1,639 research outputs found
Representations underlying skill in the discrete sequence production task: effect of hand used and hand position
Various studies suggest that movement sequences are initially learned predominantly in effector-independent spatial coordinates and only after extended practice in effector-dependent coordinates. The present study examined this notion for the discrete sequence production (DSP) task by manipulating the hand used and the position of the hand relative to the body. During sequence learning in Experiment 1, in which sequences were executed by reacting to key-specific cues, hand position appeared important for execution with the practiced but not with the unpracticed hand. In Experiment 2 entire sequences were executed by reacting to one cue. This produced similar results as in Experiment 1. These experiments support the notion that robustness of sequencing skill is based on several codes, one being a representation that is both effector and position dependent
Real-time dynamics induced by quenches across the quantum critical points in gapless Fermi systems with a magnetic impurity
The energy-dependent scattering of fermions from a localized orbital at an
energy-dependent rate gives rise to
quantum critical points (QCPs) in the pseudogap single-impurity Anderson model
separating a local moment phase with an unscreened spin moment from a
strong-coupling phase which slightly deviates from the screened phase of
standard Kondo problem. Using the time-dependent numerical renormalization
group (TD-NRG) approach we show that local dynamic properties always
equilibrate towards a steady-state value even for quenches across the QCP but
with systematic deviations from the thermal equilibrium depending on the
distance to the critical coupling. Local non-equilibrium properties are
presented for interaction quenches and hybridization quenches. We augment our
numerical data by an analytical calculation that becomes exact at short times
and find excellent agreement between the numerics and the analytical theory.
For interaction quenches within the screened phase we find a universal function
for the time-dependent local double occupancy. We trace back the discrepancy
between our results and the data obtained by a time-dependent Gutzwiller
variational approach to restrictions of the wave-function ansatz in the
Gutzwiller theory: while the NRG ground states properly account for the
formation of an extended spin moment which decouples from the system in the
unscreened phase, the Gutzwiller ansatz only allows the formation of the spin
moment on the local impurity orbital
Manipulation of the graphene surface potential by ion irradiation
We show that the work function of exfoliated single layer graphene can be
modified by irradiation with swift (E_{kin}=92 MeV) heavy ions under glancing
angles of incidence. Upon ion impact individual surface tracks are created in
graphene on SiC. Due to the very localized energy deposition characteristic for
ions in this energy range, the surface area which is structurally altered is
limited to ~ 0.01 mum^2 per track. Kelvin probe force microscopy reveals that
those surface tracks consist of electronically modified material and that a few
tracks suffice to shift the surface potential of the whole single layer flake
by ~ 400 meV. Thus, the irradiation turns the initially n-doped graphene into
p-doped graphene with a hole density of 8.5 x 10^{12} holes/cm^2. This doping
effect persists even after heating the irradiated samples to 500{\deg}C.
Therefore, this charge transfer is not due to adsorbates but must instead be
attributed to implanted atoms. The method presented here opens up a new way to
efficiently manipulate the charge carrier concentration of graphene.Comment: 6 pages, 4 figure
Geometric criticality between plaquette phases in integer-spin kagome XXZ antiferromagnets
The phase diagram of the uniaxially anisotropic antiferromagnet on the
kagom\'e lattice includes a critical line exactly described by the classical
three-color model. This line is distinct from the standard geometric classical
criticality that appears in the classical limit () of the 2D XY
model; the geometric T=0 critical line separates two unconventional
plaquette-ordered phases that survive to nonzero temperature. The
experimentally important correlations at finite temperature and the nature of
the transitions into these ordered phases are obtained using the mapping to the
three-color model and a combination of perturbation theory and a variational
ansatz for the ordered phases. The ordered phases show sixfold symmetry
breaking and are similar to phases proposed for the honeycomb lattice dimer
model and model. The same mapping and phase transition can be
realized also for integer spins but then require strong on-site
anisotropy in the Hamiltonian.Comment: 5 pages, 2 figure
Radioactive Probes of the Supernova-Contaminated Solar Nebula: Evidence that the Sun was Born in a Cluster
We construct a simple model for radioisotopic enrichment of the protosolar
nebula by injection from a nearby supernova, based on the inverse square law
for ejecta dispersion. We find that the presolar radioisotopes abundances
(i.e., in solar masses) demand a nearby supernova: its distance can be no
larger than 66 times the size of the protosolar nebula, at a 90% confidence
level, assuming 1 solar mass of protosolar material. The relevant size of the
nebula depends on its state of evolution at the time of radioactivity
injection. In one scenario, a collection of low-mass stars, including our sun,
