Magnetic Ordering, Orbital Ordering and Resonant X-ray Scattering in Perovskite Titanates

The effective Hamiltonian for perovskite titanates is derived by taking into account the three-fold degeneracy of $t_{2g}$ orbitals and the strong electron-electron interactions. The magnetic and orbital ordered phases are studied in the mean-field approximation applied to the effective Hamiltonian. A large degeneracy of the orbital states in the ferromagnetic phase is found in contrast to the case of the doubly degenerate $e_g$ orbitals. Lifting of this orbital degeneracy due to lattice distortions and spin-orbit coupling is examined. A general form for the scattering cross section of the resonant x-ray scattering is derived and is applied to the recent experimental results in YTiO$_3$. The spin wave dispersion relation in the orbital ordered YTiO$_3$ is also studied.Comment: 10 pages, 6 figure

High-temperature desorption of C60 covalently bound to 6H-SiC(0001)-(3x3)

The desorption or fragmentation temperature of C-60 bound to Si-rich-(3 x 3) and (root 3 x root 3) R30 degrees. reconstructions of 6H-SiC(0001) is investigated using inverse photoemission spectroscopy (IPES) and LEED experiments. On SiC-(3 x 3), C-60 film is found desorbed after annealing at a high temperature of 1140 K, supporting covalent bonding. Meanwhile, the Si tetramers of the (3 x 3) nanostructured substrate are recovered, as can be inferred from the full reappearance of the Mott-Hubbard surface state in the IPE spectra. SiC-(3 x 3) behaves in a singular way among the other semiconducting substrates, which covalently bind to C-60. This remarkable feature is attributed to the low density of Si dangling bonds and to the highly corrugated character of this reconstruction

Reversible hydrogenation of deuterium-intercalated quasi-free-standing graphene on SiC(0001)

Hydrogenation of deuterium-intercalated quasi-free-standing monolayer graphene on SiC(0001) is obtained and studied with low-energy electron diffraction and high-resolution electron energy loss spectroscopy. While the carbon honeycomb structure remains intact, it is shown that a significant band gap opens in the hydrogenated material. Vibrational spectroscopy evidences for hydrogen chemisorption on the quasi-free-standing graphene is provided and its thermal stability is studied

Final-state diffraction effects in angle-resolved photoemission at an organic-metal interface

In this paper it is shown that angle-resolved photoemission performed using low-energy photons on an organicmetal interface allows to clearly distinguish genuine interface states from features of substrate photoelectrons diffracted by the molecular lattice. As a model system an ordered monolayer of Zn-phthalocyanine is used as a diffraction lattice to probe the electronic band structure of a Ag(110) substrate. Photoemission close to normal emission geometry reveals strongly dispersive features absent in the pristine substrate spectra. Density functional theory modeling helped identifying these as bulk sp direct transitions undergoing surface-umklapp processes. The present results establish the important role of final-state diffraction effects in photoemission experiments at organic-inorganic interfaces

Peculiar covalent bonding of C 60 / 6H-SiC(0001)-(3 × 3) probed by photoelectron spectroscopy

International audienceHigh resolution photoemission with synchrotron radiation was used to study the interface formation of a thin layer of C60 on 6H−SiC(0001)-(3 × 3), characterized by protruding Si-tetramers. The results show that C60 is chemisorbed by orbital hybridization between the highest-occupied molecular orbital (HOMO) and the pz orbital of Si adatom at the apex of the tetramers. The covalent nature of the bonding was inferred from core level as well as valence band spectra. The Si 2p spectra reveal that a large fraction (at least 45%) of the Si adatoms remain unbound despite the reactive character of the associated dangling bonds. This is consistent with a model in which each C60 is attached to the substrate through a single covalent C60-Si bond. A binding energy shift of the core levels associated with sub-surface Si or C atoms indicates a decrease of the SiC band bending caused by a charge transfer from the C60 molecules to the substrate via the formation of donor-like interface states

Peculiar covalent bonding of C 60 / 6H-SiC(0001)-(3 × 3) probed by photoelectron spectroscopy

