Short Junctions Research Articles

Docking a flexible basket onto the core of the nuclear pore complex

The nuclear basket attaches to the nucleoplasmic side of the nuclear pore complex (NPC), coupling transcription to mRNA quality control and export. The basket expands the functional repertoire of a subset of NPCs in Saccharomyces cerevisiae by drawing a unique RNA/protein interactome. Yet, how the basket docks onto the NPC core remains unknown. By integrating AlphaFold-based interaction screens, electron microscopy and membrane-templated reconstitution, we uncovered a membrane-anchored tripartite junction between basket and NPC core. The basket subunit Nup60 harbours three adjacent short linear motifs, which connect Mlp1, a parallel homodimer consisting of coiled-coil segments interrupted by flexible hinges, and the Nup85 subunit of the Y-complex. We reconstituted the Y-complex•Nup60•Mlp1 assembly on a synthetic membrane and validated the protein interfaces in vivo. Here we explain how a short linear motif-based protein junction can substantially reshape NPC structure and function, advancing our understanding of compositional and conformational NPC heterogeneity.

Anomalous h/2e Periodicity and Majorana Zero Modes in Chiral Josephson Junctions.

Recent experiments reported that quantum Hall chiral edge state-mediated Josephson junctions (chiral Josephson junctions) could exhibit Fraunhofer oscillations with a periodicity of either h/e [Vignaud etal., Nature (London) 624, 545 (2023)NATUAS0028-083610.1038/s41586-023-06764-4] or h/2e [Amet etal., Science 352, 966 (2016)SCIEAS0036-807510.1126/science.aad6203]. While the h/e-periodic component of the supercurrent had been anticipated theoretically before, the emergence of the h/2e periodicity is still not fully understood. In this Letter, we systematically study the Fraunhofer oscillations of chiral Josephson junctions. In chiral Josephson junctions, the chiral edge states coupled to the superconductors become chiral Andreev edge states. We find that in short junctions, the coupling of the chiral Andreev edge states can trigger the h/2e-magnetic flux periodicity. Our theory resolves the important puzzle concerning the appearance of the h/2e periodicity in chiral Josephson junctions. Furthermore, we explain that when the chiral Andreev edge states couple, a pair of localized Majorana zero modes appear at the ends of the Josephson junction, which are robust and independent of the phase difference between the two superconductors. As the h/2e periodicity and the Majorana zero modes have the same physical origin in the wide junction limit, the Fraunhofer oscillation period could be useful in identifying the regime with Majorana zero modes.

Single-Molecule Junction Formation in Deep Eutectic Solvents with Highly Effective Gate Coupling.

The environment surrounding a molecular junction affects its charge-transport properties and, therefore, must be chosen with care. In the case of measurements in liquid media, the solvent must provide good solvation, grant junction stability, and, in the case of electrolyte gating experiments, allow efficient electrical coupling to the gate electrodes through control of the electrical double layer. We evaluated in this study the deep eutectic solvent mixture (DES) ethaline, which is a mixture of choline chloride and ethylene glycol (1:2), for single-molecule junction fabrication with break-junction techniques. In ethaline, we were able to (i) measure challenging and poorly soluble molecular wires, exploiting the improved solvation capabilities offered by DESs, and (ii) efficiently apply an electrostatic gate able to modulate the conductance of the junction by approximately an order of magnitude within a ∼1 V potential window. The electrochemical gating results on a Au-VDP-Au junction follow exceptionally well the single-level modeling with strong gate coupling (where VDP is 1,2-di(pyridine-4-yl)ethene). Ethaline is also an ideal solvent for the measurement of very short molecular junctions, as it grants a greatly reduced snapback distance of the metallic electrodes upon point-contact rupture. Our work demonstrates that DESs are viable alternatives to often relatively expensive ionic liquids, offering good versatility for single-molecule electrical measurements.

Impact of Junction Length on Supercurrent Resilience against Magnetic Field in InSb-Al Nanowire Josephson Junctions.

