1,564 research outputs found
Ni-impurity effects on the superconducting gap of LaSrCuO studied from the magnetic field and temperature dependence of the electronic specific heat
The magnetic field and temperature dependence of the electronic specific heat
have been systematically investigated in (LSCNO) in order to study Ni-impurity
effects on the superconducting (SC) gap. In LSCNO with =0.15 and =0.015,
the value of () at =0 K, , is
enhanced under the magnetic field applied along the -axis. The
increment of , , follows the Volovik relation
=, characteristic of the SC gap with line nodes,
with prefactor similar to that of a pure sample. The vs.
curve under =0 shows a d-wave-like SC anomaly with an abrupt increase at
and -linear dependence at , although the
-value in the vs. curve increases with increasing
Ni concentrations. Interestingly, as the SC part of , , decreases in LSCNO, is
reduced in proportion to the decrease of . These findings can
be explained phenomenologically by a simple model in which Ni impurities bring
about strong pair breaking at the edges of the coherent nodal part of the Fermi
surface but in the vicinity of the nodes of the SC gap. The reduction of the SC
condensation energy in LSCNO, evaluated from at
{0.3em}\raisebox{0.4ex}{} {-0.75em}\raisebox{-.7ex}{} {0.3em}, is also understood by the same model.Comment: 7 pages, 6 figures, accepted in Phys. Rev.
Accretion dynamics in the classical T Tauri star V2129 Oph
We analyze the photometric and spectroscopic variability of the classical T
Tauri star V2129 Oph over several rotational cycles to test the dynamical
predictions of magnetospheric accretion models. The photometric variability and
the radial velocity variations in the photospheric lines can be explained by
rotational modulation due to cold spots, while the radial velocity variations
of the He I (5876 \AA) line and the veiling variability are due to hot spot
rotational modulation. The hot and cold spots are located at high latitudes and
about the same phase, but the hot spot is expected to sit at the chromospheric
level, while the cold spot is at the photospheric level. Using the
dipole+octupole magnetic-field configuration previously proposed in the
literature for the system, we compute 3D MHD magnetospheric simulations of the
star-disk system. We use the simulation's density, velocity and scaled
temperature structures as input to a radiative transfer code, from which we
calculate theoretical line profiles at all rotational phases. The theoretical
profiles tend to be narrower than the observed ones, but the qualitative
behavior and the observed rotational modulation of the H\alpha and H\beta
emission lines are well reproduced by the theoretical profiles. The
spectroscopic and photometric variability observed in V2129 Oph support the
general predictions of complex magnetospheric accretion models with
non-axisymmetric, multipolar fields.Comment: Accepted by Astronomy and Astrophysic
Facing the wind of the pre-FUor V1331 Cyg
The mass outflows in T Tauri stars (TTS) are thought to be an effective
mechanism to remove angular momentum during the pre-main-sequence contraction
of a low-mass star. The most powerful winds are observed at the FUor stage of
stellar evolution. V1331 Cyg has been considered as a TTS at the pre-FUor
stage. We analyse high-resolution spectra of V1331 Cyg collected in 1998-2007
and 20-d series of spectra taken in 2012. For the first time the photospheric
spectrum of the star is detected and stellar parameters are derived: spectral
type G7-K0 IV, mass 2.8 Msun, radius 5 Rsun, vsini < 6 km/s. The photospheric
spectrum is highly veiled, but the amount of veiling is not the same in
different spectral lines, being lower in weak transitions and much higher in
strong transitions. The Fe II 5018, Mg I 5183, K I 7699 and some other lines of
metals are accompanied by a `shell' absorption at radial velocity of about -240
km/s. We show that these absorptions form in the post-shock gas in the jet,
i.e. the star is seen though its jet. The P Cyg profiles of H-alpha and H-beta
indicate the terminal wind velocity of about 500 km/s, which vary on
time-scales from several days to years. A model of the stellar wind is
developed to interpret the observations. The model is based on calculation of
hydrogen spectral lines using the radiative transfer code TORUS. The observed
H-alpha and H-beta line profiles and their variability can be well reproduced
with a stellar wind model, where the mass-loss rate and collimation (opening
angle) of the wind are variable. The changes of the opening angle may be
induced by small variability in magetization of the inner disc wind. The
mass-loss rate is found to vary within (6-11)x10^{-8} Msun/yr, with the
accretion rate of 2.0x10^{-6} Msun/yr.Comment: 11 pages, 12 figures; accepted for publication in MNRAS.
