Nonsymmorphic Symmetry-Protected Band Crossings in a Square-Net Metal PtPb$_4$

TL;DR

This study demonstrates the existence of nonsymmorphic symmetry-protected Dirac nodal lines in the metal PtPb$_4$, showing how crystal symmetry and spin-orbit coupling influence topological electronic states.

Contribution

It provides experimental and theoretical evidence for nonsymmorphic symmetry-protected nodal lines in PtPb$_4$, highlighting the role of symmetry and spin-orbit coupling in topological band crossings.

Findings

Nodal lines are protected by nonsymmorphic symmetry.

Spin-orbit coupling lifts degeneracy except at isolated points.

Nodal line bandwidth is smaller than DFT predictions.

Abstract

Topological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Brillouin zone boundaries in PtPb$_4$, arising from the nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal lines would remain gapless in the absence of spin-orbit coupling (SOC), the SOC in this case plays a detrimental role to topology by lifting the band degeneracy everywhere except at a set of isolated points. Nevertheless, the nodal line is observed to have a bandwidth much smaller than that found in density functional theory (DFT). Our findings reveal PtPb$_4$ to be a material system with narrow crossings approximately protected by non-symmorhpic crystalline symmetries.