Semiconducting nanowire Josephson junctions represent an attractive platform to investigate the anomalous Josephson effect and detect topological superconductivity. However, an external magnetic field generally suppresses the supercurrent through hybrid nanowire junctions and significantly limits the field range in which the supercurrent phenomena can be studied. In this work, we investigate the impact of the length of InSb-Al nanowire Josephson junctions on the supercurrent resilience against magnetic fields. We find that the critical parallel field of the supercurrent can be considerably enhanced by reducing the junction length. Particularly, in 30 nm long junctions supercurrent can persist up to 1.3 T parallel field─approaching the critical field of the superconducting film. Furthermore, we embed such short junctions into a superconducting loop and obtain the supercurrent interference at a parallel field of 1 T. Our findings are highly relevant for multiple experiments on hybrid nanowires requiring a magnetic-field-resilient supercurrent.

Current-phase relation of a short multi-mode Bi2Se3 topological insulator nanoribbon Josephson junction with ballistic transport modes

We used the asymmetric superconducting quantum interference device (SQUID) technique to extract the current phase relation (CPR) of a Josephson junction with a 3D-topological insulator (3D-TI) Bi2Se3 nanobelt as the barrier. The obtained CPR shows deviations from the standard sinusoidal CPR with a pronounced forward skewness. At temperatures below 200 mK, the junction skewness values are above the zero temperature limit for short diffusive junctions. Fitting of the extracted CPR shows that most of the supercurrent is carried by ballistic topological surface states (TSSs), with a small contribution of diffusive channels primarily due to the bulk. These findings are instrumental in engineering devices that can fully exploit the properties of the topologically protected surface states of 3D TIs.

Zero-frequency supercurrent susceptibility signatures of trivial and topological zero-energy states in nanowire junctions

We propose a method to distinguish between trivial and topological, Majorana, zero-energy states (ZESs) in both short and long superconductor-normal-superconductor junctions based on Rashba nanowires using phase-biased equilibrium transport measurements. In particular, we show how the sawtooth profile of the supercurrent, due to the Majorana oscillation suppression in the topological phase for sufficiently long superconductor regions, leads to a strong signal in its zero-frequency susceptibility for a phase difference of . This signal is notably insensitive to the chemical potential in the normal region, while trivial ZESs only causes signals in the susceptibility that is highly varying with the chemical potential, thus turning gating of the normal region into a simple experimental control knob. Furthermore, we obtain that, by tuning the junction transparency, critical currents in both short and long junctions undergo a reduction in the number of oscillations as a function of magnetic field only in the topological phase, an effect that find to be intimately linked to Majorana non-locality. Finally, we show that our results also hold at finite temperatures, thus highlighting their potential measurability under realistic experimental conditions.

The pathoanatomy of atraumatic partial distal biceps tears: a cadaveric study

Partial distal biceps tears can occur in the short and/or long heads, leading to forearm pain and weakness. Yet, the pathoanatomy of atraumatic and traumatic partial tears are not understood. The goals of this study are to determine the distal biceps partial tear frequency and tear pattern in a cohort of cadaveric specimens. Fifty three fresh frozen cadavers (average age 70.4±13.8 years, range 32-94) underwent elbow endoscopy to screen for partial tears. The partial tendon tear pattern was classified into either attritional (atraumatic), detachment on the tendon's lateral side, or avulsion (traumatic) rupture of the tendon fibers from bone on both the lateral and medial sides. The specimens were dissected and laser scanned to make 3D models. The tear location, shape, and area were calculated using gross dissection and the 3D models. Atraumatic partial distal biceps tears were identified in 40% of the specimens, 72% involved both the long and short heads, 14% long head, and 14% short head. In all tears, the tendon fibers were only detached from the lateral side. The greatest tear width occurred near the short and long head junction. Atraumatic partial distal biceps tears are common. The tear originates on the lateral side of the tendon at the short and long head junction. All the tear patterns are attritional and no specimen had surgical treatment. This finding supports the current treatment recommendation of an initial period of nonoperative care for symptomatic atraumatic partial distal biceps tears.

