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Articles 31 - 45 of 45
Full-Text Articles in Physics
Normal Transport At Positive Temperatures In Classical Hamiltonian Open Systems, P. Lafitte, Paul Ernest Parris, Stephan De Bièvre
Normal Transport At Positive Temperatures In Classical Hamiltonian Open Systems, P. Lafitte, Paul Ernest Parris, Stephan De Bièvre
Physics Faculty Research & Creative Works
We study the transport properties of classical Hamiltonian models describing the motion of an unconfined particle coupled to vibrational degrees of freedom in thermal equilibrium at zero or positive temperature. We identify and discuss conditions under which, in such systems, the particle has a well-defined diffusion constant and mobility. We will in particular point out some marked differences with the situation where the particle is confined and described with a Caldeira-Leggett model. We will more specifically report on results obtained in a classical version of the Holstein molecular crystal model, speculate on their relevance in the corresponding quantum system and …
Evaluation Of Laser-Assisted Bremsstrahlung With Dirac-Volkov Propagators, Erik Lotstedt, Ulrich D. Jentschura, Christoph H. Keitel
Evaluation Of Laser-Assisted Bremsstrahlung With Dirac-Volkov Propagators, Erik Lotstedt, Ulrich D. Jentschura, Christoph H. Keitel
Physics Faculty Research & Creative Works
We study spontaneous bremsstrahlung emission in a highly intense laser field. In this regime the interaction with the laser field has to be treated nonperturbatively by using the relativistic formalism including Dirac-Volkov propagators, while the interaction with the Coulomb field and the bremsstrahlung radiation can be treated in first-order perturbation theory. For the intermediate electron state a fully laser-dressed propagator is used, including radiative corrections to avoid singularities on the mass shell. We find that the use of the Dirac-Volkov propagator is crucial to obtain correct numerical results. The cross section of the process is evaluated for laser intensities of …
Screened-Exchange Determination Of The Electronic Properties Of Monoclinic, Tetragonal And Cubic Zirconia, Julia E. Medvedeva, C. B. Geller, D. M. Rishel, Arthur J. Freeman
Screened-Exchange Determination Of The Electronic Properties Of Monoclinic, Tetragonal And Cubic Zirconia, Julia E. Medvedeva, C. B. Geller, D. M. Rishel, Arthur J. Freeman
Physics Faculty Research & Creative Works
First-principles electronic band structure investigations of monoclinic, tetragonal, and cubic ZrO2 reveal the highly anisotropic nature of the conduction and valence band topologies in the monoclinic phase with electron and hole effective masses differing by over an order of magnitude in perpendicular directions. The planes of relatively high implied electron and hole mobilities intersect along a single crystallographic direction, making this the only direction readily available for exciton motion. Conversely, in the tetragonal and cubic phases, charge carrier effective masses are more isotropic and exciton motion is less restricted. These findings may explain recent experimental observations suggesting that exciton production …
Differential Double Capture Cross Sections In P+He Collisions, Michael Schulz, T. Vajnai, J. A. Brand
Differential Double Capture Cross Sections In P+He Collisions, Michael Schulz, T. Vajnai, J. A. Brand
Physics Faculty Research & Creative Works
We have measured differential double capture cross sections for 15 to 150 keV p+He collisions. We also analyzed differential double to single capture ratios, where we find pronounced peak structures. An explanation of these structures probably requires a quantum-mechanical description of elastic scattering between the projectile and the target nucleus. Strong final-state correlations have a large effect on the magnitude of the double capture cross sections
Electronic Band Structure And Carrier Effective Mass In Calcium Aluminates, Julia E. Medvedeva, E. N. Teasley, M. D. Hoffman
Electronic Band Structure And Carrier Effective Mass In Calcium Aluminates, Julia E. Medvedeva, E. N. Teasley, M. D. Hoffman
Physics Faculty Research & Creative Works
First-principles electronic band structure investigations of five compounds of the CaO-Al2O3 family, 3CaO·Al2O3, 12CaO·7Al2O3, CaO·Al2O3, CaO·2Al2O3, and CaO·6Al2O3, as well as CaO and alpha-, theta-, and kappa-Al2O3 are performed. We find that the conduction band in the complex oxides is formed from the oxygen antibonding p states and, although the band gap in Al2O3 is almost twice larger than in CaO, the s states of both cations. Such a hybrid nature of the conduction band leads to isotropic electron effective masses which are nearly the same for all compounds investigated. This insensitivity of the effective mass to variations in the …
Erratum: Evaluation Of Laser-Assisted Bremsstrahlung With Dirac-Volkov Propagators (Physical Review Letters (2007) 98 (043002)), Erik Lotstedt, Ulrich D. Jentschura, Christoph H. Keitel
Erratum: Evaluation Of Laser-Assisted Bremsstrahlung With Dirac-Volkov Propagators (Physical Review Letters (2007) 98 (043002)), Erik Lotstedt, Ulrich D. Jentschura, Christoph H. Keitel
Physics Faculty Research & Creative Works
No abstract provided.
