First-principles calculation of the Andreev conductance of carbon wires

Abstract

We developed a first-principles approach based on nonequilibrium Green's function (NEGF) combined with density functional theory (DFT) to investigate quantum transport properties of normal-metal-superconductor (N-S) hybrid systems. As an application of our theory, we investigated the Andreev conductance of atomic wires consisting of 4-15 carbon atoms in contact with one normal Al lead and another superconducting Al lead from first principles. Numerical results show that the Andreev conductance oscillates between an even and odd number of carbon atoms. In the presence of the superconducting lead, the self-consistent scattering potential of the N-S system can be very different from that of the corresponding normal system. Furthermore, a small change of scattering potential can give rise to a significant change of Andreev conductance. For an even number of carbon atoms, the change of scattering potential gives rise to a 4-7% difference in conductance, while when the number of carbon atoms n is odd, a 14-30% change of conductance is observed due to the potential change. We find that the charge transfer plays an important role in N-S systems. For the carbon wire with normal Al contacts, there is a significant charge transfer in real space that is responsible for the even-odd oscillation in conductance. When a superconducting lead is present, the charge is redistributed in momentum space, although it is almost not changed in real space. For even n, a 10% change of charge density at Fermi level is found mainly in the lead region. For odd n, however, the change of charge density at Fermi level is even more than 30% near the first, third, etc., carbon atoms. Since less charge density is available at Fermi level, there is a decrease in conductance for all carbon wires, especially for the wires with odd number of carbon atoms. Our results indicate that the self-consistent calculation of the scattering potential is necessary to obtain an accurate Andreev conductance of N-S hybrid structures.

First-principles investigation of quantum transport through an endohedral N@C60in the Coulomb blockade regime

“…Interesting interplay between the proximity effect and the Coulomb blockade and Kondo effects was observed. Since the NEGF-DFT method has been extended to the normal superconductor (NS) hybrid system [38], it can in principle account for the strongly correlated effect of NS systems. Actually the NEGF-DFT method is an ideal tool to study the proximity effect as well as the Andreev conductance.…”

Phenomenology of Andreev reflection from first-principles transport theory

“…This is reminiscent of the even-odd effect in C wires sandwiched between Al superconducting electrodes. Wang is changed from even to odd 15 . Finally, we study the variation of normalized conductance as a function of an applied bias, which is plotted in Fig.…”

“…Wang and co-authors have recently suggested performing a self-consistent calculation of the scattering potential to study Andreev reflection at normal metalsuperconductor junctions 15 . They calculated the conductance for carbon chains sandwiched between a normal and a superconducting Al electrode and found different values depending on whether or not the calculation was carried out self-consistent over the Hartree and exchange-correlation potential.…”

First-Principles Calculations of Spin-Triplet Andreev Reflection Spectra at Half-Metallic Ferromagnet/Superconductor Interface

“…In normal-metal/s-wave BCS superconductor heterostructure (NM/S), below the superconducting energy gap a spin up or down electron comes into NM/S interface will be reflected as a hole in the opposite spin subband, which is called spin-singlet Andreev reflection(AR) process 1 and this phenomenon has been well studied by the first-principles approach very recently. 2 If the NM is ferromagnet (F), the number of propagating states at the Fermi energy is different for majority and minority spin owing to the presence of exchange splitting. Therefore only a fraction of incident electron can be Andreev reflected and then the conductance of this spin-singlet AR is suppressed significantly.…”