76 resultados para SUB AND SUPERSOLUTIONS
Resumo:
We simulate incompressible, MHD turbulence using a pseudo-spectral code. Our major conclusions are as follows.
1) MHD turbulence is most conveniently described in terms of counter propagating shear Alfvén and slow waves. Shear Alfvén waves control the cascade dynamics. Slow waves play a passive role and adopt the spectrum set by the shear Alfvén waves. Cascades composed entirely of shear Alfvén waves do not generate a significant measure of slow waves.
2) MHD turbulence is anisotropic with energy cascading more rapidly along k<sub>⊥sub> than along k<sub>∥sub>, where k<sub>⊥sub> and k<sub>∥sub> refer to wavevector components perpendicular and parallel to the local magnetic field. Anisotropy increases with increasing k<sub>⊥sub> such that excited modes are confined inside a cone bounded by k<sub>∥sub> ∝ kγ<sub>⊥sub> where γ less than 1. The opening angle of the cone, θ(k<sub>⊥sub>) ∝ k-(1-γ)<sub>⊥sub>, defines the scale dependent anisotropy.
3) MHD turbulence is generically strong in the sense that the waves which comprise it suffer order unity distortions on timescales comparable to their periods. Nevertheless, turbulent fluctuations are small deep inside the inertial range. Their energy density is less than that of the background field by a factor θ2 (k<sub>⊥sub>)≪1.
4) MHD cascades are best understood geometrically. Wave packets suffer distortions as they move along magnetic field lines perturbed by counter propagating waves. Field lines perturbed by unidirectional waves map planes perpendicular to the local field into each other. Shear Alfvén waves are responsible for the mapping's shear and slow waves for its dilatation. The amplitude of the former exceeds that of the latter by 1/θ(k<sub>⊥sub>) which accounts for dominance of the shear Alfvén waves in controlling the cascade dynamics.
5) Passive scalars mixed by MHD turbulence adopt the same power spectrum as the velocity and magnetic field perturbations.
6) Decaying MHD turbulence is unstable to an increase of the imbalance between the flux of waves propagating in opposite directions along the magnetic field. Forced MHD turbulence displays order unity fluctuations with respect to the balanced state if excited at low k by δ(t) correlated forcing. It appears to be statistically stable to the unlimited growth of imbalance.
7) Gradients of the dynamic variables are focused into sheets aligned with the magnetic field whose thickness is comparable to the dissipation scale. Sheets formed by oppositely directed waves are uncorrelated. We suspect that these are vortex sheets which the mean magnetic field prevents from rolling up.
8) Items (1)-(5) lend support to the model of strong MHD turbulence put forth by Goldreich and Sridhar (1995, 1997). Results from our simulations are also consistent with the GS prediction γ = 2/3. The sole not able discrepancy is that the 1D power law spectra, E(k<sub>⊥sub>) ∝ k-∝<sub>⊥sub>, determined from our simulations exhibit ∝ ≈ 3/2, whereas the GS model predicts ∝ = 5/3.
Resumo:
We present a theoretical study of electronic states in topological insulators with impurities. Chiral edge states in 2d topological insulators and helical surface states in 3d topological insulators show a robust transport against nonmagnetic impurities. Such a nontrivial character inspired physicists to come up with applications such as spintronic devices [1], thermoelectric materials [2], photovoltaics [3], and quantum computation [4]. Not only has it provided new opportunities from a practical point of view, but its theoretical study has deepened the understanding of the topological nature of condensed matter systems. However, experimental realizations of topological insulators have been challenging. For example, a 2d topological insulator fabricated in a HeTe quantum well structure by Konig et al. [5] shows a longitudinal conductance which is not well quantized and varies with temperature. 3d topological insulators such as Bi<sub>2sub>Se<sub>3sub> and Bi<sub>2sub>Te<sub>3sub> exhibit not only a signature of surface states, but they also show a bulk conduction [6]. The series of experiments motivated us to study the effects of impurities and coexisting bulk Fermi surface in topological insulators. We first address a single impurity problem in a topological insulator using a semiclassical approach. Then we study the conductance behavior of a disordered topological-metal strip where bulk modes are associated with the transport of edge modes via impurity scattering. We verify that the conduction through a chiral edge channel retains its topological signature, and we discovered that the transmission can be succinctly expressed in a closed form as a ratio of determinants of the bulk Green's function and impurity potentials. We further study the transport of 1d systems which can be decomposed in terms of chiral modes. Lastly, the surface impurity effect on the local density of surface states over layers into the bulk is studied between weak and strong disorder strength limits.
