Superconductor-normal-superconductor Research Articles

High transparency induced superconductivity in field effect two-dimensional electron gases in undoped InAs/AlGaSb surface quantum wells

We report on transport characteristics of field effect two-dimensional electron gases (2DEGs) in 24 nm wide indium arsenide surface quantum wells. High quality single-subband magnetotransport with clear quantized integer quantum Hall plateaus is observed to filling factor ν = 2 in magnetic fields of up to B = 18 T, at electron densities up to 8 ×1011/cm2. Peak mobility is 11 000 cm2/Vs at 2 ×1012/cm2. Large Rashba spin–orbit coefficients up to 124 meV Å are obtained through weak anti-localization measurements. Proximitized superconductivity is demonstrated in Nb-based superconductor-normal-superconductor (SNS) junctions, yielding 78%–99% interface transparencies from superconducting contacts fabricated ex situ (post-growth), using two commonly used experimental techniques for measuring transparencies. These transparencies are on a par with those reported for epitaxially grown superconductors. These SNS junctions show characteristic voltages IcRn up to 870 μV and critical current densities up to 9.6 μA/μm, among the largest values reported for Nb-InAs SNS devices.

MgB[formula omitted] Josephson Junctions fabricated by Focused Helium Ion Beam with different irradiation doses on SiC and MgO substrates

Recently, high quality MgB2 single junctions and junction arrays fabricated by Focused Helium Ion Beam irradiation (He-FIB) have been reported. However, the effects of irradiation doses and substrates on junction properties have not been analysed systematically. In this work, 30 nm MgB2 thin films were deposited on SiC (0001) and MgO (111) substrates using Hybrid Physical–Chemical Vapour Deposition (HPCVD) technique and then etched into 2–4 μm wide microbridges. 2000–6000 ions/nm doses of He-FIB were irradiated across these microbridges, creating narrow junction barriers. R–T curves exhibit a clear foot-like structure that enlarges as irradiation dose increases. Temperature dependence of junction properties (critical current Ic, junction normal state resistance Rn, characteristic voltage Vc) shows that all junctions in our experiment exhibit superconductor–normal–superconductor (SNS) like behaviour and higher irradiation doses create thicker junction barriers. Furthermore, we analyse junction properties dependence on irradiation doses which yields an exponential change law. Finally, Fraunhofer diffraction-like patterns and Shapiro steps of junctions are also measured. Our results show MgB2 Josephson Junctions with adjustable properties can be fabricated by He-FIB on SiC and MgO substrates, which provides a solid foundation for future application devices research.

Observation of anomalous Josephson effect in nonequilibrium Andreev interferometers

The evidence of anomalous Josephson effect in superconducting-normal-superconducting (SNS) junctions requiring neither Zeeman splitting nor spin-orbit coupling is reported. We demonstrate that a spontaneous phase emerges by modifying the nonequilibrium conditions of the weak link resembling ${\ensuremath{\varphi}}_{0}$-junctions. The voltage-controlled phase shift is detected from magnetoresistance measurements of the proximitized wire connected to the weak link forming a crosslike Andreev interferometer. The interplay of Aharanov-Bohm-like and conventional Josephson currents, in agreement with recent predictions, provides the ingredients for the anomalous current-phase relation. These results represent a breakthrough for engineering superconducting nanocircuits with voltage-tunable quantum properties.

Critical current density of superconducting-normal-superconducting Josephson junctions and polycrystalline superconductors in high magnetic fields

