554 research outputs found
Strain in crystalline core-shell nanowires
The strain configuration induced by the lattice mismatch in a core-shell
nanowire is calculated analytically, taking into account the crystal anisotropy
and the difference in stiffness constants of the two materials. The method is
applied to nanowires with the wurtzite structure or the zinc-blende structure
with the hexagonal / trigonal axis along the nanowire, and the results are
compared to available numerical calculations and experimental data. It is also
applied to multishell nanowires, and to core-shell nanowires grown along the
axis of cubic semiconductors
Atomic-like spin noise in solid-state demonstrated with manganese in cadmium telluride
Spin noise spectroscopy is an optical technique which can probe spin
resonances non-perturbatively. First applied to atomic vapours, it revealed
detailed information about nuclear magnetism and the hyperfine interaction. In
solids, this approach has been limited to carriers in semiconductor
heterostructures. Here we show that atomic-like spin fluctuations of Mn ions
diluted in CdT e (bulk and quantum wells) can be detected through the Kerr
rotation associated to excitonic transitions. Zeeman transitions within and
between hyperfine multiplets are clearly observed in zero and small magnetic
fields and reveal the local symmetry because of crystal field and strain. The
linewidths of these resonances are close to the dipolar limit. The sensitivity
is high enough to open the way towards the detection of a few spins in systems
where the decoherence due to nuclear spins can be suppressed by isotopic
enrichment, and towards spin resonance microscopy with important applications
in biology and materials science
Spin waves in magnetic quantum wells with Coulomb interaction and exchange coupling
We theoretically describe the spin excitation spectrum of a two dimensional
electron gas embedded in a quantum well with localized magnetic impurities.
Compared to the previous work, we introduce equations that allow to consider
the interplay between the Coulomb interaction of delocalized electrons and the
exchange coupling between electrons and magnetic impurities. Strong
qualitative changes are found : mixed waves propagate below the single particle
continuum, an anticrossing gap is open at a specific wavevector and the kinetic
damping due to the electron motion strongly influences the coupling strength
between electrons and impurities spins
Electric-field control of the magnetic anisotropy in an ultrathin (Ga,Mn)As/(Ga,Mn)(As,P) bilayer
We report on the electric control of the magnetic anisotropy in an ultrathin
ferromagnetic (Ga,Mn)As/(Ga,Mn)(As,P) bilayer with competing in-plane and
out-of-plane anisotropies. The carrier distribution and therefore the strength
of the effective anisotropy is controlled by the gate voltage of a field effect
device. Anomalous Hall Effect measurements confirm that a depletion of carriers
in the upper (Ga,Mn)As layer results in the decrease of the in-plane
anisotropy. The uniaxial anisotropy field is found to decrease by a factor ~ 4
over the explored gate-voltage range, so that the transition to an out-of-plane
easy-axis configuration is almost reached
Modelling (001) surfaces of II-VI semiconductors
First, we present a two-dimensional lattice gas model with anisotropic
interactions which explains the experimentally observed transition from a
dominant c(2x2) ordering of the CdTe(001) surface to a local (2x1) arrangement
of the Cd atoms as an equilibrium phase transition. Its analysis by means of
transfer-matrix and Monte Carlo techniques shows that the small energy
difference of the competing reconstructions determines to a large extent the
nature of the different phases. Then, this lattice gas is extended to a model
of a three-dimensional crystal which qualitatively reproduces many of the
characteristic features of CdTe which have been observed during sublimation and
atomic layer epitaxy.Comment: 5 pages, 3 figure
Spin waves in diluted magnetic quantum wells
We study collective spin excitations in two-dimensional diluted magnetic
semiconductors, placed into external magnetic field. Two coupled modes of the
spin waves (the electron and ion modes) are found to exist in the system along
with a number of the ion spin excitations decoupled from the electron system.
We calculate analytically the spectrum of the waves taking into account the
exchange interaction of itinerant electrons both with each other and with
electrons localized on the magnetic ions. The interplay of these interactions
leads to a number of intriguing phenomena including tunable anticrossing of the
modes and a field-induced change in a sign of the group velocity of the ion
mode
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
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
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