515 research outputs found
Recoil corrections in the hydrogen isoelectronic sequence
A version of the Bethe-Salpeter equation appropriate for calculating recoil
corrections in highly charged hydrogenlike ions is presented. The nucleus is
treated as a scalar particle of charge Z, and the electron treated
relativistically. The known recoil corrections of order are
derived in both this formalism and in NRQED
The Range and Limitation of Sub-National Regime Variations under Electoral Authoritarianism:The Case of Russia
Electron-Doped Manganese Perovskites: The Polaronic State
Using the Lanczos method in linear chains we study the ground state of the
double exchange model including an antiferromagnetic super-exchange in the low
concentration limit. We find that this ground state is always inhomogeneous,
containig ferromagnetic polarons. The extention of the polaron spin distortion,
the dispersion relation and their trapping by impurities, are studied for
diferent values of the super exchange interaction and magnetic field. We also
find repulsive polaron polaron interaction.Comment: 4 pages, 6 embedded figure
A novel spin wave expansion, finite temperature corrections and order from disorder effects in the double exchange model
The magnetic excitations of the double exchange (DE) model are usually
discussed in terms of an equivalent ferromagnetic Heisenberg model. We argue
that this equivalence is valid only at a quasi--classical level -- both quantum
and thermal corrections to the magnetic properties of DE model differ from any
effective Heisenberg model because its spin excitations interact only
indirectly, through the exchange of charge fluctuations. To demonstrate this,
we perform a novel large S expansion for the coupled spin and charge degrees of
freedom of the DE model, aimed at projecting out all electrons not locally
aligned with core spins. We generalized the Holstein--Primakoff transformation
to the case when the length of the spin is by itself an operator, and
explicitly constructed new fermionic and bosonic operators to fourth order in
1/\sqrt{S}. This procedure removes all spin variables from the Hund coupling
term, and yields an effective Hamiltonian with an overall scale of electron
hopping, for which we evaluate corrections to the magnetic and electronic
properties in 1/S expansion to order O(1/S^2). We also consider the effect of a
direct superexchange antiferromagnetic interaction between core spins. We find
that the competition between ferromagnetic double exchange and an
antiferromagnetic superexchange provides a new example of an "order from
disorder" phenomenon -- when the two interactions are of comparable strength,
an intermediate spin configuration (either a canted or a spiral state) is
selected by quantum and/or thermal fluctuations.Comment: 21 pages revtex, 11 eps figure
Boundary Energies and the Geometry of Phase Separation in Double--Exchange Magnets
We calculate the energy of a boundary between ferro- and antiferromagnetic
regions in a phase separated double-exchange magnet in two and three
dimensions. The orientation dependence of this energy can significantly affect
the geometry of the phase-separated state in two dimensions, changing the
droplet shape and possibly stabilizing a striped arrangement within a certain
range of the model parameters. A similar effect, albeit weaker, is also present
in three dimensions. As a result, a phase-separated system near the percolation
threshold is expected to possess intrinsic hysteretic transport properties,
relevant in the context of recent experimental findings.Comment: 6 pages, including 4 figures; expanded versio
Proposal for the determination of nuclear masses by high-precision spectroscopy of Rydberg states
The theoretical treatment of Rydberg states in one-electron ions is
facilitated by the virtual absence of the nuclear-size correction, and
fundamental constants like the Rydberg constant may be in the reach of planned
high-precision spectroscopic experiments. The dominant nuclear effect that
shifts transition energies among Rydberg states therefore is due to the nuclear
mass. As a consequence, spectroscopic measurements of Rydberg transitions can
be used in order to precisely deduce nuclear masses. A possible application of
this approach to the hydrogen and deuterium, and hydrogen-like lithium and
carbon is explored in detail. In order to complete the analysis, numerical and
analytic calculations of the quantum electrodynamic (QED) self-energy remainder
function for states with principal quantum number n=5,...,8 and with angular
momentum L=n-1 and L=n-2 are described (j = L +/- 1/2).Comment: 21 pages; LaTe
Formation of a pentagonal particle structure from copper nanoclusters
The structure of pentagonal particles and the processes of their formation from nanoclusters with the fifthorder symmetry axes are investigated by the methods of computer modeling and scanning electron-ion microscopy using copper as an example. It is demonstrated that the mechanism of cluster growth to pentagonal particles can be realized at which the volumetric stress present in noncrystal clusters will be released without breaking of the fifth-order symmetry of the growing cluster shapeye
Instability of antiferromagnetic magnons in strong fields
We predict that spin-waves in an ordered quantum antiferromagnet (AFM) in a
strong magnetic field become unstable with respect to spontaneous two-magnon
decays. At zero temperature, the instability occurs between the threshold field
and the saturation field . As an example, we investigate the
high-field dynamics of a Heisenberg antiferromagnet on a square lattice and
show that the single-magnon branch of the spectrum disappears in the most part
of the Brillouin zone.Comment: RevTeX, 4 pages, 3 figures, accepted to PR
Spatial and temporal features of soil erosion in the forest-steppe zone of the East-European plain
Data on the rate of the erosion-accumulation processes within the sloped junctions of soils studied on key plots in Tula, Kursk, and Belgorod oblasts were analyzedyesBS
Spatial Correlation of Conduction Electrons in Metal with Complicated Geometry Of The Fermi Surface
The "density-density" correlation function of conduction electrons in metal
is investigated. It is shown, that the asymptotic behaviour of the CF depends
on the shape and the local geometry of the Fermi surface. In particular, the
exponent of power law which describes the damping of Friedel oscillations at
large r (-4 for an isotropic Fermi gas) is determined by local geometry of the
FS. The applications of the obtained results to calculations of the CF in a
metal near the electron topological transition and of the RKKY exchange
integral are considered as well.Comment: 12 pages, LaTeX, 5 figures upon request (to appear in J.Phys.:CM,
1993
- …