S. Souma

Observation of Fermi-surface–dependent nodeless superconducting gaps in Ba0.6K0.4Fe2As2

We have performed a high-resolution angle-resolved photoelectron spectroscopy study on the newly discovered superconductor Ba0.6K0.4Fe2As2 (Tc=37 K). We have observed two superconducting gaps with different values: a large gap (Δ~12 meV) on the two small hole-like and electron-like Fermi surface (FS) sheets, and a small gap (~6 meV) on the large hole-like FS. Both gaps, closing simultaneously at the bulk transition temperature (Tc), are nodeless and nearly isotropic around their respective FS sheets. The isotropic pairing interactions are strongly orbital dependent, as the ratio 2Δ/kBTc switches from weak to strong coupling on different bands. The same and surprisingly large superconducting gap due to strong pairing on the two small FSs, which are connected by the (π, 0) spin-density-wave vector in the parent compound, strongly suggests that the pairing mechanism originates from the inter-band interactions between these two nested FS sheets.

Experimental realization of a topological crystalline insulator in SnTe

A topological insulator has surface metallic states that are topologically protected by time-reversal symmetry. Tin telluride is now shown to be a ‘topological crystalline insulator’, in which the surface metallic state is instead protected by the mirror symmetry of the crystal.

Tunable Dirac cone in the topological insulator Bi 2-x Sb x Te 3-y Se y

The three-dimensional topological insulator is a quantum state of matter characterized by an insulating bulk state and gapless Dirac cone surface states. Device applications of topological insulators require a highly insulating bulk and tunable Dirac carriers, which has so far been difficult to achieve. Here we demonstrate that Bi(2-x)Sb(x)Te(3-y)Se(y) is a system that simultaneously satisfies both of these requirements. For a series of compositions presenting bulk-insulating transport behaviour, angle-resolved photoemission spectroscopy reveals that the chemical potential is always located in the bulk band gap, whereas the Dirac cone dispersion changes systematically so that the Dirac point moves up in energy with increasing x, leading to a sign change of the Dirac carriers at x~0.9. Such a tunable Dirac cone opens a promising pathway to the development of novel devices based on topological insulators.

The origin of multiple superconducting gaps in MgB2

Magnesium diboride, MgB2, has the highest transition temperature (Tc = 39 K) of the known metallic superconductors. Whether the anomalously high Tc can be described within the conventional BCS (Bardeen–Cooper–Schrieffer) framework has been debated. The key to understanding superconductivity lies with the ‘superconducting energy gap’ associated with the formation of the superconducting pairs. Recently, the existence of two kinds of superconducting gaps in MgB2 has been suggested by several experiments; this is in contrast to both conventional and high-Tc superconductors. A clear demonstration of two gaps has not yet been made because the previous experiments lacked the ability to resolve the momentum of the superconducting electrons. Here we report direct experimental evidence for the two-band superconductivity in MgB2, by separately observing the superconducting gaps of the σ and π bands (as well as a surface band). The gaps have distinctly different sizes, which unambiguously establishes MgB2 as a two-gap superconductor.

Superconducting gap symmetry of Ba0.6K0.4Fe2As2 studied by angle-resolved photoemission spectroscopy

We have performed high-resolution angle-resolved photoemission spectroscopy on the optimally-doped Ba

Band Structure and Fermi Surface of an Extremely Overdoped Iron-Based Superconductor KFe2As2

_{0.6}

Direct measurement of the out-of-plane spin texture in the Dirac-cone surface state of a topological insulator.

K

Unexpected mass acquisition of Dirac fermions at the quantum phase transition of a topological insulator

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High-resolution photoemission study of MgB2.

Fe

Fermi surface and anisotropic spin-orbit coupling of Sb(111) studied by angle-resolved photoemission spectroscopy.

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