Superconductor-normal Metal-superconductor Junctions Research Articles

Current and shot noise in a normal metal–superconductor junction driven by spin-dependent periodic pulse sequence

Current and shot noise in a normal metal–superconductor junction driven by spin-dependent periodic pulse sequence

Equal contribution of even-frequency and odd-frequency pairing to transport across normal metal–superconductor junctions

Equal contribution of even-frequency and odd-frequency pairing to transport across normal metal–superconductor junctions

Unconventional long range triplet proximity effect in planar YBa2Cu3O7/La0.7Sr0.3MnO3/YBa2Cu3O7 Josephson junctions

The proximity effect between superconductors and ferromagnets may become long range due to the generation of triplet pairs. The recent finding of a long, one micron-range unconventional Josephson effect between YBa2Cu3O7 high Tc cuprates separated by a half metallic La0.7Sr0.3MnO3 manganite ferromagnet has uncovered a novel unconventional triplet proximity effect. In this paper, we examine the temperature dependence of the critical current in planar Josephson junctions. We find that the critical current—normal resistance product follows the predictions of traditional superconductor-normal metal-superconductor junctions, which implies that triplet pairs in a ferromagnet are transported in the diffusive limit similarly to singlet pairs in a normal metal. This result calls for theoretical studies of the new triplet Josephson effect and underlines its potential in future superconducting spintronics.

Enhanced 2π-periodic Aharonov–Bohm Effect as a Signature of Majorana Zero Modes Probed by Nonlocal Measurements

We propose the $2\pi$-periodic Aharonov-Bohm (AB) effect as a nonlocal probe of Majorana zero modes (MZMs) without the restriction of fermion parity. We demonstrate the enhancement of the AB effect, where the topological protection of MZMs yields amplified and robust Andreev reflection mediated by MZMs at multiple superconductor-normal metal junctions. We investigate the influence of trivial bound states and show that a nonlocal index enables a more explicit distinction between the trivial and topological bound states than local probes.

Nonequilibrium Josephson diode effect in periodically driven SNS junctions

In typical Josephson junctions, the Josephson current is an odd function of the superconducting phase difference. Recently, diode effect in Josephson junctions is observed in experiments wherein the maximum and the minimum values of the Josephson current in the current-phase relation do not have the same magnitude. We propose a superconductor-normal metal-superconductor (SNS) junction where Josephson diode effect manifests when the normal metal region is driven. Time reversal symmetry and inversion symmetry need to be broken in the SNS junction for the diode effect to show up. We calculate long time averaged current and show that the system exhibits diode effect for two configurations of the driven SNS junction - one in which inversion symmetry is broken in the undriven part of the Hamiltonian and the other wherein both the symmetries are broken by the driving potential. In the latter configuration, a nonzero current known as anomalous current appears at the junction in absence of phase bias. In the proposed setup, the diode effect vanishes in the adiabatic limit.

Current–voltage characteristics of superconductor-normal metal-superconductor junctions

We develop a theory of current–voltage (I-U) characteristics for superconductor-normal metal-superconductor (SNS) junctions. At small voltages and sufficiently low temperatures the I-U characteristics of the junction is controlled by the inelastic relaxation time τin. In particular, the linear conductance is proportional to τin. In this regime the I-U characteristics can be expressed solely in terms of dependence of the density of states in the normal region ν(χ) on the phase difference of the order parameter across the junction. In contrast, at large voltages the I-U characteristics of the device is controlled by the elastic relaxation time τel, which is much smaller than the inelastic one.

