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Register a new userEffect of Fermi surface evolution on superconducting gap in superconducting topological insulator
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We study bulk electronic states of superconducting topological insulator, which is the promising candidate for topological superconductor. Recent experiments suggest that the three-dimensional Fermi surface evolves into two-dimensional one. We show that the superconducting energy gap structure on the Fermi surface systematically changes with this evolution. It is clarified that the bulk electronic properties such as spin-lattice relaxation rate and specific heat depend on the shape of the Fermi surface and the type of the energy gap function. These results serve as a guide to determine the pairing symmetry of Cu$_x$Bi$_2$Se$_3$.
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We study the surface state of a doped topological crystalline insulator in the superconducting state. Motivated by Sn$_{1-x}$In$_x$Te, we consider fully gapped pair potentials and calculate the surface spectral function. It is found that mirror-protected zero-energy Andreev bound states appear at the (001) surface and that these states can move along the mirror symmetric line on the surface Brillouin zone. We also show that the surface Andreev bound state changes systematically with doping due to the presence of the Dirac surface state in the normal state.
Proximity-effect-induced superconductivity was studied in epitaxial topological insulator Bi2Se3 thin films grown on superconducting NbSe2 single crystals. A point contact spectroscopy (PCS) method was used at low temperatures down to 40 mK. An induced superconducting gap in Bi2Se3 was observed in the spectra, which decreased with increasing Bi2Se3 layer thickness, consistent with the proximity effect in the bulk states of Bi2Se3 induced by NbSe2. At very low temperatures, an extra point contact feature which may correspond to a second energy gap appeared in the spectrum. For a 16 quintuple layer Bi2Se3 on NbSe2 sample, the bulk state gap value near the top surface is ~ 159 {mu}eV, while the second gap value is ~ 120 {mu}eV at 40 mK. The second gap value decreased with increasing Bi2Se3 layer thickness, but the ratio between the second gap and the bulk state gap remained about the same for different Bi2Se3 thicknesses. It is plausible that this is due to superconductivity in Bi2Se3 topological surface states induced through the bulk states. The two induced gaps in the PCS measurement are consistent with the three-dimensional bulk state and the two-dimensional surface state superconducting gaps observed in the angle-resolved photoemission spectroscopy (ARPES) measurement.
The electronic structure of nearly optimally-doped novel superconductor LaO$_{1-x}$F$_x$BiS$_2$ (${it x}$ = 0.46) was investigated using angle-resolved photoemission spectroscopy (ARPES). We clearly observed band dispersions from 2 to 6 eV binding energy and near the Fermi level (${it E}_{rm F}$), which are well reproduced by first principles calculations when the spin-orbit coupling is taken into account. The ARPES intensity map near ${it E}_{rm F}$ shows a square-like distribution around the $Gamma$(Z) point in addition to electronlike Fermi surface (FS) sheets around the X(R) point, indicating that FS of LaO$_{0.54}$F$_{0.46}$BiS$_2$ is in close proximity to the theoretically-predicted topological change.
We measured the Fermi surface (FS), band dispersion and superconducting gap in LuNi2B2C using Angle Resolved Photoemission Spectroscopy. Experimental data were compared with the tight-binding version of the Linear Muffin-Tin Orbital (LMTO) method and Linearized Augmented Plane-Wave (LAPW) calculations. We found reasonable agreement between the two calculations and experimental data. The measured FS exhibits large parallel regions with a nesting vector that agrees with a previous positron annihilation study and calculations of the generalized susceptibility. The measured dispersion curves also agree reasonably well with the TB-LMTO calculations, albeit with some differences in the strength of the hybridization. In addition, the spectrum in the superconducting state revealed a 2meV superconducting gap. The data also clearly shows the presence of a coherent peak above the chemical potential, that originates from thermally excited electrons above the energy of 2 delta. This feature was not previously observed in the Lu-based material.
We present a comprehensive study performed with high-resolution angle-resolved photoemission spectroscopy on triple-layered Bi2Sr2Ca2Cu3O10+d single crystals. By measurements above TC the Fermi surface topology defined by the Fermi level crossings of the CuO2-derived band was determined. A hole-like Fermi surface as for single and double-CuO2 layered Bi-based cuprates is found, giving new input to the current debate of the general Fermi surface topology of the high Tc superconductors. Furthermore, we present measurements of the superconducting gap of Bi-2223 and show that there are clear indications for a strong anisotropy of the superconducting gap. The universal properties of this phase in comparison to the other Bi-based cuprates will be discussed.
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