16,811 research outputs found
Size dependent line broadening in the emission spectra of single GaAs quantum dots: Impact of surface charges on spectral diffusion
Making use of droplet epitaxy, we systematically controlled the height of
self-assembled GaAs quantum dots by more than one order of magnitude. The
photoluminescence spectra of single quantum dots revealed the strong dependence
of the spectral linewidth on the dot height. Tall dots with a height of ~30 nm
showed broad spectral peaks with an average width as large as ~5 meV, but
shallow dots with a height of ~2 nm showed resolution-limited spectral lines
(<120 micro eV). The measured height dependence of the linewidths is in good
agreement with Stark coefficients calculated for the experimental shape
variation. We attribute the microscopic source of fluctuating electric fields
to the random motion of surface charges at the vacuum-semiconductor interface.
Our results offer guidelines for creating frequency-locked photon sources,
which will serve as key devices for long-distance quantum key distribution.Comment: 6 pages, 6 figures; updated figs and their description
Experimental evidence for a two-gap structure of superconducting NbSe_2: a specific heat study in external magnetic fields
To resolve the discrepancies of the superconducting order parameter in
quasi-two-dimensional NbSe_2, comprehensive specific-heat measurements have
been carried out. By analyzing both the zero-field and mixed-state data with
magnetic fields perpendicular to and parallel to the c axis of the crystal and
using the two-gap model, we conclude that (1) more than one energy scale of the
order parameter is required for superconducting NbSe_2 due to the thermodynamic
consistency; (2)delta_L=1.26 meV and delta_S=0.73 meV are obtained; (3)
N_S(0)/N(0)=11%~20%; (4) The observation of the kink in gamma(H) curve suggests
that the two-gap scenario is more favorable than the anisotropic s-wave model
to describe the gap structure of NbSe_2; and (5)delta_S is more isotropic and
has a three-dimensional-like feature and is located either on the Se or the
bonding Nb Fermi sheets.Comment: 16 pages, 4 figure
Determining Absorption, Emissivity Reduction, and Local Suppression Coefficients inside Sunspots
The power of solar acoustic waves is reduced inside sunspots mainly due to
absorption, emissivity reduction, and local suppression. The coefficients of
these power-reduction mechanisms can be determined by comparing time-distance
cross-covariances obtained from sunspots and from the quiet Sun. By analyzing
47 active regions observed by SOHO/MDI without using signal filters, we have
determined the coefficients of surface absorption, deep absorption, emissivity
reduction, and local suppression. The dissipation in the quiet Sun is derived
as well. All of the cross-covariances are width corrected to offset the effect
of dispersion. We find that absorption is the dominant mechanism of the power
deficit in sunspots for short travel distances, but gradually drops to zero at
travel distances longer than about 6 degrees. The absorption in sunspot
interiors is also significant. The emissivity-reduction coefficient ranges from
about 0.44 to 1.00 within the umbra and 0.29 to 0.72 in the sunspot, and
accounts for only about 21.5% of the umbra's and 16.5% of the sunspot's total
power reduction. Local suppression is nearly constant as a function of travel
distance with values of 0.80 and 0.665 for umbrae and whole sunspots
respectively, and is the major cause of the power deficit at large travel
distances.Comment: 14 pages, 21 Figure
Geometry-dependent electrostatics near contact lines
Long-ranged electrostatic interactions in electrolytes modify their contact
angles on charged substrates in a scale and geometry dependent manner. For
angles measured at scales smaller than the typical Debye screening length, the
wetting geometry near the contact line must be explicitly considered. Using
variational and asymptotic methods, we derive new transcendental equations for
the contact angle that depend on the electrostatic potential only at the three
phase contact line. Analytic expressions are found in certain limits and
compared with predictions for contact angles measured with lower resolution. An
estimate for electrostatic contributions to {\it line} tension is also given.Comment: 3 .eps figures, 5p
EXITE2 Observation of the SIGMA Source GRS 1227+025
We report the EXITE2 hard X-ray imaging of the sky around 3C273. A 2h
observation on May 8, 1997, shows a 260 mCrab source detected at
in each of two bands (50-70 and 70-93 keV) and located 30'
from 3C273 and consistent in position with the SIGMA source GRS1227+025. The
EXITE2 spectrum is consistent with a power law with photon index 3 and large
low energy absorption, as indicated by the GRANAT/SIGMA results. No source was
detected in more sensitive followup EXITE2 observations in 2000 and 2001 with
3 upper limits of 190 and 65 mCrab, respectively. Comparison with the
flux detected by SIGMA shows the source to be highly variable, suggesting it
may be non-thermal and beamed and thus the first example of a ``type 2''
(absorbed) Blazar. Alternatively it might be (an unprecedented) very highly
absorbed binary system undergoing accretion disk instability outbursts,
possibly either a magnetic CV, or a black hole X-ray nova.Comment: 12 pages, 4 figures, accepted for publication in Ap
New Perspectives in Sinographic Language Processing Through the Use of Character Structure
Chinese characters have a complex and hierarchical graphical structure
carrying both semantic and phonetic information. We use this structure to
enhance the text model and obtain better results in standard NLP operations.
