294 research outputs found
Bond-ordered states and -wave pairing of spinless fermions on the honeycomb lattice
Spinless fermions on the honeycomb lattice with repulsive nearest-neighbor
interactions are known to harbour a quantum critical point at half-filling,
with critical behaviour in the Gross-Neveu (chiral Ising) universality class.
The critical interaction strength separates a weak-coupling semimetallic regime
from a commensurate charge-density-wave phase. The phase diagram of this basic
model of correlated fermions on the honeycomb lattice beyond half-filling is,
however, less well established. Here, we perform an analysis of its many-body
instabilities using the functional renormalization group method with a basic
Fermi surface patching scheme, which allows us to treat instabilities in
competing channels on equal footing also away from half-filling. Between
half-filling and the van-Hove filling, the free Fermi surface is hole-like and
we again find a charge-density wave instability to be dominant at large
interactions. Moreover, its characteristics are those of the half-filled case.
Directly at the van-Hove filling the nesting property of the free Fermi surface
stabilizes a dimerized bond-order phase. At lower filling the free Fermi
surface becomes electron-like and a superconducting instability with -wave
symmetry is found to emerge from the interplay of intra-unitcell repulsion and
collective fluctuations in the proximity to the charge-density wave
instability. We estimate the extent of the various phases and extract the
corresponding order parameters from the effective low-energy Hamiltonians.Comment: 11 pages, 11 figure
A new two-strip TLC method for the quality control of technetium-99m mercaptoacetyl-triglycine (<sup>99m</sup>Tc-MAG3).
<sup>99m</sup> Tc-mercaptoacetyl-triglycine ( <sup>99m</sup> Tc-MAG3) has been used for dynamic renal imaging since about 30 years. Free pertechnetate ( <sup>99m</sup> TcO <sub>4</sub> ), colloidal <sup>99m</sup> Tc (( <sup>99m</sup> TcO <sub>2</sub> ) <sub>n</sub> ), <sup>99m</sup> Tc-tartrate (precursor), precomplexes ( <sup>99m</sup> Tc-(MAG3) <sub>x</sub> ) and lipophilic <sup>99m</sup> Tc-MAG2 are the main radiochemical impurities that may occur in the preparation. The total amount of these impurities has to be identified before release of the product for patient administration to guarantee patient safety and good image quality. The European Pharmacopoeia suggests a method based on high-pressure liquid chromatography analysis in combination with a paper chromatography. This analytical method is time consuming, expensive and requires specially trained technicians. As a consequence, it is not widely applied in nuclear medicine radiopharmacies.
We developed a simple method for radiochemical purity testing of <sup>99m</sup> Tc-MAG3. The method is based on thin layer chromatography with two strips to be developed in parallel. Method validation was carried out in comparison to the official methods of the companies and to the European Pharmacopoeia method. It was tested on specificity, accuracy, robustness and precision.
The proposed method is able to identify and quantify the sum of all impurities occurring in the preparation, respecting the acceptance criteria for the radiochemical purity defined by the official methods. Hydrophilic and lipophilic compounds are identified separately and results are obtained within less than 20 minutes. Our method is simple, cost effective, fast and is suitable for employing dose calibrators or radiometric scanners
Torus Spectroscopy of the Gross-Neveu-Yukawa Quantum Field Theory: Free Dirac versus Chiral Ising Fixed Point
We establish the universal torus low-energy spectra at the free Dirac fixed
point and at the strongly coupled {\em chiral Ising} fixed point and their
subtle crossover behaviour in the Gross-Neuveu-Yukawa field theory with
component Dirac spinors in dimensions. These fixed
points and the field theories are directly relevant for the long-wavelength
physics of certain interacting Dirac systems, such as repulsive spinless
fermions on the honeycomb lattice or -flux square lattice. The torus
spectrum has been shown previously to serve as a characteristic fingerprint of
relativistic fixed points and is a powerful tool to discriminate quantum
critical behaviour in numerical simulations. Here we use a combination of exact
diagonalization and quantum Monte Carlo simulations of strongly interacting
fermionic lattice models, to compute the critical energy spectrum on
finite-size clusters with periodic boundaries and extrapolate them to the
thermodynamic limit. Additionally, we compute the torus spectrum analytically
using the perturbative expansion in , which is in good
agreement with the numerical results, thereby validating the presence of the
chiral Ising fixed point in the lattice models at hand. We show that the strong
interaction between the spinor field and the scalar order-parameter field
strongly influences the critical torus spectrum. Building on these results we
are able to address the subtle crossover physics of the low-energy spectrum
flowing from the chiral Ising fixed point to the Dirac fixed point, and analyze
earlier flawed attempts to extract Fermi velocity renormalizations from the
low-energy spectrum.Comment: 24 pages, 14 figure
S66: A Well-balanced Database of Benchmark Interaction Energies Relevant to Biomolecular Structures
With numerous new quantum chemistry methods being developed in recent years and the promise of even more new methods to be developed in the near future, it is clearly critical that highly accurate, well-balanced, reference data for many different atomic and molecular properties be available for the parametrization and validation of these methods. One area of research that is of particular importance in many areas of chemistry, biology, and material science is the study of noncovalent interactions. Because these interactions are often strongly influenced by correlation effects, it is necessary to use computationally expensive high-order wave function methods to describe them accurately. Here, we present a large new database of interaction energies calculated using an accurate CCSD(T)/CBS scheme. Data are presented for 66 molecular complexes, at their reference equilibrium geometries and at 8 points systematically exploring their dissociation curves; in total, the database contains 594 points: 66 at equilibrium geometries, and 528 in dissociation curves. The data set is designed to cover the most common types of noncovalent interactions in biomolecules, while keeping a balanced representation of dispersion and electrostatic contributions. The data set is therefore well suited for testing and development of methods applicable to bioorganic systems. In addition to the benchmark CCSD(T) results, we also provide decompositions of the interaction energies by means of DFT-SAPT calculations. The data set was used to test several correlated QM methods, including those parametrized specifically for noncovalent interactions. Among these, the SCS-MI-CCSD method outperforms all other tested methods, with a root-mean-square error of 0.08 kcal/mol for the S66 data set
