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Calculations of the Relative Energies of the 2B1g and 2A2u States of Cyclobutanetetraone Radical Cation and Radical Anion Provide Further Evidence of a 3B2u Ground State for the Neutral Molecule: A Proposed Experimental Test of the Prediction of a Triplet Ground State for (CO)4

Description: Article on a proposed experimental test of the prediction of a triplet ground state for (CO)4.
Date: 2010
Creator: Zhou, Xin; Hrovat, David A. & Borden, Weston T.
Partner: UNT College of Arts and Sciences

Summing Planar Bosonic Open Strings

Description: In earlier work, planar graphs of massless {phi}{sup 3} theory were summed with the help of the light cone world sheet picture and the mean field approximation. In the present article, the same methods are applied to the problem of summing planar bosonic open strings. They find that in the ground state of the system, string boundaries form a condensate on the world sheet, and a new string emerges from this summation. Its slope is always greater than the initial slope, and it remains non-zero even when the initial slope is set equal to zero. If they assume the initial string tends to a field a theory in the zero slope limit, this result provides evidence for string formation in field theory.
Date: February 16, 2006
Creator: Bardakci, Korkut
Partner: UNT Libraries Government Documents Department

The Low-Energy State ofCirculating Stored Ion Beams: Crystalline Beams

Description: Molecular dynamics is employed to study the low energy states of a beam of charged particles subject to circumferentially varying guiding and focusing forces and with Coulomb forces between the particles. In a constant gradient ring, the lowest energy state is never ordered, but in an alternating gradient structure, operating below the transition energy, the lowest state is ordered. The nature and characteristics of the ground state depends upon the beam density and the ring parameters. For zero temperature the crystal remains intact for a very long time, but at nonzero temperatures it gains energy from the lattice. A critical temperature exists above which the crystal melts rapidly.
Date: March 10, 1994
Creator: Wei, J.; Li, X.-P. & Sessler, Andrew M.
Partner: UNT Libraries Government Documents Department

Quantum adiabatic computation with a constant gap is not useful in one dimension

Description: We show that it is possible to use a classical computer to efficiently simulate the adiabatic evolution of a quantum system in one dimension with a constant spectral gap, starting the adiabatic evolution from a known initial product state. The proof relies on a recently proven area law for such systems, implying the existence of a good matrix product representation of the ground state, combined with an appropriate algorithm to update the matrix product state as the Hamiltonian is changed. This implies that adiabatic evolution with such Hamiltonians is not useful for universal quantum computation. Therefore, adiabatic algorithms which are useful for universal quantum computation either require a spectral gap tending to zero or need to be implemented in more than one dimension (we leave open the question of the computational power of adiabatic simulation with a constant gap in more than one dimension).
Date: January 1, 2009
Creator: Hastings, Matthew
Partner: UNT Libraries Government Documents Department

Alternative Donor--Acceptor Stacks from Crown Ethers and Naphthalene Diimide Derivatives: Rapid, Selective Formation from Solution and Solid State Grinding

Description: Self assembling {pi}-conjugated molecules into ordered structures are of increasing interest in the field of organic electronics. One particular example is charge transfer complexes containing columnar alternative donor-acceptor (ADA) stacks, where neutral and ionic ground states can be readily tuned to modulate electrical, optical, and ferroelectrical properties. Aromatic-aromatic and charge transfer interactions have been the leading driving forces in assisting the self-assembly of ADA stacks. Various folding structures containing ADA stacks were assembled in solution with the aid of solvophobic or ion-binding interactions. Meanwhile, examples of solid ADA stacks, which are more appealing for practical use in devices, were obtained from cocrystalization of binary components or mesophase assembly of liquid crystals in bulk blends. Regardless of these examples, faster and more controllable approaches towards precise supramolecular order in the solid state are still highly desirable.
Date: January 22, 2009
Creator: Lawrence Berkeley National Laboratory. Advanced Light Source.
Partner: UNT Libraries Government Documents Department

