耦合Majorana束缚态T形双量子点中的Andreev反射

研究了连接在正常金属电极和超导电极之间的耦合Majorana束缚态(MBSs)T形双量子点结构中的Andreev反射.研究发现,对于T形双量子点结构,当入射能量等于边耦合量子点能级时Andreev反射电导出现Fano振荡,连接MBSs之后,零费米能附近出现一对新的Fano型振荡峰.如果忽略两个MBSs之间的相互作用,零费米能点的Andreev反射电导为定值1/2G_0(G_0=2e~2/h),不受量子点能级、双量子点之间耦合强度以及量子点与MBSs之间的耦合强度的影响.此外,在没有耦合MBSs的T形双量子点结构中,调节双量子点间的耦合强度可以使零费米能附近的Andreev反射电导出现由共振带向反共振带的转变,而耦合MBSs之后,又可以使反共振消失转而出现新的共振峰.     

Transport properties through double-magnetic-barrier structures in graphene

We study electrons tunneling through a double-magnetic-barrier structure on the surface of monolayer graphene.The transmission probability and the conductance are calculated by using the transfer matrix method.The results show that the normal incident transmission probability is blocked by the magnetic vector potential and the Klein tunneling region depends strongly on the direction of the incidence electron.The transmission probability and the conductance can be modulated by changing structural parameters of the barrier,such as width and height,offering a possibility to control electron beams on graphene.     

Suppression of Andreev conductance in a topological insulator–superconductor nanostep junction

When two three-dimensional topological insulators(TIs) are brought close to each other with their surfaces aligned,the surfaces form a line junction. Similarly, three TI surfaces, not lying in a single plane, can form an atomic-scale nanostep junction. In this paper, Andreev reflection in a TI–TI–superconductor nanostep junction is investigated theoretically. Because of the existence of edge states along each line junction, the conductance for a nanostep junction is suppressed. When the incident energy(ε) of an electron is larger than the superconductor gap(?), the Andreev conductance in a step junction is less than unity while for a plane junction it is unity. The Andreev conductance is found to depend on the height of the step junction. The Andreev conductance exhibits oscillatory behavior as a function of the junction height with the amplitude of the oscillations remaining unchanged when ε = 0, but decreasing for ε = ?, which is different from the case of the plane junction. The height of the step is therefore an important parameter for Andreev reflection in nanostep junctions, and plays a role similar to that of the delta potential barrier in normal metal–superconductor plane junctions.     

Break Cohesion of Metal Contacts due to Voltage Bias

The instability of metal point contacts under voltage bias is calculated based on scattering theory. When the bias is applied, the transport channels will be closed and the chemical bonds will be broken, which modify the cohesive force of the point contact. If the cross section of the contact is narrow enough, an applied bias will break the point contact, which is in agreement with the experimental results. The effect of bias-dependent force can play an important role in the stability of a nanosize device.