139,342 research outputs found
Current Carrying States in a Random Magnetic Field
We report results of a numerical study of noninteracting electrons moving in
two dimensions, in the presence of a random potential and a random magnetic
field for a sequence of finite sizes, using topological properties of the wave
functions to identify extended states. Our results are consistent with the
existence of a second order localization-delocalization transition driven by
the random potential. The critical randomness strength and localization length
exponent are estimated via a finite size scaling analysis.Comment: 4 pages, 7 eps figure
Generation of Large Moments in a Spin-1 Chain with Random Antiferromagnetic Couplings
We study the spin-1 chain with nearest neighbor couplings that are
rotationally invariant, but include both Heisenberg and biquadratic exchange,
with random strengths. We demonstrate, using perturbative renormalization group
methods as well as exact diagonalization of clusters, that the system generates
ferromagnetic couplings under certain circumstances even when all the bare
couplings are antiferromagnetic. This disorder induced instability leads to
formation of large magnetic moments at low temperatures, and is a purely
quantum mechanical effect that does not have a classical counterpart. The
physical origin of this instability, as well as its consequences, are
discussed.Comment: 4 pages, 4 eps figure
A mathematical model of the effect of a predator on species diversity
Mathematical model determines reaction between new predator and microbe competitor when the competitor is the predator's sole nutrient resource. The model utilizes differential equations to describe the interactions with the specific growth rates, and analyzes these growth rates as they are affected by population density and nutrient concentration
Edge and bulk merons in double quantum dots with spontaneous interlayer phase coherence
We have investigated nucleation of merons in double quantum dots when a
lateral distortion with a reflection symmetry is present in the confinement
potential. We find that merons can nucleate both inside and at the edge of the
dots. In addition to these merons, our results show that electron density
modulations can be also present inside the dots. An edge meron appears to have
approximately a half integer winding number.Comment: 5 pages, 4 figures, Proceedings of 17th International Conference on
High Magnetic Fields in Semiconductor Physic
NASA Langley Research Center HBCU/OMU program: 1990 student support survey
The results of a survey of students who are receiving support through the Historically Black Colleges and Universities and Other Minority Universities are given. Information is given on the race, sex, ethnic distribution, grade point average distribution, and target degree distribution
Statistical distribution of time to crack initiation and initial crack size using service data
Crack growth inspection data gathered during the service life of the C-130 Hercules airplane were used in conjunction with a crack propagation rule to estimate the distribution of crack initiation times and of initial crack sizes. A Bayesian statistical approach was used to calculate the fraction of undetected initiation times as a function of the inspection time and the reliability of the inspection procedure used
Emergent Geometry and Quantum Gravity
We explain how quantum gravity can be defined by quantizing spacetime itself.
A pinpoint is that the gravitational constant G = L_P^2 whose physical
dimension is of (length)^2 in natural unit introduces a symplectic structure of
spacetime which causes a noncommutative spacetime at the Planck scale L_P. The
symplectic structure of spacetime M leads to an isomorphism between symplectic
geometry (M, \omega) and Riemannian geometry (M, g) where the deformations of
symplectic structure \omega in terms of electromagnetic fields F=dA are
transformed into those of Riemannian metric g. This approach for quantum
gravity allows a background independent formulation where spacetime as well as
matter fields is equally emergent from a universal vacuum of quantum gravity
which is thus dubbed as the quantum equivalence principle.Comment: Invited Review for Mod. Phys. Lett. A, 17 page
Degenerate states of narrow semiconductor rings in the presence of spin orbit coupling: Role of time-reversal and large gauge transformations
The electron Hamiltonian of narrow semiconductor rings with the Rashba and
Dresselhaus spin orbit terms is invariant under time-reversal operation
followed by a large gauge transformation. We find that all the eigenstates are
doubly degenerate when integer or half-integer quantum fluxes thread the
quantum ring. The wavefunctions of a degenerate pair are related to each other
by the symmetry operation. These results are valid even in the presence of a
disorder potential. When the Zeeman term is present only some of these
degenerate levels anticross
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