991 research outputs found
Non-Gaussian Resistance Noise near Electrical Breakdown in Granular Materials
The distribution of resistance fluctuations of conducting thin films with
granular structure near electrical breakdown is studied by numerical
simulations. The film is modeled as a resistor network in a steady state
determined by the competition between two biased processes, breaking and
recovery. Systems of different sizes and with different levels of internal
disorder are considered. Sharp deviations from a Gaussian distribution are
found near breakdown and the effect increases with the degree of internal
disorder. However, we show that in general this non-Gaussianity is related to
the finite size of the system and vanishes in the large size limit.
Nevertheless, near the critical point of the conductor-insulator transition,
deviations from Gaussianity persist when the size is increased and the
distribution of resistance fluctuations is well fitted by the universal
Bramwell-Holdsworth-Pinton distribution.Comment: 8 pages, 6 figures; accepted for publication on Physica
Universal Fluctuations of the Danube Water Level: a Link with Turbulence, Criticality and Company Growth
A global quantity, regardless of its precise nature, will often fluctuate
according to a Gaussian limit distribution. However, in highly correlated
systems, other limit distributions are possible. We have previously calculated
one such distribution and have argued that this function should apply
specifically, and in many instances, to global quantities that define a steady
state. Here we demonstrate, for the first time, the relevance of this
prediction to natural phenomena. The river level fluctuations of the Danube are
observed to obey our prediction, which immediately establishes a generic
statistical connection between turbulence, criticality and company growth
statistics.Comment: 5 pages, 1 figur
The Effect of Outdoor and Indoor Group Exercise Classes on Psychological Stress in College Students: A Pilot Study with Randomization
International Journal of Exercise Science 16(5): 1012-1024, 2023. Emerging evidence suggests that outdoor group exercise may reduce stress more than indoor group exercise because the outdoor environment provides unique mental health benefits. Stress leads to illnesses and diseases, but exercise mitigates harmful impacts. This study explored differences in perceived stress and outdoor physical activity participation among college students in an indoor or outdoor group exercise class. Data were collected pre-, mid- (after four sessions), and post-intervention (after eight sessions). Seventeen participants indicated an interest in the study, but 13 signed up. Participants completed a four-week group exercise intervention that met twice weekly in outdoor and indoor conditions. Mixed ANOVAs with Tukey post hoc tests determined between-group differences in perceived stress and outdoor physical activity levels. Partial eta-squared (η2p) estimated effect sizes. Significant differences in perceived stress scores existed across time for the whole sample [f(2, 12) = 48.359, p \u3c 0.001, η2p = 0.890] and for the interaction between time and condition [f(2, 12) = 10.051, p = 0.003, η2p = 0.626]. Post hoc analysis revealed that the outdoor group’s perceived stress (p \u3c 0.001) was reduced more than the indoor group post-intervention (p = 0.028)
Competition Between Exchange and Anisotropy in a Pyrochlore Ferromagnet
The Ising-like spin ice model, with a macroscopically degenerate ground
state, has been shown to be approximated by several real materials. Here we
investigate a model related to spin ice, in which the Ising spins are replaced
by classical Heisenberg spins. These populate a cubic pyrochlore lattice and
are coupled to nearest neighbours by a ferromagnetic exchange term J and to the
local axes by a single-ion anisotropy term D. The near neighbour spin
ice model corresponds to the case D/J infinite. For finite D/J we find that the
macroscopic degeneracy of spin ice is broken and the ground state is
magnetically ordered into a four-sublattice structure. The transition to this
state is first-order for D/J > 5 and second-order for D/J < 5 with the two
regions separated by a tricritical point. We investigate the magnetic phase
diagram with an applied field along [1,0,0] and show that it can be considered
analogous to that of a ferroelectric.Comment: 7 pages, 4 figure
Dynamic behavior of magnetic avalanches in the spin-ice compound DyTiO
Avalanches of the magnetization, that is to say an abrupt reversal of the
magnetization at a given field, have been previously reported in the spin-ice
compound DyTiO. This out-of-equilibrium process, induced by
magneto-thermal heating, is quite usual in low temperature magnetization
studies. A key point is to determine the physical origin of the avalanche
process. In particular, in spin-ice compounds, the origin of the avalanches
might be related to the monopole physics inherent to the system. We have