Reduction in Inter-Hemispheric Connectivity in Disorders of Consciousness
Clinical diagnosis of disorders of consciousness (DOC) caused by brain injury poses great challenges since patients are often behaviorally unresponsive. A promising new approach towards objective DOC diagnosis may be offered by the analysis of ultra-slow (<0.1 Hz) spontaneous brain activity fluctuations measured with functional magnetic resonance imaging (fMRI) during the resting-state. Previous work has shown reduced functional connectivity within the “default network”, a subset of regions known to be deactivated during engaging tasks, which correlated with the degree of consciousness impairment. However, it remains unclear whether the breakdown of connectivity is restricted to the “default network”, and to what degree changes in functional connectivity can be observed at the single subject level. Here, we analyzed resting-state inter-hemispheric connectivity in three homotopic regions of interest, which could reliably be identified based on distinct anatomical landmarks, and were part of the “Extrinsic” (externally oriented, task positive) network (pre- and postcentral gyrus, and intraparietal sulcus). Resting-state fMRI data were acquired for a group of 11 healthy subjects and 8 DOC patients. At the group level, our results indicate decreased inter-hemispheric functional connectivity in subjects with impaired awareness as compared to subjects with intact awareness. Individual connectivity scores significantly correlated with the degree of consciousness. Furthermore, a single-case statistic indicated a significant deviation from the healthy sample in 5/8 patients. Importantly, of the three patients whose connectivity indices were comparable to the healthy sample, one was diagnosed as locked-in. Taken together, our results further highlight the clinical potential of resting-state connectivity analysis and might guide the way towards a connectivity measure complementing existing DOC diagnosis
π-π stacking tackled with density functional theory
Through comparison with ab initio reference data, we have evaluated the performance of various density functionals for describing π-π interactions as a function of the geometry between two stacked benzenes or benzene analogs, between two stacked DNA bases, and between two stacked Watson–Crick pairs. Our main purpose is to find a robust and computationally efficient density functional to be used specifically and only for describing π-π stacking interactions in DNA and other biological molecules in the framework of our recently developed QM/QM approach "QUILD". In line with previous studies, most standard density functionals recover, at best, only part of the favorable stacking interactions. An exception is the new KT1 functional, which correctly yields bound π-stacked structures. Surprisingly, a similarly good performance is achieved with the computationally very robust and efficient local density approximation (LDA). Furthermore, we show that classical electrostatic interactions determine the shape and depth of the π-π stacking potential energy surface
Dynamic Changes in Brain Functional Connectivity during Concurrent Dual-Task Performance
This study investigated the spatial, spectral, temporal and functional proprieties of functional brain connections involved in the concurrent execution of unrelated visual perception and working memory tasks. Electroencephalography data was analysed using a novel data-driven approach assessing source coherence at the whole-brain level. Three connections in the beta-band (18–24 Hz) and one in the gamma-band (30–40 Hz) were modulated by dual-task performance. Beta-coherence increased within two dorsofrontal-occipital connections in dual-task conditions compared to the single-task condition, with the highest coherence seen during low working memory load trials. In contrast, beta-coherence in a prefrontal-occipital functional connection and gamma-coherence in an inferior frontal-occipitoparietal connection was not affected by the addition of the second task and only showed elevated coherence under high working memory load. Analysis of coherence as a function of time suggested that the dorsofrontal-occipital beta-connections were relevant to working memory maintenance, while the prefrontal-occipital beta-connection and the inferior frontal-occipitoparietal gamma-connection were involved in top-down control of concurrent visual processing. The fact that increased coherence in the gamma-connection, from low to high working memory load, was negatively correlated with faster reaction time on the perception task supports this interpretation. Together, these results demonstrate that dual-task demands trigger non-linear changes in functional interactions between frontal-executive and occipitoparietal-perceptual cortices
Neuro-cognitive mechanisms of conscious and unconscious visual perception: From a plethora of phenomena to general principles
Psychological and neuroscience approaches have promoted much progress in
elucidating the cognitive and neural mechanisms that underlie phenomenal visual
awareness during the last decades. In this article, we provide an overview of
the latest research investigating important phenomena in conscious and
unconscious vision. We identify general principles to characterize conscious and
unconscious visual perception, which may serve as important building blocks for
a unified model to explain the plethora of findings. We argue that in particular
the integration of principles from both conscious and unconscious vision is
advantageous and provides critical constraints for developing adequate
theoretical models. Based on the principles identified in our review, we outline
essential components of a unified model of conscious and unconscious visual
perception. We propose that awareness refers to consolidated
visual representations, which are accessible to the entire brain and therefore
globally available. However, visual awareness not only depends
on consolidation within the visual system, but is additionally the result of a
post-sensory gating process, which is mediated by higher-level cognitive control
mechanisms. We further propose that amplification of visual representations by
attentional sensitization is not exclusive to the domain of conscious
perception, but also applies to visual stimuli, which remain unconscious.
Conscious and unconscious processing modes are highly interdependent with
influences in both directions. We therefore argue that exactly this
interdependence renders a unified model of conscious and unconscious visual
perception valuable. Computational modeling jointly with focused experimental
research could lead to a better understanding of the plethora of empirical
phenomena in consciousness research
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