Nexus fermions in topological symmorphic crystalline metals

Abstract

Topological metals and semimetals (TMs) have recently drawn significant interest. These materials give rise to condensed matter realizations of many important concepts in high-energy physics, leading to wide-ranging protected properties in transport and spectroscopic experiments. It has been wellestablished that the known TMs can be classified by the dimensionality of the topologically protected band degeneracies. While Weyl and Dirac semimetals feature zero-dimensional points, the band crossing of nodal-line semimetals forms a one-dimensional closed loop. In this paper, we identify a TM that goes beyond the above paradigms. It shows an exotic configuration of degeneracies without a welldefined dimensionality. Specifically, it consists of 0D nexus with triple-degeneracy that interconnects 1D lines with double-degeneracy. We show that, because of the novel form of band crossing, the new TM cannot be described by the established results that characterize the topology of the Dirac and Weyl nodes. Moreover, triply-degenerate nodes realize emergent fermionic quasiparticles not present in relativistic quantum field theory. We present materials candidates. Our results open the door for realizing new topological phenomena and fermions including transport anomalies and spectroscopic responses in metallic crystals with nontrivial topology beyond the Weyl/Dirac paradigm.Understanding nontrivial topology in gapless materials including metals and semimetals has recently emerged as one of the most exciting frontiers in the research of condensed matter physics and materials science [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] . Unlike conventional metals, topological metals/semimetals (TMs) are materials whose Fermi surface arises from the degeneracy of conduction and valence bands, which cannot be avoided due to their nontrivial topology. To date, the known TMs include Dirac semimetals, Weyl semimetals, and nodal-line semimetals. Dirac or Weyl semimetals have zero-dimensional (0D) band crossings, i.e., the Dirac or Weyl nodes and a Fermi surface that consists of isolated 0D points in the bulk Brillouin zone (BZ). By contrast, nodal-line semimetals feature one-dimensional (1D) band crossings and a Fermi surface that is made up of 1D closed loops in the BZ. Therefore, the band crossings serve as a key signature of nontrivial topology in metals and can be used to classify TMs. More importantly, these band crossings can give rise to fundamentally new physical phenomena. Since low-energy excitations near the Dirac or Weyl nodes mimic elementary fermions, TMs provide a unique opportunity to study important concepts of high-energy physics such as Dirac fermions, Weyl fermions, and the chiral anomaly in table-top experiments. The correspondence with high-energy physics, in turn, leads to a cornucopia of topologically protected phenomena. The resulting key experimental detectable signatures include the Dirac, Weyl or nodal-line quasiparticles in the bulk, the Fermi arc or drumhead topological surface states on the bound...

Massive fermions with low mobility in antiferromagnet orthorhombic CuMnAs single crystals

“…The WSMs with inversion symmetry broken have been shown in (Ta/Nb)(As/P) [19][20][21][22][23][24][25] , and (W/Mo)Te 2 etc [26][27][28][29] . The WSMs with spontaneous time reversal symmetry broken have also been reported in YbMnBi 2 30 , and predicted in magnetic HgCr 2 Se 4 and Heusler alloys etc [31][32][33] . The existence of Weyl points near the Fermi energy level (E F ) will lead to interesting spectroscopic and transport phenomena, such as Fermi arcs 4,16,21,22,29 , ultrahigh carrier mobility 34,35 , and chiral anomaly [36][37][38] .…”

Nearly massless Dirac fermions and strong Zeeman splitting in the nodal-line semimetal ZrSiS probed by de Haas–van Alphen quantum oscillations

“…The inversion symmetry broken Weyl states were demonstrated in transition metal monopnictides (Ta/Nb)(As/P) [7][8][9][10][11][12][13][14], photonic crystals [17], and (W/Mo)Te 2 [18][19][20][21][22][23][24][25][26][27]. The spontaneous time reversal symmetry breaking Weyl state has been reported in YbMnBi 2 [28] and predicted in magnetic Heusler alloys [29][30][31][32][33] and the magnetic members of R-Al-X (R=rare earth, X=Si, Ge) compounds [34].…”

Three-component fermions with surface Fermi arcs in tungsten carbide

“…In this work, we investigate the three-fold degeneracy proposed in the symmorphic system WC [6][7][8][9] . As illustrated in Fig.…”

“…1a, the threefold degeneracy lies in an intermediate state between Dirac and Weyl states. Recovering inversion symmetry makes two triply degenerate nodal points (TPs) merging into a single Dirac node, while applying a Zeeman field along the rotation axis drives a TP into two Weyl nodes [6][7][8][9] . In contrast to isolated Dirac and Weyl nodes, the pair of TPs is connected by a doubly degenerate band.…”

“…Theory has also proposed that in a symmorphic space group P m 6 2 (no. 187) with the tungsten carbide (WC)-type structure, the combination of rotation and mirror symmetries may protect three-fold degenerate points on the high-symmetry line [5][6][7][8][9] , which are accidental degeneracy points between the bands of one-and two-dimensional irreducible representations due to band inversion.In this work, we investigate the three-fold degeneracy proposed in the symmorphic system WC [6][7][8][9] . As illustrated in Fig.…”