Resumo:
In this work we chiefly deal with two broad classes of problems in computational materials science, determining the doping mechanism in a semiconductor and developing an extreme condition equation of state. While solving certain aspects of these questions is well-trodden ground, both require extending the reach of existing methods to fully answer them. Here we choose to build upon the framework of density functional theory (DFT) which provides an efficient means to investigate a system from a quantum mechanics description.
Zinc Phosphide (Zn<sub>3sub>P<sub>2sub>) could be the basis for cheap and highly efficient solar cells. Its use in this regard is limited by the difficulty in n-type doping the material. In an effort to understand the mechanism behind this, the energetics and electronic structure of intrinsic point defects in zinc phosphide are studied using generalized Kohn-Sham theory and utilizing the Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional for exchange and correlation. Novel 'perturbation extrapolation' is utilized to extend the use of the computationally expensive HSE functional to this large-scale defect system. According to calculations, the formation energy of charged phosphorus interstitial defects are very low in n-type Zn<sub>3sub>P<sub>2sub> and act as 'electron sinks', nullifying the desired doping and lowering the fermi-level back towards the p-type regime. Going forward, this insight provides clues to fabricating useful zinc phosphide based devices. In addition, the methodology developed for this work can be applied to further doping studies in other systems.
Accurate determination of high pressure and temperature equations of state is fundamental in a variety of fields. However, it is often very difficult to cover a wide range of temperatures and pressures in an laboratory setting. Here we develop methods to determine a multi-phase equation of state for Ta through computation. The typical means of investigating thermodynamic properties is via ’classical’ molecular dynamics where the atomic motion is calculated from Newtonian mechanics with the electronic effects abstracted away into an interatomic potential function. For our purposes, a ’first principles’ approach such as DFT is useful as a classical potential is typically valid for only a portion of the phase diagram (i.e. whatever part it has been fit to). Furthermore, for extremes of temperature and pressure quantum effects become critical to accurately capture an equation of state and are very hard to capture in even complex model potentials. This requires extending the inherently zero temperature DFT to predict the finite temperature response of the system. Statistical modelling and thermodynamic integration is used to extend our results over all phases, as well as phase-coexistence regions which are at the limits of typical DFT validity. We deliver the most comprehensive and accurate equation of state that has been done for Ta. This work also lends insights that can be applied to further equation of state work in many other materials.
Resumo:
The free neutron beta decay correlation A<sub>0sub> between neutron polarization and electron emission direction provides the strongest constraint on the ratio λ = g<sub>Asub>/g<sub>Vsub> of the Axial-vector to Vector coupling constants in Weak decay. In conjunction with the CKM Matrix element V<sub>udsub> and the neutron lifetime τ<sub>nsub>, λ provides a test of Standard Model assumptions for the Weak interaction. Leading high-precision measurements of A<sub>0sub> and τ<sub>nsub> in the 1995-2005 time period showed discrepancies with prior measurements and Standard Model predictions for the relationship between λ, τ<sub>nsub>, and V<sub>udsub>. The UCNA experiment was developed to measure A<sub>0sub> from decay of polarized ultracold neutrons (UCN), providing a complementary determination of λ with different systematic uncertainties from prior cold neutron beam experiments. This dissertation describes analysis of the dataset collected by UCNA in 2010, with emphasis on detector response calibrations and systematics. The UCNA measurement is placed in the context of the most recent τ<sub>nsub> results and cold neutron A<sub>0sub> experiments.
