Three-dimensional topological insulators are bulk insulators with Z2 topological electronic order that gives rise to conducting light-like surface states. These surface electrons are exceptionally resistant to localization by non-magnetic disorder, and have been adopted as the basis for a wide range of proposals to achieve new quasiparticle species and device functionality. Recent studies have yielded a surprise by showing that in spite of resisting localization, topological insulator surface electrons can be reshaped by defects into distinctive resonance states. Here we use numerical simulations and scanning tunnelling microscopy data to show that these resonance states have significance well beyond the localized regime usually associated with impurity bands. At native densities in the model Bi2X3 (X=Bi, Te) compounds, defect resonance states are predicted to generate a new quantum basis for an emergent electron gas that supports diffusive electrical transport.

The low temperature hidden order state of URu2Si2 has long been a subject of intense speculation, and is thought to represent an as yet undetermined many-body quantum state not realized by other known materials. Here, X-ray absorption spectroscopy (XAS) and high resolution resonant inelastic X-ray scattering (RIXS) are used to observe electronic excitation spectra of URu2Si2, as a means to identify the degrees of freedom available to constitute the hidden order wavefunction. Excitations are shown to have symmetries that derive from a correlated 5f 2 atomic multiplet basis that is modified by itinerancy. The features, amplitude and temperature dependence of linear dichroism are in agreement with ground states that closely resemble the doublet Γ5 crystal field state of uranium.The low temperature hidden order (HO) phase of URu 2 Si 2 has been a mystery for more than 25 years, and is widely anticipated to represent a novel manybody quantum state. When cooling through T HO =17.5K the material undergoes a second order phase transition, with a large loss of entropy that cannot be immediately explained by observed changes in the electronic structure [1][2][3][4]. Pinpointing the microscopic cause of this entropy change is challenging because basic properties of the atomic scale wavefunction are not decisively known. Experiments differ on whether the uranium valence state is closer to U 4+ (5f 2 ) [5,6] or U 3+ (5f 3 ) [7]. Proposed models have considered a wide range of local [8][9][10][11][12][13][14][15][16][17][18][19][20] and itinerant [21][22][23][24][25][26][27][28][29] low energy state bases for 5f electrons, and explored many exciting possibilities for the "hidden" quantum state. Here, high resolution (δE∼35meV) resonant inelastic X-ray scattering (RIXS) and X-ray absorption spectroscopy (XAS) are used to measure fundamental excitations created by resonance with the uranium 5d core level (O-edge), to identify what electronic degrees of freedom are relevant for effective models of hidden order, and what degrees of freedom are energetically gapped out.Measurements were performed on both a pristine crystalline surface cleaved in ultra high vacuum, and a cleaved surface that was oxidized by exposure to air at room temperature, promoting U 4+ valence. The dominant spectral features observed from both surfaces are shown to derive from the excitations of a freestanding 5f 2 U 4+ atom, revealing that atomically correlated Hund's rule interactions play a key role in determining the electronic degrees of freedom that can contribute to the hidden order state. However, some low energy excitations of the pristine surface are found to be extremely short lived, implying that the symmetries they represent are not strictly eliminated from the hidden order ground state. Linear dichroism in the XAS spectrum is consistent with the crystalline electric field (CEF) doublet state Γ 5 , but inconsistent with CEF singlet ground states that have been predicted as the basis of hidden order (e.g. Γ 1 , Γ 2 , Γ 3 ).Measuring XAS at the O-e...

The massless Dirac electrons found at topological insulator surfaces are thought to be influenced very little by weak, non-magnetic disorder. However, a resonance effect of strongly perturbing non-magnetic impurities has been theoretically predicted to change the dispersion and physical nature of low-energy quasiparticles, resulting in unique particle-like states that lack microscopic translational symmetry. Here we report the direct observation of impurities reshaping the surface Dirac cone of the model threedimensional topological insulator bismuth selenide. A pronounced kink-like dispersion feature is observed in disorder-enriched samples, and found to be closely associated with the anomaly caused by impurity resonance in the surface state density of states, as observed by dichroic angle-resolved photoemission spectroscopy. The experimental observation of these features, which closely resemble theoretical predictions, has significant implications for the properties of topological Dirac cones in applied scenarios that commonly feature point-defect disorder at surfaces or interfaces.

Ninety cases of early esophageal squamous carcinoma (EESC) and 27 patients with early adenocarcinoma of the gastric cardia (EAGC), who for various reasons refused surgical treatment, radiotherapy or chemotherapy, were followed up endoscopically and roentgenologically for 19 to 78 months. Of the 90 cases with EESC, 52 (58%) were still found to have superficial mucosal lesions, 8 cases progressed to an advanced stage, 27 died of cancer with an average survival time of 53.2 months, and 3 died of other non-malignant diseases. The estimated median survival time of EESC is about 75 months. Among 21 patients with EAGC, 8 (30%) were still presenting with superficial malignant changes, 6 cases progressed to advanced cancer, 11 died of carcinoma with an average survival time of 53.3 months, and 2 died of non-malignant diseases. The estimated median survival time of EAGC is about 73 months. The period for a carcinoma in situ of the esophagus and the gastric cardia to progress to an advanced stage appears to be 4-5 years.

The Bi͑111͒ surface was studied by scanning tunneling microscopy ͑STM͒, transmission electron microscopy ͑TEM͒, and angle-resolved photoemission spectroscopy ͑ARPES͒ in order to verify the existence of a recently proposed surface charge-density wave ͑CDW͒ ͓Ch. R. Ast and H. Höchst, Phys. Rev. Lett. 90, 016403 ͑2003͔͒. The STM and TEM results do not support a CDW scenario at low temperatures. Furthermore, the quasiparticle interference pattern observed in STM confirms the spin-orbit split character of the surface states which prevents the formation of a CDW, even in the case of good nesting. The dispersion of the electronic states observed with ARPES agrees well with earlier findings. In particular, the Fermi contour of the electron pocket at the center of the surface Brillouin zone is found to have a hexagonal shape. However, no gap opening or other signatures of a CDW phase transition can be found in the temperature-dependent data.