Latest content added for Digital Library Partner: UNT Librarieshttps://digital.library.unt.edu/explore/partners/UNT/browse/?fq=str_degree_discipline:Physics&start=302015-05-10T06:16:59-05:00UNT LibrariesThis is a custom feed for browsing Digital Library Partner: UNT LibrariesA Calculation of the Excitation Spectrum of Superfluid Helium-42015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504576/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504576/"><img alt="A Calculation of the Excitation Spectrum of Superfluid Helium-4" title="A Calculation of the Excitation Spectrum of Superfluid Helium-4" src="https://digital.library.unt.edu/ark:/67531/metadc504576/small/"/></a></p><p>The Hartree-Fock-Bogoliubov theory of homogeneous boson systems at finite temperatures is rederived using, a free energy variational principle. It is shown that a t-matrix naturally emerges in the theory. Phenomenological modifications are made (1) to remove the energy gap at zero momentum, and (2) to eliminate the Hartree-Fock-like terms, which dress the kinetic energy of the particle. A numerical calculation of the energy spectrum is made over a temperature range of 0.00 to 3.14 K using the Morse dipole-dipole-2 potential and the Frost-Musulin potential. The energy spectrum of the elementary excitations is calculated self-consistently. It has a phonon behavior at low momentum and a roton behavior at higher momentum, so it is in qualitative agreement with the observed energy spectrum of liquid He II. However, the temperature dependence of the spectrum is incorrectly given. At the observed density of 0.0219 atoms A-3, the depletion of the zero-momentum state at zero temperature is 40.5% for the Morse dipole-dipole-2potential, and 43.2% for the Frost- Musulin potential. The depletion increases gradually until at 3.14 K the zero momentum density becomes zero discontinuously, which indicates a transition to the ideal Bose gas.</p>Microwave Properties of Liquids and Solids, Using a Resonant Microwave Cavity as a Probe2015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504294/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504294/"><img alt="Microwave Properties of Liquids and Solids, Using a Resonant Microwave Cavity as a Probe" title="Microwave Properties of Liquids and Solids, Using a Resonant Microwave Cavity as a Probe" src="https://digital.library.unt.edu/ark:/67531/metadc504294/small/"/></a></p><p>The frequency shifts and Q changes of a resonant microwave cavity were utilized as a basis for determining microwave properties of solids and liquids. The method employed consisted of varying the depth of penetration of a cylindrical sample of the material into a cavity operating in the TM0 1 0 Mode. The liquid samples were contained in a thin-walled quartz tube. The perturbation of the cavity was achieved by advancing the sample into the cavity along the symmetry axis by employing a micrometer drive appropriately calibrated for depth of penetration of the sample. A differentiation method was used to obtain the half-power points of the cavity resonance profile at each depth of penetration. The perturbation techniques for resonant cavities were used to reduce the experimental data obtained to physical parameters for the samples. The probing frequency employed was near 9 gHz.</p>K-Shell Ionization Cross Sections of Selected Elements from Ag to La for Proton Bombardment from 0.6 to 2.0 MeV2015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504034/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504034/"><img alt="K-Shell Ionization Cross Sections of Selected Elements from Ag to La for Proton Bombardment from 0.6 to 2.0 MeV" title="K-Shell Ionization Cross Sections of Selected Elements from Ag to La for Proton Bombardment from 0.6 to 2.0 MeV" src="https://digital.library.unt.edu/ark:/67531/metadc504034/small/"/></a></p><p>The K-shell x-ray and ionization cross sections are measured for protons on Ag, Cd, Sn, Sb, Te, Ba, and La over the ion energy range of 0.6 to 2.0 MeV. The data are compared to the predictions of the PWBA, the PWBA with corrections for binding energy and/or Coulomb deflection, the BEA, and the constrained BEA predictions. In general, the non-relativistic PWBA with binding energy correction gives the best overall agreement with the measurements of proton-induced x-ray processes for the K-shell of the elements studied in this work. The data further suggest the need for relativistic PWBA treatment of the interactions in the K-shell for the range of binding energies represented by the elements investigated in this work.