Showing papers by "Argonne National Laboratory published in 1992"
TL;DR: The Ribosomal Database Project (RDP) is a curated database that offers ribosome data along with related programs and services that include phylogenetically ordered alignments of ribosomal RNA (rRNA) sequences, derived phylogenetic trees, rRNA secondary structure diagrams and various software packages for handling, analyzing and displaying alignments and trees.
Abstract: The Ribosomal Database Project (RDP) is a curated database that offers ribosome-related data, analysis services, and associated computer programs. The offerings include phylogenetically ordered alignments of ribosomal RNA (rRNA) sequences, derived phylogenetic trees, rRNA secondary structure diagrams, and various software for handling, analyzing and displaying alignments and trees. The data are available via anonymous ftp (rdp.life.uiuc.edu), electronic mail (server/rdp.life.uiuc.edu) and gopher (rdpgopher.life.uiuc.edu). The electronic mail server also provides ribosomal probe checking, approximate phylogenetic placement of user-submitted sequences, screening for chimeric nature of newly sequenced rRNAs, and automated alignment.
1,660 citations
TL;DR: The recent elucidation of the three-dimensional structure of the heat-labile enterotoxin has provided an opportunity to examine and compare the correlations between structure and function of the two toxins, which may improve understanding of the disease process itself and illuminate the role of the toxin in studies of signal transduction and G-protein function.
863 citations
TL;DR: In this paper, the effect of nitric acid concentration on the selectivity of a novel extraction chromatographic resin consisting of an octanol solution of 4,4′(5′)-bis(t-butyl-cyclohexano)-18-crown-6 sorbed on an inert polymeric support for strontium over a number of alkali, alkaline earth, and other metal cations was evaluated.
Abstract: The effect of nitric acid concentration on the selectivity of a novel extraction chromatographic resin consisting of an octanol solution of 4,4′(5′)-bis(t-butyl-cyclohexano)-18-crown-6 sorbed on an inert polymeric support for strontium over a number of alkali, alkaline earth, and other metal cations was evaluated. The effect of macro quantities of selected elements on strontium retention by the resin was also examined. The resin is shown to exhibit excellent selectivity for strontium over nearly all of the test elements; only lead and tetravalent neptunium, polonium, and plutonium show significant affinity for the material. In addition, concentrations of calcium or sodium ion up to ∼ 0.1 M. are shown not to diminish the sorption of strontium appreciably. Several useful radiochemical separation schemes devised on the basis of the results obtained are described.
686 citations
TL;DR: In this paper, the effect of macro levels of selected elements on the sorption of actinide ions by a novel extraction chromatographic resin comprised of a solution of octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide in tri-n-butyl phosphate supported on an inert polymeric substrate is described.
578 citations
TL;DR: Experimental results show that ADifOR can handle real-life codes and that ADIFOR-generated codes are competitive with divided-difference approximations of derivatives, and studies suggest that the source transformation approach to automatic differentiation may improve the time to compute derivatives by orders of magnitude.
Abstract: The numerical methods employed in the solution of many scientific computing problems require the computation of derivatives of a function f $R^N$→$R^m$ Both the accuracy and the computational requirements of the derivative computation are usually of critical importance for the robustness and speed of the numerical solution Automatic Differentiation of FORtran (ADIFOR) is a source transformation tool that accepts Fortran 77 code for the computation of a function and writes portable Fortran 77 code for the computation of the derivatives In contrast to previous approaches, ADIFOR views automatic differentiation as a source transformation problem ADIFOR employs the data analysis capabilities of the ParaScope Parallel Programming Environment, which enable us to handle arbitrary Fortran 77 codes and to exploit the computational context in the computation of derivatives Experimental results show that ADIFOR can handle real-life codes and that ADIFOR-generated codes are competitive with divided-difference approximations of derivatives In addition, studies suggest that the source transformation approach to automatic differentiation may improve the time to compute derivatives by orders of magnitude
458 citations
TL;DR: It is shown here that, by a recursive scheme related to the multilevel differentiation approach of Volin and Ostrovskii, the growth in both temporal and spatial complexity can be limited to a fixed multiple of log(T).
