5,303 research outputs found
A quasi-time-dependent radiative transfer model of OH104.9+2.4
We investigate the pulsation-phase dependent properties of the circumstellar
dust shell (CDS) of the OH/IR star OH104.9+2.4 based on radiative transfer
modeling (RTM) using the code DUSTY. Our previous study concerning simultaneous
modeling of the spectral energy distribution (SED) and near-infrared (NIR)
visibilities (Riechers et al. 2004) has now been extended by means of a more
detailed analysis of the pulsation-phase dependence of the model parameters of
OH104.9+2.4. In order to investigate the temporal variation in the spatial
structure of the CDS, additional NIR speckle interferometric observations in
the K' band were carried out with the 6 m telescope of the Special
Astrophysical Observatory (SAO). At a wavelength of 2.12 micron the
diffraction-limited resolution of 74 mas was attained. Several key parameters
of our previous best-fitting model had to be adjusted in order to be consistent
with the newly extended amount of observational data. It was found that a
simple rescaling of the bolometric flux F_bol is not sufficient to take the
variability of the source into account, as the change in optical depth over a
full pulsation cycle is rather high. On the other hand, the impact of a change
in effective temperature T_eff on SED and visibility is rather small. However,
observations, as well as models for other AGB stars, show the necessity of
including a variation of T_eff with pulsation phase in the radiative transfer
models. Therefore, our new best-fitting model accounts for these changes.Comment: 7 pages, including 5 postscript figures and 3 tables. Published in
Astronomy and Astrophysics. (v1: accepted version; v2: published version,
minor grammatical changes
Spin Dynamics in the LTT Phase of ~1/8 Doped Single Crystal La_{1.67}Eu_{0.2}Sr_{0.13}CuO_4
We present La and Cu NMR relaxation measurements in single crystal
La_{1.67}Eu_{0.2}Sr_{0.13}CuO_4. A strong peak in the La spin-lattice
relaxation rate observed in the spin ordered state is well-described by the BPP
mechanism[1] and arises from continuous slowing of electronic spin fluctuations
with decreasing temperature; these spin fluctuations exhibit XY-like anisotropy
in the ordered state. The spin pseudogap is enhanced by the static
charge-stripe order in the LTT phase.Comment: Four pages, three figure
Dzyaloshinsky-Moriya Spin Canting in the LTT Phase of La2-x-yEuySrxCuO4
The Cu spin magnetism in La2-x-yEuySrxCuO4 (x<=0.17; y<=0.2) has been studied
by means of magnetization measurements up to 14 T. Our results clearly show
that in the antiferromagnetic phase Dzyaloshinsky-Moriya (DM)superexchange
causes Cu spin canting not only in the LTO phase but also in the LTLO and LTT
phases. In La1.8Eu0.2CuO4 the canted DM-moment is about 50% larger than in pure
La2CuO4 which we attribute to the larger octahedral tilt angle. We also find
clear evidence that the size of the DM-moment does not change significantly at
the structural transition at T_LT from LTO to LTLO and LTT. The most important
change induced by the transition is a significant reduction of the magnetic
coupling between the CuO2 planes. As a consequence, the spin-flip transition of
the canted Cu spins which is observed in the LTO phase for magnetic field
perpendicular to the CuO2 planes disappears in the LTT phase. The shape of the
magnetization curves changes from the well known spin-flip type to a
weak-ferromagnet type. However, no spontaneous weak ferromagnetism is observed
even at very low temperatures, which seems to indicate that the interlayer
decoupling in our samples is not perfect. Nonetheless, a small fraction (<15%)
of the DM-moments can be remanently magnetized throughout the entire
antiferromagnetically ordered LTT/LTLO phase, i.e. for T<T_LT and x<0.02. It
appears that the remanent DM-moment is perpendicular to the CuO2 planes. For
magnetic field parallel to the CuO2 planes we find that the critical field of
the spin-flop transition decreases in the LTLO phase, which might indicate a
competition between different in-plane anisotropies. To study the Cu spin
magnetism in La2-x-yEuySrxCuO4, a careful analysis of the Van Vleck
paramagnetism of the Eu3+ ions was performed.Comment: 22 pages, 27 figure
Stripe structure, spectral feature and soliton gap in high Tc cuprates
We show that the lightly doped La_{2-x}Sr_{x}CuO_{4} can be described in
terms of a stripe magnetic structure or soliton picture. The internal
relationship between the recent neutron observation of the diagonal (x=0.05) to
vertical (x >= 0.06) stripe transition, which was predicted, and the
concomitant metal-insulator transition is clarified by this solitonic physics.
