Andreev tunneling and Josephson current in light irradiated graphene

We investigate the Andreev tunneling and Josephson current in graphene irradiated with high-frequency linearly polarized light. The corresponding stroboscopic dynamics can be solved using Floquet mechanism which results in an effective stationary theory to the problem exhibiting an anisotropic Dirac spectrum and modified pseudospin-momentum locking. When applied to an irradiated normal graphene - superconductor (NS) interface, such analysis reveal Andreev reflection (AR) to become an oscillatory function of the optical strength. Specifically we find that, by varying the polarization direction we can both suppress AR considerably or cause the Andreev transport to remain maximum at sub-gap excitation energies even in the presence of Fermi level mismatch. Furthermore, we study the optical effect on the Andreev bound states (ABS) within a short normal-graphene sheet, sandwiched between two s-wave superconductors. It shows redistribution of the low energy regime in the ABS spectrum, which in turn, has major effect in shaping the Josephson super-current. Subjected to efficient tuning, such current can be sufficiently altered even at the charge neutrality point. Our observations provide useful feedback in regulating the quantum transport in Dirac-like systems, achieved via controlled off-resonant optical irradiation on them.     

铁磁-绝缘层-d波超导结中的Andreev反射特性

考虑到铁磁层中的磁交换作用和界面散射效应,利用推广的Blonder-Tinkham-Klapwijk散射理论,计算铁磁-绝缘层-d波超导结中的准粒子输运系数与隧道谱.研究表明：铁磁层中的交换场能抑制Andreev反射,使得依赖于准粒子入射角的Andreev反射表现出瞬息波的行为,呈现一些新的隧道谱特征. 关键词： Andreev反射 d波超导 隧道谱     

Crossed Andreev reflection in graphene normal-superconductor-normal structures with pseudo-diffusive interfaces

In this Letter graphene normal-superconductor-normal heterostructures are modeled for studying the crossed Andreev reflection. A thin layer of undoped graphene with Fermi energy at the Dirac point at is assumed the interface between superconductor layer and each normal lead. The resulting contribution of the crossed Andreev reflection to the nonlocal conductance equals that of the electron elastic cotunneling. We explain this as another figure of merit for pseudodiffusive conduction at the Dirac point of the undoped layers. Also structures with only one undoped layer at the interface between the superconductor and one of the normal leads, as well as structures in which one of the leads is ferromagnetic, show pseudodiffusive conduction at the Dirac points.     

拓扑绝缘体基铁磁/f-波超导隧道结Andreev反射研究

基于Bogoliubov-de Gennes方程和Blonder-Tinkham-Klapwijk理论研究了三维拓扑绝缘体基铁磁/各向异性f-波超导隧道结的Andreev反射,其中f-波超导体选取f₁和f₂-波两种配对势.研究发现,对于f₁和f₂波,铁磁体中的磁能隙可以增强传统的Andreev逆向反射,但对Andreev镜面反射有抑制作用;但随着施加在超导体顶部电极上的栅极电位的增加,两种类型的反射都会增强.通过改变磁能隙,可以调节两种反射在准粒子输运过程中占有优势的程度.这些结果提供了一种实验检测拓扑绝缘体薄膜中镜面Andreev反射的方法.此外,隧穿电导和散粒噪声谱的差异可用于区分f₁和f₂波配对势.     

Spin-dependent quasiparticle reflection and bound States at interfaces with itinerant antiferromagnets

We find a novel channel of quasiparticle reflection from the simplest two-sublattice antiferromagnet (AF) on a bipartite lattice. Low-energy quasiparticles in a normal metal (N) experience spin-dependent retroreflection at AF/N interfaces. As a combined effect of antiferromagnetic and Andreev reflections, subgap Andreev states arise at an AF/superconductor (SC) interface. When the antiferromagnetic reflection dominates the specular one, Andreev bound states have almost zero energy on AF/s-wave superconductor (sSC) interfaces, whereas there are no low-energy subgap states on AF/d-wave superconductor (dSC) boundaries. For an sSC/AF/sSC junction, the bound states are found to split, due to the finite width of the AF interlayer, and carry the supercurrent. The theory developed in the present Letter is based on a novel quasiclassical approach, which applies to interfaces involving itinerant antiferromagnets.     

Anisotropic pairing symmetry effect on crossed Andreev reflection in a graphene-based transistor

Crossed Andreev reflection (CAR) under the influence of anisotropic pairing symmetry is considered. It is shown that CAR is sensitive to the Fermi energy and the orientation of the gap. In addition, the oscillatory period of CAR can be not only tunable by the potential energy in the superconductor region, it also can be modulated by the length of the superconductor region. The physical origination for those phenomena has also been analyzed.     

