Time-of-flight analysis of charge mobility in a Cu-phthalocyanine-based discotic liquid crystal semiconductor

We used a time-of-flight method to study the charge carrier mobility properties of a molecular-aligned discotic liquid crystal semiconductor based on Cu-phthalocyanine. The heated isotropic-phase semiconductor material was sandwiched between transparent electrodes coated onto glass substrates without conventional alignment layers. This was then cooled, and a discotic liquid crystal semiconductor cell was obtained, which we used to make mobility measurements. The material had a fixed molecular alignment due to the supercooling of the hexagonal columnar mesophase. It was clarified that the carrier mobility for electrons was as high as it was for holes at room temperature. The maximum value of negative charge mobility reached 2.60x10(-3) cm(2)/V s, although negative carrier mobility is often much lower than positive carrier mobility in other organic semiconductors, including conventional Cu-phthalocyanine vacuum-deposited films.ArticleAPPLIED PHYSICS LETTERS. 85(16):3474-3476 (2004)journal articl

Origin of the Weak Pseudo-gap Behaviors in Na_{0.35}CoO_2: Absence of Small Hole Pockets

We analyze the ``normal electronic states'' of Na_{0.35}CoO_2 based on the effective d-p model with full d-orbital freedom using the fluctuation-exchange (FLEX) approximation. They sensitively depend on the topology of the Fermi surfaces, which changes as the crystalline electric splitting (CES) due to the trigonal deformation. We succeed in reproducing the weak pseudo-gap behaviors in the density of states (DOS) and in the uniform magnetic susceptibility below 300K, assuming that six small hole-pockets predicted by LDA band calculations are absent. When they exist, on the contrary, then ``anti-pseudo-gap behaviors'' should inevitably appear. Thus, the present study strongly supports the absence of the small hole-pockets in Na_{0.35}CoO_2, as reported by recent ARPES measurements. A large Fermi surface around the \Gamma-point would account for the superconductivity in water-intercalated samples.Comment: 5pages, to appear in J. Phys. Soc. Jpn. Vol.74 (2005) No.

Time-of-flight analysis of charge mobility in a Cu-phthalocyanine-based discotic liquid crystal semiconductor

We used a time-of-flight method to study the charge carrier mobility properties of a molecular-aligned discotic liquid crystal semiconductor based on Cu-phthalocyanine. The heated isotropic-phase semiconductor material was sandwiched between transparent electrodes coated onto glass substrates without conventional alignment layers. This was then cooled, and a discotic liquid crystal semiconductor cell was obtained, which we used to make mobility measurements. The material had a fixed molecular alignment due to the supercooling of the hexagonal columnar mesophase. It was clarified that the carrier mobility for electrons was as high as it was for holes at room temperature. The maximum value of negative charge mobility reached 2.60x10(-3) cm(2)/V s, although negative carrier mobility is often much lower than positive carrier mobility in other organic semiconductors, including conventional Cu-phthalocyanine vacuum-deposited films.ArticleAPPLIED PHYSICS LETTERS. 85(16):3474-3476 (2004)journal articl

Theory of adsorbate induced surface reconstruction on W(100)

We report results of a theoretical study on an adsorbate induced surface reconstruction. Hydrogen adsorption on a W(100) surface causes a switching transition in the symmetry of the displacements of the W atoms within the ordered c(2x2) phase. This transition is modeled by an effective Hamiltonian, where the hydrogen degrees of freedom are integrated out. Based on extensive Monte Carlo renormalisation group calculations we show that the switching transition is of second order at high temperatures and of first order at low temperatures. This behavior is qualitatively explained in terms of an XY model where there is an interplay between four and eight fold anisotropy fields. We also compare the calculated phase diagrams with a simple mean field theory.Comment: CSC Preprint, 31 pages (plain TeX file, no figures

Gutzwiller Method for an Extended Periodic Anderson Model with the c-f Coulomb Interaction

We study an extended periodic Anderson model with the Coulomb interaction Ucf between conduction and f electrons by the Gutzwiller method. The crossovers between the Kondo, intermediate-valence, and almost empty f-electron regimes become sharper with Ucf, and for a sufficiently large Ucf, become first-order phase transitions. In the Kondo regime, a large enhancement in the effective mass occurs as in the ordinary periodic Anderson model without Ucf. In addition, we find that a large mass enhancement also occurs in the intermediate-valence regime by the effect of Ucf.Comment: 9 pages, 7 figure