1,090 research outputs found
Optimized Jastrow-Slater wave functions for ground and excited states: Application to the lowest states of ethene
A quantum Monte Carlo method is presented for determining multi-determinantal
Jastrow-Slater wave functions for which the energy is stationary with respect
to the simultaneous optimization of orbitals and configuration interaction
coefficients. The approach is within the framework of the so-called energy
fluctuation potential method which minimizes the energy in an iterative fashion
based on Monte Carlo sampling and a fitting of the local energy fluctuations.
The optimization of the orbitals is combined with the optimization of the
configuration interaction coefficients through the use of additional single
excitations to a set of external orbitals. A new set of orbitals is then
obtained from the natural orbitals of this enlarged configuration interaction
expansion. For excited states, the approach is extended to treat the average of
several states within the same irreducible representation of the pointgroup of
the molecule. The relationship of our optimization method with the stochastic
reconfiguration technique by Sorella et al. is examined. Finally, the
performance of our approach is illustrated with the lowest states of ethene, in
particular with the difficult case of the singlet 1B_1u state.Comment: 12 pages, 2 figure
An integrative review
In the neuroscience of language, phonemes are frequently described as
multimodal units whose neuronal representations are distributed across
perisylvian cortical regions, including auditory and sensorimotor areas. A
different position views phonemes primarily as acoustic entities with
posterior temporal localization, which are functionally independent from
frontoparietal articulatory programs. To address this current controversy, we
here discuss experimental results from neuroimaging (fMRI) as well as
transcranial magnetic stimulation (TMS) studies. On first glance, a mixed
picture emerges, with earlier research documenting neurofunctional
distinctions between phonemes in both temporal and frontoparietal sensorimotor
systems, but some recent work seemingly failing to replicate the latter.
Detailed analysis of methodological differences between studies reveals that
the way experiments are set up explains whether sensorimotor cortex maps
phonological information during speech perception or not. In particular,
acoustic noise during the experiment and ‘motor noise’ caused by button press
tasks work against the frontoparietal manifestation of phonemes. We highlight
recent studies using sparse imaging and passive speech perception tasks along
with multivariate pattern analysis (MVPA) and especially representational
similarity analysis (RSA), which succeeded in separating acoustic-phonological
from general-acoustic processes and in mapping specific phonological
information on temporal and frontoparietal regions. The question about a
causal role of sensorimotor cortex on speech perception and understanding is
addressed by reviewing recent TMS studies. We conclude that frontoparietal
cortices, including ventral motor and somatosensory areas, reflect
phonological information during speech perception and exert a causal influence
on understanding
Modelling concrete and abstract concepts using brain-constrained deep neural networks
A neurobiologically constrained deep neural network mimicking cortical areas relevant for sensorimotor, linguistic and conceptual processing was used to investigate the putative biological mechanisms underlying conceptual category formation and semantic feature extraction. Networks were trained to learn neural patterns representing specific objects and actions relevant to semantically ‘ground’ concrete and abstract concepts. Grounding sets consisted of three grounding patterns with neurons representing specific perceptual or action-related features; neurons were either unique to one pattern or shared between patterns of the same set. Concrete categories were modelled as pattern triplets overlapping in their ‘shared neurons’, thus implementing semantic feature sharing of all instances of a category. In contrast, abstract concepts had partially shared feature neurons common to only pairs of category instances, thus, exhibiting family resemblance, but lacking full feature overlap. Stimulation with concrete and abstract conceptual patterns and biologically realistic unsupervised learning caused formation of strongly connected cell assemblies (CAs) specific to individual grounding patterns, whose neurons were spread out across all areas of the deep network. After learning, the shared neurons of the instances of concrete concepts were more prominent in central areas when compared with peripheral sensorimotor ones, whereas for abstract concepts the converse pattern of results was observed, with central areas exhibiting relatively fewer neurons shared between pairs of category members. We interpret these results in light of the current knowledge about the relative difficulty children show when learning abstract words. Implications for future neurocomputational modelling experiments as well as neurobiological theories of semantic representation are discussed
Lifshitz transitions and quasiparticle de-renormalization in YbRhSi
We study the effect of magnetic fields up to 15 T on the heavy fermion state
of YbRhSi via Hall effect and magnetoresistance measurements down to 50
mK. Our data show anomalies at three different characteristic fields. We
compare our data to renormalized band structure calculations through which we
