18,205 research outputs found
Liquid-Drop Model and Quantum Resistance Against Noncompact Nuclear Geometries
The importance of quantum effects for exotic nuclear shapes is demonstrated.
Based on the example of a sheet of nuclear matter of infinite lateral
dimensions but finite thickness, it is shown that the quantization of states in
momentum space, resulting from the confinement of the nucleonic motion in the
conjugate geometrical space, generates a strong resistance against such a
confinement and generates restoring forces driving the system towards compact
geometries. In the liquid-drop model, these quantum effects are implicitly
included in the surface energy term, via a choice of interaction parameters, an
approximation that has been found valid for compact shapes, but has not yet
been scrutinized for exotic shapes.Comment: 9 pages with 3 figure
Lunar contour mapping system /lucom/ final report, 5 aug. 1964 - 18 mar. 1965
Radar sensor system for acquisition of lunar surface data - Lunar contour mapping syste
Black Holes with a Generalized Gravitational Action
Microscopic black holes are sensitive to higher dimension operators in the
gravitational action. We compute the influence of these operators on the
Schwarzschild solution using perturbation theory. All (time reversal invariant)
operators of dimension six are included (dimension four operators don't alter
the Schwarzschild solution). Corrections to the relation between the Hawking
temperature and the black hole mass are found. The entropy is calculated using
the Gibbons-Hawking prescription for the Euclidean path integral and using
naive thermodynamic reasoning. These two methods agree, however, the entropy is
not equal to 1/4 the area of the horizon.Comment: plain tex(uses phyzzx.tex), 8 pages, CALT-68-185
Small Black Holes on Branes: Is the horizon regular or singular ?
We investigate the following question: Consider a small mass, with
(the ratio of the Schwarzschild radius and the bulk curvature length) much
smaller than 1, that is confined to the TeV brane in the Randall-Sundrum I
scenario. Does it form a black hole with a regular horizon, or a naked
singularity? The metric is expanded in and the asymptotic form of
the metric is given by the weak field approximation (linear in the mass). In
first order of we show that the iteration of the weak field
solution, which includes only integer powers of the mass, leads to a solution
that has a singular horizon. We find a solution with a regular horizon but its
asymptotic expansion in the mass also contains half integer powers.Comment: Accepted for publication in PR
Initial Data for Black Holes and Black Strings in 5d
We explore time-symmetric hypersurfaces containing apparent horizons of black
objects in a 5d spacetime with one coordinate compactified on a circle. We find
a phase transition within the family of such hypersurfaces: the horizon has
different topology for different parameters. The topology varies from to
. This phase transition is discontinuous -- the topology of the
horizon changes abruptly. We explore the behavior around the critical point and
present a possible phase diagram.Comment: 4 pp, 3 figs. v3: Discussion extended including know variouse choices
of the source. The value of , errors and typos are corrected.
Conclusions clarified but ain't changed. More references added. Accepted for
publication in PR
Pinning a Domain Wall in (Ga,Mn)As with Focused Ion Beam Lithography
We utilize a focused beam of Ga+ ions to define magnetization pinning sites
in a ferromagnetic epilayer of (Ga,Mn)As. The nonmagnetic defects locally
increase the magneto-crystalline anisotropy energies, by which a domain wall is
pinned at a given position. We demonstrate techniques for manipulating domain
walls at these pinning sites as probed with the giant planar Hall effect
(GPHE). By varying the magnetic field angle relative to the crystal axes, an
upper limit is placed on the local effective anisotropy energy.Comment: 13 pages, 3 figure
Optical control of internal electric fields in band-gap graded InGaN nanowires
InGaN nanowires are suitable building blocks for many future optoelectronic
devices. We show that a linear grading of the indium content along the nanowire
axis from GaN to InN introduces an internal electric field evoking a
photocurrent. Consistent with quantitative band structure simulations we
observe a sign change in the measured photocurrent as a function of photon
flux. This negative differential photocurrent opens the path to a new type of
nanowire-based photodetector. We demonstrate that the photocurrent response of
the nanowires is as fast as 1.5 ps
Generating Spin Currents in Semiconductors with the Spin Hall Effect
We investigate electrically-induced spin currents generated by the spin Hall
effect in GaAs structures that distinguish edge effects from spin transport.
Using Kerr rotation microscopy to image the spin polarization, we demonstrate
that the observed spin accumulation is due to a transverse bulk electron spin
current, which can drive spin polarization nearly 40 microns into a region in
which there is minimal electric field. Using a model that incorporates the
effects of spin drift, we determine the transverse spin drift velocity from the
magnetic field dependence of the spin polarization.Comment: 4 pages, 4 figure
Harmonic superpositions of non-extremal p-branes
The plot of allowed p and D values for p-brane solitons in D-dimensional
supergravity is the same whether the solitons are extremal or non-extremal. One
of the useful tools for relating different points on the plot is vertical
dimensional reduction, which is possible if periodic arrays of p-brane solitons
can be constructed. This is straightforward for extremal p-branes, since the
no-force condition allows arbitrary multi-centre solutions to be constructed in
terms of a general harmonic function on the transverse space. This has also
been shown to be possible in the special case of non-extremal black holes in
D=4 arrayed along an axis. In this paper, we extend previous results to include
multi-scalar black holes, and dyonic black holes. We also consider their
oxidation to higher dimensions, and we discuss general procedures for
constructing the solutions, and studying their symmetries.Comment: Latex, 23 page
- …