Graphene nanoribbons are quasi-one-dimensional planar graphene allotropes with diverse properties dependent on their width and types of edges. Graphene nanoribbons anchored to substrates is a hybrid system, which offers novel...
moreGraphene nanoribbons are quasi-one-dimensional planar graphene allotropes with diverse properties dependent on their width and types of edges. Graphene nanoribbons anchored to substrates is a hybrid system, which offers novel opportunities for property modifications as well as experimental control. Here we present electronic structure calculations of zigzag graphene nanoribbons chemically attached via the edges to the Si or C terminated surfaces of a SiC substrate. The results show that the edge characteristics are rather robust and the properties are essentially determined by the individual nanoribbon. While the localized spin polarization of the graphene nanoribbon edge atoms is not significantly affected by the substrate, secondary energy gaps in the highest conduction and lowest valence region may emerge in the anchored structures. The van der Waals interaction together with the electrostatic interactions due to the polarity of the surface bonds are found to be important for the...
... One more NH vibration (seems N3) shows an increase of the intensity and shift from 3206 (Thy) to 3212 cm"^ (Thy-SWCNT). ... 7, 45 (2007). 16. S. Gowtham, RH Scheicher, R. Ahuja, R. Pandey, Sh. Kama, Physical Review B 76,...
more... One more NH vibration (seems N3) shows an increase of the intensity and shift from 3206 (Thy) to 3212 cm"^ (Thy-SWCNT). ... 7, 45 (2007). 16. S. Gowtham, RH Scheicher, R. Ahuja, R. Pandey, Sh. Kama, Physical Review B 76, 33401-33404 (2007). 17. Handbook. ...
We study theoretically the interactions of excitonic states with surface electromagnetic modes of a single-walled carbon nanotube. We show that these interactions result in the exciton-plasmon coupling that is significant in its strength...
moreWe study theoretically the interactions of excitonic states with surface electromagnetic modes of a single-walled carbon nanotube. We show that these interactions result in the exciton-plasmon coupling that is significant in its strength due to the presence of weakly-dispersive low-energy (~0.5-2 eV) interband surface plasmon modes and large exciton excitation energies ~1 eV in small-diameter nanotubes. We estimate the exciton-plasmon Rabi splitting to be ~0.1 eV which is close to that in organic semiconductors and much larger than that in hybrid semiconductor-metal nanoparticle molecules. We calculate the exciton absorption lineshape and show the line splitting effect as the exciton is tuned to the nearest interband surface plasmon resonance of the nanotube.
We calculate the energy exchange between phonons and electrons in a metal for very low temperatures. Our results can be applied to quantum-dot refrigerators. The formula of Allen^1 applies to the usual processes of absorption or emission...
moreWe calculate the energy exchange between phonons and electrons in a metal for very low temperatures. Our results can be applied to quantum-dot refrigerators. The formula of Allen^1 applies to the usual processes of absorption or emission of one phonon. Here we consider the energy exchange due to two phonon processes. Second order processes are expected to be important at temperatures less than 1 K. We include two different second-order processes: (i) the Compton-like scattering of phonons, and (ii) the electron-dual-phonon scattering from the second-order electron-phonon interaction. In the Compton-like process an electron emits (or absorbs) a phonon thereby making a transition into a virtual state, and then absorbs (or emits) another phonon. We have found that the process contains a singular energy denominator. The singularity is removed by introducing quasiparticle damping. Then the final formula for the energy exchange depends upon the lifetime of the electrons. For pure metals the thermal relaxation depends on the temperature of the lattice according to T^8. We perform the same calculation for electron-dual-phonon scattering and we find that the temperature dependence is T^9. A comparison with the result for an electron - single-phonon process^1 is also presented. 1. P.B.Allen, Phys.Rev.Lett. 59, 1460 (1987)
During the May 18, 1980 eruption of Mount St. Helens the Muddy River, which drains 350 km2 on the east flank of the mountain, was inundated by lahars (volcanic debris flows). Channels and flood plains were stripped of vegetation and...
