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Light quasiparticles dominate electronic transport in molecular crystal field-effect transistors

Description: We report on an infrared spectroscopy study of mobile holes in the accumulation layer of organic field-effect transistors based on rubrene single crystals. Our data indicate that both transport and infrared properties of these transistors at room temperature are governed by light quasiparticles in molecular orbital bands with the effective masses m[small star, filled]comparable to free electron mass. Furthermore, the m[small star, filled]values inferred from our experiments are in agreement with those determined from band structure calculations. These findings reveal no evidence for prominent polaronic effects, which is at variance with the common beliefs of polaron formation in molecular solids.
Date: March 1, 2007
Creator: Li, Z. Q.; Podzorov, V.; Sai, N.; Martin, Michael C.; Gershenson, M. E.; Di Ventra, M. et al.
Partner: UNT Libraries Government Documents Department


Description: The nonspherically symmetric solutions to the Bardeen-Cooper-Schrieffer theory are given a physical interpretation in terms of an anisotropic fluid model. These solutions have been used previously to predict a phase transition in liquid by He{sup 3} by Emery and Sessler and Anderson, Morel, Brueckner, and Soda. An investigation of the flow properties of such systems is made that involves the calculation of the effective mass for flow in a straight channel and the moment of inertia of a cylindrical container of the liquid. The angular dependent energy-gap characteristic of this type of theory leads to an effective mass for flow that depends on the angle between the axis of symmetry of the fluid and the direction of flow. It also vanishes as the absolute temperature tends to zero, although not as rapidly as for a spherically symmetric gap. The moment of inertia, when the symmetry direction for the fluid and the rotation axis are the same, is simply related to the mass for flow.
Date: May 16, 1960
Creator: Glassgold, A.E. & Sessler, A.M.
Partner: UNT Libraries Government Documents Department

Coexistence of Two- and Three-dimensional Shubnikov-de Haas Oscillations in Ar^+ -irradiated KTaO_3

Description: We report the electron doping in the surface vicinity of KTaO{sub 3} by inducing oxygen-vacancies via Ar{sup +}-irradiation. The doped electrons have high mobility (> 10{sup 4} cm{sup 2}/Vs) at low temperatures, and exhibit Shubnikov-de Haas oscillations with both two- and three-dimensional components. A disparity of the extracted in-plane effective mass, compared to the bulk values, suggests mixing of the orbital characters. Our observations demonstrate that Ar{sup +}-irradiation serves as a flexible tool to study low dimensional quantum transport in 5d semiconducting oxides.
Date: May 16, 2012
Creator: Harashima, S.; Bell, C.; Kim, M.; Yajima, T.; Hikita, Y. & Hwang, H.Y.
Partner: UNT Libraries Government Documents Department

A Proposal to Search for Charmed Particle Production Near Threshold

Description: We propose to search for charmed particle production thresholds by looking for anomalies in the ratio of K{sup -}/{pi}{sup -} production at P{sub T} {approx} 1 GeV/c as the incident energy is varied. Any anomalous behavior will be further investigated using a two arm effective mass spectrometer.
Date: November 1, 1974
Creator: Gross, D.; Nitz, D.; Olsen, S.; U., /Rochester; Abe, K.; Bomberowitz, R. et al.
Partner: UNT Libraries Government Documents Department

Approach for Selection of Rayleigh Damping Parameters Used for Time History Analysis

Description: Nonlinearities, whether geometric or material, need to be addressed in seismic analysis. One good analysis method that can address these nonlinearities is direct time integration with Rayleigh damping. Modal damping is the damping typically specified in seismic analysis Codes and Standards. Modal damping is constant for all frequencies where Rayleigh damping varies with frequency. An approach is proposed here for selection of Rayleigh damping coefficients to be used in seismic analyses that are consistent with given Modal damping. The approach uses the difference between the modal damping response and the Rayleigh damping response along with effective mass properties of the model being evaluated to match overall system response levels. This paper provides a simple example problem to demonstrate the approach. It also provides results for a finite element model representing an existing piping system. Displacement, acceleration, and stress results are compared from model runs using modal damping and model runs using Rayleigh damping with coefficients selected using the proposed method.
Date: July 1, 2009
Creator: Spears, R. E. & Jensen, S. R.
Partner: UNT Libraries Government Documents Department

