1,740 research outputs found
A comparative analysis of leaving home in the United States, the Netherlands and West Germany
We investigate how leaving the parental home differs between three countries with different welfare-state and housing systems: the USA, the Netherlands and West Germany. Using longitudinal survey data, we examine the transitions of leaving home to live with and without a partner. We find that, much more than in the European countries, union formation has become separated from leaving home in the USA. We also find a different impact of level of education and employment status on leaving-home patterns in the European countries with their social-welfare state system than in the US system in which market forces prevail. The differences are not just related to welfare-state systems but also to the sizes of the countries and the geographical dispersion of jobs and educational opportunities.Germany, household, leaving the parental home, Netherlands, union formation, USA
Sputter Deposition of Semiconductor Superlattices for Thermoelectric Applications
Theoretical dramatic improvement of the thermoelectric properties of materials by using quantum confinement in novel semiconductor nanostructures has lead to considerable interest in the thermoelectric community. Therefore, we are exploring the critical materials issues for fabrication of quantum confined structures by magnetron sputtering in the lead telluride and bismuth telluride families of materials. We have synthesized modulated structures from thermoelectric materials with bilayer periods of as little as 3.2 nm and shown that they are stable at deposition temperatures high enough to grow quality films. Issues critical to high quality film growth have been investigated such as nucleation and growth conditions and their effect on crystal orientation and growth morphology. These investigations show that nucleating the film at a temperature below the growth temperature of optimum electronic properties produces high quality films. Our work with sputter deposition, which is inherently a high rate deposition process, builds the technological base necessary to develop economical production of these advanced materials. High deposition rate is critical since, even if efficiencies comparable with CFC based refrigeration systems can be achieved, large quantities of quantum confined materials will be necessary for cost-competitive uses
On timelike Compton scattering at medium and high energies
We emphasize the complementarity of timelike and spacelike studies of deep
exclusive processes, taking as an example the case of timelike Compton
Scattering (TCS) i.e. the exclusive photoproduction of a lepton pair with large
invariant mass, vs deeply virtual Compton scattering (DVCS) i.e. the exclusive
leptoproduction of a real photon. Both amplitudes factorize with the same
generalized parton distributions (GPDs) as their soft parts and coefficient
functions which differ significantly at next to leading order in alpha_s. We
also stress that data on TCS at very high energy should be available soon
thanks to the study of ultraperipheral collisions at the LHC, opening a window
on quark and gluon GPDs at very small skewness.Comment: 8 pages, Presented at the workshop "30 years of strong interactions",
Spa, Belgium, 6-8 April 201
Solution of the Schr\"odinger Equation for Quantum Dot Lattices with Coulomb Interaction between the Dots
The Schr\"odinger equation for quantum dot lattices with non-cubic,
non-Bravais lattices built up from elliptical dots is investigated. The Coulomb
interaction between the dots is considered in dipole approximation. Then only
the center of mass (c.m.) coordinates of different dots couple with each other.
This c.m. subsystem can be solved exactly and provides magneto- phonon like
collective excitations. The inter-dot interaction is involved only through a
single interaction parameter. The relative coordinates of individual dots form
decoupled subsystems giving rise to intra-dot excitations. As an example, the
latter are calculated exactly for two-electron dots.
