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Description: High resolution angle-resolved photoemission studies were carried out on the surface state reported by Heimann et al. at the {bar M} point of the two-dimensional Surface Brillouin Zone of Cu(001). The symmetry of the state is shown to be odd with respect to the (100) mirror plane, as hypothesized by Heimann et al. Experimental E({rvec k}{parallel}) dispersion relations in good agreement with the earlier study are reported, and accurate determinations of peak width as a function of k{parallel} are shown to be reasonably well-fitted by a simple model. A novel temperature effect is reported along with a possible explanation.
Date: December 1, 1979
Creator: Kevan, S.D. & Shirley, D.A.
Partner: UNT Libraries Government Documents Department

Fermiology of the organic superconductor {beta}''-(ET){sub 2}SF{sub 5}CH{sub 2}CF{sub 2}SO{sub 3}.

Description: We present a detailed Fermi-surface (FS) investigation of the quasi two-dimensional (2D) organic superconductor (T{sub c} {approx} 4.5 K) {beta}{double_prime}(ET){sub 2}SF{sub 5}CH{sub 2}CF{sub 2}SO{sub 3}. In line with previous investigations, de Haas-van Alphen measurements in pulsed fields up to 60 T show a single oscillation frequency, F{sub 0} = 200 T, which corresponds to a FS size of about 5% of the first Brillouin zone. Angular dependent magnetoresistance oscillations (AMROs) are utilized for the exact determination of the in-plane FS, which is found to be a strongly elongated ellipsoid with an axes ratio of about 1:9. Transport measurements in static fields up to 33 T show an unusual temperature dependence of the Shubnikov-de Haas (SdH) signal, i.e., a decrease of the SdH amplitude with decreasing temperature.
Date: July 29, 1998
Creator: Wosnitza, J.
Partner: UNT Libraries Government Documents Department

Electronic structure of the BaFe2As2 family of iron-pnictide superconductors

Description: We use high-resolution angle-resolved photoemission to study the electronic structure of the BaFe{sub 2}As{sub 2} pnictides. We observe two electron bands and two hole bands near the X point, ({pi},{pi}) of the Brillouin zone, in the paramagnetic state for electron-doped Ba(Co{sub 0.06}Fe{sub 0.94}){sub 2}As{sub 2}, undoped BaFe{sub 2}As{sub 2}, and hole-doped Ba{sub 0.6}K{sub 0.4}Fe{sub 2}As{sub 2}. Among these bands, only the electron bands cross the Fermi level, forming two electron pockets around X while the hole bands approach but never reach the Fermi level. We show that the band structure of the BaFe{sub 2}As{sub 2} family matches reasonably well with the prediction of local-density approximation calculations after a momentum-dependent shift and renormalization. Our finding resolves a number of inconsistencies regarding the electronic structure of pnictides.
Date: June 29, 2009
Creator: Reza, Adriana; Lu, D.H.; Analytis, J.G.; Chu, J.-H.; Mo, S.-K.; He, R.-H. et al.
Partner: UNT Libraries Government Documents Department

Probing the pairing symmetry of the iron pnictides with electronic Raman scattering

Description: An important issue in the study of the iron-arsenic based superconductors is the symmetry of the superconducting gap, a problem complicated by multiple gaps on different Fermi surface sheets. Electronic Raman scattering is a flexible bulk probe which allows one in principle to determine gap magnitudes and test for gap nodes in different regions of the Brillouin zone by employing different photon polarization states. Here we calculate the clean Raman intensity for A{sub 1g}, B{sub 1g} and B{sub 2g} polarizations, and discuss the peak structures and low-energy power laws which might be expected for several popular models of the superconducting gap in these systems.
Date: April 29, 2010
Creator: Boyd, G.R.
Partner: UNT Libraries Government Documents Department

Single Dirac Cone Topological Surface State and Unusual Thermoelectric Property of Compounds from a New Topological Insulator Family