Resumo:
The prospect of terawatt-scale electricity generation using a photovoltaic (PV) device places strict requirements on the active semiconductor optoelectronic properties and elemental abundance. After reviewing the constraints placed on an "earth-abundant" solar absorber, we find zinc phosphide (α-Zn<sub>3sub>P<sub>2sub>) to be an ideal candidate. In addition to its near-optimal direct band gap of 1.5 eV, high visible-light absorption coefficient (>104 cm-1), and long minority-carrier diffusion length (>5 μm), Zn<sub>3sub>P<sub>2sub> is composed of abundant Zn and P elements and has excellent physical properties for scalable thin-film deposition. However, to date, a Zn<sub>3sub>P<sub>2sub> device of sufficient efficiency for commercial applications has not been demonstrated. Record efficiencies of 6.0% for multicrystalline and 4.3% for thin-film cells have been reported, respectively. Performance has been limited by the intrinsic p-type conductivity of Zn<sub>3sub>P<sub>2sub> which restricts us to Schottky and heterojunction device designs. Due to our poor understanding of Zn<sub>3sub>P<sub>2sub> interfaces, an ideal heterojunction partner has not yet been found.
The goal of this thesis is to explore the upper limit of solar conversion efficiency achievable with a Zn<sub>3sub>P<sub>2sub> absorber through the design of an optimal heterojunction PV device. To do so, we investigate three key aspects of material growth, interface energetics, and device design. First, the growth of Zn<sub>3sub>P<sub>2sub> on GaAs(001) is studied using compound-source molecular-beam epitaxy (MBE). We successfully demonstrate the pseudomorphic growth of Zn<sub>3sub>P<sub>2sub> epilayers of controlled orientation and optoelectronic properties. Next, the energy-band alignments of epitaxial Zn<sub>3sub>P<sub>2sub> and II-VI and III-V semiconductor interfaces are measured via high-resolution x-ray photoelectron spectroscopy in order to determine the most appropriate heterojunction partner. From this work, we identify ZnSe as a nearly ideal n-type emitter for a Zn<sub>3sub>P<sub>2sub> PV device. Finally, various II-VI/Zn<sub>3sub>P<sub>2sub> heterojunction solar cells designs are fabricated, including substrate and superstrate architectures, and evaluated based on their solar conversion efficiency.
Resumo:
An economic air pollution control model, which determines the least cost of reaching various air quality levels, is formulated. The model takes the form of a general, nonlinear, mathematical programming problem. Primary contaminant emission levels are the independent variables. The objective function is the cost of attaining various emission levels and is to be minimized subject to constraints that given air quality levels be attained.
The model is applied to a simplified statement of the photochemical smog problem in Los Angeles County in 1975 with emissions specified by a two-dimensional vector, total reactive hydrocarbon, (RHC), and nitrogen oxide, (NO<sub>xsub>), emissions. Air quality, also two-dimensional, is measured by the expected number of days per year that nitrogen dioxide, (NO<sub>2sub>), and mid-day ozone, (O<sub>3sub>), exceed standards in Central Los Angeles.
The minimum cost of reaching various emission levels is found by a linear programming model. The base or "uncontrolled" emission levels are those that will exist in 1975 with the present new car control program and with the degree of stationary source control existing in 1971. Controls, basically "add-on devices", are considered here for used cars, aircraft, and existing stationary sources. It is found that with these added controls, Los Angeles County emission levels [(1300 tons/day RHC, 1000 tons /day NO<sub>xsub>) in 1969] and [(670 tons/day RHC, 790 tons/day NO<sub>xsub>) at the base 1975 level], can be reduced to 260 tons/day RHC (minimum RHC program) and 460 tons/day NO<sub>xsub> (minimum NO<sub>xsub> program).
"Phenomenological" or statistical air quality models provide the relationship between air quality and emissions. These models estimate the relationship by using atmospheric monitoring data taken at one (yearly) emission level and by using certain simple physical assumptions, (e. g., that emissions are reduced proportionately at all points in space and time). For NO<sub>2sub>, (concentrations assumed proportional to NO<sub>xsub> emissions), it is found that standard violations in Central Los Angeles, (55 in 1969), can be reduced to 25, 5, and 0 days per year by controlling emissions to 800, 550, and 300 tons /day, respectively. A probabilistic model reveals that RHC control is much more effective than NO<sub>xsub> control in reducing Central Los Angeles ozone. The 150 days per year ozone violations in 1969 can be reduced to 75, 30, 10, and 0 days per year by abating RHC emissions to 700, 450, 300, and 150 tons/day, respectively, (at the 1969 NO<sub>xsub> emission level).
