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Journal of the Korean Physical Society https://doi.org/10.1007/s40042-020-00016-8

ORIGINAL PAPER

The low‑energy electron band structure of a two‑dimensional Dirac nodal‑line semimetal grown on a silicon surface Hyun‑Jeong Joo1 · Choongyu Hwang1   · Kyoo Kim2 Received: 10 August 2020 / Revised: 3 November 2020 / Accepted: 3 November 2020 © The Korean Physical Society 2020

Abstract The low-energy electron band structure of Cu2 Si on Si(111) has been investigated using angle-resolved photoemission spectroscopy. Cu2 Si exhibits two Dirac nodal-lines, stemming from the crossing of one electron-pocket with two hole-pockets, that are protected by mirror reflection symmetry. When Cu2 Si is placed on Si(111), the hole-pockets and their satellite bands due to the quasi-5 × 5 periodicity are clearly observed whereas the electron-pocket is observed with very weak spectral intensity. Interestingly, close to the Fermi energy, the hole-pockets exhibit almost linear energy-momentum dispersion when their spectral width is also linearly proportional to energy. These findings indicate that Cu2 Si on Si(111) can host Dirac nodal-line fermions, of which low-energy excitations might depart from those of the conventional Fermi liquid. Keywords  Dirac nodal-line fermions · Cu2Si · ARPES · Electron band structure

1 Introduction The comprehension of the Dirac fermionic nature of quasiparticles in graphene has led to the search for another type of Dirac semimetal. One of the examples that has attracted recent interests is the Dirac nodal-line semimetal, in which the valence and the conduction bands touch at extended lines that are protected by symmetries [1–3]. Depending on the symmetry, three different types of nodal lines have been proposed: those are protected by an inversion symmetry in the absence of the spin-orbit coupling [4], by a mirror symmetry in the absence of strong spin-orbit coupling [5], and by nonsymmorphic symmetries even in the presence of spin-orbit coupling [6, 7]. The existence of Dirac nodal-line fermions has been confirmed only in a few three-dimensional systems such as PbTaSe2 [8], TiB2 [9, 10], and ZrSiS [11]. When a two-dimensional form of a Dirac nodal-line semimetal can invite possible applications to flexible multifunctional devices with its exotic properties that have been discussed for graphene [12], the two-dimensional Dirac nodal-lines * Choongyu Hwang [email protected] 1



Department of Physics, Pusan National University, Busan 46241, South Korea



Korea Atomic Energy Research Institute, Daejeon 34057, South Korea

2

have been predicted in honeycomb-kagome lattice [13] and a family group of MX (M=Pd, Pt; X=S, Se, Te) [14]. Theoretical predictions on the geometric and the electronic structures of Cu2Si [15] have provoked a revisit to the reconstructed surfaces of Si [16, 17] and Cu [18] to search for Dirac nodal-line fermions in this two-dimensional system. Indeed, monolayer Cu2 Si grown on Cu(111) shows the characteristics of Dirac nodal-line fermions [5]. The low-energy electron band structure o