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THE TWO-DIMENSIONAL VALENCE ELECTRONIC STRUCTURE OF A MONOLYAER OF Ag ON Cu(00l)

Description: The metal overlayer system c(10x2)Ag/Cu(001) was studied at coverages near one monolayer with angle-resolved photoemission. The observed spectroscopic features indicate a two-dimensional d-band electronic structure that can be interpreted using a model with planar, hexagonal symmetry in which crystal field effects dominate over spin-orbit effects.
Date: May 1, 1985
Creator: Tobin, J. G.; Robey, S. W. & Shirley, D. A.
Partner: UNT Libraries Government Documents Department

GW correlation effects on plutonium quasiparticle energies: changes in crystal-field splitting

Description: We present results for the electronic structure of plutonium by using a recently developed quasiparticle self-consistent GW method (QSGW). We consider a paramagnetic solution without spin-orbit interaction as a function of volume for the face-centered cubic (fcc) unit cell. We span unit-cell volumes ranging from 10% greater than the equilibrium volume of the 8 phase to 90 % of the equivalent for the a phase of Pu. The self-consistent GW quasiparticle energies are compared to those obtained within the Local Density Approximation (LDA). The goal of the calculations is to understand systematic trends in the effects of electronic correlations on the quasiparticle energy bands of Pu as a function of the localization of the J orbitals. We show that correlation effects narrow the f bands in two significantly different ways. Besides the expected narrowing of individual f bands (flatter dispersion), we find that an even more significant effect on the f bands is a decrease in the crystal-field splitting of the different bands
Date: January 1, 2009
Creator: Albers, Robert C; Chantis, Athanasios N; Svane, Axel & Christensen, Niels E
Partner: UNT Libraries Government Documents Department

Comparative Analysis of Monochalcogenides of Actinides

Description: The series of monochalcogenides of actinides NpM, PuM, and AmM (M = S; Se; Te) has been studied with relativistic spin-polarized density functional theory. The electronic and magnetic structure of the compounds has been investigated. It has been shown that the hybridization between 5f-electrons of actinides and p-electrons of S, Se, or Te is practically absent. But there is a transfer of electrons from one element to another. This transfer, apparently, is a cause of a crystal field splitting of the bands, which is superimposed on the spin-orbit splitting. The experimental magnetic properties of the above compounds may be explained in this simple picture, obtained from non-spin-polarized calculations. Accounting for magnetism leads to the excessive spin splitting of the bands and to the disagreement with experimental data. So, present work can be considered as one more evidence, that it is necessary to develop more elaborated theory than DFT for study of magnetism in actinides and their compounds.
Date: October 5, 2005
Creator: Kutepov, A L
Partner: UNT Libraries Government Documents Department

Recovery of bypassed oil in the Dundee formation using horizontal drains. Quarterly progress report, October 1, 1995--December 31, 1995

Description: The principal objective of this project is to demonstrate the feasibility and economic success of producing oil from abandoned or nearly abandoned fields in the Dundee Formation of Central Michigan using horizontal drilling technology. A site for a horizontal well was selected in Crystal Field, a nearly-abandoned Dundee oil field in Michigan. This field had produced over 8 million barrels of oil, mostly in the 1930`s and 1940`s. At the height of development, Crystal Field produced from 193 wells, but by 1995, only seven producing wells remained, each producing less than 10 bbls/day. A horizontal well was drilled as a field demonstration pilot, funded through this DOE project, and was immensely successful. Core and logs from the Dundee interval were recovered from a vertical borehole at the same surface location. The horizontal well was brought on production at a rate of 100 bbls/day and is probably capable of producing at a higher rate. The addition of several horizontal wells, similar to the demonstration well, will likely add another 2 million bbls (or more) to the cumulative production of the field over the next few years. The presence of untapped oil in this Dundee field was dramatically demonstrated and the favorable economics were made clearly evident. If other abandoned Dundee fields are re-developed in a similar manner, the additional oil produced domestically will probably be about 80 to 100 million bbls.
Date: January 29, 1996
Creator: Wood, J.R.
Partner: UNT Libraries Government Documents Department

High resolution x-ray fluorescence spectroscopy - a new technique for site- and spin-selectivity