Electron Beam Characterization Of Carbon Nanostructures, Eric Samuel Mandell
Electron Beam Characterization Of Carbon Nanostructures, Eric Samuel Mandell
Doctoral Dissertations
"Atom-thick carbon nanostructures represent a class of novel materials that are of interest to those studying carbon's role in fossil fuel, hydrogen storage, scaled-down electronics, and other nanotechnology. Electron microscope images of "edge-on" graphene sheets show linear image features due to the projected potential of the sheets. Here, intensity profiles along these linear features can measure the curvature of the sheet, as well as the shape of the sheet (i.e. hexagonal, triangular). Also, electron diffraction powder profiles calculated for triangular graphene sheet shapes show a broadening of the low frequency edge of diffraction rings, in comparison to those calculated for …
Quantum Dot Photolithography, Raghuveer Reddy Gadipalli
Quantum Dot Photolithography, Raghuveer Reddy Gadipalli
Doctoral Dissertations
"This dissertation presents a simple method for the photolithographic patterning of silica hydrogel monoliths and planar substrates with quantum dots and inorganic semiconductor nanoparticles. We developed a method for surface patterning and bulk (3D) patterning of silica hydrogel monoliths and surface patterning of planar substrates with CdS, CdSe, PbS and PbSe quantum dots using infrared light, ultraviolet light, X-rays, and multi-photon ionization radiation. Precursor combinations were prepared which can readily dissociate with IR, UV, X-rays, and multi-photon ionization radiation. Different capping agents were used for improving quantum dot size distribution. The luminescence quantum yield of the composites can be increased …
Averaging Of The Electron Effective Mass In Multicomponent Transparent Conducting Oxides, Julia E. Medvedeva
Averaging Of The Electron Effective Mass In Multicomponent Transparent Conducting Oxides, Julia E. Medvedeva
Physics Faculty Research & Creative Works
We find that layered materials composed of various oxides of cations with s2 electronic configuration, XY2O4, X = in or Sc, Y = Ga, Zn, Al, Cd and/or Mg, exhibit isotropic electron effective mass which can be obtained via averaging over those of the corresponding single-cation oxide constituents. This effect is due to a hybrid nature of the conduction band formed from the s-states of all cations and the oxygen p-states. Moreover, the observed insensitivity of the electron effective mass to the oxygen coordination and to the distortions in the cation-oxygen chains suggests that a …
Local Defect In A Magnet With Long-Range Interactions, Jose A. Hoyos, Thomas Vojta
Local Defect In A Magnet With Long-Range Interactions, Jose A. Hoyos, Thomas Vojta
Physics Faculty Research & Creative Works
We investigate a single defect coupling to the square of the order parameter in a nearly critical magnet with long-range spatial interactions of the form r−(d+sigma), focusing on magnetic droplets nucleated at the defect while the bulk system is in the paramagnetic phase. To determine the static droplet profile, we solve a Landau-Ginzburg-Wilson action in the saddle-point approximation. Because of the long-range interaction, the droplet develops a power-law tail which is energetically unfavorable. However, as long as sigma>0, the tail contribution to the droplet free energy is subleading in the limit of large droplets; and the free energy becomes …
Angular Distributions From Photoionization Of H₂⁺, J. Colgan, O. Al-Hagan, Jerry Peacher, Don H. Madison, M. S. Pindzola, M. Foster
Angular Distributions From Photoionization Of H₂⁺, J. Colgan, O. Al-Hagan, Jerry Peacher, Don H. Madison, M. S. Pindzola, M. Foster
Physics Faculty Research & Creative Works
A study is made of the differential cross sections arising from the photoionization of H2+. Previous studies indicated surprising differences in the shapes of the angular distributions calculated from exterior complex scaling and 2C methods. To further explore these differences, we have calculated the angular distributions from the photoionization of H2+ using an independent two-body Coulomb function (2C) method and a distorted wave approach. As a final test, we also present calculations using a time-dependent technique. Our results confirm the discrepancies found previously and we present possible reasons for these differences.