performed a detailed study of the avalanche phenomena in three single crystals,
with the field oriented along the [111] direction, perpendicular to [111] and
along the [100] directions. We have measured the changing magnetization during
the avalanches and conclude that avalanches in spin ice are quite slow compared
to the avalanches reported in other systems such as molecular magnets. Our
measurements show that the avalanches trigger after a delay of about 500 ms and
that the reversal of the magnetization then occurs in a few hundreds of
milliseconds. These features suggest an unusual propagation of the reversal,
which might be due to the monopole motion. The avalanche fields seem to be
reproducible in a given direction for different samples, but they strongly
depend on the initial state of magnetization and on how the initial state was
achieved.Comment: 11 pages, 14 figure
Temperature Dependence of the Magnetic Penetration Depth in the Vortex State of the Pyrochlore Superconductor, Cd2Re2O7
We report transverse field and zero field muon spin rotation studies of the
superconducting rhenium oxide pyrochlore, Cd2Re2O7. Transverse field
measurements (H=0.007 T) show line broadening below Tc, which is characteristic
of a vortex state, demonstrating conclusively the type-II nature of this
superconductor. The penetration depth is seen to level off below about 400 mK
(T/Tc~0.4), with a rather large value of lambda (T=0)~7500A. The temperature
independent behavior below ~ 400 mK is consistent with a nodeless
superconducting energy gap. Zero-field measurements indicate no static magnetic
fields developing below the transition temperature.Comment: 4 pages, 4 figures, REVTEX 4, submitted to PR
Overlap Distribution of the Three-Dimensional Ising Model
We study the Parisi overlap probability density P_L(q) for the
three-dimensional Ising ferromagnet by means of Monte Carlo (MC) simulations.
At the critical point P_L(q) is peaked around q=0 in contrast with the double
peaked magnetic probability density. We give particular attention to the tails
of the overlap distribution at the critical point, which we control over up to
500 orders of magnitude by using the multi-overlap MC algorithm. Below the
critical temperature interface tension estimates from the overlap probability
density are given and their approach to the infinite volume limit appears to be
smoother than for estimates from the magnetization.Comment: 7 pages, RevTex, 9 Postscript figure
Magnetocaloric Study of Spin Relaxation in `Frozen' Dipolar Spin Ice Dy2Ti2O7
The magnetocaloric effect of polycrystalline samples of pure and Y-doped
dipolar spin ice Dy2Ti2O7 was investigated at temperatures from nominally 0.3 K
to 6 K and in magnetic fields of up to 2 T. As well as being of intrinsic
interest, it is proposed that the magnetocaloric effect may be used as an
appropriate tool for the qualitative study of slow relaxation processes in the
spin ice regime. In the high temperature regime the temperature change on
adiabatic demagnetization was found to be consistent with previously published
entropy versus temperature curves. At low temperatures (T < 0.4 K) cooling by
adiabatic demagnetization was followed by an irreversible rise in temperature
that persisted after the removal of the applied field. The relaxation time
derived from this temperature rise was found to increase rapidly down to 0.3 K.
The data near to 0.3 K indicated a transition into a metastable state with much
slower relaxation, supporting recent neutron scattering results. In addition,
magnetic dilution of 50 % concentration was found to significantly prolong the
dynamical response in the milikelvin temperature range, in contrast with
results reported for higher temperatures at which the spin correlations are
suppressed. These observations are discussed in terms of defects and loop
correlations in the spin ice state.Comment: 9 figures, submitted to Phys. Rev.
Long Range Order at Low Temperature in Dipolar Spin Ice
Recently it has been suggested that long range magnetic dipolar interactions
are responsible for spin ice behavior in the Ising pyrochlore magnets and . We report here numerical
results on the low temperature properties of the dipolar spin ice model,
obtained via a new loop algorithm which greatly improves the dynamics at low
temperature. We recover the previously reported missing entropy in this model,
and find a first order transition to a long range ordered phase with zero total
magnetization at very low temperature. We discuss the relevance of these
results to and .Comment: New version of the manuscript. Now contains 3 POSTSCRIPT figures as
opposed to 2 figures. Manuscript contains a more detailed discussion of the
(i) nature of long-range ordered ground state, (ii) finite-size scaling
results of the 1st order transition into the ground state. Order of authors
has been changed. Resubmitted to Physical Review Letters Contact:
[email protected]
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