Andreev Reflection In Quantum Dot Systems Coupled With Majorana Bound States

Due to their potential applications in topological quantum computation and their fundamental interest, Majorana fermions are currently attracting increased attention. Up to now, the zero bias conductance peak(ZBCP) is the unique signature that has been observed experimentally to check the existence of MBSs. Numerous theoretical and experimental studies exactly illustrate that the quantum dot(QD) structure is a good candidate for the detection of MBSs.QD systems have many unique transport properties and interesting quantum phenomena,such as quantum interference effect, Fano effect etc. In addition, comparing with a single QD,a coupled QD structure has more adjustable parameters, and thus has more important theoretical and practical value, which provides an excellent platform to detect MBSs.QDs coupled with normal metallic conductor(N) and with superconducting electrode(S)structure exhibit interesting transport properties. One of these properties is the so-called Andreev reflection(AR). Especially, in the subgap regime, the current almost entirely originates from the anomalous Andreev channel; such a spectroscopy can thus directly probe any in-gap states. In the present paper, we investigate the transport properties through QD structures between a normal and a superconducting lead. Here we focus on the effects of on AR through these QD systems.Firstly, we study the AR in an N–QD–S system coupled with MBSs. We find that for appropriate tunnel coupling with the normal and superconducting lead, the AR is enhanced by the MBSs. One can change the coupling strength between the QD and the MBS or between the two to obtain a Fano resonance. The AR conductance is always20 021/ 2()eG Gh? at thezero energy point when only QD–MBS coupling is considered, showing the robust properties of the Majorana fermions. We also study under what conditions the Fano resonant AR might appear.We also consider a T-shaped double QD structure with side-coupled to and investigatethe AR conductance through the system by adding a normal and a superconducting lead. We find that the AR conductance appears a resonant peak around zero Fermi energy when only one QD(QD1) connects to metal and superconducting leads. As a consequence of quantum interference, when another QD2 side-attached to QD1, a pair of new Fano-type resonant peaks appear and distribute aside the zero point and the Fano antiresonant point is at the energy level of the QD2. If a MBS is introduced to couple to QD2, the AR conductance shows several new features. First, a pair of new Fano-type resonance curves appear and the original ones also persist except for the position shifting. In addition, the AR conductance value at the zero Fermi energy point is exactly equal to01/ 2G in the presence of QD-MBS coupling and zero inter-MBS coupling, which is independent of the inert-dot coupling, the energy levels of QD and the strength of the QD-MBS coupling. This feature is different from that the T-shaped DQD structure side-coupled to a traditional fermions. We hope that these results will be helpful for understanding the quantum interference in MBS-assisted AR and may find significant applications, especially in quantum computation.