Masashi Nakatake

Evidence of Topological Surface State in Three-Dimensional Dirac Semimetal Cd3As2

The three-dimensional topological semimetals represent a new quantum state of matter. Distinct from the surface state in the topological insulators that exhibits linear dispersion in two-dimensional momentum plane, the three-dimensional semimetals host bulk band dispersions linearly along all directions. In addition to the gapless points in the bulk, the three-dimensional Weyl/Dirac semimetals are also characterized by “topologically protected” surface state with Fermi arcs on their surface. While Cd3As2 is proposed to be a viable candidate of a Dirac semimetal, more investigations are necessary to pin down its nature. In particular, the topological surface state, the hallmark of the three-dimensional semimetal, has not been observed in Cd3As2. Here we report the electronic structure of Cd3As2 investigated by angle-resolved photoemission measurements on the (112) crystal surface and detailed band structure calculations. The measured Fermi surface and band structure show a good agreement with the band structure calculations with two bulk Dirac-like bands approaching the Fermi level and forming Dirac points near the Brillouin zone center. Moreover, the topological surface state with a linear dispersion approaching the Fermi level is identified for the first time. These results provide experimental indications on the nature of topologically non-trivial three-dimensional Dirac cones in Cd3As2.

Surface Scattering via Bulk Continuum States in the 3D Topological Insulator Bi2Se3

We have performed scanning tunneling microscopy and differential tunneling conductance (dI/dV) mapping for the surface of the three-dimensional topological insulator Bi(2)Se(3). The fast Fourier transformation applied to the dI/dV image shows an electron interference pattern near Dirac node despite the general belief that the backscattering is well suppressed in the bulk energy gap region. The comparison of the present experimental result with theoretical surface and bulk band structures shows that the electron interference occurs through the scattering between the surface states near the Dirac node and the bulk continuum states.

High-resolution band-pass photon detector for inverse-photoemission spectroscopy

Abstract All solid state band-pass photon detectors for IPES have been developed. The detectors are composed of an optical filter for vacuum ultraviolet and a solar blind photomultiplier. A high resolution (∼0.3eV) and a high sensitivity were simultaneously realized by the best combination of elements, which are a KCl thin film, a CaF2 window and a solar-blind photomultiplier with a KCl deposited first dynode.

Unoccupied topological surface state in Bi2Te2Se

This work was financially supported by KAKENHI (Grants No. 20340092 and No. 23340105), a Grant-in-Aid for Scientific Research (B) of JSPS, and RFBR Research Project No. 13-02-92105 a. The authors acknowledge partial support from the Spanish Ministerio de Ciencia e Innovacion (Grant No. FIS2010-19609-C02-02).

PHOTOEMISSION AND INVERSE-PHOTOEMISSION STUDIES OF BI2Y3 (Y=S, SE, TE) SEMICONDUCTORS

Abstract Valence-band and conduction-band densities of states (DOSs) of Bi 2 Y 3 ( Y =S, Se, Te) semiconductors have been investigated by means of ultraviolet photoemission (UPS) and inverse-photoemission spectroscopy (IPES). With the aid of calculated partial density of states for Bi 2 Te 3 and the UPS and IPES spectra for Bi 2 Te 2 Se crystals grown by substituting Te(2) atoms in Bi 2 Te 3 with Se atoms, we assign the partial DOSs for Bi 2 Te 3 and Bi 2 Se 3 . Furthermore, it is found that Bi 2 S 3 has the largest band gap (1.1 eV) among these three Bi 2 Y 3 compounds.

A study of electronic states of trigonal and amorphous Se using ultraviolet photoemission and inverse-photoemission spectroscopies

Valence-band and conduction-band densities of states (DOSs) of trigonal and amorphous Se have been investigated by in situ measurements of ultraviolet photoemission and inverse-photoemission spectroscopies (UPS and IPES), respectively. The IPES spectra reveal, for the first time, a measure of conduction-band DOS, while the UPS spectra are fully consistent with earlier results. From comparison between theoretical DOS and a whole spectrum for trigonal Se obtained by a connection of the UPS and IPES spectra at the Fermi level, it has been shown that the coupling strength between the hybrid 4p-like orbitals of different atoms should be increased. On the basis of a comparison between IPES and core-absorption spectra for trigonal Se, a prominent peak and weak structures in the 1-5 and 5-10 eV regions above the valence-band maximum in the IPES spectrum are experimentally assigned to the 4p-like antibonding and 4d and/or 5s states, respectively. In amorphous Se, the peak at shallower energy in the 4p-like bonding bands with two peak structures is more intense in contrast to the case of the trigonal one. Such a change is attributed to a chain structure with the dihedral angle varying randomly in sign along the chain. Increases of the bonding-antibonding splitting energy of the 4p-like bands and band-gap energy in amorphous Se are ascribed to an increase of intrachain coupling strength between the hybrid 4p-like orbitals and a simultaneous decrease of the interchain coupling strength.

Resonant photoemission spectroscopy in the Mn 2p-3d excitation region of NiAs-type MnTe

Resonant photoemission experiments in the Mn 2 p -3 d excitation region of NiAs-type MnTe have been performed. The Mn 3 d , 3 p and 3 s photoemission intensities have shown remarkable resonant enhancements near the Mn 2 p -3 d threshold. Their resonant behaviors are compared with the theoretical spectra on the basis of a cluster model taking account of intraaomic multipole interactions for the Mn 3 d -3 d and 3 d -core hole orbitals, and the hybridization between the Mn 3 d and ligand p -orbitals. Calculated spectra are in qualitative agreement with experimental results except for a broad structure around 20 eV below the valence-band maximum.

The gigantic Rashba effect of surface states energetically buried in the topological insulator Bi2Te2Se

We have clarified that a topological insulator, Bi2Te2Se, shows two surface states with gigantic Rashba-type spin-splitting located at a binding energy deeper than the topological surface state. The magnitude of the Rashba parameter, as well as the momentum splitting, is found to be large enough to realize a number of nanometer-sized spintronic devices. This novel finding paves the way to studies of gigantic Rashba systems that are suitable for future spintronic applications.

Experimental verification of the surface termination in the topological insulator TlBiSe2using core-level photoelectron spectroscopy and scanning tunneling microscopy

The surface termination of the promising topological insulator TlBiSe

HIGH-RESOLUTION PHOTON DETECTION SYSTEM FOR INVERSE-PHOTOEMISSION SPECTROSCOPY

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