Digital Commons Network^™

Donor And Acceptor Concentrations In Molecular-Beam Epitaxial Gaas Grown At 300-Degrees-C And 400-Degrees-C, David C. Look, G. D. Robinson, J. R. Sizelove, C. E. Stutz

Physics Faculty Publications

The first Hall‐effect measurements on molecular beam epitaxial GaAs layers grown at the low temperatures of 300 and 400 °C are reported. Two independent methods were used to determine donor N_D and acceptor N_A concentrations and activation energy E_D0, with the following combined results: N_D≂3±1×10¹⁸, N_A≂1.5±1×10¹⁷ cm⁻³, and E_D0=0.645±0.009 eV for the 300 °C layer; N_D≂2±1×10¹⁷, N_A≂7±3×10¹⁶ cm⁻³, and E_D0=0.648±0.003 eV for the 400 °C layer. Thus, the deep donor is …

High-Voltage Variable Resistor For Ion Energy Spectroscopy, Timothy J. Gay, V. D. Irby, S. P. Yallaly

Timothy J. Gay Publications

A high-voltage variable resistor was designed, built, and implemented to modify an ion-energy spectrometer for the study of ion-atom collisions in which the projectiles change charge. The resistor is remotely switchable from 0 to 2050 MΩ and has a voltage rating of 200 kV. The design criteria and the electrical and mechanical details of the apparatus are discussed. The design and construction of an ancillary device, comprising two precision resistive-divider voltmeters, are also discussed.

Temporal Effects In Multiphoton Ionization Of Lithium, J. Greg Story, Thomas F. Gallagher

Physics Faculty Research & Creative Works

The temporal effects of multiphoton ionization are investigated using a two-color method which determines both the time and intensity at which the process occurs. We show that the total ionization efficiency depends strongly on the time at which an atom makes a transition to an excited state during an intense laser pulse. This result clearly shows that the ac-Stark-shifted bound-state resonances not only enhance the cross section for photoionization, but that the excited bound states provide temporary storage states for the atomic population during the photoionization process.

Second-Order Flexoelectric Effect In Chiral Nematic Liquid Crystals, D. W. Cronin, Rolfe G. Petschek, Eugene M. Terentjev

Faculty Scholarship

A molecular-statistical model for the second-order flexoelectric effect in chiral nematics is developed, accounting for additional chiral terms in the pair intermolecular potential U(1,2). It is shown by symmetry analysis, that there are four independent chiral terms in the flexoelectric polarization, proportional to the square of director gradients, ℘∼λ∇ n∇n. A molecular-statistical theory is developed for the second-order chiral flexoelectric constants λ. Their values, obtained in the mean-field approximation, depend on molecular dimensions, interaction parameters, and the degree of molecular chirality. Possible experimental situations are discussed where this effect can be observed.

Measurement Of The Microwave Dielectric Constant For Low‐Loss Samples With Finite Thickness Using Open‐Ended Coaxial‐Line Probes, G. Q. Jiang, W. H. Wong, E. Y. Raskovich, W. G. Clark, W. A. Hines, Jeff Sanny

Physics Faculty Works

This work addresses the effect of finite sample thickness on microwave dielectric constant measurements for thin, planar, low-loss samples using the open-ended coaxial-line probe method. Detailed measurements of the dielectric constant were carried out on a wide range of thicknesses of air samples which were backed by infinitely thick teflon and alumina dielectric media. The measurements were made at room temperature for various (50 Ω) coaxial-line dimensions, microwave frequencies 4-8 GHz, and power levels near a fraction of a mW. The results provide strong support for previously published theoretical calculations based on a boundary value problem which uses a spectral …

Photopolarimeter Based On Planar Grating Diffraction, R. M.A. Azzam, K. A. Giardina

Electrical Engineering Faculty Publications

A division-of-amplitude photopolarimeter (DOAP) is described that employs a diffraction grating in the conventional spectrometer orientation with the grating grooves normal to the plane of incidence. Four coplanar diffracted orders are used for polarimetric analysis to determine all four Stokes parameters of incident light simultaneously and virtually instantaneously (with the speed being determined solely by the photodetectors and their associated electronics); a fifth order is used for alignment by autocollimation or by use of a position-sensing quadrant detector. To sensitize the instrument for the +45° and -45° azimuths of incident linearly polarized light and for the handedness of incident circular …

Nitrogen Diffusion In Sm2Fe17 And Local Elastic And Magnetic Properties, Ralph Skomski, J.M.D. Coey

