Asymmetric Junctions Research Articles

Comparative experimental and numerical study of mixing efficiency in 3D-printed microfluidic droplet generators: T junction, cross junction, and asymmetric junctions with varying angles

In droplet-based microfluidics, rapid mixing during droplet formation enhances reaction uniformity, eliminating the need for micromixers. In this research, we conducted a comprehensive study by first employing a series of two-dimensional (2D) numerical simulations, followed by experimental investigations using 3D-printed microfluidic chips. We compared the mixing efficiency, droplet diameter, and droplet eccentricity of three different types of droplet generators: T junction, cross junction, and asymmetric droplet generators with various angles. Regarding mixing efficiency, we observed that the asymmetric droplet generators outperformed the cross junction by 30 % but fell slightly short of the mixing efficiency achieved by the T junction (1 %). Additionally, while the mixing index in the asymmetric generators closely matched that of the T junction, these asymmetric generators produced smaller droplets by 72 %. Increasing the angle in asymmetric droplet generators resulted in enhanced mixing efficiencies and an increase in droplet diameters. The asymmetric junction with a 30° angle could achieve a mixing efficiency of up to 80 %. Additionally, an analysis of the dispersed phase flow rate revealed that higher flow rates lead to larger droplet sizes and reduced mixing efficiencies. The asymmetric droplet generators improve mixing efficiency facilitating rapid reagent mixing, all while maintaining a small droplet diameter.

Coupled Thermal and Power Transport of Optical Waveguide Arrays: Photonic Wiedemann-Franz Law and Rectification Effect.

In isolated nonlinear optical waveguide arrays, simultaneous conservation of longitudinal momentum flow ("internal energy") and optical power ("particle number") of the optical modes enables study of coupled thermal and particle transport in the negative temperature regime. Based on exact numerical simulation and rationale from Landauer formalism, we predict generic photonic version of the Wiedemann-Franz law in such systems, with the Lorenz number L∝|T|^{-2}. This is rooted in the spectral decoupling of thermal and particle current, and their different temperature dependence. In addition, in asymmetric junctions, relaxation of the system toward equilibrium shows apparent asymmetry for positive and negative biases, indicating rectification behavior. This Letter illustrates the possibility of simulate nonequilibrium transport processes using optical networks, in parameter regimes difficult to reach in natural condensed matter or atomic gas systems. It also provides new insights in manipulating power and momentum flow of optical waves in artificial waveguide arrays.

Large Spin Polarization from symmetry-breaking Antiferromagnets in Antiferromagnetic Tunnel Junctions

Efficient detection of the magnetic state is a critical step towards useful antiferromagnet-based spintronic devices. Recently, finite tunneling magnetoresistance (TMR) has been demonstrated in tunnel junctions with antiferromagnetic electrodes, however, these studies have been mostly limited to junctions with two identical antiferromagnet (AFM) electrodes, where the matching of the spin-split Fermi surfaces played critical role. It remains unclear if AFMs can provide a finite net spin polarization, and hence be used as a spin polarizer or detector. In this work, we experimentally fabricate single-sided antiferromagnetic tunnel junctions consisting of one AFM electrode (Mn3Sn) and one ferromagnet (FM) electrode (CoFeB), where the spin polarized tunneling transport from AFM is detected by the FM layer. We observe a high TMR at cryogenic temperature (>100% at 10 K) in these asymmetric AFM tunnel junctions, suggesting a large effective spin polarization from Mn3Sn despite its nearly vanishing magnetization. The large TMR is consistent with recent theoretical studies where the broken symmetry in non-collinear AFMs is predicted to lift the spin degeneracy in the band structure. Our work provides strong evidence that spin polarized electrical transport can be achieved from AFMs.

