草部 浩一

We investigated the minimum energy pathways and energy barriers of reversible reaction (V-111 + H-2 <-> V-221) based upon calculations using density functional theory. We find a comparable activation barrier of around 1.3 eV for both the dissociative chemisorption and desorption processes. The charge transfer rate from a reacting hydrogen atom to the graphene is around 0.18 e per hydrogen atom in the final state. A subsequent reaction path to recover the initial structure of V-111 is realized by the migration of hydrogen atoms from V-221 onto the graphene surface. The comparable energy barrier of 1.3 eV for both adsorption and desorption suggests that this novel storage and release concept has the potential to act as a hydrogen storage system for certain applications. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Electron correlation effects caused by the topological zero mode of a<br /> hydrogenated graphene vacancy, $V_{111}$, with three adsorbed hydrogen atoms is<br /> discussed theoretically. A Kondo model is derived from the multi-reference<br /> representation of the density functional theory, where exchange scattering<br /> processes between the zero mode and low-energy modes in the Dirac cones are<br /> estimated. Even when the Dirac cone is slightly off from the charge neutral<br /> point, a finite on-site correlation energy, $U_0$, for the zero mode of an<br /> isolated $V_{111}$ allows the half-filling of the localized level giving a spin<br /> $s=1/2$. The anti-ferromagnetic Kondo screening mediated by higher order<br /> scattering processes becomes dominant in the dilute limit of the vacancies. Our<br /> estimation of relevant two body interactions certifies appearance of the Kondo<br /> effect at low temperatures.

By constructing $d_{x^2-y^2}-d_{z^2}$ two-orbital models from first<br /> principles, we have obtained a systematic correlation between the Fermi surface<br /> warping and the evaluated $T_c$ for various bilayer as well as single-layer<br /> cuprates. This reveals that smaller mixture of the $d_{z^2}$ orbital component<br /> on the Fermi surface leads to both of larger Fermi surface warping and higher<br /> $T_c$. The theoretical correlation strikingly resembles a systematic plot for<br /> the experimentally observed $T_c$ against the Fermi surface warping due to<br /> Pavarini {\it et al.} [Phys. Rev. Lett. {\bf 87}, 047003 (2001)], and the<br /> present result unambiguously indicates that the $d_{z^2}$ mixture is a key<br /> factor that determines $T_c$ in the cuprates.

To show an optimization method of element substitution for cuprate<br /> superconductors, we investigate Fermi surface shape of TlR2A2Cu3O9 with R=La,<br /> Y, and A=Li, Na, K, Rb, Cs. We adopt the generalized gradient approximation in<br /> the density-functional theory (DFT-GGA) for the study of over-doped phases of<br /> these unknown cuprates. The electronic structures of crystals optimized by<br /> DFT-GGA show systematic element dependence in a Fermi surface shape controlling<br /> parameter, r, of Cu dx2-y2 bands, where nearly absent dz2 component at the<br /> Fermi level and smaller r keeping t1 suggest enhancement of the superconducting<br /> transition temperature within the spin-fluctuation mechanism. For TlYRb2Cu3O9,<br /> smaller r by a reduction factor larger than 10% compared to a reference system<br /> of TlBa2Ca2Cu3O9 (TBCCO) appears in the outer CuO2 plane, but with 12 %<br /> reduction in t1. For TlR2Li2Cu3O9, (R=Y, La), smaller r by a factor larger than<br /> 1% appears keeping t1 as large as TBCCO in the inner plane. Our method may be<br /> used to predict an optimized material structure referencing known cuprate<br /> superconductors.

Some theorems on derivatives of the Coulomb density functional with respect to the coupling constant lambda are given. A model Fermion system with the reduced coupling constant, lambda &lt; 1, is defined to reproduce a ground-state electron density nGS(r) and the ground state energy. Fixing the charge density, possible phase transitions as level crossings occur only at discrete points along the lambda axis. We prove that Dini&apos;s lambda derivatives of the reduced energy density functional are given by expectation values of the Coulomb interaction with respect to two-state vectors at the crossing point. If the density is v-representable also for lambda &lt; 1, the accumulation of phase transition points is forbidden when lambda -&gt; 1. Our theorems ensure the existence of a converging series of models defined by the multi-reference density functional theory.