Typographical errors have been corrected after the proof stag
MHD Simulations of Magnetospheric Accretion, Ejection and Plasma-field Interaction
We review recent axisymmetric and three-dimensional (3D) magnetohydrodynamic
(MHD) numerical simulations of magnetospheric accretion, plasma-field
interaction and outflows from the disk-magnetosphere boundary.Comment: 11 pages, 8 figures, conference proceedings: "Physics at the
Magnetospheric Boundary", Geneva, Switzerland, 25-28 June, 201
Chiral charge-density-waves
We discovered the chirality of charge density waves (CDW) in 1T-TiSe by
using scanning tunnelling microscopy (STM) and optical ellipsometry. We found
that the CDW intensity becomes , where (i =1, 2, 3) is the amplitude of the tunnelling current
contributed by the CDWs. There were two states, in which the three intensity
peaks of the CDW decrease \textit{clockwise} and \textit{anticlockwise} when we
index each nesting vector in order of intensity in the Fourier transformation
of the STM images. The chirality in CDW results in the three-fold symmetry
breaking. Macroscopically, two-fold symmetry was indeed observed in optical
measurement. We propose the new generalized CDW chirality H_{CDW} \equiv
{\boldmath q_1} \cdot ({\boldmath q_2}\times {\boldmath q_3}), where
{\boldmath q_i} are the nesting vectors, which is independent of the
symmetry of components. The nonzero - the triple-{\boldmath q}
vectors do not exist in an identical plane in the reciprocal space - should
induce a real-space chirality in CDW system.Comment: 12 pages, 4 figure
High count rate {\gamma}-ray spectroscopy with LaBr3:Ce scintillation detectors
The applicability of LaBr3:Ce detectors for high count rate {\gamma}-ray
spectroscopy is investigated. A 3"x3" LaBr3:Ce detector is used in a test setup
with radioactive sources to study the dependence of energy resolution and photo
peak efficiency on the overall count rate in the detector. Digitized traces
were recorded using a 500 MHz FADC and analysed with digital signal processing
methods. In addition to standard techniques a pile-up correction method is
applied to the data in order to further improve the high-rate capabilities and
to reduce the losses in efficiency due to signal pile-up. It is shown, that
{\gamma}-ray spectroscopy can be performed with high resolution at count rates
even above 1 MHz and that the performance can be enhanced in the region between
500 kHz and 10 MHz by using pile-up correction techniques
Adaptive image ray-tracing for astrophysical simulations
A technique is presented for producing synthetic images from numerical
simulations whereby the image resolution is adapted around prominent features.
In so doing, adaptive image ray-tracing (AIR) improves the efficiency of a
calculation by focusing computational effort where it is needed most. The
results of test calculations show that a factor of >~ 4 speed-up, and a
commensurate reduction in the number of pixels required in the final image, can
be achieved compared to an equivalent calculation with a fixed resolution
image.Comment: 4 pages, 1 figure. Accepted for publication in MNRA
MHD simulations of disk-star interaction
We discuss a number of topics relevant to disk-magnetosphere interaction and
how numerical simulations illuminate them. The topics include: (1)
disk-magnetosphere interaction and the problem of disk-locking; (2) the wind
problem; (3) structure of the magnetospheric flow, hot spots at the star's
surface, and the inner disk region; (4) modeling of spectra from 3D funnel
streams; (5) accretion to a star with a complex magnetic field; (6) accretion
through 3D instabilities; (7) magnetospheric gap and survival of protoplanets.
Results of both 2D and 3D simulations are discussed.Comment: 12 pages, 10 figures, Star-Disk Interaction in Young Stars,
Proceedings of the International Astronomical Union, IAU Symposium, Volume
243. See animations at http://astro.cornell.edu/~romanova/projects.htm and at
http://astro.cornell.edu/us-rus
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