Abstract
FeSeTe has recently emerged as a leading candidate material for the two-dimensional topological superconductivity (TSC). Two reasons for the excitement are the high $T_c$ of the system and the fact that the Majorana zero modes (MZMs) inside the vortex cores live on the exposed surface rather than at the interface of a heterostructure as in the proximitized topological insulators. However, the recent scanning tunneling spectroscopy data have shown that, contrary to the theoretical expectation, the MZM does not exist inside every vortex core. Hence there are ``full'' vortices with MZMs and ``empty'' vortices without MZMs. Moreover the fraction of ``empty'' vortices increase with an increase in the magnetic field. We propose the possibility of two distinct gapped states competing for the topological surface states in FeSeTe: the TSC and half quantum anomalous Hall (hQAH). The latter is promoted by magnetic field through the alignment of magnetic impurities such as Fe interstitials. When hQAH takes over the topological surface state, the surface will become transparent to scanning tunneling microscopy and the nature of the vortex in such region will appear identical to what is expected of the vortices in the bulk, i.e., empty. Unmistakable signature of the proposed mechanism for empty vortices will be the existance of chiral Majorana modes(CMM) at the domain wall between a hQAH region and a TSC region. Such CMM should be observable by observing local density of states along a line connecting an empty vortex to a nearby full vortex.
Highlights
One exciting feature of the topological insulator (TI) is its potential to host the Majorana zero mode (MZM), which has led to many proposals [1,2,3,4] and attempts [5,6,7,8,9] to realize Majorana zero modes (MZMs) through introducing superconducting pairing to the TI surface state
Our key prediction is that the MZM that would have been in the vortex core transforms into the chiral Majorana mode (CMM) located at the boundary between the half quantum anomalous Hall (hQAH) and topological superconductivity (TSC) on the surface of FeTeSe [Fig. 1(d)]
We reveal that interstitial Fe impurities can create hQAH regions on the surface of FeSeTe, where vortex MZMs are absent but CMM will occur at the boundary
Summary
One exciting feature of the topological insulator (TI) is its potential to host the Majorana zero mode (MZM), which has led to many proposals [1,2,3,4] and attempts [5,6,7,8,9] to realize MZM through introducing superconducting pairing to the TI surface state. Uneven distribution of Fe interstitials can nucleate the hQAH regions on the surface of FeSeTe when their moments get aligned with external magnetic fields [Fig. 1(c)], preventing TSC to form in that. Such a hQAH surface state will reveal the bulk superconductivity to STM and the vortices penetrating hQAH surface will show properties of the bulk superconducting state with the topologically trivial s± pairing [33,34], i.e., becoming “empty”. More hQAH regions are nucleated on the surface of FeSeTe, providing a natural explanation of the increasing faction of empty vortices observed in experiments. Our key prediction is that the MZM that would have been in the vortex core transforms into the CMM located at the boundary between the hQAH and TSC on the surface of FeTeSe [Fig. 1(d)]. In the rest of this article, we first present our proposal using a lowenergy effective theory and support it with a numerical simulation based on a microscopic model in FeSeTe
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