2,957 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
Current-driven orbital order-disorder transition in LaMnO3
We report significant influence of electric current on the orbital
order-disorder transition in LaMnO3. The transition temperature T_OO, thermal
hysteresis in the resistivity (rho) versus temperature (T) plot around T_OO,
and latent heat L associated with the transition decrease with the increase in
current density. Eventually, at a critical current density, L reaches zero. The
transition zone, on the other hand, broadens with the increase in current
density. The states at ordered, disordered, and transition zone are all found
to be stable within the time window from ~10^-3 to ~10^4 seconds.Comment: 7 pages including 5 figures; resolution of Fig.1 is better here than
the published versio
Bird population in various ecological communities.
http://deepblue.lib.umich.edu/bitstream/2027.42/51779/1/205.pdfDescription of 205.pdf : Access restricted to on-site users at the U-M Biological Station
Multicolored quantum dimer models, resonating valence-bond states, color visons, and the triangular-lattice t_2g spin-orbital system
The spin-orbital model for triply degenerate t_2g electrons on a triangular
lattice has been shown to be dominated by dimers: the phase diagram contains
both strongly resonating, compound spin-orbital dimer states and quasi-static,
spin-singlet valence-bond (VB) states. To elucidate the nature of the true
ground state in these different regimes, the model is mapped to a number of
quantum dimer models (QDMs), each of which has three dimer colors. The generic
multicolored QDM, illustrated for the two- and three-color cases, possesses a
topological color structure, "color vison" excitations, and broad regions of
resonating VB phases. The specific models are analyzed to gain further insight
into the likely ground states in the superexchange and direct-exchange limits
of the electronic Hamiltonian, and suggest a strong tendency towards VB order
in all cases.Comment: 16 pages, 12 figure
A Potts model for the distortion transition in LaMnO
The Jahn-Teller distortive transition of \lmo is described by a modified
3-state Potts model. The interactions between the three possible orbits depends
both on the orbits and their relative orientation on the lattice. Values of the
two exchange parameters which are chosen to give the correct low temperature
phase and the correct value for the transition temperature are shown to be
consistent with microscopy theory. The model predicts a first order transitions
and also a value for the entropy above the transition in good agreement with
experiment. The theory with the same parameters also predicts the temperature
dependence of the order parameter of orbital ordering agreeing well with
published experimental results. Finally, the type of the transition is shown to
be close to one of the most disordered phases of the generalised Potts model.
The short range order found experimentally above the transition is investigated
by this model.Comment: 16 pages, 7 figures and no tables. Re-submitted to Phys. Rev.
Orbital ordering in frustrated Jahn-Teller systems
We consider the superexchange in `frustrated' Jahn-Teller systems, such as
the transition metal oxides NaNiO_2, LiNiO_2, and ZnMn_2O_4, in which
transition metal ions with doubly degenerate orbitals form a triangular or
pyrochlore lattice and are connected by the 90-degree metal-oxygen-metal bonds.
We show that this interaction is much different from a more familiar exchange
in systems with the 180-degree bonds, e.g. perovskites. In contrast to the
strong interplay between the orbital and spin degrees of freedom in
perovskites, in the 90-degree exchange systems spins and orbitals are
decoupled: the spin exchange is much weaker than the orbital one and it is
ferromagnetic for all orbital states. Due to frustration, the mean-field
orbital ground state is strongly degenerate. Quantum orbital fluctuations
select particular ferro-orbital states, such as the one observed in NaNiO_2. We
also discuss why LiNiO_2 may still behave as an orbital liquid.Comment: 5 pages, 3 figure
Dynamical Symmetry Enlargement Versus Spin-Charge Decoupling in the One-Dimensional SU(4) Hubbard Model
We investigate dynamical symmetry enlargement in the half-filled SU(4)
Hubbard chain using non-perturbative renormalization group and Quantum Monte
Carlo techniques. A spectral gap is shown to open for arbitrary Coulombic
repulsion . At weak coupling, , a SO(8) symmetry between
charge and spin-orbital excitations is found to be dynamically enlarged at low
energy. At strong coupling, , the charge degrees of freedom
dynamically decouple and the resulting effective theory in the spin-orbital
sector is that of the SO(6) antiferromagnetic Heisenberg model. Both regimes
exhibit spin-Peierls order. However, although spin-orbital excitations are
in the SO(6) regime they are in the SO(8) one. The
cross-over between these regimes is discussed.Comment: 4 pages, 2 figure
Dynamically generated dimension reduction and crossover in a spin orbital model
We study a spin orbital model in which the spin-spin interaction couples
linearly to the orbital isospin. Fluctuations drive the transition from
paramagnetic state to C type ordered state into a strongly first order one, as
observed in . At T=0, there is a FOCS to FOGS transition. Close to the
transition point, the system shows dynamically generated dimension reduction
and crossover, resulting in one or more spin reentrant transitions.Comment: Submitted to PRL. 4 pages and one figur
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