Shik Shin

Bio: Shik Shin is an academic researcher from University of Tokyo. The author has contributed to research in topics: Angle-resolved photoemission spectroscopy & Photoemission spectroscopy. The author has an hindex of 63, co-authored 706 publications receiving 18481 citations. Previous affiliations of Shik Shin include Synchrotron Radiation Center & National Institute for Materials Science.

Giant Rashba-type spin splitting in bulk BiTeI

Abstract: A very large Rashba-type spin splitting, which is a consequence of spin–orbit interaction, has been observed in the heavy-element semiconductor BiTeI. The results show the possibility, in principle, of using the material in spintronics devices in which the electron spin is controlled by electric currents.

Resonant inelastic x-ray scattering spectra for electrons in solids

Abstract: conservation are discussed. At the opposite extreme are rare-earth systems (metals and oxides), in which the 4f electrons are almost localized with strong electron correlation. The observations are interpreted based on the effects of intra-atomic multiplet coupling and weak interatomic electron transfer, which are well described with an Anderson impurity model or a cluster model. In this context a narrowing of spectral width in the excitation spectrum, polarization dependence, and the magnetic circular dichroism in ferromagnetic materials are discussed. The authors then consider transition-metal compounds, materials with electron correlation strengths intermediate between semiconductors and rare-earth systems. In these interesting cases there is an interplay of intra-atomic and interatomic electronic interactions that leads to limitations of both the band model and the Anderson impurity model. Finally, other topics in resonant x-ray emission studies of solids are described briefly.

The inhomogeneous structure of water at ambient conditions

Abstract: Small-angle X-ray scattering (SAXS) is used to demonstrate the presence of density fluctuations in ambient water on a physical length-scale of ≈1 nm; this is retained with decreasing temperature while the magnitude is enhanced. In contrast, the magnitude of fluctuations in a normal liquid, such as CCl4, exhibits no enhancement with decreasing temperature, as is also the case for water from molecular dynamics simulations under ambient conditions. Based on X-ray emission spectroscopy and X-ray Raman scattering data we propose that the density difference contrast in SAXS is due to fluctuations between tetrahedral-like and hydrogen-bond distorted structures related to, respectively, low and high density water. We combine our experimental observations to propose a model of water as a temperature-dependent, fluctuating equilibrium between the two types of local structures driven by incommensurate requirements for minimizing enthalpy (strong near-tetrahedral hydrogen-bonds) and maximizing entropy (nondirectional H-bonds and disorder). The present results provide experimental evidence that the extreme differences anticipated in the hydrogen-bonding environment in the deeply supercooled regime surprisingly remain in bulk water even at conditions ranging from ambient up to close to the boiling point.

Evidence for magnetic Weyl fermions in a correlated metal

Abstract: Experimental evidence for the realization of magnetic Weyl fermions in the strongly correlated metal Mn3Sn is reported.

Vacuum-ultraviolet reflectance and photoemission study of the metal-insulator phase transitions in VO 2 , V 6 O 13 , and V 2 O 3