Droplet motions directed by an expansion section in the T-junctions

The controlled motion of droplets in microfluidic chips is a preliminary requirement to realize their functions. The influence of the expansion section on the droplet motion is mainly investigated in the T-junction. The droplet dynamic characteristics are analyzed at the junction and the applicable flow rate of the expansion section is explored. The expansion section can reduce the entered length and motion time of the droplet when droplets flow into the channel with it, and finally avoid the possibility of droplet splitting. Even under a large difference of the branch flow rate, the expansion section can direct the droplet into its located channel. It is found that with the increase in continuous phase flow rate, the effect of the expansion section on the droplet motion behavior is gradually weakened until it disappears. Moreover, the critical conditions of it can be obtained by theoretical calculation. The expansion section can direct droplet motion in both symmetric and asymmetric junctions. However, it is mainly achieved by influencing the interfacial tension of the droplets in the symmetric junction, while the key force is related to the droplet motion in the asymmetric junction. Specifically, the expansion section influences the differential pressure force to direct the droplet in the flow into the side branch (with expansion section) mode, but it varies the interfacial tension of the droplet in the flow into the main branch mode.

Study of conductance in graphene nanochannels for symmetric and asymmetric junction configurations

Study of conductance in graphene nanochannels for symmetric and asymmetric junction configurations

Bound states in the continuum in asymmetric crossbar junctions in one-dimensional waveguides

Bound states in the continuum in asymmetric crossbar junctions in one-dimensional waveguides

Orientational Effects and Molecular-Scale Thermoelectricity Control.

The orientational effect concept in a molecular-scale junction is established for asymmetric junctions, which requires the fulfillment of two conditions: (1) design of an asymmetric molecule with strong distinct terminal end groups and (2) construction of a doubly asymmetric junction by placing an asymmetric molecule in an asymmetric junction to form a multicomponent system such as Au/Zn-TPP+M/Au. Here, we demonstrate that molecular-scale junctions that satisfy the conditions of these effects can manifest Seebeck coefficients whose sign fluctuates depending on the orientation of the molecule within the asymmetric junction in a complete theoretical investigation. Three anthracene-based compounds are investigated in three different scenarios, one of which displays a bithermoelectric behavior due to the presence of strong anchor groups, including pyridyl and thioacetate. This bithermoelectricity demonstration implies that if molecules with alternating orientations can be placed between an asymmetric source and drain, they can be potentially utilized for increasing the thermovoltage in molecular-scale thermoelectric energy generators (TEGs).

High-quality lateral monolayer-multilayer graphene junction formed by selective laser thinning for self-powered photodetection

The formation of an asymmetric junction is key to graphene-based photodetectors of high-sensitive photodetectability, because such a junction can not only facilitate the diffusion or drift of photogenerated carriers but also realize a self-powered operation. Here, a monolayer-multilayer graphene junction photodetector is accomplished by selectively thinning part of a multilayer graphene to a high-quality monolayer. Benefiting from the large photoabsorption cross section of multilayer graphene and strong asymmetry caused by the significant differences in optoelectronic properties between monolayer and multilayer graphene, the monolayer-multilayer graphene junction shows a 7-fold increase in short-circuit photocurrent as compared with that at the monolayer graphene-metal contact in scanning photocurrent images. The asymmetric configuration also enables the photodetector to work at zero bias with minimized dark current noise and stand-by power consumption. Under global illumination with visible light, a photoswitching ratio of 3.4 × 103, a responsivity of 8.8 mA W−1, a specific detectivity of 1.3 × 108 Jones and a response time of 11 ns can be obtained, suggesting a promising photoresponse. Moreover, it is worth mentioning that such a performance enhancement is achieved without compromising the broadband spectral response of graphene photodetector and it is hence applicable for long wavelength spectral range including infrared and terahertz.

Optoelectronically controlled spin-valley filter and nonlocal switch based on an asymmetrical silicene magnetic superconducting heterostructure

We investigate the effects of the circularly polarized light (CPL) and the electric field (EF) on the nonlocal transport in a silicene-based antiferromagnet/superconductor/ferromagnet (AF/S/F) asymmetrical junction. For case I (II), the CPL and the EF are applied simultaneously in the antiferromagnetic (ferromagnetic) region, whereas in the ferromagnetic (antiferromagnetic) region, only a constant EF is considered. The spin-valley-resolved conductance can be turned on or off by adjusting the CPL or the EF. The AF/S/F junction can be manipulated as a spin-locked valley filter for case I, while for case II, it can be used not only as a valley-locked spin filter but also as a nonlocal switch between two pure nonlocal processes. Such interesting nonlocal switch effect can be effectively controlled by reversing the direction of the incident energy axis, the handedness of the CPL, or the direction of the EF. These findings may open an avenue to the design and manufacture of the spintronic and valleytronic devices based on the asymmetrical silicene magnetic superconducting heterostructure.

