Abstract
We study the stability of topologically protected zero-energy flat bands at the surface of nodal noncentrosymmetric superconductors, accounting for the alteration of the gap near the surface. Within a selfconsistent mean-field theory, we show that the flat bands survive in a broad temperature range below the bulk transition temperature. There is a second transition at a lower temperature, however, below which the system spontaneously breaks time-reversal symmetry. The surface bands are shifted away from zero energy and become weakly dispersive. Simultaneously, a spin polarization and an equilibrium charge current develop in the surface region.
Highlights
The work presented in this thesis [1,2,3,4,5] is located within the broad field of condensed matter physics, which is the science of electrons in a crystal
The relevance of topological quantum matter in general was acknowledged by the Nobel Prize in Physics in 2016, which was awarded to Thouless, Haldane, and Kosterlitz “for theoretical discoveries of topological phase transitions and topological phases of matter” [10]
Topology [11, 12] is a subfield of mathematics that aims at classifying objects3 according to essential properties that do not depend on details
Summary
The topological properties of gapless electronic systems have recently attracted much attention [1,2,3,4,5,6]. The spin polarization is not primarily carried by the shifted flat bands but rather by bulk and perhaps dispersing surface states [2,11] from the region between the projected nodal rings. TRS is spontaneously broken at a much lower temperature Ts, which is signaled by a nonuniform phase of the gaps This destroys the topological protection for the flat bands, shifting them away from zero energy and giving them finite velocity. The TRSB state leads to clear experimental signatures: a splitting of the zero-bias peak in the tunneling spectrum, a nonvanishing spin polarization at the surface, and a nonvanishing equilibrium charge current parallel to the surface.
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