Enhanced Magnetism in Epitaxial SrRuO3 Thin Films

Description: We have observed enhanced magnetization in epitaxial SrRuO{sub 3} thin films compared to previously reported bulk and thin film values of 1.1-1.6 {mu}{sub B}/Ru ion. The degree of enhancement is strongly dependent on the lattice distortions imposed on the SrRuO{sub 3} films by SrTiO{sub 3}, (LaAlO{sub 3}){sub 0.3}(SrTaO{sub 3}){sub 0.7} (LSAT), and LaAlO{sub 3} substrates. A larger enhancement of magnetization for coherently strained SrRuO{sub 3} films on SrTiO{sub 3} and LSAT compared to fully relaxed films on LaAlO{sub 3} confirms the importance of the strain state in determining the magnetic ground state of the Ru ion. In particular, SrRuO{sub 3} films on (111) SrTiO{sub 3} exhibit exhanced moments as high as 3.8 {mu}{sub B}/Ru ion, thus suggesting the stabilization of a high-spin Ru{sup 4+} state.
Date: November 20, 2009
Creator: Grutter, A.J.; Wong, F.; Arenholz, E.; Liberati, M.; Vailionis, A. & Suzuki, Y.
Partner: UNT Libraries Government Documents Department

Strain Induced Magnetism in SrRuO3 Epitaxial Thin Films

Description: Epitaxial SrRuO{sub 3} thin films were grown on SrTiO{sub 3}, (LaAlO{sub 3}){sub 0.3}(SrAlO{sub 3}){sub 0.7} and LaAlO{sub 3} substrates inducing different biaxial compressive strains. Coherently strained SrRuO{sub 3} films exhibit enhanced magnetization compared to previously reported bulk and thin film values of 1.1-1.6 {micro}{sub B} per formula unit. A comparison of (001) and (110) SrRuO{sub 3} films on each substrate indicates that films on (110) oriented have consistently higher saturated moments than corresponding (001) films. These observations indicate the importance of lattice distortions in controlling the magnetic ground state in this transitional metal oxide.
Date: January 10, 2010
Creator: Grutter, A.; Wong, F.; Arenholz, E.; Liberati, M. & Suzuki, Y.
Partner: UNT Libraries Government Documents Department

Magnetic vortex dynamics on a picosecond timescale in a hexagonal permalloy pattern

Description: We have observed a motion of magnetic vortex core in a hexagonal Permalloy pattern by means of Soft X-ray microscopy. Pump-probe stroboscopic observation on a picosecond timescale has been carried out after exciting a ground state vortex structure by an external field pulse of 1 ns duration. Vortex core is excited off from the center position of the hexagonal pattern but the analysis of the core trajectory reveals that the motion is nongyrotropic.
Date: December 2, 2009
Creator: Shim, J.-H.; Kim, D.-H.; Mesler, B.; Moon, J.-H.; Lee, K.-J.; Anderson, E. H. et al.
Partner: UNT Libraries Government Documents Department


Description: Through three examples the authors illustrate some of the concepts and ingredients required for pattern formation at mesoscopic scales. Two examples built on microscopic models where mesoscopic patterns emerge from homogeneous ground states driven into instability by external forcing. In contrast, the third example builds on a mesoscopic phenomenological Ginzburg-Landan type model of solid-solid structural phase transition. Here, mesoscopic textures emerge as a result of competing length scales arising from the constraints of elastic compatibility.
Date: December 1, 2000
Partner: UNT Libraries Government Documents Department

