBCO Tapes

Josephson junction parameters extraction method for the RCLSJ model

Current-voltage (I-V) characteristics of shunted Nb/Al-AlOx/Nb Josephson junctions (JJs) can be precisely described in the resistive capacitive inductive shunted junction (RCLSJ) model derived from resistive capacitive shunted junction (RCSJ) model with additional parasitic inductance. However, the methods for extraction of current values in the RCLSJ model are not fully developed, which causes unsatisfactor accuracy of the simulations of superconducting integrated circuits. In this work, we propose an innovative method for extraction of parameters that is capable to solve inaccuracy problem. The enhancement is achieved by including the fluctuation of device parameters due to fabrication process in the analysis. A Josephson comparator is designed in the RCLSJ model to test the performance of the proposed parameter extraction method. Obtained results show that our method ensures 2 to 3 times higher simulation accuracy at frequencies above 60 GHz as compared to the traditional method.

Josephson radiation patterns in underdamped topological Josephson junctions

Josephson radiation is a useful signature for detecting Majorana zero modes in topological superconductors. We study the Josephson radiation of the underdamped topological Josephson junction within the quantum resistively and capacitively shunted junction model. We show that the quantum dynamics of the Majorana two-level system induce oscillatory patterns in the Josephson emission spectra. With the Floquet method, we obtain analytical results for these oscillatory patterns and find that they are well described by Bessel functions. We perform numerical simulations to verify the analytical results and demonstrate that these Bessel radiation patterns exist for a wide range of junction parameters.

Hierarchical equations of motion approach to hybrid fermionic and bosonic environments: Matrix product state formulation in twin space.

We extend the twin-space formulation of the hierarchical equations of motion approach in combination with the matrix product state representation [R. Borrelli, J. Chem. Phys. 150, 234102 (2019)] to nonequilibrium scenarios where the open quantum system is coupled to a hybrid fermionic and bosonic environment. The key ideas used in the extension are a reformulation of the hierarchical equations of motion for the auxiliary density matrices into a time-dependent Schrödinger-like equation for an augmented multi-dimensional wave function as well as a tensor decomposition into a product of low-rank matrices. The new approach facilitates accurate simulations of non-equilibrium quantum dynamics in larger and more complex open quantum systems. The performance of the method is demonstrated for a model of a molecular junction exhibiting current-induced mode-selective vibrational excitation.

Engineering and Characterization of an Optogenetic Model of the Human Neuromuscular Junction.

Many neuromuscular diseases, such as myasthenia gravis (MG), are associated with dysfunction of the neuromuscular junction (NMJ), which is difficult to characterize in animal models due to physiological differences between animals and humans. Tissue engineering offers opportunities to provide in vitro models of functional human NMJs that can be used to diagnose and investigate NMJ pathologies and test potential therapeutics. By incorporating optogenetic proteins into induced pluripotent stem cells (iPSCs), we generated neurons that can be stimulated with specific wavelengths of light. If the NMJ is healthy and functional, a neurochemical signal from the motoneuron results in muscle contraction. Through the integration of optogenetics and microfabrication with tissue engineering, we established an unbiased and automated methodology for characterizing NMJ function using video analysis. A standardized protocol was developed for NMJ formation, optical stimulation with simultaneous video recording, and video analysis of tissue contractility. Stimulation of optogenetic motoneurons by light to induce skeletal muscle contractions recapitulates human NMJ physiology and allows for repeated functional measurements of NMJ over time and in response to various inputs. We demonstrate this platform's ability to show functional improvements in neuromuscular connectivity over time and characterize the damaging effects of patient MG antibodies or neurotoxins on NMJ function.

Engineering and Characterization of an Optogenetic Model of the Human Neuromuscular Junction

Engineering and Characterization of an Optogenetic Model of the Human Neuromuscular Junction

Laboratory modelling of urban flooding

This paper presents two datasets obtained from laboratory experiments of urban flooding in a street network performed at the University of Liège. The experimental model represents a part of a synthetic urban district that consists of three inlets, three outlets and several three- and four- branches crossroads. The following experimental data was produced: (i) dataset 1: time-series of flow depths at model inlets and time-series of discharges at model outlets for a two-branch junction model, a two-branch bifurcation model and a district model. The datasets were generated by varying the upstream and downstream boundary conditions, i.e. flooding conditions; (ii) dataset 2 includes the same data type as dataset 1 complemented by 2D surface velocity measured using the non-intrusive LSPIV technique for eight urban form configurations in the district model. The collected data enable improving the understanding of the effect of urban forms on the urban flood processes. These two datasets are valuable for validating and improving numerical or analytical models of urban flooding and may contribute to flood risk management and flood-resilient urban design.

A bioprinted complex tissue model for myotendinous junction with biochemical and biophysical cues.

