649 research outputs found
Analysis of the passive stabilization of the long duration exposure facility
The nominal Long Duration Exposure Facility (LDEF) configurations and the anticipated orbit parameters are presented. A linear steady state analysis was performed using these parameters. The effects of orbit eccentricity, solar pressure, aerodynamic pressure, magnetic dipole, and the magnetically anchored rate damper were evaluated to determine the configuration sensitivity to variations in these parameters. The worst case conditions for steady state errors were identified, and the performance capability calculated. Garber instability bounds were evaluated for the range of configuration and damping coefficients under consideration. The transient damping capabilities of the damper were examined, and the time constant as a function of damping coefficient and spacecraft moment of inertia determined. The capture capabilities of the damper were calculated, and the results combined with steady state, transient, and Garber instability analyses to select damper design parameters
Entanglement measurement with discrete multiple coin quantum walks
Within a special multi-coin quantum walk scheme we analyze the effect of the
entanglement of the initial coin state. For states with a special entanglement
structure it is shown that this entanglement can be meausured with the mean
value of the walk, which depends on the i-concurrence of the initial coin
state. Further on the entanglement evolution is investigated and it is shown
that the symmetry of the probability distribution is reflected by the symmetry
of the entanglement distribution.Comment: 9 pages, IOP styl
Electronic Structure of Pyrochlore Iridates: From Topological Dirac Metal to Mott Insulator
In 5d transition metal oxides such as the iridates, novel properties arise
from the interplay of electron correlations and spin-orbit interactions. We
investigate the electronic structure of the pyrochlore iridates, (such as
YIrO) using density functional theory, LDA+U method, and
effective low energy models. A remarkably rich phase diagram emerges on tuning
the correlation strength U. The Ir magnetic moment are always found to be
non-collinearly ordered. However, the ground state changes from a magnetic
metal at weak U, to a Mott insulator at large U. Most interestingly, the
intermediate U regime is found to be a Dirac semi-metal, with vanishing density
of states at the Fermi energy. It also exhibits topological properties -
manifested by special surface states in the form of Fermi arcs, that connect
the bulk Dirac points. This Dirac phase, a three dimensional analog of
graphene, is proposed as the ground state of YIrO and related
compounds. A narrow window of magnetic `axion' insulator, with axion parameter
, may also be present at intermediate U. An applied magnetic field
induces ferromagnetic order and a metallic ground state.Comment: 7pages + 2pages appendices. 7 figures; see also viewpoint article by
L. Balents, "Weyl Electrons Kiss", at Physics 4, 36 (2011
Secondary periodicities of microbursts of TeV gamma rays from the Crab pulsar
Observations were made during the past several years on the Crab pulsar using the Ooty atmospheric Cerenkov array with the aim of detecting possible emission of ultra high energy gamma rays by the pulsar. During the course of these observations, it was found that the Crab pulsar emits TeV gamma rays in bursts of short duration. The microbursts of TeV gamma rays from the Crab pulsar, which were seen in the data of at least three years, also reveal interesting secondary periodicities. It was noticed at first that some bursts could be connected with the others that occurred during the same night or during the next two nights with integral number of cycles of periods 43 + or - 1 minute. Ten possible periods in the vicinity of 43 minutes were determined for all the combinations of bursts for each year. The best values of periods thus obtained were different from year to year. But when, instead of the real time, the number of Crab cycles elapsed between the bursts was used as the unit of time, two values of burst periods - 77460 and 77770 Crab cycles - were found to be significant in the data of at least two years. A Monte Carlo simulation using 1500 trial periods chosen randomly within + or - 5 minutes of the original burst period did not reveal any value of the period as significant
Doped Mott Insulators in the Triangular Lattice Hubbard Model
We investigate the evolution of the Mott insulators in the triangular lattice
Hubbard Model, as a function of hole doping in both the strong and
intermediate coupling limit. Using the density matrix renormalization group
(DMRG) method, at light hole doping , we find a
significant difference between strong and intermediate couplings. Notably, at
intermediate coupling an unusual metallic state emerges, with short ranged spin
correlations but long ranged spin-chirality order. Moreover, no clear Fermi
surface or wave-vector is observed. These features disappear on increasing
interaction strength or on further doping. At strong coupling, the 120 degree
magnetic order of the insulating magnet persists for light doping, and produces
hole pockets with a well defined Fermi surface. On further doping, SDW order and coherent hole Fermi pockets are found at
both strong and intermediate coupling. At even higher doping , the SDW order is suppressed and the spin-singlet Cooper pair
correlations are simultaneously enhanced. We interpret this as the onset of
superconductivity on suppressing magnetic order. We also briefly comment on the
strong particle hole asymmetry of the model, and contrast electron versus hole
doping.Comment: 9 pages, 11 figure
Observations on TeV gamma rays from Geminga and PSR 0950+08
The Geminga (2 CG 195+04) which exhibits a periodicity with a period of 59 to 60 s in its emission of X-rays, GeV gamma rays and TeV gamma rays was studied. During the winter of 1984 to 1985, this object was observed to see if it emits TeV gamma rays with a periodicity approx 60 s. The observations were carried out at two different sites separated by 11 Km with the Ooty Atmospheric Cerenkov Array split into two parts. Data were collected during clear moonless nights for a total duration of 15.3 hours spread over 2 months. Since the first time derivative of period is believed to be large and uncertain. The total data are subdivided into segments of duration not more than 3 days each to steer clear of the effects of P in the phase analysis. If TeV gamma ray signals are seen in each of these segments, it is possible to derive P from observed data
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