3,387 research outputs found
Carrier dynamics and coherent acoustic phonons in nitride heterostructures
We model generation and propagation of coherent acoustic phonons in
piezoelectric InGaN/GaN multi-quantum wells embedded in a \textit{pin} diode
structure and compute the time resolved reflectivity signal in simulated
pump-probe experiments. Carriers are created in the InGaN wells by ultrafast
pumping below the GaN band gap and the dynamics of the photoexcited carriers is
treated in a Boltzmann equation framework. Coherent acoustic phonons are
generated in the quantum well via both deformation potential electron-phonon
and piezoelectric electron-phonon interaction with photogenerated carriers,
with the latter mechanism being the dominant one. Coherent longitudinal
acoustic phonons propagate into the structure at the sound speed modifying the
optical properties and giving rise to a giant oscillatory differential
reflectivity signal. We demonstrate that coherent optical control of the
differential reflectivity can be achieved using a delayed control pulse.Comment: 14 pages, 11 figure
Propagating Coherent Acoustic Phonon Wavepackets in InMnAs/GaSb
We observe pronounced oscillations in the differential reflectivity of a
ferromagnetic InMnAs/GaSb heterostructure using two-color pump-probe
spectroscopy. Although originally thought to be associated with the
ferromagnetism, our studies show that the oscillations instead result from
changes in the position and frequency-dependent dielectric function due to the
generation of coherent acoustic phonons in the ferromagnetic InMnAs layer and
their subsequent propagation into the GaSb. Our theory accurately predicts the
experimentally measured oscillation period and decay time as a function of
probe wavelength.Comment: 4 pages, 4 figure
The calpastatin-derived calpain inhibitor CP1B reduces mRNA expression of matrix metalloproteinase-2 and-9 and invasion by leukemic THP-1 cells
The ubiquitous proteases Ό- and m-calpain are Ca2+-dependent cysteine endopeptidases. Besides involvement in a variety of physio(patho)logical processes, recent studies suggest a pivotal role of calpains in differentiation of hematopoietic cells and tumor cell invasion. However, the precise actions of calpains and their endogenous inhibitor, calpastatin, in these processes are only partially understood. Here we have studied the role of the calpain/calpastatin system in the invasion of leukemic cells under basal and differentiationstimulating conditions. To further differentiate the human leukaemic cell line THP-1 (monocytic), the cells were treated for 24 hours with the differentiationstimulating reagents phorbol 12-myristate 13-acetate (PMA) and dimethyl sulfoxide (DMSO). Macrophage and granulocytelike differentiation was confirmed by induction of vimentin expression as well as by microscopic and fluorescence assisted cytometric analysis. Extracellular matrix (ECM) invasion of both the basal and differentiation stimulated cells in a Matrigel assay was inhibited by preincubation of the cells with the specific calpain inhibitor CP1B for 24 hours. Inhibition of invasiveness correlated with decreased mRNA expression and secretion of the matrix metalloproteinases MMP-2 and MMP-9. In contrast, addition of CP1B only during the invasion process did neither influence transmigration nor MMP release. This is the first report showing that the calpain/calpastatin system mediates MMPmRNA expression of the leukemic THP-1 cells and as a consequence their invasiveness
Resonant Coherent Phonon Spectroscopy of Single-Walled Carbon Nanotubes
Using femtosecond pump-probe spectroscopy with pulse shaping techniques, one
can generate and detect coherent phonons in chirality-specific semiconducting
single-walled carbon nanotubes. The signals are resonantly enhanced when the
pump photon energy coincides with an interband exciton resonance, and analysis
of such data provides a wealth of information on the chirality-dependence of
light absorption, phonon generation, and phonon-induced band structure
modulations. To explain our experimental results, we have developed a
microscopic theory for the generation and detection of coherent phonons in
single-walled carbon nanotubes using a tight-binding model for the electronic
states and a valence force field model for the phonons. We find that the
coherent phonon amplitudes satisfy a driven oscillator equation with the
driving term depending on photoexcited carrier density. We compared our
theoretical results with experimental results on mod 2 nanotubes and found that
our model provides satisfactory overall trends in the relative strengths of the
coherent phonon signal both within and between different mod 2 families. We
also find that the coherent phonon intensities are considerably weaker in mod 1
nanotubes in comparison with mod~2 nanotubes, which is also in excellent
agreement with experiment.Comment: 21 pages, 22 figure
In-Situ Infrared Transmission Study of Rb- and K-Doped Fullerenes
We have measured the four IR active molecular vibrations in
as a function of doping . We observe
discontinuous changes in the vibrational spectra showing four distinct phases
(presumably , and 6). The and modes
show the largest changes shifting downward in frequency in four steps as the
doping increases. Several new very weak modes are visible in the phase
and are possibly Raman modes becoming weakly optically active. We present
quantitative fits of the data and calculate the electron-phonon coupling of the
IR mode.Comment: 3 pages, Figure 1 included, 3 more figures available by request.