We investigate the in-field critical current density ${J}_{\text{c}}(B)$ of superconducting-normal-superconducting (SNS) Josephson junctions (JJs) and polycrystalline superconducting systems with grain boundaries modeled as Josephson-type planar defects, both analytically and through computational time-dependent Ginzburg-Landau (TDGL) simulations in two and three dimensions. For very narrow SNS JJs, we derive analytic expressions for ${J}_{\text{c}}(B)$ that are high-field solutions for ${J}_{\text{c}}(B)$ for JJs across the entire applied field range up to the effective upper critical field ${B}_{\text{c2}}^{*}$. They generalize the well-known (low-field) exponential junction thickness dependence for ${J}_{\text{c}}$ from de Gennes, often used in the Josephson relation. We then extend our analytic expressions to describe wider junctions using physical arguments, and we confirm their agreement with TDGL simulations. These results are then compared with the current densities found in superconductors optimized for high-field applications. They provide an explanation for the Kramer field dependence and inverse power-law grain size dependence widely found in many low-temperature superconductors, and the power-law field dependence ${J}_{\text{c}}(B)\ensuremath{\sim}{B}^{\ensuremath{-}0.6}$ found at intermediate fields in some high-temperature superconductors including powder-in-tube ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{Ca}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$ and $R{\text{Ba}}_{2}{\text{Cu}}_{3}{\text{O}}_{7}$ tapes ($R=\text{rare}\phantom{\rule{4.pt}{0ex}}\text{earth}$). By reanalyzing critical current density data using the mathematical framework derived here and confirmed using TDGL, we enable an analysis of ${J}_{\text{c}}$ data that provides the local properties of grain boundaries in high-field superconductors and hence a deeper understanding of how grain boundaries influence ${J}_{\text{c}}$ in high magnetic fields.

Josephson Effect in MgB2/Pd/Nb Trilayer Josephson Junctions

This paper reports fabrication techniques and results of MgB2/Pd/Nb trilayer Josephson junctions. The MgB2 bottom electrode was co-evaporated by molecular beam epitaxy (MBE) technique from both magnesium and boron sources at a low substrate temperature ~ 300 °C, while the interlayer and the top niobium electrode (Pd/Nb bilayer) were deposited ex-situ using RF sputtering. The junctions exhibited and Josephson effect as well as a modulation of the critical current in a magnetic field applied in a direction normal to the junction plane. Fractional and integer Shapiro steps were observed at voltages corresponding to the frequency of the applied microwave radiation field. The products of the junctions compare well with the previously reported values. The results suggest that it should be possible to fabricate all-MgB2 and MgB2 as one of the electrodes Superconductor/Normal/Superconductor (SNS), Superconductor/Insulator/Superconductor (SIS) or even Superconductor/Ferromagnet/Superconductor (SFS) tunnel junctions with interesting characteristics and for various applications.
 Keywords: MgB2; all-MgB2; Josephson Tunnel junctions; trilayer devices; Niobium

High-Frequency Electromagnetic Emission from Non-Local Wavefunctions

In systems with non-local potentials or other kinds of non-locality, the Landauer-Büttiker formula of quantum transport leads to replacing the usual gauge-invariant current density 
 
 
 J
 
 
 with a current 
 
 
 
 
 J
 
 
 e
 x
 t
 
 
 
 
 which has a non-local part and coincides with the current of the extended Aharonov-Bohm electrodynamics. It follows that the electromagnetic field generated by this current can have some peculiar properties and in particular the electric field of an oscillating dipole can have a long-range longitudinal component. The calculation is complex because it requires the evaluation of double-retarded integrals. We report the outcome of some numerical integrations with specific parameters for the source: dipole length ∼10−7 cm, frequency 10 GHz. The resulting longitudinal field 
 
 
 
 E
 L
 
 
 
 turns out to be of the order of 
 
 
 
 10
 2
 
 
 
 to 
 
 
 
 10
 3
 
 
 
 times larger than the transverse component (only for the non-local part of the current). Possible applications concern the radiation field generated by Josephson tunnelling in thick superconductor-normal-superconductor (SNS) junctions in yttrium barium oxide (YBCO) and by current flow in molecular nanodevices.

Odd-frequency superconducting pairing in junctions with Rashba spin-orbit coupling

We consider normal-superconductor (NS) and superconductor-normal-superconductor (SNS) junctions based on one-dimensional nanowires with Rashba spin-orbit coupling and proximity-induced $s$-wave spin-singlet superconductivity and analytically demonstrate how both even- and odd-frequency and spin-singlet and -triplet superconducting pair correlations are always present. In particular, by using a fully quantum mechanical scattering approach, we show that Andreev reflection induces mixing of spatial parities at interfaces, thus being the unique process which generates odd-frequency pairing; on the other hand, both Andreev and normal reflections contribute to even-frequency pairing. We further find that locally neither odd-frequency nor spin-triplet correlations are induced, but only even-frequency spin-singlet pairing. In the superconducting regions of NS junctions, the interface-induced amplitudes decay into the bulk, with the odd-frequency components being generally much larger than the even-frequency components at low frequencies. The odd-frequency pairing also develops short and long-period oscillations due to the chemical potential and spin-orbit coupling, respectively, leading to a visible beating feature in their magnitudes. Moreover, we find that in short SNS junctions at $\pi$-phase difference and strong spin-orbit coupling, the odd-frequency spin-singlet and -triplet correlations strongly dominate with an alternating spatial pattern for a large range of sub-gap frequencies.