Abstract: Vacuum-ultraviolet reflectance and photoemission spectra of ${\mathrm{VO}}_{2}$, ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{V}}_{6}$${\mathrm{O}}_{13}$, and ${\mathrm{V}}_{2}$${\mathrm{O}}_{5}$ are measured in order to investigate the 3d-band structures and electron-correlation effects. In the case of ${\mathrm{VO}}_{2}$, drastic changes in the 3d (${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ and ${\mathit{d}}_{\mathrm{?}}$) -band structures are found in both spectra through the metal-insulator phase transition. The ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ and ${\mathit{d}}_{\mathrm{?}}$ bands are found at the Fermi level and ${\mathit{E}}_{\mathit{B}}$=1.3 eV in the photoemission spectra of metallic ${\mathrm{VO}}_{2}$. In the insulating phase, the ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ valence band in the photoemission spectra becomes empty and a rise of the ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ conduction band by about 0.5 eV is found in the reflectance spectra. This band shift through the phase transition may be a driving force of the metal-insulator transition of ${\mathrm{VO}}_{2}$. The optical band gap between the ${\mathit{d}}_{\mathrm{?}}$ valence and ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ conduction bands is obtained as 0.7 eV in the insulating phase, to which the ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$-${\mathit{d}}_{\mathrm{?}}$ correlation energy contributes partially.The splitting of the ${\mathit{d}}_{\mathrm{?}}$ band is also found in the insulating phase. This splitting energy is about 2.5 eV, while the bandwidth of the ${\mathit{d}}_{\mathrm{?}}$ band is about 1.5 eV. This large band splitting is mainly due to the correlation energy of ${\mathit{d}}_{\mathrm{?}}$ electrons of about U(${\mathit{d}}_{\mathrm{?}}$,${\mathit{d}}_{\mathrm{?}}$)=2.1 eV. On the other hand, no drastic change is found in the 3d-band structures of ${\mathrm{V}}_{6}$${\mathrm{O}}_{13}$ and ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$ except for slight changes in the bandwidths. Furthermore, the density of states at the Fermi level is rather low in these materials even in the metallic phase. These facts support the view that the electron-correlation effects are important and Mott-type metal-insulator transitions are induced in ${\mathrm{V}}_{6}$${\mathrm{O}}_{13}$ and ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$. Resonant photoemission from V3d and O2p bands are observed as the photon energy is swept through the 3p\ensuremath{\rightarrow}3d optical-absorption transition. The resonance profiles of V3d bands show the characteristic antiresonance dip, while those of O2p bands show rather broad and simple enhancements. The multiplet structures in the 3p core photoemission spectra of ${\mathrm{VO}}_{2}$ and ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$ are quantitatively analyzed with use of results from the calculations of Yamaguchi et al.

Phd by thesis

Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Weyl and Dirac semimetals in three-dimensional solids

Abstract: Weyl and Dirac semimetals are three-dimensional phases of matter with gapless electronic excitations that are protected by topology and symmetry. As three-dimensional analogs of graphene, they have generated much recent interest. Deep connections exist with particle physics models of relativistic chiral fermions, and, despite their gaplessness, to solid-state topological and Chern insulators. Their characteristic electronic properties lead to protected surface states and novel responses to applied electric and magnetic fields. The theoretical foundations of these phases, their proposed realizations in solid-state systems, and recent experiments on candidate materials as well as their relation to other states of matter are reviewed.

Electronic excitations: density-functional versus many-body Green's-function approaches

Abstract: Electronic excitations lie at the origin of most of the commonly measured spectra. However, the first-principles computation of excited states requires a larger effort than ground-state calculations, which can be very efficiently carried out within density-functional theory. On the other hand, two theoretical and computational tools have come to prominence for the description of electronic excitations. One of them, many-body perturbation theory, is based on a set of Green’s-function equations, starting with a one-electron propagator and considering the electron-hole Green’s function for the response. Key ingredients are the electron’s self-energy S and the electron-hole interaction. A good approximation for S is obtained with Hedin’s GW approach, using density-functional theory as a zero-order solution. First-principles GW calculations for real systems have been successfully carried out since the 1980s. Similarly, the electron-hole interaction is well described by the Bethe-Salpeter equation, via a functional derivative of S. An alternative approach to calculating electronic excitations is the time-dependent density-functional theory (TDDFT), which offers the important practical advantage of a dependence on density rather than on multivariable Green’s functions. This approach leads to a screening equation similar to the Bethe-Salpeter one, but with a two-point, rather than a four-point, interaction kernel. At present, the simple adiabatic local-density approximation has given promising results for finite systems, but has significant deficiencies in the description of absorption spectra in solids, leading to wrong excitation energies, the absence of bound excitonic states, and appreciable distortions of the spectral line shapes. The search for improved TDDFT potentials and kernels is hence a subject of increasing interest. It can be addressed within the framework of many-body perturbation theory: in fact, both the Green’s functions and the TDDFT approaches profit from mutual insight. This review compares the theoretical and practical aspects of the two approaches and their specific numerical implementations, and presents an overview of accomplishments and work in progress.

Topological properties and dynamics of magnetic skyrmions

Abstract: Magnetic skyrmions are particle-like nanometre-sized spin textures of topological origin found in several magnetic materials, and are characterized by a long lifetime. Skyrmions have been observed both by means of neutron scattering in momentum space and microscopy techniques in real space, and their properties include novel Hall effects, current-driven motion with ultralow current density and multiferroic behaviour. These properties can be understood from a unified viewpoint, namely the emergent electromagnetism associated with the non-coplanar spin structure of skyrmions. From this description, potential applications of skyrmions as information carriers in magnetic information storage and processing devices are envisaged.