The control cost-emission level and air quality-emission level relationships are combined in a graphical solution of the complete model to find the cost of various air quality levels. Best possible air quality levels with the controls considered here are 8 O<sub>3sub> and 10 NO<sub>2sub> violations per year (minimum ozone program) or 25 O<sub>3sub> and 3 NO<sub>2sub> violations per year (minimum NO<sub>2sub> program) with an annualized cost of $230,000,000 (above the estimated $150,000,000 per year for the new car control program for Los Angeles County motor vehicles in 1975).
Resumo:
The quality of a thermoelectric material is judged by the size of its temperature de- pendent thermoeletric-figure-of-merit (zT ). Superionic materials, particularly Zn<sub>4sub>Sb<sub>3sub> and Cu<sub>2sub>Se, are of current interest for the high zT and low thermal conductivity of their disordered, superionic phase. In this work it is reported that the super-ionic materials Ag<sub>2sub>Se, Cu<sub>2sub>Se and Cu<sub>1.97sub>Ag<sub>0.03sub>Se show enhanced zT in their ordered, normal ion-conducting phases. The zT of Ag<sub>2sub>Se is increased by 30% in its ordered phase as compared to its disordered phase, as measured just below and above its first order phase transition. The zT ’s of Cu<sub>2sub>Se and Cu<sub>1.97sub>Ag<sub>0.03sub>Se both increase by more than 100% over a 30 K temperatures range just below their super-ionic phase transitions. The peak zT of Cu<sub>2sub>Se is 0.7 at 406 K and of Cu<sub>1.97sub>Ag<sub>0.03sub>Se is 1.0 at 400 K. In all three materials these enhancements are due to anomalous increases in their Seebeck coefficients, beyond that predicted by carrier concentration measurements and band structure modeling. As the Seebeck coefficient is the entropy transported per carrier, this suggests that there is an additional quantity of entropy co-transported with charge carriers. Such co-transport has been previously observed via co-transport of vibrational entropy in bipolaron conductors and spin-state entropy in Na<sub>xsub>Co<sub>2sub>O<sub>4sub>. The correlation of the temperature profile of the increases in each material with the nature of their phase transitions indicates that the entropy is associated with the thermodynamcis of ion-ordering. This suggests a new mechanism by which high thermoelectric performance may be understood and engineered.
Resumo:
In the five chapters that follow, I delineate my efforts over the last five years to synthesize structurally and chemically relevant models of the Oxygen Evolving Complex (OEC) of Photosystem II. The OEC is nature’s only water oxidation catalyst, in that it forms the dioxygen in our atmosphere necessary for oxygenic life. Therefore understanding its structure and function is of deep fundamental interest and could provide design elements for artificial photosynthesis and manmade water oxidation catalysts. Synthetic endeavors towards OEC mimics have been an active area of research since the mid 1970s and have mutually evolved alongside biochemical and spectroscopic studies, affording ever-refined proposals for the structure of the OEC and the mechanism of water oxidation. This research has culminated in the most recent proposal: a low symmetry Mn<sub>4sub>CaO<sub>5sub> cluster with a distorted Mn<sub>3sub>CaO<sub>4sub> cubane bridged to a fourth, dangling Mn. To give context for how my graduate work fits into this rich history of OEC research, Chapter 1 provides a historical timeline of proposals for OEC structure, emphasizing the role that synthetic Mn and MnCa clusters have played, and ending with our Mn<sub>3sub>CaO<sub>4sub> heterometallic cubane complexes.
In Chapter 2, the triarylbenzene ligand framework used throughout my work is introduced, and trinuclear clusters of Mn, Co, and Ni are discussed. The ligand scaffold consistently coordinates three metals in close proximity while leaving coordination sites open for further modification through ancillary ligand binding. The ligands coordinated could be varied, with a range of carboxylates and some less coordinating anions studied. These complexes’ structures, magnetic behavior, and redox properties are discussed.
