Asymmetric Junctions Research Articles

Comparative study of symmetrical and asymmetrical B40 molecular junctions

This work investigates quantum transport in symmetrical and asymmetrical borospherene-based molecular junctions with adenine. Adenine is one of the four nucleobases of DNA and was selected because of its excellent transport properties. The density functional/non-equilibrium Green's function (DFT-NEGF) mathematical approach was utilized, which was further used to calculate quantum transport parameters including the I–V curve, transmission spectra, highest occupied molecular orbital (HOMO)–lowest occupied molecular orbital (LUMO) gap (HLG), differential conductance, and transmission pathways. We observed that both symmetrical and asymmetrical devices exhibit nonlinear behavior, thus leading to non-differential resistance. Also, the symmetrical B40 molecular device demonstrated reduced HLG relative to the asymmetrical B40 molecular device. This drop in HLG was due to a reduction in electron density. A high rectification ratio for the devices was observed at 0.2 V. Hence, symmetrical and asymmetrical B40 molecular junctions are potential candidates for use in future nanoscale devices.

Dynamic Clusters to Infer Topologic Controls on Environmental Transport of River Networks

AbstractThe knowledge of structural controls of river networks (RNs) on transport dynamics is important for modeling and predicting environmental fluxes. To investigate impacts of RN’s topology on transport processes, we introduce a systematic framework based on the concept of dynamic clusters, where the connectivity of subcatchments is assessed according to two complementary criteria: minimum‐ and maximum‐flow connectivity. Our analysis from simple synthetic RNs and several natural river basins across the United States reveals the key topological features underlying the efficiency of flux transport and aggregation. Namely, the timing of basin‐scale connectivity at low‐flow conditions is controlled by the abundance of topologically asymmetric junctions (side‐branching), which at the same time, result in a slow‐down of the flux convergence at the outlet (maximum‐flow). Our results, when compared with observed topological trends in RNs as a function of climate, indicate that humid basins exhibit topologies which are “naturally engineered” to slow‐down fluxes.

Modulation of ionic current behaviors based on a dual-channel micro/nano-pipette with ternary-form-charged model

• Ternary-Formed-Charged model is proposed for simulation of ionic behaviors of a dual-channel pipette such as a resistor, diodes, and a bipolar junction transistor. • The uneven distribution and location of surface charge density are confirmed to cause asymmetric bipolar junction transistor behavior and pseudo-bipolar junction transistor behavior. • Scale ratio of radius of the pipette to liquid bridge and the effect of surface charge at the interface between liquid bridge and air is evaluated. Dual-channel micro- or nanopipettes can be used to perform various ionic current behaviors such as a resistor, diodes, and a bipolar junction transistor (BJT) via chemical modification. These ionic current devices could be modulated by charge pattern of modification, pH, size and scale ratio of pipette to the liquid bridge, whose intrinsic mechanism is indistinct. Herein, a universal Ternary-Form-Charged (TFC) model is proposed, in which the pipettes contain three individual tunable regions with different scale, location, surface charge density while ions can flux across the charged liquid bridge connected with them. Based on the TFC model, we could obtain three basic ionic behaviors mentioned above of dual-channel pipettes and demonstrate that these basic ionic behaviors of dual-channel pipette are dependent on the number of transition zone. Transformation from the BJT behavior to a pseudo-BJT behavior is attributed to a narrow middle charged region or a thick liquid bridge. Asymmetric BJT behavior is also observed under asymmetric distribution of location and quantity of surface charge density. Furthermore, we provide the approximate size of liquid bridge: 5–8 nm and 15–20 nm for dual-channel nanopipette and micropipette, respectively. In short, the TFC model and our simulation results shed light on the mechanism of dual-channel pipette based ionic circuits and provide a useful guide to design, modify and develop novel applications of dual-channel pipettes in nanofluidic sensors.

