9,639 research outputs found
The effect of continuous, nonlinearly transformed visual feedback on rapid aiming movements
We investigated the ability to adjust to nonlinear transformations that allow people to control external systems like machines and tools. Earlier research (Verwey and Heuer 2007) showed that in the presence of just terminal feedback participants develop an internal model of such transformations that operates at a relatively early processing level (before or at amplitude specification). In this study, we investigated the level of operation of the internal model after practicing with continuous visual feedback. Participants executed rapid aiming movements, for which a nonlinear relationship existed between the target amplitude seen on the computer screen and the required movement amplitude of the hand on a digitizing tablet. Participants adjusted to the external transformation by developing an internal model. Despite continuous feedback, explicit awareness of the transformation did not develop and the internal model still operated at the same early processing level as with terminal feedback. Thus with rapid aiming movements, the type of feedback may not matter for the locus of operation of the internal model
Average persistence in random walks
We study the first passage time properties of an integrated Brownian curve
both in homogeneous and disordered environments. In a disordered medium we
relate the scaling properties of this center of mass persistence of a random
walker to the average persistence, the latter being the probability P_pr(t)
that the expectation value of the walker's position after time t has not
returned to the initial value. The average persistence is then connected to the
statistics of extreme events of homogeneous random walks which can be computed
exactly for moderate system sizes. As a result we obtain a logarithmic
dependence P_pr(t)~{ln(t)}^theta' with a new exponent theta'=0.191+/-0.002. We
note on a complete correspondence between the average persistence of random
walks and the magnetization autocorrelation function of the transverse-field
Ising chain, in the homogeneous and disordered case.Comment: 6 pages LaTeX, 3 postscript figures include
Clear-cutting eden: representations of nature in Southern fiction, 1930-1950
This dissertation examines how Southern literary representations of the natural world were influenced by, and influenced, the historical, social, and ecological changes of the 1930s and 1940s. Specifically, I examine the ways that nature is conceived of and portrayed by four authors of this era: Erskine Caldwell, Marjorie Kinnan Rawlings, Zora Neale Hurston, and William Faulkner; through their works, I investigate the intersections of race, class, and gender with the natural environment. I argue that during this time of profound regional and national upheaval there exists a climate of professed binary oppositions and that these authors’ representations of nature in their fiction reflect the tensions of such polarities as past/present, male/female, left/right, white/black, and culture/nature. Although there is no clear linear development of the way the idea of nature is used in Southern literature, the period now termed the Southern Renaissance (roughly 1930-1950) is fueled by a new wave of Southern authors who reconfigure the use of nature in their fiction in conjunction with modernist analyses of the self and the South. The relatively belated arrival of modernism in the South offers a special opportunity for studying the shift from nineteenth- to twentieth-century culture, a change that proceeded in the South in far more concentrated fashion and with greater tension and drama than in the rest of the nation. I focus on the natural environments of the texts as dynamic, expressive spaces, and I also connect the representations of the natural world in selected novels of Caldwell, Rawlings, Hurston, and Faulkner to their responses to issues of race, class, and gender while situating their works within the contexts of Southern history and literary traditions
Griffiths-McCoy singularities in the transverse field Ising model on the randomly diluted square lattice
The site-diluted transverse field Ising model in two dimensions is studied
with Quantum-Monte-Carlo simulations. Its phase diagram is determined in the
transverse field (Gamma) and temperature (T) plane for various (fixed)
concentrations (p). The nature of the quantum Griffiths phase at zero
temperature is investigated by calculating the distribution of the local
zero-frequency susceptibility. It is pointed out that the nature of the
Griffiths phase is different for small and large Gamma.Comment: 21 LaTeX (JPSJ macros included), 12 eps-figures include
Finite temperature behavior of strongly disordered quantum magnets coupled to a dissipative bath