moreDuring the May 18, 1980 eruption of Mount St. Helens the Muddy River, which drains 350 km2 on the east flank of the mountain, was inundated by lahars (volcanic debris flows). Channels and flood plains were stripped of vegetation and 0--2.5 meters of sediment were deposited. In addition, Smith Creek, a tributary of the Muddy River, was affected by the lateral blast which, in conjunction with lahars, deposited up to 15 m of sediment. The U.S. Geological Survey conducted repeat cross section surveys at least annually on 25 monumented cross sections in the study area from 1980 to 1986 and from 1994 to 1996. Channel adjustments were rapid and large; up to 12 m of incision and 8 m of aggradation within two years on Smith Creek and 3.5 m of incision on the Muddy River. Suspended sediment yields were high, 3,790,000 tons in 1982, with channel scour an important contributor. This study examined controls on the magnitude and timing of thalweg elevation and net cross section area changes. Two hypotheses were tested: (1) valley morphology is correlated to the timing and magnitude of channel adjustment, (2) channel adjustments occurred through complex response or the movement of a sediment wave. Four multiple linear regression models were run for each of the channel adjustment variables. The disturbance on Smith Creek by the blast deposition was great the minority carriers before the grain boundary, and reduce minority-carrier recombination. It was also shown that the Nitric-Phosphoric (NP) etching reduces the grain-boundary barrier heights to a value that correlates with the valence-band offsets between CdTe and Te. This effect of the NP-etch on grain boundaries extends down a minimum of 2.5 mum from the etched back contact surface. The effects of the NP-etch are also shown to be unstable and could translate to severe increases in the back-contact series resistance of devices. Finally, it was shown that there was no significant effect of the CdTe source plate usage on the inplane electrical properties, and that there is no indication of any grain-boundary barrier height inhomogeneity in any of the samples.
We study theoretically the interactions of excitonic states with surface electromagnetic modes of small-diameter (≲1 nm) semiconducting single-walled carbon nanotubes. We show that these interactions can result in strong...
moreWe study theoretically the interactions of excitonic states with surface electromagnetic modes of small-diameter (≲1 nm) semiconducting single-walled carbon nanotubes. We show that these interactions can result in strong exciton-surface-plasmon coupling. The exciton absorption lineshape exhibits the line (Rabi) splitting ˜0.1-0.3 eV as the exciton energy is tuned to the nearest interband surface plasmon resonance of the nanotube so that the mixed strongly coupled surface plasmon-exciton excitations are formed. We discuss possible ways to bring the exciton in resonance with the surface plasmon. The exciton-plasmon Rabi splitting effect we predict here for an individual carbon nanotube is close in its magnitude to that previously reported for hybrid plasmonic nanostructures artificially fabricated of organic semiconductors deposited on metallic films. We expect this effect to open up paths to new tunable optoelectronic device applications of semiconducting carbon nanotubes.
We have further advanced the theory of in-plane electrical conduction across grain-boundaries, which now includes an analytical solution to a two-step thermally-assisted tunneling model. In addition, we have modified the one-step...
moreWe have further advanced the theory of in-plane electrical conduction across grain-boundaries, which now includes an analytical solution to a two-step thermally-assisted tunneling model. In addition, we have modified the one-step thermally-assisted tunneling model to approximate the effect of grain-boundary barrier height inhomogeneity. We use these models to study the effects of the vapor CdCl2 treatment on the grain-boundary electrical properties of the thin polycrystalline films, as processed for solar cells. Compared to untreated polycrystalline CdTe, we find that the vapor CdCl2 treatment increases the dark grain-boundary barrier height and near grain-boundary doping. Also, both cases show little barrier height inhomogeneity
A model is presented for the magnon-phonon interaction in three-dimensional cubic ferromagnetic systems. The Heisenberg Hamiltonian for localized spins is used. The calculated magnon and phonon dampings are compared with the experiments...
moreA model is presented for the magnon-phonon interaction in three-dimensional cubic ferromagnetic systems. The Heisenberg Hamiltonian for localized spins is used. The calculated magnon and phonon dampings are compared with the experiments and good agreement is found. It is estimated that there is a significant broadening in the magnon linewidth at the end of the zone and that the phonon linewidth is not affected by this mechanism. The modified magnon spectrum is also evaluated and compared to experimental measurements. It is found that the model predicts softening of the magnon mode at the end of the zone consistent with the experiments. The contribution from both Mn and O atoms is included. The developed model can be applied to systems with reduced dimensions and systems with different than cubic symmetry.
