Suppression Of Andreev Reflection Research Articles

Singlet superconductivity in a single-crystal NiBi3 superconductor

Andreev reflection spectroscopy with unpolarized and highly spin-polarized currents has been utilized to study an intermetallic single-crystal superconductor NiBi3. Magnetoresistance at zero bias voltage of point contacts shows the occurrence and suppression of Andreev reflection by unpolarized and polarized current, respectively. The gap value, its symmetry and temperature dependence have been determined using an unpolarized current. The spin state in the NiBi3 sample is determined to be antiparallel using a highly spin-polarized current. The gap value 2Δ/kBT, gap symmetry and its temperature dependence, combined with the antiparallel spin state show that the bulk NiBi3 is a singlet s-wave superconductor.

Suppressed Andreev reflection and helical Andreev bound states in triplet superconductor three-dimensional topological insulator

Effect of proximity-induced unconventional p-wave superconductivity in a three-dimensional topological insulator-based S/F/S structure on the Andreev bound states (ABSs) and Josephson supercurrent is studied. We investigate, in detail, the suppression of Andreev reflection and helical ABSs in the presence of three types of triplet superconducting gap. The magnetization of ferromagnetic section is perpendicular to the surface of junction. The influence of such features on the supercurrent flow on the surface of the topological insulator is studied. We carry out our goal by introducing a relevant form of Dirac spinors for gapless renormalized by chemical potential μ excitation states. Therefore, it enables us to consider the virtual Andreev process, simultaneously, and we propose to investigate it in a tunneling conductance junction. It is shown that the results obtained in this case are completely different from those in conventional superconductivity, as s- or d-waves, for example, the magnetization is found to decrease the gap for px and px+ipy case, whereas increase it for py order. Strongly suppressed Andreev reflection is demonstrated.

Helical Andreev bound states in topological insulator f-wave Josephson junction

We study the effect of f-wave triplet superconductivity induced by proximity effect on the surface states of a three-dimensional topological insulator (3DTI). In fact, the gapless surface state for excitation spectrum of a triplet superconductor topological insulator gives rise to formation of helical Andreev bound states (ABSs) and, necessarily, suppression of Andreev reflection at the interface of a superconductor/ferromagnetic structure. By calculating the relevant form of Dirac spinors for two-dimensional time-reversal symmetric Hamiltonian, the normal and Andreev reflection coefficients and ABSs corresponding to tunneling and Josephson junctions, respectively, are obtained. It is shown, that the Andreev process vanishing occurs strongly, so that, the evanescent waves will be permissible in tunneling process. It is found that the dispersion of ABSs is an even function of electron incident angle θ for f1-wave and odd for f2-wave. In f1-order, for module of incident angle, θ>0.167π, the ABSs are independent of superconducting phase difference, ϕ, so that the Josephson current may vanish for this region. Effect of magnetization on flattening of ABSs occurs in f2 order faster than f1.

Quasiparticle Transport Properties of d-Wave Superconductor/Ferromagnet/d-Wave Superconductor Junctions

Solving the time-dependent Bogoliubov–de Gennes equations, we calculate current–voltage characteristics (CVC) and conductances (G) of clean d-wave superconductor/ferromagnet/d-wave superconductor (DFD) junctions with transparent D/F interfaces. Due to the suppression of Andreev reflection (AR) by the exchange energy \(h\) in the ferromagnetic barrier and by the presence of gap nodes in the d-wave superconductor electrodes, there is a striking difference in shape and amplitude of CVC with respect to the case of isotropic s-wave superconducting banks. This provides an indication of the superconductor order parameter symmetry. The influence of AR is much better seen in the conductance dependence on the bias voltage \(G(V)\). The conductance curves have a rich nonlinear structure in low-voltage multiple AR region. At higher voltages, the curves are almost flat, with characteristic dips located at \(eV=[2\Delta(T)\mp h]/n\), with n=1, 2, ..., Δ(T) being the maximum order parameter at the temperature \(T\). Obs...

Magnetic coupling in superconducting spin valves with strong ferromagnets

We investigate the magnetotransport behavior of ferromagnet (F)/superconductor/ferromagnet trilayers made of ferromagnetic ${\text{Ni}}_{80}{\text{Fe}}_{20}$ (Permalloy, Py) and superconducting Nb for temperatures both above and below the superconducting transition temperature ${T}_{c}$. In such devices, and for weak ferromagnets, ${T}_{c}$ depends on the relative magnetization directions of the two F layers in such a way that ${T}_{c}^{\text{P}}$ of the parallel (P) alignment is lower than ${T}_{c}^{\text{AP}}$ of the antiparallel (AP) alignment (the so-called superconducting spin-valve effect). For strong magnets, the suppression of Andreev reflection may alter this picture, but also stray field effects become important, as is known from earlier work. We compare large-area samples with microstructured ones, and find blocklike switching in the latter. We show this not to be due to a switch between the P and AP states, but rather to dipolar coupling between domains which are forming in the two Py layers, making a stray-field scenario likely. We also present measurements of the depairing (critical) current ${I}_{dp}$ and show that a similar depression of superconductivity exists far below ${T}_{c}$ as is found around ${T}_{c}$.

Superconductor-semiconductor magnetic microswitch

A hybrid superconductor-two-dimensional electron gas microdevice is presented. Its working principle is based on the suppression of Andreev reflection at the superconductor-semiconductor interface caused by a magnetic barrier generated by a ferromagnetic strip placed on top of the structure. Device switching is predicted with fields up to some mT and working frequencies of several GHz, making it promising for applications ranging from microswitches and storage cells to magnetic field discriminators.

Transmission and Andreev reflection in one-dimensional chain with randomly doped superconducting grains

We investigate the transport properties and Andreev reflection in one-dimensional (1D) systems with randomly doped superconducting grains. The superconducting grains are described by the Bogoliubov–de Gene Hamiltonian and the conductance is calculated by using the transfer matrix method and Landauer–Büttiker formula. It is found that although the quasiparticle states are localized due to the randomness and the low dimensionality, the conductance is still kept finite in the thermodynamical limit due to the Andreev reflection. We also investigate the effect of correlation of disorder in such systems and the results show the delocalization of quasiparticle states and suppression of Andreev reflection in a wide energy window.

Differential conductance of the ferromagnet/superconductor interface of DyBa2Cu3O7/La2/3Ba1/3MnO3 heterostructures

The voltage bias, temperature, and magnetic field dependence of the differential conductance of the interface between a half-metallic ferromagnet and a high-Tc superconductor have been studied in DyBa2Cu3O7/La2/3Ba1/3MnO3 heterostructures. Below the superconducting transition the differential conductance exhibits a dip at zero bias whose amplitude is a decreasing function of temperature and/or magnetic field. These results are interpreted qualitatively using a picture based on the suppression of Andreev reflection as a consequence of the high spin polarization of the carriers in the half-metallic ferromagnet.

Tunneling through a superconducting double barrier and the resonant suppression of Andreev reflection.

We consider a superconducting double barrier and show that, under certain conditions, resonant tunneling is directly observable. We study the effect of these resonances on the conductance of the structure as a function of the system parameters and temperature, in order to understand in which regimes these effects are experimentally accessible. The proposed structure provides an excellent vehicle for investigating certain theoretically predicted phenomena which have not yet been observed. These are the tunneling of an electron through the gap of a superconducting material and the phase change of an electron wave function due to Andreev reflection.