Chapter 3 explores the redox chemistry of the trimanganese system more thoroughly in the presence of a fourth Mn equivalent, finding a range of oxidation states and oxide incorporation dependent on oxidant, solvent, and Mn salt. Oxidation states from MnII<sub>4sub> to MnIIIMnIV<sub>3sub> were observed, with 1-4 O2– ligands incorporated, modeling the photoactivation of the OEC. These complexes were studied by X-ray diffraction, EPR, XAS, magnetometry, and CV.
As Ca2+ is a necessary component of the OEC, Chapter 4 discusses synthetic strategies for making highly structurally accurate models of the OEC containing both Mn and Ca in the Mn<sub>3sub>CaO<sub>4sub> cubane + dangling Mn geometry. Structural and electrochemical characterization of the first Mn<sub>3sub>CaO<sub>4sub> heterometallic cubane complex— and comparison to an all-Mn Mn<sub>4sub>O<sub>4sub> analog—suggests a role for Ca2+ in the OEC. Modification of the Mn<sub>3sub>CaO<sub>4sub> system by ligand substitution affords low symmetry Mn<sub>3sub>CaO<sub>4sub> complexes that are the most accurate models of the OEC to date.
Finally, in Chapter 5 the reactivity of the Mn<sub>3sub>CaO<sub>4sub> cubane complexes toward O- atom transfer is discussed. The metal M strongly affects the reactivity. The mechanisms of O-atom transfer and water incorporation from and into Mn<sub>4sub>O<sub>4sub> and Mn<sub>4sub>O<sub>3sub> clusters, respectively, are studied through computation and 18O-labeling studies. The μ3-oxos of the Mn<sub>4sub>O<sub>4sub> system prove fluxional, lending support for proposals of O2– fluxionality within the OEC.
Resumo:
This thesis describes a measurement of B0- B0 mixing in events produced by electron-positron annihilation at a center of mass energy of 29 GeV. The data were taken by the Mark II detector in the PEP storage ring at the Stanford Linear Accelerator Center between 1981 and 1987, and correspond to a total integrated luminosity of 224pb-1.
We used a new method, based on the kinematics of hadronic events containing two leptons, to provide a measurement of the probability, x, that a hadron, initially containing a b (b) quark decays to a positive (negative) lepton to be X = 0.17+0.15<sub>-0.08sub>, with 90% confidence level upper and lower limits of 0.38 and 0.06, respectively, including all estimated systematic errors. Because of the good separation of signal and background, this result is relatively insensitive to various systematic effects which have complicated previous measurements.
We interpret this result as evidence for the mixing of neutral B mesons. Based on existing B0<sub>dsub> mixing rate measurements, and some assumptions about the fractions of B0<sub>dsub> and B0<sub>ssub> mesons present in the data, this result favors maximal mixing of B0<sub>ssub> mesons, although it cannot rule out zero B0<sub>ssub> mixing at the 90% confidence level.
Resumo:
Pulse-height and time-of-flight methods have been used to measure the electronic stopping cross sections for projectiles of 12C, 16O, 19F, 23Na, 24Mg, and 27Al, slowing in helium, neon, argon, krypton, and xenon. The ion energies were in the range 185 keV ≤ E ≤ 2560 keV.
A semiempirical calculation of the electronic stopping cross section for projectiles with atomic numbers between 6 and 13 passing through the inert gases has been performed using a modification of the Firsov model. Using Hartree-Slater-Fock orbitals, and summing over the losses for the individual charge states of the projectiles, good agreement has been obtained with the experimental data. The main features of the stopping cross section seen in the data, such as the Z<sub>1sub> oscillation and the variation of the velocity dependence on Z<sub>1sub> and Z<sub>2sub>, are present in the calculation. The inclusion of a modified form of the Bethe-Bloch formula as an additional term allows the increase of the velocity dependence for projectile velocities above v<sub>osub> to be reproduced in the calculation.
