A novel quasi-one-dimensional topological insulator in bismuth iodide ß-Bi4I4.

Recent progress in the field of topological states of matter has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs; refs ,,,), followed by closely related ternary compounds and predictions of several weak TIs (refs ,,). However, both the conceptual richness of Z2 classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z2 topological insulator is theoretically predicted and experimentally confirmed in the ß-phase of quasi-one-dimensional bismuth iodide Bi4I4. The electronic structure of ß-Bi4I4, characterized by Z2 invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). Our angle-resolved photoemission spectroscopy measurements performed on the (001) surface reveal a highly anisotropic band-crossing feature located at the  point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.

Observation of Ultrafast Free Carrier Dynamics in Single Layer MoS2.

The dynamics of excited electrons and holes in single layer (SL) MoS2 have so far been difficult to disentangle from the excitons that dominate the optical response of this material. Here, we use time- and angle-resolved photoemission spectroscopy for a SL of MoS2 on a metallic substrate to directly measure the excited free carriers. This allows us to ascertain a direct quasiparticle band gap of 1.95 eV and determine an ultrafast (50 fs) extraction of excited free carriers via the metal in contact with the SL MoS2. This process is of key importance for optoelectronic applications that rely on separated free carriers rather than excitons.

Ultrafast Band Structure Control of a Two-Dimensional Heterostructure.

The electronic structure of two-dimensional (2D) semiconductors can be significantly altered by screening effects, either from free charge carriers in the material or by environmental screening from the surrounding medium. The physical properties of 2D semiconductors placed in a heterostructure with other 2D materials are therefore governed by a complex interplay of both intra- and interlayer interactions. Here, using time- and angle-resolved photoemission, we are able to isolate both the layer-resolved band structure and, more importantly, the transient band structure evolution of a model 2D heterostructure formed of a single layer of MoS2 on graphene. Our results reveal a pronounced renormalization of the quasiparticle gap of the MoS2 layer. Following optical excitation, the band gap is reduced by up to ∼400 meV on femtosecond time scales due to a persistence of strong electronic interactions despite the environmental screening by the n-doped graphene. This points to a large degree of tunability of both the electronic structure and the electron dynamics for 2D semiconductors embedded in a van der Waals-bonded heterostructure.