251 research outputs found
Ising universality in the two-dimensional Blume-Capel model with quenched random crystal field
Using high-precision Monte-Carlo simulations based on a parallel version of
the Wang-Landau algorithm and finite-size scaling techniques we study the
effect of quenched disorder in the crystal-field coupling of the Blume-Capel
model on the square lattice. We mainly focus on the part of the phase diagram
where the pure model undergoes a continuous transition, known to fall into the
universality class of the pure Ising ferromagnet. A dedicated scaling analysis
reveals concrete evidence in favor of the strong universality hypothesis with
the presence of additional logarithmic corrections in the scaling of the
specific heat. Our results are in agreement with an early real-space
renormalization-group study of the model as well as a very recent numerical
work where quenched randomness was introduced in the energy exchange coupling.
Finally, by properly fine tuning the control parameters of the randomness
distribution we also qualitatively investigate the part of the phase diagram
where the pure model undergoes a first-order phase transition. For this region,
preliminary evidence indicate a smoothening of the transition to second-order
with the presence of strong scaling corrections.Comment: 11 pages, 13 figures, minor correction to references appearing in
Fig. 1, to be published in Phys. Rev.
Universality in the three-dimensional random bond quantum Heisenberg antiferromagnet
The three-dimensional quenched random bond diluted quantum
Heisenberg antiferromagnet is studied on a simple-cubic lattice. Using
extensive stochastic series expansion quantum Monte Carlo simulations, we
perform very long runs for lattice up to . By
employing standard finite-size scaling method, the numerical values of the
N\'eel temperature are determined with high precision as a function of the
coupling ratio . Based on the estimated critical exponents, we find
that the critical behavior of the considered model belongs to the pure
classical Heisenberg universality class.Comment: 8 pages, 7 figure
Electronic and magnetic properties of the monolayer RuCl: A first-principles and Monte Carlo study
Recent experiments revealed that monolayer -RuCl can be obtain by
chemical exfoliation method and exfoliation or restacking of nanosheets can
manipulate the magnetic properties of the materials. In this present paper, the
electronic and magnetic properties of -RuCl monolayer are
investigated by combining first-principles calculations and Monte Carlo
simulations. From first-principles calculations, we found that the spin
configuration FM corresponds to the ground state for -RuCl,
however, the other excited zigzag oriented spin configuration has energy of 5
meV/atom higher than the ground state. Energy band gap has been obtained as
meV using PBE functionals. When spin-orbit coupling effect is taken into
account, corresponding energy gap is determined to be as meV. We also
investigate the effect of Hubbard U energy terms on the electronic band
structure of -RuCl monolayer and revealed band gap increases
approximately linear with increasing U value. Moreover, spin-spin coupling
terms (, , ) have been obtained using first principles
calculations. By benefiting from these terms, Monte Carlo simulations with
single site update Metropolis algorithm have been implemented to elucidate
magnetic properties of the considered system. Thermal variations of
magnetization, susceptibility and also specific heat curves indicate that
monolayer -RuCl exhibits a phase transition between ordered and
disordered phases at the Curie temperature K. We believe that this
study can be utilized to improve two-dimensional magnet materials
Hysteresis features of the transition-metal dichalcogenides VX2 (X = S, Se, and Te)
Very recently, it has been shown that vanadium dichalcogenides (VX2, X = S, Se and Te) monolayers show intrinsic ferromagnetism, and their critical temperatures are nearly to or beyond room temperature. Hence, they would have wide potential applications in next-generation nanoelectronic and spintronic devices. In this work, being inspired by a recent study we systematically perform Monte Carlo simulations based on single-site update Metropolis algorithm to investigate the hysteresis features of VX2 monolayers for a wide range of temperatures up to 600 K. Our simulation results indicate that, both remanence and coercivity values tend to decrease with increasing temperature. Furthermore, it is found that hysteresis curves start to evolve from rectangular at the lower temperature regions to nearly S-shaped with increasing temperature
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Cognitive Flexibility: A Default Mode Perspective