Effects Of The Final-State Electron-Ion Interactions On The Fully Differential Cross Sections For Heavy-Particle-Impact Ionization Of Helium, Don H. Madison, Jerry Peacher, M. Foster, K. Bartschat, H. P. Saha, Allison L. Harris
Effects Of The Final-State Electron-Ion Interactions On The Fully Differential Cross Sections For Heavy-Particle-Impact Ionization Of Helium, Don H. Madison, Jerry Peacher, M. Foster, K. Bartschat, H. P. Saha, Allison L. Harris
Physics Faculty Research & Creative Works
Three-dimensional fully differential cross sections for heavy-particle-impact ionization of helium are examined. Previously, the three-body distorted-wave (3DW) model has achieved good agreement with experiment in the scattering plane for small momentum transfers, but poor agreement for large momentum transfers. Poor agreement was also observed outside the scattering plane for all momentum transfers. In particular, the 3DW calculations predicted cross sections that were too small both perpendicular to the scattering plane and for large momentum transfers. The important unanswered question concerns the physical effects that cause the significant disagreement between experiment and theory. In previous works, the role of the projectile-ion …
Spatial Distribution And Magnetism In Poly-Cr-Doped Gan From First Principles, X. Y. Cui, Julia E. Medvedeva, B. Delley, C. Stampfl, Arthur J. Freeman
Spatial Distribution And Magnetism In Poly-Cr-Doped Gan From First Principles, X. Y. Cui, Julia E. Medvedeva, B. Delley, C. Stampfl, Arthur J. Freeman
Physics Faculty Research & Creative Works
Large scale density-functional theory calculations have been performed to understand the spatial distribution and magnetic coupling of Cr-doped GaN, in which exhaustive structural and magnetic configurations have been investigated by doping of up to five Cr atoms in large supercells. Our results provide direct evidence that the distribution of the doped magnetic ions is neither homogeneous nor random as widely assumed previously. Rather, under both Ga-rich and N-rich growth conditions, the Cr atoms have a strong tendency to form substitutional, embedded clusters with short-range magnetic interactions maintaining the wurtzite structure. Significantly, while the ferromagnetic state is favored for pair doping, …
Effects Of Dissipation On A Quantum Critical Point With Disorder, Jose A. Hoyos, Chetan Kotabage, Thomas Vojta
Effects Of Dissipation On A Quantum Critical Point With Disorder, Jose A. Hoyos, Chetan Kotabage, Thomas Vojta
Physics Faculty Research & Creative Works
We study the effects of dissipation on a disordered quantum phase transition with O(N) order-parameter symmetry by applying a strong-disorder renormalization group to the Landau-Ginzburg-Wilson field theory of the problem. We find that Ohmic dissipation results in a nonperturbative infinite-randomness critical point with unconventional activated dynamical scaling while super-Ohmic damping leads to conventional behavior. We discuss applications to the superconductor-metal transition in nanowires and to the Hertz theory of the itinerant antiferromagnetic transition.
Photon Angular Distribution And Nuclear-State Alignment In Nuclear Excitation By Electron Capture, Adriana Palffy, Zoltan Harman, Andrey S. Surzhykov, Ulrich D. Jentschura
Photon Angular Distribution And Nuclear-State Alignment In Nuclear Excitation By Electron Capture, Adriana Palffy, Zoltan Harman, Andrey S. Surzhykov, Ulrich D. Jentschura
Physics Faculty Research & Creative Works
The alignment of nuclear states resonantly formed in nuclear excitation by electron capture (NEEC) is studied by means of a density matrix technique. The vibrational excitations of the nucleus are described by a collective model and the electrons are treated in a relativistic framework. Formulas for the angular distribution of photons emitted in the nuclear relaxation are derived. We present numerical results for alignment parameters and photon angular distributions for a number of heavy elements in the case of E2 nuclear transitions. Our results are intended to help future experimental attempts to discern NEEC from radiative recombination, which is the …