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Articles 1 - 18 of 18
Full-Text Articles in Physics
Thermodynamics Of Modified Theories Of Gravity, Aric Hackebill
Thermodynamics Of Modified Theories Of Gravity, Aric Hackebill
Theses and Dissertations
Einstein’s equations are derived by following Jacobson’s thermodynamic method. It is seen that a family of possible field equations exist which satisfy the thermodynamic argument. Modified theories of gravity are addressed as possible candidates for replacing dark matter as an explanation for anomalous cosmological phenomena. Many of the proposed modified theories are not powerful enough to explain the currently observed phenomena and are rejected as viable theories of gravity. A surviving candidate, TeVeS, is further analyzed under the aforementioned thermodynamic argument to check for its consistency with thermodynamics.
Theoretical Investigation Of The Structures And Stability Of Gas Phase Neutral And Cationic Tixoy Clusters., Baljeet Kaur
Theoretical Investigation Of The Structures And Stability Of Gas Phase Neutral And Cationic Tixoy Clusters., Baljeet Kaur
Theses and Dissertations
Theoretical investigation of the structure and stability of neutral and cationic TixOy cluster series (where y =2x-1, 2x, 2x+1) have been performed. The lowest lying structures for the neutral clusters are usually found in the singlet state. Generally, in bulk and in the case of the neutral TixOy clusters, the 2x cluster series is relatively more abundant than the 2x-1 and the 2x+1 cluster series. But in the case of cationic TixOy clusters, the 2x-1 series is more abundant. To understand the origin of the stability of the TixO2x-1+ clusters, we use density functional theory within the NRLMOL set of …
Local Charging Behavior On Gan Surfaces, Josephus Ferguson
Local Charging Behavior On Gan Surfaces, Josephus Ferguson
Theses and Dissertations
Gallium nitride is an important III-V semiconductor which is used in many optoelectronic and high-frequency devices. The nature of the GaN surface and its electrical characteristics can impact the performance of such devices. In this study, several GaN surfaces are locally charged using an atomic force microscope, and then subsequently studied by measuring the surface potential with scanning Kelvin probe microscopy (SKPM). The charging and discharging behavior of the surface appears to be strongly influenced by surface preparation and the presence of a surface oxide layer. If a substantial oxide layer exists, then both positive and negative charging is possible …
Scalar Waves In An Almost Cylindrical Spacetime, Joseph Gordon
Scalar Waves In An Almost Cylindrical Spacetime, Joseph Gordon
Theses and Dissertations
The scalar wave equation is investigated for a scalar field propagating in a spacetime background ds²=e^{2a}(-dt²+dr²)+R(e^{-2ψ}dφ²+e^{2ψ}dz²). The metric is compactified in the radial direction. The spacetime slices of constant φ and z are foliated into outgoing null hypersurfaces by the null coordinate transformation u=t-r. The scalar field imitates the amplitude behavior of a light ray, or a gravitational wave, traveling along a null hypersurface when the area function R is a constant or is a function of u. These choices for R restrict the gravitational wave factor ψ to being an arbitrary function of u.
