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 ...

A central question in the high temperature cuprate superconductors is the fate of the parent Mott insulator upon charge doping. Here we use scanning tunneling microscopy to investigate the local electronic structure of lightly doped cuprate in the antiferromagnetic insulating regime. We show that the doped charge induces a spectral weight transfer from the high energy Hubbard bands to the low energy in-gap states. With increasing doping, a V-shaped density of state suppression occurs at the Fermi level, which is accompanied by the emergence of checkerboard charge order.The new STM perspective revealed here is the cuprates first become a charge ordered insulator upon doping. Subsequently, with further doping, Fermi surface and high temperature superconductivity grow out of it.High temperature superconductivity in the cuprates is widely believed to originate from adding charge carriers into an antiferromagnetic (AF) Mott insulator (1). Elucidating the properties of the doped Mott insulator is among the most crucial issues concerning the mechanism of superconductivity. From the electronic structure point of view, the key question is how the large Mott-Hubbard gap, or more precisely the charge transfer gap, evolves into the d-wave superconducting (SC) gap upon charge doping. This has turned out to be a formidable challenge, both theoretically and experimentally, due to the presence of strong AF fluctuations and electron correlations. A major obstacle lying between the parent Mott insulator and the optimally doped cuprate is the pseudogap phase, which exhibits a normal state gap as revealed by early spectroscopic studies (2-4). More recently, imaging and diffraction techniques show that electrons in the pseudogap phase have strong propensity towards charge (5-16) or spin order (5,(17)(18)(19). Currently neither the gap-like density of state (DOS) suppression nor the charge/spin density wave is well understood (4).Most previous experiments on the pseudogap phase focused on the underdoped regime with finite transition temperature (T c ), and aimed to address its relationship with the SC phase by probing the strength of the two orders across T c (4). Recent scanning tunneling microscopy (STM) and x-ray spectroscopy experiments provide increasing evidence that the charge order associated with the pseudogap compete with superconductivity because its feature gets suppressed as the sample enters the SC phase below T c (4,11,16). However, due to the lack of spectroscopic data spanning the energy range of both the pseudogap and charge transfer gap, less is known about lightly doped, non-SC regime standing next to the parent To address these questions, here we carry out STM investigations on lightly doped Bi 2 Sr 2-x La x CuO 6+ (La-Bi2201) in the AF insulating regime. La-Bi2201 is an ideal cuprate system which not only has a well-cleaved surface, but also can be doped towards the Mott insulating limit by varying the . Figure 1A 1C displays spatially resolved tunneling spectra dI/dV(r, V), which is roughly proportional to ...