Long and Covalently Conjugated Branched DNA Structures for in Situ Gelation in Vivo

Long and Covalently Conjugated Branched DNA Structures for in Situ Gelation in Vivo

Effect of dilute impurities on short graphene Josephson junctions

Despite the structural simplicity of graphene, its mechanical and electronic remarkable properties make this material a credible starting point for new technologies across a wide range of fields. The recent realizations of graphene-based hybrid systems, such as Josephson junctions, make graphene a promising a platform for new generations of devices for topological quantum computing and quantum sensing. To this aim, accurate control of the electronic properties of graphene Josephson junctions in the presence of disorder is essential. Here, we study the effect of a dilute homogeneous spatial distribution of non-magnetic impurities on the equilibrium supercurrent sustained by a ballistic graphene Josephson junction in the short junction limit. Within the Dirac-Bogoliubov-de Gennes approach and modeling impurities by the Anderson model we derive the supercurrent and its equilibrium power spectrum. We find a modification of the current-phase relation with a reduction of the skewness induced by disorder, and a nonmonotonic temperature dependence of the critical current. The potentialities of the supercurrent power spectrum for accurate spectroscopy of the hybridized Andreev bound states-impurities spectrum are highlighted. In the low temperature limit, the supercurrent zero frequency thermal noise directly probes the spectral function at the Fermi energy.

Josephson Diode Effect in High-Mobility InSb Nanoflags.

We report nonreciprocal dissipation-less transport in single ballistic InSb nanoflag Josephson junctions. Applying an in-plane magnetic field, we observe an inequality in supercurrent for the two opposite current propagation directions. Thus, these devices can work as Josephson diodes, with dissipation-less current flowing in only one direction. For small fields, the supercurrent asymmetry increases linearly with external field, and then it saturates as the Zeeman energy becomes relevant, before it finally decreases to zero at higher fields. The effect is maximum when the in-plane field is perpendicular to the current vector, which identifies Rashba spin-orbit coupling as the main symmetry-breaking mechanism. While a variation in carrier concentration in these high-quality InSb nanoflags does not significantly influence the supercurrent asymmetry, it is instead strongly suppressed by an increase in temperature. Our experimental findings are consistent with a model for ballistic short junctions and show that the diode effect is intrinsic to this material.

Structure Identification for Force-Induced Reaction Using Single-Molecule Conductance Measurement

Structure Identification for Force-Induced Reaction Using Single-Molecule Conductance Measurement

Tuning the topological state of a helical atom chain via a Josephson phase

By solving the Bogoliubov-de Gennes equations, we study the quasiparticle spectrum of a magnetic atom chain placed inside a short constriction-type Josephson junction. A helical magnetic order of the atomic spins is assumed, so that a topologically nontrivial state with Majorana edge modes at the ends of the chain can appear. It is found that in the presence of a nonzero Josephson phase the subgap spectrum of an infinite chain consists of four branches (Shiba bands). This spectrum is almost certainly gapped if the atomic spins form a coplanar spiral. The Majorana number of the given system is calculated analytically. It is demonstrated that a Josephson phase shift can be used to drive the system into a topologically nontrivial state and to tune the size of the topological gap. The spatial structure of Majorana edge modes is studied as well. We generalize the effective model based on discrete Bogoliubov-de Gennes equations developed by Pientka et al. for a bulk superconductor to the case of a Josephson junction. Using these discrete equations, the wave functions of Majorana zero modes are calculated analytically for a coplanar atomic spin spiral with arbitrary pitch. The wave functions exhibit an intermediate asymptotic behavior with a nonexponential fall-off with distance from the edge of the atomic chain.

Quasiparticle poisoning in trivial and topological Josephson junctions

We theoretically study a short single-channel Josephson junction between superconductors in the trivial and topological phases. The junction is assumed to be biased by a small current and subjected to quasiparticle poisoning. We find that the presence of quasiparticles leads to a voltage signal from the Josephson junction that can be observed both in the trivial and topological phases. Quite remarkably, these voltage signatures are sufficiently different in the two phases such that they can serve as means to clearly distinguish between trivial Andreev and topological Majorana bound states in the system. Moreover, these voltage signatures, in the trivial and topological phases, would allow one to directly measure the quasiparticle poisoning rates and test various approaches for protection against quasiparticle poisoning.