Flux pinning enhancement in thin films of [formula omitted

YBa2Cu3O7 (Y123) and Y3Ba5Cu8O18 (Y358) thin films were deposited by pulsed laser deposition method. XRD analysis shows both films grow in c axis orientation. Resistivity versus temperature analysis shows superconducting transition temperature was about 91.2 K and 91.5 K and transition width for Y358 and Y123 films was about 0.6 K and 1.6 K, respectively. Analysis of the temperature dependence of the AC susceptibility near the transition temperature, employing Bean's critical state model, indicates that intergranular critical current density for Y358 films is more than twice of intergranular critical current density of Y123 films. Thus, flux pining is stronger in Y358 films. Weak links in the both samples is of superconductor-normal-superconductor (SNS) type irrespective of stoichiometry.

Josephson effect through YBa2Cu3O7−δ /Au-encapsulated nanogaps

We successfully fabricated and studied planar superconductor-normal-superconductor (SNS) Josephson junctions made of YBa2Cu3O7-? (YBCO) high-TC superconductor (S) and Au normal metal (N). The crucial step in fabrication was done by using a Pt/Au bilayer encapsulation of the patterned YBCO nanogap. This procedure preserves the YBCO nanogap from the degradation due to the contact with resists, water, and chemicals involved in further processing for hybrid devices. The junctions exhibit Fraunhofer-like modulation patterns and Shapiro steps when they are irradiated by a rf field. Furthermore, we analyzed the IC(T) characteristics of our hybrid junctions employing a SINI?S (superconductor-insulator-normal conductor-insulator-superconductor) model.

Transport properties of spin-triplet superconducting monolayerMoS2

The quantum transport properties of graphene and monolayer $MoS_2$ superconductor heterostructures has been of considerable importance in the recent few years. Layered nature of molybdenum disulfide permits the superconducting correlation induction. Moreover, peculiar dynamical features of monolayer $MoS_2$, such as valence band spin-splitting in the nondegenerate $K$ and $K'$ valleys originated from strong spin-orbit coupling, and considerable direct band gap can make it potentially a useful material for electronics applications. Using the Dirac-like Hamiltonian of $MoS_2$ with taking into account the related mass asymmetry and topological contributions, we investigate the effect of spin-triplet $p$-wave pairing symmetry on the superconducting excitations, resulting in Andreev reflection process and Andreev bound state in the corresponding normal-superconductor (NS) and superconductor-normal-superconductor (SNS) structures, respectively. We study how the resulting subgap conductance and Josephson current are affected by the particular symmetry of order parameter. The signature of $p_x$-wave symmetry is found to decline the subgap superconducting energy excitations and, consequently, slightly suppress the Andreev reflection in the case of $p$-doped S region. The essential dynamical parameters $\lambda$ and $\beta$ of $MoS_2$ have significant effect on the both tunneling conductance and Josephson current. Particularly, the considered $p$-wave symmetry in the superconducting bound energies may feature the zero energy states at the interfaces. The critical current oscillations as a function of length of junction are obtained in the $p$-doped S region.

Effect of Oxygen Pressure on the Surface Roughness and Intergranular Behavior of YBCO Thin Films

YBa2Cu3 O 7−δ (YBCO) thin films were deposited by pulsed laser deposition on single-crystal LaAlO3 (100) substrates in O2 partial pressure (P O2) from 400 to 500 mTorr. The XRD data shows the presence of a-axis-oriented grains in the YBCO films deposited at P O2 = 500 mTorr. The a-axis grains lead to increase of strain. Atomic force microscopy images show as oxygen pressure has increased, average surface roughness of the films and size of droplets were increased. The grain misorientaion could be the reason for high average roughness. However, surface analysis by statistical methods reveals that three surfaces have multi-affine structure irrespective of oxygen pressure. Analysis of the temperature dependence of the AC susceptibility near the transition temperature indicates that with increasing oxygen pressure, intergranular critical current density has decreased. It is also suggested that the nature of weak links in the samples is of superconductor-normal-superconductor (SNS) type irrespective of the oxygen pressure.