Oxidation Of Pdn (N=1–7,10) Clusters Supported On Alumina/Nial(110), R. Robles, Shiv N. Khanna
Oxidation Of Pdn (N=1–7,10) Clusters Supported On Alumina/Nial(110), R. Robles, Shiv N. Khanna
Physics Publications
First-principles theoretical investigations on the oxidation of Pdn (n=1–7, 10) clusters deposited on alumina/NiAl(110) have been carried out using a gradient-corrected density-functional approach. Our studies indicate that the free Pdn clusters are compact and maintain their compact structures when deposited on the surface, undergoing only small relaxations of the Pd-Pd distance. The clusters bind to the surface via a pair of Pd atoms and with a binding energy of around 1.0 eV. Studies of oxidation through an O2molecule show that O2 occupies sites closer to the surface for Pd1, Pd4, Pd5, and Pd6 while, in other cases, the binding is …
Magnetism In Zno Nanowire With Fe/Co Codoping: First-Principles Density Functional Calculations, S. Ghosh, Q. Wang, G. P. Das, Puru Jena
Magnetism In Zno Nanowire With Fe/Co Codoping: First-Principles Density Functional Calculations, S. Ghosh, Q. Wang, G. P. Das, Puru Jena
Physics Publications
Using first-principles density functional calculations, at both generalized gradient approximation (GGA) and GGA+U levels we have investigated the electronic structure and magnetic properties of Fe/Co codoped ZnO nanowire. Here we have addressed some of the key issues such as, the preferable sites that Fe/Co can occupy, the coupling mechanism, and role of defects in coupling. We found that the spin alignment between the transition-metal atoms depends on their location. When Fe and Co atoms are nearest neighbors on the outer surface of the nanowire along [0001] direction is the lowest energy configuration with ferrimagnetic (FiM) ground state. At GGA level …
Electronic And Magnetic Properties Of A Bn Sheet Decorated With Hydrogen And Fluorine, Jian Zhou, Qian Wang, Qiang Sun, Puru Jena
Electronic And Magnetic Properties Of A Bn Sheet Decorated With Hydrogen And Fluorine, Jian Zhou, Qian Wang, Qiang Sun, Puru Jena
Physics Publications
First-principles calculations based on density-functional theory reveal some unusual properties of BN sheet functionalized with hydrogen and fluorine. These properties differ from those of similarly functionalized graphene even though both share the same honeycomb structure. (1) Unlike graphene which undergoes a metal to insulator transition when fully hydrogenated, the band gap of the BN sheet significantly narrows when fully saturated with hydrogen. Furthermore, the band gap of the BN sheet can be tuned from 4.7 to 0.6 eV and the system can be a direct or an indirect semiconductor or even a half-metal depending on surface coverage. (2) Unlike graphene, …
Crystal Field Effects On The Reactivity Of Aluminum-Copper Cluster Anions, Patrick J. Roach, W. Hunter Woodward, Arthur C. Reber, Shiv N. Khanna, A. W. Castleman Jr.
Crystal Field Effects On The Reactivity Of Aluminum-Copper Cluster Anions, Patrick J. Roach, W. Hunter Woodward, Arthur C. Reber, Shiv N. Khanna, A. W. Castleman Jr.
Physics Publications
The limits and useful modifications of the jellium model are of great interest in understanding the properties of metallic clusters, especially involving bimetallic systems. We have measured the relative reactivity of CuAl−n clusters (n=11–34) with O2. An odd-even alternation is observed that is in accordance with spin-dependant etching, and CuAl−22is observed as a “magic peak.” The etching resistance of CuAl−22 is explained by an unusually large splitting of the 2D10 subshell that occurs because of a geometric distortion of the cluster that may also be understood as a crystal field splitting of the superatomic orbitals.
First-Principles Study Of Hydrogen Adsorption In Metal-Doped Cof-10, Miao Miao Wu, Qian Wang, Qiang Sun, Puru Jena, Yoshiyuki Kawazoe
First-Principles Study Of Hydrogen Adsorption In Metal-Doped Cof-10, Miao Miao Wu, Qian Wang, Qiang Sun, Puru Jena, Yoshiyuki Kawazoe
Physics Publications
Covalent organic frameworks (COFs), due to their low-density, high-porosity, and high-stability, have promising applications in gas storage. In this study we have explored the potential of COFs doped with Li and Ca metal atoms for storing hydrogen under ambient thermodynamic conditions. Using density functional theory we have performed detailed calculations of the sites Li and Ca atoms occupy in COF-10 and their interaction with hydrogen molecules. The binding energy of Li atom on COF-10 substrate is found to be about 1.0 eV and each Li atom can adsorb up to three H2 molecules. However, at high concentration, Li atoms cluster …
Negative Ions Of Transition Metal-Halogen Clusters, Kalpataru Pradhan, Gennady L. Gutsev, Purusottam Jena
Negative Ions Of Transition Metal-Halogen Clusters, Kalpataru Pradhan, Gennady L. Gutsev, Purusottam Jena
Physics Publications
A systematic density functional theory based study of the structure and spectroscopic properties of neutral and negatively chargedMXn clusters formed by a transition metal atom M (M=Sc,Ti,V) and up to seven halogen atoms X (X=F,Cl,Br) has revealed a number of interesting features: (1) Halogen atoms are bound chemically to Sc, Ti, and V for n≤nmax, where the maximal valence nmax equals to 3, 4, and 5 for Sc, Ti, and V, respectively. For n>nmax, two halogen atoms became dimerized in the neutral species, while dimerization begins at n=5, 6, and 7 for negatively charged clusters containing Sc, Ti, and …
