4,386 research outputs found
Electronic Structures and Optical Properties of Partially and Fully Fluorinated Graphene
In this letter we study the electronic structures and optical properties of
partially and fully fluorinated graphene by a combination of abinitio G0W0
calculations and large-scale multi-orbital tight-binding simulations. We find
that for partially fluorinated graphene, the appearance of paired fluorine
atoms is more favorable than unpaired atoms. We also show that different types
of structural disorder, such as carbon vacancies, fluorine vacancies, fluorine
vacancy-clusters and fluorine armchair- and zigzag-clusters, will introduce
different types of midgap states and extra excitations within the optical gap.
Furthermore we argue that the local formation of bonds upon fluorination
can be distinguished from other disorder inducing mechanisms which do not
destroy the hybrid orbitals by measuring the polarization rotation of
passing polarized light.Comment: Final version appeared in Phys. Rev. Let
Decorated Shastry-Sutherland lattice in the spin-1/2 magnet CdCu2(BO3)2
We report the microscopic magnetic model for the spin-1/2 Heisenberg system
CdCu2(BO3)2, one of the few quantum magnets showing the 1/2-magnetization
plateau. Recent neutron diffraction experiments on this compound [M. Hase et
al., Phys. Rev. B 80, 104405 (2009)] evidenced long-range magnetic order,
inconsistent with the previously suggested phenomenological magnetic model of
isolated dimers and spin chains. Based on extensive density-functional theory
band structure calculations, exact diagonalizations, quantum Monte Carlo
simulations, third-order perturbation theory, as well as high-field
magnetization measurements, we find that the magnetic properties of CdCu2(BO3)2
are accounted for by a frustrated quasi-2D magnetic model featuring four
inequivalent exchange couplings: the leading antiferromagnetic coupling J_d
within the structural Cu2O6 dimers, two interdimer couplings J_t1 and J_t2,
forming magnetic tetramers, and a ferromagnetic coupling J_it between the
tetramers. Based on comparison to the experimental data, we evaluate the ratios
of the leading couplings J_d : J_t1 : J_t2 : J_it = 1 : 0.20 : 0.45 : -0.30,
with J_d of about 178 K. The inequivalence of J_t1 and J_t2 largely lifts the
frustration and triggers long-range antiferromagnetic ordering. The proposed
model accounts correctly for the different magnetic moments localized on
structurally inequivalent Cu atoms in the ground-state magnetic configuration.
We extensively analyze the magnetic properties of this model, including a
detailed description of the magnetically ordered ground state and its evolution
in magnetic field with particular emphasis on the 1/2-magnetization plateau.
Our results establish remarkable analogies to the Shastry-Sutherland model of
SrCu2(BO3)2, and characterize the closely related CdCu2(BO3)2 as a material
realization for the spin-1/2 decorated anisotropic Shastry-Sutherland lattice.Comment: 16 pages, 13 figures, 2 tables. Published version with additional QMC
dat
Determining the Quark Mixing Matrix From CP-Violating Asymmetries
If the Standard Model explanation of CP violation is correct, then
measurements of CP-violating asymmetries in meson decays can in principle
determine the entire quark mixing matrix.Comment: 8 pages (plain TeX), 1 figure (postscript file appended), DAPNIA/SPP
94-06, NSF-PT-94-2, UdeM-LPN-TH-94-18
SrCu(PO): A real material realization of the 1D nearest neighbor Heisenberg chain
We present evidence that crystalline Sr_2Cu(PO_4)_2 is a nearly perfect
one-dimensional (1D) spin-1/2 anti-ferromagnetic Heisenberg model (AHM) chain
compound with nearest neighbor only exchange. We undertake a broad theoretical
study of the magnetic properties of this compound using first principles (LDA,
LDA+U calculations), exact diagonalization and Bethe-ansatz methodologies to
decompose the individual magnetic contributions, quantify their effect, and fit
to experimental data. We calculate that the conditions of one-dimensionality
and short-ranged magnetic interactions are sufficiently fulfilled that Bethe's
analytical solution should be applicable, opening up the possibility to explore
effects beyond the infinite chain limit of the AHM Hamiltonian. We begin such
an exploration by examining some extrinsic effects such as impurities and