Sensitivity of Junction Width to Tidal Nonlinearity on Small and Shallow Tidal Junction

Observations of water elevation in the short and small tidal junctions of the Ota River, Japan, showed an increase in tidal nonlinearity at the apex of the junction. To quantitatively estimate the increase in nonlinearity, the barotropic hydrodynamic model was applied in an idealized junction domain, inspired by the Ota River Estuary junction. Even though the model was simplified, it successfully reproduced the increase in nonlinearity at the junction apex. A sensitivity analysis of tidal nonlinearity to the width of the upstream channel at the junction was performed by varying the upstream channel width from the same width as the branch channel width to three times the branch channel width. The relationship between the upstream channel width at the apex and tidal nonlinearity was not linear. Tidal nonlinearity was maximized when the apex width was twice the branch channel width. The convergence of the tides in the small width junction induced an increase of some positions of quarter-diurnal tidal constituent that raised the tidal nonlinearity. In the case of a wider channel, the flushing from river runoff dampen the tidal constituents, making it decrease tidal nonlinearity

Textural characteristics of mixed gels improved by structural recombination and the formation of hydrogen bonds between curdlan and carrageenan

The physicochemical properties of a single gel are strongly affected by an admixture of another ingredient as a cosolvent. Although many studies have proposed the gelation mechanism of mixed gels using scattering, spectroscopic, thermal, and rheological techniques, direct evidence at the molecular scale is still lacking. As molecular models, carrageenan and curdlan are linear polysaccharides suitable for studying gelation mechanisms of mixed gels. In this study, based on texture profile analysis and fourier transform infrared spectroscopy, we divided mixed gels into three stages. In stage 1, carrageenan molecules were dispersed into the dissociated curdlan matrix and improved the curdlan gel network. In stage 2, the dissociated curdlan competed with carrageenan for water molecules preventing the formation of single gel or mixed gels, causing the decreased textural parameters. In stage 3, owing to structural recombination and the formation of hydrogen bonds between curdlan and carrageenan, a denser gel was formed. X-ray diffraction results showed that at curdlan: carrageenan ratios from 1:1–1:3, the crystalline peak of carrageenan appeared near 2θ = 28.5° and the peak of curdlan disappeared at 2θ = 19.5°, indicating that the dissociated curdlan destroyed the original crystalline domains of curdlan and dispersed into the carrageenan matrix. Scanning electron microscopic analysis revealed that at a curdlan: carrageenan ratio of 1:3, shorter and smaller junction zones facilitated a considerably more compact and dense structure. Based on molecular docking, curdlan was found to dissociate into a single helical chain, bonding with double- or single-helical carrageenan. Thus, structural recombination and new intermolecular hydrogen bonds between curdlan and carrageenan afforded considerably short and small junction zones.

Tunable effective length of fractional Josephson junctions

Topological Josephson junctions (TJJs) have been a subject of widespread interest due to their hosting of Majorana zero modes. In long junctions, i.e. junctions where the junction length exceeds the superconducting coherence length, TJJs manifest themselves in specific features of the critical current (Beenakker 2013 Phys. Rev. Lett. 110 017003). Here we propose to couple the helical edge states mediating the TJJ to additional channels or quantum dots, by which the effective junction length can be increased by tunable parameters associated with these couplings, so that such measurements become possible even in short junctions. Besides effective low-energy models that we treat analytically, we investigate realizations by a Kane–Mele model with edge passivation and treat them numerically via tight binding models. In each case, we explicitly calculate the critical current using the Andreev bound state spectrum and show that it differs in effectively long junctions in the cases of strong and weak parity changing perturbations (quasiparticle poisoning).

Emission from Josephson junctions with Gaussian distribution of critical currents