Electronic Structure And Properties Of Isoelectronic Magic Clusters: Al13x (X=H,Au,Li,Na,K,Rb,Cs), Yeon Jae Ko, Anisha Shakya, Haopeng Wang, Andrej Grubisic, Weijun Zheng, Matthias Gotz, Gerd Gantefor, Kit H. Bowen, Puru Jena, Boggavarapu Kiran
Electronic Structure And Properties Of Isoelectronic Magic Clusters: Al13x (X=H,Au,Li,Na,K,Rb,Cs), Yeon Jae Ko, Anisha Shakya, Haopeng Wang, Andrej Grubisic, Weijun Zheng, Matthias Gotz, Gerd Gantefor, Kit H. Bowen, Puru Jena, Boggavarapu Kiran
Physics Publications
The equilibrium structure, stability, and electronic properties of the Al13X (X=H,Au,Li,Na,K,Rb,Cs) clusters have been studied using a combination of photoelectron spectroscopy experiment and density functional theory. All these clusters constitute 40 electron systems with 39 electrons contributed by the 13 Al atoms and 1 electron contributed by each of the X (X=H,Au,Li,Na,K,Rb,Cs) atom. A systematic study allows us to investigate whether all electrons contributed by the X atoms are alike and whether the structure, stability, and properties of all the magic clusters are similar. Furthermore, quantitative agreement between the calculated and the measured electron affinities and vertical detachment energies enable …
Enhanced Magnetic Moments Of Alkali Metal Coated Sc Clusters: New Magnetic Superatoms, Kalpataru Pradhan, J. Ulises Reveles, Prasenjit Sen, S. N. Khanna
Enhanced Magnetic Moments Of Alkali Metal Coated Sc Clusters: New Magnetic Superatoms, Kalpataru Pradhan, J. Ulises Reveles, Prasenjit Sen, S. N. Khanna
Physics Publications
It is shown that the magnetic moments of Sc atoms can be significantly enhanced by combining them with alkali atoms. We present results of first principles electronic structure calculations of ScNan (1≤n≤12) clusters that indicate that a ScNa12 cluster consisting of a Sc atom surrounded by 12 Na atoms forming a compact icosahedral structure has a spin magnetic moment of 3μB that is three times that of an isolated Sc atom. This unusual behavior is analyzed in terms of the filling of the supershells 1S, 1P,… controlled by the nature and size of the alkali atoms and the more localized …
Magnetic Moment And Local Moment Alignment In Anionic And/Or Oxidized Fen Clusters, D. R. Roy, R. Robles, S. N. Khanna
Magnetic Moment And Local Moment Alignment In Anionic And/Or Oxidized Fen Clusters, D. R. Roy, R. Robles, S. N. Khanna
Physics Publications
First principles studies on the ground statestructure, binding energy, spin multiplicity, and the noncollinearity of local spin moments in Fen and Fen− clusters and their oxides, viz., FenO2and FenO2− have been carried out within a density functional formalism. The ground states of Fen and Fen− clusters have collinear spins with a magnetic moment of around 3.0 μB per atom. The O2 molecule is found to be dissociatively absorbed and its most significant effect on spin occurs in Fe2, where Fe2O2 and Fe2O2− show antiferromagnetic and noncollinear spin arrangements, respectively. The calculated adiabatic electron affinity and the vertical transitions from the …
Structures Of Neutral And Anionic Au16 Clusters Revisited, Gang Chen, Qian Wang, Qiang Sun, Yoshiyuki Kawazoe, Puru Jena
Structures Of Neutral And Anionic Au16 Clusters Revisited, Gang Chen, Qian Wang, Qiang Sun, Yoshiyuki Kawazoe, Puru Jena
Physics Publications
Using the density functional theory and generalized gradient approximation for exchange and correlation potential, we have performed an extensive search to obtain the ground stategeometries of both neutral and anionic Au16 cluster by considering over 200 low lying isomers. For the neutral Au16 cluster we found a new Td compact structure to be the lowest energy configuration, which is nearly degenerate in total energy with the previously reported Cs and C2v noncage structures. While the Au16− cluster, in agreement with previous calculations, is found to have a Td hollow cage structure, an isomer with a planar structure is found to …
Probing The Existence Of Energetically Degenerate Cluster Isomers By Chemical Tagging, Qian Wang, Qiang Sun, Puru Jena
Probing The Existence Of Energetically Degenerate Cluster Isomers By Chemical Tagging, Qian Wang, Qiang Sun, Puru Jena
Physics Publications
Current methods for identifying the ground state geometry of a cluster require synergy between theory and experiment. However, this becomes a difficult problem when the accuracy of the theoretical methods is not sufficient to distinguish between nearly degenerate isomers. Using density functional theory based calculations, we show that the near degeneracy between the planar and the cage structures can be lifted by tagging these with halogens and superhalogens moieties such as Cl and BO2. The energy of the planar Au16− isomer is lowered from 0.15 eV before tagging to 0.51–0.55 eV after tagging, thus providing a way to probe its …
The Applicability Of Three-Dimensional Aromaticity In Bisnn- Zintl Analogues, Penee A. Clayborne, Ujjwal Gupta, Arthur C. Reber, Joshua J. Melko, Shiv N. Khanna, A. W. Castleman Jr.