Ratchet Effect in Triangle-Shaped Vertical Nb/TiO/Nb Josephson Junctions

We fabricate vertical Nb/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /Nb Josephson junctions with the shape of right-angled isosceles triangles of different sizes. The TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> layer is primarily metallic, indicating that our junctions belong to the superconductor-normal metal-superconductor (S-N-S) junction. When applying bipolar dc currents across the junctions in parallel magnetic fields, there are many obvious differences between the positive and negative parts of the current-voltage characteristics. We attributed this asymmetry behavior (or ratchet behavior) mainly to the asymmetric pinnings for Josephson vortex penetration at the asymmetric edges (the right-angled corner and the hypotenuse) of the triangled junction, in addition to the effect of current-induced magnetic self-fields. For comparison with superconducting thin-film ratchets, the rectified dc voltages of our junctions with applied ac currents are investigated at different temperatures, magnetic fields, and driving frequencies. The waveforms of the time-resolved voltage with sinusoid currents are also measured. The ratchet effect in the junctions decreases monotonically with increasing temperature, increases to the peak first, and then decreases with the increasing magnetic field. It remains almost unchanged in the driving frequency range of 0.1 to 100 kHz. Compared with Abrikosov vortex ratchets, the ratchet effect in the junctions is much more sensitive to temperatures far from <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , demonstrating the larger temperature-regulating window. This article shows the distinct influence of the asymmetric junction boundaries on the characteristics of the junctions under certain conditions, which can be useful for further understanding and inspiring new applications of Josephson junctions.

Quasi-periodic Andreev reflection in topological nodal-line semimetal–superconductor junctions

The Andreev reflection in topological nodal-line semimetal–superconductor (TNLSM–SC) junctions is studied based on the Landauer–Büttiker formula combined with the non-equilibrium Green’s function method. It is found that the Andreev reflection coefficient can reach large values of several tens for a TNLSM–SC junction. In the energy gap, two new peaks for the Andreev reflection appear. Outside the energy gap, the Andreev reflection shows a quasi-periodic variation. With the incident energy increasing, the Andreev reflection decreases periodically. This phenomenon originates from the peculiar energy spectrum of the TNLSMs. The value and quasi-period of the Andreev reflection coefficient depend on the size of the system in different directions. These anomalous Andreev reflections are helpful to study the transport properties in normal metal–superconductor junctions.

Strengthened proximity effect at grain boundaries to enhance inter-grain supercurrent in Ba1-xKxFe2As2 superconductors

Iron-based superconductors have great potential for high-power applications due to their prominent high-field properties. One of the central issues in enhancing the critical current density of iron-based superconducting wires is to reveal the roles and limitations of grain boundaries in supercurrent transport. Here, we finely tuned the electronic properties of grain boundaries by doping Ba1-xKxFe2As2 superconductors in a wide range (0.25 ≤ x ≤ 0.598). It is found that the intra-grain Jcintra peaks near x∼0.287, while the inter-grain Jcinter has a maximum at about x∼0.458. Remarkably, the grain boundary transparency parameter defined as ε = Jcinter/Jcintra rises monotonically with doping. Through detailed microscopic analysis, we suggest that the FeAs segregation phase commonly existing at grain boundaries and the adjacent grains constitute superconductor-normal metal-superconductor (SNS) Josephson junctions which play a key role in transporting supercurrent. A sandwich model based on the proximity effect and the SNS junction is proposed to well interpret our data. It is found that overdoping in superconducting grains largely strengthens the proximity effect and consequently enhances the intergrain supercurrent. Our results will shed new insights and inspirations for improving the application parameters of iron-based superconductors by grain boundary engineering.

Vortex control in superconducting Corbino geometry networks

In superconductors, vortices induced by a magnetic field are nucleated randomly due to some random fluctuations or pinned by impurities or boundaries, impeding the development of vortex based quantum devices. Here, we propose a superconducting structure which allows to nucleate and control vortices on-demand by controlling magnetic fields and currents. Using time-dependent Ginzburg Landau theory, we study a driven vortex motion in two-dimensional Corbino geometries of superconductor-normal metal-superconductor Josephson junctions. We remedy the randomness of nucleation by introducing normal conducting rails to the Corbino disk to guide the nucleation process and motion of vortices towards the junction. We elaborate on the consequences of rail-vortex and vortex-vortex interactions to the quantization of resistance across the junction. Finally, we simulate the nucleations and manipulations of two and four vortices in Corbino networks, and discuss its application to Majorana zero mode braiding operations. Our study provides a potential route towards quantum computation with non-Abelian anyons.

Gapless vortex bound states in superconducting topological semimetals.