Description: Angle resolved photoemission spectroscopy (ARPES) study on TlBiTe2 and TlBiSe2 from a Thallium-based III-V-VI2 ternary chalcogenides family revealed a single surface Dirac cone at the center of the Brillouin zone for both compounds. For TlBiSe{sub 2}, the large bulk gap ({approx} 200meV) makes it a topological insulator with better mechanical properties than the previous binary 3D topological insualtor family. For TlBiTe{sub 2}, the observed negative bulk gap indicates it as a semi-metal, rather than a narrow gap semi-conductor as conventionally believed; this semi-metality naturally explains its mysteriously small thermoelectric figure of merit comparing to other compounds in the family. Finally, the unique band structures of TlBiTe{sub 2} also suggests it as a candidate for topological superconductors.
Date: August 18, 2011
Creator: Chen, Y
Partner: UNT Libraries Government Documents Department

Electronic Structure of Crystalline 4He at High Pressures

Description: Using inelastic X-ray scattering techniques, we have succeeded in probing the high-pressure electronic structure of helium crystal at 300 K which has the widest known electronic energy bandgap of all materials, that was previously inaccessible to measurements due to the extreme energy and pressure range. We observed rich electron excitation spectrum, including a cut-off edge above 23 eV, a sharp exciton peak showing linear volume dependence, and a series of excitations and continuum at 26 to 45 eV. We determined electronic dispersion along the {Gamma}-M direction over two Brillouin zones, and provided a quantitative picture of the helium exciton beyond the simplified Wannier-Frenkel description.
Date: January 10, 2011
Creator: Mao, Ho Kwang; Shirley, Eric L.; Ding, Yang; Eng, Peter; Cai, Yong Q.; Chow, Paul et al.
Partner: UNT Libraries Government Documents Department

Phonon frequencies and elastic constants of cubic Pu from electronic structure theory

Description: The phonon frequencies and elastic constants of plutonium are calculated using a model for the electronic structure that treats the valance electrons as a pseudopotential and the f-electrons in tight-binding theory. An effective interaction between ions is presented with electron screening treated in the Thomas-Fermi approximation and the f-electrons contributing bonding and repulsive terms to the potential. The phonon frequencies and elastic constants are calculated using the face-centered cubic lattice structure for both the {alpha}-, and {delta}-phases of Pu. The electronic structure predicts the qualitative behavior of the elastic constants and the transverse branches of the phonon dispersion curves in agreement with experimental values of the elastic constants for B-phase Pu.
Date: November 1, 1996
Creator: Straub, G.K.
Partner: UNT Libraries Government Documents Department

Beyond the Brillouin limit with the Penning fusion experiment

Description: Several years ago, it was proposed that a dense nonneutral plasma could be produced in a Penning trap. Nonneutral plasmas have excellent confinement. Thus, such a dense plasma might produce simultaneously high density and good confinement (as needed for fusion). Recently, this theoretical conjecture has been demonstrated in a small (3 mm radius) electron experiment (PFX). Densities up to 35 times the Brillouin density (limiting number density in a static trap) have been inferred from the observed strong (100:1) spherical focussing. Electrons are injected at low energy from a single pole of the sphere. A surprising observation is the self-organization of the system into a spherical state, which occurs precisely when the trap parameters are adjusted to produce a spherical well. This organization is observed by a bootstrapping which produces a hysteresis. Additional observations which confirm the dense spherical focus are energy-scattered electrons and deflections of an electron probe beam by the space charge of the central focus.
Date: November 9, 1996
Creator: Barnes, D.C.; Mitchell, T.B. & Schauer, M.M.
Partner: UNT Libraries Government Documents Department


Description: In the heavy fermion CeRhIn{sub 5} and the isostructural compound LaRhIn{sub 5} the extra 4f electron in Ce dramatically alters the band structure near EF, suggesting that the 4f's participate in band formation. ARPES data indicates that correlation effects are mostly evident along the {Lambda}-Z direction in the Brillouin zone. Very good agreement to GGA band calculations is found.
Date: June 1, 2001
Partner: UNT Libraries Government Documents Department

Evidence against a charge density wave on Bi(111)

