5,585 research outputs found
Temporal evolution of oscillating coronal loops
Context. Transverse oscillations of coronal structures are currently
intensively studied to explore the associated magnetohydrodynamic wave physics
and perform seismology of the local medium. Aims. We make a first attempt to
measure the thermodynamic evolution of a sample of coronal loops that undergo
decaying kink oscillations in response to an eruption in the corresponding
active region. Methods. Using data from the six coronal wavelengths of SDO/AIA,
we performed a differential emission measure (DEM) analysis of 15 coronal loops
before, during, and after the eruption and oscillation. Results. We find that
the emission measure, temperature, and width of the DEM distribution undergo
significant variations on time scales relevant for the study of transverse
oscillations. There are no clear collective trends of increases or decreases
for the parameters we analysed. The strongest variations of the parameters
occur during the initial perturbation of the loops, and the influence of
background structures may also account for much of this variation. Conclusions.
The DEM analysis of oscillating coronal loops in erupting active regions shows
evidence of evolution on time scales important for the study of the
oscillations. Further work is needed to separate the various observational and
physical mechanisms that may be responsible for the variations in temperature,
DEM distribution width, and total emission measure.Comment: Accepted in A&
Preparation, Structure, and Reactivity of Nonstabilized Organoiron Compounds. Implications for Iron-Catalyzed Cross Coupling Reactions
A series of unprecedented organoiron complexes of the formal oxidation states −2, 0, +1, +2, and +3 is presented, which are largely devoid of stabilizing ligands and, in part, also electronically unsaturated (14-, 16-, 17- and 18-electron counts). Specifically, it is shown that nucleophiles unable to undergo β-hydride elimination, such as MeLi, PhLi, or PhMgBr, rapidly reduce Fe(3+) to Fe(2+) and then exhaustively alkylate the metal center. The resulting homoleptic organoferrate complexes [(Me4Fe)(MeLi)][Li(OEt2)]2 (3) and [Ph4Fe][Li(Et2O)2][Li(1,4-dioxane)] (5) could be characterized by X-ray crystal structure analysis. However, these exceptionally sensitive compounds turned out to be only moderately nucleophilic, transferring their organic ligands to activated electrophiles only, while being unable to alkylate (hetero)aryl halides unless they are very electron deficient. In striking contrast, Grignard reagents bearing alkyl residues amenable to β-hydride elimination reduce FeXn (n = 2, 3) to clusters of the formal composition [Fe(MgX)2]n. The behavior of these intermetallic species can be emulated by structurally well-defined lithium ferrate complexes of the type [Fe(C2H4)4][Li(tmeda)]2 (8), [Fe(cod)2][Li(dme)]2 (9), [CpFe(C2H4)2][Li(tmeda)] (7), [CpFe(cod)][Li(dme)] (11), or [Cp*Fe(C2H4)2][Li(tmeda)] (14). Such electron-rich complexes, which are distinguished by short intermetallic Fe−Li bonds, were shown to react with aryl chlorides and allyl halides; the structures and reactivity patterns of the resulting organoiron compounds provide first insights into the elementary steps of low valent iron-catalyzed cross coupling reactions of aryl, alkyl, allyl, benzyl, and propargyl halides with organomagnesium reagents. However, the acquired data suggest that such C−C bond formations can occur, a priori, along different catalytic cycles shuttling between metal centers of the formal oxidation states Fe(+1)/Fe(+3), Fe(0)/Fe(+2), and Fe(−2)/Fe(0). Since these different manifolds are likely interconnected, an unambiguous decision as to which redox cycle dominates in solution remains difficult, even though iron complexes of the lowest accessible formal oxidation states promote the reactions most effectively
Explicit determination of a 727-dimensional root space of the hyperbolic Lie algebra
The 727-dimensional root space associated with the level-2 root \bLambda_1
of the hyperbolic Kac--Moody algebra is determined using a recently
developed string theoretic approach to hyperbolic algebras. The explicit form
of the basis reveals a complicated structure with transversal as well as
longitudinal string states present.Comment: 12 pages, LaTeX 2
Conformal Solutions Of Duality Invariant 2D Magnetohydrodynamic Turbulence
We consider possible conformal field theory (CFT) descriptions of the various
inertial ranges that exist in duality invariant Magnetohydrodynamics. Such
models arise as effective theories of dyonic plasmas in 3 dimensions in which
all fields are independent of the third coordinate. We find new constraints on
the allowed CFT's compared to those that may describe turbulence in
plasmas of electric charges only. The predictions from CFT concerning
equipartition of energy amongst the electric and magnetic fields are discussed,
and quantities exhibiting universal scaling are derived.Comment: Latex, 8 page
Duality Invariant Magnetohydrodynamics And Dyons
The theory of magnetohydrodynamics is extended to the cases of a plasma of
separate magnetic and electric charges, as well as to a plasma of dyons
respectively. In both these cases the system possesses electric-magnetic
duality symmetry. In the former case we find that because of the existence of
two independent generalized Ohm's law equations, the limit of infinite electric
and magnetic conductivity results in the vanishing of both electric and
magnetic fields, as well as the corresponding currents. In the dyonic case, we
find that the resulting duality-invariant system of equations are equivalent to
those of ordinary MHD, after suitable field redefinitions.Comment: 11 pages, late
Comparing league formats with respect to match importance in Belgian football
Recently, most clubs in the highest Belgian football division have become convinced that the format of their league should be changed. Moreover, the TV station that broadcasts the league is pleading for a more attractive competition. However, the clubs have not been able to agree on a new league format, mainly because they have conflicting interests. In this paper, we compare the current league format, and three other formats that have been considered by the Royal Belgian Football Association. We simulate the course of each of these league formats, based on historical match results. We assume that the attractiveness of a format is determined by the importance of its games; our importance measure for a game is based on the number of teams for which this game can be decisive to reach a given goal. Furthermore, we provide an overview of how each league format aligns with the expectations and interests of each type of club
Chiral Plaquette Polaron Theory of Cuprate Superconductivity
Ab-initio density functional calculations on explicitly doped
La(2-x)Sr(x)CuO4 find doping creates localized holes in out-of-plane orbitals.