We find that the vortex bound states in superconducting topological semimetals are gapless owing to topological massless excitations in their normal states. We demonstrate this universal result in a variety of semimetals, including Dirac and Weyl semimetals, three-fold degenerate spin-1 fermions, spin-3/2 Rarita-Schwinger-Weyl fermion semimetals and other exotic fermion semimetals. The formation of these gapless bound states is closely related to their Andreev specular reflection and propagating Andreev modes in π-phase superconductor-normal metal-superconductor junctions. We further demonstrate that these gapless states are topologically protected and can be derived from a topological pumping process.

Higher harmonic resistance oscillations in micro-bridge superconducting Nb ring

We studied resistance oscillations in two types of superconducting mesoscopic Nb rings. In a simple superconducting ring device, a resistance oscillation with a period of the quantized magnetic flux was clearly observed. On the other hand, in a micro-bridge ring device where two-narrow parts are embedded in parallel and work as superconductor-normal metal–superconductor junctions, higher harmonic resistance oscillations were obtained when the measurement current was well-tuned. We argue that such higher harmonic resistance oscillations can be detected even in the micro-bridge Nb superconducting ring device where the device size is much larger than the coherence length of Nb.

Nb/Au/NbSe2 hybrid Josephson junctions

We fabricated thin film Nb/Au/NbSe2 hybrid Josephson junctions by a dry transfer technique with only a single lithography step in the process. A thin Au layer is deposited on Nb to prevent oxidation during the fabrication process. The superconducting van der Waals (vdW) material NbSe2 is placed on top of the Nb/Au layer. Electrical transport properties are studied in detail and are compared to numerical simulations. The current–voltage characteristics we observe are typical for superconductor-normal metal-superconductor junctions and we do not observe an effect of a potential insulating barrier between NbSe2 and Au. The modulation of the critical current in magnetic fields indicates an inhomogeneous distribution of supercurrent density, presumably caused by the roughness of the Nb/Au interface to NbSe2. Nevertheless, we observe clear integer Shapiro steps, indicating a sinusoidal current phase relation of junctions. We do not see an impact of the anisotropic s-wave and the charge density wave in NbSe2 on the Josephson effects. Our method presents a viable and simple way to fabricate hybrid Josephson junctions between superconducting films and two-dimensional (2D) vdW superconducting materials, which might offer a chance for widespread applications.

Exciting odd-frequency equal-spin triplet correlations at metal-superconductor interfaces

We predict the occurrence of odd frequency equal-spin-triplet correlations at a normal metal-superconductor junction. This result is significant because equal-spin-triplet correlations are associated with the presence of dissipation-less pure spin current. Inserting a spin-flipper at the interface of a normal metal-superconductor junction excites equal-spin-triplet correlations. The existence of odd frequency equal-spin-triplet correlations in absence of odd frequency mixed spin-triplet correlations is the main take-home message of this work. It tallies well with the measured local magnetization density of states and spin-polarized local density of states at the interface. The importance of spin-flip scattering to the obtained results is manifest when we compare our normal metal-spin flipper-superconductor junction to other hybrid junctions where either only spin mixing or both spin mixing and spin-flip scattering are present.

Enhanced Andreev reflection in Kekulé-Y patterned graphene

We theoretically investigate the electron-hole conversions in normal metal-superconductor junctions based on the Kekul\'e-Y (Kek-Y) patterned graphene. It is shown that, in contrast to the Klein tunneling in the pristine graphene, the perfect Andreev reflection with almost unit efficiency can be extended to a large range of incident angles in the Kek-Y patterned graphene in single-Dirac cone phase with the bias voltage in the gap. The normalized zero-bias conductance is also obtained.

Andreev reflection in Fermi-arc surface states of Weyl semimetals

Fermi arc surface states are the hallmark of Weyl semimetals, whose identification is usually challenged by their coexistence with gapless bulk states. Surface transport measurements by fabricating setups on the sample boundary provide a natural solution to this problem. Here, we study the Andreev reflection (AR) in a planar normal metal-superconductor junction on the Weyl semimetal surface with a pair of Fermi arcs. For a conserved transverse momentum, the occurrence of normal reflection depends on the relative orientation between the Fermi arcs and the normal of the junction, which is a direct result of the disconnected Fermi arcs. Consequently, a crossover from the suppressed to perfect AR occurs with varying the orientation of the planar junction, giving rise to a change from double-peak to plateau structure in conductance spectra. Moreover, such a crossover can be facilitated by imposing a magnetic field, making electrons slide along the Fermi arcs so as to switch between two regimes of the AR. Our results provide a decisive signature for the detection of Fermi arcs and open the possibilities of exploring novel phenomenology through their interplay with superconductivity.