</p>Test of Gauge Invariance: Charged Harmonic Oscillator in an Electromagnetic Field2015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504158/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504158/"><img alt="Test of Gauge Invariance: Charged Harmonic Oscillator in an Electromagnetic Field" title="Test of Gauge Invariance: Charged Harmonic Oscillator in an Electromagnetic Field" src="https://digital.library.unt.edu/ark:/67531/metadc504158/small/"/></a></p><p>The gauge-invariant formulation of quantum mechanics is compared to the conventional approach for the case of a one-dimensional charged harmonic oscillator in an electromagnetic field in the electric dipole approximation. The probability of finding the oscillator in the ground state or excited states as a function of time is calculated, and the two approaches give different results. On the basis of gauge invariance, the gauge-invariant formulation of quantum mechanics gives the correct probability, while the conventional approach is incorrect for this problem. Therefore, expansion coefficients or a wave function cannot always be interpreted as probability amplitudes. For a physical interpretation as probability amplitudes the expansion coefficients must be gauge invariant.</p>Shubnikov-de Haas Effect Under Uniaxial Stress: A New Method for Determining Deformation Potentials and Band Structure Information in Semiconductors2015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504123/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504123/"><img alt="Shubnikov-de Haas Effect Under Uniaxial Stress: A New Method for Determining Deformation Potentials and Band Structure Information in Semiconductors" title="Shubnikov-de Haas Effect Under Uniaxial Stress: A New Method for Determining Deformation Potentials and Band Structure Information in Semiconductors" src="https://digital.library.unt.edu/ark:/67531/metadc504123/small/"/></a></p><p>The problem with which this investigation is concerned is that of demonstrating the applicability of a particular theory and technique to two materials of different band structure, InSb and HgSe, and in doing so, determining the deformation potentials of these materials. The theory used in this investigation predicts an inversion-asymmetry splitting and an anisotropy of the Fermi surface under uniaxial stress. No previous studies have ever verified the existence of an anisotropy of the Fermi surface of semiconductors under stress. In this work evidence will be given which demonstrates this anisotropy. Although the inversion-asymmetry splitting parameter has been determined for some materials, no value has ever been reported for InSb. The methods presented in this paper allow a value of the splitting parameter to be determined for InSb.</p>A Technique for Increasing the Optical Strength of Single-Crystal NaCl and KCl Through Temperature Cycling2015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504009/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504009/"><img alt="A Technique for Increasing the Optical Strength of Single-Crystal NaCl and KCl Through Temperature Cycling" title="A Technique for Increasing the Optical Strength of Single-Crystal NaCl and KCl Through Temperature Cycling" src="https://digital.library.unt.edu/ark:/67531/metadc504009/small/"/></a></p><p>This thesis relates a technique for increasing the optical strength of NaCl and KCl single-crystal samples. The 1.06-μm pulsed laser damage thresholds were increased by factors as large as 4.6 for a bulk NaCl single-crystal sample. The bulk laser damage breakdown threshold (LDBT) of the crystal was measured prior to and after heat treatment at 800*C using a Nd:YAG laser operating at 1.06 μm. Bulk and surface LDBTs were also studied on samples annealed at 400° C. These samples showed differences in damage morphology on both cleaved and polished surfaces, and the cleaved surfaces had improved damage thresholds. However, neither the polished surfaces nor the bulk showed improved threshold at the lower annealing temperature.</p>A Theoretical Investigation of Bound Roton Pairs in Superfluid Helium-42015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc501017/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc501017/"><img alt="A Theoretical Investigation of Bound Roton Pairs in Superfluid Helium-4" title="A Theoretical Investigation of Bound Roton Pairs in Superfluid Helium-4" src="https://digital.library.unt.edu/ark:/67531/metadc501017/small/"/></a></p><p>The Bogoliubov theory of excitations in superfluid helium is used to study collective modes at zero temperature. A repulsive delta function shell potential is used in the quasiparticle excitation energy spectrum to fit the observed elementary excitation spectrum, except in the plateau region. The linearized equation of motion method is used to obtain the secular equation for a collective mode consisting of a linear combination of one and two free quasiparticles of zero total momentum. It is shown that in this case for high-lying collective modes, vertices involving three quasiparticles cancel, and only vertices involving four quasiparticles are important. A decomposition into various angular momentum states is then made. Bound roton pairs in the angular momentum D-state observed in light-scattering experiments exist only for an attractive coupling between helium atoms in this oversimplified model. Thus, the interaction between particles can be reinterpreted as a phenomenological attractive coupling between quasiparticles, in order to explain the Raman scattering from bound roton pairs in superfluid helium.