Rikiya Yoshida

Quadratic Fermi node in a 3D strongly correlated semimetal

Strong spin–orbit coupling fosters exotic electronic states such as topological insulators and superconductors, but the combination of strong spin–orbit and strong electron–electron interactions is just beginning to be understood. Central to this emerging area are the 5d transition metal iridium oxides. Here, in the pyrochlore iridate Pr2Ir2O7, we identify a non-trivial state with a single-point Fermi node protected by cubic and time-reversal symmetries, using a combination of angle-resolved photoemission spectroscopy and first-principles calculations. Owing to its quadratic dispersion, the unique coincidence of four degenerate states at the Fermi energy, and strong Coulomb interactions, non-Fermi liquid behaviour is predicted, for which we observe some evidence. Our discovery implies that Pr2Ir2O7 is a parent state that can be manipulated to produce other strongly correlated topological phases, such as topological Mott insulator, Weyl semimetal, and quantum spin and anomalous Hall states.

Single-scattering spectroscopy for extremely dense colloidal suspensions by use of a low-coherence interferometer

Single-scattering spectroscopy by use of a low-coherence interferometer is introduced to measure the power spectra of light scattered from extremely dense colloidal suspensions. The power spectrum of a heterodyne component can be obtained by subtraction of the power spectrum of a homodyne component from the measured power spectrum. The heterodyne power spectrum for light scattered from the medium is shown to coincide with the single-scattering spectrum to a depth of up to a few times the mean-free path length. Therefore single-scattering spectroscopy is newly proposed as a means by which to analyze the characteristics of extremely dense colloidal suspensions.

Electronic structure of superconducting FeSe studied by high-resolution photoemission spectroscopy

We have performed soft x-ray and ultrahigh-resolution laser-excited photoemission measurements on tetragonal FeSe, which was recently identified as a superconductor. Energy dependent study of valence band is compared to band structure calculations and yields a reasonable assignment of partial densities of states. However, the sharp peak near the Fermi level slightly deviates from the calculated energy position, giving rise to the necessity of self-energy correction. We have also performed ultrahigh-resolution laser photoemission experiment on FeSe and observed the suppression of intensity around the Fermi level upon cooling.

Translational Symmetry Breaking and Gapping of Heavy-Quasiparticle Pocket in URu2Si2

URu2Si2 is a uranium compound that exhibits a so-called ‘hidden-order’ transition at ~17.5 K. However, the order parameter of this second-order transition as well as many of its microscopic properties remain unclarified despite considerable research. One of the key questions in this regard concerns the type of spontaneous symmetry breaking occurring at the transition; although rotational symmetry breaking has been detected, it is not clear whether another type of symmetry breaking also occurs. Another key question concerns the property of Fermi-surface gapping in the momentum space. Here we address these key questions by a momentum-dependent observation of electronic states at the transition employing ultrahigh-resolution three-dimensional angle-resolved photoemission spectroscopy. Our results provide compelling evidence of the spontaneous breaking of the lattices translational symmetry and particle-hole asymmetric gapping of a heavy quasiparticle pocket at the transition.

Multiple phosphorus chemical sites in heavily phosphorus-doped diamond

We have performed high-resolution core level photoemission spectroscopy on a heavily phosphorus (P)-doped diamond film in order to elucidate the chemical sites of doped-phosphorus atoms in diamond. P 2p core level study shows two bulk components, providing spectroscopic evidence for multiple chemical sites of doped-phosphorus atoms. This indicates that only a part of doped-phosphorus atoms contribute to the formation of carriers. From a comparison with band calculations, possible origins for the chemical sites are discussed.

Electronic Structures of CeM2Al10 (M = Fe, Ru, and Os) Studied by Soft X-ray Resonant and High-Resolution Photoemission Spectroscopies

We have performed a photoemission spectroscopy (PES) study of CeM2Al10 (M = Fe, Ru, and Os) to directly observe the electronic structure involved in the unusual magnetic ordering. Soft X-ray resonant (SXR) PES provides spectroscopic evidence of the hybridization between conduction and Ce 4f electrons (c–f hybridization) and the order of the hybridization strength (Ru < Os < Fe). High-resolution (HR) PES of CeRu2Al10 and CeOs2Al10, as compared with that of CeFe2Al10, identifies two structures that can be ascribed to structures induced by the c–f hybridization and the antiferromagnetic ordering, respectively. Although the c–f hybridization-induced structure is a depletion of the spectral intensity (pseudogap) around the Fermi level (EF) with an energy scale of 20–30 meV, the structure related to the antiferromagnetic ordering is observed as a shoulder at approximately 10–11 meV within the pseudogap. The energies of the shoulder structures of CeRu2Al10 and CeOs2Al10 are approximately half of the optical gap ...We have performed a photoemission spectroscopy (PES) study of CeM2Al10 (M = Fe, Ru, and Os) to directly observe the electronic structure involved in the unusual magnetic ordering. Soft X-ray resonant (SXR) PES provides spectroscopic evidence of the hybridization between conduction and Ce 4f electrons (c–f hybridization) and the order of the hybridization strength (Ru < Os < Fe). High-resolution (HR) PES of CeRu2Al10 and CeOs2Al10, as compared with that of CeFe2Al10, identifies two structures that can be ascribed to structures induced by the c–f hybridization and the antiferromagnetic ordering, respectively. Although the c–f hybridization-induced structure is a depletion of the spectral intensity (pseudogap) around the Fermi level (EF) with an energy scale of 20–30 meV, the structure related to the antiferromagnetic ordering is observed as a shoulder at approximately 10–11 meV within the pseudogap. The energies of the shoulder structures of CeRu2Al10 and CeOs2Al10 are approximately half of the optical gap ...