Description: X-ray spectroscopy has long been used to elucidate electronic and structural information of molecules. One of the weaknesses of x-ray absorption is its sensitivity to all of the atoms of a particular element in a sample. Through out this thesis, a new technique for enhancing the site- and spin-selectivity of the x-ray absorption has been developed. By high resolution fluorescence detection, the chemical sensitivity of K emission spectra can be used to identify oxidation and spin states; it can also be used to facilitate site-selective X-ray Absorption Near Edge Structure (XANES) and site-selective Extended X-ray Absorption Fine Structure (EXAFS). The spin polarization in K fluorescence could be used to generate spin selective XANES or spin-polarized EXAFS, which provides a new measure of the spin density, or the nature of magnetic neighboring atoms. Finally, dramatic line-sharpening effects by the combination of absorption and emission processes allow observation of structure that is normally unobservable. All these unique characters can enormously simplify a complex x-ray spectrum. Applications of this novel technique have generated information from various transition-metal model compounds to metalloproteins. The absorption and emission spectra by high resolution fluorescence detection are interdependent. The ligand field multiplet model has been used for the analysis of K{alpha} and K{beta} emission spectra. First demonstration on different chemical states of Fe compounds has shown the applicability of site selectivity and spin polarization. Different interatomic distances of the same element in different chemical forms have been detected using site-selective EXAFS.
Date: December 1, 1996
Creator: Wang, Xin
Partner: UNT Libraries Government Documents Department

Recovery of bypassed oil in the Dundee Formation using horizontal drains. Annual report, March 1996--March 1997

Description: This Class II field project has demonstrated that economic quantities of hydrocarbons can be produced from abandoned or nearly abandoned fields in the Dundee Formation of Central Michigan using horizontal drilling technology. The site selected for the demonstration horizontal well was Crystal Field, a nearly abandoned Dundee oil field in Montcalm County, Michigan. This field had produced over 8 million barrels of oil, mostly in the 1930`s and 1940`s. At the height of development, Crystal Field produced from 193 wells, but by 1995, only seven producing wells remained, each producing less than 10 bbls/day. A horizontal well, the TOW 1-3, drilled as a field demonstration pilot was successful, producing at rate of 100 bbls of oil per day with a zero water cut. Although the well is capable of producing at a of 500+ bbls/day, the production rate is being kept low deliberately to try to prevent premature water coning. Cumulative production exceeded 50,000 bbls of oil by the end of April, 1997 and lead to the permitting and licensing of several dozen Dundee wells by project end. Twelve of these permits were for continued development of Crystal Field. Two long horizontal wells were drilled successfully in Crystal after the TOW 1-3, but were disappointing economically. Core and logs from the Dundee interval were recovered from a vertical borehole at the same surface location. The addition of several horizontal wells will likely add another 2 million bbls (or more) to the cumulative production of the field over the next few years. If other abandoned Dundee fields are re-developed in a similar manner, the additional oil produced could exceed 80 million barrels.
Date: April 1, 1998
Partner: UNT Libraries Government Documents Department

Recovery of bypassed oil in the Dundee Formation using horizontal drains. Quarterly report, July 1, 1996--September 30, 1996

Description: The principal objective of this project is to demonstrate the feasibility and economic success of producing oil from abandoned or nearly abandoned fields in the Dundee Formation of Central Michigan using horizontal drilling technology. A site for a horizontal well was selected in Crystal Field, a nearly-abandoned Dundee oil field in Michigan. This field had produced over 8 million barrels of oil, mostly in the 1930`s and 1940`s. At the height of development, Crystal Field produced from 193 wells, but by 1995, only seven producing wells remained, each producing less than 10 bbls/day. A horizontal well was drilled as a field demonstration pilot, funded through this DOE project, and was immensely successful. Core and logs from the Dundee interval were recovered from a vertical borehole at the same surface location. The horizontal well was brought on production at a rate of 100 bbls/day and is probably capable of producing at a higher rate. The addition of several horizontal wells, similar to the demonstration well, will likely add another 2 million bbls (or more) to the cumulative production of the field over the next few years. The presence of untapped oil in this Dundee field was dramatically demonstrated and the favorable economics were made clearly evident. If other abandoned Dundee fields are re-developed in a similar manner, the additional oil produced could exceed 80 Trillion barrels. Horizontal drilling will likely revolutionize the development of old carbonate fields such as those in the Dundee of Michigan.
Date: October 31, 1996
Creator: Wood, J.R.
Partner: UNT Libraries Government Documents Department