Abstract: In its basic form the reverse mode of automatic differentiation yields gradient vectors at a small multiple of the computational work needed to evaluate the underlying scalar function. The practical applicability of this temporal complexity result, due originally to Linnainmaa, seemed to be severely limited by the fact that the memory requirement of the basic implementation is proportional to the run timeT, of the original evaluation program. It is shown here that, by a recursive scheme related to the multilevel differentiation approach of Volin and Ostrovskii, the growth in both temporal and spatial complexity can be limited to a fixed multiple of log(T). Other compromises between the run time and memory requirement are possible, so that the reverse mode becomes applicable to computational problems of virtually any size.
406 citations
TL;DR: The physics of flux lines in the cuprate superconductors pinned by columnar defects is mapped onto boson localization in two dimensions, and a scaling theory for the irreversibility line which separates them is proposed.
Abstract: The physics of flux lines in the cuprate superconductors pinned by columnar defects is mapped onto boson localization in two dimensions. The theory predicts a Bose glass phase with an infinite tilt modulus and zero linear resistivity, as well as an entangled flux liquid. We describe correlations and transport in these phases, and propose a scaling theory for the irreversibility line which separates them.
373 citations
TL;DR: An electron donor-acceptor-donor molecule consisting of two porphyrin donors rigidly attached to the two-electron acceptor N,N'-diphenyl-3,4,9,10-perylenebis(dicarboximide) acts as a light intensity—dependent molecular switch on a picosecond time scale.
Abstract: An electron donor-acceptor-donor molecule consisting of two porphyrin donors rigidly attached to the two-electron acceptor N,N'-diphenyl-3,4,9,10-perylenebis(dicarboximide) acts as a light intensity-dependent molecular switch on a picosecond time scale. Excitation of the porphyrins within this molecule with subpicosecond laser pulses results in single or double reduction of the acceptor depending on the light intensity. The singly and doubly reduced electron acceptors absorb light strongly at 713 and 546 nanometers, respectively. Because these absorption changes are produced solely by electron transfers, this molecular switch effectively has no moving parts and switches significantly faster than photochromic molecules that must undergo changes in molecular structure.
371 citations
TL;DR: Observations suggest that Trp residues are involved in the translocation of protein through the membrane and that following translocation, Trp residue serve as anchors on the periplasmic side of the membrane.
Abstract: Membrane proteins have a significantly higher Trp content than do soluble proteins. This is especially true for the M and L subunits of the photosynthetic reaction center from purple bacteria. The Trp residues are not uniformly distributed through the membrane but are concentrated at the periplasmic side of the complex. In addition, Trp residues are not randomly aligned. Within the protein subunits, many form hydrogen bonds with carbonyl oxygens of the main chain, thereby stabilizing the protein. On the surface of the molecule, they are correctly positioned to form hydrogen bonds with the lipid head groups while their hydrophobic rings are immersed in the lipid part of the bilayer. These observations suggest that Trp residues are involved in the translocation of protein through the membrane and that following translocation, Trp residues serve as anchors on the periplasmic side of the membrane.
352 citations
TL;DR: It is proposed that most ionic-crystal surfaces undergo a simple zero-temperature reconstruction, a prediction validated by computer simulation of surfaces in rocksalt-structured materials.
Abstract: Based on the manner in which large Coulomb terms with opposite signs cancel each other at long range, it is proposed that most ionic-crystal surfaces undergo a simple zero-temperature reconstruction, a prediction validated by computer simulation of surfaces in rocksalt-structured materials.
291 citations
TL;DR: The melting transition in twinned and untwinned single crystals is measured resistively in fields up to 8 T as a function of the angle between the c axis and the a-b plane.
Abstract: The melting transition in twinned and untwinned single crystals is measured resistively in fields up to 8 T as a function of the angle between the c axis and the a-b plane. The angular dependence follows the Lindemann criterion with ${\mathit{c}}_{\mathit{L}}$=0.15. The suppression of melting by strong pinning by twin boundaries is demonstrated.
TL;DR: It is shown that singlet pairing correlations develop above {ital T}{sub {ital c}}, and the normal state of a short coherence length superconductor deviates from a canonical Fermi liquid, which provides a simple, qualitative explanation for the spin-gap behavior observed in several high-{ital T}.
Abstract: We study the normal state of the 2D attractive Hubbard model using quantum Monte Carlo simulations. We show that singlet pairing correlations develop above ${\mathit{T}}_{\mathit{c}}$, and the normal state of a short coherence length superconductor deviates from a canonical Fermi liquid. In the intermediate U regime, the spin susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathit{s}}$ is strongly temperature dependent, and the low-frequency spectral weight, as measured by the NMR relaxation rate 1/${\mathit{T}}_{1}$T, is shown to track ${\mathrm{\ensuremath{\chi}}}_{\mathit{s}}$. This provides a simple, qualitative explanation for the spin-gap behavior observed in several high-${\mathit{T}}_{\mathit{c}}$ systems.
