Crossed Andreev reflection in graphene-based ferromagnet-superconductor structures

We report a theoretical investigation of the spin-polarized transport of relativistic electrons through a three-terminal graphene-based superconductor bipolar transistor with ferromagnetic leads. It is found that the magnetoresistance in such a system can be improved largely in comparison with that in the corresponding two-terminal structure due to the existence of the special crossed Andreev reflection, which is quite different from that in the conventional three-terminal ferromagnet-superconductor devices. The physical origination for such a phenomenon has also been analyzed. We also find that the non-local conductivity can not only exhibit different feature for parallel and antiparallel alignment, it is also easily tuned by the external magnetic field and the bias voltage.     

Andreev reflections and magnetoresistance in ferromagnet-superconductor mesoscopic structures

An analysis is made of the change in the resistance of a nanostructure consisting of a diffusive ferromagnetic (F) wire and normal metal electrodes, due to the onset of superconductivity (S) in the normal electrode and Andreev scattering processes. The superconducting transition results in an additional contact resistance arising from the necessity to match the spin-polarized current in the F-wire to the spinless current in the S reservoir, which is comparable to the resistance of a piece of F wire with length equal to the spin relaxation length. It is also shown that in the absence of spin relaxation the resistance of a two-domain structure is the same for a ferro-or antiferromagnetic configuration if one electrode is in the superconducting state. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 7, 497–502 (10 April 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

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.     

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.     

Crossed Andreev reflection in a quantum dot coupled to a topological superconducting single-stranded DNA

We investigate the crossed Andreev reflection (CAR) through a quantum dot (QD) coupled to topological superconducting single-stranded DNA (ssDNA). It is found that the topological nontrivial states appear in the QD due to leakage of the Majorana zero mode. Majorana zero mode can be identified by measuring the CAR. This device can be used as a Majorana zero mode detector that relies on the system parameters, such as the spin orbit coupling, the twist angle, molecular length. A high efficiency Cooper pair splitter can be realized by regulating the magnitude and direction of the gate voltage. In additions, the signature of CAR is robust against the Coulomb blockade and the disorder induced by distinct amino acids. This work provides an alternative method for detection of Majorana zero mode in ssDNA.     

宽角度反射式相位延迟器的设计

金属增强型反射镜在入射光非正入射的时候,两个不同的偏振态之间会产生不同的相移。利用最优技术设计了一种相位延迟器,其工作波长在640-670nm之间,入射角在40-50°范围内时,反射率>99%且相移为90°±20°。波长在670nm附近时相移对入射角不敏感。膜层厚度误差对相移影响最大。     

Andreev level qubit

We investigate the dynamics of a two-level Andreev bound state system in a transmissive quantum point contact embedded in an rf SQUID. Coherent coupling of the Andreev levels to the circulating supercurrent allows manipulation and readout of the level states. The two-level Hamiltonian for the Andreev levels is derived, and the effect of interaction with the quantum fluctuations of the induced flux is studied. We also consider an inductive coupling of qubits and discuss the relevant SQUID parameters for qubit operation and readout.     

Signals for specular Andreev reflection

We report a theoretical investigation of the spin-dependent Andreev reflection at the interface of a graphene-based ferromagnet/superconductor junction. It is found that the ferromagnetic exchange interaction in the ferromagnet can suppress Andreev retroreflection but enhance the specular Andreev reflection. There is a transition between the specular Andreev reflection and Andreev retroreflection at which the shot noise vanishes and the Fano factor has a universal value. The present work provides a new method of detecting the specular Andreev reflection, which can be experimentally tested within the present-day technique.     

Thermal conductance of Andreev interferometers

We calculate the thermal conductance G(T) of diffusive Andreev interferometers, which are hybrid loops with one superconducting arm and one normal-metal arm. The presence of the superconductor suppresses G(T); however, unlike a conventional superconductor, G(T)/G(T)(N) does not vanish as the temperature T-->0, but saturates at a finite value that depends on the resistance of the normal-superconducting interfaces, and their distance from the path of the temperature gradient. The reduction of G(T) is determined primarily by the suppression of the density of states in the proximity-coupled normal metal along the path of the temperature gradient. G(T) is also a strongly nonlinear function of the thermal current, as found in recent experiments.     

Andreev reflection in three dimensions

正When an electron with negative charge-e is incident from a normal metal to an interface with a superconductor,it may be reflected as a hole with positive charge e at the interface.In such a way,a pair of electrons can be transferred to the superconductor and form a Cooper pair.This process of electron-hole conversion is called Andreev reflection(AR)[1].In two dimensions where the normal metal-superconductor junction is assumed to be along the x direction,     

Chladni figures in Andreev billiards

We study wave functions and their nodal patterns in Andreev billiards consisting of a normal-conducting (N) ballistic quantum dot in contact with a superconductor (S). The bound states in such systems feature an electron and a hole component which are coherently coupled by the scattering of electrons into holes at the S-N interface. The wave function “lives” therefore on two sheets of configuration space, each of which features, in general, distinct nodal patterns. By comparing the wave functions and their nodal patterns for holes and electrons detailed tests of semiclassical predictions become possible. One semiclassical theory based on ideal Andreev retroreflection predicts the electron- and hole eigenstates to perfectly mirror each other. We probe the limitations of validity of this model both in terms of the spectral density of the eigenstates and the shape of the wavefunctions in the electron and hole sheet. We identify cases where the Chladni figures for the electrons and holes drastically differ from each other and explain these discrepancies by limitations of the retroreflection picture.     

Resonant tunneling through Andreev levels

We use a semiclassical approach for analyzing the tunneling transport through a normal conductor in contact with superconducting mirrors. Our analysis of the electron–hole propagation along semiclassical trajectories shows that resonant transmission through Andreev levels is possible resulting in an excess, low-energy quasiparticle contribution to the conductance. The excess conductance oscillates with the phase difference between the superconductors having maxima at odd multiples of π for temperatures much below the Thouless temperature.     

Interaction-induced renormalization of Andreev reflection

We analyze the charge transport between a one-dimensional weakly interacting electron gas and a superconductor within the scaling approach in the basis of scattering states. We derive the renormalization group equations, which fully account for the intrinsic energy dependence due to Andreev reflection. A strong renormalization of the corresponding reflection phase is predicted even for a perfectly transparent metal-superconductor interface. The interaction-induced suppression of the Andreev conductance is shown to be highly sensitive to the normal-state resistance, providing a possible explanation of experiments with carbon-nanotube/superconductor junctions by Morpurgo et al. [Science 286, 263 (1999)].     

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.