Revised Proposal to Search for Heavy Resonances

Description: During October 1974, we submitted a most timely proposal to search for narrow, heavy resonances by measuring the effective mass spectrum of {pi}{pi}, {pi}K, KK, {pi}p, Kp and {bar p}p pairs. The subsequent discovery of a heavy, narrow resonance at SLAC and BNL caused much excitement and raised many new questions. In part, this may have been the reason why our proposal (No. 357) did not receive the consideration we feel it strongly deserved. We therefore resubmit our proposal, with additional clarification and improvement, and hope that the Program Advisory Committee and the Director will act favorably on our request to make these measurements.
Date: December 1, 1974
Creator: Jovanovic, D.; Lundy, R.; /Fermilab; Finley, D.; Loeffler, F.; Shibata, E. et al.
Partner: UNT Libraries Government Documents Department

MSW without matter

Description: We examine the effects of a scalar field, coupled only to neutrinos, on oscillations among weak interaction current eigenstates. The existence of a real scalar field is manifested as effective masses for the neutrino mass3 eigenstates, the same for F, as for v. Under some conditions, this can lead to a vanishing of {delta}m{sup 2}, giving rise to MSW-like effects. We present an idealized example and show that it may be possible to resolve the apparent discrepancy in spectra required by reprocess nucleosynthesis in the mantles of supernovae and by Solar neutrino solutions.
Date: September 1, 1996
Creator: Goldman, T.; McKellar, B.H.J. & Stephneson, G.J. Jr.
Partner: UNT Libraries Government Documents Department

Photoemission in strongly correlated crystalline f-electron systems: A need for a new approach

Description: The unusual properties of heavy fermion (or heavy electron) materials have sparked an avalanche of research over the last two decades in order to understand the basic phenomena responsible for these properties. Photoelectron spectroscopy (often referred to as PES in the following sections), the most direct measurement of the electronic structure of a material, should in principle be able to shed considerable light on this matter. In general the distinction between a localized and a band-like state is trivially observed in band dispersion. Much of the past work was performed on poly-crystalline samples, scraped in-situ to expose a clean surface for PES. There have since been considerable advances both in the quality of specimens as well as experimental resolution, which raise questions regarding these conclusions. Much of the past work on poly-crystalline samples has been reported in several review articles, most notably Allen et al., and it is not necessary here to review those efforts again, with the exception of subsequent work performed at high resolution. The primary focus of the present review will be on new measurements obtained on single crystals, cleaved or prepared in situ and measured at high resolution, which seem to suggest that agreement with the GS and NCA approximations is less than perfect, and that perhaps the starting models need to be modified, or that even an entirely new approach is called for. Of the promising new models the Periodic Anderson Model is most closely related to the SIM. Indeed, at high temperatures it reverts to the SIM. However, the charge polaron model of Liu (1997) as well as the two-electron band model of Sheng and Cooper (1995) cannot yet be ruled out. Inasmuch as the bulk of the single crystal work was performed by the Los Alamos group, this review will draw heavily on ...
Date: December 1, 1998
Creator: Arko, A.J.; Joyce, J.J. & Sarrao, J.
Partner: UNT Libraries Government Documents Department

Confined excitons, phonons and their interactions in Ge nanocrystals embedded in SiO2

Description: The authors report the resonant Raman scattering of the optical phonon in Ge nanocrystals with radius ranging from 2 to 5 nm. They have observed the effect of quantum confinement on both the optical phonon and the E{sub 1} exciton. The confinement energy of the E{sub 1} exciton has been explained within the effective mass approximation.
Date: December 31, 2000
Creator: Teo, K.L.; Kwok, S.H. & Yu, P.Y.
Partner: UNT Libraries Government Documents Department

Comparison of quantum confinement effects between quantum wires and dots

Description: Dimensionality is an important factor to govern the electronic structures of semiconductor nanocrystals. The quantum confinement energies in one-dimensional quantum wires and zero-dimensional quantum dots are quite different. Using large-scale first-principles calculations, we systematically study the electronic structures of semiconductor (including group IV, III-V, and II-VI) surface-passivated quantum wires and dots. The band-gap energies of quantum wires and dots have the same scaling with diameter for a given material. The ratio of band-gap-increases between quantum wires and dots is material-dependent, and slightly deviates from 0.586 predicted by effective-mass approximation. Highly linear polarization of photoluminescence in quantum wires is found. The degree of polarization decreases with the increasing temperature and size.
Date: March 30, 2004
Creator: Li, Jingbo & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Band anticrossing in dilute nitrides

