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 well-established 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 well-defined 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.
Highlights
Oscillation due to Fermi arcs[20, 21], the Kerr and Faraday rotations in optical experiments[22], and topological superconductivity and Majorana fermions when superconductivity is induced via doping or proximity effect[23,24,25,26]. Because all these fascinating properties arise from the band crossings, there has been growing interest in the search for new Topological metals and semimetals (TMs) with new types of band crossings[27, 28], including a classification of 3, 6, and 8-fold band degeneracies that appear at high-symmetry points in non-symmorphic crystals[28]
We find that the new TM features a pair of triply-degenerate nodes, which are interconnected by multi-segments of lines with two-fold degeneracy
To explicitly reveal the novelty of the new TM, we show that the new band crossing cannot be describe by the established results in topological band theory, which have successfully characterized the topology of the Dirac and Weyl nodes
Summary
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. Oscillation due to Fermi arcs[20, 21], the Kerr and Faraday rotations in optical experiments[22], and topological superconductivity and Majorana fermions when superconductivity is induced via doping or proximity effect[23,24,25,26] Because all these fascinating properties arise from the band crossings, there has been growing interest in the search for new TMs with new types of band crossings[27, 28], including a classification of 3-, 6-, and 8-fold band degeneracies that appear at high-symmetry points in non-symmorphic crystals[28]. Our results highlight the exciting possibilities to realize new particles beyond high-energy textbook examples and to search for new topologically protected low-energy phenomena in transport and spectroscopic experiments beyond the Weyl/Dirac paradigm
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