First of all, to tackle the problem of graphical variation we define
allographic classes of characters. Next, the relation of inclusion of a
subcharacter in a characters, provides us with a directed graph of allographic
classes. We provide this graph with two weights: semanticity (semantic relation
between subcharacter and character) and phoneticity (phonetic relation) and
calculate "most semantic subcharacter paths" for each character. Finally,
adding the information contained in these paths to unigrams we claim to
increase the efficiency of text mining methods. We evaluate our method on a
text classification task on two corpora (Chinese and Japanese) of a total of 18
million characters and get an improvement of 3% on an already high baseline of
89.6% precision, obtained by a linear SVM classifier. Other possible
applications and perspectives of the system are discussed.Comment: 17 pages, 5 figures, presented at CICLing 201
Spin gap formation in the quantum spin systems TiOX, X=Cl and Br
In the layered quantum spin systems TiOCl and TiOBr the magnetic
susceptibility shows a very weak temperature dependence at high temperatures
and transition-induced phenomena at low temperatures. There is a clear
connection of the observed transition temperatures to the distortion of the
octahedra and the layer separation. Band structure calculations point to a
relation of the local coordinations and the dimensionality of the magnetic
properties. While from magnetic Raman scattering only a small decrease of the
magnetic exchange by -5-10% is derived comparing TiOCl with TiOBr, the
temperature dependence of the magnetic susceptibility favors a much bigger
change.Comment: 5 figures, 15 pages, further information see
http://www.peter-lemmens.d
Discovery of X-ray pulsations from "next Geminga" - PSR J1836+5925
We report the X-ray pulsation of ~173.3 ms for the "next Geminga", PSR
J1836+5925, with recent XMM-Newton investigations. The X-ray periodicity is
consistent wtih the gamma-ray ephemeris at the same epoch. The X-ray folded
light curve has a sinusoidal structure which is different from the
double-peaked gamma-ray pulse profile. We have also analysed the X-ray
phase-averaged spectra which shows the X-ray emission from PSR J1836+5925 is
thermal dominant. This suggests the X-ray pulsation mainly originates from the
modulated hot spot on the stellar surface.Comment: 7 pages, 3 figures, 1 table, accepted for publication in ApJ Lette
Nonmagnetic impurity perturbation to the quasi-two-dimensional quantum helimagnet LiCu2O2
A complete phase diagram of Zn substituted quantum quasi-two-dimensional
helimagnet LiCu2O2 has been presented. Helical ordering transition temperature
(T_h) of the original LiCu2O2 follows finite size scaling for less than ~ 5.5%
Zn substitution, which implies the existence of finite helimagnetic domains
with domain boundaries formed with nearly isolated spins. Higher Zn
substitution > 5.5% quenches the long-range helical ordering and introduces an
intriguing Zn level dependent magnetic phase transition with slight thermal
hysteresis and a universal quadratic field dependence for T_c (Zn > 0.055,H).
The magnetic coupling constants of nearest-neighbor (nn) J1 and
next-nearest-neighbor (nnn) J2 (alpha=J2/J1) are extracted from high
temperature series expansion (HTSE) fitting and N=16 finite chain exact
diagonalization simulation. We have also provided evidence of direct
correlation between long-range helical spin ordering and the magnitude of
electric polarization in this spin driven multiferroic material
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