Source: School of Physics
Written by: School of Physics
Edited by: Tan Rongyu, Wang Dongmei

In recent years, the realization of Majorana zero modes (MZMs) and the application of them in topological quantum computation are an important research direction in the condensed matter physics community. While MZMs can naturally be realized in intrinsic topological superconductors, such as at the two ends of a one-dimensional p-wave superconductor or in the vortex cores of a two-dimensional chiral p-wave superconductor, it is unfortunate that intrinsic topological superconductors are lacking in nature. In 2008, Liang Fu and Charles L. Kane made a theoretical breakthrough. They showed that effective two-dimensional p-wave superconductivity could be achieved by taking the Dirac surface states of a topological insulator in proximity to a conventional s-wave superconductor. Once the Dirac surface states are gapped by the superconductivity, the vortices generated by an external magnetic field will harbor MZMs at their cores, as shown in Fig.1(a).

Time-reversal invariant Dirac semimetals (DSMs) and Weyl semimetals (WSMs) in three dimensions are two classes of topological materials which have recently attracted great attention. Compared to topological insulators, they show remarkable difference in both bulk and boundary physics. In the bulk, the energy bands of DSMs and WSMs do not have a global gap, instead they touch at some isolated points in the Brillouin zone. On the boundary, the surface states of DSMs and WSMs are manifested as Fermi arcs which are open and in general spin-polarized rather than spin-momentum locked, as shown in Fig.1(b). Because of these remarkable difference, when the Fermi arcs are gapped by s-wave superconductivity, the superconductivity is no longer of the two-dimensional p-wave nature. Accordingly, it seems quite “natural’’ to expect that superconducting DSMs and WSMs should not be able to realize MZMs in the vortices. However, in collaboration with Dr. Zhigang Wu and Dr. Wen Huang, Dr. Zhongbo Yan demonstrated that in superconducting DSMs and WSMs, when the vortex lines are terminated at surfaces with Fermi arcs, the two ends of the vortex line can each bind one robust MZMs in the weakly doped regime. Furthermore, they also revealed that the Zeeman field could greatly enhance the topological gap protecting the MZMs, which is in sharp contrast to the situation in superconducting topological insulators where the Zeeman field is found to decrease the topological gap. This work greatly broadens the scope of materials for the realization and study of MZMs, and it was recently published on line in Physics Review Letters.


This work was supported by the Start Grant of Sun Yat-sen University, National Natural Science Foundation of China, and Key-Area Research and Development Program of Guangdong Province.

The linkage of the paper is: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.257001