Analysis of the unconventional chiral fermions in a non-centrosymmetric chiral crystal $\textbf {PtAl}$

TL;DR

This study investigates the electronic structure and topological properties of the chiral crystal PtAl, revealing non-trivial chiral fermions, Berry phases, and surface states that could impact future quantum materials research.

Contribution

It provides the first detailed experimental and theoretical analysis of chiral fermions and topological states in PtAl, a non-centrosymmetric chiral crystal.

Findings

The smallest Fermi pocket has a non-trivial Berry phase of 1.16π.

PtAl hosts higher-fold chiral fermions at high-symmetry points.

Long helical Fermi arcs connect chiral points on the surface.

Abstract

Symmetry-protected non-trivial states in chiral topological materials hold immense potential for fundamental science and technological advances. Here, we report electrical transport, quantum oscillations, and electronic structure results of a single crystal of chiral quantum material $\rm PtAl$. Based on the de Haas-van Alphen (dHvA) oscillations, we show that the smallest Fermi pocket ($\alpha$) possesses a non-trivial Berry phase $1.16$$\pi$. The band associated with this Fermi pocket carries a linear energy dispersion over a substantial energy window of $\sim$700 meV that is further consistent with the calculated optical conductivity. First-principles calculations unfold that $\rm PtAl$ is a higher-fold chiral fermion semimetal where structural chirality drives the chiral fermions to lie at the high-symmetry $\Gamma$ and $R$ points of the cubic Brillouin zone. In the absence of spin-orbit coupling, the band crossings at $\Gamma$ and $\rm R$ points are three- and four-fold degenerate with a chiral charge of $-2$ and $+2$, respectively. The inclusion of spin-orbit coupling transforms these crossing points into four- and six-fold degenerate points with a chiral charge of $-4$ and $+4$. Nontrivial surface states on the $(001)$ plane connect the bulk projected chiral points through the long helical Fermi arcs that spread over the entire Brillouin zone.