Nonmagnetic crystal-electric-field ground state in the heavy-fermion compound PrInAg{sub 2}

Description: The authors have performed inelastic neutron scattering measurements that confirm that the crystal-electric-field split ground state in the heavy-fermion compound PrInAg{sub 2} is a nonmagnetic, non-Kramers doublet. This implies that a quadrupolar Kondo interaction is responsible for the enhanced thermodynamic properties observed at low temperatures. They also observe anomalous broadening of the inelastic peaks and suggest two possible causes for this broadening.
Date: June 18, 1998
Creator: Kelley, T.M.; Beyermann, W.P.; Robinson, R.A.; Nakotte, H.; Canfield, P.C. & Trouw, F.
Partner: UNT Libraries Government Documents Department

Modeling crystal-field interaction for f-elements in LaCl{sub 3}.

Description: The results of crystal field calculations in the framework of exchange charge model (ECM) are reported for trivalent lanthanide and actinide ions doped into LaCl{sub 3}. Whereas the scalar strength of the model crystal field parameters are consistent with that previously determined by fitting the experimental data, the sign of the second-order parameter is found to be negative, in contrast to previous reports. The contribution from long-range electrostatic interactions exceeds that from the nearest neighboring ligands and leads to the negative sign of the second-order crystal field parameter. Other interaction mechanisms including overlap, covalence, and charge exchange are less important to the second order parameter, but dominate the fourth- and sixth-order parameters. This work provides a consistent interpretation of the previously controversial experimental results for both lanthanide and actinide ions in this classical host.
Date: July 14, 1997
Creator: Liu, G. K. & Zhorin, V. V.
Partner: UNT Libraries Government Documents Department

Crystalline electric fields in mixed valent systems

Description: The inelastic neutron studies of rare-earth-based mixed valent systems have all shown remarkably similar results: a broad quasielastic line with half width on the order of 10 MeV. This width exhibits a strong temperature dependence in those systems which undergo a valence transition and is only weakly temperature dependent in those systems which show no transition. A surprising result was the absence of crystalline electric field (CEF) excitations. Recent measurements on the alloy Ce/sub .9-x/La/sub x/Th/sub .1/ have revealed the existence of CEF excitations. For x = 0, the valence transition is strongly first order and occurs near T/sub 0/ approx. 150 K. The inelastic spectra exhibit the typical broad quasielastic scattering. As x increases, T/sub 0/ decreases due to internal pressure effects, and a well-defined, but broad, excitation appears near E = 15 MeV. This is interpreted as a CEF excitation between the GAMMA/sub 7/ and GAMMA/sub 8/ levels of the Ce/sup 3/+ ion. For x = 0.40, the valence transition is almost completely suppressed and the excitation becomes even sharper.
Date: January 1, 1980
Creator: Shapiro, S.M.
Partner: UNT Libraries Government Documents Department

Angle-dependent Ni2+ x-ray magnetic linear dichroism: Interfacialcoupling revisited

Description: Using x ray magnetic linear dichroism (XMLD) for magnetometry requires detailed knowledge of its dependence on the relative orientation of polarization, magnetic moments, and crystallographic axes. We show that Ni{sup 2+} L{sub 2,3} XMLD in cubic lattices has to be described as linear combination of two fundamental spectra - not one as previously assumed. The spectra are calculated using atomic multiplet theory and the angular dependence is derived from crystal field symmetry. Applying our results to Co/NiO(001) interfaces, we find perpendicular coupling between Ni and Co moments.
Date: August 25, 2007
Creator: Arenholz, Elke; van der Laan, Gerrit; Chopdekar, Rajesh V. & Suzuki, Yuri
Partner: UNT Libraries Government Documents Department

