Self-assembled indium linear chains on the Si(111) surface are found to exhibit instability of the metallic phase and 1D charge density wave (CDW). The room-temperature metallic phase of these chains undergoes a temperature-induced, reversible transition into a semiconducting phase. The 1D CDW along the chains is observed directly in real space by scanning tunneling microscopy at low temperature. The Fermi contours of the metallic phase measured by angle-resolved photoemission exhibit a perfect nesting predicting precisely the CDW periodicity.

Honeycomb structures of group IV elements can host massless Dirac fermions with nontrivial Berry phases. Their potential for electronic applications has attracted great interest and spurred a broad search for new Dirac materials especially in monolayer structures. We present a detailed investigation of the ${\beta }_{12}$ sheet, which is a borophene structure that can form spontaneously on a Ag(111) surface. Our tight-binding analysis revealed that the lattice of the ${\beta }_{12}$ sheet could be decomposed into two triangular sublattices in a way similar to that for a honeycomb lattice, thereby hosting Dirac cones. Furthermore, each Dirac cone could be split by introducing periodic perturbations representing overlayer-substrate interactions. These unusual electronic structures were confirmed by angle-resolved photoemission spectroscopy and validated by first-principles calculations. Our results suggest monolayer boron as a new platform for realizing novel high-speed low-dissipation devices.

The electronic structure of Bi(001) ultrathin films (thickness $\ge 7$ bilayers) on $\mathrm{Si}(111)\mathrm{\text{−}}7\times 7$ was studied by angle-resolved photoemission spectroscopy and first-principles calculations. In contrast with the semimetallic nature of bulk Bi, both the experiment and theory demonstrate the metallic character of the films with the Fermi surface formed by spin-orbit-split surface states (SSs) showing little thickness dependence. Below the Fermi level, we clearly detected quantum well states (QWSs) at the $\overline{M}$ point, which were surprisingly found to be non-spin-orbit split; the films are “electronically symmetric” despite the obvious structural nonequivalence of the top and bottom interfaces. We found that the SSs hybridize with the QWSs near $\overline{M}$ and lose their spin-orbit-split character.

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.

The search for metallic boron allotropes has attracted great attention in the past decades and recent theoretical works predict the existence of metallicity in monolayer boron. Here, we synthesize the ${\beta }_{12}$-sheet monolayer boron on a Ag(111) surface and confirm the presence of metallic boron-derived bands using angle-resolved photoemission spectroscopy. The Fermi surface is composed of one electron pocket at the $\overline{S}$ point and a pair of hole pockets near the $\overline{X}$ point, which is supported by the first-principles calculations. The metallic boron allotrope in ${\beta }_{12}$ sheet opens the way to novel physics and chemistry in material science.