formed in a group or cluster with an intermediate- to high-mass star that ended
its life as a supernova while our sun was still a protostar, a starless core,
or perhaps a diffuse cloud. Using recent observations of protostars to estimate
the size of the protosolar nebula constrains the distance of the supernova at
0.02 to 1.6 pc. The supernova distance limit is consistent with the scales of
low-mass stars formation around one or more massive stars, but it is closer
than expected were the sun formed in an isolated, solitary state. Consequently,
if any presolar radioactivities originated via supernova injection, we must
conclude that our sun was a member of such a group or cluster that has since
dispersed, and thus that solar system formation should be understood in this
context. In addition, we show that the timescale from explosion to the creation
of small bodies was on the order of 1.8 Myr (formal 90% confidence range of 0
to 2.2 Myr), and thus the temporal choreography from supernova ejecta to
meteorites is important. Finally, we can not distinguish between progenitor
masses from 15 to 25 solar masses in the nucleosynthesis models; however, the
20 solar mass model is somewhat preferred.Comment: ApJ accepted, 19 pages, 3 figure
Spin Waves in Quantum Antiferromagnets
Using a self-consistent mean-field theory for the Heisenberg
antiferromagnet Kr\"uger and Schuck recently derived an analytic expression for
the dispersion. It is exact in one dimension () and agrees well with
numerical results in . With an expansion in powers of the inverse
coordination number () we investigate if this expression can be
{\em exact} for all . The projection method of Mori-Zwanzig is used for the
{\em dynamical} spin susceptibility. We find that the expression of Kr\"uger
and Schuck deviates in order from our rigorous result. Our method is
generalised to arbitrary spin and to models with easy-axis anisotropy \D.
It can be systematically improved to higher orders in . We clarify its
relation to the expansion.Comment: 8 pages, uuencoded compressed PS-file, accepted as Euro. Phys. Lette
Context-dependent motor skill and the role of practice
Research has shown that retrieval of learned information is better when the original learning context is reinstated during testing than when this context is changed. Recently, such contextual dependencies have also been found for perceptual-motor behavior. The current study investigated the nature of context-dependent learning in the discrete sequence production task, and in addition examined whether the amount of practice affects the extent to which sequences are sensitive to contextual alterations. It was found that changing contextual cuesâbut not the removal of such cuesâhad a detrimental effect on performance. Moreover, this effect was observed only after limited practice, but not after extensive practice. Our findings support the notion of a novel type of context-dependent learning during initial motor skill acquisition and demonstrate that this context-dependence reduces with practice. It is proposed that a gradual development with practice from stimulus-driven to representation-driven sequence execution underlies this practice effect
Cognitive Processing in New and Practiced Discrete Keying Sequences
This study addresses the role of cognitive control in the initiation and execution of familiar and unfamiliar movement sequences. To become familiar with two movement sequences participants first practiced two discrete key press sequences by responding to two fixed series of 6-key specific stimuli. In the ensuing test phase they executed these two familiar and also two unfamiliar keying sequences while there was a two-third chance a tone was presented together with one randomly selected key specific stimulus in each sequence. In the counting condition of the test phase participants counted the low pitched (i.e., target) tones. By and large the results support the dual processor model in which the prime role of the cognitive processor shifts from executing to initiating sequences while the gradual development of motor chunks allows a motor processor to execute the sequences. Yet, the results extend this simple model by suggesting that with little practice sequence execution is based also on some non-cognitive (perhaps associative) learning mechanism and, for some participants, on the use of explicit sequence knowledge. Also, after extensive practice the cognitive processor appears to still contribute to slower responses. The occurrence of long interkey intervals was replicated suggesting that fixed 6-key sequences include several motor chunks. Yet, no indication was found that the cognitive processor is responsible for concatenating these chunks
Graphene on Si(111)7x7
We demonstrate that it is possible to mechanically exfoliate graphene under
ultra high vacuum conditions on the atomically well defined surface of single
crystalline silicon. The flakes are several hundred nanometers in lateral size
and their optical contrast is very faint in agreement with calculated data.
Single layer graphene is investigated by Raman mapping. The G and 2D peaks are
shifted and narrowed compared to undoped graphene. With spatially resolved
Kelvin probe measurements we show that this is due to p-type doping with hole
densities of n_h \simeq 6x10^{12} cm^{-2}. The in vacuo preparation technique
presented here should open up new possibilities to influence the properties of
graphene by introducing adsorbates in a controlled way.Comment: 8 pages, 7 figure
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