2,450 research outputs found
A synoptic study of Sudden Phase Anomalies (SPA's) effecting VLF navigation and timing
Sudden phase anomalies (SPA's) observed on VLF recordings are related to sudden ionospheric disturbances due to solar flares. Results are presented for SPA statistics on 500 events observed in New York during the ten year period 1961 to 1970. Signals were at 10.2kHz and 13.6kHz emitted from the OMEGA transmitters in Hawaii and Trinidad. A relationship between SPA frequency and sun spot number was observed. For sun spot number near 85, about one SPA per day will be observed somewhere in the world. SPA activity nearly vanishes during periods of low sun spot number. During years of high solar activity, phase perturbations observed near noon are dominated by SPA effects beyond the 95th percentile. The SPA's can be represented by a rapid phase run-off which is approximately linear in time, peaking in about 6 minutes, and followed by a linear recovery. Typical duration is 49 minutes
Omega VLF timing revision 1
The report specifically discusses time dissemination techniques, including epoch determination, frequency determination, and ambiguity resolution. It also discusses operational considerations including equipment, path selection, and adjustment procedure. epoch (the actual location or timing of periodic events) is shown to be both maintainable and calibratable by the techniques described to better than 3-microsecond accuracy; and frequency (the uniformity of the time scale) to about one part in 10 to the 12th power
Oxygen-stripes in La0.5Ca0.5MnO3 from ab initio calculations
We investigate the electronic, magnetic and orbital properties of
La0.5Ca0.5MnO3 perovskite by means of an ab initio electronic structure
calculation within the Hartree-Fock approximation. Using the experimental
crystal structure reported by Radaelli et al. [Phys. Rev B 55, 3015 (1997)], we
find a charge-ordering stripe-like ground state. The periodicity of the
stripes, and the insulating CE-type magnetic structure are in agreement with
neutron x-ray and electron diffraction experiments. However, the detailed
structure is more complex than that envisaged by simple models of charge and
orbital order on Mn d-levels alone, and is better described as a charge-density
wave of oxygen holes, coupled to the Mn spin/orbital order.Comment: 4 pages, 3 figures. Version accepted for publication in PR
Structural, orbital, and magnetic order in vanadium spinels
Vanadium spinels (ZnV_2O_4, MgV_2O_4, and CdV_2O_4) exhibit a sequence of
structural and magnetic phase transitions, reflecting the interplay of lattice,
orbital, and spin degrees of freedom. We offer a theoretical model taking into
account the relativistic spin-orbit interaction, collective Jahn-Teller effect,
and spin frustration. Below the structural transition, vanadium ions exhibit
ferroorbital order and the magnet is best viewed as two sets of
antiferromagnetic chains with a single-ion Ising anisotropy. Magnetic order,
parametrized by two Ising variables, appears at a tetracritical point.Comment: v3: streamlined introductio
Thermodynamics of the one-dimensional SU(4) symmetric spin-orbital model
The ground state properties and the thermodynamics of the one-dimensional
SU(4) symmetric spin system with orbital degeneracy are investigated using the
quantum Monte Carlo loop algorithm. The spin-spin correlation functions exhibit
a 4-site periodicity, and their low temperature behavior is controlled by two
correlation lengths that diverge like the inverse temperature, while the
entropy is linear in temperature and its slope is consistent with three gapless
modes of velocity . The physical implications of these results are
discussed.Comment: 4 pages, 4 figures, RevTe
Finite temperature spin-dynamics and phase transitions in spin-orbital models
We study finite temperature properties of a generic spin-orbital model
relevant to transition metal compounds, having coupled quantum Heisenberg-spin
and Ising-orbital degrees of freedom. The model system undergoes a phase
transition, consistent with that of a 2D Ising model, to an orbitally ordered
state at a temperature set by short-range magnetic order. At low temperatures
the orbital degrees of freedom freeze-out and the model maps on to a quantum
Heisenberg model. The onset of orbital excitations causes a rapid scrambling of
the spin spectral weight away from coherent spin-waves, which leads to a sharp
increase in uniform magnetic susceptibility just below the phase transition,
reminiscent of the observed behavior in the Fe-pnictide materials.Comment: 4 page
Double-exchange via degenerate orbitals
We consider the double-exchange for systems in which doped electrons occupy
degenerate orbitals, treating the realistic situation with double degenerate
orbitals. We show that the orbital degeneracy leads in general to
formation of anisotropic magnetic structures and that in particular, depending
on the doping concentration, the layered magnetic structures of the A-type and
chain-like structures of the C-type are stabilized. The phase-diagram that we
obtain provides an explanation for the experimentally observed magnetic
structures of some over-doped (electron-doped) manganites of the type
NdSrMnO, PrSrMnO and SmCaMnO
with .Comment: 4 pages, 1 figur
Elementary excitations of the symmetric spin-orbital model: The XY limit
The elementary excitations of the 1D, symmetric, spin-orbital model are
investigated by studying two anisotropic versions of the model, the pure XY and
the dimerized XXZ case, with analytical and numerical methods. While they
preserve the symmetry between spin and orbital degrees of freedom, these models
allow for a simple and transparent picture of the low--lying excitations: In
the pure XY case, a phase separation takes place between two phases with
free--fermion like, gapless excitations, while in the dimerized case, the
low-energy effective Hamiltonian reduces to the 1D Ising model with gapped
excitations. In both cases, all the elementary excitations involve simultaneous
flips of the spin and orbital degrees of freedom, a clear indication of the
breakdown of the traditional mean-field theory.Comment: Revtex, two figure
Two-dimensional gapless spin liquids in frustrated SU(N) quantum magnets
A class of the symmetrically frustrated SU(N) models is constructed for
quantum magnets based on the generators of SU(N) group. The total Hamiltonian
lacks SU(N) symmtry. A mean field theory in the quasi-particle representation
is developed for spin liquid states. Numerical solutions in two dimension
indicate that the ground states are gapless and the quasi-particles are Dirac
particles. The mechanism may be helpful in exploring the spin liquid phases in
the spin-1 bilinear-biquadratic model and the spin-orbital model in higher
dimensions.Comment: 9 pages, 3 figures, to appear in New Journal of Physic
Low-energy sector of the S=1/2 Kagome antiferromagnet
Starting from a modified version of the the S=1/2 Kagome antiferromagnet to
emphasize the role of elementary triangles, an effective Hamiltonian involving
spin and chirality variables is derived. A mean-field decoupling that retains
the quantum nature of these variables is shown to yield a Hamiltonian that can
be solved exactly, leading to the following predictions: i) The number of low
lying singlet states increase with the number of sites N like 1.15 to the power
N; ii) A singlet-triplet gap remains in the thermodynamic limit; iii) Spinons
form boundstates with a small binding energy. By comparing these properties
with those of the regular Kagome lattice as revealed by numerical experiments,
we argue that this description captures the essential low energy physics of
that model.Comment: 4 pages including 3 figure
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