We investigated the optical properties of (Ca,Sr)RuO3 films on the borderline of a metal-insulator (M-I) transition. Our results show all of the predicted characteristics for a metallic Mott-Hubbard system, including (i) a mass enhancement in dc-limit, (ii) an U/2 excitation, and (iii) an U excitation. Also, a self-consistency is found within the Gutzwiller-Brinkman-Rice picture for the Mott transition. Our finding displays that electron correlation should be important even in 4d materials.PACS numbers: 71.30.+h, 71.27.+a, 71.10.Fd, Correlation between electrons in transition and rare earth metal compounds has attracted lots of attentions. In general, the more localized the electron wave function is, the stronger the correlation effects are. As a result, correlation effects are believed to be much more important in describing 3d electrons than 4d or 5d electrons.A metal-insulator (M-I) transition driven by electron correlation was proposed by Mott and subsequently investigated intensively.[1] Since the Hubbard model was proposed in early 1960's, it has been widely accepted as the simplest model which can describe correlation effects. Although the model is composed of only two parameters, i.e. inter-site hopping energy t (= W/z) and on-site Coulomb repulsive energy U , it has not been exactly solved yet except for one dimensional case. [W and z are the bandwidth and the coordination number, respectively.] Up to several years ago, different approaches provided limited insights into different aspects of the M-I transition. However, recent theoretical progresses, including a slave-boson approach, infinite dimension limit approaches with several techniques, and numerical calculations for finite size systems, started to provide a coherent picture. [2] According to the traditional Gutzwiller-BrinkmanRice (GBR) picture, [3] the Mott M-I transition from a metallic side can be described by narrowing and disappearing of a Fermi liquid quasi-particle (QP) band at a critical value of correlation strength, (U/W ) c . Under this strong renormalization, an effective mass, m * , [4] of the QP is related by:Recent theoretical works predict that one particle spectral function A(ω) for the metallic phase will be split into lower (LHB) and upper (UHB) Hubbard bands, in addition to the QP band located at zero frequency. Fig. 1(a) shows the schematic diagram of A(ω). Then, the corresponding optical conductivity spectra σ 1 (ω) can be easily predicted and displayed in Fig. 1(b). Note that σ 1 (ω) in a metallic side has three pronounced features: (i) a "QP peak" near zero frequency, (ii) an "U/2 peak" due to optical transitions between QP band and LHB (or UHB), and (iii) an "U peak" due to a transition between LHB and UHB.FIG. 1. Schematic diagrams of (a) one particle spectral function and (b) optical conductivity, for 4/6-filled metallic Mott-Hubbard system. Dotted lines indicate the contributions from O(2p) band.In this letter, we will report optical properties of (Ca,Sr)RuO 3 films, where four electrons occupy triply degenerate t 2g...

No abstract

For the first time, vertically suspended and stretched carbon nanotube network junctions were fabricated in large quantity via the directed assembly strategy using only conventional microfabrication facilities. In this process, surface molecular patterns on the side-wall of the Al structures were utilized to guide the assembly and alignment of carbon nanotubes in the solution. We also performed extensive experimental (electrical and mechanical) analysis and theoretical simulation about the vertically suspended single-walled carbon nanotube network junctions. The junctions exhibited semiconductor-like conductance behavior. Furthermore, we demonstrated gas sensing and electromechanical sensing using these devices.

At micro- and nanoscales, materials with high Young's moduli and low densities are of great interest for high-frequency micromechanical resonator devices. Incorporating carbon nanotubes (CNTs), with their unmatched properties, has added functionality to many man-made composites. We report on the fabrication of < or = 100-nm-thick laminates by sputter-deposition of aluminium onto a two-dimensional single-walled CNT network. These nanolaminates--composed of Al, its native oxide Al(2)O(3) and CNTs--are fashioned, in a scalable manner, into suspended doubly clamped micromechanical beams. Dynamic flexural measurements show marked increases in resonant frequencies for nanolaminates with Al-CNT laminae. Such increases, further supported by quasi-static flexural measurements, are partly attributable to enhancements in elastic properties arising from the addition of CNTs. As a consequence, these nanolaminate micromechanical resonators show significant suppression of mechanical nonlinearity and enhanced strength, both of which are advantageous for practical applications and analogous to biological nanocomposites, similarly composed of high-aspect-ratio, mechanically superior mineral platelets in a soft protein matrix.