In the musculoskeletal system, the myotendinous junction (MTJ) is optimally designed from the aspect of force transmission generated from a muscle through a tendon onto the bone to induce movement. Although the MTJ is a key complex tissue in force transmission, the realistic fabrication, and formation of complex tissues can be limited. To obtain the MTJ construct, we prepared two bioinks, muscle‐ and tendon‐derived decellularized extracellular matrix (dECM), which can induce myogenic and tenogenic differentiation of human adipose‐derived stem cells (hASCs). By using a modified bioprinting process supplemented with a nozzle consisting of a single‐core channel and double‐sheath channels, we can achieve three different types of MTJ units, composed of muscle, tendon, and interface zones. Our results indicated that the bioprinted dECM‐based constructs induced hASCs to myogenic and tenogenic differentiation. In addition, a significantly higher MTJ‐associated gene expression was detected at the MTJ interface with a cell‐mixing zone than in the other interface models. Based on the results, the bioprinted MTJ model can be a potential platform for understanding the interaction between muscle and tendon cells, and even the bioprinting method can be extensively applied to obtain complex tissues.

A Non-Signalized Junction Model for Agent-Based Simulations of Car–Pedestrian Mode Mass Evacuations

During major disasters, such as a subduction earthquake and the associated tsunami, combinations of uncommon conditions such as non-functioning traffic signals, a large number of pedestrians on traffic lanes, and debris scattered on roads can be widespread. It is vital to take these uncommon conditions into account since they can significantly influence the evacuation progress. Agent-Based Models (ABMs) with capabilities to reproduce evacuees’ behaviors as emergent phenomena is promising method to simulate combinations of such rare conditions. This paper presents a new model to cover the current research gap in accurately modeling car–car and car–pedestrian interactions at non-signalized junctions. Specifically, the details of accurately approximating car trajectories at junctions and automated construction, approximating free-flow speed of cars along curved trajectories, and accurately calculating the points of collision and time to collision are presented. As a demonstrative application, we simulated a hypothetical evacuation scenario with non-functioning traffic signals in which different numbers of slow evacuees are allowed to use cars. While the ABM is yet to be thoroughly validated, the presented demonstrative scenarios indicates that a considerable number of the needy can be allowed to use cars for evacuation if their routes and evacuation start time window are well planned.

Image encryption with a Josephson junction model embedded in FPGA

Image encryption with a Josephson junction model embedded in FPGA

Phase-dependent microwave response of a graphene Josephson junction

Gate-tunable Josephson junctions embedded in a microwave environment provide a promising platform to in-situ engineer and optimize novel superconducting quantum circuits. The key quantity for the circuit design is the phase-dependent complex admittance of the junction, which can be probed by sensing an rf SQUID with a tank circuit. Here, we investigate a graphene-based Josephson junction as a prototype gate-tunable element enclosed in a SQUID loop that is inductively coupled to a superconducting resonator operating at 3 GHz. With a concise circuit model that describes the dispersive and dissipative response of the coupled system, we extract the phase-dependent junction admittance corrected for self-screening of the SQUID loop. We decompose the admittance into the current-phase relation and the phase-dependent loss and as these quantities are dictated by the spectrum and population dynamics of the supercurrent-carrying Andreev bound states, we gain insight to the underlying microscopic transport mechanisms in the junction. We theoretically reproduce the experimental results by considering a short, diffusive junction model that takes into account the interaction between the Andreev spectrum and the electromagnetic environment, from which we deduce a lifetime of ~17 ps for non-equilibrium populations.

Local accumulation time for diffusion in cells with gap junction coupling.

In this paper we analyze the relaxation to steady state of intracellular diffusion in a pair of cells with gap-junction coupling. Gap junctions are prevalent in most animal organs and tissues, providing a direct diffusion pathway for both electrical and chemical communication between cells. Most analytical models of gap junctions focus on the steady-state diffusive flux and the associated effective diffusivity. Here we investigate the relaxation to steady state in terms of the so-called local accumulation time. The latter is commonly used to estimate the time to form a protein concentration gradient during morphogenesis. The basic idea is to treat the fractional deviation from the steady-state concentration as a cumulative distribution for the local accumulation time. One of the useful features of the local accumulation time is that it takes into account the fact that different spatial regions can relax at different rates. We consider both static and dynamic gap junction models. The former treats the gap junction as a resistive channel with effective permeability μ, whereas the latter represents the gap junction as a stochastic gate that randomly switches between an open and closed state. The local accumulation time is calculated by solving the diffusion equationin Laplace space and then taking the small-s limit. We show that the accumulation time is a monotonically increasing function of spatial position, with a jump discontinuity at the gap junction. This discontinuity vanishes in the limit μ→∞ for a static junction and β→0 for a stochastically gated junction, where β is the rate at which the gate closes. Finally, our results are generalized to the case of a linear array of cells with nearest-neighbor gap junction coupling.