Hourglass-like Nodal Net Semimetal in Ag2BiO3

Abstract: Based on first-principles calculation and analysis of crystal symmetries, we propose a kind of hourglass-like nodal net (HNN) semimetal in centrosymmetric Ag2BiO3 that is constructed by two hourglass-like nodal chains (HNCs) at mutually orthogonal planes in the extended Brillouin zone (BZ) when the weak spin-orbit coupling (SOC) mainly from the 6s orbital of Bi atoms is ignored. The joint point in the nodal net structure is a special double Dirac point located at the BZ corner. Different from previous HNN [Bzdusek et al., Nature 538, 75 (2016)] where the SOC and double groups nonsymmorphic symmetries are necessary, and also different from the accidental nodal net, this HNN structure is inevitably formed and guaranteed by spinless nonsymmorphic symmetries, and thus robust against any symmetry-remaining perturbations. The Fermi surface in Ag2BiO3 consisting of a torus-like electron pocket and a torus-like hole pocket may lead to unusual transport properties. A simple four-band tight-binding model is built to reproduce the HNN structure. For a semi-infinite Ag2BiO3 , the "drumhead" like surface states with nearly flat dispersions are demonstrated on (001) and (100) surfaces, respectively. If such weak SOC effect is taken into consideration, this HNN structure will be slightly broken, left a pair of hourglass-like Dirac points at the two-fold screw axis. This type of hourglass-like Dirac semimetal is symmetry-enforced and does not need band inversion anymore. Our discovery provides a new platform to study novel topological semimetal states from nonsymmorphic symmetries.