96 research outputs found
Spectroscopic Properties of a Biologically Relevant [Fe2(μ-O)2] Diamond Core Motif with a Short Iron-Iron Distance
Diiron cofactors in enzymes perform diverse challenging transformations. The structures of high valent intermediates (Q in methane monooxygenase and X in ribonucleotide reductase) are debated since Fe−Fe distances of 2.5–3.4 Å were attributed to “open” or “closed” cores with bridging or terminal oxido groups. We report the crystallographic and spectroscopic characterization of a FeIII2(μ-O)2 complex (2) with tetrahedral (4C) centres and short Fe−Fe distance (2.52 Å), persisting in organic solutions. 2 shows a large Fe K-pre-edge intensity, which is caused by the pronounced asymmetry at the TD FeIII centres due to the short Fe−μ−O bonds. A ≈2.5 Å Fe−Fe distance is unlikely for six-coordinate sites in Q or X, but for a Fe2(μ-O)2 core containing four-coordinate (or by possible extension five-coordinate) iron centres there may be enough flexibility to accommodate a particularly short Fe−Fe separation with intense pre-edge transition. This finding may broaden the scope of models considered for the structure of high-valent diiron intermediates formed upon O2 activation in biology
"Level Curvature" Distribution for Diffusive Aharonov-Bohm Systems: analytical results
We calculate analytically the distributions of "level curvatures" (LC) (the
second derivatives of eigenvalues with respect to a magnetic flux) for a
particle moving in a white-noise random potential.
We find that the Zakrzewski-Delande conjecture is still valid even if the
lowest weak localization corrections are taken into account. The ratio of mean
level curvature modulus to mean dissipative conductance is proved to be
universal and equal to in agreement with available numerical data.Comment: 12 pages. Submitted to Phys.Rev.
Energy level dynamics in systems with weakly multifractal eigenstates: equivalence to 1D correlated fermions
It is shown that the parametric spectral statistics in the critical random
matrix ensemble with multifractal eigenvector statistics are identical to the
statistics of correlated 1D fermions at finite temperatures. For weak
multifractality the effective temperature of fictitious 1D fermions is
proportional to (1-d_{n})/n, where d_{n} is the fractal dimension found from
the n-th moment of inverse participation ratio. For large energy and parameter
separations the fictitious fermions are described by the Luttinger liquid model
which follows from the Calogero-Sutherland model. The low-temperature
asymptotic form of the two-point equal-parameter spectral correlation function
is found for all energy separations and its relevance for the low temperature
equal-time density correlations in the Calogero-Sutherland model is
conjectured.Comment: 4 pages, Revtex, final journal versio
Spectral Correlations from the Metal to the Mobility Edge
We have studied numerically the spectral correlations in a metallic phase and
at the metal-insulator transition. We have calculated directly the two-point
correlation function of the density of states . In the metallic phase,
it is well described by the Random Matrix Theory (RMT). For the first time, we
also find numerically the diffusive corrections for the number variance
predicted by Al'tshuler and Shklovski\u{\i}. At the
transition, at small energy scales, starts linearly, with a slope
larger than in a metal. At large separations , it is found to
decrease as a power law with and , in good agreement with recent microscopic
predictions. At the transition, we have also calculated the form factor , Fourier transform of . At large , the number variance
contains two terms \tilde{K}(0)t \to 0$.Comment: 7 RevTex-pages, 10 figures. Submitted to PR
Which Kubo formula gives the exact conductance of a mesoscopic disordered system?
