Hysteretic Critical State in Coplanar Josephson Junction with Monolayer Graphene Barrier

Coplanar Al/graphene/Al junctions fabricated on the same graphene sheet deposited on silicon carbide (SiC), show robust Josephson coupling at subKelvin temperature, when the separations between the electrodes is below 400 nm. Remarkably, a hysteretic Critical State sets in when ramping an orthogonal magnetic field, with a sudden collapse of the Josephson critical current I _c when turning the field on, and a revival of I _c when inverting the sweep. Similar hysteresis can be found in granular superconducting films which may undergo the Berezinskii-Kosterlitz-Thouless transition. Here, we give quantitative arguments to prove that this odd behavior of the magnetoconductance gives evidence for an incipient Berezinskii-Kosterlitz-Thouless transition with drift and pinning of fluctuating free vortices induced by the current bias.     

Spin-Orbit Coupling Effects in the dc Josephson Current in the s Wave Superconductor/Insulator/d Wave Superconductor Junction

The Bogoliubov-de Gennes equation is applied to the study of the spin-orbit coupling (SOC) effects in s-wave superconductor/insulator/d-wave superconductor junctions. It is found that the dc Josephson currents show a nonmonotonic dependence on SOC scattering strength. The phase-current relationship for the spin-up and spin-down channels deviates substantially from sin2φ because of the spin-orbit scattering, so the periods of oscillation curves are 2π for the total Josephson current at α=π/4. An alternative method to confirm the effects of the spin-orbit scattering is the measurement of the phase-current relationship.     

Anisotropic Supercurrent in Strained Graphene Josephson Junction

The Weyl?CDirac fermions in graphene-based SG/I/SG (SG denoting superconducting graphene; I, an insulator) junctions subjected to strains become highly asymmetric. The effect of the strain-induced symmetry change of the fermions on the supercurrent is studied. The highly anisotropic velocity near the critical deformation causes the critical current parallel to the direction of strain to oscillate as a function of gate voltage at a very high frequency. The same anisotropic velocity destroys the oscillation of the gate-dependent critical current when the current is perpendicular to the direction of strain. The direction-dependent supercurrent is due to the strain turning the two-dimensional massless relativistic electrons into nearly one-dimensional massless relativistic fermions near the critical deformation. This work reveals that a new effect can be induced by applying strain on a graphene-based Josephson junction.     

Effect of Virtual Andreev Reflection on Tunneling in Normal/Superconductor Graphene Junction

The Andreev reflection (AR) process and the tunneling conductance in graphene-based normal metal/superconductor junction are studied, where both massless and massive fermions are considered. As a result, the evanescent type of AR or the virtual Andreev reflection (VAR), is found to take place at the interface above the critical angle or in case where AR decays due to massive energy gap. Significantly, current of the junction is found to exist in the evanescent AR regime. In this work, we modify tunneling conductance formalism by including the current induced by the evanescent type of AR process (or VAR), which has not been taken into account in previous studies. In contrast to the previous work, our formula leads to a new result due to the influence of the VAR when the energy of the quasiparticles greater than the superconducting gap.     

Temperature Dependence of the Josephson Current in the d-Wave Superconductor Junction Based on the 2-D Extended Hubbard Model

We present a theoretical study of the Josephson current in the d_x ^{2-y 2}-wave superconductor junction. We calculate the current-phase relation and the temperature dependence of the Josephson current for the (100) oriented and the (110) oriented junction using the two-dimensional (2-D) extended Hubbard model. We obtain the anomalous temperature dependence of the current in the (110) oriented junction which has been reported within the quasiclassical theory.     

Tunneling of Graphene Massive Dirac Fermions Through a Double Barrier

We study the tunneling of Dirac fermions in graphene through a double barrier potential. This is allowing the carriers to have an effective mass inside the barrier as generated by a lattice miss-match with the boron nitride substrate. The consequences of this gap opening on the transmission are investigated and the realization of resonant tunneling conditions is analyzed.     

Dependence of Josephson Current on Orientational Angle in Diffusive p-Wave Superconductor Junctions

We calculate Josephson current in p-wave superconductor (P)/diffusive normal metal (DN)/insulator/diffusive normal metal/p-wave superconductor junctions, using quasi-classical Green’s function theory. We study the dependence of the critical current on the orientational angle at various temperatures. We find that the critical current in these junctions are proportional to cos?αcos?β. Here, α and β are angles between the direction perpendicular to the interfaces, located between the DN and P, and the crystal axis of left and right P respectively.     