identify Lifshitz transitions associated with the heavy fermion bands. The Hall
measurements indicate that the de-renormalization of the quasiparticles, {\it
i.e} the destruction of the local Kondo singlets, occurs smoothly while the
Lifshitz transitions occur within rather confined regions of the magnetic
field.Comment: 7 pages, 5 figure
On Silicon Carbide Grains as the Carrier of the 21 Micron Emission Feature in Post-Asymptotic Giant Branch Stars
The mysterious 21mu emission feature seen in 12 proto-planetary nebulae
(PPNe) remains unidentified since its first detection in 1989. Over a dozen of
candidate materials have been proposed within the past decade, but none of them
has received general acceptance. Very recently, silicon carbide (SiC) grains
with impurities were suggested to be the carrier of this enigmatic feature,
based on recent laboratory data that doped SiC grains exhibit a resonance at
\~21mu. This proposal gains strength from the fact that SiC is a common dust
species in carbon-rich circumstellar envelopes. However, SiC dust has a strong
vibrational band at ~11.3mu. We show in this Letter that in order to be
consistent with the observed flux ratios of the 11.3mu feature to the 21mu
feature, the band strength of the 21mu resonance has to be very strong, too
strong to be consistent with current laboratory measurements. But this does not
yet readily rule out the SiC hypothesis since recent experimental results have
demonstrated that the 21mu resonance of doped SiC becomes stronger as the C
impurity increases. Further laboratory measurements of SiC dust with high
fractions of C impurity are urgently needed to test the hypothesis of SiC as
the carrier of the 21mu feature.Comment: 14 pages, 3 figures, accepted for publication in ApJ
D-string on near horizon geometries and infinite conformal symmetry
We show that the symmetries of effective D-string actions in constant dilaton
backgrounds are directly related to homothetic motions of the background
metric. In presence of such motions, there are infinitely many nonlinearly
realized rigid symmetries forming a loop (or loop like) algebra. Near horizon
(AdS) D3 and D1+D5 backgrounds are discussed in detail and shown to provide 2d
interacting field theories with infinite conformal symmetry.Comment: 5 pages, revtex, no figures; symmetry transformations for BI action
added, coupling of D-string to RR 2-form in D1-D5 background corrected; final
version, to appear in Phys. Rev. Let
Incoherent vector vortex-mode solitons in self-focusing nonlinear media
We suggest a novel type of composite spatial optical soliton created by a
coherent vortex beam guiding a partially incoherent light beam in a
self-focusing nonlinear medium. We show that the incoherence of the guided mode
may enhance, rather than suppress, the vortex azimuthal instability, and also
demonstrate strong destabilization of dipole-mode solitons by partially
incoherent light
A Mismatch Negativity Study of (A)Grammatical and Meaningful/less Mini- Constructions
Clinical language performance and neurophysiological correlates of language
processing were measured before and after intensive language therapy in
patients with chronic (time post stroke >1 year) post stroke aphasia (PSA). As
event-related potential (ERP) measure, the mismatch negativity (MMN) was
recorded in a distracted oddball paradigm to short spoken sentences. Critical
‘deviant’ sentence stimuli where either well-formed and meaningful, or
syntactically, or lexico-semantically incorrect. After 4 weeks of speech-
language therapy (SLT) delivered with high intensity (10.5 h per week),
clinical language assessment with the Aachen Aphasia Test battery demonstrated
significant linguistic improvements, which were accompanied by enhanced MMN
responses. More specifically, MMN amplitudes to grammatically correct and
meaningful mini-constructions and to ‘jabberwocky’ sentences containing a
pseudoword significantly increased after therapy. However, no therapy-related
changes in MMN responses to syntactically incorrect strings including
agreement violations were observed. While MMN increases to well-formed
meaningful strings can be explained both at the word and construction levels,
the neuroplastic change seen for ‘jabberwocky’ sentences suggests an
explanation in terms of constructions. The results confirm previous reports
that intensive SLT leads to improvements of linguistic skills in chronic
aphasia patients and now demonstrate that this clinical improvement is
associated with enhanced automatic brain indexes of construction processing,
although no comparable change is present for ungrammatical strings.
Furthermore, the data confirm that the language-induced MMN is a useful tool
to map functional language recovery in PSA
Interplay between Kondo suppression and Lifshitz transitions in YbRhSi at high magnetic fields
We investigate the magnetic field dependent thermopower, thermal
conductivity, resistivity and Hall effect in the heavy fermion metal YbRh2Si2.
In contrast to reports on thermodynamic measurements, we find in total three
transitions at high fields, rather than a single one at 10 T. Using the Mott
formula together with renormalized band calculations, we identify Lifshitz
transitions as their origin. The predictions of the calculations show that all
experimental results rely on an interplay of a smooth suppression of the Kondo
effect and the spin splitting of the flat hybridized bands.Comment: 5 pages, 4 figure
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