The topological materials have attracted much attention for their unique electronic structure and peculiar physical properties. ZrTe5 has host a long-standing puzzle on its anomalous transport properties manifested by its unusual resistivity peak and the reversal of the charge carrier type. It is also predicted that single-layer ZrTe5 is a two-dimensional topological insulator and there is possibly a topological phase transition in bulk ZrTe5. Here we report high-resolution laser-based angle-resolved photoemission measurements on the electronic structure and its detailed temperature evolution of ZrTe5. Our results provide direct electronic evidence on the temperature-induced Lifshitz transition, which gives a natural understanding on underlying origin of the resistivity anomaly in ZrTe5. In addition, we observe one-dimensional-like electronic features from the edges of the cracked ZrTe5 samples. Our observations indicate that ZrTe5 is a weak topological insulator and it exhibits a tendency to become a strong topological insulator when the layer distance is reduced.

High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and superconducting gap of the (Tl0.58Rb0.42)Fe1.72Se2 superconductor with a T(c) = 32  K. The Fermi surface topology consists of two electronlike Fermi surface sheets around the Γ point which is distinct from that in all other iron-based superconductors reported so far. The Fermi surface around the M point shows a nearly isotropic superconducting gap of ∼12  meV. The large Fermi surface near the Γ point also shows a nearly isotropic superconducting gap of ∼15  meV, while no superconducting gap opening is clearly observed for the inner tiny Fermi surface. Our observed new Fermi surface topology and its associated superconducting gap will provide key insights and constraints into the understanding of the superconductivity mechanism in iron-based superconductors.

The physical property investigation (like transport measurements) and ultimate application of the topological insulators usually involve surfaces that are exposed to ambient environment (1 atm and room temperature). One critical issue is how the topological surface state will behave under such ambient conditions. We report high resolution angle-resolved photoemission measurements to directly probe the surface state of the prototypical topological insulators, Bi 2 Se 3 and Bi 2 Te 3 , upon exposing to various environments. We find that the topological order is robust even when the surface is exposed to air at room temperature. However, the surface state is strongly modified after such an exposure. Particularly, we have observed the formation of two-dimensional quantum well states near the exposed surface of the topological insulators. These findings provide key information in understanding the surface properties of the topological insulators under ambient environment and in engineering the topological surface state for applications.T he topological insulators represent a novel state of matter where the bulk is insulating but the surface is metallic, which is expected to be robust due to topological protection (1-5). The topological surface state exhibits unique electronic structure and spin texture that provide a venue not only to explore novel quantum phenomena in fundamental physics (6-10) but also to show potential applications in spintronics and quantum computing (2,5,11). The angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental tool to directly identify and characterize topological insulators (12). A number of three-dimensional topological insulators have been theoretically predicted and experimentally identified by ARPES (13-21); some of their peculiar properties have been revealed by scanning tunneling microscopy (STM) (22-26). The application of the topological surface states depends on the surface engineering that can be manipulated by incorporation of nonmagnetic (27-31) or magnetic (27, 28, 31-33) impurities or gas adsorptions (27,(33)(34)(35). While the ARPES and STM measurements usually involve the fresh surface obtained by cleaving samples in situ under ultrahigh vacuum, for the transport and optical techniques, which are widely used to investigate the intrinsic quantum behaviors of the topological surface state (36-40), and particularly the ultimate applications of the topological insulators, the surface is usually exposed to ambient conditions (1 atm air and room temperature) or some gas protection environment. It is therefore crucial to investigate whether the topological order can survive under the ambient conditions and, furthermore, whether and how the surface state may be modified after such exposures. Results and DiscussionWe start by first looking at the electronic structure of the prototypical topological insulators Bi 2 ðSe;TeÞ 3 under ultrahigh vacuum. The Fermi surface and the band structure of the Bi 2 ðSe 3−x Te x Þ topological insulators depend sensitively on ...