Intrinsic Sources and Functional Impacts of Asymmetry at Electrical Synapses

Electrical synapses couple inhibitory neurons across the brain, underlying a variety of functions that are modifiable by activity. Despite recent advances, many functions and contributions of electrical synapses within neural circuitry remain underappreciated. Among these are the sources and impacts of electrical synapse asymmetry. Using multi-compartmental models of neurons coupled through dendritic electrical synapses, we investigated intrinsic factors that contribute to effective synaptic asymmetry and that result in modulation of spike timing and synchrony between coupled cells. We show that electrical synapse location along a dendrite, input resistance, internal dendritic resistance, or directional conduction of the electrical synapse itself each alter asymmetry as measured by coupling between cell somas. Conversely, we note that asymmetrical gap junction (GJ) conductance can be masked by each of these properties. Furthermore, we show that asymmetry modulates spike timing and latency of coupled cells by up to tens of milliseconds, depending on direction of conduction or dendritic location of the electrical synapse. Coordination of rhythmic activity between two cells also depends on asymmetry. These simulations illustrate that causes of asymmetry are diverse, may not be apparent in somatic measurements of electrical coupling, influence dendritic processing, and produce a variety of outcomes on spiking and synchrony of coupled cells. Our findings highlight aspects of electrical synapses that should always be included in experimental demonstrations of coupling, and when assembling simulated networks containing electrical synapses.

Efficient domain wall motion in asymmetric magnetic tunnel junctions with vertical current flow

In this paper, we study the domain wall motion induced by vertical current flow in asymmetric magnetic tunnel junctions. The domain wall motion in the free layer is mainly dictated by the current-induced field-like torque acting on it. We show that as we increase the MTJ asymmetry, by considering dissimilar ferromagnetic contacts, a linear-in-voltage field-like torque behavior is accompanied by an enhancement in the domain wall displacement efficiency and a higher degree of bidirectional propagation. Our analysis is based on a combination of a quantum transport model and magnetization dynamics as described by the Landau–Lifshitz–Gilbert equation, along with comparison to the intrinsic characteristics of a benchmark in-plane current injection domain wall device.

Negative tension controls stability and structure of intermediate filament networks

Networks, whose junctions are free to move along the edges, such as two-dimensional soap froths and membrane tubular networks of endoplasmic reticulum are intrinsically unstable. This instability is a result of a positive tension applied to the network elements. A paradigm of networks exhibiting stable polygonal configurations in spite of the junction mobility, are networks formed by bundles of Keratin Intermediate Filaments (KIFs) in live cells. A unique feature of KIF networks is a, hypothetically, negative tension generated in the network bundles due to an exchange of material between the network and an effective reservoir of unbundled filaments. Here we analyze the structure and stability of two-dimensional networks with mobile three-way junctions subject to negative tension. First, we analytically examine a simplified case of hexagonal networks with symmetric junctions and demonstrate that, indeed, a negative tension is mandatory for the network stability. Another factor contributing to the network stability is the junction elastic resistance to deviations from the symmetric state. We derive an equation for the optimal density of such networks resulting from an interplay between the tension and the junction energy. We describe a configurational degeneration of the optimal energy state of the network. Further, we analyze by numerical simulations the energy of randomly generated networks with, generally, asymmetric junctions, and demonstrate that the global minimum of the network energy corresponds to the irregular configurations.

Droplet Dynamics in Asymmetric Microfluidic Junctions

Droplet Dynamics in Asymmetric Microfluidic Junctions

Strengthened Spin–Valley Polarization and Negative Magnetoresistance Based on an Asymmetrical Double-Ferromagnetic Wse2 Junction

Strengthened Spin–Valley Polarization and Negative Magnetoresistance Based on an Asymmetrical Double-Ferromagnetic Wse2 Junction

Nonlinear Fermi liquid transport through a quantum dot in asymmetric tunnel junctions

We study the nonlinear conductance through a quantum dot, specifically its dependence on the asymmetries in the tunnel couplings and bias voltages $V$, at low energies. Extending the microscopic Fermi liquid theory for the Anderson impurity model, we obtain an exact formula for the steady current $I$ up to terms of order ${V}^{3}$ in the presence of these asymmetries. The coefficients for the nonlinear terms are described in terms of a set of the Fermi liquid parameters: the phase shift, static susceptibilities, and three-body correlation functions of electrons in the quantum dots, defined with respect to the equilibrium ground state. We calculate these correlation functions, using the numerical renormalization group approach (NRG), over a wide range of impurity-electron filling that can be controlled by a gate voltage in real systems. The NRG results show that the order ${V}^{2}$ nonlinear current is enhanced significantly in the valence fluctuation regime. It is caused by the order $V$ energy shift of the impurity level, induced in the presence of the tunneling or bias asymmetry. Furthermore, in the valence fluctuation regime, we also find that the order ${V}^{3}$ nonlinear current exhibits a shoulder structure, for which the three-body correlations that evolve for large asymmetries play an essential role.

