751 research outputs found
Excitonic lasing in semiconductor quantum wires
Direct experimental evidences for excitonic lasing is obtained in optically
pumped V-groove quantum wire structures. We demonstrate that laser emission at
a temperature of 10 K arises from a population inversion of localized excitons
within the inhomogenously-broadened luminescence line. At the lasing threshold,
we estimate a maximum exciton density of about 1.8 105cm-1.Comment: 11 pages, 4 figures, submitted to Phys. Rev.
An alternate model for magnetization plateaus in the molecular magnet V_15
Starting from an antiferromagnetic Heisenberg Hamiltonian for the fifteen
spin-1/2 ions in V_15, we construct an effective spin Hamiltonian involving
eight low-lying states (spin-1/2 and spin-3/2) coupled to a phonon bath. We
numerically solve the time-dependent Schrodinger equation of this system, and
obtain the magnetization as a function of temperature in a time-dependent
magnetic field. The magnetization exhibits unusual patterns of hysteresis and
plateaus as the field sweep rate and temperature are varied. The observed
plateaus are not due to quantum tunneling but are a result of thermal
averaging. Our results are in good agreement with recent experimental
observations.Comment: Revtex, 4 pages, 5 eps figure
String-inspired Teleparallel Cosmology
The present paper represents an attempt for a very generic string inspired
theory of gravitation, based on a stringy action in the teleparallel gravity
which includes a specific functional which depends on the scalar field and its
kinetic energy, as well as the torsion and boundary terms, embedding also
possible effects from the teleparallel Gauss--Bonnet invariants. We focus our
study on FLRW cosmology. After we deduce the cosmological equations for the
associated generic theory of gravitation, we focus on string inspired couplings
which are studied by considering different analytical techniques. The first
analytical technique is based on the linear stability theory, by introducing
proper dimensionless variables which enables us to study the structure of the
phase space and the associated physical effects. In this case, we have obtained
different cosmological solutions which correspond to matter and dark energy
dominated solutions, achieving a possible transition between matter and dark
energy dominated epochs. For each type of cosmological solutions, we have
discussed the corresponding physical features, attaining viable constraints for
the coupling constants due to dynamical effects. The dynamical study of the
physical features included also a numerical analysis by fine--tuning the
initial conditions deep into the matter era, obtaining possible trajectories
for the effective equation of state for specific coupling functions.Comment: 18 pages. Matches published version in NP
Structural relaxation of E' gamma centers in amorphous silica
We report experimental evidence of the existence of two variants of the E'
gamma centers induced in silica by gamma rays at room temperature. The two
variants are distinguishable by the fine features of their line shapes in
paramagnetic resonance spectra. These features suggest that the two E' gamma
differ for their topology. We find a thermally induced interconversion between
the centers with an activation energy of about 34 meV. Hints are also found for
the existence of a structural configuration of minimum energy and of a
metastable state.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Recommended from our members
Model based heritability scores for high-throughput sequencing data
Supplementary materials. (PDF 1370 KB
Properties of low-lying states in some high-nuclearity Mn, Fe and V clusters: Exact studies of Heisenberg models
Using an efficient numerical scheme that exploits spatial symmetries and spin
parity, we have obtained the exact low-lying eigenstates of exchange
Hamiltonians for the high nuclearity spin clusters, Mn_{12}, Fe_8 and V_{15}.
The largest calculation involves the Mn_{12} cluster which spans a Fock space
of a hundred million. Our results show that the earlier estimates of the
exchange constants need to be revised for the Mn_{12} cluster to explain the
level ordering of low-lying eigenstates. In the case of the Fe_8 cluster,
correct level ordering can be obtained which is consistent with the exchange
constants for the already known clusters with butterfly structure. In the
V_{15} cluster, we obtain an effective Hamiltonian that reproduces exactly, the
eight low-lying eigenvalues of the full Hamiltonian.Comment: Revtex, 12 pages, 16 eps figures; this is the final published versio
- …