Narrow Results

Motivated by a recent experiment [R. S. Keizer, et. al., Nature 439, 825 (2006)], we study the Josephson effect in superconductor/diffusive half metal/superconductor junctions. The Josephson π-state is more stable than the 0-junction when the spin-flip scattering at the junction interface opens the Josephson channel of the odd-frequency Cooper pairs.

Perspective for qubits Josephson Mo-Re alloy-oxide–Pb, Mo-Re alloy-normal metal–oxide–MoRe alloy, and NbN -normal metal–oxide–Pb junctions have been fabricated and investigated.

Using the equivalent single-particle multi-channel network and the Landauer formula, we theoretically study anti-resonances in conductance of a normal metal–superconductor junction with a side-coupled quantum dot. The transport properties depend on the interplay between the Coulomb blockade effect and the Andreev reflection. It is found that the calculated dependence of the conductance on the gate voltage of dot exhibits two anti-resonant conductance dips. This behavior is caused by the destructive interference of the wave directly transmitted through the normal metal–superconductor junction and the wave reflected from the dot. Moreover, we find that the shape of two anti-resonance profile is symmetric, due to the Andreev reflection, depending on the strength of coupling between the quantum dot and normal metal.

We analyze the full counting statistics (FCS) of a superconducting junction in nonequilibrium in the limit of small interface transparency. In this limit we treat both supercurrent and multiple Andreev reflections on equal footing and show how to generalize previous results for both phenomena. Furthermore, we also allow for different gaps of both superconductors and investigate the intermediate regime which allows to make contact with previous results on normal-superconductor heterostructures. We also compare our predictions in this regime to experimental data.

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 p_x and p_x+ip_y case, whereas increase it for p_y order. Strongly suppressed Andreev reflection is demonstrated.

To study how Andreev reflection (AR) occurs between a superconductor and a three-dimensional topological insulator (TI), we use superconducting Nb tips to perform point-contact AR spectroscopy at 4.2 K on as-grown single crystals of Bi₂Se₃. Scanning tunneling spectroscopy and scanning tunneling microscopy are also used to characterize the superconducting tip and both the doping level and surface condition of the TI sample. The point-contact measurements show clear spectral signatures of AR, as well as a depression of zero-bias conductance with decreasing junction impedance. The latter observation can be attributed to interfacial Rashba spin-orbit coupling, and the presence of bulk bands at the Fermi level in our samples suggests that bulk states of Bi₂Se₃ are involved in the observed AR.

We consider a superconductor/double-ferromagnet/superconductor (S/I₁/F/I_C/F'/I₂/S) ballistic junction with thin insulating layers in the interfaces. An extensive investigation is carried out for the possibility of 0- or π-junction and the important phaseshift. The average exchange field for the two ferromagnets has limited use, only for weak exchange fields. Several analytical formulas are obtained for the energy spectrum in simple cases with interface potential (a) absent, (b) between ferromagnets only, (c) double resonance barrier between S and F layers. They give reasonable agreement even outside their range of validity. For the case of strong S/F barriers we find a strong interplay of Andreev bound states and interbarrier resonances.

The quantization of the current in a superconducting quantum point contact is reviewed and the critical current is discussed at different temperatures depending on the carrier concentration as well by suggesting a constant potential in the semiconductor and then a Maxwell potential. When the Fermi wavelength is comparable with the constriction width we showed that the critical current has a step-like variation as a function of the constriction width and the carrier concentration.

Combining the first-principles noncollinear calculations of scattering matrices with Andreev approximation, we investigated the spin-triplet Andreev reflection (AR) spectra for the interface between half-metallic ferromagnet Co₂MnSi and s-wave BCS superconductor Al with and without interfacial roughness, where the orientations of magnetic moments near the interface are randomly distributed. The calculated results show that the AR spectra have peak structures near zero bias for the clean interface with relative weak magnetic disorder. With the increasing degree of interfacial roughness or magnetic disorder, these subgap peaks of conductance spectra will be washed out. The results also show that the value of subgap conductance spectrum can be raised significantly by the magnetic disorder. Finally, our calculations reveal that the long-range spin-triplet AR in Co₂MnSi/Al(001) interface can be enhanced by a small amount of interfacial roughness.

Using the standard nonequilibrium Green’s function techniques, we investigate the effect of Rashba spin-orbit interaction (RSOI) and ferromagnetic electrode on the spin accumulation in the parallel-coupled double quantum dots coupled with a ferromagnetic and a superconducting electrode. It is demonstrated that FM electrode cannot induce the spin polarization of Andreev reflection (AR) current, but can induce the spin accumulation in the QDs. However, RSOI can lead to the spin polarization of AR current as well as the spin accumulation in the QDs. In the existence of RSOI, complete spin-polarized QD can be achieved with negative bias voltage $V$, which is the most significant advantage of our device. When energy levels ${\epsilon }_1={\epsilon }_2=0$ and the interdot coupling strength $t_c=0.01$, the maximum value of spin accumulation in this paper is obtained as 0.7. The results may be useful on the design of spintronic devices.

The influence of time-varying fields on the transport through a mesoscopic device has been investigated. This mesoscopic device is modeled as a quantum dot coupled to superconducting reservoirs via quantum point contact. The effect of a magnetic field and the Andreev reflection process were taken into account. The conductance was deduced by using Landuaer–Buttiker equation. A numerical calculation has been performed that shows a resonant tunneling behavior. Such investigation is important for fabricating photoelectron mesoscopic devices.

We study the transport properties of a single InAs self-assembled quantum dot contacted with superconducting leads. The charging energy E_c of the quantum dot is much larger than the superconducting gap energy Δ. For the dot whose tunnel coupling Γ to the lead is much larger than Δ but smaller than E_c, we observe enhancement of first-order Andreev reflections by the Kondo effect. We find that the zero-bias conductance measured for various Δ's and Kondo temperature T_K's collapses onto a single curve with Δ/T_K as the only relevant energy scale, providing experimental evidence for universal scaling in this system. On the other hand, for the dot with Γ comparable to E_c we observe a supercurrent flowing through the dot, reflecting the charge fluctuation sufficiently greater that one.