43,685 research outputs found
Precision polarimetry with real-time mitigation of optical-window birefringence
Optical-window birefringence is frequently a major obstacle in experiments
measuring changes in the polarization state of light traversing a sample under
investigation. It can contribute a signal indistinguishable from that due to
the sample and complicate the analysis. Here, we explore a method to measure
and compensate for the birefringence of an optical window using the reflection
from the last optical surface before the sample. We demonstrate that this
arrangement can cancel out false signals due to the optical-window
birefringence-induced ellipticity drift to about 1%, for the values of total
ellipticity less than 0.25 rad
A technique for constructing spectral reflectance curves from Viking lander camera multispectral data
A technique for evaluating the construction of spectral reflectance curves from multispectral data obtained with the Viking lander cameras is presented. The multispectral data is limited to 6 channels in the wave-length range 0.4 to 1.1 microns, and several of the channels suffer from appreciable out-of-band response. The technique represents the estimated reflectance curves as a linear combination of known basic functions with coefficients determined to minimize the error in the representation, and it permits all channels, with and without out-of-band response, to contribute equally valid information. The technique is evaluated for known spectral reflectance curves of 8 materials felt likely to be present on the Martian surface. The technique provides an essentially exact fit if the the reflectance curve has no pronounced maxima and minima. Even if the curve has pronounced maxima and minima, the fit is good and reveals the most dominant features. Since only 6 samples are available some short period features are lost. This loss is almost certainly due to undersampling rather than out-of-band channel response
4D visualization of embryonic, structural crystallization by single-pulse microscopy
In many physical and biological systems the transition from an amorphous to ordered native structure involves complex energy landscapes, and understanding such transformations requires not only their thermodynamics but also the structural dynamics during the process. Here, we extend our 4D visualization method with electron imaging to include the study of irreversible processes with a single pulse in the same ultrafast electron microscope (UEM) as used before in the single-electron mode for the study of reversible processes. With this augmentation, we report on the transformation of amorphous to crystalline structure with silicon as an example. A single heating pulse was used to initiate crystallization from the amorphous phase while a single packet of electrons imaged selectively in space the transformation as the structure continuously changes with time. From the evolution of crystallinity in real time and the changes in morphology, for nanosecond and femtosecond pulse heating, we describe two types of processes, one that occurs at early time and involves a nondiffusive motion and another that takes place on a longer time scale. Similar mechanisms of two distinct time scales may perhaps be important in biomolecular folding
D-brane orbiting NS5-branes
We study real time dynamics of a Dp-brane orbiting a stack of NS5-branes. It
is generally known that a BPS D-brane moving in the vicinity of NS5-branes
becomes unstable due to the presence of tachyonic degree of freedom induced on
the D-brane. Indeed, the D-brane necessarily falls into the fivebranes due to
gravitational attraction and eventually collapses into a pressureless fluid.
Such a decay of the D-brane is known to be closely related to the rolling
tachyon problem. In this paper we show that in special cases the decay of
D-brane caused by gravitational attraction can be avoided. Namely for certain
values of energy and angular momentum the D-brane orbits around the fivebranes,
maintaining certain distance from the fivebranes all the time, and the process
of tachyon condensation is suppressed. We show that the tachyonic degree of
freedom induced on such a D-brane really disappears and the brane returns to a
stable D-brane.Comment: 12 pages, latex, added referenc
Correlation Functions of Conserved Currents in N = 2 Superconformal Theory
Using a manifestly supersymmetric formalism, we determine the general
structure of two- and three- point functions of the supercurrent and the
flavour current of N = 2 superconformal field theories. We also express them in
terms of N = 1 superfields and compare to the generic N = 1 correlation
functions. A general discussion of the N = 2 supercurrent superfield and the
multiplet of anomalies and their definition as derivatives with respect to the
supergravity prepotentials is also included.Comment: 43 pages, latex, no figures, v.2: section 4.2 extende
The Outer Shock of the Oxygen-Rich Supernova Remnant G292.0+1.8: Evidence for the Interaction with the Stellar Winds from its Massive Progenitor
We study the outer-shock structure of the oxygen-rich supernova remnant
G292.0+1.8, using a deep observation with the Chandra X-ray Observatory. We
measure radial variations of the electron temperature and emission measure that
we identify as the outer shock propagating into a medium with a radially
decreasing density profile. The inferred ambient density structure is
consistent with models for the circumstellar wind of a massive progenitor star
rather than for a uniform interstellar medium. The estimated wind density n_H =
0.1 ~ 0.3 cm^-3) at the current outer radius (~7.7 pc) of the remnant is
consistent with a slow wind from a red supergiant (RSG) star. The total mass of
the wind is estimated to be ~ 15 - 40 solar mass (depending on the estimated
density range), assuming that the wind extended down to near the surface of the
progenitor. The overall kinematics of G292.0+1.8 are consistent with the
remnant expanding through the RSG wind.Comment: 9 pages (2-column), 5 figures, accepted for Ap
Semiconductor resonator solitons above band gap
We show experimentally the existence of bright and dark spatial solitons in
semiconductor resonators for excitation above the band gap energy. These
solitons can be switched on, both spontaneously and with address pulses,
without the thermal delay found for solitons below the band gap which is
unfavorable for applications. The differences between soliton properties above
and below gap energy are discussed.Comment: 4 pages, 7 figure
Consequences of critical interchain couplings and anisotropy on a Haldane chain
Effects of interchain couplings and anisotropy on a Haldane chain have been
investigated by single crystal inelastic neutron scattering and density
functional theory (DFT) calculations on the model compound SrNiVO.
Significant effects on low energy excitation spectra are found where the
Haldane gap (; where is the intrachain exchange
interaction) is replaced by three energy minima at different antiferromagnetic
zone centers due to the complex interchain couplings. Further, the triplet
states are split into two branches by single-ion anisotropy. Quantitative
information on the intrachain and interchain interactions as well as on the
single-ion anisotropy are obtained from the analyses of the neutron scattering
spectra by the random phase approximation (RPA) method. The presence of
multiple competing interchain interactions is found from the analysis of the
experimental spectra and is also confirmed by the DFT calculations. The
interchain interactions are two orders of magnitude weaker than the
nearest-neighbour intrachain interaction = 8.7~meV. The DFT calculations
reveal that the dominant intrachain nearest-neighbor interaction occurs via
nontrivial extended superexchange pathways Ni--O--V--O--Ni involving the empty
orbital of V ions. The present single crystal study also allows us to
correctly position SrNiVO in the theoretical - phase
diagram [T. Sakai and M. Takahashi, Phys. Rev. B 42, 4537 (1990)] showing where
it lies within the spin-liquid phase.Comment: 12 pages, 12 figures, 3 tables PRB (accepted). in Phys. Rev. B (2015
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