Description: The Bi(111) surface was studied by scanning tunneling microscopy (STM), transmission electron microscopy (TEM) and angle-resolved photoemission (ARPES) in order to verify the existence of a recently proposed surface charge density wave (CDW) [Ch. R. Ast and H. Hoechst Phys. Rev. Lett. 90, 016403 (2003)]. The STM and TEM results to not support a CDW scenario at low temperatures. Furthermore, the quasiparticle interference pattern observed in STM confirms the spin-orbit split character of the surface states which prevents the formation of a CDW, even in the case of good nesting. The dispersion of the electronic states observed with ARPES agrees well with earlier findings. In particular, the Fermi contour of the electron pocket at the centre of the surface Brillouin zone is found to have a hexagonal shape. However, no gap opening or other signatures of a CDW phase transition can be found in the temperature-dependent data.
Date: May 1, 2005
Creator: Kim, T.K.; Wells, J.; Kirkegaard, C.; Li, Z.; Hoffmann, S.V.; Gayone, J.E. et al.
Partner: UNT Libraries Government Documents Department

Extracting the spectral function of the cuprates by a full two-dimensional analysis: Angle-resolved photoemission spectra of Bi2Sr2CuO6

Description: Recently, angle-resolved photoemission spectroscopy (ARPES) has revealed a dispersion anomaly at high binding energy near 0.3-0.5 eV in various families of the high-temperature superconductors. For further studies of this anomaly we present a new two-dimensional fitting-scheme and apply it to high-statistics ARPES data of the strongly-overdoped Bi{sub 2}Sr{sub 2}CuO{sub 6} cuprate superconductor. The procedure allows us to extract the self-energy in an extended energy and momentum range. It is found that the spectral function of Bi{sub 2}Sr{sub 2}CuO{sub 6} can be parameterized using a small set of tight-binding parameters and a weakly-momentum-dependent self-energy up to 0.7 eV in binding energy and over the entire first Brillouin zone. Moreover the analysis gives an estimate of the momentum dependence of the matrix element, a quantity, which is often neglected in ARPES analyses.
Date: April 30, 2010
Creator: Meevasana, W.
Partner: UNT Libraries Government Documents Department


Description: An important question regarding the technique of angle-resolved photoemission (ARP) is the extent to which it can be used to determine experimental valence-band dispersion relations E{sub i}({rvec k}) for single crystalline solids. In the case of the 3d and 4d transition metals, studies of copper, nickel, palladium, and silver, show that a model based on the assumption of direct interband transitions (direct-transition model) may be used, in conjunction with an appropriate final-state dispersion relation E{sub f}({rvec k}), to elucidate E{sub i}({rvec k}) for these materials along several high symmetry lines (primarily {Gamma}{Lambda}L) in k-space. To answer this question more generally, we have undertaken an extensive study of the valence band structures of other transition metals along various k-space lines. To date, studies have been extended to the (111) faces of the 5d metals Pt and Au along with the Pt(100) ((5 x 20) surface structure) face, and the (110) and (100) faces of Ag. The experiments were all conducted at SSRL, using synchrotron radiation in the range 6 eV < h{nu} < 34 eV. The results of these studies, combined with our previous Ag(111) work at these energies, allow us to invoke important conclusions concerning the relationships between ARP data, E{sub i}({rvec k}) and E{sub f}({rvec k}) for these materials. Several are summarized. For each crystal face investigated, the direct-transition model, along with a simple quasi-free-electron E{sub f}({rvec k}), was sufficient to determine experimental E{sub i}({rvec k}) relations along the appropriate k-space line that were in general agreement with theoretical RAPW band structure calculations. Essentially, we required E{sub f}({rvec k}) to be of the form (h{sup 2}/2m*)|{rvec k} + {rvec G}|{sup 2} + V{sub o}, where {rvec G} is a reciprocal lattice vector, fitting this relation to the appropriate calculated bulk conduction band near the center of the line under ...
Date: June 1, 1980
Creator: Davis, R.F.; Mills, K.A.; Thornton, G.; Kevan, S.D. & Shirley, D.A.
Partner: UNT Libraries Government Documents Department

