14,695 research outputs found
Nongaussian fluctuations arising from finite populations: Exact results for the evolutionary Moran process
The appropriate description of fluctuations within the framework of
evolutionary game theory is a fundamental unsolved problem in the case of
finite populations. The Moran process recently introduced into this context
[Nowak et al., Nature (London) 428, 646 (2004)] defines a promising standard
model of evolutionary game theory in finite populations for which analytical
results are accessible. In this paper, we derive the stationary distribution of
the Moran process population dynamics for arbitrary games for the
finite size case. We show that a nonvanishing background fitness can be
transformed to the vanishing case by rescaling the payoff matrix. In contrast
to the common approach to mimic finite-size fluctuations by Gaussian
distributed noise, the finite size fluctuations can deviate significantly from
a Gaussian distribution.Comment: 4 pages (2 figs). Published in Physical Review E (Rapid
Communications
Vortices in fermion droplets with repulsive dipole-dipole interactions
Vortices are found in a fermion system with repulsive dipole-dipole
interactions, trapped by a rotating quasi-two-dimensional harmonic oscillator
potential. Such systems have much in common with electrons in quantum dots,
where rotation is induced via an external magnetic field. In contrast to the
Coulomb interactions between electrons, the (externally tunable) anisotropy of
the dipole-dipole interaction breaks the rotational symmetry of the
Hamiltonian. This may cause the otherwise rotationally symmetric exact
wavefunction to reveal its internal structure more directly.Comment: 5 pages, 5 figure
Conserved Matter Superenergy Currents for Orthogonally Transitive Abelian G2 Isometry Groups
In a previous paper we showed that the electromagnetic superenergy tensor,
the Chevreton tensor, gives rise to a conserved current when there is a
hypersurface orthogonal Killing vector present. In addition, the current is
proportional to the Killing vector. The aim of this paper is to extend this
result to the case when we have a two-parameter Abelian isometry group that
acts orthogonally transitive on non-null surfaces. It is shown that for
four-dimensional Einstein-Maxwell theory with a source-free electromagnetic
field, the corresponding superenergy currents lie in the orbits of the group
and are conserved. A similar result is also shown to hold for the trace of the
Chevreton tensor and for the Bach tensor, and also in Einstein-Klein-Gordon
theory for the superenergy of the scalar field. This links up well with the
fact that the Bel tensor has these properties and the possibility of
constructing conserved mixed currents between the gravitational field and the
matter fields.Comment: 15 page
Three-Omega Thermal-Conductivity Measurements with Curved Heater Geometries
The three-omega method, a powerful technique to measure the thermal
conductivity of nanometer-thick films and the interfaces between them, has
historically employed straight conductive wires to act as both heaters and
thermometers. When investigating stochastically prepared samples such as
two-dimensional materials and nanomembranes, residue and excess material can
make it difficult to fit the required millimeter-long straight wire on the
sample surface. There are currently no available criteria for how diverting
three-omega heater wires around obstacles affects the validity of the thermal
measurement. In this Letter, we quantify the effect of wire curvature by
performing three-omega experiments with a wide range of frequencies using both
curved and straight heater geometries on SiO/Si samples. When the heating
wire is curved, we find that the measured Si substrate thermal conductivity
changes by only 0.2%. Similarly, we find that wire curvature has no significant
effect on the determination of the thermal resistance of a 65 nm SiO
layer, even for the sharpest corners considered here, for which the largest
measured ratio of the thermal penetration depth of the applied thermal wave to
radius of curvature of the heating wire is 4.3. This result provides useful
design criteria for three-omega experiments by setting a lower bound for the
maximum ratio of thermal penetration depth to wire radius of curvature.Comment: 4 pages, 3 figure
Fe/V and Fe/Co (001) superlattices: growth, anisotropy, magnetisation and magnetoresistance
Some physical properties of bcc Fe/V and Fe/Co (001) superlattices are
reviewed. The dependence of the magnetic anisotropy on the in-plane strain
introduced by the lattice mismatch between Fe and V is measured and compared to
a theoretical derivation. The dependence of the magnetic anisotropy (and
saturation magnetisation) on the layer thickness ratio Fe/Co is measured and a
value for the anisotropy of bcc Co is derived from extrapolation. The
interlayer exchange coupling of Fe/V superlattices is studied as a function of
the layer thickness V (constant Fe thickness) and layer thickness of Fe
(constant V thickness). A region of antiferromagnetic coupling and GMR is found
for V thicknesses 12-14 monolayers. However, surprisingly, a 'cutoff' of the
antiferromagnetic coupling and GMR is found when the iron layer thickness
exceeds about 10 monolayers.Comment: Proceedings of the International Symposium on Advanced Magnetic
Materials (ISAMM'02), October 2-4, 2002, Halong Bay, Vietnam. REVTeX style; 4
pages, 5 figure
Signatures of Valley Kondo Effect in Si/SiGe Quantum Dots
We report measurements consistent with the valley Kondo effect in Si/SiGe
quantum dots, evidenced by peaks in the conductance versus source-drain voltage
that show strong temperature dependence. The Kondo peaks show unusual behavior
in a magnetic field that we interpret as arising from the valley degree of
freedom. The interplay of valley and Zeeman splittings is suggested by the
presence of side peaks, revealing a zero-field valley splitting between 0.28 to
0.34 meV. A zero-bias conductance peak for non-zero magnetic field, a
phenomenon consistent with valley non- conservation in tunneling, is observed
in two samples.Comment: 16 pages, 7 figure
Microscopic origin of Heisenberg and non-Heisenberg exchange interactions in ferromagnetic bcc Fe
By means of first principles calculations we investigate the nature of
exchange coupling in ferromagnetic bcc Fe on a microscopic level. Analyzing the
basic electronic structure reveals a drastic difference between the
orbitals of and symmetries. The latter ones define the shape of
the Fermi surface, while the former ones form weakly-interacting impurity
levels. We demonstrate that, as a result of this, in Fe the orbitals
participate in exchange interactions, which are only weakly dependent on the
configuration of the spin moments and thus can be classified as
Heisenberg-like. These couplings are shown to be driven by Fermi surface
nesting. In contrast, for the states the Heisenberg picture breaks down,
since the corresponding contribution to the exchange interactions is shown to
strongly depend on the reference state they are extracted from. Our analysis of
the nearest-neighbour coupling indicates that the interactions among
states are mainly proportional to the corresponding hopping integral and thus
can be attributed to be of double-exchange origin.Comment: 5 pages, 4 figure
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