415 research outputs found
Investigation of the Jahn-Teller Transition in TiF3 using Density Functional Theory
We use first principles density functional theory to calculate electronic and
magnetic properties of TiF3 using the full potential linearized augmented plane
wave method. The LDA approximation predicts a fully saturated ferromagnetic
metal and finds degenerate energy minima for high and low symmetry structures.
The experimentally observed Jahn-Teller phase transition at Tc=370K can not be
driven by the electron-phonon interaction alone, which is usually described
accurately by LDA.
Electron correlations beyond LDA are essential to lift the degeneracy of the
singly occupied Ti t2g orbital. Although the on-site Coulomb correlations are
important, the direction of the t2g-level splitting is determined by the
dipole-dipole interactions. The LDA+U functional predicts an aniferromagnetic
insulator with an orbitally ordered ground state. The input parameters U=8.1 eV
and J=0.9 eV for the Ti 3d orbital were found by varying the total charge on
the TiF ion using the molecular NRLMOL code. We estimate the
Heisenberg exchange constant for spin-1/2 on a cubic lattice to be
approximately 24 K. The symmetry lowering energy in LDA+U is about 900 K per
TiF3 formula unit.Comment: 7 pages, 9 figures, to appear in Phys. Rev.
Theory of coherent transport by an ultra-cold atomic Fermi gas through linear arrays of potential wells
Growing interest is being given to transport of ultra-cold atomic gases
through optical lattices generated by the interference of laser beams. In this
connection we evaluate the phase-coherent transport of a spin-polarized gas of
fermionic atoms along linear structures made from potential wells set in four
alternative types of sequence. These are periodic chains of either identical
wells or pairs of different wells, and chains of pairs of wells arranged in
either a Fibonacci quasi-periodic sequence or a random sequence. The
transmission coefficient of fermionic matter is evaluated in a T-matrix
scattering approach by describing each array through a tight-binding
Hamiltonian and by reducing it to an effective dimer by means of a
decimation/renormalization method. The results are discussed in comparison with
those pertaining to transport by Fermi-surface electrons coupled to an outgoing
lead and by an atomic Bose-Einstein condensate. Main attention is given to (i)
Bloch oscillations and their mapping into alternating-current flow through a
Josephson junction; (ii) interference patterns that arise on period doubling
and their analogy with beam splitting in optical interferometry; (iii)
localization by quasi-periodic disorder inside a Fibonacci-ordered structure of
double wells; and (iv) Anderson localization in a random structure of double
wells.Comment: 14 pages, 4 figure
Modeling of complex oxide materials from the first principles: systematic applications to vanadates RVO3 with distorted perovskite structure
"Realistic modeling" is a new direction of electronic structure calculations,
where the main emphasis is made on the construction of some effective
low-energy model entirely within a first-principle framework. Ideally, it is a
model in form, but with all the parameters derived rigorously, on the basis of
first-principles electronic structure calculations. The method is especially
suit for transition-metal oxides and other strongly correlated systems, whose
electronic and magnetic properties are predetermined by the behavior of some
limited number of states located near the Fermi level. After reviewing general
ideas of realistic modeling, we will illustrate abilities of this approach on
the wide series of vanadates RVO3 (R= La, Ce, Pr, Nd, Sm, Gd, Tb, Yb, and Y)
with distorted perovskite structure. Particular attention will be paid to
computational tools, which can be used for microscopic analysis of different
spin and orbital states in the partially filled t2g-band. We will explicitly
show how the lifting of the orbital degeneracy by the monoclinic distortion
stabilizes C-type antiferromagnetic (AFM) state, which can be further
transformed to the G-type AFM state by changing the crystal distortion from
monoclinic to orthorhombic one. Two microscopic mechanisms of such a
stabilization, associated with the one-electron crystal field and electron
correlation interactions, are discussed. The flexibility of the orbital degrees
of freedom is analyzed in terms of the magnetic-state dependence of interatomic
magnetic interactions.Comment: 23 pages, 13 figure
Block bond-order potential as a convergent moments-based method
The theory of a novel bond-order potential, which is based on the block
Lanczos algorithm, is presented within an orthogonal tight-binding
representation. The block scheme handles automatically the very different
character of sigma and pi bonds by introducing block elements, which produces
rapid convergence of the energies and forces within insulators, semiconductors,
metals, and molecules. The method gives the first convergent results for
vacancies in semiconductors using a moments-based method with a low number of
moments. Our use of the Lanczos basis simplifies the calculations of the band
energy and forces, which allows the application of the method to the molecular
dynamics simulations of large systems. As an illustration of this convergent
O(N) method we apply the block bond-order potential to the large scale
simulation of the deformation of a carbon nanotube.Comment: revtex, 43 pages, 11 figures, submitted to Phys. Rev.