Some theorems on derivatives of the Coulomb density functional with respect to the coupling constant lambda are given. A model Fermion system with the reduced coupling constant, lambda &lt; 1, is defined to reproduce a ground-state electron density nGS(r) and the ground state energy. Fixing the charge density, possible phase transitions as level crossings occur only at discrete points along the lambda axis. We prove that Dini&apos;s lambda derivatives of the reduced energy density functional are given by expectation values of the Coulomb interaction with respect to two-state vectors at the crossing point. If the density is v-representable also for lambda &lt; 1, the accumulation of phase transition points is forbidden when lambda -&gt; 1. Our theorems ensure the existence of a converging series of models defined by the multi-reference density functional theory.

We have developed a new method of calculating the van der Waals interaction in isolated systems on the bases of the van der Waals density functional theory. This method is designed to be absolutely nonempirical, applicable to any three-dimensional isolated system and implementable into various first-principles electronic state calculation codes utilizing local density approximation (LDA). The validity of this method is tested by comparing the adiabatic potentials of an Ar dimer and a Kr dimer obtained by this method to those obtained in experiments and other theoretical studies. We find that the LDA error is largely mitigated in both equilibrium bond length and cohesive energy.

We have developed a new method of calculating the van der Waals interaction in isolated systems on the bases of the van der Waals density functional theory. This method is designed to be absolutely nonempirical, applicable to any three-dimensional isolated system and implementable into various first-principles electronic state calculation codes utilizing local density approximation (LDA). The validity of this method is tested by comparing the adiabatic potentials of an Ar dimer and a Kr dimer obtained by this method to those obtained in experiments and other theoretical studies. We find that the LDA error is largely mitigated in both equilibrium bond length and cohesive energy.

Stable oxygen sites on a PdO film over a Pd(100) thin structure with a (root 5 x root 5)R27 degrees surface unit cell are determined using the first-principles electronic structure calculations with the generalized gradient approximation. The adsorbed monatomic oxygen goes to a site bridging two twofold-coordinated Pd atoms or to a site bridging a twofold-coordinated Pd atom and a fourfold-coordinated Pd atom. Estimated reaction energies of CO oxidation by reduction of the oxidized PdO film and N2O reduction mediated by oxidation of the PdO film are both exothermic. Motion of the adsorbed oxygen atom between the two stable sites is evaluated using the nudged elastic band method, where an energy barrier for a translational motion of the adsorbed oxygen may become similar to 0.45 eV, which is low enough to allow fluxionality of the surface oxygen at high temperatures. The oxygen fluxionality is allowed by the existence of twofold-coordinated Pd atoms on the PdO film, whose local structure has a similarity to that of Pd catalysts for the Suzuki-Miyaura cross-coupling. Although NOx (including NO2 and NO) reduction is not always catalyzed by the PdO film only, we conclude that continual redox reactions may happen mediated by oxygen-adsorbed PdO films over a Pd surface structure, when the influx of NOx and CO continues, and when the reaction cycle is kept on a well-designed oxygen surface.

We propose a possible charge fluctuation effect expected in layered superconducting materials. In the multireference density functional theory, relevant fluctuation channels for the Josephson coupling between superconducting layers include the interlayer pair hopping derived from the Coulomb repulsion. When interlayer single-electron tunneling processes are irrelevant in the Kohn-Sham electronic band structure calculation, the two-body effective interactions stabilize a superconducting phase. This state is also regarded as a valence-bond solid in a bulk electronic state. The hidden order parameters coexist with the superconducting order parameter when the charging effect of a layer is comparable to the pair hopping. Relevant material structures favorable for the pair-hopping mechanism are discussed.

We theoretically investigate electronic transport properties of a distorted carbon nanotube and estimate conductance shift for the sub-band conduction. In this simulation nanotubes are distorted in such a way that they make shapes of various vibrational modes. We find that the shift depends on the way of distortion. Thus, if the distortion is induced by selective excitation of a phonon mode by a monochrome light, we may expect clear response in the electronic current through the biased nanotube, which implies its possible application as a current switching device. (C) 2009 The Japan Society of Applied Physics

We theoretically investigate electronic transport properties of a distorted carbon nanotube and estimate conductance shift for the sub-band conduction. In this simulation nanotubes are distorted in such a way that they make shapes of various vibrational modes. We find that the shift depends on the way of distortion. Thus, if the distortion is induced by selective excitation of a phonon mode by a monochrome light, we may expect clear response in the electronic current through the biased nanotube, which implies its possible application as a current switching device. (C) 2009 The Japan Society of Applied Physics