A bioinspired broadband self-powered photodetector based on photo-pyroelectric-thermoelectric effect able to detect human radiation

The self-powered broadband photodetectors based on pyro-phototronic effect or photothermoelectric effect are promising for diverse applications. Here, we propose a polar bear-inspired self-powered and uncooled broadband photodetector based on the coupled effect of pyro-photoronic and photothermoelectric effect, which integrates wurtzite structured CdS pyroelectric-thermoelectric materials with energy-harvest and heat-storing functional structures, besides cooperates with asymmetric Schottky junction by constructing an asymmetric pair of Ag electrodes. The peak-to-peak current, specific responsivity and detectivity are all enhanced by 11 times, attributed to the synergy between photopyroelectric effect and photothermoelectric effect. Significantly, for a low-power long-wave infrared radiation, it still achieves outstanding response on human radiation, when the separation between human finger and detector is extended to 10 cm. This study provides a novel design to achieve the synergy of photopyroelectric effect and photothermoelectric effect, and opens up a new avenue towards upgrading the performance of optoelectronic devices and energy harvesters for extensive applications.

Insights on asymmetric BTB-based molecular junctions: Effect of electrode coupling

NEGF andDFTcalculations within Fisher Lee formalism are usedto explore the effect of the asymmetry and the strength of coupling with the electrode on transport properties of (1-(2-bisthienyl)benzene) oligomers. Electronic analysis of metal–molecule interactions reveals the ionic nature of Ti-C bonds inducing an interfacial dipole. The Tid-orbitals are found to be strongly coupled to the lowest unoccupied orbitalof BTB, thus facilitating charge transfer from Ti to the molecule.The hole transport mechanism is found in the cases of Au-(BTB)n-Au and Au-(BTB)n-Ti while possible combined hole and electron transport is predicted in the case of Ti-(BTB)n-Ti.

The effects of covalent coupling strength on the electron transport properties and rectification in graphene/porphine/graphene molecular junctions

He et al. demonstrated dehydrogenative coupling of single porphines to graphene edges by scanning probe technique with submolecular resolution, revealing specific bonding motifs and electronic features directly, which created a new hybrid material with tunable functionalities [Nat. Chem. 9 (1) (2017) 33–38]. Motivated by this work, we study the electron transport properties of four types molecular junctions with a single porphine molecule coupled between two graphene nanoribbons as have been observed in the above experiment by density functional theory calculations. It is found that both the electron transmission near the Fermi level and the current at low bias can be enhanced by manipulating the covalent coupling between the porphine molecule and graphene nanoribbons. Based on this behavior, we design three asymmetric junctions to realize rectification effect, and as anticipated, a perfect rectifier is obtained by one of them. The mechanism is well understood by the distributions of local density of states. The design of molecular devices with covalent coupling provides an efficient method to tune the electron transmission states, which is of great importance in developing integrated circuits in future.

Low Resistance Asymmetric III-Nitride Tunnel Junctions Designed by Machine Learning

The tunnel junction (TJ) is a crucial structure for numerous III-nitride devices. A fundamental challenge for TJ design is to minimize the TJ resistance at high current densities. In this work, we propose the asymmetric p-AlGaN/i-InGaN/n-AlGaN TJ structure for the first time. P-AlGaN/i-InGaN/n-AlGaN TJs were simulated with different Al or In compositions and different InGaN layer thicknesses using TCAD (Technology Computer-Aided Design) software. Trained by these data, we constructed a highly efficient model for TJ resistance prediction using machine learning. The model constructs a tool for real-time prediction of the TJ resistance, and the resistances for 22,254 different TJ structures were predicted. Based on our TJ predictions, the asymmetric TJ structure (p-Al0.7Ga0.3N/i-In0.2Ga0.8N/n-Al0.3Ga0.7N) with higher Al composition in p-layer has seven times lower TJ resistance compared to the prevailing symmetric p-Al0.3Ga0.7N/i-In0.2Ga0.8N/n-Al0.3Ga0.7N TJ. This study paves a new way in III-nitride TJ design for optical and electronic devices.