Anisotropic x-ray magnetic linear dichroism at the Fe L2,3 edgesin Fe3O4

Description: X-ray magnetic linear dichroism at the Fe L{sub 2,3} edges of the ferrimagnet Fe{sub 3}O{sub 4} was found to exhibit a strong dependence on the relative orientation of external magnetic field, x-ray polarization, and crystalline axes. Spectral shape and magnitude of the effect were determined for Fe{sub 3}O{sub 4}(011) and Fe{sub 3}O{sub 4}(001) thin films varying the in-plane orientation of field and polarization. All dichroism spectra can be described as a linear combination of three fundamental spectra which in turn give a good agreement with calculated spectra using atomic multiplet theory. The angular dependence of the magnetic dichroism reflects the crystal field symmetry. It can be used to estimate the crystal field splitting and allows determining the spin quantization axis.
Date: April 30, 2006
Creator: Arenholz, Elke; van der Laan, Gerrit; Chopdekar, Rajesh V. & Suzuki, Yuri
Partner: UNT Libraries Government Documents Department

High-Pressure Evolution of Fe2O3 Electronic Structure Revealed by X-ray Absorption

Description: We report the first high pressure measurement of the Fe K-edge in hematite (Fe{sub 2}O{sub 3}) by X-ray absorption spectroscopy in partial fluorescence yield geometry. The pressure-induced evolution of the electronic structure as Fe{sub 2}O{sub 3} transforms from a high-spin insulator to a low-spin metal is reflected in the x-ray absorption pre-edge. The crystal field splitting energy was found to increase monotonically with pressure up to 48 GPa, above which a series of phase transitions occur. Atomic multiplet, cluster diagonalization, and density-functional calculations were performed to simulate the pre-edge absorption spectra, showing good qualitative agreement with the measurements. The mechanism for the pressure-induced phase transitions of Fe{sub 2}O{sub 3} is discussed and it is shown that ligand hybridization significantly reduces the critical high-spin/low-spin gap pressure.
Date: August 12, 2011
Creator: Kao, Chi-Chang
Partner: UNT Libraries Government Documents Department

Anisotropic intermediate valence in Yb2M3Ga9 (M = Rh, Ir)

Description: The intermediate valence compounds Yb{sub 2}M{sub 3}Ga{sub 9} (M = Rh, Ir) exhibit an anisotropic magnetic susceptibility. We report measurements of the temperature dependence of the 4f occupation number, n{sub f}(T), for Yb{sub 2}M{sub 3}Ga{sub 9} as well as the magnetic inelastic neutron scattering spectrum S{sub mag}({Delta}E) at 12 and 300 K for Yb{sub 2}Rh{sub 3}Ga{sub 9}. Both n{sub f}(T) and S{sub mag}({Delta}E) were calculated for the Anderson impurity model with crystal field terms within an approach based on the non-crossing approximation. These results corroborate the importance of crystal field effects in these materials; they also suggest that Anderson lattice effects are important to the physics of Yb{sub 2}M{sub 3}Ga{sub 9}.
Date: April 26, 2005
Creator: Christianson, A.D.; Lawrence, J.M.; Lobos, A.M.; Aligia, A.A.; Bauer, E.D.; Moreno, N.O. et al.
Partner: UNT Libraries Government Documents Department

Coherent orbital waves during an Ultrafast Photo-induced Isulator-metal Transition in a magnetoresistive manganite

Description: Photo-excitation can drive strongly correlated electron insulators into competing conducting phases1,2, resulting in giant and ultrafast changes of their electronic and magnetic properties. The underlying non-equilibrium dynamics involve many degrees of freedom at once, whereby sufficiently short optical pulses can trigger the corresponding collective modes of the solid along temporally coherent pathways. The characteristic frequencies of these modes range between the few GHz of acoustic vibrations3 to the tens or even hundreds of THz for purely electronic excitations. Virtually all experiments so far have used 100 fs or longer pulses, detecting only comparatively slow lattice dynamics4,5. Here, we use sub-10-fs optical pulses to study the photo-induced insulator-metal transition in the magneto-resistive manganite Pr0.7Ca0.3MnO3. At room temperature, we find that the time-dependent pathway towards the metallic phase is accompanied by coherent 31 THz oscillations of the optical reflectivity, significantly faster than all lattice vibrations. These high-frequency oscillations are suggestive of coherent orbital waves6,7, crystal-field excitations triggered here by impulsive stimulated Raman scattering. Orbital waves are likely to be initially localized to the small polarons of this room-temperature manganite, coupling to other degrees of freedom at longer times, as photo-domains coalesce into a metallic phase.
Date: June 1, 2007
Creator: ULTRAS-INFM-CNR Dipartimento di Fisica, Politecnico di Milano, Italy
Partner: UNT Libraries Government Documents Department