Band anticrossing effects in highly mismatched semiconductor alloys

Description: The first five chapters of this thesis focus on studies of band anticrossing (BAC) effects in highly electronegativity- mismatched semiconductor alloys. The concept of bandgap bowing has been used to describe the deviation of the alloy bandgap from a linear interpolation. Bowing parameters as large as 2.5 eV (for ZnSTe) and close to zero (for AlGaAs and ZnSSe) have been observed experimentally. Recent advances in thin film deposition techniques have allowed the growth of semiconductor alloys composed of significantly different constituents with ever- improving crystalline quality (e.g., GaAs{sub 1-x}N{sub x} and GaP{sub 1-x}N{sub x} with x {approx}< 0.05). These alloys exhibit many novel and interesting properties including, in particular, a giant bandgap bowing (bowing parameters > 14 eV). A band anticrossing model has been developed to explain these properties. The model shows that the predominant bowing mechanism in these systems is driven by the anticrossing interaction between the localized level associated with the minority component and the band states of the host. In this thesis I discuss my studies of the BAC effects in these highly mismatched semiconductors. It will be shown that the results of the physically intuitive BAC model can be derived from the Hamiltonian of the many-impurity Anderson model. The band restructuring caused by the BAC interaction is responsible for a series of experimental observations such as a large bandgap reduction, an enhancement of the electron effective mass, and a decrease in the pressure coefficient of the fundamental gap energy. Results of further experimental investigations of the optical properties of quantum wells based on these materials will be also presented. It will be shown that the BAC interaction occurs not only between localized states and conduction band states at the Brillouin zone center, but also exists over all of k-space. Finally, taking ZnSTe and ZnSeTe as examples, ...
Date: September 9, 2002
Creator: Wu, Junqiao
Partner: UNT Libraries Government Documents Department

Neutron study of fracton excitations in percolating antiferromagnets

Description: The authors report the results of an inelastic neutron scattering experiment on nearly-percolating Heisenberg antiferromagnets (RbMn{sub c}Mg{sub 1{minus}o}F{sub 3}), in which the Mn concentrations (C = 0.31, 0.34 and 0.39) are very close to the percolation threshold (c{sub p} = 0.312). A broad peak superimposed on Ising-cluster excitations was observed throughout the Brillouin zone. The intensity of a broad peak increased on approaching the percolation threshold. The origin of this broad peak is attributed to the excitation of fractons in a percolating network.
Date: June 27, 1997
Creator: Ikeda, H.; Takahashi, M.; Fernandez-Baca, J.A. & Nicklow, R.M.
Partner: UNT Libraries Government Documents Department

Constant Q measurements of collective modes in Mg{sub 70}Zn{sub 30} glass

Description: In this study the Neutron Brillouin Scattering technique has been used to measure longitudinal excitations in a magnesium-zinc glass at momentum transfers within the first pseudo-Brillouin zone. The measurements were performed at room temperature and constant momentum transfer, which enables the data to be readily and reliably compared with theory. The experimental results taken down to Q = 6.2 nm{sup {minus}1} and E = 27 meV show a 5 meV downward shift in the dispersion energy of the longitudinal optic mode of the glass when compared to theoretical predictions.
Date: November 1, 1997
Creator: Benmore, C.J.; Sweeney, S.; Robinson, R.A.; Egelstaff, P.A. & Suck, J.B.
Partner: UNT Libraries Government Documents Department


Description: High resolution angle-resolved photoemission spectroscopy of highly overdoped Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} with a T{sub c} = 51K indicates that the basic transport processes in this material are fundamentally different from both the lesser doped cuprates as well as model metallic compounds. The overdoped sample has sharp ARPES peaks at the Fermi energy throughout the Brillouin zone even in the normal state, unlike the lesser-doped compounds. In particular, the spectra near ({pi},0) point show the presence of a sharp peak well above T{sub c}. The ARPES lineshapes, and thus the self energy, at a given energy are almost independent of k. Further, the quasiparticle scattering rate at the Fermi energy seems to be closely tied to direct resistivity measurements. This leads us to the conclusion that overdoped Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} is best described as a quasiparticle liquid. However, the energy dependence of the scattering rates is quite similar to that found in the lesser-doped compounds and quite different from that seen in a typical metal.
Date: July 23, 2001
Partner: UNT Libraries Government Documents Department

Electron-like Fermi surface and Remnant (pi,0) features in overdoped La(1.78)Sr(0.22)CuO4

Description: We have performed an angle-resolved photoemission study of overdoped La1.78Sr0.22CuO4, and have observed sharp nodal quasiparticle peaks in the second Brillouin zone that are comparable to data from Bi2Sr2CaCu2O8+d. The data analysis using energy distribution curves, momentum distribution curves and intensity maps all show evidence of an electron-like Fermi surface, which is well explained by band structure calculations. Evidence for many-body effects are also found in the substantial spectral weight remaining below the Fermi level around (pi,0), where the band is predicted to lie above EF.
Date: January 1, 2001
Creator: Yoshida, T.; Zhou, X.J.; Nakamura, M.; Kellar, S.A.; Bogdanov, P.V.; Lu, E.D. et al.
Partner: UNT Libraries Government Documents Department