Control of Spin-Valley Current in Strain-Engineered Graphene Magnetic Junction

We investigate the spin-valley current in a strain-engineered graphene magnetic system, normal region (N)/strain region(S)/ferromagnetic region (F)/normal region (N) junction. Locally strained region S leads to valley-dependent current, and ferromagnetic region F leads to the spin-dependent current. We find that pure valley current can be created easily by applying the real-vector potential that is equal to the pseudo-vector potential caused by strains in the S-region. In this work, we focus on the spin current in each valley when exchange field is applied in the F-region, and find that the linear control of spin-valley polarization by gate potential is possible. It is also found that when the current is carried only by the carriers in one valley (pure valley current), pure spin-up current can be linearly altered to pure spin-down current by tuning the gate potential, in the case of very large-F-thickness junction. Our work is applicable for devising controllable spin-valley-current electronics circuits.     

Detecting Current Noise with a Josephson Junction in the Macroscopic Quantum Tunneling Regime

We discuss the use of a hysteretic Josephson junction to detect current fluctuations with frequencies below the plasma frequency of the junction. These adiabatic fluctuations are probed by switching measurements observing the noise-affected average rate of macroscopic quantum tunneling of the detector junction out of its zero-voltage state. In a proposed experimental scheme, frequencies of the noise are limited by an on-chip filtering circuit. The third cumulant of current fluctuations at the detector is related to an asymmetry of the switching rates.      

Transport Properties of Topological Insulator-Based Ferromagnet/f-Wave Superconductor Junction

We investigate effect of magnetically-induced relativistic mass and also anisotropic f-wave pair coupling on the tunneling conductance on the surface of a 3D topological insulator ferromagnet/superconductor junction, which two types of pairing for superconductivity are possible. A topological insulator as a new state of condensed-matter caused by spin–orbit interaction and time-reversal symmetry has a bulk band gap and gapless surface states. We use the BTK formalism to find the charge carriers behavior. Due to two different nature of order parameters of the f-wave superconductivity, the tunneling conductance is found to be linearly dependent on the magnetic gap in terms of f ₁ and the exponential for f ₂. It is shown that the conversion of the conductance peak from ZBCP to ZBCD occurs in the f ₁ case with increasing m, while this is not observed in f ₂. Also, we find that the conductance behaves as a unit step function for the superconductor electrostatic potential in f ₁, and this should be usable in nanoelectronic switch devices. In addition, we illustrate how the magnetic gap affects the transmission coefficient in quite different behaviors for order parameters.     

Controlled Single-Cooper-Pair Charging Effects in a Small Josephson Junction Array

We report on measurements of single-Cooper-pair charging effects in small Josephson junction arrays, and the experimental techniques that were used. We succeeded in having complete control over the array's electrostatic parameters; offset charges were accurately compensated, and the poisoning of 2e-periodic effects by quasiparticles was circumvented. This allowed for a controlled study of the array's coherent ground state. A few measurements gave results which were not fully 2e-periodic due to interesting parity effects. The arrays are in the regime where the energy scales for the Josephson effect and single-charge effects are comparable.     