The model which allows to obtain the spectrum of emission of systems of Josephson junctions with the inhomogeneous distributions of critical currents along junctions is developed. With the use of this model we study electrical properties of systems in which junctions have the Gaussian distributions of critical currents. In particular, IV-characteristics and power of emission from inhomogeneous junctions with dimensions smaller than the Josephson depth of penetration of magnetic field have been investigated. We showed that for such junctions the dependence of emitted power on voltage (i.e. the spectrum of emission) had maxima at voltages corresponded to Fiske steps in the whole range of voltages, though in the IV-characteristics particularities (nuclei of zero-field steps) were not seen and they could be revealed only in derivatives of these curves. The comparison of our results with similar results which we obtained earlier for long junctions allows to suppose that the investigated mechanism of the formation of zero-field steps is general and it is valid for both long and short junctions. We investigated the averaged on random realizations height of some maximum of emitted power at different values of the Gaussian standard deviation of critical currents and found the square dependence of this height on the dimensionless parameter which characterizes the standard deviation. This result was in agreement with the theory of zero-field steps. We also considered electrical properties and power of emission from the stack of two long interacting with each other Josephson junctions in magnetic field. Each of the junctions had small (about 10-3 %) Gaussian distribution of critical currents. We found that if magnetic field was absent then there were only normal modes in the system (namely, the in-phase mode and the anti-phase mode). Zero-field steps were formed at voltages corresponded to the split even Fiske step. There were only normal modes in the system also when the relation of magnetic field to the value of the magnetic field at which the critical current becomes zero was more than 0.6. When this relation was smaller, other modes existed as well. We supposed that some normal modes could be destroyed because due to magnetic field standing waves were formed at both odd and even Fiske steps, so some modes could be locked with standing waves.

Multiple andreev reflections in topological insulator nanoribbons

Superconducting proximity junctions made of topological insulator (TI) nanoribbons (NRs) provide a useful platform for studying topological superconductivity. We report on the fabrication and measurement of Josephson junctions (JJs) using Sb-doped Bi2Se3 NRs in contact with Al electrodes. Aharonov–Bohm and Altshuler–Aronov–Spivak oscillations of the axial magneto-conductance of TI NR were observed, indicating the existence of metallic surface states along the circumference of the TI NR. We observed the supercurrent in the TI NR JJ and subharmonic gap structures of the differential conductance due to multiple Andreev reflections. The interface transparency of the TI NR JJs estimated based on the excess current reaches τ = 0.83, which is among the highest values reported for TI JJs. The temperature dependence of critical current is consistent with the short and ballistic junction model confirming the formation of highly transparent superconducting contacts on the TI NR. Our observations would be useful for exploring topological Josephson effects in TI NRs.

Lévy noise effects on Josephson junctions

We review three different approaches to investigate the non-equilibrium stochastic dynamics of a Josephson junction affected by Lévy-distributed current fluctuations. First, we study the lifetime in the metastable superconducting state of current-biased short and long junctions, in the presence of Gaussian and Lévy noise sources. We highlight the noise-induced nonmonotonic behavior of the mean switching time as a function of noise intensity and driving frequency, that is the noise enhanced stability and the stochastic resonant activation, respectively. Then, we characterize the Lévy noise source through the average voltage drop across a current-biased junction. The voltage measurement versus the noise intensity allows to infer the value of the stability index that characterizes Lévy-distributed fluctuations. The numerical calculation of the average voltage drop across the junction well agrees with the analytical estimate of the average velocity for Lévy-driven escape processes from a metastable state. Finally, we look at the distribution of switching currents out of the zero-voltage state, when a Lévy noise signal is added to a linearly ramped bias current. The analysis of the cumulative distribution function of the switching currents gives information on both the Lévy stability index and the intensity of fluctuations. We present also a theoretical model to catch the features of the Lévy signal from a measured distribution of switching currents. The phenomena discussed in this work can pave the way for an effective and reliable Josephson-based scheme to characterize Lévy components eventually embedded in an unknown noisy signal.

Thermodynamics in topological Josephson junctions

We study the thermodynamic properties of topological Josephson junctions using a quantum spin Hall (QSH) insulator-based junction as an example. In particular, we propose that phase-dependent measurements of the heat capacity offer an alternative to Josephson-current measurements to demonstrate key topological features. Even in an equilibrium situation, where the fermion parity is not conserved, the heat capacity exhibits a pronounced double peak in its phase dependence as a signature of the protected zero-energy crossing in the Andreev spectrum. This double-peak feature is robust against changes of the tunneling barrier and thus allows one to distinguish between topological and trivial junctions. At short time scales fermion parity is conserved and the heat capacity is $4\pi$-periodic in the superconducting phase difference. We propose a dispersive setup coupling the Josephson junction to a tank LC circuit to measure the heat capacity of the QSH-based Josephson junction sufficiently fast to detect the $4\pi$-periodicity. Although explicitly calculated for a short QSH-based Josephson junction, our results are also applicable to long as well as nanowire-based topological Josephson junctions.