351 research outputs found
Isotope shift calculations for atoms with one valence electron
This work presents a method for the ab initio calculation of isotope shift in
atoms and ions with one valence electron above closed shells. As a zero
approximation we use relativistic Hartree-Fock and then calculate correlation
corrections. The main motivation for developing the method comes from the need
to analyse whether different isotope abundances in early universe can
contribute to the observed anomalies in quasar absorption spectra. The current
best explanation for these anomalies is the assumption that the fine structure
constant, alpha, was smaller at early epoch. We test the isotope shift method
by comparing the calculated and experimental isotope shift for the alkali and
alkali-like atoms Na, MgII, K, CaII and BaII. The agreement is found to be
good. We then calculate the isotope shift for some astronomically relevant
transitions in SiII and SiIV, MgII, ZnII and GeII.Comment: 11 page
Theoretical analysis of neutron scattering results for quasi-two dimensional ferromagnets
A theoretical study has been carried out to analyse the available results
from the inelastic neutron scattering experiment performed on a quasi-two
dimensional spin-1/2 ferromagnetic material . Our formalism is based
on a conventional semi-classical like treatment involving a model of an ideal
gas of vortices/anti-vortices corresponding to an anisotropic XY Heisenberg
ferromagnet on a square lattice. The results for dynamical structure functions
for our model corresponding to spin-1/2, show occurrence of negative values in
a large range of energy transfer even encompassing the experimental range, when
convoluted with a realistic spectral window function. This result indicates
failure of the conventional theoretical framework to be applicable to the
experimental situation corresponding to low spin systems. A full quantum
formalism seems essential for treating such systems.Comment: 16 pages, 6 figures, 1 Table Submitted for publicatio
Charge disproportionation in YNiO : ESR and susceptibility study
We present a study of the magnetic properties of YNiO in the
paramagnetic range, above and below the metal-insulator (MI) transition. The dc
susceptibility, (measured up to 1000 K) is a decreasing function of
T for 150 K (the N\'{e}el temperature) and we observe two different
Curie-Weiss regimes corresponding to the metallic and insulator phases. In the
metallic phase, this behaviour seems to be associated with the small ionic
radius of Y% . The value of the Curie constant for T T allows
us to discard the possibility of Ni localization. An electron spin
resonance (ESR) spectrum is visible in the insulator phase and only a fraction
of the Ni ions contributes to this resonance. We explain the ESR and behaviour for T T in terms of charge disproportionation of
the type 2Ni Ni+Ni that is compatible with the
previously observed structural transition across T.Comment: 10 pages, 4 figures, submitted to Phys. Rev.
Microscopic transition potential: Determination of and coupling constants
A transition potential, based on an effective
quark-quark interaction and a constituent quark cluster model for baryons, is
derived in the Born-Oppenheimer approach. The potential shows significant
differences with respect to those obtained by a direct scaling of the
nucleon-nucleon interaction. From its asymptotic behavior we extract the values
of and coupling constants in a
particular coupling schemeComment: 15 eps figures, Accepted for publication in Phys. Rev.
Nonlocal calculation for nonstrange dibaryons and tribaryons
We study the possible existence of nonstrange dibaryons and tribaryons by
solving the bound-state problem of the two- and three-body systems composed of
nucleons and deltas. The two-body systems are , , and
, while the three-body systems are , ,
, and . We use as input the nonlocal ,
, and potentials derived from the chiral quark cluster
model by means of the resonating group method. We compare with previous results
obtained from the local version based on the Born-Oppenheimer approximation.Comment: 19 pages. To be published in Physical Review
Interplay of superexchange and orbital degeneracy in Cr-doped LaMnO3
We report on structural, magnetic and Electron Spin Resonance (ESR)
investigations in the manganite system LaMn_{1-x}Cr_{x}O_{3} (x<=0.5). Upon
Cr-doping we observe a reduction of the Jahn-Teller distortion yielding less
distorted orthorhombic structures. A transition from the Jahn-Teller distorted
O' to the pseudocubic O phase occurs between 0.3<x<0.4. A clear connection
between this transition and the doping dependence of the magnetic and ESR
properties has been observed. The effective moments determined by ESR seem
reduced with respect to the spin-only value of both Mn^{3+} and Cr^{3+} ions
Origin of Life
The evolution of life has been a big enigma despite rapid advancements in the
fields of biochemistry, astrobiology, and astrophysics in recent years. The
answer to this puzzle has been as mind-boggling as the riddle relating to
evolution of Universe itself. Despite the fact that panspermia has gained
considerable support as a viable explanation for origin of life on the Earth
and elsewhere in the Universe, the issue remains far from a tangible solution.