Ralph Skomski Publications

The reaction between nitrogen gas and an intermetallic compound is studied, with particular reference to Sm₂Fe₁₇, treating the Sm₂Fe₁₇N_y, system as a gas-solid solution. A simple lattice gas model is used to describe the reaction of nitrogen atoms with the metal lattice in terms of the net reaction energy U₀= - 57 ± 5 kJ/mol. The equilibrium nitrogen concentration is calculated as a function of nitrogenation temperature and gas pressure. Refined diffusion parameters D₀= 1.02 mm₂/s and E_a= 133 kJ/mol, determined by …

Enhanced Retroreflection Of Light From Rough Metal Surfaces, Sivakumar Natarajan

Masters Theses

The optical constants n and k measured by Johnson and Christy (1972) for the noble metals (gold, silver and copper) are used to analyze the relationship between the surface characteristics and the peak height and half-width of the backscattering peak which is observed in the angular distribution of the intensity of diffusely reflected light at a vacuum-metal interface in a direction antiparallel to the original incident beam. Surfaces with surface roughness of heights less than 200 A to 300 A will be considered and a recently developed perturbation treatment will be used in these studies.

The backscattering of light from …

Large Amplitude L=1 Coherent Structures In Non-Neutral Plasmas Confined In A Cylindrical Trap, Ross L. Spencer, Grant W. Mason

Faculty Publications

The computation of l= 1 coherent structures in non-neutral plasmas with arbitrary density profiles and for large displacements of the plasma from the symmetry axis of a confining cylindrical trap is described. As the structures are displaced from the axis, they revolve about the symmetry axis with a frequency that typically increases with displacement. The plasma also is distorted into an approximately elliptical shape. The frequency shifts and the eccentricities as a function of displacement, plasma size, and the shape of the density profile are both computed numerically and calculated analytically. The results are shown to be consistent with data …

Existence Of High-Order Superlattices In Orientationally Ordered C_{60}, J. E. Fischer, D. E. Luzzi, K. Kniaź, A. R. Mcghie, D. A. Ricketts-Foot, W. R. Romanow, Gavin B. M. Vaughan, Dayin Li, Allan L. Smith, Robert M. Strongin, M. A. Cichy, Laurent Bard, Amos B. Smith Iii

Chemistry Faculty Publications and Presentations

We correlate the presence or absence of high-order modulated structures in solid C₆₀ with the differential scanning calorimetry signature of the orientational ordering transition and other diagnostic data. We have searched for, but not observed, the 2a face-centered cubic structure recently observed in low-temperature electron and single-crystal x-ray diffraction. We conclude that the intrinsic ground state is the previously reported simple cubic structure [space groupPa3¯, a(14 K)=14.06 Å], and that other structures may be stabilized by defects or impurities.

Coherence Peak And Superconducting Energy Gap In Rb_{3}C_{60} Observed By Muon Spin Relaxation, R. F. Kiefl, W. A. Macfarlane, K. H. Chow, S. Dunsiger, T. L. Duty, T. M. S. Johnston, J. W. Schneider, J. Sonier, Laurent Brard, Robert M. Strongin, J. E. Fischer, Amos B. Smith Iii

Chemistry Faculty Publications and Presentations

Muon spin relaxation resulting from spin exchange scattering of endohedral muonium (μ+e-) with thermal electronic excitations has been observed in the fullerene superconductor Rb₃C₆₀. The temperature dependence of T₁^-1 shows a coherence peak just below T_c and can be fit to the conventional Hebel-Slichter theory for spin relaxation in a superconductor with a broadened BCS density of states. The average energy gap for electronic excitations, Δ/k_B=53(4) K or 2Δ/ k_BT_c=3.6(3), is consistent with the BCS weak coupling limit.

The Relationship Between Precipitation And Temperature Over The Contiguous United States, Weining Zhao, M. A. K. Khalil

Physics Faculty Publications and Presentations

The correlation between monthly total precipitation and monthly mean temperature over the 80-year period from 1905 to 1984 at nearly 1000 stations in the contiguous United States was calculated and analyzed. Both local and field significances were tested by using statistical methods. Areas of both negative and positive precipitation-temperature correlations were found. Over most of the United States, summer precipitation and temperature were negatively correlated, which indicates that warm summers tended to be dryer. This is particularly true in the central and southern Great Plains. The area south of the Great Lakes covering the eastern portion of the Corn Belt …

Non-Contact Measurement For Thermal Conductivity Of Diamond-Like And Hard Carbon Thin Films, Ganming Qin