The Applicability Of Three-Dimensional Aromaticity In Bisnn- Zintl Analogues, Penee A. Clayborne, Ujjwal Gupta, Arthur C. Reber, Joshua J. Melko, Shiv N. Khanna, A. W. Castleman Jr.
Physics Publications
Three-dimensional aromaticity is shown to play a role in the stability of deltahedral Zintl clusters and here we examine the connection between aromaticity and stability. In order to gain further insight, we have studied Zintl analogs comprised of bismuthdopedtin clusters with photoelectron spectroscopy and theoretical methods. To assign aromaticity, we examine the ring currents induced around the cage by using the nucleus independent chemical shift. In the current study, BiSn4− is a stable cluster and fits aromatic criteria, while BiSn5− is found to fit antiaromatic criteria and has reduced stability. The more stable clusters exhibit an aromatic character which originates …
Communications: Chain And Double-Ring Polymeric Structures: Observation Of Alnh3n+1− (N=4–8) And Al4h14, Xiang Li, Andrej Grubisic, Kit H. Bowen, Anil K. Kandalam, Boggavarapu Kiran, Gerd Gantefor, Puru Jena
Communications: Chain And Double-Ring Polymeric Structures: Observation Of Alnh3n+1− (N=4–8) And Al4h14, Xiang Li, Andrej Grubisic, Kit H. Bowen, Anil K. Kandalam, Boggavarapu Kiran, Gerd Gantefor, Puru Jena
Physics Publications
A pulsed arc discharge source was used to prepare gas-phase, aluminum hydride cluster anions, AlnHm−, exhibiting enhanced hydrogen content. The maximum number of hydrogen atoms in AlnHm− species was m=3n+1 for n=5–8, i.e., AlnH3n+1−, and m=3n+2 for n=4, i.e., Al4H14−, as observed in their mass spectra. These are the most hydrogen-rich aluminum hydrides to be observed thus far, transcending the 3:1 hydrogen-to-aluminum ratio in alane. Even more striking, ion intensities for AlnHm− species with m=3n+1 and m=3n+2 hydrogen atoms were significantly higher than those of nearby AlnHm− mass peaks for which m<3n+1, i.e., the ion intensities for AlnH3n+1− and for Al4H14− deviated from the roughly bell-shaped ion intensity patterns seen for most AlnHm−species, in which m ranges from 1 to 3n. Calculations based on density functional theory showed that AlnH3n+1− clusters have chain and/or double-ring polymericstructures.
Surface Photovoltage In Undoped N-Type Gan, M. A. Reshchikov, M. Foussekis, A. A. Baski
Surface Photovoltage In Undoped N-Type Gan, M. A. Reshchikov, M. Foussekis, A. A. Baski
Physics Publications
Steady-state and transient surface photovoltage (SPV) in undoped GaN is studied in vacuum and air ambient at room temperature and 400 K with a Kelvin probe. The results are explained within a phenomenological model accounting for the accumulation of photogenerated holes at the surface, capture of free electrons from the bulk over the near-surface potential barrier, and emission of electrons from surface states into the bulk. Simple analytical expressions are obtained and compared with experimental results. In particular, the proposed model explains the logarithmic decay of the SPV after stopping illumination. Internal and external mechanisms of the SPV are discussed …