Evolution of the electronic structure across the filling-control and bandwidth-control metal-insulator transitions in pyrochlore-type Ru oxides

We have performed photoemission and soft x-ray absorption studies of pyrochlore-type Ru oxides, namely, the filling-control system Sm$_{2-x}$Ca$_x$Ru$_2$O$_7$ and the bandwidth-control system Sm$_{2-x}$Bi$_x$Ru$_2$O$_7$, which show insulator-to-metal transition with increasing Ca and Bi concentration, respectively. Core levels and the O 2$p$ valence band in Sm$_{2-x}$Ca$_x$Ru$_2$O$_7$ show almost the same amount of monotonous upward energy shifts with Ca concentration, which indicates that the chemical potential is shifted downward due to hole doping. The Ru 4$d$ band in Sm$_{2-x}$Ca$_x$Ru$_2$O$_7$ is also shifted toward the Fermi level ($E_F$) with hole doping and the density of states (DOS) at $E_F$ increases. The core levels in Sm$_{2-x}$Bi$_x$Ru$_2$O$_7$, on the other hand, do not show clear energy shifts except for the Ru 3$d$ core level, whose line shape change also reflects the increase of metallic screening with Bi concentration. We observe pronounced spectral weight transfer from the incoherent to the coherent parts of the Ru 4d $t_{2g}$ band with Bi concentration, which is expected for a bandwidth-control Mott-Hubbard system. The increase of the DOS at $E_F$ is more abrupt in the bandwidth-control Sm$_{2-x}$Bi$_x$Ru$_2$O$_7$ than in the filling-control Sm$_{2-x}$Ca$_x$Ru$_2$O$_7$, in accordance with a recent theoretical prediction. Effects of charge transfer between the Bi 6$sp$ band and the Ru 4$d$ band are also discussed.Comment: 11 pages, 6 figure

Renormalized mean-field t-J model of high-Tc superconductivity: comparison with experiment

Using an advanced version of the renormalized mean-field theory (RMFT) for the t-J model, we examine spin-singlet superconducting (SC) state of $d_{x^2 - y^2}$-symmetry. Overall doping dependence of the SC gap magnitude is in good agreement with experimental results for $\text{Bi}_{2}\text{Sr}_{2}\text{Ca} \text{Cu}_2 \text{O}_{8 + \delta}$ (BSCCO) and $\text{La}_{2-x}\text{Sr}_{x}\text{Cu} \text{O}_{4}$ (LSCO) compounds at the optimal doping and in the overdoped regime. We also calculate the dispersion relation for the Bogoliubov quasiparticles and compare our findings both with the angle resolved photoemission data for the cuprates, as well as with the variational Monte Carlo and other mean-field studies. Within the method proposed by Fukushima [cf. Phys. Rev. B \textbf{78}, 115105 (2008)], we analyze different forms of the t-J Hamiltonian, i.e. modifications caused by the form of exchange interaction, and by the presence of three-site terms. It is shown that although the former has a small influence, the latter suppresses strongly the superconductivity. We also analyze the temperature dependence of the gap magnitude and compare the results with those of the recently introduced finite-temperature renormalized mean-field theory (TRMFT) of Wang et al. [cf. Phys. Rev. B \textbf{82}, 125105 (2010)].Comment: 7 pages, 6 figures, 2 tables. Submitted to Physical Review

Orbital Symmetry and Electron Correlation in Na_{x}CoO_2

Measurements of polarization-dependent soft x-ray absorption reveal that the electronic states determining the low-energy excitations of Na$_{x}$CoO$_2$ have predominantly $a_{1g}$ symmetry with significant O $2p$ character. A large transfer of spectral weight observed in O $1s$ x-ray absorption provides spectral evidence for strong electron correlations in the layered cobaltates. Comparing Co $2p$ x-ray absorption with calculations based on a cluster model, we conclude that Na$_{x}$CoO$_2$ exhibits a charge-transfer electronic character rather than a Mott-Hubbard character