Theses

In this research work, non-contact temperature measurement was applied to determine the thermal conductivity of diamond-like and hard carbon thin films. Dielectric materials widely used as thin films in device manufacturing are SiO₂, Si₃N₄, polymer, and etc. However, their heat dissipating capacity is not good for power devices. It is necessary to develop a new material for this purpose. Diamond crystal is a high quality dielectric material. It has the highest room-temperature thermal conductivity [k=20W (cm · K) at 20°C] among all materials. In addition, it has high electrical resistivity ( >10¹⁶ Ω …

Light Diffraction By Field-Induced Non-Periodic Magnetic Domain Structures In Fecl2, Christian Binek

Christian Binek Publications

The magnetic-field-induced mixed phase of the meta magnet FeCl2 gives rise to reduced transmittivity of circularly polarized light. Within the framework of the diffraction theory of thin random magnetic phase gratings and the assumption of field-dependent refractive indices of the antiferromagnetic domains the field dependence of the transmission is perfectly modeled.

Avalanches And The Renormalization Group For Pinned Charge-Density Waves, Alan Middleton, Onuttom Narayan

Physics - All Scholarship

The critical behavior of charge-density waves (CDWs) in the pinned phase is studied for applied fields increasing toward the threshold field, using recently developed renormalization group techniques and simulations of automaton models. Despite the existence of many metastable states in the pinned state of the CDW, the renormalization group treatment can be used successfully to find the divergences in the polarization and the correlation length, and, to first order in an $\epsilon = 4-d$ expansion, the diverging time scale. The automaton models studied are a charge-density wave model and a ``sandpile'' model with periodic boundary conditions; these models are found …

Photoemission Studies Of Co- And Fe-Based Compounds With The Thmn12 Structure, A.S. Fernando, J.P. Woods, Sitaram Jaswal, B.M. Patterson, D. Welipitiya, A.S. Nazareth, David J. Sellmyer

David Sellmyer Publications

The electronic structures of NdFe₁₁Ti, NdCo₁₀V₂, and YCo₁₀Cr₂ have been studied with photoemission and spin-polarized calculations. The changes in these electronic structures upon nitrogenation have also been investigated. In the Fe compound, the Fe 3d states dominate the calculated density of states near the Fermi-edge, and the N(2p) peak is evident at around 6.3 eV. There is no shift in Fe 3d peaks visible in these compounds upon nitrogenation. Other than small energy shifts in the peak positions, there is an overall agreement between experimental data and the calculated …

Magnetic Properties Of Epitaxial Single Crystal Ultrathin Fe3Si Films On Gaas (001), Sy_Hwang Liou, S.S. Malhotra, J.X. Shen, M. Hong, J. Kwo, H.S. Chen, J.P. Mannaerts

Sy-Hwang Liou Publications

Magnetic properties of Fe₃Si films with thickness from 2 to 210 monolayers (ML) epitaxially grown on GaAs (001) were studied using a superconducting quantum interference device and alternating gradient force magnetometers. Growth of these single-crystal intermetallic compound films were carried out in a multichamber molecular beam epitaxy (MBE) system. The samples were covered in situ with Au 50 Å thick to prevent oxidation when the samples were removed from the MBE chamber. All the films are ferromagnetic even for samples as thin as 2 ML. The easy magnetization direction of the films is parallel to the film surface. …

Electronic Structure Of Sm2Fe17NX Compounds, J.P. Woods, A.S. Fernando, Sitaram Jaswal, B.M. Patterson, D. Welipitiya, David J. Sellmyer

David Sellmyer Publications

Sm₂Fe₁₇N_x is of considerable current interest as a permanent-magnet material because of its enhanced Curie temperature and uniaxial anisotropy. The electronic structures of Sm₂Fe₁₇N_x for x=0 and x~2.6 have been studied with photoemission and spin-polarized calculations. The materials are prepared by arc melting and nitrogen is introduced by ion implantation. The nitrogen concentration is quantified with Auger electron spectroscopy. The Sm 4f electrons with binding energies between 6 and 10 eV are investigated with resonant photoemission using photon energies near 140 eV. The major features of ultra-violet …

Experimental Determination Of The Unoccupied Bands Of W(110), Dongqi Li, Peter A. Dowben, J.E. Ortega, F.J. Himpsel

Peter Dowben Publications

Inverse photoemission from W(110) has been used to determine the dispersion of the unoccupied energy bands along the tungsten ΓΣN line. The critical points are determined to be Γ₁₂ at 2.6 eV, N₁ at 2.2 eV, and the lower limit of N₄ at 3.2 eV. The upper bound of the lifetime broadening is estimated as 0.3-0.4 eV at 2-3 eV above E_F. The image state is observed at 4.5 eV above E_F, or 0.7 eV in binding energy, relative to the vacuum level.

