(1 − / (0)) with ϕ 0 and ξ(0) being the flux quantum and the GL coherent length at zero temperature, respectively, we obtained ξ(0) = 2.0 nm, which is around two times shorter than the bulk value 3 .8 Getting back to the thickness dependence of T c on in Figs. 2c and 2d, we note that the first single step superconducting transition appears at the thickness of 9.6 nm and 5.9 nm for sample A and N, respectively. The appearance of high-T C superconductivity in the rather thicker conditions offers us to consider that how the electric-field effect contributes to the high-T C superconductivity. Here, we examined the effect of electrostatic doping on the superconductivity in both ultrathin and thicker conditions by different experimental schemes in the same device set-up as sample A. At first, samples B, C on SrTiO 3 and M on MgO were etched at T = 245 K to induce the superconductivity as shown in Fig. 3a, the top panel of Fig. 3c and Fig. 3d (red line data), respectively. The thicknesses of samples B, C and M are tuned to the one/two-unit-cell (the red square in Fig. 2c), 9.4 nm (the blue triangle in Fig. 2c) and 3.7 nm (the green circle in Fig. 2d), respectively, by monitoring the leakage current under the assumption that the same etching rate with sample A holds. After the detection of the high-T C superconducting behavior, we examined the electrostatic effect with removing and then applying V G at T = 220 K (along the blue arrow in Fig. 1b; Note that no electrochemical etching occurs at this temperature as mentioned above).For sample B expectedly in the one/two-unit-cell condition, the initial insulating behavior was not recovered by removing V G from 5 V to 3 and 0 V as shown in Fig. 3b. 9 This result is a good evidence for the thickness reached close to one/two-unit-cell, because the high-T C superconductivity under V G = 0 V is consistent with the result of previous reports on FeSe/SrTiO 3 with charge transfer solely from the SrTiO 3 substrate 8 .Interestingly, the increase of T c on from 43.5 K to 46.3 K as a result of reduced V G implicates the over-doping effect by electrostatic doping. In contrast, in sample C under the thicker 9.4 nm condition, the superconductivity vanishes and the R s -T curve is back to the insulating behavior when V G is removed (blue line in middle panel of Fig. 3c).The reappearance of insulating behavior indicates the uniform etching without local ultrathin region providing a high-T C superconducting current pass. Then, application of V G = 5 V recovers the superconductivity again (blue line in bottom panel of Fig. 3c), demonstrating a reversible control of insulator-superconductor transition by electrostatic mean. These series of results in sample C suggests that the observable thickness condition for high-T c on superconductivity is widely expanded from one/two-unit-cell to above 10-unit-cells by the application of electric field. Additionally, T c on in sample M decreases with reducing the accumulated charge as shown in Fig. 3d. Therefore, we suppose that the high-T C supercondu...

We have performed angle-resolved photoemission spectroscopy (ARPES) of LaSb and CeSb, a candidate of topological insulator. Using soft-x-ray photons, we have accurately determined the three-dimensional bulk band structure and revealed that the band inversion at the Brillouin-zone corner -a prerequisite for realizing topological-insulator phase -is absent in both LaSb and CeSb. Moreover, unlike the ARPES data obtained with soft-x-ray photons, those with vacuum ultraviolet (VUV) photons were found to suffer significant kz broadening. These results suggest that LaSb and CeSb are topologically trivial semimetals, and unusual Dirac-cone-like states observed with VUV photons are not of the topological origin.

We study the effect of oxygen vacancies on the electronic structure of the model strongly correlated metal SrVO3. By means of angle-resolved photoemission (ARPES) synchrotron experiments, we investigate the systematic effect of the UV dose on the measured spectra. We observe the onset of a spurious dose-dependent prominent peak at an energy range were the lower Hubbard band has been previously reported in this compound, raising questions on its previous interpretation. By a careful analysis of the dose dependent effects we succeed in disentangling the contributions coming from the oxygen vacancy states and from the lower Hubbard band. We obtain the intrinsic ARPES spectrum for the zero-vacancy limit, where a clear signal of a lower Hubbard band remains. We support our study by means of state-of-the-art ab initio calculations that include correlation effects and the presence of oxygen vacancies. Our results underscore the relevance of potential spurious states affecting ARPES experiments in correlated metals, which are associated to the ubiquitous oxygen vacancies as extensively reported in the context of a two-dimensional electron gas (2DEG) at the surface of insulating d 0 transition metal oxides. A major challenge of modern physics is to understand the fascinating phenomena in strongly-correlated transition metal oxides (TMOs), which emerge in the neighborhood of the Mott insulator state. Some preeminent examples that have gathered the interest for almost 30 years are high temperature superconductivity, colossal magnetoresistance, heavy fermion physics and, of course, the Mott metal-insulator transition itself [1]. Significant theoretical progress was made with the introduction of Dynamical Mean Field Theory (DMFT) and its combination with ab initio Density Functional methods (LDA+DMFT), which allows treatment of the interactions promoting itinerancy and localization of electrons on equal footing [2][3][4]. Among the most emblematic achievements of DMFT is the prediction of a Hubbard satellite, which splits off of the conduction band of a metal. This satellite results from the partial localization of conduction electrons due to their mutual Coulomb repulsion. Early DMFT studies also showed that it is the precursor of the localized electronic states of a Mott insulator [5]. Since then, these predictions promoted a large number of studies using photoemission spectroscopy, which is a technique to directly probe the presence of Hubbard bands. In this context, the TMO system SrVO 3 has emerged as the drosophila model system to test the predictions of strongly correlated electron theories. In fact, SrVO 3 is arguably the simplest correlated metal. It is a simple cubic perovskite, with nominally one electron per V site, which occupies a 3 fold degenerate t 2g conduction band. While the presence of a satellite in the photoemission spectra of Ni metal was already well known, in the context of correlated TMOs, the Hubbard band was originally reported in a systematic investigation of Ca 1−x Sr x VO 3 [6], which was follo...

One of key challenges in current material research is to search for new topological materials with inverted bulk-band structure. In topological insulators, the band inversion caused by strong spin-orbit coupling leads to opening of a band gap in the entire Brillouin zone, whereas an additional crystal symmetry such as point-group and nonsymmorphic symmetries sometimes prohibits the gap opening at/on specific points or line in momentum space, giving rise to topological semimetals. Despite many theoretical predictions of topological insulators/semimetals associated with such crystal symmetries, the experimental realization is still relatively scarce. Here, using angle-resolved photoemission spectroscopy with bulk-sensitive soft x-ray photons, we experimentally demonstrate that hexagonal pnictide CaAgAs belongs to a new family of topological insulators characterized by the inverted band structure and the mirror reflection symmetry of crystal. We have established the bulk valence-band structure in three-dimensional Brillouin zone, and observed the Dirac-like energy band and ring-torus Fermi surface associated with the line node, where bulk valence and conducting bands cross on a line in the momentum space under negligible spin-orbit coupling. Intriguingly, we found that no other bands cross the Fermi level and therefore the low-energy excitations are solely characterized by the Dirac-like band. CaAgAs provides an excellent platform to study the interplay among low-energy electron dynamics, crystal symmetry, and exotic topological properties.