# Observation of multiple types of topological fermions in PdBiSe

## TL;DR

This study reports the experimental observation of four distinct types of topological fermions in PdBiSe, expanding the understanding of topological phases and fermionic excitations in condensed matter physics.

## Contribution

The paper provides the first experimental demonstration of multiple topological fermions coexisting in a single material, confirmed through density functional theory and photoemission spectroscopy.

## Key findings

- Identification of Weyl, Rarita-Schwinger-Weyl, double class-II three-component, and charge-2 fourfold fermions in PdBiSe.
- Coexistence of multiple topological fermions in one material.
- Potential for exploring exotic fermionic properties and interactions.

## Abstract

Topological semimetals with different types of band crossings provide a rich platform to realize novel fermionic excitations, known as topological fermions. In particular, some fermionic excitations can be direct analogues of elementary particles in quantum field theory when both obey the same laws of physics in the low-energy limit. Examples include Dirac and Weyl fermions, whose solid-state realizations have provided new insights into long-sought phenomena in high-energy physics. Recently, theorists predicted new types of fermionic excitations in condensed-matter systems without any high-energy counterpart, and their existence is protected by crystalline symmetries. By studying the topology of the electronic structure in PdBiSe using density functional theory calculations and bulk-sensitive soft X-ray angle-resolved photoemission spectroscopy, we demonstrate a coexistence of four different types of topological fermions: Weyl, Rarita-Schwinger-Weyl, double class-II three-component, and charge-2 fourfold fermions. Our discovery provides a remarkable platform to realize multiple novel fermions in a single solid, charting the way forward to studies of their potentially exotic properties as well as their interplay.

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Source: https://tomesphere.com/paper/1905.11285