Effect of ordered array of magnetic dots on the dynamics of Josephson vortices in stacked SNS Josephson junctions under DC and AC current

We use the anisotropic time-dependent Ginzburg-Landau theory to investigate the effect of a square array of out-of-plane magnetic dots on the dynamics of Josephson vortices (fluxons) in artificial stacks of superconducting-normal-superconducting (SNS) Josephson junctions in the presence of external DC and AC currents. Periodic pinning due to the magnetic dots distorts the triangular lattice of fluxons and results in the appearance of commensurability features in the current-voltage characteristics of the system. For the larger values of the magnetization, additional peaks appear in the voltage-time characteristics of the system due to the creation and annihilation of vortex-antivortex pairs. Peculiar changes in the response of the system to the applied current is found resulting in a “superradiant” vortex-flow state at large current values, where a rectangular lattice of moving vortices is formed. Synchronizing the motion of fluxons by adding a small ac component to the biasing dc current is realized. However, we found that synchronization becomes difficult for large magnetization of the dots due to the formation of vortex-antivortex pairs.

SNS junctions in nanowires with spin-orbit coupling: Role of confinement and helicity on the subgap spectrum

We study normal transport and the subgap spectrum of superconductor-normal-superconductor (SNS) junctions made of semiconducting nanowires with strong Rashba spin-orbit coupling. We focus, in particular, on the role of confinement effects in long ballistic junctions. In the normal regime, scattering at the two contacts gives rise to two distinct features in conductance: Fabry-Perot resonances and Fano dips. The latter arise in the presence of a strong Zeeman field $B$ that removes a spin sector in the leads (helical leads), but not in the central region. Conversely, a helical central region between nonhelical leads exhibits helical gaps of half-quantum conductance, with superimposed helical Fabry-Perot oscillations. These normal features translate into distinct subgap states when the leads become superconducting. In particular, Fabry-Perot resonances within the helical gap become parity-protected zero-energy states (parity crossings), well below the critical field ${B}_{c}$ at which the superconducting leads become topological. As a function of Zeeman field or Fermi energy, these zero modes oscillate around zero energy, forming characteristic loops, which evolve continuously into Majorana bound states as $B$ exceeds ${B}_{c}$. The relation with the physics of parity crossings of Yu-Shiba-Rusinov bound states is discussed.

Multiple Andreev reflection and critical current in topological superconducting nanowire junctions

We study transport in a voltage-biased superconductor–normal–superconductor (SNS) junction made of semiconducting nanowires with strong spin–orbit coupling, as it transitions into a topological superconducting phase for increasing Zeeman field. Despite the absence of a fractional steady-state ac Josephson current in the topological phase, the dissipative multiple Andreev reflection current Idc at different junction transparencies is particularly revealing. It exhibits unique features related to topology, such as gap inversion, the formation of Majorana bound states and fermion-parity conservation. Moreover, the critical current Ic, which remarkably does not vanish at the critical point where the system becomes gapless, provides direct evidence of the topological transition.

Interference effect on a Josephson junction with magnetic interfaces

We study the proximity effect in a superconductor-normal-superconductor (SNS) layer with spin-active interfaces, paying particular attention to the effect of interference on the transition between 0 and π states of a Josephson junction. We compute the supercurrent density in the ballistic limit by using the Green’s function formalism without making the quasiclassical approximation to study the interference effect due to coherent scattering of quasiparticles from the interfaces. The supercurrent is strongly influenced by various factors such as the thickness of the normal layer, the strength of the interface potential, and the orientation of the interface spin. We also discuss the symmetry of the induced superconducting pair amplitude for the 0 and the π states. The detailed features of the effect of interference on the Josephson junction can be important for applications of quantum devices.