In/Si(111): Self-assembled one and two-dimensional electrongases

Description: We present angle-resolved photoemission measurements forultrathin In films on Si(111). Depending on the coverage, this systemself-organizes into a metallic monolayer with either 4x1 or sqrt7 x sqrt3symmetry relative to the substrate. Electronically, they behave likeideal one- and two-dimensional electron gases (1DEG and 2DEG),respectively. The 4x1 system has atomic chains of In whose energy bandsdisperse only parallel to the chains, while for the sqrt7 x sqrt3 system,the dominant reciprocal space features (in both diffraction andbandstructure) resemble a pseudo-square lattice with only weakersecondary features relating to the sqrt7 x sqrt3 periodicity. In bothmaterials the electrons show coupling to the structure. The 1DEG couplesstrongly to phonons of momentum 2kF, leading to an 8x"2" Peierls-likeinsulating ground state. The 2DEG appears to be partially stabilized byelectron gap formation at the sqrt 7 x sqrt3 zone boundary.
Date: January 22, 2001
Creator: Rotenberg, Eli; Yeom, H. W.; Takeda, S.; Matsuda, I.; Horikoshi, K.; Schaefer, J. et al.
Partner: UNT Libraries Government Documents Department

Modified magnetic ground state in NiMn2O4 thin films

Description: We demonstrate the stabilization of a magnetic ground state in epitaxial NiMn2O4 (NMO) thin films not observed in their bulk counterpart. Bulk NMO exhibits a magnetic transition from a paramagnetic phase to a collinear ferrimagnetic moment configuration below 110 K and to a canted moment configuration below 70 K. By contrast, as-grown NMO films exhibit a single magnetic transition at 60 K and annealed films exhibit the magnetic behavior found in bulk. Cation inversion and epitaxial strain are ruled out as possible causes for the new magnetic ground state in the as-grown films. However, a decrease in the octahedral Mn{sup 4+}:Mn{sup 3+} concentration is observed and likely disrupts the double exchange that produces the magnetic state at intermediate temperatures. X-ray magnetic circular dichroism and bulk magnetometry indicate a canted ferrimagnetic state in all samples at low temperature. Together these results suggest that the collinear ferrimagnetic state observed in bulk NMO at intermediate temperatures is suppressed in the as grown NMO thin films due to a decrease in octahedral Mn{sup 4+} while the canted moment ferrimagnetic ordering is preserved below 60 K.
Date: August 3, 2010
Creator: Nelson-Cheeseman, B. B.; Chopdekar, R. V.; Toney, M. F.; Arenholz, E.; Suzuki, Y. & Iwata, J.M.
Partner: UNT Libraries Government Documents Department


Description: The 3d4f levels in the spectrum of triply ionized vanadium, V IV, were located by L. Iglesias. She located these levels by identifying transitions from the 3d{sup 2} ground configuration to the 3d4p levels, then to the 3d4d levels and finally to the 3d4f levels. She also identified the transitions from the 3d4d levels to the 3d5p levels, continued up to the 3d5d levels, then back down to the 3d4f levels. Though the 3d4f levels were well established by two routes, the direct transitions from the ground state were not observed, being beyond her experimental range which stopped at 675 {angstrom}. We have photographed the spectrum of vanadium in the region of 190-650 {angstrom} and the direct transitions from 3d{sup 2} to 3d4f have been observed. The spectrum was excited with a vacuum sliding-spark discharge between vanadium metal electrodes separated by a quartz spacer as described previously. Peak discharge current was 1000 {angstrom}. The spectrum was photographed on Kodak SWR plates using the 10 {center_dot} 7 m grazing incidence spectrograph at the National Bureau of Standards in Washington, D.C. The plate factor in the region of interest is about 0.27 {angstrom}/mm. The plates were measured on a Grant comparator. Lines of yttrium IV and V, oxygen III and carbon IV were used for reference standards. The same plates were used to obtain the spectrum of vanadium V as reported previously by van Deurzen. Table I contains 19 transitions from the ground configuration, 3d{sup 2}, to the 3d4f configuration and 3 transitions from the ground configuration to the 3dSp configuration. Column 1 in the table is our measured vacuum wavelengths. The intensities in column 2 are visual estimates on a scale of 0 to 9. Column 3 lists the wavenumber of the measured lines. The classifications of the transitions and the ...
Date: September 1, 1977
Creator: Shalimoff, G. V. & Conway, J.G.
Partner: UNT Libraries Government Documents Department