The phase diagram with the unidentified transition lines between
antiferromagnetic to stripe phases, the doping dependence of the modulation
period, the origin of the mid-infrared optical absorption are investigated
comparatively with other single layer systems: La_{2-x}Sr_{x}NiO_{4} and
(La,Nd)_{2-x}Sr_{x}CuO_{4}. The novel type of quasi-particles and holes is
fully responsible for metallic conduction and ultimately superconductivity.Comment: 4 pages RevTex, 5 figure
Phantom for Evaluating Accuracy of Image Registration Software
Provided is a phantom for evaluating the accuracy of image registration software based on a result of matching tomograms of a predetermined position of the phantom, taken using two or more imaging apparatuses. Accordingly, it is possible to more efficiently evaluate the accuracy of the image registration software by comparing the tomograms with one another using a three-dimensional analysis. In addition, it is possible to facilitate the comparison of the tomograms with one another by installing a plurality of indicating bars in the phantom so that their cross sections can appear on each of the tomograms
Shear stress fluctuations in the granular liquid and solid phases
We report on experimentally observed shear stress fluctuations in both
granular solid and fluid states, showing that they are non-Gaussian at low
shear rates, reflecting the predominance of correlated structures (force
chains) in the solidlike phase, which also exhibit finite rigidity to shear.
Peaks in the rigidity and the stress distribution's skewness indicate that a
change to the force-bearing mechanism occurs at the transition to fluid
behaviour, which, it is shown, can be predicted from the behaviour of the
stress at lower shear rates. In the fluid state stress is Gaussian distributed,
suggesting that the central limit theorem holds. The fibre bundle model with
random load sharing effectively reproduces the stress distribution at the yield
point and also exhibits the exponential stress distribution anticipated from
extant work on stress propagation in granular materials.Comment: 11 pages, 3 figures, latex. Replacement adds journal reference and
addresses referee comment
Suppression of Antiferromagnetic Order by Light Hole Doping in La_2Cu_{1-x}Li_xO_4: A ^{139}La NQR Study
^{139}La nuclear quadrupole resonance measurements in lightly doped
La_2Cu_{1-x}Li_xO_4 have been performed to reveal the dependence of the
magnetic properties of the antiferromagnetic CuO_2 planes on the character of
the doped holes and their interactions with the dopant. A detailed study shows
that the magnetic properties are remarkably insensitive to the character of the
dopant impurity. This indicates that the added holes form previously
unrecognized collective structures.Comment: 4 pages, 3 figures. Slightly modified version, as accepted for
publication in Physical Review Letter
Superconductivity of the spin ladder system: Are the superconducting pairing and the spin-gap formation of the same origin?
Pressure-induced superconductivity in a spin-ladder cuprate
SrCaCuO has not been studied on a microscopic level so
far although the superconductivity was already discovered in 1996. We have
improved high-pressure technique with using a large high-quality crystal, and
succeeded in studying the superconductivity using Cu nuclear magnetic
resonance (NMR). We found that anomalous metallic state reflecting the
spin-ladder structure is realized and the superconductivity possesses a
s-wavelike character in the meaning that a finite gap exists in the
quasi-particle excitation: At pressure of 3.5GPa we observed two excitation
modes in the normal state from the relaxation rate . One gives rise
to an activation-type component in , and the other -linear
component linking directly with the superconductivity. This gapless mode likely
arises from free motion of holon-spinon bound states appearing by hole doping,
and the pairing of them likely causes the superconductivity.Comment: to be published in Phys. Rev. Let
^{17}O and ^{51}V NMR for the zigzag spin-1 chain compound CaV2O4
V NMR studies on CaV2O4 single crystals and O NMR studies on
O-enriched powder samples are reported. The temperature dependences of
the O NMR line width and nuclear spin-lattice relaxation rate give
strong evidence for a long-range antiferromagnetic transition at Tn = 78 K in
the powder. Magnetic susceptibility measurements show that Tn = 69 K in the
crystals. A zero-field V NMR signal was observed at low temperatures (f
237 MHz at 4.2 K) in the crystals. The field swept spectra with the
field in different directions suggest the presence of two antiferromagnetic
substructures. Each substructure is collinear, with the easy axes of the two
substructures separated by an angle of 19(1) degree, and with their average
direction pointing approximately along the b-axis of the crystal structure. The
two spin substructures contain equal number of spins. The temperature
dependence of the ordered moment, measured up to 45 K, shows the presence of an
energy gap Eg in the antiferromagnetic spin wave excitation spectrum.
Antiferromagnetic spin wave theory suggests that Eg lies between 64 and 98 K.Comment: 11 pages, 14 figures. v2: 2 new figures; version published in Phys.
Rev.
Charge order and low frequency spin dynamics in lanthanum cuprates revealed by Nuclear Magnetic Resonance
We report detailed 17O, 139La, and 63Cu Nuclear Magnetic Resonance (NMR) and
Nuclear Quadrupole Resonance (NQR) measurements in a stripe ordered
La1.875Ba0.125CuO4 single crystal and in oriented powder samples of
La1.8-xEu0.2SrxCuO4. We observe a partial wipeout of the 17O NMR intensity and
a simultaneous drop of the 17O electric field gradient (EFG) at low
temperatures where the spin stripe order sets in. In contrast, the 63Cu
intensity is completely wiped out at the same temperature. The drop of the 17O
quadrupole frequency is compatible with a charge stripe order. The 17O spin
lattice relaxation rate shows a peak similar to that of the 139La, which is of
magnetic origin. This peak is doping dependent and is maximal at x ~ 1/8.Comment: submitted to European Physical Journal Special Topic
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