Description: Alloying III-V compounds with small amounts of nitrogen leads to dramatic reduction of the fundamental band-gap energy in the resulting dilute nitride alloys. The effect originates from an anti-crossing interaction between the extended conduction-band states and localized N states. The interaction splits the conduction band into two nonparabolic subbands. The downward shift of the lower conduction subband edge is responsible for the N-induced reduction of the fundamental band-gap energy. The changes in the conduction band structure result in significant increase in electron effective mass and decrease in the electron mobility, and lead to a large enhance of the maximum doping level in GaInNAs doped with group VI donors. In addition, a striking asymmetry in the electrical activation of group IV and group VI donors can be attributed to mutual passivation process through formation of the nearest neighbor group-IV donor nitrogen pairs.
Date: December 23, 2003
Creator: Shan, W.; Yu, K.M.; Walukiewicz, W.; Wu, J.; Ager III, J.W. & Haller, E.E.
Partner: UNT Libraries Government Documents Department

Band anticrossing in highly mismatched semiconductor alloys

Description: The basic theoretical aspects of the band anticrossing effects in highly electronegativity-mismatched semiconductor alloys are reviewed. The many-impurity Anderson model treated in the coherent potential approximation is applied to the semiconductor alloys, in which metallic anion atoms are partially substituted by atoms of a highly electronegative element. Analytical solutions for the Green's function describe dispersion relations and state broadening effects for the restructured conduction band. The solutions are identical to those obtained from the physically intuitive and widely used two-level band anticrossing model. It is shown that the model explains key experimental observations including the unusual composition and pressure dependence of the interband optical transitions and the large enhancement of the electron effective mass.
Date: July 26, 2002
Creator: Walukiewicz, W.
Partner: UNT Libraries Government Documents Department


Description: The magnitude of the energy gap in nuclear matter associated with a highly correlated ground state of the type believed to be important in the theory of superconductivity has been evaluated theoretically. The integral equation of Cooper, Mills, and Sessler is linearized and transformed into a form suitable for numerical solution. The energy gap, calculated by using an appropriate single-particle potential and the Gammel-Thaler two-body potential, is found to be a very strong function of the density of nuclear matter, and of the effective mass at the Fermi surface. It is concluded that the magnitude of the energy gap for nuclear matter should not be compared directly with experimental values for finite nuclei, although the results suggest that if the theory is extended to apply to finite nuclei it probably would be in agreement with experiment.
Date: January 31, 1960
Creator: Emery, V.J. & Sessler, A.M.
Partner: UNT Libraries Government Documents Department

Predicting dissolution patterns in variable aperture fractures: 1. Development and evaluation of an enhanced depth-averaged computational model

Description: Water-rock interactions within variable-aperture fractures can lead to dissolution of fracture surfaces and local alteration of fracture apertures, potentially transforming the transport properties of the fracture over time. Because fractures often provide dominant pathways for subsurface flow and transport, developing models that effectively quantify the role of dissolution on changing transport properties over a range of scales is critical to understanding potential impacts of natural and anthropogenic processes. Dissolution of fracture surfaces is controlled by surface-reaction kinetics and transport of reactants and products to and from the fracture surfaces. We present development and evaluation of a depth-averaged model of fracture flow and reactive transport that explicitly calculates local dissolution-induced alterations in fracture apertures. The model incorporates an effective mass transfer relationship that implicitly represents the transition from reaction-limited dissolution to transport-limited dissolution. We evaluate the model through direct comparison to previously reported physical experiments in transparent analog fractures fabricated by mating an inert, transparent rough surface with a smooth single crystal of potassium dihydrogen phosphate (KDP), which allowed direct measurement of fracture aperture during dissolution experiments using well-established light transmission techniques [Detwiler, et al., 2003]. Comparison of experiments and simulations at different flow rates demonstrate the relative impact of the dimensionless Peclet and Damkohler numbers on fracture dissolution and the ability of the computational model to simulate dissolution. Despite some discrepancies in the small-scale details of dissolution patterns, the simulations predict the evolution of large-scale features quite well for the different experimental conditions. This suggests that our depth-averaged approach to simulating fracture dissolution provides a useful approach for extending laboratory results that are often limited in scale to scales that are more representative of geologic processes of interest.
Date: April 21, 2006
Creator: Detwiler, R L & Rajaram, H
Partner: UNT Libraries Government Documents Department