Model for l/f Flux Noise in SQUIDs and Qubits

Description: We propose a model for 1/f flux noise in superconducting devices (f is frequency). The noise is generated by the magnetic moments of electrons in defect states which they occupy for a wide distribution of times before escaping. A trapped electron occupies one of the two Kramers-degenerate ground states, between which the transition rate is negligible at low temperature. As a result, the magnetic moment orientation is locked. Simulations of the noise produced by a plausible density of randomly oriented defects yield 1/f noise magnitudes in good agreement with experiments.
Date: January 19, 2007
Creator: Koch, Roger H.; DiVincenzo, David P. & Clarke, John
Partner: UNT Libraries Government Documents Department

Direct observation of photoinduced bent nitrosyl excited-state complexes

Description: Ground state structures with side-on nitrosyl ({eta}{sup 2}-NO) and isonitrosyl (ON) ligands have been observed in a variety of transition-metal complexes. In contrast, excited state structures with bent-NO ligands have been proposed for years but never directly observed. Here we use picosecond time-resolved infrared spectroscopy and density functional theory (DFT) modeling to study the photochemistry of Co(CO){sub 3}(NO), a model transition-metal-NO compound. Surprisingly, we have observed no evidence for ON and {eta}{sup 2}-NO structural isomers, but have observed two bent-NO complexes. DFT modeling of the ground and excited state potentials indicates that the bent-NO complexes correspond to triplet excited states. Photolysis of Co(CO){sub 3}(NO) with a 400-nm pump pulse leads to population of a manifold of excited states which decay to form an excited state triplet bent-NO complex within 1 ps. This structure relaxes to the ground triplet state in ca. 350 ps to form a second bent-NO structure.
Date: June 28, 2008
Creator: Sawyer, Karma R.; Steele, Ryan P.; Glascoe, Elizabeth A.; Cahoon, James F.; Schlegel, Jacob P.; Head-Gordon, Martin et al.
Partner: UNT Libraries Government Documents Department

Black holes in supergravity: the non-BPS branch

Description: We construct extremal, spherically symmetric black hole solutions to 4D supergravity with charge assignments that preclude BPS-saturation. In particular, we determine the ground state energy as a function of charges and moduli. We find that the mass of the non-BPS black hole remains that of a marginal bound state of four basic constituents throughout the entire moduli space and that there is always a non-zero gap above the BPS bound.
Date: October 25, 2007
Creator: Gimon, Eric; Gimon, Eric G.; Larsen, Finn & Simon, Joan
Partner: UNT Libraries Government Documents Department

Effects of d-electrons in pseudopotential screened-exchange density functional calculations

Description: We report a theoretical study on the role of shallow d states in the screened-exchange local density approximation (sX-LDA) band structure of binary semiconductor systems. We found that the inaccurate pseudo-wavefunctions can lead to (1) an overestimation of the screened-exchange interaction between the localized d states and the delocalized higher energy s and p states and (2) an underestimation of the screened-exchange interaction between the d states. The resulting sX-LDA band structures have substantially smaller band gaps compared with experiments. We correct the pseudo-wavefunctions of d states by including the semicore s and p states of the same shell in the valence states. The correction of pseudo-wavefunctions yields band gaps and the d state binding energy with good agreements with experiments and the full potential linearized augmented planewave (FLAPW) calculations. Compared with the quasi-particle GW method, our sX-LDA results shows not only similar quality on the band gaps but also much better d state binding energy. Combined with its capability of ground state structure calculation, the sX-LDA is expected to be a valuable theoretical tool for the II-VI and III-V (especially the III-N) bulk semiconductors and nanostructure studies.
Date: August 11, 2008
Creator: Wang, Lin-Wang; Lee, Byounghak; Canning, Andrew & Wang, Lin-Wang
Partner: UNT Libraries Government Documents Department