Spin transport properties in ferromagnet/superconductor junctions on topological insulator

The spin-dependent Andreev reflection is investigated theoretically by analyzing the electronic transport in a thin-film topological insulator (TI) ferromagnet/superconductor (FM/SC) junction. The tunneling conductance and shot noise are calculated based on the Dirac-Bogoliubov-de Gennes equation and Blonder-Tinkham-Klapwijk theory. It is found that the magnetic gap in ferromagnet can enhance the Andreev retro-reflection but suppress the specular Andreev reflection. The gate potential applied to the electrode on top of superconductor can enhance the two types of reflections. There is a transition between the two types of reflections at which both the tunneling conductance and differential shot noise become zero. These results provide a method to realize and detect experimentally the intra-band specular Andreev reflection in thin film TI-based FM/SC structures.     

Elementary Andreev processes in a driven superconductor–normal metal contact

We investigate the full counting statistics of a voltage-driven normal metal(N)–superconductor(S) contact. In the low-bias regime below the superconducting gap, the NS contact can be mapped onto a purely normal contact, albeit with doubled voltage and counting fields. Hence in this regime the transport characteristics can be obtained by the corresponding substitution of the normal metal results. The elementary processes are single Andreev transfers and electron- and hole-like Andreev transfers. Considering Lorentzian voltage pulses we find an optimal quantization for half-integer Levitons.     

Specular Andreev reflection in graphene

By combining the Dirac equation of relativistic quantum mechanics with the Bogoliubov-de Gennes equation of superconductivity we investigate the electron-hole conversion at a normal-metal-superconductor interface in graphene. We find that the Andreev reflection of Dirac fermions has several unusual features: (1) the electron and hole occupy different valleys of the band structure; (2) at normal incidence the electron-hole conversion happens with unit efficiency in spite of the large mismatch in Fermi wavelengths at the two sides of the interface; and, most fundamentally: (3) away from normal incidence the reflection angle may be the same as the angle of incidence (retroreflection) or it may be inverted (specular reflection). Specular Andreev reflection dominates in weakly doped graphene, when the Fermi wavelength in the normal region is large compared to the superconducting coherence length.     

Resonant Andreev reflection in a normal-metal/quantum-dot/superconductor system with coupled Majorana bound states

Andreev reflection(AR) in a normal-metal/quantum-dot/superconductor(N–QD–S) system with coupled Majorana bound states(MBSs) is investigated theoretically. We find that in the N–QD–S system, the AR can be enhanced when coupling to the MBSs is incorporated. Fano line-shapes can be observed in the AR conductance spectrum when there is an appropriate QD–MBS coupling or MBS–MBS coupling. The AR conductance is always e~2/2h at the zero Fermi energy point when only QD–MBSs coupling is considered. In addition, the resonant AR occurs when the MBS–MBS coupling roughly equals to the QD energy level. We also find that an AR antiresonance appears when the QD energy level approximately equals to the sum of the QD–MBS coupling and the MBS–MBS coupling. These features may serve as characteristic signatures for the probe of MBSs.     

Surface bound states and spin currents in noncentrosymmetric superconductors

We investigate the ground state properties of a noncentrosymmetric superconductor near a surface. We determine the spectrum of Andreev bound states due to surface-induced mixing of bands with opposite spin helicities for a Rashba-type spin-orbit coupling. We find that the order parameter suppression qualitatively changes the bound state spectrum. The spin structure of Andreev states leads to a spin supercurrent along the interface, which is strongly enhanced compared to the normal state spin current. Particle and hole coherence amplitudes show Faraday-like rotations of the spin along quasiparticle trajectories.     

Doubled full shot noise in quantum coherent superconductor-semiconductor junctions

We performed low temperature shot noise measurements in superconductor (TiN) strongly disordered normal metal (heavily doped Si) weakly transparent junctions. We show that the conductance has a maximum due to coherent multiple Andreev reflections at low energy and that the shot noise is then twice the Poisson noise (S = 4eI). When the subgap conductance reaches its minimum at finite voltage the shot noise changes to the normal value (S = 2eI) due to a large quasiparticle contribution.     

A pure spin-current injector of semiconductor quantum dots with Andreev reflection and Rashba spin-orbit coupling

We propose a four-terminal device consisting of two parallel quantum dots with Rashba spin-orbit interaction (RSOI), coupled to two side superconductor leads and two common ferromagnetic leads, respectively. The two ferromagnetic leads and two quantum dots form a ring threaded by Aharonov-Bohm (AB) flux. This device possesses normal quasiparticle transmission between the two ferromagnetic leads, and normal and crossed Andreev reflections providing conductive holes. For the appropriate spin polarization of the ferromagnetic leads, RSOI and AB flux, the pure spin-up (or spin-down) current without net charge current in the right lead, which is due to the equal numbers of electrons and holes with the same spin-polarization moving along the same direction, can be obtained by adjusting the gate voltage, which may be used in practice as a pure spin-current injector.     

Crossed Andreev reflection in a graphene bipolar transistor

We investigate the crossed Andreev reflections between two graphene leads connected by a narrow superconductor. When the leads are, respectively, of the n and p type, we find that electron elastic cotunneling and local Andreev reflection are both eliminated even in the absence of any valley-isospin or spin polarizations. We further predict oscillations of both diagonal and cross conductances as a function of the distance between the graphene-superconductor interfaces.     