Synthesis of cadmium telluride quantum wires and the similarity of their band gaps to those of equidiameter cadmium telluride quantum dots

Description: High-quality colloidal CdTe quantum wires having purposefully controlled diameters in the range of 5-11 nm are grown by the solution-liquid-solid (SLS) method, using Bi-nanoparticle catalysts, cadmium octadecylphosphonate and trioctylphosphine telluride as precursors, and a TOPO solvent. The wires adopt the wurtzite structure, and grow along the [002] direction (parallel to the c axis). The size dependence of the band gaps in the wires are determined from the absorption spectra, and compared to the experimental results for high-quality CdTe quantum dots. In contrast to the predictions of an effective-mass approximation, particle-in-a-box model, and previous experimental results from CdSe and InP dot-wire comparisons, the band gaps of CdTe dots and wires of like diameter are found to be experimentally indistinguishable. The present results are analyzed using density functional theory under the local-density approximation by implementing a charge-patching method. The higher-level theoretical analysis finds the general existence of a threshold diameter, above which dot and wire band gaps converge. The origin and magnitude of this threshold diameter is discussed.
Date: July 11, 2008
Creator: Wang, Lin-Wang; Sun, Jianwei; Wang, Lin-Wang & Buhro, William E.
Partner: UNT Libraries Government Documents Department

Supercritical Stability, Transitions, and (Pseudo)tachyons

Description: Highly supercritical strings (c >> 15) with a time-like linear dilaton provide a large class of solutions to string theory, in which closed string tachyon condensation is under control (and follows the worldsheet renormalization group flow). In this note we analyze the late-time stability of such backgrounds, including transitions between them. The large friction introduced by the rolling dilaton and the rapid decrease of the string coupling suppress the back-reaction of naive instabilities. In particular, although the graviton, dilaton, and other light fields have negative effective mass squared in the linear dilaton background, the decaying string coupling ensures that their condensation does not cause large back-reaction. Similarly, the copious particles produced in transitions between highly supercritical theories do not back-react significantly on the solution. We discuss these features also in a somewhat more general class of time-dependent backgrounds with stable late-time asymptotics.
Date: January 3, 2007
Creator: Aharony, Ofer & Silverstein, Eva
Partner: UNT Libraries Government Documents Department

Single particle wavefunction localizations in bulged CdSenanowires

Description: Using atomistic empirical pseudopotentials, we havecalculated the electronic structures of CdSe nanowires with a bulgedarea. The localized state wavefunctions and their binding energies arecalculated, and their dependences on the bulged area shape are analyzed.We find that both the binding energy and the wavefunction localizationstrongly depend on the bulged area shape, with the most compact shapeproduces the largest binding energy and strongest wavefunctionlocalization. We also find that the top of the valence band state has aweaker localization than the bottom of the conduction band state due toan effective mass anisotropy.
Date: July 2, 2006
Creator: Zhao, Zhengji; Wang, Lin-Wang & Wu, Fengmin
Partner: UNT Libraries Government Documents Department

Tuning a double quantum well Fermi surface with in-plane magnetic fields

Description: A double quantum well (QW) subject to in-plane magnetic fields B{sub {parallel}} has the dispersion curves of its two QWs shifted in k-space. When the QWs are strongly coupled, an anticrossing and partial energy gap occur, yielding a tunable multi-component Fermi surface. We report measurements of the resultant features in the conductance, capacitive density of states, and giant deviations in cyclotron effective masses.
Date: July 1, 1995
Creator: Simmons, J.A.; Harff, N.E.; Eiles, T.M.; Lyo, S.K. & Klem, J.F.
Partner: UNT Libraries Government Documents Department