Electronic Structure of the Novel Filled Skutterudite PrPt 4 Ge 12 Superconductor

We have performed soft x-ray photoemission spectroscopy (SXPES) and resonant photoemission spectroscopy (RPES) of the filled skutterudite superconductor PrPt 4 Ge 12 in order to study the electronic structure of valence band and the character of Pr 4f. SXPES of PrPt 4 Ge 12 measured with 1200 eV photon energy, where spectral contribution of Pr 4f is negligible, was found nearly identical with that of LaPt 4 Ge 12 , indicating similarity of Pt–Ge derived electronic states of the two compounds. Good correspondence with band calculations allows us to ascribe the dominant Ge 4p character of the density of states at the Fermi level (E F ). Pr 3d → 4f RPES shows that, although Pr 4f electrons in PrPt 4 Ge 12 are not as strongly hybridized with conduction electrons near E F as in PrFe4P12, there are finite Pr 4f contribution to the states near E F in PrPt 4 Ge 12 . These PES results give the information of fundamental electronic structure for understanding the physical properties of the novel filled skutterudite superconductor PrPt 4 Ge 12 .

Te concentration dependent photoemission and inverse-photoemission study of FeSe1−xTex

Abstract We have characterized the electronic structure of FeSe1−xTex for various x values using soft x-ray photoemission spectroscopy (SXPES), high-resolution photoemission spectroscopy (HRPES) and inverse photoemission spectroscopy (IPES). The SXPES valence band spectral shape shows that the 2 eV feature in FeSe, which was ascribed to the lower Hubbard band in previous theoretical studies, becomes less prominent with increasing x. HRPES exhibits systematic x dependence of the structure near the Fermi level (EF): its splitting near EF and filling of the pseudogap in FeSe. IPES shows two features, near EF and approximately 6 eV above EF; the former may be related to the Fe 3d states hybridized with chalcogenide p states, while the latter may consist of plane-wave-like and Se d components. In the incident electron energy dependence of IPES, the density of states near EF for FeSe and FeTe has the Fano lineshape characteristic of resonant behavior. These compounds exhibit different resonance profiles, which may reflect the differences in their electronic structures. By combining the PES and IPES data the on-site Coulomb energy was estimated at 3.5 eV for FeSe.

Soft X-ray Core-Level Photoemission Study of Boron Sites in Heavily Boron-Doped Diamond Films

We have performed high-resolution soft x-ray B 1 s core-level photoemission spectroscopy of heavily boron-doped diamond films in order to study the chemical sites of doped-boron atoms and their relation to the superconducting transition temperature ( T c ). We find that B 1 s core-level spectra exhibit several fine structures and three bulk components can be resolved from photon energy dependent studies and spectral fittings. These results indicate existence of several chemical environments of doped-boron atoms. One of the bulk components having the lowest binding energy can be assigned to a signal from substitutionally doped-boron atoms. Comparisons of bulk components with T c suggest spectroscopic evidence for the importance of substitutionally doped-boron atoms for effective carrier and superconductivity.

Electronic Structure of Noncentrosymmetric Superconductor Li2(PdxPt1-x)3B Studied by Photoemission Spectroscopy

We have performed x-ray photoemission spectroscopy on the system of noncentrosymmetric superconductor, Li 2 (Pd x Pt 1- x ) 3 B. For Li 2 Pt 3 B, we found two major peaks with two other weak components, and the band calculations were in agreement with the observation. The assignment of valence band features using the calculated partial density of states determined that Pt 5 d and B 2 p contribute to the density of states at the Fermi level. The effect of antisymmetric spin–orbit coupling on the band structure might have been probed, and the analysis on the effect of Pt incorporation into the system indicates the smooth evolution of electronic structures. We presented the measurements of core levels (Pd 3 d , Pt 4 f , and B 1 s ) and discussed the chemical bonding states and electronic structures from them.