石墨烯/硅肖特基结太阳能电池的研究进展

太阳能电池利用光伏效应直接将光能转变成电能,能有效地解决未来能源危机和环境污染,符合可持续发展的理念.传统的硅基太阳能电池存在需要高温过程,工艺复杂,发电成本无法与火电和水电相抗衡等问题.针对上述问题,近年来研究人员开发了诸多新型太阳能电池以降低制造成本,其中采用石墨烯作为透明电极的石墨烯/硅肖特基结太阳能电池被认为是新一代低成本、高效率的太阳能电池.然而,石墨烯功函数较低、方阻较高,载流子沿界面复合严重,并且平面硅反射率较高,导致石墨烯/硅肖特基结太阳能电池的效率远低于传统硅基太阳能电池.因此,近年来,主要研究重点在石墨烯掺杂改性、抑制界面处的载流子复合和降低器件的反射率等方面.目前,石墨烯/硅肖特基结太阳能电池的光电转换效率(PCE)已由1.65％提升到16.61％.目前,成功应用于提升器件性能的石墨烯掺杂剂主要有HNO3、金属纳米粒子和双(三氟甲磺酰基)酰胺(TFSA)等.其中,HNO3应用最为广泛,但其稳定性较差,采用金属纳米粒子等物理掺杂可以同时提升器件的PCE和稳定性.在石墨烯和硅之间引入Al2 O3、MoS2、量子点等界面层和表面钝化,可以有效地减少硅表面的悬空键,抑制载流子复合,从而提高器件的性能.此外,研究人员通过在石墨烯表面引入TiO2、PMMA、MgF2/ZnS等减反射膜,或在硅表面引入纳米线、多孔硅等微结构,来降低器件的反射率,提高其对光的利用率.本文总结了近年来石墨烯/硅太阳能电池的研究进展,简要介绍了器件的结构和原理,重点介绍了石墨烯掺杂、石墨烯层数选择、硅的纳米或微米结构、减反射膜和界面优化等手段,分析了目前石墨烯/硅肖特基结太阳能电池商业化所面临的问题并对其提出展望,以期为制备效率高和稳定性强的新型石墨烯/硅肖特基结太阳能电池提供一定参考.     

Numerical Simulations of the Fluxon Dynamics of a Two-Dimensional Josephson Junction System

A direct perturbation method applied to a long Josephson junction molded by one-dimensional sine-Gordon equation is presented and the modulation of perturbations on fluxon velocity and stable state is studied. Simulations of a two-dimensional Josephson junction with dc bias current in an external magnetic field provide an I–V characteristic curve for the system and determine the various dynamic behaviors, including the periodic, quasiperiodic, and chaotic motions. Physical explanations for the behaviors are presented. The numerical results are in good agreement with the energetic analysis.     

Effects of Radiative Corrections on a Quantum Disordered Self-dual Josephson Junction Array

The quantum effective potential of a three-dimensional Abelian Maxwell-mixed-Chern-Simons gauge theory coupled to complex fields and massless fermions is investigated at zero temperature. The insulating phase of the related self-dual Josephson junction array is found to be stable against gauge field fluctuations since these do not induce symmetry breaking terms in the one-loop effective potential. Conversely, coupling to gapless fermions is shown to generate correction terms in the effective potential which change the symmetry of the ground state and favor transitions between the insulating and superconducting states.     

Switching Dynamics of an Underdamped Josephson Junction Coupled to a Microwave Cavity

Current-biased Josephson junctions are promising candidates for the detection of single photons in the microwave frequency domain. With modern fabrication technologies, the switching properties of the junction can be adjusted to achieve quantum limited sensitivity. Namely, the width of the switching current distribution can be reduced well below the current amplitude produced by a single photon trapped inside a superconducting cavity. However, for an effective detection a strong junction cavity coupling is required, providing nonlinear system dynamics. We compare experimental findings for our prototype device with a theoretical analysis aimed to describe the switching dynamics of junctions under microwave irradiation. Measurements are found in qualitative agreement with our simulations.     

Josephson Current in Nb/InAs/Nb Highly Transmissive Ballistic Junctions

Highly transmissive ballistic junctions are demonstrated between two superconducting Nb electrodes and the two-dimensional electron gas formed at an InAs/AISb heterojunction. A reproducible fabrication protocol is presented yielding good critical supercurrent values. Current–voltage characteristics were measured down to 0.4 K and the observed supercurrent behavior was analyzed within a ballistic model in the clean limit. This investigation allows us to demonstrate an intrinsic interface transmissivity exceeding 86%.     

Cross-Whisker Intrinsic Josephson Junction as a Probe of Symmetry of the Superconducting Order Parameter

As a test of the superconducting order parameter, we have developed an intrinsic Josephson junction by the name of cross-whisker junction. This junction was made using two Bi₂Sr₂CaCu₂O_8+d single crystal whiskers. Two whiskers were connected at their c planes with various cross-angles. Angular dependence of the critical current densities shows d-wave-like fourfold-symmetry. However, the angular dependence is much stronger than that of the conventional d_x ^{2-y 2} wave. The Jc shows its smallest value around 45 deg, which suggests that the Josephson penetration depth becomes longer. We have successfully observed a Fraunhofer pattern in the cross-whisker junction with cross-angle 45 deg.