Artificial van der Waals heterostructures constitute an emerging field that promises to design systems with properties on demand. Stacking patterns and the utilization of different types of chemically inert layers can deliver novel...
moreArtificial van der Waals heterostructures constitute an emerging field that promises to design systems with properties on demand. Stacking patterns and the utilization of different types of chemically inert layers can deliver novel materials and devices. Despite the relatively weak van der Waals interaction, which does not affect the electronic properties around the Fermi level, our first-principles calculations show significant changes in the higher conduction and deeper valence regions in the considered graphene/silicene, graphene/MoS2, and silicene/MoS2 systems. Such changes are linked to strong out-of-plane hybridization effects and van der Waals interactions. We also find that the interface coupling significantly affects the vibrational properties of the heterostructures when compared to the individual constituents. Specifically, the van der Waals coupling is found to be a major factor for the stability of the system. The emergence of shear and breathing modes, as well as the t...
We have studied the electronic structure of the Ca_2RuO4 using the full-potential linearized augmented plane-wave method, which is an implementation of the density functional theory with the local spin density approximation. The code that...
moreWe have studied the electronic structure of the Ca_2RuO4 using the full-potential linearized augmented plane-wave method, which is an implementation of the density functional theory with the local spin density approximation. The code that is used is WIEN97. The material has a reach variety of magnetic and structural phases at different temperatures. The electronic band structure and the density of states for different phases are presented. We compare our results with results for other representatives of the 4d-family of the ruthenates.
The interaction energy due to electromagnetic field fluctuations between two infinitely long straight parallel dielectric-diamagnetic cylinders immersed in a medium is considered. We make use of the mode summation method for the...
moreThe interaction energy due to electromagnetic field fluctuations between two infinitely long straight parallel dielectric-diamagnetic cylinders immersed in a medium is considered. We make use of the mode summation method for the calculations. We investigate the energy dependence on the cylindrical radial curvature and dielectric response of the involved materials. It is shown that the sign of the interaction energy can be changed by a suitable choice of the dielectric properties of the involved objects. The condition for the relation of the material's dielectric properties for repulsive interaction is obtained to be the same as the one for planar materials.
A new array camera system was developed using a 58 x 62 pixel Si:Ga (gallium doped silicon) DRO (direct readout) photoconductor array detector manufactured by Hughes/Santa Barbara Research Center (SBRC). The camera system is a broad band...
moreA new array camera system was developed using a 58 x 62 pixel Si:Ga (gallium doped silicon) DRO (direct readout) photoconductor array detector manufactured by Hughes/Santa Barbara Research Center (SBRC). The camera system is a broad band photometer designed for 5 to 14 micron imaging with large ground-based optical telescopes. In a typical application a 10 micron photon flux of about 10(exp 9) photons sec(exp -1) m(exp -2) microns(exp -1) arcsec(exp -2) is incident in the telescope focal plane, while the detector well capacity of these arrays is 10(exp 5) to 10 (exp 6) electrons. However, when the real efficiencies and operating conditions are accounted for, the 2-channel 3596 pixel array operates with about 1/2 full wells at 10 micron and 10% bandwidth with high duty cycle and no real experimental compromises.
Cd2Os2O7 crystallizes in the pyrochlore structure and undergoes a metal-insulator transition (MIT) near 226 K. We have characterized the MIT in Cd2Os2O7 using x-ray diffraction, resistivity at ambient and high pressure, specific heat,...
moreCd2Os2O7 crystallizes in the pyrochlore structure and undergoes a metal-insulator transition (MIT) near 226 K. We have characterized the MIT in Cd2Os2O7 using x-ray diffraction, resistivity at ambient and high pressure, specific heat, magnetization, thermopower, Hall coefficient, and thermal conductivity. Both single crystals and polycrystalline material were examined. The MIT is accompanied by no change in crystal symmetry and a change in unit-cell volume of less than 0.05%. The resistivity shows little temperature dependence above 226 K, but increases by 3 orders of magnitude as the sample is cooled to 4 K. The specific heat anomaly resembles a mean-field transition and shows no hysteresis or latent heat. Cd2Os2O7 orders magnetically at the MIT. The magnetization data are consistent with antiferromagnetic order, with a small parasitic ferromagnetic component. The Hall and Seebeck coefficients are consistent with a semiconducting gap opening at the Fermi energy at the MIT. We have also performed electronic structure calculations on Cd2Os2O7. These calculations indicate that Cd2Os2O7 is metallic, with a sharp peak in the density of states at the Fermi energy. We interpret the data in terms of a Slater transition. In this scenario, the MIT is produced by a doubling of the unit cell due to the establishment of antiferromagnetic order. A Slater transition-unlike a Mott transition-is predicted to be continuous, with a semiconducting energy gap opening much like a BCS gap as the material is cooled below TMIT.