ARPES Study of the Metal-Insulator Transition in Bismuth Cobaltates

We present an angle-resolved photoemission spectroscopy (ARPES) study of a Mott-Hubbard-type bismuth cobaltate system across a metal-insulator transition. By varying the amount of Pb substitution, and by doping with Sr or Ba cation, a range of insulating to metallic properties is obtained. We observe a systematic change in the spectral weight of the coherent and incoherent parts, accompanied by an energy shift of the incoherent part. The band dispersion also shows the emergence of a weakly dispersing state at the Fermi energy with increasing conductivity. These changes correspond with the changes in the temperature-dependent resistivity behavior. We address the nature of the coherent-incoherent parts in relation to the peak-dip-hump feature seen in cuprates superconductors

Hall Transport in Granular Metals and Effects of Coulomb Interactions

We present a theory of Hall effect in granular systems at large tunneling conductance $g_{T}\gg 1$. Hall transport is essentially determined by the intragrain electron dynamics, which, as we find using the Kubo formula and diagrammatic technique, can be described by nonzero diffusion modes inside the grains. We show that in the absence of Coulomb interaction the Hall resistivity $\rho_{xy}$ depends neither on the tunneling conductance nor on the intragrain disorder and is given by the classical formula $\rho_{xy}=H/(n^* e c)$, where $n^*$ differs from the carrier density $n$ inside the grains by a numerical coefficient determined by the shape of the grains and type of granular lattice. Further, we study the effects of Coulomb interactions by calculating first-order in $1/g_T$ corrections and find that (i) in a wide range of temperatures T \gtrsim \Ga exceeding the tunneling escape rate \Ga, the Hall resistivity $\rho_{xy}$ and conductivity \sig_{xy} acquire logarithmic in $T$ corrections, which are of local origin and absent in homogeneously disordered metals; (ii) large-scale ``Altshuler-Aronov'' correction to \sig_{xy}, relevant at T\ll\Ga, vanishes in agreement with the theory of homogeneously disordered metals.Comment: 29 pages, 16 figure

Disorder Effects in the Bipolaron System Ti4_{4}O7_{7} Studied by Photoemission Spectroscopy

We have performed a photoemission study of Ti$_{4}$O$_{7}$ around its two transition temperatures so as to cover the metallic, high-temperature insulating (bipolaron-liquid), and low-temperature insulating (bipolaron-crystal) phases. While the spectra of the low-temperature insulating phase show a finite gap at the Fermi level, the spectra of the high-temperature insulating phase are gapless, which is interpreted as a soft Coulomb gap due to dynamical disorder. We suggest that the spectra of the high-temperature disordered phase of Fe$_{3}$O$_{4}$, which exhibits a charge order-disorder transition (Verwey transition), can be interpreted in terms of a Coulomb gap.Comment: 4 pages, 3 epsf figures embedde

Structural importance of Stone-Thrower-Wales defects in rolled and flat graphenes from surface-enhanced Raman scattering

We first survey the historical aspects of the term Stone-Thrower-Wales (STW) defect and its experimental identification. Physicochemical properties associated with the STW defect have been extensively investigated theoretically as well. However, it is difficult to verify the predicted properties by means of experiments. Here we demonstrate an experimental way to probe the vibrational properties of STW defects in single-wall carbon nanotubes (SWCNTs) using surface-enhanced Raman scattering (SEAS). We also performed density functional theory calculations to support our interpretation of the SERS spectra. The characteristic fluctuations of peak intensities and frequencies are ascribed to dynamic motion of an STW defect in the hexagonal SWCNT lattice. The role of an STW defect at edges is also discussed in terms of its relevance to the stability and O-2 reactivity of flat and curved graphene structures.ArticleCARBON. 50(9):3274-3279 (2012)journal articl

Non-Abelian Vortices on Cylinder -- Duality between vortices and walls

We investigate vortices on a cylinder in supersymmetric non-Abelian gauge theory with hypermultiplets in the fundamental representation. We identify moduli space of periodic vortices and find that a pair of wall-like objects appears as the vortex moduli is varied. Usual domain walls also can be obtained from the single vortex on the cylinder by introducing a twisted boundary condition. We can understand these phenomena as a T-duality among D-brane configurations in type II superstring theories. Using this T-duality picture, we find a one-to-one correspondence between the moduli space of non-Abelian vortices and that of kinky D-brane configurations for domain walls.Comment: 33 pages, 17 figures, v2: references added, typos corrected, the final version published in PR