Experimental realization of two-dimensional Dirac nodal line fermions in monolayer Cu2Si

Abstract

Topological nodal line semimetals, a novel quantum state of materials, possess topologically nontrivial valence and conduction bands that touch at a line near the Fermi level. The exotic band structure can lead to various novel properties, such as long-range Coulomb interaction and flat Landau levels. Recently, topological nodal lines have been observed in several bulk materials, such as PtSn4, ZrSiS, TlTaSe2 and PbTaSe2. However, in two-dimensional materials, experimental research on nodal line fermions is still lacking. Here, we report the discovery of two-dimensional Dirac nodal line fermions in monolayer Cu2Si based on combined theoretical calculations and angle-resolved photoemission spectroscopy measurements. The Dirac nodal lines in Cu2Si form two concentric loops centred around the Γ point and are protected by mirror reflection symmetry. Our results establish Cu2Si as a platform to study the novel physical properties in two-dimensional Dirac materials and provide opportunities to realize high-speed low-dissipation devices.

Topological node line semimetal state in two-dimensional tetragonal allotrope of Ge and Sn

“…On the other hand, Honeycomb-Kagome lattice Hg 3 As 2 [31], Lieb lattice BeH 2 and Be 2 C [32], single layer MX (M=Pd, Pt; X=S, Se, Te) [33], single layer B 2 C [34], 2D compounds X 2 Y (X=Ca, Sr and Ba; Y=As, Sb and Bi) [35], β 12 -borophene [36] and pentagonal group p-IVX 2 (IV=C, Si, Ge, Sn, Pb; X=S, Se, Te) [37] were predicted to be 2D node line semimetals, in which the conduction and valence bands touch near the Fermi level at extended loops. More intriguingly, single layer Cu 2 Si [38] and CuSe [39] have been experimentally demonstrated to be 2D node line semimetals. Up to now, single layer HfGeTe [40] have been theoretically proposed to be 2D Z 2 topological metal.…”

Computational Prediction of the Low‐Temperature Ferromagnetic Semiconducting 2D SiN Monolayer

“…It has been already shown that the computational prediction of new materials with unimaginable properties can set a direction for the experimenters who in subsequent works will find a way to synthesize these new materials. A typical example is the planar hexacoordinate Cu 2 Si monolayer, which was predicted computationally in 2015 and, in just 2 years, fabricated experimentally . Both experimental and theoretical results show fascinating properties.…”

Symmetry demanded topological nodal-line materials

“…However, because SOC is intrinsically small in Cu 2 Si, its effect is also small (gap openings ≤15 meV). Unlike many other proposed materials which are hard to synthesize, especially in thin film form, the experimental synthesis of monolayer Cu 2 Si on Cu(111) surface by chemical vapor deposition has been realized decades ago, and ARPES has been measured confirming the band crossings at both sides of [102].…”

“…Some predicted centrosymmetric DNL materials include cubic antiperovskite materials Cu 3 NX [83,84], CaTe [92], LaX [93], Ca 3 P 2 [94], CaP 3 family [95], BaSn 2 [96,97], AlB 2 -type diborides [98][99][100], and 3D carbon allotrope materials with negligible SOC such as Mackay-Terrones crystals [54] and hyperhoneycomb lattices [101]. In addition, two-dimensional DNL materials have also been proposed in monolayer Cu 2 Si [102] and honeycomb-kagome lattice [103]. Tables 2 and 3 show the crystal structures, electronic band structures without and with SOC, as well as nodal line distributions without SOC within a BZ.…”

“…Cu 2 Si [102]: Cu 2 Si is composed of a honeycomb Cu lattice and a triangular Si lattice, in which Si and Cu atoms are coplanar in monolayer (Table 3). It is in space group P63/mmc No.…”