629 research outputs found
Superconducting junctions from non-superconducting doped CuO layers
The theoretical approach proposed recently for description of redistribution
of electronic charge in multilayered selectively doped systems is modified for
a system with finite number of layers. A special attention is payed to the case
of a finite heterostructure made of copper-oxide layers which are all
non-superconducting (including non-conducting) because of doping levels being
beyond the well-known characteristic interval for superconductivity. Specific
finite structures and doping configurations are proposed to obtain atomically
thin superconducting heterojunctions of different compositions.Comment: 4 pages, 3 figures, two bibliography references were update
Proposal for an optical laser producing light at half the Josephson frequency
We describe a superconducting device capable of producing laser light in the
visible range at half of the Josephson generation frequency with the optical
phase of the light locked to the superconducting phase difference. It consists
of two single-level quantum dots embedded into a p-n semiconducting
heterostructure and surrounded by a cavity supporting a resonant optical mode.
We study decoherence and spontaneous switching in the device.Comment: 4+3 pages, 3+1 figure
Remarks on supersymmetry of quantum systems with position-dependent effective masses
We apply the supersymmetry approach to one-dimensional quantum systems with
spatially-dependent mass, by including their ordering ambiguities dependence.
In this way we extend the results recently reported in the literature.
Furthermore, we point out a connection between these systems and others with
constant masses. This is done through convenient transformations in the
coordinates and wavefunctions.Comment: 8 pages, 1 figur
Diffusing opinions in bounded confidence processes
We study the effects of diffusing opinions on the Deffuant et al. model for
continuous opinion dynamics. Individuals are given the opportunity to change
their opinion, with a given probability, to a randomly selected opinion inside
an interval centered around the present opinion. We show that diffusion induces
an order-disorder transition. In the disordered state the opinion distribution
tends to be uniform, while for the ordered state a set of well defined opinion
clusters are formed, although with some opinion spread inside them. If the
diffusion jumps are not large, clusters coalesce, so that weak diffusion favors
opinion consensus. A master equation for the process described above is
presented. We find that the master equation and the Monte-Carlo simulations do
not always agree due to finite-size induced fluctuations. Using a linear
stability analysis we can derive approximate conditions for the transition
between opinion clusters and the disordered state. The linear stability
analysis is compared with Monte Carlo simulations. Novel interesting phenomena
are analyzed
Evolution and extinction dynamics in rugged fitness landscapes
Macroevolution is considered as a problem of stochastic dynamics in a system
with many competing agents. Evolutionary events (speciations and extinctions)
are triggered by fitness records found by random exploration of the agents'
fitness landscapes. As a consequence, the average fitness in the system
increases logarithmically with time, while the rate of extinction steadily
decreases. This dynamics is studied by numerical simulations and, in a simpler
mean field version, analytically. We also study the effect of externally added
`mass' extinctions. The predictions for various quantities of paleontological
interest (life-time distributions, distribution of event sizes and behavior of
the rate of extinction) are robust and in good agreement with available data.
Brief version of parts of this work have been published as Letters. (PRL 75,
2055, (1995) and PRL, 79, 1413, (1997))Comment: 30 pages 9 figures LaTe
Local Manipulation of Nuclear Spin in a Semiconductor Quantum Well
The shaping of nuclear spin polarization profiles and the induction of
nuclear resonances are demonstrated within a parabolic quantum well using an
externally applied gate voltage. Voltage control of the electron and hole wave
functions results in nanometer-scale sheets of polarized nuclei positioned
along the growth direction of the well. RF voltages across the gates induce
resonant spin transitions of selected isotopes. This depolarizing effect
depends strongly on the separation of electrons and holes, suggesting that a
highly localized mechanism accounts for the observed behavior.Comment: 18 pages, 4 figure
Treating some solid state problems with the Dirac equation
The ambiguity involved in the definition of effective-mass Hamiltonians for
nonrelativistic models is resolved using the Dirac equation. The multistep
approximation is extended for relativistic cases allowing the treatment of
arbitrary potential and effective-mass profiles without ordering problems. On
the other hand, if the Schrodinger equation is supposed to be used, our
relativistic approach demonstrate that both results are coincidents if the
BenDaniel and Duke prescription for the kinetic-energy operator is implemented.
Applications for semiconductor heterostructures are discussed.Comment: 06 pages, 5 figure
A squeeze-like operator approach to position-dependent mass in quantum mechanics
We provide a squeeze-like transformation that allows one to remove a position
dependent mass from the Hamiltonian. Methods to solve the Schr\"{o}dinger
equation may then be applied to find the respective eigenvalues and
eigenfunctions. As an example, we consider a position-dependent-mass that leads
to the integrable Morse potential and therefore to well-known solutions
Love kills: Simulations in Penna Ageing Model
The standard Penna ageing model with sexual reproduction is enlarged by
adding additional bit-strings for love: Marriage happens only if the male love
strings are sufficiently different from the female ones. We simulate at what
level of required difference the population dies out.Comment: 14 pages, including numerous figure
Closed form solution for a double quantum well using Gr\"obner basis
Analytical expressions for spectrum, eigenfunctions and dipole matrix
elements of a square double quantum well (DQW) are presented for a general case
when the potential in different regions of the DQW has different heights and
effective masses are different. This was achieved by Gr\"obner basis algorithm
which allows to disentangle the resulting coupled polynomials without
explicitly solving the transcendental eigenvalue equation.Comment: 4 figures, Mathematica full calculation noteboo
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