Observation of a Chiral Wave Function in Twofold Degenerate Quadruple Weyl System BaPtGe

TL;DR

This study experimentally demonstrates a chiral wave function in the bulk phonon spectrum of BaPtGe, revealing a previously unknown quadruple Weyl node and showcasing IXS as a tool to explore topological properties in quantum materials.

Contribution

The paper provides the first experimental evidence of a chiral wave function associated with a quadruple Weyl node in a phononic system, using high-resolution IXS and first principles calculations.

Findings

Identification of a twofold degenerate quadruple Weyl node in BaPtGe

Observation of non-trivial phonon dynamical structure factor S(Q,ω)

Validation of IXS as a method to detect topological wave functions

Abstract

Topological states in quantum materials are defined by non-trivial topological invariants, such as the Chern number, which are properties of their bulk wave functions. A remarkable consequence of topological wave functions is the emergence of edge modes, a phenomenon known as bulk-edge correspondence, that gives rise to quantized or chiral physical properties. While edge modes are widely presented as signatures of non-trivial topology, how bulk wave functions can manifest explicitly topological properties remains unresolved. Here, using high-resolution inelastic x-ray spectroscopy (IXS) combined with first principles calculations, we report experimental signatures of chiral wave functions in the bulk phonon spectrum of BaPtGe, which we show to host a previously undiscovered twofold degenerate quadruple Weyl node. The chirality of the degenerate phononic wave function yields a non-trivial phonon dynamical structure factor, S(Q,$\omega$), along high-symmetry directions, that is in excellent agreement with numerical and model calculations. Our results establish IXS as a powerful tool to uncover topological wave functions, providing a key missing ingredient in the study of topological quantum matter.