Tight-binding parameters for charge transfer along DNA
We systematically examine all the tight-binding parameters pertinent to
charge transfer along DNA. The molecular structure of the four DNA bases
(adenine, thymine, cytosine, and guanine) is investigated by using the linear
combination of atomic orbitals method with a recently introduced
parametrization. The HOMO and LUMO wavefunctions and energies of DNA bases are
discussed and then used for calculating the corresponding wavefunctions of the
two B-DNA base-pairs (adenine-thymine and guanine-cytosine). The obtained HOMO
and LUMO energies of the bases are in good agreement with available
experimental values. Our results are then used for estimating the complete set
of charge transfer parameters between neighboring bases and also between
successive base-pairs, considering all possible combinations between them, for
both electrons and holes. The calculated microscopic quantities can be used in
mesoscopic theoretical models of electron or hole transfer along the DNA double
helix, as they provide the necessary parameters for a tight-binding
phenomenological description based on the molecular overlap. We find that
usually the hopping parameters for holes are higher in magnitude compared to
the ones for electrons, which probably indicates that hole transport along DNA
is more favorable than electron transport. Our findings are also compared with
existing calculations from first principles.Comment: 15 pages, 3 figures, 7 table
Two-Point Functions and Boundary States in Boundary Logarithmic Conformal Field Theories
Our main aim in this thesis is to address the results and prospects of
boundary logarithmic conformal field theories: theories with boundaries that
contain the above Jordan cell structure. We have investigated c_{p,q} boundary
theory in search of logarithmic theories and have found logarithmic solutions
of two-point functions in the context of the Coulomb gas picture. Other
two-point functions have also been studied in the free boson construction of
BCFT with SU(2)_k symmetry. In addition, we have analyzed and obtained the
boundary Ishibashi state for a rank-2 Jordan cell structure [hep-th/0103064].
We have also examined the (generalised) Ishibashi state construction and the
symplectic fermion construction at c=-2 for boundary states in the context of
the c=-2 triplet model. The differences between two constructions are
interpreted, resolved and extended beyond each case.Comment: Ph.D. Thesis (University of Oxford), 96 pages, the layout is modified
from the origina
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in âs = 7 TeV pp collisions with the ATLAS detector
A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fbâ1 of protonâproton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results
Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC
Measurements of inclusive jet suppression in heavy ion collisions at the LHC
provide direct sensitivity to the physics of jet quenching. In a sample of
lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated
luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with
a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the
transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the
anti-kt algorithm with values for the distance parameter that determines the
nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of
the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp.
Jet production is found to be suppressed by approximately a factor of two in
the 10% most central collisions relative to peripheral collisions. Rcp varies
smoothly with centrality as characterized by the number of participating
nucleons. The observed suppression is only weakly dependent on jet radius and
transverse momentum. These results provide the first direct measurement of
inclusive jet suppression in heavy ion collisions and complement previous
measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables,
submitted to Physics Letters B. All figures including auxiliary figures are
available at
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02
Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment
This paper describes an analysis of the angular distribution of W->enu and
W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with
the ATLAS detector at the LHC in 2010, corresponding to an integrated
luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and
the missing transverse energy, the W decay angular distribution projected onto
the transverse plane is obtained and analysed in terms of helicity fractions
f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV
and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw
> 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour,
are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017
+/- 0.030, where the first uncertainties are statistical, and the second
include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables,
revised author list, matches European Journal of Physics C versio
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