International audienceHigh resolution photoemission with synchrotron radiation was used to study the interface formation of a thin layer of C60 on 6H−SiC(0001)-(3 × 3), characterized by protruding Si-tetramers. The results show that C60 is chemisorbed by orbital hybridization between the highest-occupied molecular orbital (HOMO) and the pz orbital of Si adatom at the apex of the tetramers. The covalent nature of the bonding was inferred from core level as well as valence band spectra. The Si 2p spectra reveal that a large fraction (at least 45%) of the Si adatoms remain unbound despite the reactive character of the associated dangling bonds. This is consistent with a model in which each C60 is attached to the substrate through a single covalent C60-Si bond. A binding energy shift of the core levels associated with sub-surface Si or C atoms indicates a decrease of the SiC band bending caused by a charge transfer from the C60 molecules to the substrate via the formation of donor-like interface states

Approaching Truly Freestanding Graphene: The Structure of Hydrogen-Intercalated Graphene on 6H-SiC(0001)

We measure the adsorption height of hydrogen-intercalated quasifreestanding monolayer graphene on the (0001) face of 6H silicon carbide by the normal incidence x-ray standing wave technique. A density functional calculation for the full (63√×63√)−R30° unit cell, based on a van der Waals corrected exchange correlation functional, finds a purely physisorptive adsorption height in excellent agreement with experiments, a very low buckling of the graphene layer, a very homogeneous electron density at the interface, and the lowest known adsorption energy per atom for graphene on any substrate. A structural comparison to other graphenes suggests that hydrogen-intercalated graphene on 6H−SiC(0001) approaches ideal graphene.Peer reviewe

Self-organised growth of molecular arrays at surfaces

International audienceThe autonomous ordering and assembly of atoms and molecules on atomically well-defined surfaces allow creating a wide range of surface nanostructures, opening an alternative 'bottom-up' route to the traditional 'top-down' fabrication methods of the microelectronics industry now approaching their fundamental limits. This review summarises some recent efforts of our team to grow molecular arrays on metal, insulating or semiconductor surfaces. In a fundamental approach, two-dimensional surface arrays of nanometre size have been obtained under ultrahigh vacuum by evaporation of molecules, functionalised to favour the intermolecular links rather than molecule-substrate ones. Intermolecular links such as hydrogen bonds, covalent or coordination bonding were profitably used to create various molecular networks. Alternatively, we also investigated molecular self-assembly from the solution whose architectures are mainly fixed by the molecule-substrate adsorption forces. Molecular assemblies were characterised using Scanning Near-Field Microscopies (Scanning Tunnelling Microscopy, non-contact-Atomic Force Microscopy), whereas electronic and vibrational properties were investigated by surface spectroscopy such as Ultra-Violet and X-ray Photoelectron Spectroscopy, infrared or Surface-Enhanced Raman Spectroscopy

Detection of somaclonal variation by random amplified polymorphic DNA analysis during micropropagation of Phalaenopsis bellina (Rchb.f.) Christenson

Phalaenopsis bellina (Rchb.f.) Christenson orchid species are known for their beautiful flower shape, graceful inflorescence and fragrance. Protocorm-like bodies (PLBs) of P. bellina were induced from leaf segments. The PLBs were then subjected to proliferation using ½ strength Murashige and Skoog (MS) media with two subcultures at three months intervals. Twelve decamer random amplified polymorphic DNA (RAPD) primers were used to study somaclonal variation among the mother plant, the initially induced PLBs and proliferated PLBs after 3 and 6 months in culture. Eight out of twelve primers produced 172 bands with 18 polymorphic bands in all the treatments. The amplified products varied between 125 to 8000 bp. Among the primers used, P 16 produced the highest number of bands (29), while primer OPU 10 produced the lowest number (15). The range of similarity coefficient was from 0.83 to 1.0 among the different sub-cultures and mother plant (MP). It was found that minimal or no changes occurred between the MP and the PLBs produced after 3 months of induction. The induced PLBs were then subcultured for six months for proliferation and this resulted in about 17% dissimilarity with MP. It is reported that micropropagation of P. bellina can be carried out successfully using ½ strength MS media for 6 months but further proliferation may result in somaclonal variation which might change the prolific characteristic of this orchids