14,768 research outputs found
A critical analysis of vacancy-induced magnetism in mono and bilayer graphene
The observation of intrinsic magnetic order in graphene and graphene-based
materials relies on the formation of magnetic moments and a sufficiently strong
mutual interaction. Vacancies are arguably considered the primary source of
magnetic moments. Here we present an in-depth density functional theory study
of the spin-resolved electronic structure of (monoatomic) vacancies in graphene
and bilayer graphene. We use two different methodologies: supercell
calculations with the SIESTA code and cluster-embedded calculations with the
ALACANT package. Our results are conclusive: The vacancy-induced extended
magnetic moments, which present long-range interactions and are capable of
magnetic ordering, vanish at any experimentally relevant vacancy concentration.
This holds for -bond passivated and un-passivated reconstructed
vacancies, although, for the un-passivated ones, the disappearance of the
magnetic moments is accompanied by a very large magnetic susceptibility. Only
for the unlikely case of a full -bond passivation, preventing the
reconstruction of the vacancy, a full value of 1 for the extended
magnetic moment is recovered for both mono and bilayer cases. Our results put
on hold claims of vacancy-induced ferromagnetic or antiferromagnetic order in
graphene-based systems, while still leaving the door open to -type
paramagnetism.Comment: Submitted to Phys. Rev B, 9 page
Vortex density fluctuations in quantum turbulence
We compute the frequency spectrum of turbulent superfluid vortex density
fluctuations and obtain the same Kolmogorov scaling which has been observed in
a recent experiment in Helium-4. We show that the scaling can be interpreted in
terms of the spectrum of reconnecting material lines. The calculation is
performed using a vortex tree algorithm which considerably speeds up the
evaluation of Biot-Savart integrals.Comment: 7 Pages, 7 figure
A construction of integer-valued polynomials with prescribed sets of lengths of factorizations
For an arbitrary finite set S of natural numbers greater 1, we construct an
integer-valued polynomial f, whose set of lengths in Int(Z) is S. The set of
lengths of f is the set of all natural numbers n, such that f has a
factorization as a product of n irreducibles in Int(Z)={g in Q[x] | g(Z)
contained in Z}.Comment: To appear in Monatshefte f\"ur Mathematik; 11 page
Neutral and ionic dopants in helium clusters: interaction forces for the and
The potential energy surface (PES) describing the interactions between
and and an extensive
study of the energies and structures of a set of small clusters,
, have been presented by us in a previous series of
publications [1-3]. In the present work we want to extend the same analysis to
the case of the excited and of the
ionized Li moiety. We thus show here calculated
interaction potentials for the two title systems and the corresponding fitting
of the computed points. For both surfaces the MP4 method with cc-pV5Z basis
sets has been used to generate an extensive range of radial/angular coordinates
of the two dimensional PES's which describe rigid rotor molecular dopants
interacting with one He partner
Kinetic energy cascades in quasi-geostrophic convection in a spherical shell
We consider triadic nonlinear interaction in the Navier-Stokes equation for
quasi-geostrophic convection in a spherical shell. This approach helps
understanding the origin of kinetic energy transport in the system and the
particular scheme of mode interaction, as well as the locality of the energy
transfer. The peculiarity of convection in the sphere, concerned with
excitation of Rossby waves, is considered. The obtained results are compared
with our previous study in Cartesian geometry
Multifractality of the Feigenbaum attractor and fractional derivatives
It is shown that fractional derivatives of the (integrated) invariant measure
of the Feigenbaum map at the onset of chaos have power-law tails in their
cumulative distributions, whose exponents can be related to the spectrum of
singularities . This is a new way of characterizing multifractality
in dynamical systems, so far applied only to multifractal random functions
(Frisch and Matsumoto (J. Stat. Phys. 108:1181, 2002)). The relation between
the thermodynamic approach (Vul, Sinai and Khanin (Russian Math. Surveys 39:1,
1984)) and that based on singularities of the invariant measures is also
examined. The theory for fractional derivatives is developed from a heuristic
point view and tested by very accurate simulations.Comment: 20 pages, 5 figures, J.Stat.Phys. in pres
Non-equilibrium temperatures in steady-state systems with conserved energy
We study a class of non-equilibrium lattice models describing local
redistributions of a globally conserved quantity, which is interpreted as an
energy. A particular subclass can be solved exactly, allowing to define a
statistical temperature T_{th} along the same lines as in the equilibrium
microcanonical ensemble. We compute the response function and find that when
the fluctuation-dissipation relation is linear, the slope T_{FD}^{-1} of this
relation differs from the inverse temperature T_{th}^{-1}. We argue that T_{th}
is physically more relevant than T_{FD}, since in the steady-state regime, it
takes equal values in two subsystems of a large isolated system. Finally, a
numerical renormalization group procedure suggests that all models within the
class behave similarly at a coarse-grained level, leading to a new parameter
which describes the deviation from equilibrium. Quantitative predictions
concerning this parameter are obtained within a mean-field framework.Comment: 16 pages, 2 figures, submitted to Phys. Rev.
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