TL;DR: The paragenetic sequence (from uraninite to schoepite to uranyl silicates) is identical to those observed in weathered Uraninite deposits as discussed by the authors.
TL;DR: In this paper, a classification scheme for resinite is proposed, based on the structural characteristics of the original resinite from which the resinite was derived, which can be reasonably inferred from analytical data concerning the structure and composition of the individual sample.
TL;DR: In this article, an efficient and general method for the computation of analytic energy gradients and energy response properties for general MRCI (multireference configuration interaction) and ACPF (averaged coupled pair functional) wave functions is presented.
Abstract: An efficient and general method for the computation of analytic energy gradients and energy response properties for general MRCI (multireference configuration interaction) and ACPF (averaged coupled pair functional) wave functions is presented. This methodology includes a general approach, based on successive orbital transformations, for the inclusion of the effects of various orbital resolution (canonicalization) constraints. Initial implementation in the columbus Program System demonstrates, particularly for large‐scale multireference wave functions, that the additional computational effort required for the energy gradient is a small fraction of that required for the energy. For polyatomic molecules, the computational resources required for the energy gradient do not depend explicitly on the number of constituent atoms. This combination of features represents a major step forward in the computation and characterization of molecular potential energy surfaces.
TL;DR: A direct correlation between the shape of {ital M}({ital H}) and the field-dependent thermal relaxation rate is demonstrated and a signature of the thermal softening boundary at which thermal fluctuations on the scale of the coherence length {xi} are relevant is identified.
Abstract: We construct a phase diagram'' of the vortex-solid phase of Y-Ba-Cu-O single crystals by making a first overall connection of the shape of the magnetic hysteresis {ital M}({ital H},{ital T}) with the single-vortex (1D) and collective (3D) pinning regimes. The crossovers between different regimes are visualized by drawing contours of constant critical current density {ital J}{sub {ital c}} in the {ital H}-{ital T} plane. We identify the transition from 1D to 3D pinning, and from the nonlocal into a local behavior of vortex bundles in the collective pinning regime. A direct correlation between the shape of {ital M}({ital H}) and the field-dependent thermal relaxation rate is demonstrated. We also identify a signature of the thermal softening boundary at which thermal fluctuations on the scale of the coherence length {xi} are relevant.
TL;DR: The first direct measurements of {ital T}{sub {ital c}} as a function of uniaxial stress along the crystal axes for an untwinned YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} single crystal are presented.
Abstract: We present the first direct measurements of ${\mathit{T}}_{\mathit{c}}$ as a function of uniaxial stress ${\mathit{p}}_{\mathit{i}}$ (i=a,b,c) along the crystal axes for an untwinned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ single crystal. ${\mathit{T}}_{\mathit{c}}$ is linear in ${\mathit{p}}_{\mathit{i}}$ with slopes ${\mathit{dT}}_{\mathit{c}}$/${\mathit{dp}}_{\mathit{a}}$=-2.0\ifmmode\pm\else\textpm\fi{}0.2 K/GPa, ${\mathit{dT}}_{\mathit{c}}$/${\mathit{dp}}_{\mathit{b}}$=+1.9\ifmmode\pm\else\textpm\fi{}0.2 K/GPa, and ${\mathit{dT}}_{\mathit{c}}$/${\mathit{dp}}_{\mathit{c}}$=-0.3\ifmmode\pm\else\textpm\fi{}0.1 K/GPa. These results confirm predictions based on thermal expansion measurements [C. Meingast et al., Phys. Rev. Lett. 67, 1634 (1991)] and show that the small hydrostatic pressure dependence of ${\mathit{T}}_{\mathit{c}}$ in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ is due to a cancellation of large and opposite effects in the a-b plane.
TL;DR: This work considers a 3/4-filled, two-band discrete tight-binding Peierls-Hubbard model for an isolated chain of a halogen-bridged, mixed-valence, transition-metal linear-chain complex (HMMC or {ital MX} chain), employing the adiabatic approximation in which the quantum fluctuations associated with phonons are implicitly treated as an external field for the electrons.
