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 {\Gamma} point and are protected by mirror reflection symmetry. Our results establish Cu2Si as a new platform to study the novel physical properties in two-dimensional Dirac materials and provide new opportunities to realize high-speed low-dissipation devices.
@article{arxiv.1611.09578,
title = {Discovery of two-dimensional Dirac nodal line fermions in monolayer Cu2Si},
author = {Baojie Feng and Botao Fu and Shusuke Kasamatsu and Suguru Ito and Peng Cheng and Cheng-Cheng Liu and Ya Feng and Shilong Wu and Sanjoy K. Mahatha and Polina Sheverdyaeva and Paolo Moras and Masashi Arita and Osamu Sugino and Tai-Chang Chiang and Kenya Shimada and Koji Miyamoto and Taichi Okuda and Kehui Wu and Lan Chen and Yugui Yao and Iwao Matsuda},
journal= {arXiv preprint arXiv:1611.09578},
year = {2018}
}