712 research outputs found
Bidirectional transport on a dynamic lattice
Bidirectional variants of stochastic many particle models for transport by
molecular motors show a strong tendency to form macroscopic clusters on static
lattices. Inspired by the fact that the microscopic tracks for molecular motors
are dynamical, we study the influence of different types of lattice dynamics on
stochastic bidirectional transport. We observe a transition toward efficient
transport (corresponding to the dissolution of large clusters) controlled by
the lattice dynamics.Comment: 5 pages, 5 figure
Particle interactions and lattice dynamics: Scenarios for efficient bidirectional stochastic transport?
Intracellular transport processes driven by molecular motors can be described
by stochastic lattice models of self-driven particles. Here we focus on
bidirectional transport models excluding the exchange of particles on the same
track. We explore the possibility to have efficient transport in these systems.
One possibility would be to have appropriate interactions between the various
motors' species, so as to form lanes. However, we show that the lane formation
mechanism based on modified attachment/detachment rates as it was proposed
previously is not necessarily connected to an efficient transport state and is
suppressed when the diffusivity of unbound particles is finite. We propose
another interaction mechanism based on obstacle avoidance that allows to have
lane formation for limited diffusion. Besides, we had shown in a separate paper
that the dynamics of the lattice itself could be a key ingredient for the
efficiency of bidirectional transport. Here we show that lattice dynamics and
interactions can both contribute in a cooperative way to the efficiency of
transport. In particular, lattice dynamics can decrease the interaction
threshold beyond which lanes form. Lattice dynamics may also enhance the
transport capacity of the system even when lane formation is suppressed.Comment: 25 pages, 17 figures, 2 table
Pressure-induced structural phase transition in the Bechgaard-Fabre salts
The crystal structures of the quasi-one-dimensional organic salts
(TMTTF)PF and (TMTSF)PF were studied by pressure-dependent
x-ray diffraction up to 10 GPa at room temperature. The unit-cell parameters
exhibit a clear anomaly due to a structural phase transition at 8.5 and 5.5 GPa
for (TMTTF)PF and (TMTSF)PF, respectively
Spontaneous symmetry breaking in a two-lane model for bidirectional overtaking traffic
First we consider a unidirectional flux \omega_bar of vehicles each of which
is characterized by its `natural' velocity v drawn from a distribution P(v).
The traffic flow is modeled as a collection of straight `world lines' in the
time-space plane, with overtaking events represented by a fixed queuing time
tau imposed on the overtaking vehicle. This geometrical model exhibits platoon
formation and allows, among many other things, for the calculation of the
effective average velocity w=\phi(v) of a vehicle of natural velocity v.
Secondly, we extend the model to two opposite lanes, A and B. We argue that the
queuing time \tau in one lane is determined by the traffic density in the
opposite lane. On the basis of reasonable additional assumptions we establish a
set of equations that couple the two lanes and can be solved numerically. It
appears that above a critical value \omega_bar_c of the control parameter
\omega_bar the symmetry between the lanes is spontaneously broken: there is a
slow lane where long platoons form behind the slowest vehicles, and a fast lane
where overtaking is easy due to the wide spacing between the platoons in the
opposite direction. A variant of the model is studied in which the spatial
vehicle density \rho_bar rather than the flux \omega_bar is the control
parameter. Unequal fluxes \omega_bar_A and \omega_bar_B in the two lanes are
also considered. The symmetry breaking phenomenon exhibited by this model, even
though no doubt hard to observe in pure form in real-life traffic, nevertheless
indicates a tendency of such traffic.Comment: 50 pages, 16 figures; extra references adde
Enhancement of spin mixing conductance in La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub>/LaNiO<sub>3</sub>/SrRuO<sub>3</sub> heterostructures
Spin pumping and the effective spin-mixing conductance in heterostructures based on magnetic oxide trilayers composed of La0.7Sr0.3MnO3 (LSMO), LaNiO3 (LNO), and SrRuO3 (SRO) are investigated. The heterostructures serve as a model system for an estimation of the effective spin-mixing conductance at the different interfaces. The results show that by introducing a LNO interlayer between LSMO and SRO, the total effective spin-mixing conductance increases due to the much more favorable interface of LSMO/LNO with respect to the LSMO/SRO interface. Nevertheless, the spin current into the SRO does not decrease because of the spin diffusion length of λLNO ≈ 3.2nm in the LNO. This value is two times higher than that of SRO. The results show the potential of using oxide interfaces to tune the effective spin-mixing conductance in heterostructures and to bring novel functionalities into spintronics by implementing complex oxides
Traditionally taught students learn; actively engaged students remember
A common narrative in physics education research is that students taught in lecture-based classes learn less than those taught with activity-based reformed methods. We show this narrative is simplistic and misses important dynamics of student learning. In particular, we find students of both methods show equal short-term learning gains on a conceptual question dealing with electric potential. For traditionally taught students, this learning rapidly decays on a time scale of weeks, vanishing by the time of the typical end-of-term post-test. For students in reform-based classes, however, the knowledge is retained and may even be enhanced by subsequent instruction. This difference
explains the many previous pre- and post-test studies that have found minimal learning gains in lecture-based courses. Our findings suggest a more nuanced model of student learning, one that is sensitive to time-dependent effects such as forgetting and interference. In addition, the findings suggest that lecture-based courses, by incorporating aspects designed to reinforce student understanding of previously covered topics, might approach the long-term learning found in research-based pedagogies
A novel heavy-fermion state in CaCu_3Ru_4O12
We have measured susceptibility, specific heat, resistivity, and thermopower
of CaCuTiRuO and CaCuMnRuO, and
have found that CaCuRuO can be regarded as a heavy-fermion oxide
in d-electron systems. The Kondo temperature is near 200 K, and the
susceptibility (1.4 emu/Cu mol) and the electron specific heat
coefficient (28 mJ/Cu molK) are moderately enhanced. The resistivity is
proportional to at low temperatures, and satisfies the Kadowaki-Woods
relation. The heavy-fermion state comes from the interaction between the
localized moment of Cu 3d and the conduction electron of Ru 4d. An
insulator-metal transition occurs between and 4 in
CaCuTiRuO, which can be regarded as a transition from
magnetic insulator to heavy-fermion metal.Comment: 4 pages, 5 figures, submitted to J. Phys. Soc. Jp
Trees over Infinite Structures and Path Logics with Synchronization
We provide decidability and undecidability results on the model-checking
problem for infinite tree structures. These tree structures are built from
sequences of elements of infinite relational structures. More precisely, we
deal with the tree iteration of a relational structure M in the sense of
Shelah-Stupp. In contrast to classical results where model-checking is shown
decidable for MSO-logic, we show decidability of the tree model-checking
problem for logics that allow only path quantifiers and chain quantifiers
(where chains are subsets of paths), as they appear in branching time logics;
however, at the same time the tree is enriched by the equal-level relation
(which holds between vertices u, v if they are on the same tree level). We
separate cleanly the tree logic from the logic used for expressing properties
of the underlying structure M. We illustrate the scope of the decidability
results by showing that two slight extensions of the framework lead to
undecidability. In particular, this applies to the (stronger) tree iteration in
the sense of Muchnik-Walukiewicz.Comment: In Proceedings INFINITY 2011, arXiv:1111.267
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