The terminase enzyme from bacteriophage lambda is responsible for the insertion of viral DNA into the confined space within the capsid. The enzyme is composed of the virally encoded proteins gpA (73.3 kDa) and gpNu1 (20.4 kDa) isolated as...
moreThe terminase enzyme from bacteriophage lambda is responsible for the insertion of viral DNA into the confined space within the capsid. The enzyme is composed of the virally encoded proteins gpA (73.3 kDa) and gpNu1 (20.4 kDa) isolated as a gpA(1).gpNu1(2) holoenzyme complex. Lambda terminase possesses a site-specific nuclease activity, an ATP-dependent DNA strand-separation activity, and an ATPase activity that must work in concert to effect genome packaging. We have previously characterized the ATPase activity of the holoenzyme and have identified catalytic active sites in each enzyme subunit [Tomka and Catalano (1993) Biochemistry 32, 11992-11997; Hwang et al. (1996) Biochemistry 35, 2796-2803]. We have noted that GTP stimulates the ATPase activity of the enzyme, and terminase-mediated GTP hydrolysis has been observed. The studies presented here describe a kinetic analysis of the GTPase activity of lambda terminase. GTP hydrolysis by the enzyme requires divalent metal, is optimal at alkaline pH, and is strongly inhibited by salt. Interestingly, while GTP can bind to the enzyme in the absence of DNA, GTP hydrolysis is strictly dependent on the presence of polynucleotide. Unlike ATP hydrolysis that occurs at both subunits of the holoenzyme, a single catalytic site is observed in the steady-state kinetic analysis of GTPase activity (k(cat) approximately 37 min(-)(1); K(m) approximately 500 microM). Moreover, while GTP stimulates ATP hydrolysis (apparent K(D) approximately 135 microM for GTP binding), all of the adenosine nucleotides examined strongly inhibit the GTPase activity of the enzyme. The data presented here suggest that the two "NTPase" catalytic sites in terminase holoenzyme communicate, and we propose a model describing allosteric interactions between the two sites. The biological significance of this interaction with respect to the assembly and disassembly of the multiple nucleoprotein packaging complexes required for virus assembly is discussed.
New effects due to the electron-phonon interaction in some low-dimensional tight-binding systems are discussed. A sheet of graphite (two-dimensional) and an armchair single wall carbon nanotube (SWNT) (quasi-one dimensional) are taken as...
moreNew effects due to the electron-phonon interaction in some low-dimensional tight-binding systems are discussed. A sheet of graphite (two-dimensional) and an armchair single wall carbon nanotube (SWNT) (quasi-one dimensional) are taken as examples. The geometrical structure and the linear dispersion of the energy with respect to the electron wave vector are expected to play a significant role. For the ordinary
A theoretical model for the magnon-phonon interaction in cubic three-dimensional antiferromagnetic and ferromagnetic systems is presented. The starting point is the Heisenberg Hamiltonian for localized spins. The proposed model is only...
moreA theoretical model for the magnon-phonon interaction in cubic three-dimensional antiferromagnetic and ferromagnetic systems is presented. The starting point is the Heisenberg Hamiltonian for localized spins. The proposed model is only for the acoustic part of the magnon and phonon spectra. Using Green's functions technique the real and imaginery parts of the magnon and phonon self-energies are calculated. The effects
We study theoretically the interactions of excitonic states with surface electromagnetic modes of small-diameter (˜1nm) semiconducting single-walled carbon nanotubes (CNs). We show that these interactions can result in strong...
moreWe study theoretically the interactions of excitonic states with surface electromagnetic modes of small-diameter (˜1nm) semiconducting single-walled carbon nanotubes (CNs). We show that these interactions can result in strong exciton-interband-surface-plasmon coupling in individual CNs. This results in the exciton emission line (Rabi) splitting ˜0.1eV as the exciton energy is tuned to the nearest interband plasmon resonance of the CN [1].