This paper examines the various prevailing hypotheses regarding origin of life
like abiogenesis, RNA World, Iron-sulphur World, and panspermia; and concludes
that delivery of life-bearing organic molecules by the comets in the early
epoch of the Earth alone possibly was not responsible for kick-starting the
process of evolution of life on our planet.Comment: 32 pages, 8 figures,invited review article, minor additio
Spin Dynamics In Perovskites, Pyrochlores, And Layered Manganites
High temperature electron spin resonance (ESR) and magnetic susceptibility (Ï) are analyzed for manganites related with colossal magnetoresistance (CMR). The properties of compounds with different crystalline structures: three-dimensional (3D) perovskites, pyrochlore, and La1.2Sr1.8Mn2O7, a two-dimensional layer, are compared. In the paramagnetic regime, and outside the critical regions associated with phase transitions, the temperature dependence of the ESR linewidth presents a universal behavior dominated by the variations of Ï(T), ÎHpp(T) = [C/TÏ(T)]ÎHpp(â). The high temperature limit of the linewidth, ÎHpp(â), is related to the parameters of the Hamiltonian describing the interactions of the spin system. The role played by magnetic anisotropy, isotropic superexchange, and double exchange is revealed and discussed in the analysis of the experimental data. In CMR and non-CMR pyrochlores, ÎHpp(â)âÏ2 p/J where J is proportional to the Curie-Weiss temperature, including the hybridization mechanism producing CMR. Instead, ÎHpp(â) of CMR perovskites seems not to be affected by the double-exchange interaction. In contrast with the 3D perovskites, the ESR linewidth and resonance field of La1.2Sr1.8Mn2O7, a bilayer compound, although isotropic at high temperatures, becomes anisotropic for Tc= 125 K<T<Tpâ450 K. © 2000 American Institute of Physics.879 II58105812Causa, M.T., (1998) Phys. Rev. B, 58, p. 3233Lofland, (1997) Phys. Lett. A, 233, p. 476Causa, M.T., Alejandro, G., Tovar, M., Pagliuso, P.G., Rettori, C., Oseroff, S.B., Subramanian, M.A., (1999) J. Appl. Phys., 85, p. 5408Anderson, P.W., Weiss, P.R., (1953) Rev. Mod. Phys., 25, p. 269Zener, C., (1951) Phys. Rev., 82, p. 403Shimakawa, Y., Kubo, Y., Hamada, N., Jorgensen, J.D., Hu, Z., Short, S., Nohara, M., Takagi, H., (1999) Phys. Rev. B, 59, p. 1249. , and references thereinVentura, C., Alascio, B., (1997) Phys. Rev. B, 56, p. 14533Huber, D.L., Alejandro, G., Caneiro, A., Causa, M.T., Prado, F., Tovar, M., Oseroff, S.B., (1999) Phys. Rev. B, 60, p. 12155Chauvet, O., Goglio, G., Molinie, P., Corraze, B., Brohan, L., (1998) Phys. Rev. Lett., 81, p. 1102. , and references thereinMoreno, N.O., Pagliuso, P.G., Rettori, C., Gardner, J.S., Sarrao, J.L., Thompson, J.D., GarcĂa-Flores, A., Oseroff, S.B., unpublishe
Implementation of the Random Forest Method for the Imaging Atmospheric Cherenkov Telescope MAGIC
The paper describes an application of the tree classification method Random
Forest (RF), as used in the analysis of data from the ground-based gamma
telescope MAGIC. In such telescopes, cosmic gamma-rays are observed and have to
be discriminated against a dominating background of hadronic cosmic-ray
particles. We describe the application of RF for this gamma/hadron separation.
The RF method often shows superior performance in comparison with traditional
semi-empirical techniques. Critical issues of the method and its implementation
are discussed. An application of the RF method for estimation of a continuous
parameter from related variables, rather than discrete classes, is also
discussed.Comment: 16 pages, 8 figure
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