10,874 research outputs found
Precision digital solar aspect sensor
The development of a digital solar aspect sensor with a resolution of approximately 14 arc-seconds is discussed. An interpolation technique was used to generate the fine angle measurements. The sensor and its mode of operation are described. The electronic and mechanical design of the sensor were completed, and two flight units, one for the OAO 4 and the other for determining the attitude of a spinning spacecraft, are being fabricated
A simple model illustrating the impossibility of measuring off-shell effects
We consider a simple model and make use of field transformations in
Lagrangian field theories to illustrate the impossibility of measuring
off-shell effects in nucleon-nucleon bremsstrahlung and related processes.Comment: 3 pages, Latex2e, uses espcrc1.sty, contribution presented at the
16th International Conference on Few-Body Problems in Physics, Taipei,
Taiwan, 6-10 March 200
On the entropy of plasmas described with regularized -distributions
In classical thermodynamics the entropy is an extensive quantity, i.e.\ the
sum of the entropies of two subsystems in equilibrium with each other is equal
to the entropy of the full system consisting of the two subsystems. The
extensitivity of entropy has been questioned in the context of a theoretical
foundation for the so-called -distributions, which describe plasma
constituents with power-law velocity distributions. We demonstrate here, by
employing the recently introduced {\it regularized -distributions},
that entropy can be defined as an extensive quantity even for such
power-law-like distributions that truncate exponentially.Comment: Preprint accepted for publication in Phys. Rev.
Optimized time-dependent perturbation theory for pulse-driven quantum dynamics in atomic or molecular systems
We present a time-dependent perturbative approach adapted to the treatment of
intense pulsed interactions. We show there is a freedom in choosing secular
terms and use it to optimize the accuracy of the approximation. We apply this
formulation to a unitary superconvergent technique and improve the accuracy by
several orders of magnitude with respect to the Magnus expansion.Comment: 4 pages, 2 figure
Single electron transistors with high quality superconducting niobium islands
Deep submicron Al-AlOx-Nb tunnel junctions and single electron transistors
with niobium islands were fabricated by electron beam gun shadow evaporation.
Using stencil masks consisting of the thermostable polymer polyethersulfone
(PES) and germanium, high quality niobium patterns with good superconducting
properties and a gap energy of up to 2Delta = 2.5 meV for the niobium were
achieved. The I(U) characteristics of the transistors show special features due
to tunneling of single Cooper pairs and significant gate modulation in both the
superconducting and the normal state.Comment: 4 pages, 4 figure
Picosecond pump–probe and polarization techniques in supersonic molecular beams: Measurement of ultrafast vibrational-rotational dephasing and coherence
In the last few years, the time-resolved dynamics of collisionless intramolecular vibrational-energy redistribution (IVR) [1] has been probed [2] using picosecond excitation and fluorescence detection. By this method new information on IVR, coherence and photochemical changes (e.g., trans-cis isomerization) has been obtained. [2,3] However, in a number of cases the (early time) primary step following picosecond excitation could not be resolved simply because the time resolution was limited to ~50ps
Strong Coupling Theory of Two Level Atoms in Periodic Fields
We present a new convergent strong coupling expansion for two-level atoms in
external periodic fields, free of secular terms. As a first application, we
show that the coherent destruction of tunnelling is a third-order effect. We
also present an exact treatment of the high-frequency region, and compare it
with the theory of averaging. The qualitative frequency spectrum of the
transition probability amplitude contains an effective Rabi frequency.Comment: 4 pages with 3 figure
Role of multiorbital effects in the magnetic phase diagram of iron-pnictides
We elucidate the pivotal role of the bandstructure's orbital content in
deciding the type of commensurate magnetic order stabilized within the
itinerant scenario of iron-pnictides. Recent experimental findings in the
tetragonal magnetic phase attest to the existence of the so-called charge and
spin ordered density wave over the spin-vortex crystal phase, the latter of
which tends to be favored in simplified band models of itinerant magnetism.
Here we show that employing a multiorbital itinerant Landau approach based on
realistic bandstructures can account for the experimentally observed magnetic
phase, and thus shed light on the importance of the orbital content in deciding
the magnetic order. In addition, we remark that the presence of a hole pocket
centered at the Brillouin zone's -point favors a magnetic stripe
rather than a tetragonal magnetic phase. For inferring the symmetry properties
of the different magnetic phases, we formulate our theory in terms of magnetic
order parameters transforming according to irreducible representations of the
ensuing D point group. The latter method not only provides
transparent understanding of the symmetry breaking schemes but also reveals
that the leading instabilities always belong to the subset
of irreducible representations, independent of their C or C nature.Comment: 11 pages, 6 figure
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