Akari Takayama

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.

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

We have performed spin- and angle-resolved photoemission spectroscopy of Bi(2)Te(3) and present the first direct evidence for the existence of the out-of-plane spin component on the surface state of a topological insulator. We found that the magnitude of the out-of-plane spin polarization on a hexagonally deformed Fermi surface of Bi(2)Te(3) reaches maximally 25% of the in-plane counterpart, while such a sizable out-of-plane spin component does not exist in the more circular Fermi surface of TlBiSe(2), indicating that the hexagonal deformation of the Fermi surface is responsible for the deviation from the ideal helical spin texture. The observed out-of-plane polarization is much smaller than that expected from the existing theory, suggesting that an additional ingredient is necessary for correctly understanding the surface spin polarization in Bi(2)Te(3).

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.

Tunable Spin Polarization in Bismuth Ultrathin Film on Si(111)

We performed a spin- and angle-resolved photoemission spectroscopy of bismuth ultrathin film on Si(111) with various film thickness d. We found that the spin polarization of spin-split Rashba surface states near the Brillouin-zone boundary, which is high (0.7) at d = 40 BL (bilayers), is gradually reduced on decreasing d and almost vanishes at d = 8 BL. This finding provides a novel method to generate spin-polarized electrons with tunable spin-polarization.

Two-Dimensional Superconductor with a Giant Rashba Effect: One-Atom-Layer Tl-Pb Compound on Si(111).

A one-atom-layer compound made of one monolayer of Tl and one-third monolayer of Pb on a Si(111) surface having √3×√3 periodicity was found to exhibit a giant Rashba-type spin splitting of metallic surface-state bands together with two-dimensional superconducting transport properties. Temperature-dependent angle-resolved photoelectron spectroscopy revealed an enhanced electron-phonon coupling for one of the spin-split bands. In situ micro-four-point-probe conductivity measurements with and without magnetic field demonstrated that the (Tl, Pb)/Si(111) system transformed into the superconducting state at 2.25 K, followed by the Berezinskii-Kosterlitz-Thouless mechanism. The 2D Tl-Pb compound on Si(111) is believed to be the prototypical object for prospective studies of intriguing properties of the superconducting 2D system with lifted spin degeneracy, bearing in mind that its composition, atomic and electron band structures, and spin texture are already well established.

Relationship between Fermi surface warping and out-of-plane spin polarization in topological insulators: A view from spin- and angle-resolved photoemission

We have performed spin- and angle-resolved photoemission spectroscopy of the topological insulator Pb(Bi,Sb)2Te4 (Pb124) and observed significant out-of-plane spin polarization on the hexagonally warped Dirac-cone surface state. To put this into context, we carried out quantitative analysis of the warping strengths for various topological insulators (Pb124, Bi2Te3, Bi2Se3, and TlBiSe2) and elucidated that the out-of-plane spin polarization Pz is systematically correlated with the warping strength. However, the magnitude of Pz is found to be only half of that expected from the kp theory when the warping is strong, which points to the possible role of many-body effects. Besides confirming a universal relationship between the spin polarization and the surface state structure, our data provide an empirical guiding principle for tuning the spin polarization in topological insulators.

Ultrahigh-resolution spin-resolved photoemission spectrometer with a mini Mott detector.

We have developed an ultrahigh-resolution spin-resolved photoemission spectrometer with a highly efficient mini Mott detector and an intense xenon plasma discharge lamp. The spectrometer achieves the energy resolutions of 0.9 and 8 meV for non-spin-resolved and spin-resolved modes, respectively. Three-dimensional spin-polarization is determined by using a 90° electron deflector situated before the Mott detector. The performance of spectrometer is demonstrated by observation of a clear Rashba splitting of the Bi(111) surface states.

Proving Nontrivial Topology of Pure Bismuth by Quantum Confinement

The topology of pure Bi is controversial because of its very small (∼10  meV) band gap. Here we perform high-resolution angle-resolved photoelectron spectroscopy measurements systematically on 14-202 bilayer Bi films. Using high-quality films, we succeed in observing quantized bulk bands with energy separations down to ∼10  meV. Detailed analyses on the phase shift of the confined wave functions precisely determine the surface and bulk electronic structures, which unambiguously show nontrivial topology. The present results not only prove the fundamental property of Bi but also introduce a capability of the quantum-confinement approach.

Spin polarization of gapped Dirac surface states near the topological phase transition in TlBi(S(1-x)Se(x))2.

We performed systematic spin- and angle-resolved photoemission spectroscopy of TlBi(S(1-x)Se(x))(2) which undergoes a topological phase transition at x ~ 0.5. In TlBiSe(2) (x = 1.0), we revealed a helical spin texture of Dirac-cone surface states with an intrinsic in-plane spin polarization of ~0.8. The spin polarization still survives in the gapped surface states at x > 0.5, although it gradually weakens upon approaching x = 0.5 and vanishes in the nontopological phase. No evidence for the out-of-plane spin polarization was found, irrespective of x and momentum. The present results unambiguously indicate the topological origin of the gapped Dirac surface states, and also impose a constraint on models to explain the origin of mass acquisition of Dirac fermions.

Topological proximity effect in a topological insulator hybrid

It is well known that a topologically protected gapless state appears at an interface between a topological insulator and an ordinary insulator; however, the physics of the interface between a topological insulator and a metal has largely been left unexplored. Here we report a novel phenomenon termed topological proximity effect, which occurs between a metallic ultrathin film and a three-dimensional topological insulator. We study one bilayer of bismuth metal grown on the three-dimensional topological insulator material TlBiSe2, and by using spin- and angle-resolved photoemission spectroscopy, we found evidence that the topological Dirac-cone state migrates from the surface of TlBiSe2 to the attached one-bilayer Bi. We show that such a migration of the topological state occurs as a result of strong spin-dependent hybridization of the wave functions at the interface, which is also supported by our first-principles calculations. This discovery points to a new route to manipulating the topological properties of materials.