784 research outputs found
Spin 3/2 Pentaquarks
We investigate the possible existence of the spin 3/2 pentaquark states using
interpolating currents with K-N color-octet structure in the framework of QCD
finite energy sum rule (FESR). We pay special attention to the convergence of
the operator product expansion
Chiral Symmetry in Light-front QCD
The definition of chiral transformations in light-front field theory is very
different from the conventional form in equal-time formalism. We study the
consistency of chiral transformations and chiral symmetry in light-front QCD
and derive a complete new light-front axial-vector current for QCD. The
breaking of chiral symmetry in light-front QCD is only associated with helicity
flip interaction between quarks and gluons. Remarkably, the new axial-vector
current does not contain the pion pole part so that the associate chiral charge
smoothly describes pion transitions for various hadronic processes.Comment: 15 pages, no figure, JHEP style, added reference and corrected typos
and some changed conten
Electronic structure study of double perovskites FeReO (A=Ba,Sr,Ca) and SrMoO (M=Cr,Mn,Fe,Co) by LSDA and LSDA+U
We have implemented a systematic LSDA and LSDA+U study of the double
perovskites FeReO (A=Ba,Sr,Ca) and SrMoO
(M=Cr,Mn,Fe,Co) for understanding of their intriguing electronic and magnetic
properties. The results suggest a ferrimagnetic (FiM) and half-metallic (HM)
state of FeReO (A=Ba,Sr) due to a pdd- coupling between the
down-spin Re/Fe orbitals via the intermediate O
ones, also a very similar FiM and HM state of SrFeMoO.
In contrast, a decreasing Fe component at Fermi level () in the
distorted CaFeReO partly accounts for its nonmetallic behavior,
while a finite - coupling between the down-spin
Re/Fe orbitals being present at serves to
stabilize its FiM state. For SrCrMoO compared with
SrFeMoO, the coupling between the down-spin Mo/Cr
orbitals decreases as a noticeable shift up of the Cr 3d
levels, which is likely responsible for the decreasing value and weak
conductivity. Moreover, the calculated level distributions indicate a
Mn(Co)/Mo ionic state in SrMnMoO
(SrCoMoO), in terms of which their antiferromagnetic insulating
ground state can be interpreted. While orbital population analyses show that
owing to strong intrinsic pd covalence effects, SrMoO
(M=Cr,Mn,Fe,Co) have nearly the same valence state combinations, as accounts
for the similar M-independent spectral features observed in them.Comment: 21 pages, 3 figures. to be published in Phys. Rev. B on 15th Se
Green’s function method to the ground state properties of a two-component Bose–Einstein condensate
The elementary excitation spectrum of a two-component Bose–Einstein condensate is obtained by Green’s function method. It is found to have two branches. In the long-wave limit, the two branches of the excitation spectrum are reduced to one phonon excitation and one single-particle excitation. With the obtained excitation spectrum and the Green’s functions, the depletion of the condensate and the ground state energy have also been calculated in this paper
Comparison of s- and d-wave gap symmetry in nonequilibrium superconductivity
Recent application of ultrafast pump/probe optical techniques to
superconductors has renewed interest in nonequilibrium superconductivity and
the predictions that would be available for novel superconductors, such as the
high-Tc cuprates. We have reexamined two of the classical models which have
been used in the past to interpret nonequilibrium experiments with some
success: the mu* model of Owen and Scalapino and the T* model of Parker.
Predictions depend on pairing symmetry. For instance, the gap suppression due
to excess quasiparticle density n in the mu* model, varies as n^{3/2} in d-wave
as opposed to n for s-wave. Finally, we consider these models in the context of
S-I-N tunneling and optical excitation experiments. While we confirm that
recent pump/probe experiments in YBCO, as presently interpreted, are in
conflict with d-wave pairing, we refute the further claim that they agree with
s-wave.Comment: 14 pages, 11 figure
Non-linear macroscopic polarization in III-V nitride alloys
We study the dependence of macroscopic polarization on composition and strain
in wurtzite III-V nitride ternary alloys using ab initio density-functional
techniques. The spontaneous polarization is characterized by a large bowing,
strongly dependent on the alloy microscopic structure. The bowing is due to the
different response of the bulk binaries to hydrostatic pressure, and to
internal strain effects (bond alternation). Disorder effects are instead minor.
Deviations from parabolicity (simple bowing) are of order 10 % in the most
extreme case of AlInN alloy, much less at all other compositions. Piezoelectric
polarization is also strongly non-linear. At variance with the spontaneous
component, this behavior is independent of microscopic alloy structure or
disorder effects, and due entirely to the non-linear strain dependence of the
bulk piezoelectric response. It is thus possible to predict the piezoelectric
polarization for any alloy composition using the piezoelectricity of the parent
binaries.Comment: RevTex 7 pages, 7 postscript figures embedde
Accurate calculation of polarization-related quantities in semiconductors
We demonstrate that polarization-related quantities in semiconductors can be
predicted accurately from first-principles calculations using the appropriate
approach to the problem, the Berry-phase polarization theory. For III-V
nitrides, our test case, we find polarizations, polarization differences
between nitride pairs, and piezoelectric constants quite close to their
previously established values. Refined data are nevertheless provided for all
the relevant quantities.Comment: RevTeX 4 pages, no figure
Scale dependence of the quark masses and mixings: leading order
We consider the Renormalization Group Equations (RGE) for the couplings of
the Standard Model and its extensions. Using the hierarchy of the quark masses
and of the Cabibbo-Kobayashi-Maskawa (CKM) matrix our argument is that a
consistent approximation for the RGE should be based on the parameter . We consider the RGE in the approximation where we
neglect all the relative terms of the order and higher.
Within this approximation we find the exact solution of the evolution equations
of the quark Yukawa couplings and of the vacuum expectation value of the Higgs
field. Then we derive the evolution of the observables: quark masses, CKM
matrix, Jarlskog invariant, Wolfenstein parameters of the CKM matrix and the
unitarity triangle. We show that the angles of the unitarity triangle remain
constant. This property may restrict the possibility of new symmetries or
textures at the grand unification scale.Comment: 15 pages, 4 figures, author of one reference adde
A Real-Space Full Multigrid study of the fragmentation of Li11+ clusters
We have studied the fragmentation of Li11+ clusters into the two
experimentally observed products (Li9+,Li2) and (Li10+,Li) The ground state
structures for the two fragmentation channels are found by Molecular Dynamics
Simulated Annealing in the framework of Local Density Functional theory.
Energetics considerations suggest that the fragmentation process is dominated
by non-equilibrium processes. We use a real-space approach to solve the
Kohn-Sham problem, where the Laplacian operator is discretized according to the
Mehrstellen scheme, and take advantage of a Full MultiGrid (FMG) strategy to
accelerate convergence. When applied to isolated clusters we find our FMG
method to be more efficient than state-of-the-art plane wave calculations.Comment: 9 pages + 6 Figures (in gzipped tar file
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