Two-dimensional (2D) topological insulators (TIs) are promising platforms for low-dissipation spintronic devices based on the quantum-spin-Hall (QSH) effect, but experimental realization of such systems with a large band gap suitable for room-temperature applications has proven difficult. Here, we report the successful growth on bilayer graphene of a quasi-freestanding WSe2 single layer with the 1T′ structure that does not exist in the bulk form of WSe2. Using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we observe a gap of 129 meV in the 1T′ layer and an in-gap edge state located near the layer boundary. The system′s 2D TI characters are confirmed by first-principles calculations. The observed gap diminishes with doping by Rb adsorption, ultimately leading to an insulator–semimetal transition. The discovery of this large-gap 2D TI with a tunable band gap opens up opportunities for developing advanced nanoscale systems and quantum devices.

No abstract

Here we report the evolution of bulk band structure and surface states in rare earth monobismuthides with partially filled f shell. Utilizing synchrotron-based photoemission spectroscopy, we determined the three-dimensional bulk band structure and identified the bulk band inversions near the X points, which, according to the topological theory, could give rise to nontrivial band topology with odd number of gapless topological surface states. Near the surface  point, no clear evidence for predicted gapless topological surface state is observed due to its strong hybridization with the bulk bands. Near the M point, the two surface states, due to projections from two inequivalent bulk band inversions, interact and give rise to two peculiar sets of gapped surface states. The bulk band inversions and corresponding surface states can be tuned substantially by varying rare earth elements, in good agreement with density-functional theory calculations assuming local f electrons. Our study therefore establishes rare earth mono-bismuthides as an interesting class of materials possessing tunable electronic properties and magnetism, providing a promising platform to search for novel properties in potentially correlated topological materials.

No abstract

Scanning-tunneling-microscopy images of annealed Ge(111) show large c(2&8) domains with protrusions that match closely the arrangement of adatom and rest-atom dangling bonds in the simple adatom model. The unit cell, however, appears asymmetric. or "buckled"; namely, the two adatoms or the two rest atoms within the unit cell show significant diA'erences in apparent height. Both the adatom and rest-atom dangling bonds can be simultaneously observed with either bias polarity, indicating that both are partially filled. These results will be discussed in relation to the proposed charge transfer between the adatoms and rest atoms.The Si(111)-(7X7)surface appears to be nearly fully understood, yet there remain unresolved issues for the closely related Ge(111)-c(2X8) surface. The geometrical structure of the Ge(111)-c(2X8) surface is generally considered to be optimally described by a simple adatom model in which the (1 x 1) substrate is decorated with germanium adatoms in alternating (2 x 2) and c (2 x 4) subunits. In this model, each c(2X8) primitive unit cell of eight (1 xi) sites contains two adatoms and two rest atoms, totaling four dangling bonds. Verification of this model has been provided by studies using x-ray diffraction, ' scanning tunneling microscopy (STM), and medium-energy ion scattering.In the STM study of Becker et al. , a pattern of protrusions was observed which matches the arrangement of either the rest atoms or the adatoms, depending of the sample-to-tip bias polarity, but not both simultaneously This o. bservation, combined with information available from a model calculation, ' lead to the following suggestions: (i) The simple adatom model is valid. (ii) There is a complete electron transfer from the adatoms to the rest atoms resulting in a fully occupied rest-atom dangling-bond state and a completely empty adatotn dangling-bond state. A surface electronic band at a binding energy of about 0.8 eV observed by ultraviolet photoemission was ascribed to the occupied restatom dangling-bond state in analogy to the Si(ill)-(7 x 7) case. Two questions remain concerning the above tnodel. The first question arose from a more recent photoemission study revealing the existence of another occupied surface-state band with a dangling-bond character, which could not fit into the model. The second question had to do with the high Coulomb energy needed for a complete charge transfer from the adatoms to the rest atoms. A fully ionic surface appears to be highly unlikely 6 In the work reported here, we examined the Ge(111)-c(2X8) surface with STM under various bias conditions and found features not reported before, which could provide an explanation in connection with the above-mentioned questions. Our results confirm that the simple adatom model is valid except that the c(2X8) unit cell appears "buckled" or asymmetric; namely, the two adatoms or the two rest atoms in a unit cell show significant differences in apparent height depending on the imaging conditions. This observation is somewhat similar to the familiar c...