T. Shimojima

Giant Rashba-type spin splitting in bulk BiTeI

There has been increasing interest in phenomena emerging from relativistic electrons in a solid, which have a potential impact on spintronics and magnetoelectrics. One example is the Rashba effect, which lifts the electron-spin degeneracy as a consequence of spin-orbit interaction under broken inversion symmetry. A high-energy-scale Rashba spin splitting is highly desirable for enhancing the coupling between electron spins and electricity relevant for spintronic functions. Here we describe the finding of a huge spin-orbit interaction effect in a polar semiconductor composed of heavy elements, BiTeI, where the bulk carriers are ruled by large Rashba-like spin splitting. The band splitting and its spin polarization obtained by spin- and angle-resolved photoemission spectroscopy are well in accord with relativistic first-principles calculations, confirming that the spin splitting is indeed derived from bulk atomic configurations. Together with the feasibility of carrier-doping control, the giant-Rashba semiconductor BiTeI possesses excellent potential for application to various spin-dependent electronic functions.

Redox Control and High Conductivity of Nickel Bis(dithiolene) Complex π‑Nanosheet: A Potential Organic Two-Dimensional Topological Insulator

A bulk material comprising stacked nanosheets of nickel bis(dithiolene) complexes is investigated. The average oxidation number is -3/4 for each complex unit in the as-prepared sample; oxidation or reduction respectively can change this to 0 or -1. Refined electrical conductivity measurement, involving a single microflake sample being subjected to the van der Pauw method under scanning electron microscopy control, reveals a conductivity of 1.6 × 10(2) S cm(-1), which is remarkably high for a coordination polymeric material. Conductivity is also noted to modulate with the change of oxidation state. Theoretical calculation and photoelectron emission spectroscopy reveal the stacked nanosheets to have a metallic nature. This work provides a foothold for the development of the first organic-based two-dimensional topological insulator, which will require the precise control of the oxidation state in the single-layer nickel bisdithiolene complex nanosheet (cf. Liu, F. et al. Nano Lett. 2013, 13, 2842).

Octet-Line Node Structure of Superconducting Order Parameter in KFe2As2

An Eight-Noded Monster In superconductors, electrons are bound into pairs, and the exact form of that pairing and the resulting energy gap can vary, depending on the details of the electron-electron interaction and the band structure of the material. The energy gaps of the recently discovered iron-based superconductors exhibit a variety of pairing functions. KFe2As2 has been suggested to have a d-wave gap, similar to cuprate superconductors. Okazaki et al. (p. 1314) use laser-based angle-resolved photoemission spectroscopy (ARPES) to map out the superconducting gap on three Fermi surfaces (FS) of the compound. They find a different gap structure on each, with the middle FS gap vanishing at eight distinct positions (nodes). It appears that the gap respects the tetragonal symmetry of the crystal, indicating (although the details may vary) the all iron-based superconductors have an extended s-wave–symmetric pairing—a finding that will help understanding of unconventional superconductivity. Laser-based photoemission spectroscopy is used to map out the pairing gap of an iron-based superconductor. In iron-pnictide superconductivity, the interband interaction between the hole and electron Fermi surfaces (FSs) is believed to play an important role. However, KFe2As2 has three zone-centered hole FSs and no electron FS but still exhibits superconductivity. Our ultrahigh-resolution laser angle-resolved photoemission spectroscopy unveils that KFe2As2 is a nodal s-wave superconductor with highly unusual FS-selective multi-gap structure: a nodeless gap on the inner FS, an unconventional gap with “octet-line nodes” on the middle FS, and an almost-zero gap on the outer FS. This gap structure may arise from the frustration between competing pairing interactions on the hole FSs causing the eightfold sign reversal. Our results suggest that the A1g superconducting symmetry is universal in iron-pnictides, in spite of the variety of gap functions.

Orbital-dependent modifications of electronic structure across the magnetostructural transition in BaFe2As2.

Laser angle-resolved photoemission spectroscopy (ARPES) is employed to investigate the temperature (T) dependence of the electronic structure in BaFe2As2 across the magnetostructural transition at T{N} approximately 140 K. A drastic transformation in Fermi surface (FS) shape across T{N} is observed, as expected by first-principles band calculations. Polarization-dependent ARPES and band calculations consistently indicate that the observed FSs at k{z} approximately pi in the low-T antiferromagnetic state are dominated by the Fe3d{zx} orbital, leading to the twofold electronic structure. These results indicate that magnetostructural transition in BaFe2As2 accompanies orbital-dependent modifications in the electronic structure.

