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      Observation of Topological Surface State in High Temperature Superconductor MgB2

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          Abstract

          The hunt for the benchmark topological superconductor (TSc) has been an extremely active research subject in condensed matter research, as it may host Majorana Fermions and enable topological quantum computation. However, so far essentially all TSc candidates have very low transition temperatures (Tc) and/or are very sensitive to disorder and dopant levels, both of which have greatly hampered the progress of this field. Here, we use Angle-resolved Photoemission Spectroscopy (ARPES) to show the presence of Dirac Nodal Lines (DNLs) and corresponding topological surface states (TSSs) on the (010) faces of the Tc=39K s-wave BCS superconductor MgB2. Not only is this nearly triple the current record4 of superconducting Tc among all candidate TScs, but the nature of these DNL states should make them highly tolerant against disorder and inadvertent doping variations. This makes MgB2 a promising high temperature platform for the search and engineering of Majorana fermions.

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          A large energy-gap oxide topological insulator based on the superconductor BaBiO3

          Mixed-valent perovskite oxides based on BaBiO3 (BBO) are, like cuperates, well-known high-Tc superconductors. Recent ab inito calculations have assigned the high-Tc superconductivity to a correlation-enhanced electron--phonon coupling mechanism, stimulating the prediction and synthesis of new superconductor candidates among mixed-valent thallium perovskites. Existing superconductivity has meant that research has mainly focused on hole-doped compounds, leaving electron-doped compounds relatively unexplored. Here we demonstrate through ab inito calculations that BBO emerges as a topological insulator (TI) in the electron-doped region, where the spin-orbit coupling (SOC) effect is significant. BBO exhibits the largest topological energy gap of 0.7 eV among currently known TI materials, inside which Dirac-type topological surface states (TSSs) exit. As the first oxide TI, BBO is naturally stable against surface oxidization and degrading, different from chalcoginide TIs. An extra advantage of BBO lies in its ability to serve an interface between the TSSs and the superconductor for the realization of Majorana Fermions.
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            Influence of Mg deficiency in MgB2 single crystals on crystal structure and superconducting properties

            The effects of high-temperature vacuum-annealing induced Mg deficiency in MgB2 single crystals grown under high pressure were investigated. As the annealing temperature was increased from 800 to 975 C, the average Mg content in the MgB2 crystals systematically decreased, while Tc remains essentially unchanged and the superconducting transition slightly broadens from 0.55 K to 1.3 K. The reduction of the superconducting volume fraction was noticeable already after annealing at 875 C. Samples annealed at 975 C are partially decomposed and the Mg site occupancy is decreased to 0.92 from 0.98 in as-grown crystals. Annealing at 1000 C completely destroys superconductivity. X-ray diffraction analysis revealed that the main final product of decomposition is polycrystalline MgB4 and thus the decomposition reaction of MgB2 can be described as 2MgB2(s) = MgB4(s) + Mg(g). First-principles calculations of the Mg1-x(VMg)xB2 electronic structure, within the supercell approach, show a small downshift of the Fermi level. Holes induced by the vacancies go to both sigma and pi bands. These small modifications are not expected to influence Tc, in agreement with observations. The significant reduction of the superconducting volume fraction without noticeable Tc reduction indicates the coexistence, within the same crystal, of superconductive and non-superconductive electronic phases, associated with regions rich and poor in Mg vacancies.
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              Measurement of the Penetration Depth and Coherence Length of MgB\(_\text{2}\) in All Directions Using Transmission Electron Microscopy

               ,  ,   (2015)
              We demonstrate that images of flux vortices in a superconductor taken with a transmission electron microscope can be used to measure the penetration depth and coherence length in all directions at the same temperature and magnetic field. This is particularly useful for MgB\(_2\), where these quantities vary with the applied magnetic field and values are difficult to obtain at low field or in the \(c\)-direction. We obtained images of flux vortices from an MgB\(_2\) single crystal cut in the \(ac\) plane by focussed ion beam milling and tilted to \(45^\circ\) with respect to the electron beam about its \(a\) axis. A new method was developed to simulate these images which accounted for vortices with a non-zero core in a thin, anisotropic superconductor and a simplex algorithm was used to make a quantitative comparison between the images and simulations to measure the penetration depths and coherence lengths. This gave penetration depths \(\Lambda_{ab}=100\pm 35\) nm and \(\Lambda_c=120\pm 15\) nm at 10.8 K in a field of 4.8 mT. The large error in \(\Lambda_{ab}\) is a consequence of tilting the sample about \(a\) and had it been tilted about \(c\), the errors would be reversed. Thus, obtaining the most precise values requires taking images of the flux lattice with the sample tilted in more than one direction. In a previous paper, we obtained a more precise value using a sample cut in the \(ab\) plane. Using this value gives \(\Lambda_{ab}=107\pm 8\) nm, \(\Lambda_c=120\pm 15\) nm, \(\xi_{ab}=39\pm 11\) nm and \(\xi_c=35\pm 10\) nm which agree well with measurements made using other techniques. The experiment required two days to conduct and does not require large-scale facilities. It was performed on a very small sample: \(30\times 15\) microns and 200 nm thick so this method could prove useful for characterising new superconductors where only small single crystals are available.
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                Author and article information

                Journal
                23 May 2018
                Article
                1805.09240

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

                Custom metadata
                cond-mat.supr-con

                Condensed matter

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