787 research outputs found
Low-temperature specific heat for ferromagnetic and antiferromagnetic orders in CaRu1-xMnxO3
Low-temperature specific heat of CaRu1-xMnxO3 was measured to clarify the
role of d electrons in ferromagnetic and antiferromagnetic orders observed
above x=0.2. Specific heat divided by temperature C_p/T is found to roughly
follow a T^2 function, and relatively large magnitudes of electronic specific
heat coefficient gamma were obtained in wide x range. In particular, gamma is
unchanged from the value at x=0 (84 mJ/K^2 mol) in the paramagnetic state for
x<=0.1, but linearly reduced with increasing x above x= 0.2. These features of
gamma strongly suggest that itinerant d electrons are tightly coupled with the
evolution of magnetic orders in small and intermediate Mn concentrations.Comment: 4 pages, 2 figures, to be published in J. Phys.: Conf. Ser. (SCES
2011, Cambridge, UK
Shape-independent scaling of excitonic confinement in realistic quantum wires
The scaling of exciton binding energy in semiconductor quantum wires is
investigated theoretically through a non-variational, fully three-dimensional
approach for a wide set of realistic state-of-the-art structures. We find that
in the strong confinement limit the same potential-to-kinetic energy ratio
holds for quite different wire cross-sections and compositions. As a
consequence, a universal (shape- and composition-independent) parameter can be
identified that governs the scaling of the binding energy with size. Previous
indications that the shape of the wire cross-section may have important effects
on exciton binding are discussed in the light of the present results.Comment: To appear in Phys. Rev. Lett. (12 pages + 2 figures in postscript
Excitons in T-shaped quantum wires
We calculate energies, oscillator strengths for radiative recombination, and
two-particle wave functions for the ground state exciton and around 100 excited
states in a T-shaped quantum wire. We include the single-particle potential and
the Coulomb interaction between the electron and hole on an equal footing, and
perform exact diagonalisation of the two-particle problem within a finite basis
set. We calculate spectra for all of the experimentally studied cases of
T-shaped wires including symmetric and asymmetric GaAs/AlGaAs and
InGaAs/AlGaAs structures. We study in detail the
shape of the wave functions to gain insight into the nature of the various
states for selected symmetric and asymmetric wires in which laser emission has
been experimentally observed. We also calculate the binding energy of the
ground state exciton and the confinement energy of the 1D quantum-wire-exciton
state with respect to the 2D quantum-well exciton for a wide range of
structures, varying the well width and the Al molar fraction . We find that
the largest binding energy of any wire constructed to date is 16.5 meV. We also
notice that in asymmetric structures, the confinement energy is enhanced with
respect to the symmetric forms with comparable parameters but the binding
energy of the exciton is then lower than in the symmetric structures. For
GaAs/AlGaAs wires we obtain an upper limit for the binding energy
of around 25 meV in a 10 {\AA} wide GaAs/AlAs structure which suggests that
other materials must be explored in order to achieve room temperature
applications. There are some indications that
InGaAs/AlGaAs might be a good candidate.Comment: 20 pages, 10 figures, uses RevTeX and psfig, submitted to Physical
Review
Bound states of L-shaped or T-shaped quantum wires in inhomogeneous magnetic fields
The bound state energies of L-shaped or T-shaped quantum wires in inhomogeous
magnetic fields are found to depend strongly on the asymmetric parameter
, i.e. the ratio of the arm widths. Two effects of
magnetic field on bound state energies of the electron are obtained. One is the
depletion effect which purges the electron out of the OQD system. The other is
to create an effective potential due to quantized Landau levels of the magnetic
field. The bound state energies of the electron in L-shaped or T-shaped quantum
wires are found to depend quadratically (linearly) on the magnetic field in the
weak (strong) field region and are independent of the direction of the magnetic
field. A simple model is proposed to explain the behavior of the magnetic
dependence of the bound state energy both in weak and strong magnetic field
regions.Comment: 4 pages, 4 figure
Gain in a quantum wire laser of high uniformity
A multi-quantum wire laser operating in the 1-D ground state has been
achieved in a very high uniformity structure that shows free exciton emission
with unprecedented narrow width and low lasing threshold. Under optical pumping
the spontaneous emission evolves from a sharp free exciton peak to a
red-shifted broad band. The lasing photon energy occurs about 5 meV below the
free exciton. The observed shift excludes free excitons in lasing and our
results show that Coulomb interactions in the 1-D electron-hole system shift
the spontaneous emission and play significant roles in laser gain.Comment: 4 pages, 4 figures, prepared by RevTe
Suzaku X-Ray Imaging and Spectroscopy of Cassiopeia A
Suzaku X-ray observations of a young supernova remnant, Cassiopeia A, were
carried out. K-shell transition lines from highly ionized ions of various
elements were detected, including Chromium (Cr-Kalpha at 5.61 keV). The X-ray
continuum spectra were modeled in the 3.4--40 keV band, summed over the entire
remnant, and were fitted with a simplest combination of the thermal
bremsstrahlung and the non-thermal cut-off power-law models. The spectral fits
with this assumption indicate that the continuum emission is likely to be
dominated by the non-thermal emission with a cut-off energy at > 1 keV. The
thermal-to-nonthermal fraction of the continuum flux in the 4-10 keV band is
best estimated as ~0.1. Non-thermal-dominated continuum images in the 4--14 keV
band were made. The peak of the non-thermal X-rays appears at the western part.
The peak position of the TeV gamma-rays measured with HEGRA and MAGIC is also
shifted at the western part with the 1-sigma confidence. Since the location of
the X-ray continuum emission was known to be presumably identified with the
reverse shock region, the possible keV-TeV correlations give a hint that the
accelerated multi-TeV hadrons in Cassiopeia A are dominated by heavy elements
in the reverse shock region.Comment: Publ. Astron. Soc. Japan 61, pp.1217-1228 (2009
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