Equation of state of hadronic matter with dibaryons in an effective quark model

Description: The equation of state of symmetric nuclear matter with the inclusion of non-strange dibaryons is studied. They pay special attention to the existence of a dibaryon condensate at zero temperature. These calculations have been performed in an extended quark-meson coupling model with density-dependent parameters, which takes into account the finite size of nucleons and dibaryons. A first-order phase-transition to pure dibaryon matter has been found. The corresponding critical density is strongly dependent on the value of the dibaryon mass. The density behavior of the nucleon and dibaryon effective masses and confining volumes have also been discussed.
Date: June 1, 1998
Creator: Aguirre, R. & Schvellinger, M.
Partner: UNT Libraries Government Documents Department

A study of parameters useful for describing plasma-opening switches

Description: Plasma opening switches (POS) have been used continually and studied since their introduction in 1975. During that period they have performed well for prepulse suppression and sharpening the front of the power pulse. Their use for long conduction time and rapid opening to stand off high voltage in the same POS has met with very limited success. There has been a large theoretical effort involving models and particle-in-cell simulations (PICS), but the connection between theory and experiment has been tenuous at best, and convincing agreement with experiment has been minimal. The authors believe progress toward long conduction and rapid opening would be faster if macroscopic physical parameters describing the physics of the switch were used to compare experiment to simulation. One of these parameters (electron flow impedance) has been used to describe the electrical characteristics of the POS. This parameter provides a good description of both the standard POS (SPOS) and the magnetically controlled POS (MCPOS) because its value is sensibly independent of load current. An additional parameter, the effective mass of the plasma, was measured in one MCPOS experiment. In this article they describe other parameters important to operation of the SPOS and the MCPOS, and parameters important in designing PICS used to study these devices.
Date: September 1, 1998
Creator: Mendel, C.W. Jr.; Seidel, D.B. & Rosenthal, S.E.
Partner: UNT Libraries Government Documents Department

Conduction band mass determinations for n-type InGaAs/InAlAs single quantum wells

Description: The authors report the measurement of the conduction band mass in n-type single 27-ML-wide InGaAs/InAlAs quantum well lattice matched to InP using two methods: (1) Magnetoluminescence spectroscopy and (2) far-infrared cyclotron resonance. The magnetoluminescence method utilizes Landau level transitions between 0 and 14 T at 1.4 K. The far infrared cyclotron resonance measurements were made at 4.2 K and to fields as large up to 18 T. The 2D-carrier density N{sub 2D} = 3 {times} 10{sup 11} cm{sup {minus}2} at low temperatures. The magnetoluminescence technique yielded an effective conduction-band mass of m{sub c} = 0.062m{sub 0} while the far infrared cyclotron resonance measurements gave m{sub c} = 0.056m{sub 0}, where m{sub 0} is the free electron mass. Both measurements show no evidence for any significant conduction-band nonparabolicity.
Date: May 1, 1998
Creator: Jones, E.D.; Reno, J.L.; Kotera, Nobuo & Wang, Y.
Partner: UNT Libraries Government Documents Department


Description: The bremsstrahlung emitted by an electron scattered in a Coulomb field was first calculated by Bethe and Heitler. The total cross section for production of photons with wave number between k and k + dk by a nonrelativistic electron of kinetic energy {epsilon} is d{sigma}/dk dk = 16/3 Z{sup 2}r{sub 0}{sup 2} (e{sup 2}/hc) (mc{sup 2}/{epsilon})log ({radical} {epsilon}/hck + {radical} {epsilon}/hck -1) dk/k, where Ze is the charge of the (heavy) ion, and r{sub 0} is the classical electron radius. Bremsstrahlung in a plasma has been computed by a number of authors in the approximation of replacing the Coulomb field by a cut-off Coulomb or static Debye potential. It is the purpose of this communication to call attention to another important effect of the medium upon the rate of emission of bremsstrahlung. This may be described as a modification of the relation of the photon's energy to its wave number, due to the index of refraction of the medium. Equivalently, we note that one must include in the calculation of bremsstrahlung in a medium the photon-medium interactions which result in the 'clothing' of a 'bare' photon. The replacement of a particle by a quasiparticle has long been known to be important in the description of strongly interacting systems of massive particles such as liquid helium; the effect can be particularly dramatic for a photon because the medium gives a nonzero effective mass to the quasi-photon.
Date: January 15, 1963
Creator: Stack, John D. & Sessler, Andrew M.
Partner: UNT Libraries Government Documents Department