Spin-orbital ground states of superconducting doped topological insulators: A Majorana platform

Abstract

The ${\mathrm{Bi}}_{2}$ ${\mathrm{Se}}_{3}$ class of topological insulators has recently been shown to undergo a superconducting transition upon hole or electron doping (${\mathrm{Cu}}_{x}$-${\mathrm{Bi}}_{2}$${\mathrm{Se}}_{3}$ with ${\mathrm{T}}_{C}={3.8}^{\ifmmode^\circ\else\textdegree\fi{}}$K and ${\mathrm{Pd}}_{x}$-${\mathrm{Bi}}_{2}$${\mathrm{Te}}_{3}$ with ${\mathrm{T}}_{C}={5.5}^{\ifmmode^\circ\else\textdegree\fi{}}$K), raising the possibilities that these are ``topological superconductors'' or realize a superconducting state that can be used as Majorana platform. We use angle resolved photoemission spectroscopy to examine elements of band structure that determine the spin-orbital ground states of superconducting ${\mathrm{Cu}}_{x}$-${\mathrm{Bi}}_{2}$${\mathrm{Se}}_{3}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Te}}_{3}$, observing that the spin-momentum locked topological surface states remain well defined and non-degenerate with respect to bulk electronic states at the Fermi level in optimally doped ${\mathrm{Cu}}_{x}$-${\mathrm{Bi}}_{2}$${\mathrm{Se}}_{3}$. The implications of this unconventional superconducting surface topology are discussed, and we explore the possibility of realizing the same topology in superconducting variants of ${\mathrm{Bi}}_{2}$${\mathrm{Te}}_{3}$. Characteristics of the experimentally measured three dimensional bulk states are examined with respect to the superconducting state and topological properties, showing that a single Majorana fermion zero mode is expected to be bound at each superconducting vortex on the crystal surface. Systematic measurements also reveal intriguing renormalization and charge correlation instabilities of the surface-localized electronic modes.