Simulation of Conducted Responses in Microvascular Networks: Role of Current Rectification by Endothelial Cell Gap Junctions

Neuronal activity in the brain stimulates local increases in blood flow. Such flow increases require dilation of feeding arterioles, which can be initiated by upstream conducted responses in the form of electrical currents transmitted along the endothelial lining of the vessels. One potential trigger for this response is hyperpolarization of capillary endothelial cells in active regions resulting from local elevation of extracellular K+ concentration. The conducted hyperpolarization causes arteriolar dilation. For this mechanism to be effective in delivering increased flow to a specific location, the conducted response must travel in the upstream direction only, without reentry into adjacent parallel flow pathways. Such specificity in the response to activation has been observed in skeletal muscle, and may result from partial rectification of current in gap junctions connecting adjacent endothelial cells. The goal of this work is to simulate the spread of hyperpolarizing currents along vessel walls in microvascular networks of the cerebral circulation, including the effects of rectification in gap junctions. A theoretical model was developed for the spread of electrical current in the endothelial cells of a network of microvessels in response to local K+- induced hyperpolarization. Transmembrane currents are represented in the model by a KIR-channel conductance, a non-voltage-dependent K+ conductance, and a background conductance. Currents along vessel walls are represented assuming asymmetric gap junction conductance between adjacent endothelial cells, with higher conductance for downstream currents (causing upstream hyperpolarization) than for upstream currents. The model was applied to a synthetic network of 45 segments, assuming 20 nS conductance for upstream propagation, while a range of values was used for downstream propagation. The model shows effective upstream propagation of conducted responses when the gap junction conductance for downstream propagation is 1 nS or less. This result was confirmed in a simulation of a reconstructed murine cerebral cortex network with 4881 segments. In conclusion, partial current rectification by endothelial gap junctions can result in preferential upstream propagation of conducted responses, consistent with experimental observations of local flow regulation. Supported by NIH grant U01 HL133362. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Efficiency of diode effect in asymmetric inline long Josephson junctions

An effective superconducting diode—that is an element whose critical current depends upon the polarity—is achieved with a special configuration of a long Josephson junction and a control line. The proposed geometry is simple, based on the well-established asymmetric inline long Josephson junction, and can be realized using traditional superconductors without the need of magnetic materials. The performance of the diode, as measured by the efficiency, only depends on the normalized length and the control current intensity. At the optimal working point, the efficiency reaches about 76% and could be further improved at the expenses of the circuit simplicity. Finally, when a sinusoidal current is applied to the device, calculations with realistic fabrication parameters show the rectification of a sinusoidal current with a frequency in the MHz region.

Suppressing the Conductance of Single-Molecule Junctions Fabricated by sp2 C-H Bond Metalation.

High-conducting single-molecule junctions have attracted a great deal of attention, but insulating single-molecule junctions, which are critical in molecular circuits, have been less investigated due to the long-standing challenges. Herein, the in situ formation of a Au-C linker via electrical-potential-mediated sp2 C-H bond metalation of polyfluoroarenes with the assistance of scanning tunneling microscope-based break junction technique is reported. This metalation process is bias-dependent and occurs with an electropositive electrode, and the formed junction is highly oriented. Surprisingly, these polyfluoroarenes exhibit unexpected low conductance even under short molecular lengths and are superior molecular insulators. Flicker noise analysis and DFT calculations confirm that the insulating properties of polyfluoroarenes are ascribed to their multiple fluorine substituents. Our results pave a way for constructing oriented asymmetric molecular junctions and provide an efficient strategy to suppress the single-molecule conductance, which will aid in the design of molecular insulators and advance the development of self-integrating functional molecular circuits.