Bulk polycrystalline chalcogenides were prepared from chalcogenide nanocrystals synthesized in high yields by a variety of chemical processes. Doping of the nanocrystals allowed for the modification of the carrier concentration in the...
moreBulk polycrystalline chalcogenides were prepared from chalcogenide nanocrystals synthesized in high yields by a variety of chemical processes. Doping of the nanocrystals allowed for the modification of the carrier concentration in the bulk composites. Densification employing spark plasma sintering dimensionally integrated nanoscale grains within a bulk matrix, resulting in a uniform dispersion of nonconglomerated nanocrystals. Transport properties of the resulting dimensional nanocomposites were evaluated through temperature dependent resistivity, Hall, Seebeck coefficient, and thermal conductivity measurements. These properties were investigated as a function of nanocrystal size and composition. The results are discussed in terms of the potential for thermoelectric applications.
A model is presented for the magnon-phonon interaction in three-dimensional cubic ferromagnetic systems. The Heisenberg Hamiltonian for localized spins is used. The calculated magnon and phonon dampings are compared with the experiments...
moreA model is presented for the magnon-phonon interaction in three-dimensional cubic ferromagnetic systems. The Heisenberg Hamiltonian for localized spins is used. The calculated magnon and phonon dampings are compared with the experiments and good agreement is found. It is estimated that there is a significant broadening in the magnon linewidth at the end of the zone and that the phonon linewidth is not affected by this mechanism. The modified magnon spectrum is also evaluated and compared to experimental measurements. It is found that the model predicts softening of the magnon mode at the end of the zone consistent with the experiments. The contribution from both Mn and O atoms is included. The developed model can be applied to systems with reduced dimensions and systems with different than cubic symmetry.
We present theoretical investigations of temperature-dependent Casimir interactions of a graphene flake between substrates in a fluid. By properly choosing the materials, we propose that the graphene can be suspended in the fluid due to...
moreWe present theoretical investigations of temperature-dependent Casimir interactions of a graphene flake between substrates in a fluid. By properly choosing the materials, we propose that the graphene can be suspended in the fluid due to the balance between the Casimir, buoyancy and gravitational forces. The graphene properties, such as the Dirac-like nature of the carriers and universal optical conductivity, have a profound effect on the Casimir force making it completely thermal at room temperature. Since thermal contributions to the Casimir interaction in most materials are usually small, the graphene system offers a unique opportunity to demonstrate such effects without going to extreme temperatures. We show that the equilibrium position of the suspended flake is temperature dependent. We suggest that this maybe a promising system for observing thermal Casimir effects via levitation.
Efficiency-limiting defects in photovoltaic devices are readily probed by time-resolved spectroscopy. This paper presents the first direct optical measurements of the relaxation and recombination pathways of photoexcited carriers in the...
moreEfficiency-limiting defects in photovoltaic devices are readily probed by time-resolved spectroscopy. This paper presents the first direct optical measurements of the relaxation and recombination pathways of photoexcited carriers in the CdS window layer of CdTe/CdS polycrystalline thin films. We utilize the complimentary techniques of femtosecond time-resolved differential absorption (TRDA) and picosecond time-resolved photoluminescence (TRPL) to determine the relaxation and recombination
The functionalization of carbon nanotubes (CNTs) by molecular adsorption is of scientific interest and also of importance in potential applications as sensors. We have studied theoretically the interactions between CNTs and organic...
moreThe functionalization of carbon nanotubes (CNTs) by molecular adsorption is of scientific interest and also of importance in potential applications as sensors. We have studied theoretically the interactions between CNTs and organic aromatic molecules that are derived from benzene by addition of different functional groups (e.g. CH3, OH, NO2). We perform density functional ab initio calculations based on the plane-wave
Understanding the adsorption of aromatic molecules on carbon nanotubes is important for nanotube functionalization. We perform ab-initio plane-wave calculations for the adsorption of benzene derivatives such as nitrobenzene, aniline and...
moreUnderstanding the adsorption of aromatic molecules on carbon nanotubes is important for nanotube functionalization. We perform ab-initio plane-wave calculations for the adsorption of benzene derivatives such as nitrobenzene, aniline and toluene, using pseudopotentials in the local density approximation. We find that the minimum energy configurations of the molecules are flat along the nanotube, in agreement with experiments on polar molecules