Excess currents larger than the point contact limit in normal-metal/superconducting junctions

In a point contact NS junction, perfect Andreev reflection occurs over a range of voltages equal to the superconducting energy gap, producing an excess current of I_exc =  (4 / 3)(2 eΔ / h). If the superconductor has a finite width, rather than the infinite width of the point contact, one cannot neglect superfluid flow inside the superconducting contact. The energy range available for perfect Andreev reflections then becomes larger than the superconducting gap, since superfluid flow alters the dispersion relation inside the finite width superconductor. We find a maximum excess current of approximately (7 / 3)(2 eΔ / h) when the width of the superconductor is approximately 7 / 3 times the width of the normal metal.     

Double Andreev reflections at surface states of the topological insulators with hexagonal warping

We study the Andreev reflection(AR)at the interface of the topological insulator with hexagonal warping and superconductor junction.Due to the hexagonal warping effect,the double ARs are found in a certain range of the incident angle,where for one incident electron beam,two beams of holes are reflected back.Interestingly,both the beams of holes are reflected as retro-AR on the same side of the normal line of the interface but with different reflection angles,different from the previously reported double AR with one retro-AR and one specular-AR.The double reflections owing to the warping effect show the optics-like property of the Dirac fermion and can stimulate the double reflections of light in anisotropic crystals.In addition,we find that the double ARs are dependent on the hexagonal warping parameter nonmonotonically,and in an intermediate strength the double AR phenomenon is prominent,providing a possibility to explore the warping parameter of topological insulators.     

Re_3W的点接触安德烈夫反射谱研究

本文通过对不同晶体结构Re_3W样品的点接触测量和对比研究,证实具有中心对称结构和非中心对称结构的Re_3W都是弱耦合Bardeen-Cooper-Schrieffer超导体,同时发现在两个相表面都可以形成很理想的点接触结,即电子通过界面时受到的非弹性散射很弱.将Re_3W样品置于大气环境近六个月后重新进行测量,仍然能够得到类似的结果,表明Re_3W具有很好的稳定性. Re_3W的这种优良特性,不仅可通过点接触实验得到的参数推算出Re_3W两个相的费米速度,而且提供了一种简单的方法,可以在点接触实验中利用Re_3W来印证针尖材料的费米速度和测量其自旋极化率等.作为尝试,本文用Re_3W/Ni点接触结测量了铁磁性金属Ni的自旋极化率,得到了与前人报道一致的结果.     

Andreev reflection and bound states in topological insulator based planar and step Josephson junctions

A superconductor-topological insulator-superconductor (S/TI/S) junction having normal region at angle θ is studied theoretically to investigate the junction angle dependency of the Andreev reflection and the formation of the Andreev bound states in the step and planar S/TI/S structures. It is found that the Andreev reflection becomes θ dependent only in the presence of the potential barrier at the TI/S interface. In particular, the step and planar TI/S junction have totally different conductive behavior with bias voltage and potential barrier in the regime of retro and specular Andreev reflection. Interestingly, we find that the elliptical cross section of Dirac cone, an important feature of topological insulator with step surface defect, affects the Fabry-Perot resonance of the Andreev reflection induced Andreev bound states (which become Majorana zero energy states at low chemical potential) in the step S/TI/S structure. Unlike the usual planar S/TI/S structures, we find these ellipticity affected Andreev bound states lead to non-monotonic Josephson super-current in the step S/TI/S structure whose non-monotonicity can be controlled with the use of the potential barrier, which may find applications in nanoelectronics.     

正常金属/超导/正常金属双垒隧道结中的量子相干输运

考虑到量子相干效应和界面散射效应 ,利用 L ambert理论模型 ,计算正常金属 /绝缘层 /超导 /绝缘层 /正常金属双垒隧道结中的准粒子输运系数和隧道谱。研究表明 :( 1)所有的准粒子输运系数和电导谱在超导能隙之上都随能量作周期性振荡 ,其振荡周期依赖于超导层的厚度 ;( 2 )在超导能隙之上 Andreev反射系数随能量呈现周期性消失现象 ;( 3)在绝缘层势垒强度取很大的隧道极限下 ,超导层中会形成一系列的准粒子束缚态 ,其位置由量子化条件决定 ;( 4)界面散射效应不仅能压低各子能隙电导峰 ,还能使子能隙电导峰劈裂为两个峰。     

Enhancement of subgap conductance in a graphene superconductor junction by valley polarization

We theoretically study the differential conductance of a graphene/graphene superconductor junction, where the valley polarization of Dirac electrons is considered in the nonsuperconducting region. It is shown that the subgap conductance will increase monotonically with the valley-polarization strength when the chemical potential μ is near the Dirac point μ≤ 3?(? is the superconducting gap), whereas it will decrease monotonically when μ is far away from the Dirac point, μ≥ 5?.The former case is induced by the specular Andreev reflection while the retro-reflection accounts for the later result. Our findings may shed light on the control of conductance of a graphene superconductor junction by valley polarization.