222 research outputs found
Event related potentials reveal that increasing perceptual load leads to increased responses for target stimuli and decreased responses for irrelevant stimuli
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Ab-initio study of gap opening and screening effects in gated bilayer graphene
The electronic properties of doped bilayer graphene in presence of bottom and
top gates have been studied and characterized by means of Density Functional
Theory calculations. Varying independently the bottom and top gates it is
possible to control separately the total doping charge on the sample, and the
average external electric field acting on the bilayer. We show that, at fixed
doping level, the band gap at the K point in the Brillouin zone depends
linearly on the average electric field, whereas the corresponding
proportionality coefficient has a non-monotonic dependence on doping. We find
that the DFT-calculated band gap at K, for small doping levels, is roughly half
of the band gap obtained with standard Tight Binding approach. We show that
this discrepancy arises from an underestimate, in the TB model, of the
screening of the system to the external electric field. In particular, on the
basis of our DFT results we observe that, when the bilayer graphene is in
presence of an external electric field, both an interlayer and an intralayer
screening occur. Only the interlayer screening is included in TB calculations,
while both screenings are fundamental for the description of the band gap
opening. We finally provide a general scheme to obtain the full band structure
of the gated bilayer graphene, for an arbitrary value of the external electric
field and of the doping.Comment: 15 pages 20 figures, accepted for publication in Physical Review
Giant non-adiabatic effects in layer metals: Raman spectra of intercalated graphite explained
The occurrence of non-adiabatic effects in the vibrational properties of
metals have been predicted since the 60's, but hardly confirmed experimentally.
We report the first fully \emph{ab initio} calculations of non-adiabatic
frequencies of a number of layer and conventional metals. We suggest that
non-adiabatic effects can be a feature of the vibrational Raman spectra of any
bulk metal, and show that they are spectacularly large (up to 30% of the phonon
frequencies) in the case of layer metals, such as superconducting ,
and other graphite intercalated compounds. We develop a framework
capable to estimate the electron momentum-relaxation time of a given system,
and thus its degree of non-adiabaticity, in terms of the experimentally
observed frequencies and linewidths.Comment: 4 pages, 3 figures, 1 tabl
Structure and stability of graphene nanoribbons in oxygen, carbon dioxide, water, and ammonia
We determine, by means of density functional theory, the stability and the
structure of graphene nanoribbon (GNR) edges in presence of molecules such as
oxygen, water, ammonia, and carbon dioxide. As in the case of
hydrogen-terminated nanoribbons, we find that the most stable armchair and
zigzag configurations are characterized by a non-metallic/non-magnetic nature,
and are compatible with Clar's sextet rules, well known in organic chemistry.
In particular, we predict that, at thermodynamic equilibrium, neutral GNRs in
oxygen-rich atmosphere should preferentially be along the armchair direction,
while water-saturated GNRs should present zigzag edges. Our results promise to
be particularly useful to GNRs synthesis, since the most recent and advanced
experimental routes are most effective in water and/or ammonia-containing
solutions.Comment: accepted for publication in PR
Structure, Stability, Edge States and Aromaticity of Graphene Ribbons
We determine the stability, the geometry, the electronic and magnetic
structure of hydrogen-terminated graphene-nanoribbons edges as a function of
the hydrogen content of the environment by means of density functional theory.
Antiferromagnetic zigzag ribbons are stable only at extremely-low ultra-vacuum
pressures. Under more standard conditions, the most stable structures are the
mono- and di-hydrogenated armchair edges and a zigzag edge reconstruction with
one di- and two mono-hydrogenated sites. At high hydrogen-concentration
``bulk'' graphene is not stable and spontaneously breaks to form ribbons, in
analogy to the spontaneous breaking of graphene into small-width nanoribbons
observed experimentally in solution. The stability and the existence of exotic
edge electronic-states and/or magnetism is rationalized in terms of simple
concepts from organic chemistry (Clar's rule)Comment: 4 pages, 3 figures, accepted for publication by Physical Review
Letter
Modeling the overalternating bias with an asymmetric entropy measure
Psychological research has found that human perception of randomness is biased. In particular, people consistently show the overalternating bias: they rate binary sequences of symbols (such as Heads and Tails in coin flipping) with an excess of alternation as more random than prescribed by the normative criteria of Shannon's entropy. Within data mining for medical applications, Marcellin proposed an asymmetric measure of entropy that can be ideal to account for such bias and to quantify subjective randomness. We fitted Marcellin's entropy and Renyi's entropy (a generalized form of uncertainty measure comprising many different kinds of entropies) to experimental data found in the literature with the Differential Evolution algorithm. We observed a better fit for Marcellin's entropy compared to Renyi's entropy. The fitted asymmetric entropy measure also showed good predictive properties when applied to different datasets of randomness-related tasks. We concluded that Marcellin's entropy can be a parsimonious and effective measure of subjective randomness that can be useful in psychological research about randomness perception
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