A simple-structured silicon photodetector possessing asymmetric Schottky junction for NIR imaging

In this work, we design a minimalist Au/Si/Ag photodetector (PD) with asymmetric Schottky junctions (ASJ) combined with asymmetric geometrical contacts (AGC). Compared with the AGC-only PD (Ag/Si/Ag), the Au/Si/Ag PD possesses an efficient photodetection in visible to near-infrared (NIR) region. The experimental results show that the Au/Si/Ag PD exhibits a high responsivity of 424.3 mA/W, a low dark current of 8.5 pA, a high detectivity of 3.35×1013 Jones, and a wide linear dynamic range (LDR) of 187 dB, at the wavelength of 980 nm with 0 V bias voltage. That means this PD has a competitive performance compared to the reported Si-based PDs. Using this high-performance PD, we successfully demonstrate an NIR transmission imaging application, proving its NIR imaging capability. More importantly, the Au/Si/Ag PD has the advantage of facile preparation, i.e. neither expensive new materials nor complex processes are needed, providing a new route for the construction of low-cost self-powered PDs as NIR imaging sensors.

Multiple control of few-layer Janus MoSSe systems

In this computational work based on density functional theory we study the electronic and electron transport properties of asymmetric multi-layer MoSSe junctions, known as Janus junctions. Focusing on 4-layer systems, we investigate the influence of electric field, electrostatic doping, strain, and interlayer stacking on the electronic structure. We discover that a metal to semiconductor transition can be induced by an out-of-plane electric field. The critical electric field for such a transition can be reduced by in-plane biaxial compressive strain. Due to an intrinsic electric field, a 4-layer MoSSe can rectify out-of-plane electric current. The rectifying ratio reaches 34.1 in a model junction Zr/4-layer MoSSe/Zr. This ratio can be further enhanced by increasing the number of MoSSe layers. In addition, we show a drastic sudden vertical compression of 4-layer MoSSe due to in-plane biaxial tensile strain, indicating a second phase transition. Furthermore, an odd-even effect on electron transmission at the Fermi energy for Zr/$n$-layer MoSSe/Zr junctions with $n=1, \, 2,\, 3, \,\dots,\, 10$ is observed. These findings reveal the richness of physics in this asymmetric system and strongly suggest that the properties of 4-layer MoSSe are highly tunable, thus providing a guide to future experiments relating materials research and nanoelectronics.

Current-induced torques in magnetic Weyl semimetal tunnel junctions.

We study the current-induced torques in asymmetric magnetic tunnel junctions containing a conventional ferromagnet and a magnetic Weyl semimetal contact. The Weyl semimetal hosts chiral bulk states and topologically protected Fermi arc surface states which were found to govern the voltage behavior and efficiency of current-induced torques. We report how bulk chirality dictates the sign of the non-equilibrium torques acting on the ferromagnet and discuss the existence of large field-like torques acting on the magnetic Weyl semimetal which exceeds the theoretical maximum of conventional magnetic tunnel junctions. The latter are derived from the Fermi arc spin texture and display a counter-intuitive dependence on the Weyl nodes separation. Our results shed light on the new physics of multilayered spintronic devices comprising of magnetic Weyl semimetals, which might open doors for new energy efficient spintronic devices.

A junctional PACSIN2/EHD4/MICAL-L1 complex coordinates VE-cadherin trafficking for endothelial migration and angiogenesis

Angiogenic sprouting relies on collective migration and coordinated rearrangements of endothelial leader and follower cells. VE-cadherin-based adherens junctions have emerged as key cell-cell contacts that transmit forces between cells and trigger signals during collective cell migration in angiogenesis. However, the underlying molecular mechanisms that govern these processes and their functional importance for vascular development still remain unknown. We previously showed that the F-BAR protein PACSIN2 is recruited to tensile asymmetric adherens junctions between leader and follower cells. Here we report that PACSIN2 mediates the formation of endothelial sprouts during angiogenesis by coordinating collective migration. We show that PACSIN2 recruits the trafficking regulators EHD4 and MICAL-L1 to the rear end of asymmetric adherens junctions to form a recycling endosome-like tubular structure. The junctional PACSIN2/EHD4/MICAL-L1 complex controls local VE-cadherin trafficking and thereby coordinates polarized endothelial migration and angiogenesis. Our findings reveal a molecular event at force-dependent asymmetric adherens junctions that occurs during the tug-of-war between endothelial leader and follower cells, and allows for junction-based guidance during collective migration in angiogenesis.