The critical current density of an SNS Josephson-junction in high magnetic fields

Although the functional form of the critical current density (Jc) of superconducting–normal–superconducting (SNS) Josephson-junctions (J-Js) has long been known in the very low field limit (e.g. the sinc function), includes the local properties of the junction and has been confirmed experimentally in many systems, there have been no such general solutions available for high fields. Here, we derive general analytic equations for Jc in zero field and in high fields across SNS J-Js for arbitrary resistivity of the superconductor and the normal layer which are consistent with the literature results available in limiting cases. We confirm the validity of the approach using both computational solutions to time-dependent Ginzburg–Landau (TDGL) theory applied to SNS junctions and experimental Jc data for an SNS PbBi–Cd–PbBi junction. We suggest that since SNS junctions can be considered the basic building blocks for the description of the grain boundaries of polycrystalline materials because they both provide flux-flow channels, this work may provide a mathematical framework for high Jc technological polycrystalline superconductors in high magnetic fields.

Suppression of Andreev current due to transverse current flow in an InAs two-dimensional electrons

In a superconductor–normal–superconductor (SNS) structure, the effect of transverse current across the normal part on the transport through Andreev bound states (ABSs) has been examined. Here, a ballistic InAs two-dimensional electron system (2DES) is used as the N-layer to form ABSs. At the same time the 2DES has strong spin–orbit interaction, hence there should emerge the spin-Hall effect associated with the transverse current. We have observed strong reduction of characteristic oscillation in the conductance versus bias voltage, which may be attributed to spin polarization due to the spin-Hall effect.

Corbino-geometry Josephson weak links in thin superconducting films

I consider a Corbino-geometry SNS (superconducting-normal-superconducting) Josephson weak link in a thin superconducting film, in which current enters at the origin, flows outward, passes through an annular Josephson weak link, and leaves radially. In contrast to sandwich-type annular Josephson junctions, in which the gauge-invariant phase difference obeys the sine-Gordon equation, here the gauge-invariant phase difference obeys an integral equation. I present exact solutions for the gauge-invariant phase difference across the weak link when it contains an integral number N of Josephson vortices and the current is zero. I then study the dynamics when a current is applied, and I derive the effective resistance and the viscous drag coefficient; I compare these results with those in sandwich-type junctions. I also calculate the critical current when there is no Josephson vortex in the weak link but there is a Pearl vortex nearby.

Mesoscopic conductance fluctuations in InAs nanowire-based SNS junctions

We report a systematic experimental study of mesoscopic conductance fluctuations in superconductor/normal/superconductor (SNS) devices Nb/InAs-nanowire/Nb. These fluctuations far exceed their value in the normal state and strongly depend on temperature even in the low-temperature regime. This dependence is attributed to high sensitivity of perfectly conducting channels to dephasing and the SNS fluctuations thus provide a sensitive probe of dephasing in a regime where normal transport fails to detect it. Further, the conductance fluctuations are strongly nonlinear in bias voltage and reveal subgap structure. The experimental findings are qualitatively explained in terms of multiple Andreev reflections in chaotic quantum dots with imperfect contacts.

Synthesis and characterization of excess magnesium MgB 2 superconductor under inert carbon environment

The structural, transport and magnetic properties of MgB 2 superconductor heavily blended with Mg is studied. The samples are synthesized with a new approach in both, pressed carbon environment and in flowing argon. The excess magnesium used is observed to play dual role: one being the prevention of Mg losses during the synthesis process and hence maintaining the stoichiometry of MgB 2 phase, and second being the formation of Mg milieu probably all around the MgB 2 grains to give a dense structure. Excess Mg also improves the grain connectivity by going into the pores and there by minimizing the insulating junctions. The residual resistivity of the sample is observed to decrease from 57.02 μΩ cm to 10.042 μΩ cm as it is progressively filled with superconductor–normal–superconductor (SNS) type junctions amongst the grains by the virtue of increased magnesium content. The synthesized samples devoid of porosity show the superconducting transition, T c in the range of 39–34 K as of clean MgB 2 samples, though overloaded with Mg. The excess Mg resulted in enhanced critical current density, J c from 6.8 × 10 3 A cm −2 to 5.9 × 10 4 A cm −2 at 20 K and 10 kOe, with reasonable decrease in the superconducting transition. Thus our samples being overloaded with Mg impart mechanical strength and competitive superconducting properties, which forms a part of interest.