Katsuaki Sugawara

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.

High-quality three-dimensional nanoporous graphene.

We report three-dimensional (3D) nanoporous graphene with preserved 2D electronic properties, tunable pore sizes, and high electron mobility for electronic applications. The complex 3D network comprised of interconnected graphene retains a 2D coherent electron system of massless Dirac fermions. The transport properties of the nanoporous graphene show a semiconducting behavior and strong pore-size dependence, together with unique angular independence. The free-standing, large-scale nanoporous graphene with 2D electronic properties and high electron mobility holds great promise for practical applications in 3D electronic devices.

High-temperature superconductivity in potassium-coated multilayer FeSe thin films

The recent discovery of possible high-temperature (T(c)) superconductivity over 65 K in a monolayer FeSe film on SrTiO3 (refs 1-6) triggered a fierce debate on how superconductivity evolves from bulk to film, because bulk FeSe crystal exhibits a T(c) of no higher than 10 K (ref. 7). However, the difficulty in controlling the carrier density and the number of FeSe layers has hindered elucidation of this problem. Here, we demonstrate that deposition of potassium onto FeSe films markedly expands the accessible doping range towards the heavily electron-doped region. Intriguingly, we have succeeded in converting non-superconducting films with various thicknesses into superconductors with T(c) as high as 48 K. We also found a marked increase in the magnitude of the superconducting gap on decreasing the FeSe film thickness, indicating that the interface plays a crucial role in realizing the high-temperature superconductivity. The results presented provide a new strategy to enhance and optimize T(c) in ultrathin films of iron-based superconductors.

Superconducting Calcium-Intercalated Bilayer Graphene

We report the direct evidence for superconductivity in Ca-intercalated bilayer graphene C6CaC6, which is regarded as the thinnest limit of Ca-intercalated graphite. We performed the electrical transport measurements with the in situ 4-point-probe method in ultrahigh vacuum under zero- or nonzero-magnetic field for pristine bilayer graphene, Li-intercalated bilayer graphene (C6LiC6) and C6CaC6 fabricated on SiC substrate. We observed that the zero-resistance state occurs in C6CaC6 with the onset temperature (T(c)(onset)) of 4 K, while the T(c)(onset) is gradually decreased upon applying the magnetic field. This directly proves the superconductivity origin of the zero resistance in C6CaC6. On the other hand, both pristine bilayer graphene and C6LiC6 exhibit nonsuperconducting behavior, suggesting the importance of intercalated atoms and its species to drive the superconductivity.We report the superconductivity in Ca-intercalated bilayer graphene C6CaC6, the thinnest limit of Ca graphite intercalation compound. We performed in situ electrical transport measurements on pristine bilayer graphene, C6LiC6 and C6CaC6 fabricated on SiC substrate under zero and nonzero magnetic field. While both bilayer graphene and C6LiC6 show non-superconducting behavior, C6CaC6 exhibits the superconductivity with transition temperature (Tc) of 4.0 K. The observed Tc in C6CaC6 and the absence of superconductivity in C6LiC6 show a good agreement with the theoretical prediction, suggesting the importance of a free-electron-like metallic band at the Fermi level to drive the superconductivity.

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

High-resolution angle-resolved photoemission spectroscopy has been performed on Sb(111) to elucidate the origin of anomalous electronic properties in group-V semimetal surfaces. The surface was found to be metallic despite the semimetallic character of bulk. We clearly observed two surface-derived Fermi surfaces which are likely spin split, demonstrating that the spin-orbit interaction plays a dominant role in characterizing the surface electronic states of group-V semimetals. The universality or dissimilarity of the electronic structure in Bi and Sb is discussed in relation to the granular superconductivity, electron-phonon coupling, and surface charge or spin density wave.

Superconducting gap and pseudogap in iron-based layered superconductor La(O1-xFx)FeAs

We report high-resolution photoemission spectroscopy of newly-discovered iron-based layered superconductor La(O 0.93 F 0.07 )FeAs ( T c = 24 K). We found that the superconducting gap shows a marked deviation from the isotropic s -wave symmetry. The estimated gap size at 5 K is 3.6 meV in the s - or axial p -wave case, while it is 4.1 meV in the polar p - or d -wave case. We also found a pseudogap of 15–20 meV above T c , which is gradually filled-in with increasing temperature and closes at temperature far above T c similarly to copper-oxide high-temperature superconductors.