Enhanced mixing efficiency and reduced droplet size with novel droplet generators

Nowadays, droplet microfluidics has become widely utilized for high-throughput assays. Efficient mixing is crucial for initiating biochemical reactions in many applications. Rapid mixing during droplet formation eliminates the need for incorporating micromixers, which can complicate the chip design. Furthermore, immediate mixing of substances upon contact can significantly improve the consistency of chemical reactions and resulting products. This study introduces three innovative designs for droplet generators that achieve efficient mixing and produce small droplets. The T-cross and cross-T geometries combine cross and T junction mixing mechanisms, resulting in improved mixing efficiency. Numerical simulations were conducted to compare these novel geometries with traditional T and cross junctions in terms of mixing index, droplet diameter, and eccentricity. The cross-T geometry exhibited the highest mixing index and produced the smallest droplets. For the flow rate ratio of 0.5, this geometry offered a 10% increase in the mixing index and a decrease in the droplet diameter by 10% compared to the T junction. While the T junction has the best mixing efficiency among traditional droplet generators, it produces larger droplets, which can increase the risk of contamination due to contact with the microchannel walls. Therefore, the cross-T geometry is highly desirable in most applications due to its production of considerably smaller droplets. The asymmetric cross junction offered a 8% increase in mixing index and around 2% decrease in droplet diameter compared to the conventional cross junction in flow rate ratio of 0.5. All novel geometries demonstrated comparable mixing efficiency to the T junction. The cross junction exhibited the lowest mixing efficiency and produced larger droplets compared to the cross-T geometry (around 1%). Thus, the novel geometries, particularly the cross-T geometry, are a favorable choice for applications where both high mixing efficiency and small droplet sizes are important.

Sm, Pt asymmetric n- and p-type contacts in WSe2 phototransistor for high-performance broadband photodetection

The development of high-performance broadband photodetectors working at room temperature is still attractive. The Schottky barrier phototransistor based on asymmetric junction seems to be endowed with such potential—as photodetectors with low device power consumption and high photoresponse; however, it is rarely studied. Herein, a Sm–WSe2–Pt phototransistor with asymmetric metal contacts is constructed, and it is systematically investigated for their electronic and photoelectronic tunability via gate voltage, wavelength, and illumination power density. It was found that the tunable photogating process dominates the photoresponse mechanism, which allows for an excellent broadband photodetection from 300 to 1000 nm wavelength. In addition, the responsivity (R) and specific detectivity (D*) at 450 nm can reach 1723 A/W and 2.3 × 1013 Jones, respectively, while that of infrared illumination of 900 nm can reach 4.7 A/W and 3.1 × 1010 Jones, respectively. In addition, the device exhibits obvious photoresponse at zero bias, the R and D* can reach up to 27 mA/W and 8.5 × 1010 Jones, which realizes self-driven photodetection. This work provides an optimal option for realizing high-integrated, high-performance, low-power-consuming, and room-temperature-working broadband photodetectors.

Effects of Morphologically Asymmetric Junctions on the Effective Refractory Period of the Accessory Pathway in Wolff-Parkinson-White Syndrome: A Simulation Study

Effects of Morphologically Asymmetric Junctions on the Effective Refractory Period of the Accessory Pathway in Wolff-Parkinson-White Syndrome: A Simulation Study

A solar-blind vacuum-ultraviolet photodetector based on free-standing lamellar aluminum nitride single crystal

High-quality aluminum nitride (AlN) crystals are the key material for the development of high-performance solid-state solar-blind vacuum-ultraviolet (VUV) photodetectors. However, the commonly used epitaxial method to grow AlN crystals would limit this development due to the existence of indispensable substrates. This study addresses this issue using free-standing lamellar AlN single crystals that are grown using the physical vapor transport method. The large lateral dimension of the crystal enables the construction of an Au-AlN-graphene van der Waals heterojunction, which can function as a vertical VUV photodetector with the graphene serving as the light window. The asymmetric junctions formed on the two sides of the crystal and the limited penetration of the VUV endow the device with a bias polarity-dependent photoresponse feature arising from different photoelectric processes. Furthermore, the device demonstrates a high responsivity of 5.77 A W−1 and a high specific detectivity of 1.71 × 1013 cm Hz1/2 W−1 under the illumination of a 193 nm laser. The high crystallinity of the AlN guarantees a high spectral selectivity of responsivity with a 193 nm/280 nm rejection ratio of 3 × 102. This work would inspire the development of wide-bandgap-semiconductor-based VUV photodetectors in terms of methodology and mechanism.

