10,106 research outputs found
Fractionalization of minimal excitations in integer quantum Hall edge channels
A theoretical study of the single electron coherence properties of Lorentzian
and rectangular pulses is presented. By combining bosonization and the Floquet
scattering approach, the effect of interactions on a periodic source of voltage
pulses is computed exactly. When such excitations are injected into one of the
channels of a system of two copropagating quantum Hall edge channels, they
fractionalize into pulses whose charge and shape reflects the properties of
interactions. We show that the dependence of fractionalization induced
electron/hole pair production in the pulses amplitude contains clear signatures
of the fractionalization of the individual excitations. We propose an
experimental setup combining a source of Lorentzian pulses and an Hanbury Brown
and Twiss interferometer to measure interaction induced electron/hole pair
production and more generally to reconstruct single electron coherence of these
excitations before and after their fractionalization.Comment: 18 pages, 10 figures, 1 tabl
Raman scattering through surfaces having biaxial symmetry
Magnetic Raman scattering in two-leg spin ladder materials and the
relationship between the anisotropic exchange integrals are analyzed by P. J.
Freitas and R. R. P. Singh in Phys. Rev. B, {\bf 62}, 14113 (2000). The angular
dependence of the two-magnon scattering is shown to provide information for the
magnetic anisotropy in the Sr_14Cu_24O_41 and La_6Ca_8Cu_24O_41 compounds. We
point out that the experimental results of polarized Raman measurements at
arbitrary angles with respect to the crystal axes have to be corrected for the
light ellipticity induced inside the optically anisotropic crystals. We refer
quantitatively to the case of Sr_14Cu_24O_41 and discuss potential implications
for spectroscopic studies in other materials with strong anisotropy.Comment: To be published as a Comment in Phys. Rev.
The Next Geminga: Deep Multiwavelength Observations of a Neutron Star Identified with 3EG J1835+5918
We describe Chandra, HST, and radio observations that reveal a radio-quiet
but magnetospherically active neutron star in the error circle of the
high-energy gamma-ray source 3EG J1835+5918, the brightest of the unidentified
EGRET sources at high Galactic latitude. A Chandra ACIS-S spectrum of the
ultrasoft X-ray source RX J1836.2+5925, suggested by Mirabal & Halpern as the
neutron star counterpart of 3EG J1835+5918, requires two components: a
blackbody of T~3x10^5 K and a hard tail that can be parameterized as a power
law of photon index Gamma~2. An upper limit of d < 800 pc can be derived from
the blackbody fit under an assumption of R = 10 km. Deep optical imaging with
the HST STIS CCD failed to detect this source to a limit of V > 28.5, thus
f_X/f_V > 6000 and d > 250 pc assuming the X-ray fitted temperature for the
full surface. Repeated observations with the 76 m Lovell telescope at Jodrell
Bank place an upper limit of < 0.1 mJy on the flux density at 1400 MHz for a
pulsar with P > 0.1 s, and < 0.25 mJy for a ~10 ms pulsar at the location of RX
J1836.2+5925. All of this evidence points to an older, possibly more distant
version of the highly efficient gamma-ray pulsar Geminga, as the origin of the
gamma-rays from 3EG J1835+5918.Comment: 4 pages, 4 figures, accepted for publication in ApJ Letter
Charge Ordering and Spin Dynamics in NaV2O5
We report high-resolution neutron inelastic scattering experiments on the
spin excitations of NaV2O5. Below Tc, two branches associated with distinct
energy gaps are identified. From the dispersion and intensity of the spin
excitation modes, we deduce the precise zig-zag charge distribution on the
ladder rungs and the corresponding charge order (about 0.6). We argue that the
spin gaps observed in the low-T phase of this compound are primarily due to the
charge transfer.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Light-curve modelling constraints on the obliquities and aspect angles of the young Fermi pulsars
In more than four years of observation the Large Area Telescope on board the
Fermi satellite has identified pulsed -ray emission from more than 80
young pulsars, providing light curves with high statistics. Fitting the
observations with geometrical models can provide estimates of the magnetic
obliquity and aspect angle , yielding estimates of the
radiation beaming factor and luminosity. Using -ray emission geometries
(Polar Cap, Slot Gap, Outer Gap, One Pole Caustic) and radio emission geometry,
we fit -ray light curves for 76 young pulsars and we jointly fit their
-ray plus radio light curves when possible. We find that a joint radio