The intra/extradimensional set-shifting task (IED) provides a reliable assessment of cognitive flexibility, the shifting of attention to select behaviorally relevant stimuli in a given context. Impairments in this domain were previously reported in patients with altered neurotransmitter systems such as schizophrenia and Parkinson's disease. Consequently, corticostriatal connections were implicated in the mediation of this function. In addition, parts of the default mode network (DMN), namely the medial prefrontal and posterior cingulate/precuneus cortices, are also being progressively described in association with set-shifting paradigms. Nevertheless, a definitive link between cognitive flexibility and DMN connectivity remains to be established. To this end, we related resting state functional magnetic resonance imaging (fMRI)-based functional connectivity of DMN with IED task performance in a healthy population, measured outside the scanner. The results demonstrated that greater posterior cingulate cortex/precuneus (DMN) connectivity with the ventromedial striatopallidum at rest correlated with fewer total adjusted errors on the IED task. This finding points to a relationship between DMN and basal ganglia connectivity for cognitive flexibility, further highlighting this network's potential role in adaptive human cognition.The Evelyn Trust (RUAG/018) supported this research. Additionally, DV received funding from the Yousef Jameel Academic Program; DKM is funded by the NIHR Cambridge Biomedical Centre (RCZB/004), and an NIHR Senior Investigator Award (RCZB/014), and EAS is supported by the Stephen Erskine Fellowship Queens’ College, Cambridge. We would also like to thank Dr. Guy Williams and Victoria Lupson and the rest of the staff in the Wolfson Brain Imaging Centre (WBIC) at Addenbrooke’s Hospital for their assistance in scanning. Finally, we thank all the participants for their contribution to this studyThis is the author accepted manuscript. The final version is available from Mary Ann Liebert via http://dx.doi.org/10.1089/brain.2015.038
Default Mode Dynamics for Global Functional Integration.
UNLABELLED: The default mode network (DMN) has been traditionally assumed to hinder behavioral performance in externally focused, goal-directed paradigms and to provide no active contribution to human cognition. However, recent evidence suggests greater DMN activity in an array of tasks, especially those that involve self-referential and memory-based processing. Although data that robustly demonstrate a comprehensive functional role for DMN remains relatively scarce, the global workspace framework, which implicates the DMN in global information integration for conscious processing, can potentially provide an explanation for the broad range of higher-order paradigms that report DMN involvement. We used graph theoretical measures to assess the contribution of the DMN to global functional connectivity dynamics in 22 healthy volunteers during an fMRI-based n-back working-memory paradigm with parametric increases in difficulty. Our predominant finding is that brain modularity decreases with greater task demands, thus adapting a more global workspace configuration, in direct relation to increases in reaction times to correct responses. Flexible default mode regions dynamically switch community memberships and display significant changes in their nodal participation coefficient and strength, which may reflect the observed whole-brain changes in functional connectivity architecture. These findings have important implications for our understanding of healthy brain function, as they suggest a central role for the DMN in higher cognitive processing. SIGNIFICANCE STATEMENT: The default mode network (DMN) has been shown to increase its activity during the absence of external stimulation, and hence was historically assumed to disengage during goal-directed tasks. Recent evidence, however, implicates the DMN in self-referential and memory-based processing. We provide robust evidence for this network's active contribution to working memory by revealing dynamic reconfiguration in its interactions with other networks and offer an explanation within the global workspace theoretical framework. These promising findings may help redefine our understanding of the exact DMN role in human cognition.This research was supported by the Evelyn Trust (RUAG/018). In addition, DV received funding from the Yousef Jameel Academic Program; DKM is supported by the NIHR Cambridge Biomedical Centre (RCZB/004), and an NIHR Senior Investigator Award (RCZB/014), and EAS is funded by the Stephen Erskine Fellowship Queens’ College Cambridge.This is the author accepted manuscript. The final version is available from Society for Neuroscience via http://dx.doi.org/10.1523/JNEUROSCI.2135-15.201
Distinct patterns of thought mediate the link between brain functional connectomes and well-being
Ongoing thought patterns constitute important aspects of both healthy and abnormal human cognition. However, the neural mechanisms behind these daily experiences and their contribution to well-being remain a matter of debate. Here, using resting-state fMRI and retrospective thought sampling in a large neurotypical cohort (n = 211), we identified two distinct patterns of thought, broadly describing the participants’ current concerns and future plans, that significantly explained variability in the individual functional connectomes. Consistent with the view that ongoing thoughts are an emergent property of multiple neural systems, network-based analysis highlighted the central importance of both unimodal and transmodal cortices in the generation of these experiences. Importantly, while state-dependent current concerns predicted better psychological health, mediating the effect of functional connectomes, trait-level future plans were related to better social health, yet with no mediatory influence. Collectively, we show that ongoing thoughts can influence the link between brain physiology and well-being
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