A versatile system for ultrahigh resolution, low temperature, and polarization dependent Laser-angle-resolved photoemission spectroscopy

We have developed a low temperature ultrahigh resolution system for polarization dependent angle-resolved photoemission spectroscopy (ARPES) using a vacuum ultraviolet (vuv) laser (hnu=6.994 eV) as a photon source. With the aim of addressing low energy physics, we show the system performance with angle-integrated PES at the highest energy resolution of 360 mueV and the lowest temperature of 2.9 K. We describe the importance of a multiple-thermal-shield design for achieving the low temperature, which allows a clear measurement of the superconducting gap of tantalum metal with a T(c)=4.5 K. The unique specifications and quality of the laser source (narrow linewidth of 260 mueV, high photon flux), combined with a half-wave plate, facilitates ultrahigh energy and momentum resolution polarization dependent ARPES. We demonstrate the use of s- and p-polarized laser-ARPESs in studying the superconducting gap on bilayer-split bands of a high T(c) cuprate. The unique features of the quasi-continuous-wave vuv laser and low temperature enables ultrahigh-energy and -momentum resolution studies of the spectral function of a solid with large escape depth. We hope the present work helps in defining polarization dependent laser excited angle-resolved photoemission spectroscopy as a frontier tool for the study of electronic structure and properties of materials at the sub-meV energy scale.

Orbital-Independent Superconducting Gaps in Iron Pnictides

Bulk photoemission studies of iron pnictides suggest a role for orbital fluctuations in creating the superconducting state. The origin of superconductivity in the iron pnictides has been attributed to antiferromagnetic spin ordering that occurs in close combination with a structural transition, but there are also proposals that link superconductivity to orbital ordering. We used bulk-sensitive laser angle–resolved photoemission spectroscopy on BaFe2(As0.65P0.35)2 and Ba0.6K0.4Fe2As2 to elucidate the role of orbital degrees of freedom on the electron-pairing mechanism. In strong contrast to previous studies, an orbital-independent superconducting gap magnitude was found for the hole Fermi surfaces. Our result is not expected from the superconductivity associated with spin fluctuations and nesting, but it could be better explained invoking magnetism-induced interorbital pairing, orbital fluctuations, or a combination of orbital and spin fluctuations. Regardless of the interpretation, our results impose severe constraints on theories of iron pnictides.

Strongly spin-orbit coupled two-dimensional electron gas emerging near the surface of polar semiconductors.

We investigate the two-dimensional highly spin-polarized electron accumulation layers commonly appearing near the surface of n-type polar semiconductors BiTeX (X=I, Br, and Cl) by angular-resolved photoemission spectroscopy. Because of the polarity and the strong spin-orbit interaction built in the bulk atomic configurations, the quantized conduction-band subbands show giant Rashba-type spin splitting. The characteristic 2D confinement effect is clearly observed also in the valence bands down to the binding energy of 4 eV. The X-dependent Rashba spin-orbit coupling is directly estimated from the observed spin-split subbands, which roughly scales with the inverse of the band-gap size in BiTeX.

Lifting ofxz/yzorbital degeneracy at the structural transition in detwinned FeSe

We study superconducting FeSe (Tc = 9 K) exhibiting the tetragonal-orthorhombic structural transition (Ts = 90 K) without any antiferromagnetic ordering, by utilizing angle-resolved photoemission spectroscopy. In the detwinned orthorhombic state, the energy position of the dyz orbital band at the Brillouin zone corner is 50 meV higher than that of dxz, indicating the orbital order similar to NaFeAs and BaFe2As2 families. Evidence of orbital order also appears in the hole bands at the Brillouin zone center. Precisely measured temperature dependence using strain-free samples shows that the onset of the orbital ordering (To) occurs very close to Ts, thus suggesting that the electronic nematicity above Ts is considerably weaker in FeSe compared to BaFe2As2 family.

Momentum-dependent sign inversion of orbital order in superconducting FeSe

We investigate the electronic reconstruction across the tetragonal-orthorhombic structural transition in FeSe by employing polarization-dependent angle-resolved photoemission spectroscopy on detwinned single crystals. Across the structural transition, the electronic structures around the

Superconductivity in an electron band just above the Fermi level: possible route to BCS-BEC superconductivity

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