100 research outputs found
Influence of free carriers on exciton ground states in quantum wells
The influence of free carriers on the ground state of the exciton at zero
magnetic field in a quasi-two-dimensional quantum well doped with electrons is
considered in the framework of the random phase approximation. The effects of
the exciton-charge-density interaction and the inelastic scattering processes
due to the Hartree-Fock electron-electron exchange interaction are taken into
account. The effect of phase-space filling is considered using an approximate
approach. The results of the calculation are compared with the experimental
data available.Comment: 8 pages, 5 fugure
Diffusivities and kinetics of short-range and long-range orderings in Ni-Fe permalloys
The microscopic model of atomic diffusion is considered to describe the
short-range order relaxation kinetics within the f.c.c.-Ni-Fe Permalloys. The
model takes into account both the discrete and anisotropic characters of atomic
jumps within the long-range field of concentration heterogeneities of the
interacting atoms. The diffusion coefficients and activation energies for the
disordered Ni-Fe permalloy are estimated with the evaluated probabilities of
atomic jumps. As shown, the increasing of a temperature with a fixed
composition influences on the 'potential' field of interatomic interaction
ambiguously: the field 'potential' increases for defined coordination shells
and decreases for some of other ones. Although the temperature increasing
promotes the increasing of any atomic-probabilities jumps generally, but
decreasing of the action of 'potential' field generated by the atoms of defined
element and caused by its concentration heterogeneities onto the distant sites
results in increasing of the atomic-jumps' probabilities of just this element,
first of all, into the sites, which are more distant from the 'source' of
heterogeneity. Within the framework of the static concentration waves' method
along with the self-consistent field approximation, the Onsager-type kinetics
equation is obtained to describe the long-range order relaxation by the
L12-type superstructure. To calculate diffusivities for the ordered Ni3Fe
permalloy, the independent, diffraction experimental data of the long-range
order parameter relaxation are used. Theoretical curves of the long-range order
time evolution for the non-stoichiometric f.c.c.-Ni-Fe permalloys are plotted.
Decreasing of the concentration of alloying element results in decelerating of
the long-range order parameter change and in increasing of its relaxation time
Femtosecond study of the interplay between excitons, trions, and carriers in (Cd,Mn)Te quantum wells
We present an absorption study of the neutral and positively charged exciton
(trion) under the influence of a femtosecond, circularly polarized, resonant
pump pulse. Three populations are involved: free holes, excitons, and trions,
all exhibiting transient spin polarization. In particular, a polarization of
the hole gas is created by the formation of trions. The evolution of these
populations is studied, including the spin flip and trion formation processes.
The contributions of several mechanisms to intensity changes are evaluated,
including phase space filling and spin-dependent screening. We propose a new
explanation of the oscillator strength stealing phenomena observed in p-doped
quantum wells, based on the screening of neutral excitons by charge carriers.
We have also found that binding heavy holes into charged excitons excludes them
from the interaction with the rest of the system, so that oscillator strength
stealing is partially blockedComment: 4 pages, 4 figure
p-Type doping of II-VI heterostructures from surface states: application to ferromagnetic CdMnTe quantum wells
We present a study of p-type doping of CdTe and CdMnTe quantum
wells from surface states. We show that this method is as efficient as usual
modulation doping with nitrogen acceptors, and leads to hole densities
exceeding cm. Surface doping was successfully applied
to obtain carrier-induced ferromagnetism in a CdMnTe quantum well.
The observed temperature dependence of photoluminescence spectra, and the
critical temperature, correspond well to those previously reported for
ferromagnetic quantum wells doped with nitrogen.Comment: 4 figure
Microphotoluminescence study of disorder in ferromagnetic (Cd,Mn)Te quantum well
Microphotoluminescence mapping experiments were performed on a modulation
doped (Cd,Mn)Te quantum well exhibiting carrier induced ferromagnetism. The
zero field splitting that reveals the presence of a spontaneous magnetization
in the low-temperature phase, is measured locally; its fluctuations are
compared to those of the spin content and of the carrier density, also measured
spectroscopically in the same run. We show that the fluctuations of the carrier
density are the main mechanism responsible for the fluctuations of the
spontaneous magnetization in the ferromagnetic phase, while those of the Mn
spin density have no detectable effect at this scale of observation.Comment: 4 pages, 3 figure
Carrier-induced ferromagnetism in p-Zn1-xMnxTe
We present a systematic study of the ferromagnetic transition induced by the
holes in nitrogen doped Zn1-xMnxTe epitaxial layers, with particular emphasis
on the values of the Curie-Weiss temperature as a function of the carrier and
spin concentrations. The data are obtained from thorough analyses of the
results of magnetization, magnetoresistance and spin-dependent Hall effect
measurements. The experimental findings compare favorably, without adjustable
parameters, with the prediction of the Rudermann-Kittel-Kasuya-Yosida (RKKY)
model or its continuous-medium limit, that is, the Zener model, provided that
the presence of the competing antiferromagnetic spin-spin superexchange
interaction is taken into account, and the complex structure of the valence
band is properly incorporated into the calculation of the spin susceptibility
of the hole liquid. In general terms, the findings demonstrate how the
interplay between the ferromagnetic RKKY interaction, carrier localization, and
intrinsic antiferromagnetic superexchange affects the ordering temperature and
the saturation value of magnetization in magnetically and electrostatically
disordered systems.Comment: 14 pages, 10 figure
Subnanosecond spectral diffusion of a single quantum dot in a nanowire
We have studied spectral diffusion of the photoluminescence of a single CdSe
quantum dot inserted in a ZnSe nanowire. We have measured the characteristic
diffusion time as a function of pumping power and temperature using a recently
developed technique [G. Sallen et al, Nature Photon. \textbf{4}, 696 (2010)]
that offers subnanosecond resolution. These data are consistent with a model
where only a \emph{single} carrier wanders around in traps located in the
vicinity of the quantum dot
- …