Revision of the statistical mechanics of phonons to include phonon line widths

Description: Zubarev in 1960 obtained the smeared Bose-Einstein (B-E) function in order to take into account the fact that the eigenenergy associated with a fixed phonon wave vector q and fixed polarization index j is not precisely defined but instead, is smeared by phonon-phonon and phonon-electron interactions. The ratio GAMMA(qj)/..omega..(qj) is often quite small, i.e., of the order of 0.01 or less, where GAMMA is the phonon linewidth and h-bar ..omega.. is the eigenenergy. However, in strongly anharmonic crystals GAMMA/..omega.. may be as large as 0.3 at certain points of the Brillouin zone. In such dramatic cases one would suspect that such phonon linewidths would have some observable effect on the thermodynamic properties. The purpose of this work is to derive the expression for the average free energy per mode for a crystal having large phonon linewidths and to test the properties of the thermodynamic functions derivable from the average free energy per mode. (WHK)
Date: January 1, 1983
Creator: Overton, W.C. Jr.
Partner: UNT Libraries Government Documents Department

First-principles thermoelasticity of transition metals at high pressure I. Tantalum prototype in the quasi-harmonic limit

Description: The thermoelastic properties of bcc tantalum have been investigated over a broad range of pressures (up to 10 Mbar) and temperatures (up to 26,000 K) using a new first-principles approach that accurately accounts for cold, electron-thermal, and ion-thermal contributions in materials where anharmonic effects are small. Specifically, we have combined ab initio full-potential linear-muffin-tin-orbital (FP-LMTO) electronic-structure calculations for the cold and electron-thermal contributions to the elastic moduli with phonon contributions for the ion-thermal part calculated using model generalized pseudopotential theory (MGPT). For the latter, a summation of terms over the Brillouin zone is performed within the quasi-harmonic approximation, where each term is composed of a strain derivative of the phonon frequency at a particular k point. At ambient pressure, the resulting temperature dependence of the Ta elastic moduli is in excellent agreement with ultrasonic measurements. The experimentally observed anomalous behavior of C{sub 44} at low temperatures is shown to originate from the electron-thermal contribution. At higher temperatures, the main contribution to the temperature dependence of the elastic moduli comes from thermal expansion, but inclusion of the electron- and ion-thermal contributions is essential to obtain quantitative agreement with experiment. In addition, the pressure dependence of the moduli at ambient temperature compares well with recent diamond-anvil cell measurements to 1.05 Mbar. Moreover, the calculated longitudinal and bulk sound velocities in polycrystalline Ta at higher pressure and temperature in the vicinity of shock melting ({approx} 3 Mbar) agree well with data obtained from shock experiments. However, at high temperatures along the melt curve above 1 Mbar, the B{prime} shear modulus becomes negative indicating the onset of unexpectedly strong anharmonic effects. Finally, the assumed temperature dependence of the Steinberg-Guinan strength model obtained from scaling with the bulk shear modulus is examined at ambient pressure.
Date: April 25, 2006
Creator: Orlikowski, D; Soderlind, P & Moriarty, J A
Partner: UNT Libraries Government Documents Department

Band Structure Parameters and Fermi Resonances of Exciton-Polaritons in CsI and CsBr under Hydrostatic Pressure

Description: Most alkali halides crystallize in the fcc sodium chloride structure. In contrast, with the exception of CsF, the Cs-halides form the simple cubic cesium chloride (CsCl) structure at ambient conditions and they have a substantially different electronic structure than other alkali halides; in particular, they have several nearly degenerate electronic levels near the Brillouin zone center. Highly resolved Three-Photon Spectroscopy (TPS) measurements allow direct observation of the near band edge structure and, in the case of CsI, probe more states than one-photon techniques. A number of interesting phenomena, among them level repulsion (Fermi resonance), occur as these levels are tuned through one another by application of hydrostatic pressure. To the best of our knowledge, this has been observed for CsBr for the first time. Doubling the photon energy range compared to a previous publication [see Yoo et al. PRL 84, 3875 (2000)] allows direct observation of the n=1, 2 and 3 exciton-polariton members of the {Lambda}{sub 8}{sup -}-{Lambda}{sub 6}{sup +} transition in CsI and lets us establish unambiguous values for the bandgap (6.139 eV), binding energy (0.265 eV) and their pressure dependence up to 7 kbar. Similarly to CsI, the CsBr linewidth of the lowest {Lambda}{sub 4}{sup -} polariton (A) decreases upon compression.
Date: November 29, 2005
Creator: Lipp, M J; Yoo, C H; Strachan, D & Daniels, W B
Partner: UNT Libraries Government Documents Department

