1,440 research outputs found
Advanced microwave radiometer antenna system study
The practicability of a multi-frequency antenna for spaceborne microwave radiometers was considered in detail. The program consisted of a comparative study of various antenna systems, both mechanically and electronically scanned, in relation to specified design goals and desired system performance. The study involved several distinct tasks: definition of candidate antennas that are lightweight and that, at the specified frequencies of 5, 10, 18, 22, and 36 GHz, can provide conical scanning, dual linear polarization, and simultaneous multiple frequency operation; examination of various feed systems and phase-shifting techniques; detailed analysis of several key performance parameters such as beam efficiency, sidelobe level, and antenna beam footprint size; and conception of an antenna/feed system that could meet the design goals. Candidate antennas examined include phased arrays, lenses, and optical reflector systems. Mechanical, electrical, and performance characteristics of the various systems were tabulated for ease of comparison
Absolute conservation law for black holes
In all 2d theories of gravity a conservation law connects the (space-time
dependent) mass aspect function at all times and all radii with an integral of
the matter fields. It depends on an arbitrary constant which may be interpreted
as determining the initial value together with the initial values for the
matter field. We discuss this for spherically reduced Einstein-gravity in a
diagonal metric and in a Bondi-Sachs metric using the first order formulation
of spherically reduced gravity, which allows easy and direct fixations of any
type of gauge. The relation of our conserved quantity to the ADM and Bondi mass
is investigated. Further possible applications (ideal fluid, black holes in
higher dimensions or AdS spacetimes etc.) are straightforward generalizations.Comment: LaTex, 17 pages, final version, to appear in Phys. Rev.
Universal conservation law and modified Noether symmetry in 2d models of gravity with matter
It is well-known that all 2d models of gravity---including theories with
nonvanishing torsion and dilaton theories---can be solved exactly, if matter
interactions are absent. An absolutely (in space and time) conserved quantity
determines the global classification of all (classical) solutions. For the
special case of spherically reduced Einstein gravity it coincides with the mass
in the Schwarzschild solution. The corresponding Noether symmetry has been
derived previously by P. Widerin and one of the authors (W.K.) for a specific
2d model with nonvanishing torsion. In the present paper this is generalized to
all covariant 2d theories, including interactions with matter. The related
Noether-like symmetry differs from the usual one. The parameters for the
symmetry transformation of the geometric part and those of the matterfields are
distinct. The total conservation law (a zero-form current) results from a two
stage argument which also involves a consistency condition expressed by the
conservation of a one-form matter ``current''. The black hole is treated as a
special case.Comment: 3
The Complete Solution of 2D Superfield Supergravity from graded Poisson-Sigma Models and the Super Pointparticle
Recently an alternative description of 2d supergravities in terms of graded
Poisson-Sigma models (gPSM) has been given. As pointed out previously by the
present authors a certain subset of gPSMs can be interpreted as "genuine"
supergravity, fulfilling the well-known limits of supergravity, albeit deformed
by the dilaton field. In our present paper we show that precisely that class of
gPSMs corresponds one-to-one to the known dilaton supergravity superfield
theories presented a long time ago by Park and Strominger. Therefore, the
unique advantages of the gPSM approach can be exploited for the latter: We are
able to provide the first complete classical solution for any such theory. On
the other hand, the straightforward superfield formulation of the point
particle in a supergravity background can be translated back into the gPSM
frame, where "supergeodesics" can be discussed in terms of a minimal set of
supergravity field degrees of freedom. Further possible applications like the
(almost) trivial quantization are mentioned.Comment: 48 pages, 1 figure. v3: after final version, typos correcte
Classical and Quantum Integrability of 2D Dilaton Gravities in Euclidean space
Euclidean dilaton gravity in two dimensions is studied exploiting its
representation as a complexified first order gravity model. All local classical
solutions are obtained. A global discussion reveals that for a given model only
a restricted class of topologies is consistent with the metric and the dilaton.
A particular case of string motivated Liouville gravity is studied in detail.
Path integral quantisation in generic Euclidean dilaton gravity is performed
non-perturbatively by analogy to the Minkowskian case.Comment: 27 p., LaTeX, v2: included new refs. and a footnot
Area spectrum in Lorentz covariant loop gravity
We use the manifestly Lorentz covariant canonical formalism to evaluate
eigenvalues of the area operator acting on Wilson lines. To this end we modify
the standard definition of the loop states to make it applicable to the present
case of non-commutative connections. The area operator is diagonalized by using
the usual shift ambiguity in definition of the connection. The eigenvalues are
then expressed through quadratic Casimir operators. No dependence on the
Immirzi parameter appears.Comment: 12 pages, RevTEX; improved layout, typos corrected, references added;
changes in the discussion in sec. IIIB and
Evolution of magnetism in Yb(Rh_(1-x)Co_x)2Si2
We present a study of the evolution of magnetism from the quantum critical
system YbRh2Si2 to the stable trivalent Yb system YbCo2Si2. Single crystals of
Yb(Rh_(1-x)Co_x)2Si2 were grown for 0 < x < 1 and studied by means of magnetic
susceptibility, electrical resistivity, and specific heat measurements, as well
as photoemission spectroscopy. The results evidence a complex magnetic phase
diagram, with a non-monotonic evolution of T_N and two successive transitions
for some compositions resulting in two tricritical points. The strong
similarity with the phase diagram of YbRh2Si2 under pressure indicates that Co
substitution basically corresponds to the application of positive chemical
pressure. Analysis of the data proves a strong reduction of the Kondo
temperature T_K with increasing Co content, T_K becoming smaller than T_N for x
~ 0.5, implying a strong localization of the 4f electrons. Furthermore,
low-temperature susceptibility data confirm a competition between ferromagnetic
and antiferromagnetic exchange. The series Yb(Rh_(1-x)Co_x)2Si2 provides an
excellent experimental opportunity to gain a deeper understanding of the
magnetism at the quantum critical point in the vicinity of YbRh2Si2 where the
antiferromagnetic phase disappears (T_N=>0).Comment: 11 pages, 9 figure
Quantum Averaging I: Poincar\'e--von Zeipel is Rayleigh--Schr\"odinger
An exact analogue of the method of averaging in classical mechanics is
constructed for self--adjoint operators. It is shown to be completely
equivalent to the usual Rayleigh--Schr\"odinger perturbation theory but gives
the sums over intermediate states in closed form expressions. The anharmonic
oscillator and the Henon--Heiles system are treated as examples to illustrate
the quantum averaging method.Comment: 12 pages, LaTeX, to appear in Journ. Phys.
Virtual black hole phenomenology from 2d dilaton theories
Equipped with the tools of (spherically reduced) dilaton gravity in first
order formulation and with the results for the lowest order S-matrix for s-wave
gravitational scattering (P. Fischer, D. Grumiller, W. Kummer, and D.
Vassilevich, gr-qc/0105034) new properties of the ensuing cross-section are
discussed. We find CPT invariance, despite of the non-local nature of our
effective theory and discover pseudo-self-similarity in its kinematic sector.
After presenting the Carter-Penrose diagram for the corresponding virtual
black hole geometry we encounter distributional contributions to its
Ricci-scalar and a vanishing Einstein-Hilbert action for that configuration.
Finally, a comparison is done between our (Minkowskian) virtual black hole and
Hawking's (Euclidean) virtual black hole bubbles.Comment: 17 pages, 13 figure
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