Interstitial Modification Of Rare-Earth Intermetallics, Ralph Skomski, S. Murray, S. Brennan, J.M.D. Coey

Ralph Skomski Publications

Interstitial modification of R₂Fe₁₇Z_x is studied with R=Y, Sm, and Z=N, C, B, Si, and S. Equilibrium interstitial concentrations are calculated using a lattice gas model. For gas-solid reaction a low solubility is obtained at high temperatures, whereas a solid-solid reaction is predicted to yield R₂Fe₁₇Z_1.5 in the high-temperature limit. Theoretical results are compared with data on Sm₂Fe₁₇C_x produced by solid-solid reaction at 500 °C. Volume expansions up to 4.8% were obtained, which corresponds to x≈2. The concentrations of interstitial boron and silicon are much …

Characterization, Transport, And Magnetic Properties Of Co/Pd Multilayers Produced By Pulsed Laser Deposition, M. Enrech, Ralph Skomski, J.M.D. Coey, J.G. Lunney

Ralph Skomski Publications

Metallic Co/Pd multilayers with equal layer thickness and periods ranging from 5 to 150 A were deposited using pulsed laser deposition. They have an fcc (111) texture and the diffraction patterns give clear evidence of a modulated period structure. From the period dependence of the in-plane electrical resistivity a diffuse interface between cobalt and palladium of thickness b= 6 ± 2 Å has been deduced. Magnetic, magnetoresistance, and Kerr-effect measurements indicate in-plane magnetization. In low fields (B₀ < 10 mT) the magnetoresistanceis highly anisotropic, varying by ~2% in the transverse configuration and + 1% in the longitudinal configuration due to orbital moment of cobalt in the interface region. The shape of the Kerr-effect major hysteresis loop can be explained by the in-plane component of the anisotropy of the cobalt layers.

Magnetic Reversal In Sm2Fe17NY Permanent Magnets, Ralph Skomski, K.-H. Müller, P.A.P. Wendhausen, J.M.D. Coey

Ralph Skomski Publications

Coercivity and nucleation fields for three-dimensional inhomogenities are calculated and discussed. For soft spherical inclusions in a hard magnetic matrix a (δ /R) ² dependence of the nucleation field on inclusion radius R and domain wall width δ has been obtained. To calculate propagation fields for spherical inhomogenities, a nonlinear model has been used. The corresponding coercive field can be much higher than the nucleation field, particularly if the magnitude of the anisotropy changes very steeply. Soft inclusions with sharp interfaces are ineffective at producing complete magnetic reversal. For partly nitrided particles coercivity and nucleation field are predicted …

On The Role Of Zn In Sm2Fe17NX Permanent Magnets, P.A.P. Wendhausen, D. Eckert, A. Handstein, K.-H. Müller, G. Leitner, Ralph Skomski

Ralph Skomski Publications

Coercivities up to 1.9 T were achieved by bonding micron-size Sm₂Fe₁₇N_x particles with zinc. X-ray analysis of Zn-bonded samples indicates the presence of the ZnFe Γ phase and no unreacted Zn, after heat treatment at about 390 °C for 100 min for samples with Zn content varying from 5 to 35 wt. %. The increase of coercivity occurs only for a more intense heat treatment, e.g., at 390 °C! for 300 min or 425 °C for 100 min. Differential scanning calorimetry (DSC) shows two exothermic peaks at 388 and 426 °C, but no endothermic peak …

Nanostructural Effects And Interface Magnetism In Co/Pd Multilayers, Ping He, Z.-S. Shan, John A. Woollam, David J. Sellmyer

David Sellmyer Publications

A series of Co/Pd multilayers were made by dc magnetron sputter deposition on Al foil substrates. For these multilayered samples, Co layer thicknesses were less than 4 Å and Pd layers were varied from 4 to 22 Å. Sputtering rates were controlled by either sputtering power (10–50 W) or Ar sputtering pressure (3–15 mTorr). In both cases, lower deposition rates yielded higher perpendicular coercivity up to 2.6 kOe. Structures of the samples were studied using conventional θ -2θ x-ray diffractometry (XRD). It has been found that magnetic properties such as coercivity and saturation magnetization are sensitive to interfacial structures. A …

Behavior Of Disordered Co-Pd, Co-Ag, And Co-Mo Alloys In Multilayer Interfaces, Z.S. Shan, Ping He, C. Moore, John A. Woollam, David J. Sellmyer