LDRD final report on Bloch Oscillations in two-dimensional nanostructure arrays for high frequency applications.

Description: We have investigated the physics of Bloch oscillations (BO) of electrons, engineered in high mobility quantum wells patterned into lateral periodic arrays of nanostructures, i.e. two-dimensional (2D) quantum dot superlattices (QDSLs). A BO occurs when an electron moves out of the Brillouin zone (BZ) in response to a DC electric field, passing back into the BZ on the opposite side. This results in quantum oscillations of the electron--i.e., a high frequency AC current in response to a DC voltage. Thus, engineering a BO will yield continuously electrically tunable high-frequency sources (and detectors) for sensor applications, and be a physics tour-de-force. More than a decade ago, Bloch oscillation (BO) was observed in a quantum well superlattice (QWSL) in short-pulse optical experiments. However, its potential as electrically biased high frequency source and detector so far has not been realized. This is partially due to fast damping of BO in QWSLs. In this project, we have investigated the possibility of improving the stability of BO by fabricating lateral superlattices of periodic coupled nanostructures, such as metal grid, quantum (anti)dots arrays, in high quality GaAs/Al{sub x}Ga{sub 1-x}As heterostructures. In these nanostructures, the lateral quantum confinement has been shown theoretically to suppress the optical-phonon scattering, believed to be the main mechanism for fast damping of BO in QWSLs. Over the last three years, we have made great progress toward demonstrating Bloch oscillations in QDSLs. In the first two years of this project, we studied the negative differential conductance and the Bloch radiation induced edge-magnetoplasmon resonance. Recently, in collaboration with Prof. Kono's group at Rice University, we investigated the time-domain THz magneto-spectroscopy measurements in QDSLs and two-dimensional electron systems. A surprising DC electrical field induced THz phase flip was observed. More measurements are planned to investigate this phenomenon. In addition to their potential device applications, periodic ...
Date: September 1, 2008
Creator: Lyo, Sungkwun Kenneth; Pan, Wei; Reno, John Louis; Wendt, Joel Robert & Barton, Daniel Lee
Partner: UNT Libraries Government Documents Department

Particle-Hole Symmetry Breaking in the Pseudogap State of Bi2201

Description: In conventional superconductors, a gap exists in the energy absorption spectrum only below the transition temperature (T{sub c}), corresponding to the energy price to pay for breaking a Cooper pair of electrons. In high-T{sub c} cuprate superconductors above T{sub c}, an energy gap called the pseudogap exists, and is controversially attributed either to pre-formed superconducting pairs, which would exhibit particle-hole symmetry, or to competing phases which would typically break it. Scanning tunnelling microscopy (STM) studies suggest that the pseudogap stems from lattice translational symmetry breaking and is associated with a different characteristic spectrum for adding or removing electrons (particle-hole asymmetry). However, no signature of either spatial or energy symmetry breaking of the pseudogap has previously been observed by angle-resolved photoemission spectroscopy (ARPES). Here we report ARPES data from Bi2201 which reveals both particle-hole symmetry breaking and dramatic spectral broadening indicative of spatial symmetry breaking without long range order, upon crossing through T* into the pseudogap state. This symmetry breaking is found in the dominant region of the momentum space for the pseudogap, around the so-called anti-node near the Brillouin zone boundary. Our finding supports the STM conclusion that the pseudogap state is a broken-symmetry state that is distinct from homogeneous superconductivity.
Date: August 19, 2011
Creator: Hashimoto, M.; He, R. -H.; Tanaka, K.; Testaud, J. P.; Meevasana, W.; Moore, R. G. et al.
Partner: UNT Libraries Government Documents Department