75,656 research outputs found
Signature of a silver phase percolation threshold in microscopically phase separated ternary Ge0.15Se0.85-xAgx (0 <= x <= 0.20) glasses
Temperature modulated Alternating Differential Scanning Calorimetric (ADSC)
studies show that Se rich Ge0.15Se0.85-xAgx (0 <= x <= 0.20) glasses are
microscopically phase separated, containing Ag2Se phases embedded in a
Ge0.15Se0.85 backbone. With increasing silver concentration, Ag2Se phase
percolates in the Ge-Se matrix, with a well-defined percolation threshold at x
= 0.10. A signature of this percolation transition is shown up in the thermal
behavior, as the appearance of two exothermic crystallization peaks. Density,
molar volume and micro-hardness measurements, undertaken in the present study,
also strongly support this view of percolation transition. The super-ionic
conduction observed earlier in these glasses at higher silver proportions, is
likely to be connected with the silver phase percolation.Comment: 4 pages, 7 figure
Exact solution of a model of qubit dephasing due to telegraph noise
We present a general and exact formalism for finding the evolution of a
quantum system subject to external telegraph noise. The various qubit
decoherence rates are determined by the eigenvalues of a transfer matrix. The
formalism can be applied to a qubit subject to an arbitrary combination of
dephasing and relaxational telegraph noise, in contrast to existing
non-perturbative methods that treat only one or the other of these limits. We
present 3 applications: 1) We obtain the full qubit dynamics on time scales
short compared with the enviromental correlation times. In the strong coupling
cases this reveals unexpected oscillations and induced magnetization
components; 2) We find in strong coupling case strong violations of the widely
used relation 1/T = 1/2T + 1/T, which is a result of
perturbation theory; 3) We discuss the effects of bang-bang and spin-echo
controls of the qubit dynamics in general settings of the telegraph noises.
%The result shows that these methods are not very effective in %reducing
decoherence arising from a single telegraph noise. Finally, we discuss the
extension of the method to the cases of many telegraph noise sources and
multiple qubits. The method still works when white noise is also present.Comment: 7 pages, 6 figures, revised and extende
Electronic and magnetic properties of substitutional Mn clusters in (Ga,Mn)As
The magnetization and hole distribution of Mn clusters in (Ga,Mn)As are
investigated by all-electron total energy calculations using the projector
augmented wave method within the density-functional formalism. It is found that
the energetically most favorable clusters consist of Mn atoms surrounding one
center As atom. As the Mn cluster grows the hole band at the Fermi level splits
increasingly and the hole distribution gets increasingly localized at the
center As atom. The hole distribution at large distances from the cluster does
not depend significantly on the cluster size. As a consequence, the spin-flip
energy differences of distant clusters are essentially independent of the
cluster size. The Curie temperature is estimated directly from these
spin-flip energies in the mean field approximation. When clusters are present
estimated values are around 250 K independent of Mn concentration whereas
for a uniform Mn distribution we estimate a of about 600 K.Comment: 7 pages, 5 figures, 2 tables; Revised manuscript 26. May 200
On the X-Ray Light Curve, Pulsed-Radio Emission, and Spin Frequency Evolution of the Transient Anomalous X-Ray Pulsar Xte J1810--197 During its X-Ray Outburst
We show that: (i) the long-term X-ray outburst light curve of the transient
AXP XTE J1810-197 can be accounted for by a fallback disk that is evolving
towards quiescence through a disk instability after having been heated by a
soft gamma-ray burst, (ii) the spin-frequency evolution of this source in the
same period can also be explained by the disk torque acting on the
magnetosphere of the neutron star, (iii) most significantly, recently observed
pulsed-radio emission from this source coincides with the epoch of minimum
X-ray luminosity. This is natural in terms of a fallback disk model, as the
accretion power becomes so low that it is not sufficient to suppress the beamed