Emphasis is layed on qualitative effects like: i) Influence of the magnetic
field on the lattice instability due to inter-dot interaction, ii) Closing of
the gap between the lower and the upper c.m. mode at B=0 for elliptical dots
due to dot interaction, and iii) Kinks in the single dot excitation energies
(versus magnetic field) due to change of ground state angular momentum. It is
shown that for obtaining striking qualitative effects one should go beyond
simple cubic lattices with spherical dots. We also prove a more general version
of the Kohn Theorem for quantum dot lattices. It is shown that for observing
effects of electron- electron interaction between the dots in FIR spectra
(breaking Kohn's Theorem) one has to consider dot lattices with at least two
dot species with different confinement tensors.Comment: 11 figures included as ps-file
PT-Symmetry Quantum Electrodynamics--PTQED
The construction of -symmetric quantum electrodynamics is
reviewed. In particular, the massless version of the theory in 1+1 dimensions
(the Schwinger model) is solved. Difficulties with unitarity of the -matrix
are discussed.Comment: 11 pages, 1 figure, contributed to Proceedings of 6th International
Workshop on Pseudo-Hermitian Hamiltonians in Quantum Physic
Molecular Dynamics Study of Bamboo-like Carbon Nanotube Nucleation
MD simulations based on an empirical potential energy surface were used to
study the nucleation of bamboo-like carbon nanotubes (BCNTs). The simulations
reveal that inner walls of the bamboo structure start to nucleate at the
junction between the outer nanotube wall and the catalyst particle. In
agreement with experimental results, the simulations show that BCNTs nucleate
at higher dissolved carbon concentrations (i.e., feedstock pressures) than
those where non-bamboolike carbon nanotubes are nucleated
Roto-vibrational spectrum and Wigner crystallization in two-electron parabolic quantum dots
We provide a quantitative determination of the crystallization onset for two
electrons in a parabolic two-dimensional confinement. This system is shown to
be well described by a roto-vibrational model, Wigner crystallization occurring
when the rotational motion gets decoupled from the vibrational one. The Wigner
molecule thus formed is characterized by its moment of inertia and by the
corresponding sequence of rotational excited states. The role of a vertical
magnetic field is also considered. Additional support to the analysis is given
by the Hartree-Fock phase diagram for the ground state and by the random-phase
approximation for the moment of inertia and vibron excitations.Comment: 10 pages, 8 figures, replaced by the published versio
Electron spin coherence in semiconductors: Considerations for a spin-based solid state quantum computer architecture
We theoretically consider coherence times for spins in two quantum computer
architectures, where the qubit is the spin of an electron bound to a P donor
impurity in Si or within a GaAs quantum dot. We show that low temperature
decoherence is dominated by spin-spin interactions, through spectral diffusion
and dipolar flip-flop mechanisms. These contributions lead to 1-100 s
calculated spin coherence times for a wide range of parameters, much higher
than former estimates based on measurements.Comment: Role of the dipolar interaction clarified; Included discussion on the
approximations employed in the spectral diffusion calculation. Final version
to appear in Phys. Rev.
Phase separating binary fluids under oscillatory shear
We apply lattice Boltzmann methods to study the segregation of binary fluid
mixtures under oscillatory shear flow in two dimensions. The algorithm allows
to simulate systems whose dynamics is described by the Navier-Stokes and the
convection-diffusion equations. The interplay between several time scales
produces a rich and complex phenomenology. We investigate the effects of
different oscillation frequencies and viscosities on the morphology of the
phase separating domains. We find that at high frequencies the evolution is
almost isotropic with growth exponents 2/3 and 1/3 in the inertial (low
viscosity) and diffusive (high viscosity) regimes, respectively. When the
period of the applied shear flow becomes of the same order of the relaxation
time of the shear velocity profile, anisotropic effects are clearly
observable. In correspondence with non-linear patterns for the velocity
profiles, we find configurations where lamellar order close to the walls
coexists with isotropic domains in the middle of the system. For particular
values of frequency and viscosity it can also happen that the convective
effects induced by the oscillations cause an interruption or a slowing of the
segregation process, as found in some experiments. Finally, at very low
frequencies, the morphology of domains is characterized by lamellar order
everywhere in the system resembling what happens in the case with steady shear.Comment: 1 table and 12 figures in .gif forma
Delocalization in harmonic chains with long-range correlated random masses
We study the nature of collective excitations in harmonic chains with masses
exhibiting long-range correlated disorder with power spectrum proportional to
, where is the wave-vector of the modulations on the random
masses landscape. Using a transfer matrix method and exact diagonalization, we
compute the localization length and participation ratio of eigenmodes within
the band of allowed energies. We find extended vibrational modes in the
low-energy region for . In order to study the time evolution of an
initially localized energy input, we calculate the second moment of
the energy spatial distribution. We show that , besides being dependent
of the specific initial excitation and exhibiting an anomalous diffusion for
weakly correlated disorder, assumes a ballistic spread in the regime
due to the presence of extended vibrational modes.Comment: 6 pages, 9 figure
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