989 research outputs found
Density Functional Study of Calcium Nitride
The high-pressure behavior of Ca3N2 is studied up 100 GPa using density functional theory. Evaluation of many hypothetical polymorphs of composition A3X2 leads us to propose four high-pressure polymorphs for both α- and β-Ca3N2: (1) an anti-Rh2O3−II structure at 5 GPa, (2) an anti-B-sesquioxide structure at 10 GPa, (3) an anti-A-sesquioxide structure at 27 GPa, and (4) a hitherto unknown hexagonal structure (P63/mmc), derived from the post-perovskite structure of CaIrO3, at 38 GPa. The development of the density and bulk modulus under pressure has been examined
From hard exclusive meson electroproduction to deeply virtual Compton scattering
We systematically evaluate observables for hard exclusive electroproduction
of real photons and compare them to experiment using a set of Generalized
Parton Distributions (GPDs) whose parameters are constrained by Deeply Virtual
Meson Production data, nucleon form factors and parton distributions. The
Deeply Virtual Compton Scattering amplitudes are calculated to leading-twist
accuracy and leading order in QCD perturbation theory while the leptonic tensor
is treated exactly, without any approximation. This study constitutes a check
of the universality of the GPDs. We summarize all relevant details on the
parametrizations of the GPDs and describe its use in the handbag approach of
the aforementioned hard scattering processes. We observe a good agreement
between predictions and measurements of deeply virtual Compton scattering on a
wide kinematic range, including most data from H1, ZEUS, HERMES, Hall A and
CLAS collaborations for unpolarized and polarized targets when available. We
also give predictions relevant for future experiments at COMPASS and JLab after
the 12 GeV upgrade.Comment: 37 pages, 12 figures v2 : fixed typos, updated to future published
version (EPJC). v3 : fixed typo in Eq. (59
A density functional study of the high-pressure chemistry of MSiN2(M = Be,Mg, Ca)
Normal pressure modifications and tentative high-pressure phases of the nitridosilicates MSiN2 with M = Be, Mg, or Ca have been thoroughly studied by density functional methods. At ambient pressure, BeSiN2 and MgSiN2 exhibit an ordered wurtzite variant derived from idealized filled β-cristobalite by a C1-type distortion. At ambient pressure, the structure of CaSiN2 can also be derived from idealized filled β-cristobalite by a different type of distortion (D1-type). Energy–volume calculations for all three compounds reveal transition into an NaCl superstructure under pressure, affording sixfold coordination for Si. At 76 GPa BeSiN2 forms an LiFeO2-type structure, corresponding to the stable ambient-pressure modification of LiFeO2, while MgSiN2 and CaSiN2 adopt an LiFeO2-type structure, corresponding to a metastable modification (24 and 60 GPa, respectively). For both BeSiN2 and CaSiN2 intermediate phases appear (for BeSiN2 a chalcopyrite-type structure and for CaSiN2 a CaGeN2-type structure). These two tetragonal intermediate structures are closely related, differing mainly in their c/a ratio. As a consequence, chalcopyrite-type structures exhibit tetrahedral coordination for both cations (M and Si), whereas in CaGeN2-type structures one cation is tetrahedrally (Si) and one bisdisphenoidally (M) coordinated. Both structure types, chalcopyrite and CaGeN2, can also be derived from idealized filled β-cristobalite through a B1-type distortion. The group–subgroup relation of the BeSiN2/MgSiN2, the CaSiN2, the chalcopyrite, the CaGeN2 and the idealized filled β-cristobalite structure is discussed and the displacive phase transformation pathways are illustrated. The zero-pressure bulk moduli were calculated for all phases and have been found to be comparable to compounds such as α- Si3N4, CaIrO3 and Al4C3. Furthermore, the thermodynamic stability of BeSiN2, MgSiN2 and CaSiN2 against phase agglomerates of the binary nitrides M3N2 and Si3N4 under pressure are examined
Mixing of pseudoscalar mesons and isospin symmetry breaking
Mixing of the pseudoscalar mesons is discussed in the quark-flavor basis with
the hypothesis that the basis decay constants follow the pattern of particle
state mixing. The divergences of the axial vector currents which embody the
axial vector anomaly, combined with this hypothesis provide a calculational
scheme for the parameters describing the mixing of the pion, eta and eta'
mesons. Phenomenological applications of this mixing scheme are presented with
particular interest focussed on isospin symmetry breaking in QCD estimated as
eta and eta' admixtures to the pion. In contrast to previous work a possible
difference in the basis decay constants f_u and f_d is considered and
consequences of this potentially large effect on the strength of isospin
symmetry breaking is discussed.Comment: 10 pages, (using LATEX with w-ijmpa.sty), invited talk presented at
MESON 2004, 8th Intern. Workshop on Meson Production, Properties and
Interaction
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