Abstract
The protected helical surface states in thin films of topological insulators (TI) are subject to inter-surface hybridisation. This leads to gap opening and spin texture changes as witnessed in photoemission and quasiparticle interference investigations. Theoretical studies show that universally the hybridisation energy exhibits exponential decay as well as sign oscillations as a function of film thickness, depending on the effective band parameters of the material. When a step is introduced in the TI film e.g. by profiling the substrate such that the hybridisation has different signs on both sides of the step, 1D bound states appear within the hybridisation gap which decay exponentially with distance from the step. The step bound states have linear dispersion and inherit the helical spin locking from the surface states and are therefore non-degenerate. When the substrate becomes an s-wave superconductor Majorana zero modes located at the step ends are created inside the superconducting gap. The proposed scenario involves just a suitably stepped interface of superconductor and TI and therefore may be a most simple device being able to host Majorana zero modes.
Highlights
The surfaces of strong topological insulators (TIs) like Bi2Se3, Bi2Te3, and Sb2Te3 carry spin-locked nondegenerate helical surface states
In this work we have shown that the helical surface states in thin films of topological insulators may be manipulated in an interesting and promising way
This leads to a hybridization energy that both decays exponentially and oscillates with film thickness depending on materials parameters and simultaneously leads to a gap in the 2D helical surface states
Summary
The surfaces of strong topological insulators (TIs) like Bi2Se3, Bi2Te3, and Sb2Te3 carry spin-locked nondegenerate helical surface states. If the thicknesses to the left (dL ) and right (dR) of the step are chosen in such a way that the hybridizations tL(dL ) and tR(dR) have opposite signs on the two sides of the step, a helical nondegenerate bound state within the hybridization gap may appear which is spatially located at the step and has linear dispersion If it were found experimentally, this would entail a further interesting speculative possibility: Once the substrate of the step-profiled TI becomes a simple s-wave spin singlet superconductor, the proximity effect will open a superconducting gap in the dispersion of the nondegenerate (spin-locked) step state. To be a very simple way to realize these states in a realistic geometry
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