Resumo:
Trace volatile organic compounds emitted by biogenic and anthropogenic sources into the atmosphere can undergo extensive photooxidation to form species with lower volatility. By equilibrium partitioning or reactive uptake, these compounds can nucleate into new aerosol particles or deposit onto already-existing particles to form secondary organic aerosol (SOA). SOA and other atmospheric particulate matter have measurable effects on global climate and public health, making understanding SOA formation a needed field of scientific inquiry. SOA formation can be done in a laboratory setting, using an environmental chamber; under these controlled conditions it is possible to generate SOA from a single parent compound and study the chemical composition of the gas and particle phases. By studying the SOA composition, it is possible to gain understanding of the chemical reactions that occur in the gas phase and particle phase, and identify potential heterogeneous processes that occur at the surface of SOA particles. In this thesis, mass spectrometric methods are used to identify qualitatively and qualitatively the chemical components of SOA derived from the photooxidation of important anthropogenic volatile organic compounds that are associated with gasoline and diesel fuels and industrial activity (C12 alkanes, toluene, and o-, m-, and p-cresols). The conditions under which SOA was generated in each system were varied to explore the effect of NO<sub>xsub> and inorganic seed composition on SOA chemical composition. The structure of the parent alkane was varied to investigate the effect on the functionalization and fragmentation of the resulting oxidation products. Relative humidity was varied in the alkane system as well to measure the effect of increased particle-phase water on condensed-phase reactions. In all systems, oligomeric species, resulting potentially from particle-phase and heterogeneous processes, were identified. Imines produced by reactions between (NH<sub>4sub>)<sub>2sub>SO<sub>4sub> seed and carbonyl compounds were identified in all systems. Multigenerational photochemistry producing low- and extremely low-volatility organic compounds (LVOC and ELVOC) was reflected strongly in the particle-phase composition as well.
Resumo:
The kinetics of the reduction of O<sub>2sub> by Ru(NH<sub>3sub>)<sub>6sub>+2 as catalyzed by cobalt(II) tetrakis(4-N-methylpyridyl)porphyrin are described both in homogeneous solution and when the reactants are confined to Nafion coatings on graphite electrodes. The catalytic mechanism is determined and the factors that can control the total reduction currents at Nafion-coated electrodes are specified. A kinetic zone diagram for analyzing the behavior of catalyst-mediator-substrate systems at polymer coated electrodes is presented and utilized in identifying the current-limiting processes. Good agreement is demonstrated between calculated and measured reduction currents at rotating disk electrodes. The experimental conditions that will yield the optimum performance of coated electrodes are discussed, and a relationship is derived for the optimal coating thickness.
The relation between the reduction potentials of adsorbed and unadsorbed cobalt(III) tetrakis(4-N-methylpyridyl)porphyrin and those where it catalyzes the electroreduction of dioxygen is described. There is an unusually large change in the formal potential of the Co(III) couple upon the adsorption of the porphyrin on the graphite electrode surface. The mechanism in which the (inevitably) adsorbed porphyrin catalyzes the reduction of O<sub>2sub> is in accord with a general mechanistic scheme proposed for most monomeric cobalt porphyrins.
Four new dimeric metalloporphyrins (prepared in the laboratory of Professor C. K. Chang) have the two porphyrin rings linked by an anthracene bridge attached to meso positions. The electrocatalytic behavior of the diporphyrins towards the reduction of O<sub>2sub> at graphite electrodes has been examined for the following combination of metal centers: Co-Cu, Co-Fe, Fe-Fe, Fe-H<sub>2sub>. The Co-Cu diporphyrin catalyzes the reduction of O<sub>2sub> to H<sub>2sub>O<sub>2sub> but no further. The other three catalysts all exhibit mixed reduction pathways leading to both H<sub>2sub>O<sub>2sub> and H<sub>2sub>O. However, the pathways that lead to H<sub>2sub>O do not involve H<sub>2sub>O<sub>2sub> as an intermediate. A possible mechanistic scheme is offered to account for the observed behavior.
