14,197 research outputs found
Statics and Dynamics of Yukawa Cluster Crystals on Ordered Substrates
We examine the statics and dynamics of particles with repulsive Yukawa
interactions in the presence of a two-dimensional triangular substrate for
fillings of up to twelve particles per potential minimum. We term the ordered
states Yukawa cluster crystals and show that they are distinct from the
colloidal molecular crystal states found at low fillings. As a function of
substrate and interaction strength at fixed particle density we find a series
of novel crystalline states that we characterize using the structure factor.
For fillings greater than four, shell and ring structures form at each
potential minimum and can exhibit sample-wide orientational order. A disordered
state can appear between ordered states as the substrate strength varies. Under
an external drive, the onsets of different orderings produce clear changes in
the critical depinning force, including a peak effect phenomenon that has
generally only previously been observed in systems with random substrates. We
also find a rich variety of dynamic ordering transitions that can be observed
via changes in the structure factor and features in the velocity-force curves.
The dynamical states encompass a variety of moving structures including
one-dimensional stripes, smectic ordering, polycrystalline states, triangular
lattices, and symmetry locking states. Despite the complexity of the system, we
identify several generic features of the dynamical phase transitions which we
map out in a series of phase diagrams. Our results have implications for the
structure and depinning of colloids on periodic substrates, vortices in
superconductors and Bose-Einstein condensates, Wigner crystals, and dusty
plasmas.Comment: 14 pages, 17 postscript figure
Reversible Vortex Ratchet Effects and Ordering in Superconductors with Simple Asymmetric Potential Arrays
We demonstrate using computer simulations that the simplest vortex ratchet
system for type-II superconductors with artificial pinning arrays, an
asymmetric one-dimensional (1D) potential array, exhibits the same features as
more complicated two-dimensional vortex ratchets that have been studied in
recent experiments. We show that the 1D geometry, originally proposed by Lee et
al. [Nature 400, 337 (1999)], undergoes multiple reversals in the sign of the
ratchet effect as a function of vortex density, substrate strength, and ac
drive amplitude, and that the sign of the ratchet effect is related to the type
of vortex lattice structure present. When the vortex lattice is highly ordered,
an ordinary vortex ratchet effect occurs which is similar to the response of an
isolated particle in the same ratchet geometry. In regimes where the vortices
form a smectic or disordered phase, the vortex-vortex interactions are relevant
and we show with force balance arguments that the ratchet effect can reverse in
sign. The dc response of this system features a reversible diode effect and a
variety of vortex states including triangular, smectic, disordered and square.Comment: 10 pages, 12 postscript figures. Version to appear in Phys. Rev.
Spectral variability in early-type binary X-ray systems
Theoretical models for the ionization of trace elements in a strong stellar wind by a compact binary X-ray source and for the resulting orbital phase dependence of the emergent soft X-ray spectra and the profiles of ultraviolet resonance lines are presented. Model results agree qualitatively with the X-ray and ultraviolet spectra of the system 4U 0900-40/HD 77581 and explain the suppression of the absorption profiles of the Si IV upsilon 1394 and C IV upsilon 1548 lines when the X-ray sources is in front of the star. The model predicts that the absorption profiles of the N V upsilon 1239 and O VI upsilon 1032 lines will be enhanced rather than suppressed during this orbital phase. We predict phase-dependent linear polarization in the resonance lines profiles. Future observations of these phase dependent effects in early-type binary X-ray systems may be used to investigate the dynamics of stellar winds and their interactions with the X-ray source
Ad Hoc American Studies: Michigan and the Hidden History of a Movement
This essay uses the history of the American Culture program at the University of Michigan as an occasion for rethinking the early years of American Studies more generally. Our archival research has found that the program was founded in 1935—seventeen years earlier than previously thought—making it the oldest American Studies degree program in the world. These forgotten years challenge conventional genealogies tracing the field’s origins to the “myth and symbol” school at Harvard. Instead, Michigan’s history reveals a deep engagement with interdisciplinary movements in the 1930s that shared an interest in critiquing American democracy, particularly human ecology, institutionalist economics, and ethnic studies. The sheer variety of intellectual activities that found a home in American Studies suggests that the movement is best understood, from the start, as a disparate set of projects bound by a resistance to artificial institutional boundaries that hindered engagement with a wider public. Our analysis of Michigan’s program, from the 1930s to the 1980s, pushes back against intellectual histories of American Studies that emphasize disciplinary coherence. We instead argue for the ongoing importance of short-term coalitions and collaborations that unfolded in the interstices between disciplines and foregrounded the social and political relevance of academic inquiry into American culture
Multiscaling at Point J: Jamming is a Critical Phenomenon
We analyze the jamming transition that occurs as a function of increasing
packing density in a disordered two-dimensional assembly of disks at zero
temperature for ``Point J'' of the recently proposed jamming phase diagram. We
measure the total number of moving disks and the transverse length of the
moving region, and find a power law divergence as the packing density increases
toward a critical jamming density. This provides evidence that the T = 0
jamming transition as a function of packing density is a {\it second order}
phase transition. Additionally we find evidence for multiscaling, indicating
the importance of long tails in the velocity fluctuations.Comment: 4 pages, 5 figures; extensive new numerical data; final version in
press at PR
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