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.

We produced a Japanese translation of the 15-item myasthenia gravis (MG)-specific quality of life (QOL) scale (MG-QOL15), assessed its reliability and validity, and examined clinical factors affecting the self-perceived QOL in MG. Consecutive 327 patients with MG seen at six neurological centers were evaluated. All patients completed an MG-QOL15 Japanese version (MG-QOL15-J), the Beck Depression Inventory-Second Edition (BDI-II), and a generic health-related QOL questionnaire, the SF-36. Disease severity was determined according to the MG Foundation of America (MGFA) quantitative MG score and the MG composite. The MG-QOL15-J exhibited adequate internal reliability, test-retest repeatability, and concurrent validity with SF-36, disease severity, and known-patient groups categorized by MGFA postintervention status. Multivariate analysis revealed severity, dose of oral corticosteroids, and BDI-II as independent factors negatively affecting QOL. The MG-QOL15-J is anticipated to be a valuable clinical measure of QOL in Japanese patients with MG.

2D anisotropic Dirac cones are observed in χ borophene, a monolayer boron sheet, using high-resolution angle-resolved photoemission spectroscopy. The Dirac cones are centered at the X and X' points. The data also reveal that the hybridization between borophene and Ag(111) is very weak, which explains the preservation of the Dirac cones. As χ borophene has been predicated to be a superconductor, the results may stimulate further research interest in the novel physics of borophene, such as the interplay between Cooper pairs and the massless Dirac fermions.

The topology of pure Bi is controversial because of its very small (∼10 meV) band gap. Here we perform high-resolution angle-resolved photoelectron spectroscopy measurements systematically on 14−202 bilayer Bi films. Using high-quality films, we succeed in observing quantized bulk bands with energy separations down to ∼10 meV. Detailed analyses on the phase shift of the confined wave functions precisely determine the surface and bulk electronic structures, which unambiguously show nontrivial topology. The present results not only prove the fundamental property of Bi but also introduce a capability of the quantum-confinement approach. [5]. Even now, numbers of novel quantum phenomena have been intensively reported on this system [6][7][8][9][10][11][12][13]. In spite of the enormous amount of research, one fundamental property of Bi has been controversial: its electronic topology. Because of its huge spin-orbit coupling (SOC) [14], Bi has also been a central element in designing topological materials such as Bi 1−x Sb x , Bi 2 Se 3 , Na 3 Bi, and β-Bi 4 I 4 [15][16][17][18][19]. A combination of SOC and several symmetries produces topologically protected electronic states with inherent spin splitting. Despite the essential role in topological studies, a pure Bi crystal itself had long been believed topologically trivial based on several calculations [20][21][22][23][24][25][26], which had been considered to agree with transport [27] and angleresolved photoelectron spectroscopy (ARPES) measurements [22,28,29]. However, a recent high-resolution ARPES result suggests the surface bands are actually different from previously calculated ones and Bi possesses a nontrivial topology [30,31]. New transport measurements also imply the presence of topologically protected surface states [32,33].Nevertheless, the recent ARPES result has not yet been conclusive because it lacks clear peaks of bulk bands [30,31]. In principle, surface-normal bulk dispersions can be measured by changing the incident photon energy, where the momentum resolution is determined from the uncertainty relation ∆z · ∆k z ≥ 1/2 (Ref.[34]). (∆z is an escape depth of photoelectrons.) However, the Dirac dispersion of Bi is so sharp against this resolution that hν-dependent spectra show no clear peak [29][30][31]. This is a serious problem because Bi has a very small (∼10 meV [21,26]) band gap and a slight energy shift in bulk bands can easily transform a nontrivial case [ Fig. 1(d)] into a trivial case [ Fig. 1(e)]. In short, to unambiguously identify the topology of Bi, one must precisely determine both the surface and bulk electronic structures. One promising approach is using a thin film geometry, where quantumwell state (QWS) subbands are formed inside bulk band projections [35,36]. Although QWSs originate from bulk states, they possess a two-dimensional character and can be clearly observed in ARPES measurements.In this Letter, we performed high-resolution ARPES

The low photon energy of driving fields in the multi-terahertz (multi-THz) spectral window has proven particularly suitable to