radio emission from XTE J1810-197.Comment: 13 pages, 2 Figures, accepted for publication in Ap
A possible disk mechanism for the 23d QPO in Mkn~501
Optically thin two-temperature accretion flows may be thermally and viscously
stable, but acoustically unstable. Here we propose that the O-mode instability
of a cooling-dominated optically thin two-temperature inner disk may explain
the 23-day quasi-periodic oscillation (QPO) period observed in the TeV and
X-ray light curves of Mkn~501 during its 1997 high state. In our model the
relativistic jet electrons Compton upscatter the disk soft X-ray photons to TeV
energies, so that the instability-driven X-ray periodicity will lead to a
corresponding quasi-periodicity in the TeV light curve and produce correlated
variability. We analyse the dependence of the instability-driven
quasi-periodicity on the mass (M) of the central black hole, the accretion rate
() and the viscous parameter () of the inner disk. We
show that in the case of Mkn~501 the first two parameters are constrained by
various observational results, so that for the instability occurring within a
two-temperature disk where , the quasi-period is expected to
lie within the range of 8 to 100 days, as indeed the case. In particular, for
the observed 23-day QPO period our model implies a viscosity coefficient
, a sub-Eddington accretion rate and a transition radius to the outer standard disk of , and predicts a period variation due to the
motion of the instability region.Comment: 18 pages, 1 figure, accepted by AP
Spatial variability of Holocene changes in the annual precipitation pattern - a model-data synthesis for the Asian monsoon region
Interhemispheric comparison of average substorm onset locations: evidence for deviation from conjugacy
Based on 2760 well-defined substorm onsets in the Northern Hemisphere and 1432 in the Southern Hemisphere observed by the FUV Imager on board the IMAGE spacecraft, a detailed statistical study is performed including both auroral regions. This study focuses on the hemispheric comparisons. Southward pointing interplanetary magnetic field (IMF) is favorable for substorm to occur, but still 30% of the events are preceded by northward IMF. The magnetic latitude (MLat) of substorm onset depends mainly on the merging electric field (<I>E</sub>m</sub></I>) with a relationship of |dMLat|= &minus;5.2 <I>E<sub>m</sub></I><sup>0.5</sup>, where dMLat is the deviation from onset MLat. In addition, seasonal effects on onset MLat are also detected, with about 2 degrees higher latitudes during solstices than equinoxes. Both IMF <I>B<sub>y</sub></I> and solar illumination have a significant influence on the magnetic local time (MLT) of onsets. An average relation, dMLT=0.25 <I>B<sub>y</sub></I> between IMF <I>B<sub>y</sub></I> and the deviation from onset MLT, was found. The <I>B<sub>y</sub></I> dependence varies slightly with the onset latitude. At lower latitudes (higher activity) it is reduced. After removal of the relationship with IMF <I>B<sub>y</sub></I> a linear relationships remains between the solar zenith angle and onset MLT with dMLT=1 min/deg. Therefore, both solar illumination and IMF <I>B<sub>y</sub></I> can contribute to hemispheric longitudinal displacements of substorm onset locations from conjugacy. No indications for systematic latitudinal displacements between the hemispheres have been found
Magnetic Excitations and Continuum of a Field-Induced Quantum Spin Liquid in -RuCl
We report on terahertz spectroscopy of quantum spin dynamics in
-RuCl, a system proximate to the Kitaev honeycomb model, as a
function of temperature and magnetic field. An extended magnetic continuum
develops below the structural phase transition at K. With the onset
of a long-range magnetic order at K, spectral weight is transferred to
a well-defined magnetic excitation at meV, which is
accompanied by a higher-energy band at meV. Both
excitations soften in magnetic field, signaling a quantum phase transition at
T where we find a broad continuum dominating the dynamical response.
Above , the long-range order is suppressed, and on top of the continuum,
various emergent magnetic excitations evolve. These excitations follow clear
selection rules and exhibit distinct field dependencies, characterizing the
dynamical properties of the field-induced quantum spin liquid
- …