Abstract: We consider a 3/4-filled, two-band discrete tight-binding Peierls-Hubbard model for an isolated chain of a halogen-bridged, mixed-valence, transition-metal linear-chain complex (HMMC or {ital MX} chain). We have employed the adiabatic approximation in which the quantum fluctuations associated with phonons are implicitly treated as an external field for the electrons, and treat electron-electron effects in the Hartree-Fock approximation. We investigate ground states as functions of the model parameters and doping-induced and photoinduced excitations---kinks, polarons, bipolarons, and excitons. Results for several experimental observables, including the lattice distortion, the excess charge and spin densities of defects, and the optical absorption, are compiled. For the ground state, we find that the bond-order-wave (BOW) portion of the one-band phase diagram is eliminated from the two-band phase diagram, in agreement with the lack of real materials in the pure BOW phase. The extent of electron-hole asymmetry and of spatial localization or delocalization of defects is explored. Two separate solitons or polarons are compared with corresponding bipolarons. We demonstrate explicitly the need to employ the two-band model for a realistic modeling of the {ital MX} systems, focusing on three specific systems: (a) highly distorted, valence-localized (strongly charge-disproportionated) PtCl, (b) moderately distorted PtBr, and (c) weakly distorted, valence-delocalizedmore » (weak charge-density wave) PtI. The compilation of results reported here constitutes a reference resource against which the rapidly expanding experimental data can be compared.« less
TL;DR: In this paper, the total photoproduction cross section was determined from a measurement of electroproduction with the ZEUS detector at HERA, and the Q2 values of the virtual photons were in the range 10−7
TL;DR: Electronic structure and energetics of (0001) and (1{bar 1}02) surfaces of sapphire, {alpha}-Al{sub 2}O{sub 3}, were calculated using the self-consistent-field discrete variational (DV) method in the local-density framework to determine the surface atomic termination.
Abstract: Electronic structure and energetics of (0001) and (11\ifmmode\bar\else\textasciimacron\fi{}02) surfaces of sapphire, \ensuremath{\alpha}-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$, were calculated using the self-consistent-field discrete variational (DV) method in the local-density framework. Clusters of a size of 70--120 atoms embedded in the semi-infinite host lattice were used to model the sapphire free surfaces. Calculations were performed on all possible terminating (0001) and (11\ifmmode\bar\else\textasciimacron\fi{}02) surfaces obtained from cleaving a sapphire single crystal. The energetic calculations show that the surface with the lowest cleavage energy is terminated with an Al layer for the (0001) surface, while it is terminated with an O layer for the (11\ifmmode\bar\else\textasciimacron\fi{}02) surface. The concept of the surface building block, useful in determining the surface atomic termination, was proposed. Two unoccupied surface bands derived from the top-layer Al atoms are found within the bulk band gap at 2 and 8 eV below the conduction-band minimum (CBM) for the (0001) surface. For the (11\ifmmode\bar\else\textasciimacron\fi{}02) surface, the occupied valence states derived from 2p states of the top-layer O atoms are found \ensuremath{\sim}4.0 eV below the CBM, which may explain the 4.7-eV energy-loss feature found experimentally on a sapphire (11\ifmmode\bar\else\textasciimacron\fi{}02) surface.
TL;DR: Comparison with rotation rates seen in NMR suggests that the dynamics below 260 K is more complex than jumps between equivalent molecular configurations.
Abstract: We report the studies of the elasticity of ${\mathrm{O}}_{60}$ single crystals. For sublimed fcc crystals, Young's modulus has an 8% jump at the first-order transition at 260 K. At \ensuremath{\sim}160 K there is a frequency-dependent elastic anomaly resulting from time dependent stress relaxation. Comparison with rotation rates seen in NMR suggests that the dynamics below 260 K is more complex than jumps between equivalent molecular configurations. Solvent grown monoclinic crystals have a second-order transition at 242 K, while the 160-K glass transition remains unchanged.
01 Aug 1992-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: The Fragment Mass Analyzer (FMA) as discussed by the authors is a recoil mass spectrometer, 8 m in length, which will be used to separate nuclear reaction products from the primary beam ad disperse them by mass/charge (m/q) at the focal plane.