plus -ray fit strategy is important to obtain (, )
estimates that can explain simultaneous radio and -ray emission. The
intermediate-to-high altitude magnetosphere models, Slot Gap, Outer Gap, and
One pole Caustic, are favoured in explaining the observations. We find no
evolution of on a time scale of a million years. For all emission
geometries our derived -ray beaming factors are generally less than one
and do not significantly evolve with the spin-down power. A more pronounced
beaming factor vs. spin-down power correlation is observed for Slot Gap model
and radio-quiet pulsars and for the Outer Gap model and radio-loud pulsars. For
all models, the correlation between -ray luminosity and spin-down power
is consistent with a square root dependence. The -ray luminosities
obtained by using our beaming factors not exceed the spin-down power. This
suggests that assuming a beaming factor of one for all objects, as done in
other studies, likely overestimates the real values. The data show a relation
between the pulsar spectral characteristics and the width of the accelerator
gap that is consistent with the theoretical prediction for the Slot Gap model.Comment: 90 pages, 80 figures (63 in Appendices), accepted for publication in
Astronomy and Astrophysic
Magnetic and thermodynamic properties of Sr_{2}LaFe_{3}O_{9}
Using a Dirac-Heisenberg Hamiltonian with biquadratic exchange interactions,
we study the effect of iron disproportionation on the magnetic ordering, and
describe the first-order magnetic transition occurring in the perovskite
Sr_{2}LaFe_{3}O_{9}. Upon fitting the experimental data, we give an estimate of
the exchange integrals for the antiferromagntic and ferromagnetic interactions,
in agreement with previous works on kindered compounds. Spin-wave theory yields
a magnon spectrum with a gapless antiferromagnetic mode together with two
gapped ferromagnetic ones.Comment: 8 pages of RevTex, 5 figures (available upon request), submitted to
J. Mag. Mag. Ma
Surface effects on the orbital order in the single layered manganite La0.5Sr1.5MnO4
We report the first observation of `orbital truncation rods' -- the
scattering arising from the termination of bulk orbital order at the surface of
a crystal. The x-ray measurements, performed on a cleaved, single-layered
perovskite, La0.5Sr1.5MnO4, reveal that while the crystallographic surface is
atomically smooth, the orbital `surface' is much rougher, with an r.m.s.
deviation from the average `surface' of ~0.7nm. The temperature dependence of
this scattering shows evidence of a surface-induced second order transition.Comment: 13 pages, 4 figure
Normal State Magnetic Properties of Ni and Zn Substituted in YBa_{2}Cu_{3} O_{6+x}: Hole-Doping Dependence
We present SQUID susceptibility data on Zn and Ni substituted
YBa_{2}Cu_{3}O_{6+x}. Cross-checks with NMR yield an unprecedented accuracy in
the estimate of the magnetic susceptibility associated with the substituants,
from the underdoped to the lightly overdoped case. This allows us to determine
the Weiss temperature \theta for YBCO: its value is very small for all hole
dopings n_h. Since in conventional metals, the Kondo temperature,
, magnetic screening effects would not be expected for ; in contrast, increasing n_h produces a reduction of the small moment
induced by Zn^{2+} and a nearly constant effective moment for Ni^{2+}
corresponding to a spin 1/2 rather than to a spin 1.Comment: 4 pages, 5 figures, to be published in Europhysics Letter
Absolute magnitudes and kinematics of CP stars from Hipparcos data
The position in the HR diagram and the kinematic characteristics of different
kinds of CP stars of the upper main sequence are obtained using the LM method
(Luri et al., 1996). Most of the CP stars are main sequence stars occupying the
whole width of the sequence. From a kinematic point of view, they belong to the
young disk population (ages < 1.5 Gyr). It has also been found that, on
kinematic grounds, the behaviour of lambda Bootis stars is similar to the one
observed for normal stars of the same spectral range. On the other hand, roAp
and noAp stars show the same kinematic characteristics. The peculiar velocity
distribution function has been decomposed into a sum of three dimensional
gaussians and the presence of Pleiades, Sirius and Hyades moving groups has
been clearly established. Finally, a small number of CP stars are found to be
high-velocity objects.Comment: 8 pages, 1 figure, to appear in: Proc. of the 26th workshop of the
European Working Group on CP stars, eds. P. North, A. Schnell and J.
Ziznovsky, Contrib. Astr. Obs. Skalnate Pleso Vol. 27, No
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