Quantum Goos-Hänchen switch in graphene junctions

Goos-Hänchen (GH) shift, an interference phenomenon describing the lateral shift of the reflected beam along the interface during total internal reflection, has attracted great interests in the field of quantum transport in two-dimensional materials. In particular, the GH effect generates a novel pseudospin-dependent scattering effect in graphene, which in turn results in an conductance step in the bipolar junctions. Here, we reveal that a barrier region with effective barrier strength χ can greatly enrich the GH effect in graphene junctions. In contrast to the conventional case where the negative shift is allowed only in the p-n junction, the thin barrier enables both negative and positive spatial shifts in pIn and nIn junctions, where I represents the barrier region. More interestingly, the lowest channel degeneracy can be efficiently varied by tuning χ in both the symmetric pInIp and the asymmetric pnIp junctions, leading to a highly controllable switch that switches the conductance between and . These results advance the knowledge of GH effects in electronic materials and suggest experimental avenues for its observation and manipulation.

Asymmetrical Schottky Junction Built by Metal/Conducting Polymer Targeting Efficient Flexible Direct Current Tribovoltaic Generator

Direct current (DC) from mechanical energy built on the tribovoltaic effect is promising for the development of self‐powered flexible electronics. A semiconductor triboelectric generator is one of the optimized solutions for DC output. However, it remains underutilized because of its low output power and insufficient insight into the fundamental principles of charge generation and transport at dynamic interfaces. Herein, a flexible fabric‐based DC generator that relies on a metal–semiconducting polymer interface constructed by an asymmetric Schottky junction between a poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS)‐coated fabric and an aluminum (Al) foil is reported. The dependence of the electrical output performance of the device on the doping carrier concentration using different types of PEDOT:PSS to modulate the Schottky barrier height, validating both the work function and conductivity of the triboelectric material, which plays a fundamental role in DC generation, is explored. A large work function difference is required to create a high built‐in electric field for efficient separation of electron–hole pairs. Decent conductivity also fulfills an integral role in the innate carrier‐transport capability. Accordingly, an open‐circuit voltage (Voc) of 800 mV, a short‐circuit current (Isc) of 80 μA, and a power density of 6.7 mW cm−2 are yielded simultaneously.

Giant tunnel magnetoresistance induced by thermal bias

We analyze the spin-resolved transport and, in particular, the tunnel magnetoresistance of an asymmetric ferromagnetic tunnel junction with an embedded quantum dot or molecule subject to thermal and voltage bias in the nonlinear response regime. We demonstrate that such system exhibits a giant tunnel magnetoresistance effect that can be tuned by gate and bias voltages. Large values of magnetoresistance are associated with the interplay between the Kondo correlations and the ferromagnetic-contact-induced exchange field. In particular, we show that the nonequilibrium current in the parallel and antiparallel magnetic configuration of the system changes sign at different values of the voltage and thermal bias. This gives rise to giant values of magnetoresistance, the sign of which can be controlled by the applied sources.

Droplet dynamics in asymmetric microfluidic junctions

Droplet-based microfluidics has received increasing interest over the last two decades, due to its extensive applications in drug delivery, cell encapsulation, protein crystallization, biochemical synthesis and high-throughput screening. As one of the key units in the microfluidic channels, asymmetric microfluidic junctions provide more opportunities to control and organize the droplets with different sizes, however, few studies have been reported to explore the effect of the junction angle on droplet behaviour in the asymmetric junctions. This work presents a numerical investigation of the droplet behaviour in asymmetric microfluidic junctions. The effect of the junction angle on the droplet breakup is explored in detail. Droplet breakup and non-breakup are found in asymmetric junctions. In the regime of the asymmetric breakup, the volume of the daughter droplet in the uphill branching channel is always larger than that in the downhill branching channel. Interestingly, in the regime of non-breakup, droplets are sorted into the uphill or downhill branching channels, suggesting that this asymmetric junction could be used for passive droplet sorting. The physical mechanisms behind droplet breakup and sorting are explored. Both the junction angle and the initial tip-to-tip length of the droplet play essential roles in the droplet behaviour in the asymmetric junctions. A phase diagram of the droplet behaviour is presented in this work. A scaling model is proposed to predict the transition between the breakup and non-breakup of the droplets in the asymmetric junctions.