How does contrasting dependence of impurity-atom diffusivity on the density of host disordered medium arise?

We report results of molecular dynamics investigations into neutral impurity diffusing within an amorphous solid as a function of the size of the diffusant and density of the host amorphous matrix. We find that self diffusivity exhibits an anomalous maximum as a function of the size of the impurity species. An analysis of properties of the impurity atom with maximum diffusivity shows that it is associated with lower mean square force, reduced backscattering of velocity autocorrelation function, near-exponential decay of the intermediate scattering function (as compared to stretched-exponential decay for other sizes of the impurity species) and lower activation energy. These results demonstrate the existence of size-dependent diffusivity maximum in disordered solids. Further, we show that the diffusivity maximum is observed at lower impurity diameters with increase in density. This is explained in terms of the Levitation parameter and the void structure of the amorphous solid. We demonstrate that these results imply contrasting dependence of self diffusivity (D) on the density of the amorphous matrix, p. D increases with p for small sizes of the impurity but shows an increase followed by a decrease for intermediate sizes of the impurity atom. For large sizes of the impurity atom, D decreases with increase in p. These contrasting dependence arises naturally from the existence of Levitation Effect.

Effect of density discontinuity on Alfvén internal gravity waves

The stability characteristics of Alfvén Internal gravity waves for an inviscid, nondissipative, Boussinesq fluid undergoing shear in the presence of a density discontinuity with and without a rigid boundary is studied.

Enhancement of Tunneling Density of States at a Junction of Three Luttinger Liquid Wires

We study the tunneling density of states (TDOS) for a junction of three Tomonaga-Luttinger liquid wires. We show that there are fixed points which allow for the enhancement of the TDOS, which is unusual for Luttinger liquids. The distance from the junction over which this enhancement occurs is of the order of x=v/(2 omega), where v is the plasmon velocity and omega is the bias frequency. Beyond this distance, the TDOS crosses over to the standard bulk value independent of the fixed point describing the junction. This finite range of distances opens up the possibility of experimentally probing the enhancement in each wire individually.

Radiation induced depinning of a charge density wave system

The frequency-dependent response of a pinned charge density wave is considered in terms of forced vibration of an oscillator held in an anharmonic well. It is shown that the effective pinning-frequency can be reduced by applying a d.c. field. If a strong a.c. field, superposed on a d.c. field is applied on such a system “jumps” can be observed in the frequency dependent response of the system. The conditions at which these “jumps” occur are investigated with reference to NbSe3. The possibility of observing such phenomena in other systems like superionic conductors, non-linear dielectrics like ferroelectrics is pointed out. The characteristics are expressed in terms of some “scaled variables” — in terms of which the characteristics show a universal behaviour.

Isolation of hen's egg yolk very low density lipoproteins by DEAE-cellulose chromatography

A procedure has been developed for the isolation of very low density lipoproteins from hen's egg yolk plasma using DEAE-cellulose chromatography. This procedure is rapid and does not require ultracentrifugation and should, therefore, serve as a useful procedure for use in laboratories where this facility does not exist.

Study of linear and nonlinear optical properties of dendrimers using density matrix renormalization group method

We have used the density matrix renormalization group (DMRG) method to study the linear and nonlinear optical responses of first generation nitrogen based dendrimers with donor acceptor groups. We have employed Pariser–Parr–Pople Hamiltonian to model the interacting pi electrons in these systems. Within the DMRG method we have used an innovative scheme to target excited states with large transition dipole to the ground state. This method reproduces exact optical gaps and polarization in systems where exact diagonalization of the Hamiltonian is possible. We have used a correction vector method which tacitly takes into account the contribution of all excited states, to obtain the ground state polarizibility, first hyperpolarizibility, and two photon absorption cross sections. We find that the lowest optical excitations as well as the lowest excited triplet states are localized. It is interesting to note that the first hyperpolarizibility saturates more rapidly with system size compared to linear polarizibility unlike that of linear polyenes.

Radiation induced depinning of a charge density wave system

The frequency-dependent response of a pinned charge density wave is considered in terms of forced vibration of an oscillator held in an anharmonic well. It is shown that the effective pinning-frequency can be reduced by applying a d.c. field. If a strong a.c. field, superposed on a d.c. field is applied on such a system “jumps” can be observed in the frequency dependent response of the system. The conditions at which these “jumps” occur are investigated with reference to NbSe3. The possibility of observing such phenomena in other systems like superionic conductors, non-linear dielectrics like ferroelectrics is pointed out. The characteristics are expressed in terms of some “scaled variables” — in terms of which the characteristics show a universal behaviour

CO2-laser induced X-ray emission from high density gaseous targets

A theory for the emission of X-rays from a high density gaseous plasma interacting with CO2 laser is given. It predicts a sharp increase in the X-ray intensity for densities close to the critical.

Theory of CO2-laser induced x-ray emission from high density gaseous targets

Abstract is not available.