A model for superconductivity is developed based on the assumption that doping
leads to the formation of holes on a four-site Cu plaquette composed of the
out-of-plane A1 orbitals apical O pz, planar Cu dz2, and planar O psigma. This
is in contrast to the assumption of hole doping into planar Cu dx2-y2 and O
psigma orbitals as in the t-J model. Interaction of holes with the d9 spin
background leads to chiral polarons with either a clockwise or anti-clockwise
charge current. When the polaron plaquettes percolate through the crystal at
x~0.05 for LaSrCuO, a Cu dx2-y2 and planar O psigma band is formed. Spin
exchange Coulomb repulsion with chiral polarons leads to D-wave
superconductivity. The equivalent of the Debye energy in phonon
superconductivity is the maximum energy separation between a chiral polaron and
its time-reversed partner. An additive skew-scattering contribution to the Hall
effect is induced by chiral polarons and leads to a temperature dependent Hall
effect that fits the measured values for LaSrCuO. The integrated imaginary
susceptibility satisfies omega/T scaling due to chirality and spin-flip
scattering of polarons along with a uniform distribution of polaron energy
splittings. The derived functional form is compatible with experiments. The
static spin structure factor is computed and is incommensurate with a
separation distance from (pi,pi) given by ~(2pi)x. Coulomb scattering of the
x2-y2 band with polarons leads to linear resistivity. Coupling of the x2-y2
band to the undoped Cu d9 spins leads to the ARPES pseudogap and its doping and
temperature dependence.Comment: 32 pages, 17 figure
Optimized Synthesis, Structural Investigations, Ligand Tuning and Synthetic Evaluation of Silyloxy-Based Alkyne Metathesis Catalysts
Nitride- and alkylidyne complexes of molybdenum endowed with triarylsilanolate ligands are excellent (pre)catalysts for alkyne-metathesis reactions of all sorts, since they combine high activity with an outstanding tolerance toward polar and/or sensitive functional groups. Structural and reactivity data suggest that this promising application profile results from a favorable match between the characteristics of the high-valent molybdenum center and the electronic and steric features of the chosen Ar3SiO groups. This interplay ensures a well-balanced level of Lewis acidity at the central atom, which is critical for high activity. Moreover, the bulky silanolates, while disfavoring bimolecular decomposition of the operative alkylidyne unit, do not obstruct substrate binding. In addition, Ar3SiO groups have the advantage that they are more stable within the coordination sphere of a high-valent molybdenum center than tert-alkoxides, which commonly served as ancillary ligands in previous generations of alkyne metathesis catalysts. From a practical point of view it is important to note that complexes of the general type [(Ar3SiO)3MoΞX] (X = N, CR; R = aryl, alkyl, Ar = aryl) can be rendered air-stable with the aid of 1,10-phenanthroline, 2,2′-bipyridine or derivatives thereof. Although the resulting adducts are themselves catalytically inert, treatment with Lewis acidic additives such as ZnCl2 or MnCl2 removes the stabilizing N-donor ligand and gently releases the catalytically active template into the solution. This procedure gives excellent results in alkyne metathesis starting from air-stable and hence user-friendly precursor complexes. The thermal and hydrolytic stability of representative molybdenum alkylidyne and -nitride complexes of this series was investigated and the structure of several decomposition products elucidated
A New Ligand Design Based on London Dispersion Empowers Chiral Bismuth–Rhodium Paddlewheel Catalysts
Heterobimetallic bismuth–rhodium paddlewheel complexes with phenylglycine ligands carrying TIPS-groups at the meta-positions of the aromatic ring exhibit outstanding levels of selectivity in reactions of donor/acceptor and donor/donor carbenes; at the same time, the reaction rates are much faster and the substrate scope is considerably wider than those of previous generations of chiral [BiRh] catalysts. As shown by a combined experimental, crystallographic, and computational study, the new catalysts draw their excellent application profile largely from the stabilization of the chiral ligand sphere by London dispersion (LD) interactions of the peripheral silyl substituents
Landau quantization effects in the charge-density-wave system (Per)(mnt) (where Au and Pt)
A finite transfer integral orthogonal to the conducting chains of a
highly one-dimensional metal gives rise to empty and filled bands that simulate
an indirect-gap semiconductor upon formation of a commensurate
charge-density-wave (CDW). In contrast to semiconductors such as Ge and Si with
bandgaps eV, the CDW system possesses an indirect gap with a greatly
reduced energy scale, enabling moderate laboratory magnetic fields to have a
major effect. The consequent variation of the thermodynamic gap with magnetic
field due to Zeeman splitting and Landau quantization enables the electronic
bandstructure parameters (transfer integrals, Fermi velocity) to be determined
accurately. These parameters reveal the orbital quantization limit to be
reached at T in (Per)(mnt) salts, making them highly
unlikely candidates for a recently-proposed cascade of field-induced
charge-density wave states
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