562 research outputs found
Dilution effects in HoYSnO: from the Spin Ice to the single-ion magnet
A study of the modifications of the magnetic properties of
HoYSnO upon varying the concentration of diamagnetic
Y ions is presented. Magnetization and specific heat measurements show
that the Spin Ice ground-state is only weakly affected by doping for , even if non-negligible changes in the crystal field at Ho occur.
In this low doping range SR relaxation measurements evidence a
modification in the low-temperature dynamics with respect to the one observed
in the pure Spin Ice. For , or at high temperature, the dynamics
involve fluctuations among Ho crystal field levels which give rise to a
characteristic peak in Sn nuclear spin-lattice relaxation rate. In this
doping limit also the changes in Ho magnetic moment suggest a variation
of the crystal field parameters.Comment: 4 pages, 5 figures, proceedings of HFM2008 Conferenc
GUT-Scale Primordial Black Holes: Consequences and Constraints
A population of very light primordial black holes which evaporate before
nucleosynthesis begins is unconstrained unless the decaying black holes leave
stable relics. We show that gravitons Hawking radiated from these black holes
would source a substantial stochastic background of high frequency
gravititational waves ( Hz or more) in the present universe. These
black holes may lead to a transient period of matter dominated expansion. In
this case the primordial universe could be temporarily dominated by large
clusters of "Hawking stars" and the resulting gravitational wave spectrum is
independent of the initial number density of primordial black holes.Comment: 4 pages; grey body factors included in graviton emission
calculations, and a couple of references added, but the conclusions are
unchanged. v3 Minor changes to references and wording; final versio
Correlations Between Lag, Luminosity, and Duration in Gamma-ray Burst Pulses
We derive a new peak lag vs. peak luminosity relation in gamma-ray burst
(GRB) pulses. We demonstrate conclusively that GRB spectral lags are pulse
rather than burst properties and show how the lag vs. luminosity relation
determined from CCF measurements of burst properties is essentially just a
rough measure of this newly derived relation for individual pulses. We further
show that most GRB pulses have correlated properties: short-lag pulses have
shorter durations, are more luminous, and are harder within a burst than
long-lag pulses. We also uncover a new pulse duration vs. pulse peak luminosity
relation, and indicate that long-lag pulses often precede short-lag pulses.
Although most pulse behaviors are supportive of internal shocks (including
long-lag pulses), we identify some pulse shapes that could result from external
shocks.Comment: 14 pages, 4 figures, 1 table; accepted for publication in
Astrophysical Journal Letter
Stabilization of single-electron pumps by high magnetic fields
We study the effect of perpendicular magnetic fields on a single-electron
system with a strongly time-dependent electrostatic potential. Continuous
improvements to the current quantization in these electron pumps are revealed
by high-resolution measurements. Simulations show that the sensitivity of
tunnel rates to the barrier potential is enhanced, stabilizing particular
charge states. Nonadiabatic excitations are also suppressed due to a reduced
sensitivity of the Fock-Darwin states to electrostatic potential. The
combination of these effects leads to significantly more accurate current
quantization
The discovery of polarization in the afterglow of GRB 990510 with the ESO Very Large Telescope
Following a BeppoSAX alert (Piro 1999a) and the discovery of the OT at SAAO
(Vreeswijk et al. 1999a), we observed GRB 990510 with the FORS instrument on
ESO's VLT Unit 1 (`Antu'). The burst is unremarkable in gamma rays, but in
optical is the first one to show good evidence for jet-like outflow (Stanek et
al. 1999, Harrison et al. 1999). We report the detection of significant linear
polarization in the afterglow: it is (1.6 +/- 0.2)% 0.86 days after trigger,
and after 1.81 days is consistent with that same value, but much more
uncertain. The polarization angle is constant on a time scale of hours, and may
be constant over one day. We conclude that the polarization is intrinsic to the
source and due to the synchrotron nature of the emission, and discuss the
random and ordered field geometries that may be responsible for it.Comment: submitted to ApJ Lett., 5 pages including 2 figures, uses
emulateapj.st
Spin Dynamics at Very Low Temperature in Spin Ice DyTiO
We have performed AC susceptibility and DC magnetic relaxation measurements
on the spin ice system DyTiO down to 0.08 K. The relaxation time of
the magnetization has been estimated below 2 K down to 0.08 K. The spin
dynamics of DyTiO is well described by using two relaxation times
( (short time) and (long time)). Both and increase on cooling. Assuming the Arrhenius law in the
temperature range 0.5-1 K, we obtained an energy barrier of 9 K. Below 0.5 K,
both and show a clear deviation from the thermal
activated dynamics toward temperature independent relaxation, suggesting a
quantum dynamics.Comment: 4 page
Magnetic Monopole Dynamics in Spin Ice
One of the most remarkable examples of emergent quasi-particles, is that of
the "fractionalization" of magnetic dipoles in the low energy configurations of
materials known as "spin ice", into free and unconfined magnetic monopoles
interacting via Coulomb's 1/r law [Castelnovo et. al., Nature, 451, 42-45
(2008)]. Recent experiments have shown that a Coulomb gas of magnetic charges
really does exist at low temperature in these materials and this discovery
provides a new perspective on otherwise largely inaccessible phenomenology. In
this paper, after a review of the different spin ice models, we present
detailed results describing the diffusive dynamics of monopole particles
starting both from the dipolar spin ice model and directly from a Coulomb gas
within the grand canonical ensemble. The diffusive quasi-particle dynamics of
real spin ice materials within "quantum tunneling" regime is modeled with
Metropolis dynamics, with the particles constrained to move along an underlying
network of oriented paths, which are classical analogues of the Dirac strings
connecting pairs of Dirac monopoles.Comment: 26 pages, 12 figure
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