In both research and textbook literature one often finds two ``different''
Kubo formulas for the zero-temperature conductance of a non-interacting Fermi
system. They contain a trace of the product of velocity operators and
single-particle (retarded and advanced) Green operators: or . The study investigates the relationship between
these expressions, as well as the requirements of current conservation, through
exact evaluation of such quantum-mechanical traces for a nanoscale (containing
1000 atoms) mesoscopic disordered conductor. The traces are computed in the
semiclassical regime (where disorder is weak) and, more importantly, in the
nonperturbative transport regime (including the region around
localization-delocalization transition) where concept of mean free path ceases
to exist. Since quantum interference effects for such strong disorder are not
amenable to diagrammatic or nonlinear -model techniques, the evolution
of different Green function terms with disorder strength provides novel insight
into the development of an Anderson localized phase.Comment: 7 pages, 5 embedded EPS figures, final published version (note: PRB
article has different title due to editorial censorship
Lectures on Chiral Disorder in QCD
I explain the concept that light quarks diffuse in the QCD vacuum following
the spontaneous breakdown of chiral symmetry. I exploit the striking analogy to
disordered electrons in metals, identifying, among others, the universal regime
described by random matrix theory, diffusive regime described by chiral
perturbation theory and the crossover between these two domains.Comment: Lectures given at the Cargese Summer School, August 6-18, 200
Multifractality and critical fluctuations at the Anderson transition
Critical fluctuations of wave functions and energy levels at the Anderson
transition are studied for the family of the critical power-law random banded
matrix ensembles. It is shown that the distribution functions of the inverse
participation ratios (IPR) are scale-invariant at the critical point,
with a power-law asymptotic tail. The IPR distribution, the multifractal
spectrum and the level statistics are calculated analytically in the limits of
weak and strong couplings, as well as numerically in the full range of
couplings.Comment: 14 pages, 13 eps figure
Spatializing the Ecological Leviathan: Territorial Strategies and the Production of Regional Natures
This paper explores a dual absence – the absence of the state within contemporary geographical analyses of nature; and the absence of nature within contemporary explorations of state power. We argue that the modern state continues to play a crucial role in framing social interactions with nature, while nature is still vital to states within their realization of different forms of material and ideological power. In order to reconnect analyses of the state and nature, this paper combines work on the production of nature and state strategy with Lefebvre’s recently translated writings on state space and territory. By focusing on the production of territory (or state space), we explore the interaction of the state and nature in the context of the political management of social and ecological space. We unravel the spatial entanglements of the state and nature through an analysis of the British state’s territorial strategies within the West Midlands region. By considering three key historical periods within the history of the West Mid-lands we reveal how the emergence of the regional space called the West Midlands is a product of the ongoing spatial dialectics of state and nature therein
Measurement of the View the tt production cross-section using eμ events with b-tagged jets in pp collisions at √s = 13 TeV with the ATLAS detector
This paper describes a measurement of the inclusive top quark pair production cross-section (σtt¯) with a data sample of 3.2 fb−1 of proton–proton collisions at a centre-of-mass energy of √s = 13 TeV, collected in 2015 by the ATLAS detector at the LHC. This measurement uses events with an opposite-charge electron–muon pair in the final state. Jets containing b-quarks are tagged using an algorithm based on track impact parameters and reconstructed secondary vertices. The numbers of events with exactly one and exactly two b-tagged jets are counted and used to determine simultaneously σtt¯ and the efficiency to reconstruct and b-tag a jet from a top quark decay, thereby minimising the associated systematic uncertainties. The cross-section is measured to be:
σtt¯ = 818 ± 8 (stat) ± 27 (syst) ± 19 (lumi) ± 12 (beam) pb,
where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, the integrated luminosity and the LHC beam energy, giving a total relative uncertainty of 4.4%. The result is consistent with theoretical QCD calculations at next-to-next-to-leading order. A fiducial measurement corresponding to the experimental acceptance of the leptons is also presented
Search for TeV-scale gravity signatures in high-mass final states with leptons and jets with the ATLAS detector at sqrt [ s ] = 13TeV
A search for physics beyond the Standard Model, in final states with at least one high transverse momentum charged lepton (electron or muon) and two additional high transverse momentum leptons or jets, is performed using 3.2 fb−1 of proton–proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015 at √s = 13 TeV. The upper end of the distribution of the scalar sum of the transverse momenta of leptons and jets is sensitive to the production of high-mass objects. No excess of events beyond Standard Model predictions is observed. Exclusion limits are set for models of microscopic black holes with two to six extra dimensions
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