Single photon induced symmetry breaking of H2 dissociation

Description: H{sub 2}, the smallest and most abundant molecule in the universe, has a perfectly symmetric ground state. What does it take to break this symmetry? Here we show that the inversion symmetry can be broken by absorption of a linearly polarized photon, which itself has inversion symmetry. In particular, the emission of a photoelectron with subsequent dissociation of the remaining H{sub 2}{sup +} fragment shows no symmetry with respect to the ionic H+ and neutral H atomic fragments. This result is the consequence of the entanglement between symmetric and antisymmetric H{sub 2}{sup +} states resulting from autoionization. The mechanisms behind this symmetry breaking are general for all molecules.
Date: December 6, 2006
Creator: Martin, F.; Fernandez, J.; Havermeier, T.; Foucar, L.; Weber, Th; Kreidi, K. et al.
Partner: UNT Libraries Government Documents Department

Hunting the Scalar Glueball: Prospects for BES III

Description: The search for the ground state scalar glueball $G_0$ isreviewed. Spin zero glueballs will have unique dynamical properties ifthe $<G_0|\overline qq>$ amplitude is suppressed by chiralsymmetry, as it is to all orders in perturbation theory: for instance,mixing of $G_0$ with $\overline qq$ mesons would be suppressed, radiative$\jp$ decay would be a filter for new physics in the spin zero channel,and the decay $G_0 \rightarrow \overline KK$ could be enhanced relativeto $G_0 \rightarrow \pi \pi$. These properties are consistent with theidentification of $f_0(1710)$ as the largely unmixed ground state scalarglueball, while recent BES data implies that $f_0(1500)$ does not containthe dominant glueball admixture. Three hypotheses are discussed: that$G_0$ is 1) predominantly $f_0(1500)$ or 2) predominantly $f_0(1710)$ or3) is strongly mixed between $f_0(1500)$ and $f_0(1710)$.
Date: September 18, 2006
Creator: Chanowitz, Michael S.
Partner: UNT Libraries Government Documents Department

Coulomb excitation of the 242mAm isomer

Description: The {sup 242m}Am isomer, a well-known candidate for photo-depopulation research, has been studied in this first ever Coulomb excitation of a nearly pure ({approx} 98%) isomer target. Thirty new states, including a new rotational band built on a K{sup {pi}} = 6{sup -} state have been identified. Strong K-mixing results in nearly equal populations of the K{sup {pi}} = 5{sup -} and 6{sup -} states. Newly identified states have been assigned to the K{sup {pi}} = 3{sup -} rotational band, the lowest states of which are known to decay into the ground-state band. Implications regarding K-mixing and Coulomb excitation paths to the ground state are discussed.
Date: October 24, 2006
Creator: Hayes, A; Cline, D; Moody, K; Wu, C; Becker, J; Carpenter, M et al.
Partner: UNT Libraries Government Documents Department

E2 Transition Probabilities in 114Te: a Conundrum

Description: Lifetimes in {sup 114}Te were determined using the recoil distance Doppler-shift technique with a plunger device coupled to five HP Ge detectors enhanced by one Euroball Cluster detector. The experiment was carried out at the Cologne FN Tandem facility using the {sup 93}Nb({sup 24}Mg,p2n) reaction at 90 MeV. The differential decay curve method in coincidence mode was employed to derive lifetimes for seven excited states, while the lifetime of an isomeric state was obtained in singles mode. The resulting E2 transition probabilities are shown to be very anomalous in comparison with the vibrational energy spacings of the ground state band.
Date: May 13, 2005
Creator: Moller, O; Warr, N; Jolie, J; Dewald, A; Fitzler, A; Linnemann, A et al.
Partner: UNT Libraries Government Documents Department

