No abstract

A simple empirical relation connects the critical temperature of the order-disorder transition T c to the dipolar reorientation rate T' 1 (T) of CN" molecules by the concept of a critical reorientation rate T C ' 1 (T C ).This relation is derived from experimental data on four pure alkali cyanides, the dipole-diluted (KBr)^ :(KCN) X system, and the mixed alkali cyanides (KCN) 1 _ X :(NaCN) x and (RbCN)^ :(KCN) X . The observed disappearance of the order-disorder transition at a certain x value occurs in all mixed systems, when the spread of relaxation rates exceeds a particular value.PACS numbers: 64.70. Kb, 77.40.+i, The study of the reorientation and ordering behavior of diatomic molecules in ionic solids is a field of intense current interest. CN" molecular ions have become phototypical model cases in these studies, because they can be incorporated -in connection with alkali and halide ions-into a variety of regular and irregular solid-state structures. The case of a regular CN" sublattice -the pure alkali cyanides-is characterized at high temperatures by an orientationaily disordered structure, which transforms by a first-order transition at T c into a state of parallel (ferroelastic) order. 1 Dilution of the CrT dipole concentration by halide substitution-in systems such as (KCN) X iCKBR)^ shifts T c gradually to lower temperatures, until at a critical CN" concentration x c ail measurable indications of a longrange order disappear abruptly (observed first in mixed KC1: KCN). 2 It has been speculated that systems with x

No abstract

No abstract

No abstract

Ab initio density functional calculations were performed for a toroidal carbon C 120 nanostructure. Hydrogen molecules, n ¼ 1-15, were added inside the nanotorus and for each one of these systems a geometry optimization was obtained. The cohesive energy shows that these structures are energetically stable. For example, the binding energies are À34.95 and À36.19 Hartrees and the interatomic distances HAH are 0.753 and 0.772 Å for 1 and 14 molecules, respectively. Considering only molecular hydrogen, we have always seen so far weak physisorption into the C 120 nanotorus. There is no chemisorption until the number oh hydrogen molecules are increased to 14. In this case, four hydrogen atoms are chemisorbed. With 15 molecules, there are 10 hydrogen atoms chemisorbed just at the inner nanotorus surface forming 10 HAC bondings with bond length close to that in methane.

No abstract

We report the observation of photoconduction and a strong nonlinear optical absorptive response exhibited by multi-wall carbon nanotubes. An aerosol pyrolysis method was employed for the preparation of the samples. Measurements of the optical transmittance with 7 ns pulses at 1064 nm wavelength allowed us to identify a two-photon absorption effect as the main mechanism of third-order nonlinearity. Photoconductive experiments at 445 nm wavelength seem to confirm the possibility for generating non-resonant multi-photonic absorption processes in the multi-wall carbon nanotubes. By the optical control of the conductivity in the nanotubes, we implement an optoelectronic amplitude modulator device with potential applications for sharp selective functionalities.