Fabrication of Li-intercalated bilayer graphene

We have succeeded in fabricating Li-intercalated bilayer graphene on silicon carbide. The low-energy electron diffraction from Li-deposited bilayer graphene shows a sharp 3×3R30° pattern in contrast to Li-deposited monolayer graphene. This indicates that Li atoms are intercalated between two adjacent graphene layers and take the same well-ordered superstructure as in bulk C6Li. The angle-resolved photoemission spectroscopy has revealed that Li atoms are fully ionized and the π bands of graphene are systematically folded by the superstructure of intercalated Li atoms, producing a snowflake-like Fermi surface centered at the Γ point. The present result suggests a high potential of Li-intercalated bilayer graphene for application to a nano-scale Li-ion battery.

Ca intercalated bilayer graphene as a thinnest limit of superconducting C6Ca

Success in isolating a 2D graphene sheet from bulky graphite has triggered intensive studies of its physical properties as well as its application in devices. Graphite intercalation compounds (GICs) have provided a platform of exotic quantum phenomena such as superconductivity, but it is unclear whether such intercalation is feasible in the thinnest 2D limit (i.e., bilayer graphene). Here we report a unique experimental realization of 2D GIC, by fabricating calcium-intercalated bilayer graphene C6CaC6 on silicon carbide. We have investigated the structure and electronic states by scanning tunneling microscopy and angle-resolved photoemission spectroscopy. We observed a free-electron–like interlayer band at the Brillouin-zone center, which is thought to be responsible for the superconductivity in 3D GICs, in addition to a large π* Fermi surface at the zone boundary. The present success in fabricating Ca-intercalated bilayer graphene would open a promising route to search for other 2D superconductors as well as to explore its application in devices.

Direct Observation of Dirac Cone in Multilayer Silicene Intercalation Compound CaSi2

Calcium-intercalated multilayer silicene CaSi2 exhibits a massless Dirac-cone π-electron-band dispersion like graphene, while the Dirac point is about 2 eV away from the Fermi level due to diiimide-based charge transfer from the Ca atoms to the silicene layers. This indicates that the graphene-like electronic structure with a massless Dirac cone is stably formed in the metal-intercalated multilayer silicene.

Unconventional Charge-Density-Wave Transition in Monolayer 1T-TiSe2.

Reducing the dimension in materials sometimes leads to unexpected discovery of exotic and/or pronounced physical properties such as quantum Hall effect in graphene and high-temperature superconductivity in iron-chalcogenide atomically thin films. Transition-metal dichalcogenides (TMDs) provide a fertile ground for studying the interplay between dimensionality and electronic properties, since they exhibit a variety of electronic phases like semiconducting, superconducting, and charge-density-wave (CDW) states. Among TMDs, bulk 1T-TiSe2 has been a target of intensive studies due to its unusual CDW properties with the periodic lattice distortions characterized by the three-dimensional (3D) commensurate wave vector. Clarifying the ground states of its two-dimensional (2D) counterpart is of great importance not only to pin down the origin of CDW, but also to find unconventional physical properties characteristic of atomic-layer materials. Here, we show the first experimental evidence for the realization of 2D CDW phase without Fermi-surface nesting in monolayer 1T-TiSe2. Our angle-resolved photoemission spectroscopy (ARPES) signifies an electron pocket at the Brillouin-zone corner above the CDW-transition temperature (TCDW ∼ 200 K), while, below TCDW, an additional electron pocket and replica bands appear at the Brillouin-zone center and corner, respectively, due to the back-folding of bands by the 2 × 2 superstructure potential. Similarity in the spectral signatures to bulk 1T-TiSe2 implies a common driving force of CDW, i.e., exciton condensation, whereas the larger energy gap below TCDW in monolayer 1T-TiSe2 suggests enhancement of electron-hole coupling upon reducing dimensionality. The present result lays the foundation for the electronic-structure engineering based with atomic-layer TMDs.