Low-energy excitations, symmetry breaking and specific heat in YbBiPt

Description: The heavy fermion compound YbBiPt has a very large linear coefficient of specific heat {gamma} = 8 Jmol{sup {minus}1} K{sup {minus}2} and this is understood, to first order, in terms of the observed low-energy neutron scattering response. However, at low temperatures, symmetry forbidden splittings at 1 and 2 meV respectively are observed. These levels give good qualitative agreement with the measured specific heat, but poor quantitative agreement. Indeed, the specific heat drops more rapidly with temperature that can be accounted for assuming a temperature-independent density of states. The authors also present new low-temperature crystallographic data, which rule out any significant structural distortions.
Date: December 31, 1998
Creator: Robinson, R.A.; Christianson, A.; Nakotte, H.; Beyermann, W.P. & Canfield, P.C.
Partner: UNT Libraries Government Documents Department

Spectroscopy and dynamics of 5f states of Es{sup 3+} in LaF{sub 3}

Description: Using time- and wavelength-resolved laser-induced fluorescence methods, the 5f state spectroscopy and photodynamics of {sup 253}Es{sup 3+} in LaF{sub 3} have been investigated. Based on an effective operator Hamiltonian model and approximating the metal ion site symmetry as C{sub 2V}, a set of crystal field parameters has been obtained that fit the 56 assigned levels associated with the 7 states of Es{sup 3+} that were observed. The {sup 5}F{sub 5} emitting state of Es{sup 3+} exhibited a decay rate that approached the expected purely radiative decay of the state. This suggests that the shorter lifetime previously found for this state of Es{sup 3+} in LaCl{sub 3} arose from radiation damage induced by alpha decay of {sup 253}Es.
Date: November 1, 1997
Creator: Beitz, J.V.; Williams, C.W. & Liu, G.K.
Partner: UNT Libraries Government Documents Department

Quasi-two-dimensional quantum states of H{sub 2} in stage-2 Rb-intercalated graphite

Description: Inelastic-incoherent-neutron scattering can be a valuable nanostructural probe of H{sub 2}-doped porous materials, provided the spectral peaks can be interpreted in terms of crystal-field-split hydrogen-molecule energy levels, which represent a signature of the local symmetry. Inelastic-neutron-scattering measurements as well as extensive theoretical analyses have been performed on stage-2 Rb-intercalated graphite (Rb-GIC), with physisorbed H{sub 2}, HD, and D{sub 2}, a layered porous system with abundant spectral peaks, to assess whether the crystal-field-state picture enables a quantitative understanding of the observed structure. Potential-energy surfaces for molecular rotational and translational motion, as well as the intermolecular interactions of hydrogen molecules in Rb-GIC, were calculated within local-density-functional theory (LDFT). Model potentials, parameterized using results of the LDFT calculations, were employed in schematic calculations of rotational and translational excited state spectra of a single physisorbed H{sub 2} molecule in Rb-GIC. Results of the analysis are basically consistent with the assignment by Stead et al. of the lowest-lying peak at 1.4 meV to a rotational-tunneling transition of an isotropic hindered-rotor oriented normal to the planes, but indicate a small azimuthal anisotropy and a lower barrier than for the isotropic case. Based on the experimental isotope shifts and the theoretically predicted states, they conclude that spectral peaks at 11 and 22 meV are most likely related to center of mass excitations.
Date: October 30, 1995
Creator: Smith, A. P.; Benedek, R.; Trouw, F. R.; Minkoff, M. & Yang, L. H.
Partner: UNT Libraries Government Documents Department