1,344 research outputs found
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
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
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
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
Long-term morbidity and follow-up after choledochal malformation surgery; A plea for a quality of life study
Much about the aetiology, pathophysiology, natural course and optimal treatment of choledochal malformation remains under debate. Surgeons continuously strive to optimize their roles in the management of choledochal malformation. Nowadays the standard treatment is complete cyst excision followed by Rouxen-Y hepaticojejunostomy, be it via a laparotomy, laparoscopy or robot-assisted procedure. Whatever surgical endeavor is undertaken, it will be a major operation, with significant morbidity. It is important to realize that especially in asymptomatic cases, this is considered prophylactic surgery, aimed at preventing symptoms but even more important the development of malignancy later in life. A clear overview of long-term outcomes is therefore necessary. This paper aims to review the long-term outcomes after surgery for choledochal malformation. We will focus on biliary complications such as cholangitis, the development of malignancy and quality of life. We will try and identify factors related to a worse outcome. Finally, we make a plea for a large scale study into quality and course of life after resection of a choledochal malformation, to help patients, parents and their treating physicians to come to a well-balanced decision regarding the treatment of a choledochal malformation. (C) 2020 The Authors. Published by Elsevier Inc
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