</p>Proton-Induced L-shell X-Rays of Pr, Sm, Eu, Gd, and Dy2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500284/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500284/"><img alt="Proton-Induced L-shell X-Rays of Pr, Sm, Eu, Gd, and Dy" title="Proton-Induced L-shell X-Rays of Pr, Sm, Eu, Gd, and Dy" src="https://digital.library.unt.edu/ark:/67531/metadc500284/small/"/></a></p><p>Characteristic L-shell x rays of the five rare earths Pr, Sm, Eu, Gd, and Dy were studied in this work. The x rays were produced by ionization from 0.3 to 2.0 MeV protons from the 2.0 MV Van de Graaff at North Texas State University. Total L-shell ionization and x-ray production cross sections were measured for Sm and compared to the BEA, CBEA and PWBA theories. Total L-shell ionization cross sections were measured for Pr, Eu, Gd, and Dy and compared to the BEA, CBEA, and PWBA. The CBEA and PWBA fit the samarium data well for both ionization and x-ray production cross sections. The BEA was generally 40 per cent lower than the data. The CBEA and the PWBA also fit the ionization cross section data for Pr, Eu, Gd and Dy, while the BEA was generally 40 per cent lower than the data.</p>K-Shell Ionization Cross Sections of Selected Elements from Fe to As for Proton Bombardment from 0.5 to 2.0 MeV2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500606/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500606/"><img alt="K-Shell Ionization Cross Sections of Selected Elements from Fe to As for Proton Bombardment from 0.5 to 2.0 MeV" title="K-Shell Ionization Cross Sections of Selected Elements from Fe to As for Proton Bombardment from 0.5 to 2.0 MeV" src="https://digital.library.unt.edu/ark:/67531/metadc500606/small/"/></a></p><p>The problem with which this investigation is concerned is that of making experimental measurements of proton-induced K-shell x-ray production cross sections and to study the dependence of these cross sections upon the energy of the incident proton. The measurements were made by detection of the characteristic x-rays emitted as a consequence of the ionization of the K-shell of the atom. The method for relating this characteristic x-ray emission to the x-ray production cross section is discussed in this work.</p>Collision Broadening in the Microwave Rotational Spectrum of Gaseous Monomeric Formaldehyde2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500274/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500274/"><img alt="Collision Broadening in the Microwave Rotational Spectrum of Gaseous Monomeric Formaldehyde" title="Collision Broadening in the Microwave Rotational Spectrum of Gaseous Monomeric Formaldehyde" src="https://digital.library.unt.edu/ark:/67531/metadc500274/small/"/></a></p><p>A source-modulation microwave spectrograph was utilized to measure line width parameters for several spectral lines in the pure rotational spectrum of formaldehyde (H₂CO). The spectrograph featured high-gain ac amplification and phase-sensitive detection, and was capable of measuring microwave lines having absorption coefficients as small as 10⁻⁷ cm⁻¹ with a frequency resolution on the order of 30 kHz. Center frequencies of the measured lines varied from 4,830 MHz to 72,838 MHz; hence, most of the observations were made on transitions between K-doublets in the rotational spectrum. Corrections were applied to the measured line width parameters to account for Doppler broadening and, where possible, for deviations due to magnetic hyperfine structure in some of the K-doubled lines. Low modulation voltages and low microwave power levels were used to minimize modulation and saturation broadenings; other extraneous broadenings were found to be insignificant. The primary broadening mechanism at low gas pressure is pressure broadening, and a review of this topic is included. Line width parameters for the several observed transitions were determined by graphing half-widths versus pressure for each spectral line, and performing a linear least-squares fit to the data points. Repeatability measurements indicated the accuracy of the line width parameters to be better than ±10 percent. The reasons for this repeatability spread are discussed, Broadening of each line was measured for self- and foreign-gas broadening by atomic helium and diatomic hydrogen. Effective collision diameters were calculated for each broadening interaction, based on the observed rates of broadening.</p>