Testing the Physics of Nuclear Isomers

Description: For much of the past century, physicists have searched for methods to control the release of energy stored in an atom's nucleus. Nuclear fission reactors have been one successful approach, but finding other methods to capitalize on this potential energy source have been elusive. One possible source being explored is nuclear isomers. An isomer is a long-lived excited state of an atom's nucleus--a state in which decay back to the nuclear ground state is inhibited. The nucleus of an isomer thus holds an enormous amount of energy. If scientists could develop a method to release that energy instantaneously in a gamma-ray burst, rather than slowly over time, they could use it in a nuclear battery. Research in the late 1990s indicated that scientists were closer to developing such a method--using x rays to trigger the release of energy from the nuclear isomer hafnium-178m ({sup 178m}Hf). To further investigate these claims, the Department of Energy (DOE) funded a collaborative project involving Lawrence Livermore, Los Alamos, and Argonne national laboratories that was designed to reproduce those earlier results.
Date: January 25, 2006
Creator: Hazi, A
Partner: UNT Libraries Government Documents Department

Magnesium substitutions in rare-earth metal germanides with the Gd5Si4 type. Synthesis, structure determination and magnetic properties of RE5-xMgxGe4 (RE=Gd-Tm, Lu and Y)

Description: A series of magnesium-substituted rare-earth metal germanides with a general formula RE{sub 5-x}Mg{sub x}Ge{sub 4} (x {approx} 1.0-2.3; RE =Gd-Tm, Lu, Y) have been synthesized by high-temperature reactions and structurally characterized by single-crystal X-ray diffraction. These compounds crystallize with the common Gd{sub 5}Si{sub 4} type in the orthorhombic space group Pnma (No. 62; Z =4; Pearson's code oP36) and do not appear to undergo temperature-induced crystallographic phase transitions down to 120 K. Replacing rare-earth metal atoms with Mg, up to nearly 45 % at., reduces the valence electron count and is clearly expressed in the subtle changes of the Ge-Ge and metal-metal bonding. Magnetization measurements as a function of the temperature and the applied field reveal complex magnetic structures at cryogenic temperatures, and Curie-Weiss paramagnetic behavior at higher temperatures. The observed local moment magnetism is consistent with RE+ ground states in all cases. In the magnetically ordered phases, the magnetization cannot reach saturation in fields up to 50 kOe. The structural trends across the series and the variations of hte magnetic properties as a function of the Mg content are also discussed. KEYWORDS: Rare-earth intermetallics, germanides, crystal structure,Gd{sub 5}Si{sub 4} type.
Date: January 1, 2009
Creator: Sarrao, J L; Thompson, Joe D; Tobash, P H & Bobev, S
Partner: UNT Libraries Government Documents Department

Tuning magnetic frustration on the diamond lattice of the A-site magnetic spinels CoA12-xGax04: lattice expansion versus site disorder

Description: The spinels CoB{sub 2}O{sub 4} with magnetic Co{sup 2+} ions on the diamond lattice A site can be frustrated because of competing near-neighbor (J{sub 1}) and next-near neighbor (J{sub 2}) interactions. Here we describe attempts to tune the relative strengths of these interactions by substitution on the non-magnetic B-site. The system we employ is CoAl{sub 2-x}Ga{sub x}O{sub 4}, where Al is systematically replaced by the larger Ga, ostensibly on the B site. As expected, Ga substitution expands the lattice, resulting in Co atoms on the A-site being pushed further from one other and thereby weakening magnetic interactions. In addition, Ga distributes between the B and the A site in a concentration dependent manner displacing an increasing amount of Co from the A site with increasing x. This increased inversion, which is confirmed by neutron diffraction studies carried out at room temperature, affects magnetic ordering very significantly, and changes the nature of the ground state. Modeling of the magnetic coupling illustrates the complexity that arises from the cation site disorder.
Date: January 1, 2008
Creator: Proffen, Thomas E; Melot, Brent C; Page, Katharine; Seshadri, Ramzy; Stoudenmire, E M; Balents, Leon et al.
Partner: UNT Libraries Government Documents Department