Nematic superconductivity in CuxBi2Se3 : Surface Andreev bound states

Abstract

We study theoretically the topological surface states (TSSs) and the possible surface Andreev bound states (SABSs) of CuxBi2Se3 which is known to be a topological insulator at x = 0. The superconductivity (SC) pairing of this compound is assumed to have the broken spin-rotation symmetry, similar to that of the A-phase of 3 He as suggested by recent nuclear-magnetic resonance experiments. For both spheroidal and corrugated cylindrical Fermi surfaces with the hexagonal warping terms, we show that the bulk SC gap is rather anisotropic; the minimum of the gap is negligibly small as comparing to the maximum of the gap. This would make the fully-gapped pairing effectively nodal. For a clean system, our results indicate the bulk of this compound to be a topological superconductor with the SABSs appearing inside the bulk SC gap. The zero-energy SABSs which are Majorana fermions, together with the TSSs not gapped by the pairing, produce a zero-energy peak in the surface density of states (SDOS). The SABSs are expected to be stable against short-range nonmagnetic impurities, and the local SDOS is calculated around a nonmagnetic impurity. The relevance of our results to experiments is discussed.

Reveal the nematic topological superconductivity from the anisotropic unconventional Josephson effect: Theory and application to doped Bi2Se3

“…The possible spintriple pairings include Δ z belonging to A 1u representation and {Δ x , Δ y } in the two-dimensional (2D) E u representation of D 3d crystallographic point group [47]. Note that the Δ x and Δ y pairings can spontaneously break the continuous rotational symmetry down to two-fold, giving rise to the nematic superconductivity, and the warping terms R 1 and R 2 are essential for the presence of a full gap for the Δ y and Δ x pairings, respectively [48]. The rotational symmetry breaking observed in the nuclear magnetic resonance [2] and scanning tunneling microscopy measurements [49] suggests the existence of Δ x or Δ y pairings.…”

Lifshitz transition in dirty nematic superconductor

“…Such symmetry breaking arises from the nematic superconducting order parameter within E u representation [8,9]. Unconventional nematic superconductivity gives rise to several intriguing phenomena such as surface Andreev bound states [10][11][12], half-quantum vortices [13,14], spin (nematic) vortices [15], spontaneous strain and magnetization [16], vestigial order [17], unconventional Higgs modes [18], anisotropic quasiparticle interference [19,20].…”

Pauli paramagnetism of triplet Cooper pairs in a nematic superconductor

“…Orientation of the nematicity axis shows the direction of two-fold symmetry breaking [11,20]. Nematic order parameter brings several interesting features such as vestigial nematic order [21], surface Andreev bound states [22], unconventional Higgs mode [23], and quasiparticle interference [24,25].…”