defects
Orbital-spin order and the origin of structural distortion in MgTiO
We analyze electronic, magnetic, and structural properties of the spinel
compound MgTiO using the local density approximation+U method. We show
how MgTiO undergoes to a canted orbital-spin ordered state, where
charge, spin and orbital degrees of freedom are frozen in a geometrically
frustrated network by electron interactions. In our picture orbital order
stabilize the magnetic ground state and controls the degree of structural
distortions. The latter is dynamically derived from the cubic structure in the
correlated LDA+U potential. Our ground-state theory provides a consistent
picture for the dimerized phase of MgTiO, and might be applicable to
frustrated materials in general.Comment: 6 pages, 6 figure
Role of C in MgC_xNi_3 investigated from first principles
The influence of vacancies in the sub-lattice of , on its
structural, electronic and magnetic properties are studied by means of the
density-functional based Korringa-Kohn-Rostoker Green's function method
formulated in the atomic sphere approximation. Disorder is taken into account
by means of coherent-potential approximation. Characterizations representing
the change in the lattice properties include the variation in the equilibrium
lattice constants, bulk modulus and pressure derivative of the bulk modulus,
and that of electronic structure include the changes in the, total, partial and
-resolved density of states. The incipient magnetic properties are
studied by means of fixed-spin moment method of alloy theory, together in
conjunction with the phenomenological Ginzburg-Landau equation for magnetic
phase transition. The first-principles calculations reveal that due to the
breaking of the - bonds, some of the 3d states, which were lowered
in energy due to strong hybridization, are transfered back to higher energies
thereby increasing the itinerant character in the material. The Bloch spectral
densities evaluated at the high symmetry points however reveal that the charge
redistribution is not uniform over the cubic Brillouin zone, as new states are
seen to be created at the point, while a shift in the states on the
energy scale are seen at other high symmetry points
About the modern "experimental value" of W boson width.
It is shown that the methods which have been used up to now to determine the
width from the data confirm the SM predictions for some
combinations of various phenomenological parameters, however, they do not give
an independent value for the width. Moreover, the accuracy that could be
achieved in future experimental checks of SM predictions for such quantities is
limited by effects which require detailed theoretical study.Comment: Latex, 8 pages
Triple product asymmetries in K, D_(s) and B_(s) decays
One distinguishes between "true" CP violating triple product (TP) asymmetries
which require no strong phases and "fake" asymmetries which are due to strong
phases but require no CP violation. So far a single true TP asymmetry has been
measured in . A general discussion is presented for
T-odd TP asymmetries in four-body decays. It is shown that TP asymmetries
vanish for two identical and kinematically indistinguishable particles in the
final state. Two examples are and . A non-zero TP asymmetry can be expected when non-trivial
kinematic correlations exist, as in the decay . Triple
product asymmetries measured in charmed particle decays indicate an interesting
pattern of final-state interactions. We reiterate a discussion of TP
asymmetries in meson decays to two vector mesons each decaying to a
pseudoscalar pair, extending results to decays where one vector meson decays
into a lepton pair. We derive expressions for time-dependent TP asymmetries for
neutral B decays to flavorless states in terms of the neutral mass
difference and the width-difference . Time-integrated
true CP violating asymmetries, measurable for untagged decays, are shown
to be suppressed by neither nor
if transversity amplitudes for CP-even and CP-odd states involve different weak
phases. In contrast, fake asymmetries require flavor tagging and are suppressed
by the former ratio when time-integrated. We apply our results to and data and suggest an application for .Comment: minor corrections, submitted to Phys. Rev.
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