4,019 research outputs found
Stars creating a gravitational repulsion
In the framework of the Theory of General Relativity, models of stars with an
unusual equation of state where is the mass density
and is the pressure, are constructed. These objects create outside
themselves the forces of gravitational repulsion. The equilibrium of such stars
is ensured by a non-standard balance of forces. Negative mass density, acting
gravitationally on itself, creates an acceleration of the negative mass,
directed from the center. Therefore in the absence of pressure such an object
tends to expand. At the same time, the positive pressure, which falls just like
in ordinary stars from the center to the surface, creates a force directed from
the center. This force acts on the negative mass density, which causes
acceleration directed the opposite of the acting force, that is to the center
of the star. This acceleration balances the gravitational repulsion produced by
the negative mass. Thus, in our models gravity and pressure change roles: the
negative mass tends to create a gravitational repulsion, while the gradient of
the pressure acting on the negative mass tends to compress the star. In this
paper, we construct several models of such a star with various equations of
state.Comment: 6 pages, 4 figure
Numbers of donors and acceptors from transport measurements in graphene
A method is suggested to separately determine the surface density of
positively and negatively charged impurities that limit the mobility in a
graphene monolayer. The method is based on the exact result for the transport
cross-section, according to which the massless carriers are scattered more
strongly when they are attracted to a charged impurity than when they are
repelled from it.Comment: 3 pages, 1 figur
Transverse field effect in graphene ribbons
It is shown that a graphene ribbon, a ballistic strip of carbon monolayer,
may serve as a quantum wire whose electronic properties can be continuously and
reversibly controlled by an externally applied transverse voltage. The electron
bands of armchair-edge ribbons undergo dramatic transformations: The Fermi
surface fractures, Fermi velocity and effective mass change sign, and
excitation gaps are reduced by the transverse field. These effects are manifest
in the conductance plateaus, van Hove singularities, thermopower, and activated
transport. The control over one-dimensional bands may help enhance effects of
electron correlations, and be utilized in device applications.Comment: 4 pages, 3 figure
Distribution of averages in a correlated Gaussian medium as a tool for the estimation of the cluster distribution on size
Calculation of the distribution of the average value of a Gaussian random
field in a finite domain is carried out for different cases. The results of the
calculation demonstrate a strong dependence of the width of the distribution on
the spatial correlations of the field. Comparison with the simulation results
for the distribution of the size of the cluster indicates that the distribution
of an average field could serve as a useful tool for the estimation of the
asymptotic behavior of the distribution of the size of the clusters for "deep"
clusters where value of the field on each site is much greater than the rms
disorder.Comment: 15 pages, 6 figures, RevTe
Oscillation of linear ordinary differential equations: on a theorem by A. Grigoriev
We give a simplified proof and an improvement of a recent theorem by A.
Grigoriev, placing an upper bound for the number of roots of linear
combinations of solutions to systems of linear equations with polynomial or
rational coefficients.Comment: 16 page
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