Observation of sixfold degenerate fermions in PdSb$_2$

TL;DR

This paper reports the direct observation of sixfold degenerate fermions in PdSb$_2$ using high-resolution ARPES, confirming theoretical predictions and highlighting a new type of topological fermion beyond high energy physics.

Contribution

The study provides the first experimental visualization of sixfold degenerate fermions in a condensed matter system, confirming their existence and properties through ARPES and first-principles calculations.

Findings

Sixfold degenerate bands observed at the R point in PdSb$_2$

Confirmation of quadratic dispersion of sixfold degenerate crossing

Demonstration of topological fermions beyond high energy physics constraints

Abstract

Three types of fermions have been extensively studied in topological quantum materials: Dirac, Weyl, and Majorana fermions. Beyond the fundamental fermions in high energy physics, exotic fermions are allowed in condensed matter systems residing in three-, six- or eightfold degenerate band crossings. Here, we use angle-resolved photoemission spectroscopy to directly visualize three-doubly-degenerate bands in PdSb$_2$. The ultrahigh energy resolution we are able to achieve allows for the confirmation of all the sixfold degenerate bands at the R point, in remarkable consistency with first-principles calculations. Moreover, we find that this sixfold degenerate crossing has quadratic dispersion as predicted by theory. Finally, we compare sixfold degenerate fermions with previously confirmed fermions to demonstrate the importance of this work: our study indicates a topological fermion beyond the constraints of high energy physics.