We study the effect of dissipation on the infinite randomness fixed point and
the Griffiths-McCoy singularities of random transverse Ising systems in chains,
ladders and in two-dimensions. A strong disorder renormalization group scheme
is presented that allows the computation of the finite temperature behavior of
the magnetic susceptibility and the spin specific heat. In the case of Ohmic
dissipation the susceptibility displays a crossover from Griffiths-McCoy
behavior (with a continuously varying dynamical exponent) to classical Curie
behavior at some temperature . The specific heat displays Griffiths-McCoy
singularities over the whole temperature range. For super-Ohmic dissipation we
find an infinite randomness fixed point within the same universality class as
the transverse Ising system without dissipation. In this case the phase diagram
and the parameter dependence of the dynamical exponent in the Griffiths-McCoy
phase can be determined analytically.Comment: 23 pages, 12 figure
Critical Behavior and Griffiths-McCoy Singularities in the Two-Dimensional Random Quantum Ising Ferromagnet
We study the quantum phase transition in the two-dimensional random Ising
model in a transverse field by Monte Carlo simulations. We find results similar
to those known analytically in one-dimension. At the critical point, the
dynamical exponent is infinite and the typical correlation function decays with
a stretched exponential dependence on distance. Away from the critical point
there are Griffiths-McCoy singularities, characterized by a single,
continuously varying exponent, z', which diverges at the critical point, as in
one-dimension. Consequently, the zero temperature susceptibility diverges for a
RANGE of parameters about the transition.Comment: 4 pages RevTeX, 3 eps-figures include
Random antiferromagnetic quantum spin chains: Exact results from scaling of rare regions
We study XY and dimerized XX spin-1/2 chains with random exchange couplings
by analytical and numerical methods and scaling considerations. We extend
previous investigations to dynamical properties, to surface quantities and
operator profiles, and give a detailed analysis of the Griffiths phase. We
present a phenomenological scaling theory of average quantities based on the
scaling properties of rare regions, in which the distribution of the couplings
follows a surviving random walk character. Using this theory we have obtained
the complete set of critical decay exponents of the random XY and XX models,
both in the volume and at the surface. The scaling results are confronted with
numerical calculations based on a mapping to free fermions, which then lead to
an exact correspondence with directed walks. The numerically calculated
critical operator profiles on large finite systems (L<=512) are found to follow
conformal predictions with the decay exponents of the phenomenological scaling
theory. Dynamical correlations in the critical state are in average
logarithmically slow and their distribution show multi-scaling character. In
the Griffiths phase, which is an extended part of the off-critical region
average autocorrelations have a power-law form with a non-universal decay
exponent, which is analytically calculated. We note on extensions of our work
to the random antiferromagnetic XXZ chain and to higher dimensions.Comment: 19 pages RevTeX, eps-figures include
Random quantum magnets with long-range correlated disorder: Enhancement of critical and Griffiths-McCoy singularities
We study the effect of spatial correlations in the quenched disorder on
random quantum magnets at and near a quantum critical point. In the random
transverse field Ising systems disorder correlations that decay algebraically
with an exponent rho change the universality class of the transition for small
enough rho and the off-critical Griffiths-McCoy singularities are enhanced. We
present exact results for 1d utilizing a mapping to fractional Brownian motion
and generalize the predictions for the critical exponents and the generalized
dynamical exponent in the Griffiths phase to d>=2.Comment: 4 pages RevTeX, 1 eps-figure include
Attractors in fully asymmetric neural networks
The statistical properties of the length of the cycles and of the weights of
the attraction basins in fully asymmetric neural networks (i.e. with completely
uncorrelated synapses) are computed in the framework of the annealed
approximation which we previously introduced for the study of Kauffman
networks. Our results show that this model behaves essentially as a Random Map
possessing a reversal symmetry. Comparison with numerical results suggests that
the approximation could become exact in the infinite size limit.Comment: 23 pages, 6 figures, Latex, to appear on J. Phys.
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