Resumo:
Part I
The electric birefringence of dilute DNA solutions has been studied in considerable detail and on a large number of samples, but no new and reliable information was discovered concerning the tertiary structure of DNA. The large number of variables which effect the birefringence results is discussed and suggestions are made for further work on the subject.
The DNA molecules have been aligned in a rapidly alternating (10 to 20 kc/sec) square wave field confirming that the orientation mechanism is that of counterion polarization. A simple empirical relation between the steady state birefringence, Δn<sub>stsub>, and the square of the electric field, E, has been found: Δn<sub>stsub> = E2/(a E2 + b), where a = 1/Δn<sub>ssub> and b = (E2/Δn<sub>stsub>)<sub>E→osub>. Δn<sub>ssub> is the birefringence extrapolated to infinite field strength.
The molecules show a distribution of relaxation times from 10-4 to 0.2 sec, which is consistent with expectations for flexible coil molecules. The birefringence and the relaxation times decrease with increasing salt concentrations. They also depend on the field strength and pulse duration in a rather non-reproducible manner, which may be due in part to changes in the composition of the solution or in the molecular structure of the DNA (other than denaturation). Further progress depends on the development of some control over these effects.
Part II
The specificity of the dissociation of reconstituted and native deoxyribonucleohistones (DNH) by monovalent salt solutions has been investigated. A novel zone ultracentrifugation method is used in which the DNH is sedimented as a zone through a preformed salt gradient, superimposed on a stabilizing D<sub>2sub>O (sucrose) density gradient. The results, obtained by scanning the quartz sedimentation tubes in a spectrophotometer, were verified by the conventional, preparative sedimentation technique. Procedures are discussed for the detection of microgram quantities of histones, since low concentrations must be used to prevent excessive aggregation of the DNH.
The data show that major histone fractions are selectively dissociated from DNH by increasing salt concentrations: Lysine rich histone (H I) dissociates gradually between 0.1 and 0.3 F, slightly lysine rich histone (H II) dissociates as a narrow band between 0.35 and 0.5 F, and arginine rich histone (H III, H IV) dissociates gradually above 0.5 F NaClO<sub>4sub>.
The activity of the partially dissociated, native DNH in sustaining RNA synthesis, their mobility and their unusual heat denaturation and renaturation behavior are described. The two-step melting behavior of the material indicates that the histones are non-randomly distributed along the DNA, but the implications are that the uncovered regions are not of gene-size length.
Resumo:
From the tunneling characteristics of a tin-tin oxide-lead junction, a direct measurement has been made of the energy-gap variation for a superconductor carrying a current in a compensated geometry. Throughout the region investigated – several temperatures near T<sub>csub> and down to a reduced temperature t = 0.8 –the observed current dependence agrees quite well with predictions based on the Ginzburg-Landau-Gor’kov theory. Near T<sub>csub> the predicted temperature dependence is also well verified, though deviations are observed at lower temperatures; even for the latter, the data are internally consistent with the temperature dependence of the experimental critical current. At the lowest temperature investigated, t = 0.8, a small “Josephson” tunneling current allowed further a direct measurement of the electron drift velocity at low current densities. From this, a preliminary experimental value of the critical velocity, believed to be the first reported, can be inferred in the basis of Ginzburg-Landau theory. For tin at t = 0.8, we find v<sub>csub> = 87 m/sec. This value does not appear fully consistent with those predicted by recent theories for superconductors with short electronic mean-free-paths.
Resumo:
A.G. Vulih has shown how an essentially unique intrinsic multiplication can be defined in certain types of Riesz spaces (vector lattices) L. In general, the multiplication is not universally defined in L, but L can always be imbedded in a large space L# in which multiplication is universally defined.
If ф is a normal integral in L, then ф can be extended to a normal integral on a large space L<sub>1sub>(ф) in L#, and L<sub>1sub>(ф) may be regarded as an abstract integral space. A very general form of the Radon-Nikodym theorem can be proved in L<sub>1sub>(ф), and this can be used to give a relatively simple proof of a theorem of Segal giving a necessary and sufficient condition that the Radon-Nikodym theorem hold in a measure space.
In another application, the multiplication is used to give a representation of certain Riesz spaces as rings of operators on a Hilbert space.