3,852 research outputs found
Universality in snowflake aggregation
Aggregation of ice crystals is a key process governing precipitation. Individual ice crystals exhibit considerable diversity of shape, and a wide range of physical processes could influence their aggregation; despite this we show that a simple computer model captures key features of aggregate shape and size distribution reported recently from cirrus clouds. The results prompt a new way to plot the experimental size distributions leading to remarkably good dynamical scaling. That scaling independently confirms that there is a single dominant aggregation mechanism at play, albeit our model (based on undeflected trajectories to contact) does not capture its form exactly
Gait analysis by high school students
Human walking is a complicated motion. Movement scientists have developed various research methods to study gait. This article describes how a high school student collected and analysed high quality gait data in much the same way that movement scientists do, via the recording and measurement of motions with a video analysis tool and via electromyography, i.e., the measurement of muscle activity. Physics, biology, and mathematics come together in this practical investigation work at a rather high level. It shows that science learning at school can resemble science practice in research laboratories, provided that students have adequate tools
Effects of in-chain and off-chain substitutions on spin fluctuations in the spin-Peierls compound CuGeO_3
The effect of in-chain and off-chain substitutions on 1D spin fluctuations in
the spin-Peierls compound CuGeO_3 has been studied using Raman scattering in
order to understand the interplay between defect induced states, enhanced
spin-spin correlations and the ground state of low dimensional systems.
In-chain and off-chain substitutions quench the spin-Peierls state and induce
3D antiferromagnetic order at T\leq 5 K. Consequently a suppression of a 1D
gap-induced mode as well as a constant intensity of a spinon continuum are
observed at low temperatures. A 3D two-magnon density of states now gradually
extends to higher temperatures T\leq 60K compared with pure CuGeO_3. This
effect is more pronounced in the case of off-chain substitutions (Si) for which
a N\'eel state occurs over a larger substitution range, starting at very low
concentrations. Besides, additional low energy excitations are induced. These
effects, i.e. the shift of a dimensional crossover to higher temperatures are
due to an enhancement of the spin-spin correlations induced by a small amount
of substitutions. The results are compared with recent Monte Carlo studies on
substituted spin ladders, pointing to a similar instability of coupled,
dimerized spin chains and spin ladders upon substitution.Comment: 14 pages, 6 eps figures, to be published in PR
Charge-density-wave order parameter of the Falicov-Kimball model in infinite dimensions
In the large-U limit, the Falicov-Kimball model maps onto an effective Ising
model, with an order parameter described by a BCS-like mean-field theory in
infinite dimensions. In the small-U limit, van Dongen and Vollhardt showed that
the order parameter assumes a strange non-BCS-like shape with a sharp reduction
near T approx T_c/2. Here we numerically investigate the crossover between
these two regimes and qualitatively determine the order parameter for a variety
of different values of U. We find the overall behavior of the order parameter
as a function of temperature to be quite anomalous.Comment: (5 pages, 3 figures, typeset with ReVTeX4
Orbital-selective Mott transitions in the anisotropic two-band Hubbard model at finite temperatures
The anisotropic degenerate two-orbital Hubbard model is studied within
dynamical mean-field theory at low temperatures. High-precision calculations on
the basis of a refined quantum Monte Carlo (QMC) method reveal that two
distinct orbital-selective Mott transitions occur for a bandwidth ratio of 2
even in the absence of spin-flip contributions to the Hund exchange. The second
transition -- not seen in earlier studies using QMC, iterative perturbation
theory, and exact diagonalization -- is clearly exposed in a low-frequency
analysis of the self-energy and in local spectra.Comment: 4 pages, 5 figure
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