Kouichi Kusakabe

Direct Imaging of Monovacancy-Hydrogen Complexes in a Single Graphitic Layer

Understanding how foreign chemical species bond to atomic vacancies in graphene layers can advance our ability to tailor the electronic and magnetic properties of defective graphenic materials. Here we use ultra-high vacuum scanning tunneling microscopy (UHV-STM) and density functional theory to identify the precise structure of hydrogenated single atomic vacancies in a topmost graphene layer of graphite and establish a connection between the details of hydrogen passivation and the electronic properties of a single atomic vacancy. Monovacancy-hydrogen complexes are prepared by sputtering of the graphite surface layer with low energy ions and then exposing it briefly to an atomic hydrogen environment. High-resolution experimental UHV-STM imaging allows us to determine unambiguously the positions of single missing atoms in the defective graphene lattice and, in combination with the ab initio calculations, provides detailed information about the distribution of low-energy electronic states on the periphery of the monovacancy-hydrogen complexes. We found that a single atomic vacancy where each sigma-dangling bond is passivated with one hydrogen atom shows a well-defined signal from the non-bonding pi-state which penetrates into the bulk with a (\sqrt 3 \times \sqrt 3)R30^ \circ periodicity. However, a single atomic vacancy with full hydrogen termination of sigma-dangling bonds and additional hydrogen passivation of the extended pi-state at one of the vacancys monohydrogenated carbon atoms is characterized by complete quenching of low-energy localized states. In addition, we discuss the migration of hydrogen atoms at the periphery of the monovacancy-hydrogen complexes which dramatically change the vacancys low-energy electronic properties, as observed in our low-bias high-resolution STM imaging.

Theoretical Evidences for Enhanced Superconducting Transition Temperature of CaSi2 in a High-Pressure AlB2 Phase

By means of first-principles calculations, we studied stable lattice structures and estimated superconducting transition temperature of CaSi2 at high pressure. Our simulation showed stability of the AlB2 structure in a pressure range above 17 GPa. In this structure, doubly degenerated optical phonon modes, in which the neighboring silicon atoms oscillate alternately in a silicon plane, show prominently strong interaction with the conduction electrons. In addition there exists a softened optical mode (out-of-plan motion of silicon atoms), whose strength of the electron-phonon interaction is nearly the same as the above mode. The density of states at the Fermi level in the AlB2 structure is higher than that in the trigonal structure. These findings and the estimation of the transition temperature strongly suggest that higher Tc is expected in the AlB2 structure than the trigonal structures which are known so far.By means of first-principles calculations, we studied stable lattice structures and estimated superconducting transition temperature of CaSi 2 at high pressure. Our simulation shows stability of the AlB 2 structure in a pressure range above 17 GPa. In this structure, doubly degenerated optical phonon modes, in which the neighboring silicon atoms oscillate alternately in a silicon plane, show prominently strong interaction with the conduction electrons. In addition there exists a softened optical mode (out-of-plan motion of silicon atoms), whose strength of the electron–phonon interaction is nearly the same as the above mode. The density of states at the Fermi level in the AlB 2 structure is higher than that in the trigonal structure. These findings and the estimation of the transition temperature strongly suggest that higher T c is expected in the AlB 2 structure than the trigonal structures which are known so far.

Origin of enhanced superconducting transition temperature through structural transformation in CaSi2

Using the first-principles lattice dynamics, we have studied physical origin of enhancement in the superconducting transition temperature Tc of CaSi2 with structural phase transition. Optimization results show that CaSi2 has the AlB2 structure as an optimized structure above 17GPa. The electron-phonon interaction is enhanced, when CaSi2 takes the AlB2 structure compared to phase III. Especially, an E2g Einstein mode and a softened optical B2g mode are important.

Time-dependent resonant UHF CI approach for the photo-induced dynamics of the multi-electron system confined in 2D QD

We extend the static multi-reference description (resonant UHF) to the dynamic system in order to include the correlation effect over time, and simplify the TD Schrodinger equation (TD-CI) into a time-developed rate equation where the TD external field Ĥ′(t) is then incorporated directly in the Hamiltonian without any approximations. We apply this TD-CI method to the two-electron ground state of a 2D quantum dot (QD) under photon injection and study the resulting two-electron Rabi oscillation.

Analysis of phonon modes strongly coupled to electrons in high Tc superconducting phases of calcium

For a zigzag crystal structure of calcium in the phase V, we estimated the superconducting transition temperature Tc by the use of the Allen-Dynes formula. If we set the effective screened Coulomb repulsion constant μ* at 0.1, we obtain Tc=16.37K at 120 GPa and Tc=17.15K at 140 GPa. In order to clarify the origin of such high values of Tc, we analyzed a partial electron-phonon coupling constant at each phonon mode. As the result we found that an optical mode at the G point strongly interacts with electrons and it induces the high Tc in the phase V. The phonon mode can exist only in the particular structure like the zigzag structure.

Ab initio study on the high superconducting transition temperature in calcium under high pressure

For calcium in the phases IV and V, we estimated the superconducting transition temperature T c by the use of the Allen–Dynes formula. Setting the effective screened Coulomb repulsion constant μ* at 0.1 in the formula, we obtained T c =23.42 K at 100 GPa for Ca-IV and T c =15.87 K at 120 GPa for Ca-V. In order to clarify the origin of such high values of T c , first, we investigated the band character of electrons and found that the high T c is not necessarily related to the so called s–d transfer. Then we analyzed the electron–phonon coupling at each phonon mode in Ca-V where the highest T c in elements has been experimentally observed. As a result, we discovered that an optical mode at the Γ point has the strongest electron–phonon coupling. Such phonon mode can exist only in the complex crystal structure of Ca-V, and the result shows that the high T c seems to be closely linked with the complex crystal structures like Ca-IV and Ca-V. †This paper was presented at the XLVIth European High Pressure Research Group (EHPRG 46) Meeting, Valencia (Spain), 7–12 September, 2008.