Observation of multiple types of topological fermions in PdBiSe

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 highenergy 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.

Chiral separation effect for spin 3/2 fermions

“…Massless charged (quasi)spin 3/2 quasiparticles emerge due to fourfold band crossings (while a Weyl point emerges due to a two-fold band crossing), which were experimentally discovered in a number of crystals, e.g. CoSi, RhSi [27][28][29], AlPt [30], PdGa [43], PdBiSe [31]. For a multi-fold band crossing node, we find that the CSE conductivity σ s is dictated by the node's Chern number C s (the subscript s labels a type of the node, as explained in section 4; in the context of topological semimetals, we assume that a quasiparticle is of unit charge e = 1)…”

Symmetry-protected topological phase for spin-tensor-momentum-coupled ultracold atoms

“…We show that the system exhibits different types of TDPs, including a single unpaired unconventional TDP hosting nontrivial middle band emerged by quadratic SVMC and STMC. In addition, several two-fold WPs appear in our system, which provides a controllable platform to explore the coexistence of multiple types of topological fermions [49][50][51][52][53][54][55][56][57]. Remarkably, the exotic Fermi arcs connecting TDP and WPs are observed.…”

Strain-induced topological charge control in multifold fermion systems

“…Though first-principle calculations predicted the presence of such unconventional chiral fermions in several space groups 17,[20][21][22]27 , only a few materials such as AlPt 28 , PdGa 29 , PdBiSe 30 , PtGa 31 , RhSn 32 , CoSi 23,24,33 , and RhSi 24 have been experimentally realized, where the surface and bulk electronic properties have been visualized by scanning tunnelling microscopy and angle resolved photoemission spectroscopy. The family of transition metal silicides (MSi, M = Co, Rh) is one of the prominent proposed and experimentally verified material candidates to host unconventional chiral fermions [20][21][22]34,35 .…”