Abstract: The Fragment Mass Analyzer (FMA) is currently being brought into operation at the Argonne Tandem Linear Accelerator System (ATLAS) The FMA is a recoil mass spectrometer, 8 m in length, which will be used to separate nuclear reaction products from the primary beam ad disperse them by mass/charge (m/q) at the focal plane It has a solid angle of 8 msr, an energy acceptance of {plus minus}20% around the central energy, and an m/q acceptance of {plus minus}7% around the central mass The FMA will allow the study of gamma rays originating from weak fusion-reaction channels by gating the gamma spectra by the desired ion identified at the FMA focal plane Production and decay of nuclei far from stability will be studied at the FMA focal plane by implanting exotic recoils directly into detectors or by using a fast tape system to transport the recoils to shielded detector systems With its capability of rotating from {minus}5 to +45 degrees around the target, the FMA will also be used for reaction mechanism studies Beta-NMR and nuclear moment measurements will be made behind the focal plane A facility description and a progress report on the commissioning of the FMA are presented 13more » refs, 9 figs, 1 tab« less
TL;DR: In this paper, the properties of nanocrystalline ZnO have been examined using nanindenter techniques to determine hardness, Young's modulus, and strain rate sensitivity.
Abstract: A number of nanocrystalline ceramics have been fabricated by the gas phase condensation technique. The mechanical properties of one of the first ceramics produced by this method, nanophase TiO{sub 2},have been discussed in an earlier study. This paper reports a similar study undertaken to examine the properties of nanocrystalline ZnO. Nanoindenter techniques are used to determine hardness, Young's modulus, and strain rate sensitivity in ultra-fine grained ZnO. Significant properties variations are experienced within a given sample, indicating a large degree of microstructural inhomogeneity. Nevertheless, a distinct evolution in properties can be observed as a function of sintering temperature. Young's modulus and hardness values increase almost linearly with increasing sintering temperature, and, in addition, there also appears to be a linear correlation between the development of the two materials properties. In contrast, strain rate sensitivity is shown to have an inverse dependence on sintering temperature. This dependence appears to be linked to the strong influence of grain size on strain rate sensitivity, so that the lower sintering temperatures, which provide the finer grain sizes, tend to promote strain rate sensitivity. The results of this study are strikingly similar to those obtained earlier for nanophase TiO{sub 2}, and they indicate that themore » earlier results could probably be generalized to a much broader range of nanocrystalline ceramics.« less
TL;DR: In this paper, the high pressure phase is stable in the nanostructured grains at ambient conditions resulting from the additional hydrostatic pressure component due to the Gibbs-Thomson effect, and at temperatures exceeding 600°C monoclinic γ-transforms to cubic α-Y2O3.
TL;DR: In this paper, the ionization potentials of copper clusters (Cu n ) up to 150 atoms have been bracketed using laser photoionization, and large decreases in IP at n = 2, 8, 20, 30, 34, 40 and 60 atoms are observed, as well as a strong odd-even alternation extending beyond 100 atoms.
TL;DR: A comprehensive review of the literature on uranium and plutonium electrorefining in molten salts is given in this paper, which covers work published from 1943 to November 1991 and discusses electrodeposition and electrodissolution at solid and liquid metal electrodes as well as mass transfer in liquid metal and molten salt phases.
TL;DR: In this paper, the authors report the major results to date of an ongoing study of the mechanical behavior of nanocrystalline metals produced by the inert-gas condensation method; some results have been reported elsewhere.
TL;DR: This article addresses the problem of indexing and retrieving first-order predicate calculus terms in the context of automated deduction programs to find variants, generalizations, instances, and terms that unify with a given term.
Abstract: This article addresses the problem of indexing and retrieving first-order predicate calculus terms in the context of automated deduction programs. The four retrieval operations of concern are to find variants, generalizations, instances, and terms that unify with a given term. Discrimination-tree indexing is reviewed, and several variations are presented. The path-indexing method is also reviewed. Experiments were conducted on large sets of terms to determine how the properties of the terms affect the performance of the two indexing methods. Results of the experiments are presented.
TL;DR: Insoluble plutonium- and americium-bearing colloidal particles formed during simulated weathering of a high-level nuclear waste glass indicate that models of actinide mobility and repository integrity, which assume complete solubility of actsinides in ground water, underestimate the potential for radionuclide release into the environment.
Abstract: Insoluble plutonium- and americium-bearing colloidal particles formed during simulated weathering of a high-level nuclear waste glass. Nearly 100 percent of the total plutonium and americium in test ground water was concentrated in these submicrometer particles. These results indicate that models of actinide mobility and repository integrity, which assume complete solubility of actinides in ground water, underestimate the potential for radionuclide release into the environment. A colloid-trapping mechanism may be necessary for a waste repository to meet long-term performance specifications.