A one-dimensional model of liquid laminar flows with large Reynolds numbers in tapered microchannels

In this article, we construct a novel one-dimensional model of microfluidic laminar flows in tapered circular and rectangular channels assuming the flow in channels fully developed. In the model, we take into account the inertance and dynamic pressure terms. The model can be used for a wide range of flows: from the pure capillary flow regime, where the capillary forces are the main driver of the liquid in the channel, to the external pressure flow regime where the external pressure applied to the liquid at the entrance to the channel is much larger than the capillary pressure in the channel, so that the capillary force can be ignored. We apply the model to rectangular Y-shape junctions, where the base channel is connected to a reservoir and the end channels are exposed to atmospheric air. We show that, in asymmetric Y-shape junctions, there can be a time of “meniscus arrest,” where only one of the two channels with a smaller radius fills, and, the other one, with a larger radius, is arrested. The time of meniscus arrest decreases with an increase in the applied external pressure; when this pressure becomes large enough, the meniscus arrest disappears. In this article, we also investigate the applicability of the fully developed flow approximation assumed in the model.

Anomalous Josephson effect and quantum anomaly in inversion asymmetric Weyl semimetals

We study a Josephson junction involving an inversion-asymmetric Weyl semimetal in presence of time-reversal symmetric (TRS) or time-reversal symmetry broken tilt in the Weyl spectra. We reveal that both types of tilts in the Weyl nodes lead to a Josephson $0$-$\pi$ transition and a zero bias valley/chiral supercurrent. Strikingly, the TRS tilt gives rise to a pure valley Josephson current (VJC) and TRS broken tilt induces a pure chirality Josephson current (CJC) in this system. The VJC and CJC are the manifestation of valley symmetry broken and $\mathbb{Z}_2$ symmetry broken by the respective tilt. We obtain the reversal of a pure VJC and pure CJC even in the zero bias condition controllable by the junction length. Our analysis of controllability of valley and chirality dependent transport in an inversion asymmetric Weyl semimetal junction could allow applications in valleytronics and chiralitytronics, respectively. The tilt induced Josephson effect provides an alternative route for supercurrent $0$-$\pi$ transition, different from the conventional ferromagnetism Josephson junctions where the spin polarization is essential. In the long junction and zero temperature limit, VJC and CJC are associated with a quantum anomaly which is manifested through a discontinuous jump in the current in absence of TRS and TRS breaking tilts, respectively.

Z-Piezo Pulse-Modulated STM Break Junction: Toward Single-Molecule Rectifiers with Dissimilar Metal Electrodes.

Fabricating single-molecule junctions with asymmetric metal electrodes is significant for realizing single-molecule diodes, but it remains a big challenge. Herein, we develop a z-piezo pulse-modulated scanning tunneling microscopy break junction (STM-BJ) technique to construct a robust asymmetric junction with different metal electrodes. The asymmetric Ag/BPY-EE/Au single-molecule junctions exhibit a middle conductance value in between those of the two individual symmetric metal electrode junctions, which is consistent with the order of calculated energy-dependent transmission coefficient T(E) of the asymmetric junctions at EF. Furthermore, the single-molecule conductance of Ag/BPY-EE/Au decreases by about 70% when reversing the bias voltage from 100 to -100 mV, and a clear asymmetric I-V feature at the single-molecule level is observed for these junctions. This rectifying behavior could be ascribed to a different interfacial coupling of molecules at the two end electrodes, which is confirmed by the different displacement of T(E) at the two bias voltages. Other asymmetric junctions exhibit similar rectifying behavior. The current work provides a feasible way to fabricate hybrid junctions based on asymmetric metal electrodes and investigate their electron transport toward the design of molecular rectifiers.