Pseudo Dirac Nodal Sphere: Unusual Electronic Structure and Material Realization

TL;DR

This paper introduces the concept of a pseudo Dirac nodal sphere (PDNS), a higher-dimensional topological semimetal state characterized by a spherical band crossing, and identifies material candidates to realize this novel electronic structure.

Contribution

It generalizes topological semimetals to a higher-symmetry PDNS state with a spherical band crossing, expanding the understanding of electronic topological phases.

Findings

Theoretical demonstration of two types of PDNS states based on crystalline symmetries.

Identification of material candidates (strained MH3 and Si3N2) to realize PDNS states.

Discovery of unique electronic properties and low-energy excitations in PDNS semimetals.

Abstract

Topological semimetals (TSMs) in which conduction and valence bands cross at zero-dimensional (0D) Dirac nodal points (DNPs) or 1D Dirac nodal lines (DNLs), in 3D momentum space, have recently drawn much attention due to their exotic electronic properties. Here we generalize the TSM state further to a higher-symmetry and higher-dimensional pseudo Dirac nodal sphere (PDNS) state, with the band crossings forming a 2D closed sphere at the Fermi level. The PDNS state is characterized with a spherical backbone consisting of multiple crossing DNLs while band degeneracy in between the DNLs is approximately maintained by weak interactions. It exhibits some unique electronic properties and low-energy excitations, such as collective plasmons different from DNPs and DNLs. Based on crystalline symmetries, we theoretically demonstrate two possible types of PDNS states, and identify all the possible band crossings with pairs of 1D irreducible representations to form the PDNS states in 32 point groups. Importantly, we discover that strained MH3 (M= Y, Ho, Tb, Nd) and Si3N2 are materials candidates to realize these two types of PDNS states, respectively. As a high-symmetry-required state, the PDNS semimetal can be regarded as the "parent phase" for other topological gapped and gapless states.