Emergence of exceptional points and their spectroscopic signature in a Dirac semimetal–dirty superconductor heterojunction

We theoretically investigate the emergence of non-hermitian physics at the heterojunction of a type-II Dirac semi-metal (DSM) and a dirty superconductor (DSC). The non-hermiticity is introduced in the DSM through the self-energy term incorporated via the dirtiness of the superconducting material. This causes the spectra of the effective Hamiltonian to become complex, which gives rise to the appearance of the exceptional points (EPs). This complex self energy, apart from having a frequency dependence, also acquires spatial dependence as well, which is unique and can provide interesting effects related to non-hermitian physics in spectral function analysis. At an appropriate distance from the normal metal-superconductor junction of the DSC, non-hermitian degeneracies appear and a single Dirac point splits into two EPs. In the spectral function analysis, apart from the EPs, a Fermi-arc like structure also emerges, which connects the two degeneracies (EPs). The results discussed here are distinctive and possibly can be realized in spectroscopy measurements.

Conductance asymmetries in mesoscopic superconducting devices due to finite bias

Tunneling conductance spectroscopy in normal metal-superconductor junctions is an important tool for probing Andreev bound states in mesoscopic superconducting devices, such as Majorana nanowires. In an ideal superconducting device, the subgap conductance obeys specific symmetry relations, due to particle-hole symmetry and unitarity of the scattering matrix. However, experimental data often exhibits deviations from these symmetries or even their explicit breakdown. In this work, we identify a mechanism that leads to conductance asymmetries without quasiparticle poisoning. In particular, we investigate the effects of finite bias and include the voltage dependence in the tunnel barrier transparency, finding significant conductance asymmetries for realistic device parameters. It is important to identify the physical origin of conductance asymmetries: in contrast to other possible mechanisms such as quasiparticle poisoning, finite-bias effects are not detrimental to the performance of a topological qubit. To that end we identify features that can be used to experimentally determine whether finite-bias effects are the source of conductance asymmetries.

Electrical and thermal transport in antiferromagnet-superconductor junctions

We demonstrate that antiferromagnet-superconductor (AF-S) junctions show qualitatively different transport properties than normal metal-superconductor (N--S) and ferromagnet-superconductor (F-S) junctions. We attribute these transport features to presence of two new scattering processes in AF--S junctions, i.e., specular reflection of holes and retroreflection of electrons. Using the Blonder-Tinkham-Klapwijk formalism, we find that the electrical and thermal conductance depend nontrivially on antiferromagnetic exchange strength. Furthermore, we show that the interplay between the N\'eel vector direction and the interfacial Rashba spin-orbit coupling leads to a large anisotropic magnetoresistance. The unusual transport properties make AF--S interfaces unique among the traditional condensed-matter-system-based superconducting junctions.

Anomalous Andreev reflection in an 8- Pmmn borophene-based superconducting junction

We investigate the Andreev reflection in an 8-$Pmmn$ borophene-based normal metal-superconductor junction by solving the Dirac--Bogoliubov--de Gennes equation. There are three unusual features for Andreev reflection of anisotropic massless Dirac fermion: (1) The Andreev retroreflection generates in the interband conversion. (2) The specular Andreev reflection generates in the intraband conversion. (3) The electron-hole conversion with unit efficiency happens at a nonzero incident angle of electron regardless of the magnitude of doping in normal and superconducting regions. These three unique properties are opposite to the ones in a graphene-based normal metal-superconductor junction due to the anisotropic band structure of 8-$Pmmn$ borophene. Interestingly, we can modulate the Andreev reflection by adjusting the direction of junction. Our work gives an insight into the understanding of Andreev reflection of Dirac fermion.