Observation and Assignment of Silent and Higher Order Vibrations in the Infrared Transmission of C60 Crystals

We report the measurement of infrared transmission of large C60 single crystals. The spectra exhibit a very rich structure with over 180 vibrational absorptions visible in the 100 - 4000 cm-1 range. Many silent modes are observed to have become weakly IR-active. We also observe a large number of higher order combination modes. The temperature (77K - 300K) and pressure (0 - 25KBar) dependencies of these modes were measured and are presented. Careful analysis of the IR spectra in conjunction with Raman scattering data showing second order modes and neutron scattering data, allow the selection of the 46 vibrational modes C60. We are able to fit *all* of the first and second order data seen in the present IR spectra and the previously published Raman data (~300 lines total), using these 46 modes and their group theory allowed second order combinations.Comment: REVTEX v3.0 in LaTeX. 12 pages. 8 Figures by request. c60lon

Four Lessons in Versatility or How Query Languages Adapt to the Web

Exposing not only human-centered information, but machine-processable data on the Web is one of the commonalities of recent Web trends. It has enabled a new kind of applications and businesses where the data is used in ways not foreseen by the data providers. Yet this exposition has fractured the Web into islands of data, each in different Web formats: Some providers choose XML, others RDF, again others JSON or OWL, for their data, even in similar domains. This fracturing stifles innovation as application builders have to cope not only with one Web stack (e.g., XML technology) but with several ones, each of considerable complexity. With Xcerpt we have developed a rule- and pattern based query language that aims to give shield application builders from much of this complexity: In a single query language XML and RDF data can be accessed, processed, combined, and re-published. Though the need for combined access to XML and RDF data has been recognized in previous work (including the W3C’s GRDDL), our approach differs in four main aspects: (1) We provide a single language (rather than two separate or embedded languages), thus minimizing the conceptual overhead of dealing with disparate data formats. (2) Both the declarative (logic-based) and the operational semantics are unified in that they apply for querying XML and RDF in the same way. (3) We show that the resulting query language can be implemented reusing traditional database technology, if desirable. Nevertheless, we also give a unified evaluation approach based on interval labelings of graphs that is at least as fast as existing approaches for tree-shaped XML data, yet provides linear time and space querying also for many RDF graphs. We believe that Web query languages are the right tool for declarative data access in Web applications and that Xcerpt is a significant step towards a more convenient, yet highly efficient data access in a “Web of Data”

B4-B1 phase transition of GaN under isotropic and uniaxial compression

The question raised recently about the mechanisms of the pressure-induced B4-B1 transition of GaN is investigated using a density functional-based metadynamics method. A homogeneous deformation path through a tetragonal intermediate structure is found to be energetically favorable under hydrostatic pressure conditions. This transition path is initialized by the soft phonon modes that appear along the transverse acoustic branches near the transition point. However, under certain nonhydrostatic pressure conditions an elastic instability replaces the phonon instability to become a dominant mechanism for the structural transformation of GaN. The phase transition would avoid the tetragonal path when a uniaxial stress is applied on the c axis but follows a different path through a hexagonal intermediate structure. The selection of the transformation mechanisms, determined by specific stress conditions applied, highlights the important role the competition between phonon and elastic instabilities plays in the reconstructive phase transitions. \ua9 2013 American Physical Society.Peer reviewed: YesNRC publication: Ye

NMR shielding constants for hydrogen guest molecules in structure II clathrates

Proton NMR shielding constants and chemical shifts for hydrogen guests in small and large cages of structure II clathrates are calculated using density-functional theory and the gauge-invariant atomic-orbital method. Shielding constants are calculated at the B3LYP level with the 6-311++G(d,p) basis set. The calculated chemical shifts are corrected with a linear regression to reproduce the experimental chemical shifts of a set of standard molecules. The calculated chemical shifts of single hydrogen molecules in the small and large structure II cages are 4.94 and 4.84 ppm, respectively, which show that within the error range of the method the H2 guest molecules in the small and large cages cannot be distinguished. Chemical shifts are also calculated for double occupancy of the hydrogen guests in small cages, and double, triple, and quadruple occupancy in large cages. Multiple occupancy changes the chemical shift of the hydrogen guests by approximately 0.2 ppm. The relative effects of other guest molecules and the cage on the chemical shift are studied for the cages with multiple occupancies.Peer reviewed: YesNRC publication: Ye

PHASE TRANSITIONS OF ICE V AND VI

Les transitions de phase de la glace V et VI dans le domaine 90-175 K ont été étudiées par calorimétrie. Les transitions dans le domaine 105-140 K qui sont semblables à des transitions ordre-désordre, ont été identifiées. Les variations d'enthalpie de celles-ci ainsi que les transformations des glaces V et VI en glace Ic à plus hautes températures ont été déterminées.Phase transitions of ice V and VI in the range 90-175 K have been investigated by heat-flow calorimetry. Transitions in the range 105-140 K, which are likely order-disorder, have been identified and enthalpy changes of these and the transformations of ice V and VI to ice Ic at higher temperatures have been determined

EFFECT OF PRESSURE ON THE STRUCTURE OF MOLECULES

On a déterminé par spectroscopie infrarouge l'effet de la pression sur les angles de rotation internes du gauche-1,2-dichloréthane et du s-trithiane. L'angle de rotation interne du gauche-1,2-dichloréthane varie au taux de -2,7° kbar-1 jusqu'à 5 kbars, et la molécule de s-trithiane devient plate sous une pression d'environ 50 kbars.The effect of pressure on the internal rotation angles of gauche-1,2-dichloroethane and s-trithiane has been determined by infrared spectroscopy. The internal rotation angle of gauche-1,2-dichloroethane changes at the rate of -2.7° kbar-1 up to 5 kbar, and s-trithiane becomes a flat molecule at ~50 kbar

Finite temperature multiscale computational modeling of materials at nanoscale

The A multiscale computational method (CADD) is presented for modeling of materials at nanoscale whereby a continuum region containing defects is coupled to a fully atomistic region. The method reduces the degree of freedom in simulations of mechanical behavior of nanomaterials without sacrificing important physics. Applications to nanoindentation are used to validate and demonstrate the capabilities of the model

Multiscale modeling of solids at the nanoscale: Dynamic approach

One major class of multiscale models directly couples a region described with full atomistic detail to a surrounding region modeled using continuum concepts and finite element methods. Here, the development of a new dynamic approach to such coupled atomistic-continuum models is discussed with insight into the key ideas and features, with emphasis on fundamental difficulties involved in dynamic multiscale models. Simulations of nanoindentation in single crystals are performed to demonstrate the power of the developed method in capturing both long-range dislocation plasticity and short-range atomistic phenomena during single or cyclic loading without the computational cost of full atomistic simulations. The effects of several process variables are investigated, including system temperature and rate of indentation. The deformation mechanisms and the surface evaluation that occur during a series of single and cyclic indentation simulations are discussed