REVTEX v3.0 IRC60DO
Polarization dependence of coherent phonon generation and detection in highly-aligned single-walled carbon nanotubes
We have investigated the polarization dependence of the generation and
detection of radial breathing mode (RBM) coherent phonons (CP) in
highly-aligned single-walled carbon nanotubes. Using polarization-dependent
pump-probe differential-transmission spectroscopy, we measured RBM CPs as a
function of angle for two different geometries. In Type I geometry, the pump
and probe polarizations were fixed, and the sample orientation was rotated,
whereas, in Type II geometry, the probe polarization and sample orientation
were fixed, and the pump polarization was rotated. In both geometries, we
observed a very nearly complete quenching of the RBM CPs when the pump
polarization was perpendicular to the nanotubes. For both Type I and II
geometries, we have developed a microscopic theoretical model to simulate CP
generation and detection as a function of polarization angle and found that the
CP signal decreases as the angle goes from 0 degrees (parallel to the tube) to
90 degrees (perpendicular to the tube). We compare theory with experiment in
detail for RBM CPs created by pumping at the E44 optical transition in an
ensemble of single-walled carbon nanotubes with a diameter distribution
centered around 3 nm, taking into account realistic band structure and
imperfect nanotube alignment in the sample
Dark-bright magneto-exciton mixing induced by Coulomb interaction in strained quantum wells
Coupled magneto-exciton states between allowed (`bright') and forbidden
(`dark') transitions are found in absorption spectra of strained
InGaAs/GaAs quantum wells with increasing magnetic field up to
30 T. We found large (~ 10 meV) energy splittings in the mixed states. The
observed anticrossing behavior is independent of polarization, and sensitive
only to the parity of the quantum confined states. Detailed experimental and
theoretical investigations indicate that the excitonic Coulomb interaction
rather than valence band complexity is responsible for the splittings. In
addition, we determine the spin composition of the mixed states.Comment: 4 pages, 4 figure
Theory of coherent acoustic phonons in InGaN/GaN multi-quantum wells
A microscopic theory for the generation and propagation of coherent LA
phonons in pseudomorphically strained wurzite (0001) InGaN/GaN multi-quantum
well (MQW) p-i-n diodes is presented. The generation of coherent LA phonons is
driven by photoexcitation of electron-hole pairs by an ultrafast Gaussian pump
laser and is treated theoretically using the density matrix formalism. We use
realistic wurzite bandstructures taking valence-band mixing and strain-induced
piezo- electric fields into account. In addition, the many-body Coulomb
ineraction is treated in the screened time-dependent Hartree-Fock
approximation. We find that under typical experimental conditions, our
microscopic theory can be simplified and mapped onto a loaded string problem
which can be easily solved.Comment: 20 pages, 17 figure
Scattering length of the ground state Mg+Mg collision
We have constructed the X 1SIGMAg+ potential for the collision between two
ground state Mg atoms and analyzed the effect of uncertainties in the shape of
the potential on scattering properties at ultra-cold temperatures. This
potential reproduces the experimental term values to 0.2 inverse cm and has a
scattering length of +1.4(5) nm where the error is prodominantly due to the
uncertainty in the dissociation energy and the C6 dispersion coefficient. A
positive sign of the scattering length suggests that a Bose-Einstein condensate
of ground state Mg atoms is stable.Comment: 15 pages, 3 figures, Submitted Phys. Rev.
- âŠ