Critical test for Altshuler-Aronov theory: Evolution of the density of states singularity in double perovskite Sr2FeMoO6 with controlled disorder

With high-resolution photoemission spectroscopy measurements, the density of states (DOS) near the Fermi level (E-F) of double perovskite Sr2FeMoO6 having different degrees of Fe/Mo antisite disorder has been investigated with varying temperature. The DOS near E-F showed a systematic depletion with increasing degree of disorder, and recovered with increasing temperature. Altshuler-Aronov (AA) theory of disordered metals well explains the dependences of the experimental results. Scaling analysis of the spectra provides experimental indication for the functional form of the AA DOS singularity.

Investigation of inter-ion interactions in N,N,N',N'-tetramethylethylenediammonium dithiocyanate via experimental and theoretical charge density studies

The crystal structure of the N,N,N',N'-tetramethylethylenediammonium dithiocyanate salt has been examined by experimental charge density studies from high-resolution X-ray diffraction data. The corresponding results are compared with multipole refinements, using theoretical structure factors obtained from a periodic density functional theory calculation at the B3LYP level with a 6-31G** basis set. The salt crystallizes in space group P (1) over bar and contains only a single ion pair with an inversion center in the cation. The salt has thus one unique classical N+-H center dot center dot center dot(NCS)(-) hydrogen bond but also has six other weaker interactions: four C-H center dot center dot center dot S, one C-H center dot center dot center dot N, and one C-H center dot center dot center dot C-pi. The nature of all these interactions has been examined topologically using Bader's quantum theory of "atoms in molecules" and all eight of the Koch-Popelier criteria. The experimental and theoretical approaches agree well and both show that the inter-ion interactions, even in this simplest of systems, play an integrated and complex role in the packing of the ions in the crystal. Electrostatic potential maps are derived from experimental charge densities. This is the first time such a system has been examined in detail by these methods.

Chemically Programmed Ultrahigh Density Two-Dimensional Semiconductor Superlattice Array

Designing an ultrahigh density linear superlattice array consisting of periodic blocks of different semiconductors in the strong confinement regime via a direct synthetic route remains an unachieved challenge in nanotechnology. We report a general synthesis route for the formulation of a large-area ultrahigh density superlattice array that involves adjoining multiple units of ZnS rods by prolate US particles at the tips. A single one-dimensional wire is 300-500 nm long and consists of periodic quantum wells with a barrier width of 5 nm provided by ZnS and a well width of 1-2 nm provided by CdS, defining a superlattice structure. The synthesis route allows for tailoring of ultranarrow laserlike emissions (fwhm approximate to 125 meV) originating from strong interwell energy dispersion along with control of the width, pitch, and registry of the superlattice assembly. Such an exceptional high-density superlattice array could form the basis of ultrahigh density memories in addition to offering opportunities for technological advancement in conventional heterojunction-based device applications.

'Wet N2O oxidation' process and interface state density characterization of nanoscale nitrided SiO2 for flash memory application

In this paper we first present the 'wet N2O' furnace oxidation process to grow nitrided tunnel oxides in the thickness range 6 to 8 nm on silicon at a temperature of 800 degrees C. Electrical characteristics of MOS capacitors and MOSFETs fabricated using this oxide as gate oxide have been evaluated and the superior features of this oxide are ascertained The frequency response of the interface states, before and after subjecting the MOSFET gate oxide to constant current stress, is studied using a simple analytical model developed in this work.

Seismic response of reinforced soil retaining wall models: Influence of backfill relative density

This paper presents the results of shaking table tests on model reinforced soil retaining walls in the laboratory. The influence of backfill relative density on the seismic response was studied through a series of laboratory model tests on retaining walls. Construction of model retaining walls in the laminar box mounted on shaking table, instrumentation and results from the shaking table tests are described in detail. Three types of walls: wrap- and rigid-faced reinforced soil walls and unreinforced rigid-faced walls constructed to different densities were tested for a relatively small excitation. Wrap-faced walls are further tested for higher base excitation at different frequencies and relative densities. It is observed from these tests that the effect of backfill density on the seismic performance of reinforced retaining walls is pronounced only at very low relative density and at the higher base excitation. The walls constructed with higher backfill relative density showed lesser face deformations and more acceleration amplifications compared to the walls constructed with lower densities when tested at higher base excitation. The response of wrap- and rigid-faced retaining walls is not much affected by the backfill relative density when tested at smaller base excitation. The effects of facing rigidity were evaluated to a limited extent. Displacements in wrap-faced walls are many times higher compared to rigid-faced walls. The results obtained from this study are helpful in understanding the relative performance of reinforced soil retaining walls constructed to when subjected to smaller and higher base excitation for the range of relative density employed in the testing program. (C) 2007 Elsevier Ltd. All rights reserved.

Evaluation of methods to estimate foliage density in the understorey of a tropical evergreen forest

Foliage density and leaf area index are important vegetation structure variables. They can be measured by several methods but few have been tested in tropical forests which have high structural heterogeneity. In this study, foliage density estimates by two indirect methods, the point quadrat and photographic methods, were compared with those obtained by direct leaf counts in the understorey of a wet evergreen forest in southern India. The point quadrat method has a tendency to overestimate, whereas the photographic method consistently and ignificantly underestimates foliage density. There was stratification within the understorey, with areas close to the ground having higher foliage densities.