There has been growing interest in searching for exotic self-conjugate, charge-neutral low-energy fermionic quasi-particles, known as Majorana fermions (MFs) in solid state systems. Their signatures have been proposed and potentially observed at edges of topological superconcuctors with non-trivial topological invariant in the bulk electronic band structure. Much effort have been focused on realizing MFs in odd-parity superconductors made of strong spin-orbit coupled materials in proximity to conventional superconductors. In this paper, Prof. Chung (a NCTU professor and the coordinator of NCTS thematic group on topology and strong correlations) and collaborators propose a novel mechanism for realizing MFs in 2D spin-singlet topological superconducting state induced by doping a correlated quantum spin Hall insulator (QSHI) on honeycomb lattice.
At half-filling, a QSHI is a two-dimensional topological insulator supporting a pair of counter-propagating (helical) metallic edge states while the bulk remains insulating. Graphene-based materials are attractive candidates to realize QSHIs when doping with heavy elements (indium or thallium) to enhance the spin-orbit coupling. Upon charge doping, QSHIs may become topological superconductors. Via a renormalized mean-field approach, the system is found to exhibit time-reversal symmetry (TRS) breaking chiral d−wave (d+i d’-wave) superconductivity near half-filling in the limit of large on-site repulsion. Surprisingly, however, at large spin-orbit coupling, the system undergoes a topological phase transition into a new topological phase protected by a topological invariant, called the “pseudo-spin Chern number”. This feature can be viewed as a persistent extension of the quantum spin Hall phase upon doping. From bulk-edge correspondence, this phase is featured by the presence of two pairs of counter-propagating helical Majorana modes per edge, instead of two co-propagating (or chiral) edge modes as expected from chiral d-wave superconductors. Prof. Chung’s proposal offers a novel route to realize Majorana fermions in graphene-based materials doped with heavy adatoms.
Reference:
Helical Majorana fermions in dx2−y2 + idxy-wave topological superconductivity of doped correlated quantum spin Hall insulators, Shih-Jye Sun, Chung-Hou Chung*, Yung-Yeh Chang, Wei-Feng Tsai, and Fu-Chun Zhang, Scientific Reports, 6, Article number: 24102 (2016). http://www.nature.com/articles/srep24102
Figure Caption: (a). Excitation spectrum of 2D chiral d-wave topological superconductors on a zigzag ribbon of honeycomb lattice. (b). Decay of Majorana fermions as topological edge states near the zero-energy modes of the linearly-dispersed spectrum in (a). (c). Schematic plot of the helical (counter-propagating) Majorana edge modes on a zigzag nano-ribbon (ZGNR).