David Sellmyer Publications

Local environment effects are important for understanding interfacial magnetism in artificially structured multilayers. For this reason, stable and/or metastable disordered alloys Co_xM_1-x (M=Pd, Ag, Mo) were fabricated in a dc cosputtering system. The magnetization as a function of composition and temperature were measured and analyzed in terms of a mean-field model. The local environment effect on the Co moment from its Ag (or Mo) neighbors and the Pd moment from its Co neighbors are studied. It is found that the polarized Pd moment increases linearly with an increasing number of nearest Co neighbors and the Co moment …

Magnetization Reversal And Defects In Co/Pt Multilayers, J.X. Shen, Roger D. Kirby, K. Wierman, Z.-S. Shan, David J. Sellmyer, T. Suzuki

Roger Kirby Publications

(Co 3 Å/Pt 10 Å)× N (N=8, 16, and 30 bilayers) multilayer thin films were prepared by sputtering onto an 850-Å-thick SiN layer that had been deposited on a silicon (111) substrate. We used the polar Kerr effect to measure the time dependence of magnetization reversal over the temperature range 90–300 K. Direct domain observations were also carried out. The results show that the domain expansion process depends strongly on the number of bilayers. Uniform domain expansion was found only in the thinner samples. From the time dependence of the magnetization reversal measurements over the temperature range 90–300 …

Magnetization Reversal In Co/Cr Multilayer Films, D. Wang, Roger D. Kirby, B. W. Robertson, David J. Sellmyer

Roger Kirby Publications

Magnetization reversal of Co/Cr multilayer films is studied by utilizing initial magnetization curves, minor loops, isothermal and dc demagnetization remanence curves, and temperature dependence of the loops. Electron microscope observations reveal that the films consist of columnar grains of size from 400 to 800 Å. It is suggested that the magnetization reversal is dominated by rotation of single-domain grains. Numerical calculations are performed based on the Stoner–Wohlfarth coherent rotation model for noninteracting, single-domain particles of uniaxial anisotropy. The calculated results agree qualitatively with the experimental results. To investigate the quantitative differences, magnetic interactions among the grains are measured in terms …

Interface Magnetism And Superparamagnetism Of Co/Cu Multilayers, Z.S. Shan, S. Nafis, John A. Woollam, Sy_Hwang Liou, David J. Sellmyer

Sy-Hwang Liou Publications

The magnetic properties of Co/Cu multilayers were investigated from 5 to 380 K and analyzed in terms of the interface magnetism and mean-field model. The interface, which is about 1.2 Å thick, is nonmagnetic at room temperature and becomes magnetic at 5 K with the average magnetization of 40% of the pure Co magnetization. The samples of X Å Co/10 Å Cu behaved superparamagnetically as X ranged from 4 to 6.5 Å and did not show superparamagnetic behavior for thinner or thicker Co layer thickness.

Crystal Field In Nitrogenated Rare-Earth Intermetallics, Ralph Skomski, M.D. Kuz'min, J.M.D. Coey

Ralph Skomski Publications

The crystal-field in Sm₂Fe₁₇N_{3- δ} and Sm(Fe₁₁Ti)N_{1- δ} due to interstitial nitrogen has been investigated. Intrinsic parametrization in the superposition model allows separation of the crystal field created by a neighboring nitrogen atom from a purely geometrical factor, which is different for Sm₂Fe₁₇N_{3- δ} and Sm( Fe₁₁Ti) N_{1- δ} Using published magnetic data, values for the intrinsic parameter A₂ per nitrogen atom of A₂=200± 60 Ka₀^-2 and A₂=270±60 Ka₀^-2 for Sm₂Fe₁₇N …

Photoluminescence And Absorption Studies Of Defects In Cdte And Znxcd1-Xte Crystals, Cheryl Barnett Davis, David D. Allred, A. Reyes-Mena, Jesus González-Hernández, Ovidio González, Bret C. Hess, Worth P. Allred

Faculty Publications

We have studied at cryogenic temperatures photoluminescence features which lie more than 0.15 eV below the band edge in ZnxCd1-xTe (0≤x≤0.09) crystals. The same features, namely a defect band which lies at about 0.13-0.20 eV below the band-gap energy and a peak at 1.1 eV, that are observed in pure CdTe samples are observed in these alloy materials. In annealed samples we observe that the 1.1 eV feature, which has been attributed to tellurium vacancies, increases with fast cooling. Increased concentrations of tellurium vacancies can be understood in terms of the phase diagram of CdTe which indicates that higher concentrations …