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We calculate the lifetime of conduction band excited states in self-assembled quantum dots by taking into account LO-phonon-electron interaction and various anharmonic phonon couplings. We show that polaron relaxation cannot be accurately described by a semi-classical model. The contributions of different anharmonic decay channels are shown to depend strongly on the polaron energy. We calculate the energy dependence of polaron lifetime and compare our results to available experimental measurements of polaron decay time in InAs/GaAs quantum dots., Comment: 5 pages, 4 figure, accepted for publication in Phys. Rev. B (Rapid Com.)

Polaron dephasing processes are investigated in InAs/GaAs dots using far-infrared transient four wave mixing (FWM) spectroscopy. We observe an oscillatory behaviour in the FWM signal shortly (< 5 ps) after resonant excitation of the lowest energy conduction band transition due to coherent acoustic phonon generation. The subsequent single exponential decay yields long intraband dephasing times of 90 ps. We find excellent agreement between our measured and calculated FWM dynamics, and show that both real and virtual acoustic phonon processes are necessary to explain the temperature dependence of the polarization decay., Comment: 10 pages, 4 figures, submitted to Phys Rev Lett

We report on calculations of broadening effects in QCL due to alloy scattering. The output of numerical calculations of alloy broadened Landau levels compare favorably with calculations performed at the self-consistent Born approximation. Results for Landau level width and optical absorption are presented. A disorder activated forbidden transition becomes significant in the vicinity of crossings of Landau levels which belong to different subbands. A study of the time dependent survival probability in the lowest Landau level of the excited subband is performed. It is shown that at resonance the population relaxation occurs in a subpicosecond scale., Comment: 7 pages, 8 figures

Magnetic tunnel junctions (MTJs) with partially oxidized 9 \AA AlO$_x$-barriers were recently shown to have the necessary characteristics to be used as magnetoresistive sensors in high-density storage devices. Here we study dielectric breakdown in such underoxidized magnetic tunnel junctions, focusing on its dependence on tunnel junction area and oxidation time. A clear relation between breakdown mechanism and junction area is observed for the MTJs with the highest studied oxidation time: samples with large areas fail usually due to extrinsic causes (characterized by a smooth resistance decrease at dielectric breakdown). Small area junctions fail mainly through an intrinsic mechanism (sharp resistance decrease at breakdown). However, this dependence changes for lower oxidation times, with extrinsic breakdown becoming dominant. In fact, in the extremely underoxidized magnetic tunnel junctions, failure is exclusively related with extrinsic causes, independently of MTJ-area. These results are related with the presence of defects in the barrier (weak spots that lead to intrinsic breakdown) and of metallic unoxidized Al nanoconstrictions (leading to extrinsic breakdown)., Comment: 3 pages, 3 figures, accepted in IEEE Trans. Magn

We present a detailed study on domain imaging, Kerr effect magnetometry (MOKE) and magnetoresistance (MR), for a series of 20 nm Co$_{73.8}$Fe$_{16.2}$B$_{10}$ thin films, both as-deposited (amorphous) and annealed (crystalline). By considering the two different (orthogonal) in-plane magnetization components, obtained by MOKE measurements, we were able to study the uniaxial anisotropy induced during CoFeB-deposition and to discriminate the magnetization processes under a magnetic field parallel and perpendicular to such axis. MOKE magnetic imaging enabled us to observe the dominant magnetization processes, namely domain wall motion and moment rotation. These processes were correlated with the behavior of the magnetoresistance, which depends both on short-range spin disorder electron scattering and on the angle between the electrical current and the spontaneous magnetization ($\emph{\textbf{M}}_{S}$). A simple numerical treatment based on Stoner-Wolfarth model enables us to satisfactorily predict the magnetization behaviour observed in these films. A comparison between the results in Co$_{73.8}$Fe$_{16.2}$B$_{10}$ films and the previous ones obtained in annealed Co$_{80}$Fe$_{20}$ films, show that the introduction of boron in CoFe reduces significatively the coercive and saturation fields along the easy axis (e.g. $H_{c}$ from $\sim$ 2 down to $\sim$ 0.5 kAm$^{-1}$). Also, the magnetization along the hard axis saturates at lower fields. We conclude that amorphous and nanocrystalline CoFeB films show low coercive fields and abrupt switching, as well as absence of short range spin disorder effects after switching when compared with Co$_{80}$Fe$_{20}$.

A MOKE magnetometry unit simultaneously sensitive to both in-plane magnetization components, based on an intensity differential detection method, allows us to observe the uniaxial anisotropy impressed during CoFe-deposition and to discriminate the magnetization processes under a magnetic field parallel and perpendicular to such axes. Our MOKE imaging unit, using a CCD camera for Kerr effect domain visualization provides direct evidence on the dominant M-processes, namely domain wall motion and moment rotation. Further magnetic information was obtained by AMR measurements due to the dependence of the electrical resistivity on the short-range spin disorder and also on the angle between the electrical current direction (I) and the spontaneous magnetization ($\emph{\textbf{M}}_{S}$)., Comment: 5 pages, 2 figures

Exciton spin relaxation is investigated in single epitaxially grown semiconductor quantum dots in order to test the expected spin relaxation quenching in this system. We study the polarization anisotropy of the photoluminescence signal emitted by isolated quantum dots under steady-state or pulsed non-resonant excitation. We find that the longitudinal exciton spin relaxation time is strikingly short ($\leq$100 ps) even at low temperature. This result breaks down the picture of a frozen exciton spin in quantum dots.

We present the experimental evidence of giant optical anisotropy in single InAs quantum dots. Polarization-resolved photoluminescence spectroscopy reveals a linear polarization ratio with huge fluctuations, from one quantum dot to another, in sign and in magnitude with absolute values up to 82%. Systematic measurements on hundreds of quantum dots coming from two different laboratories demonstrate that the giant optical anisotropy is an intrinsic feature of dilute quantum-dot arrays., Comment: submitted to Applied Physics Letters

By combining band gap engineering with the self-organized growth of quantum dots, we present a scheme of adjusting the mid-infrared absorption properties to desired energy transitions in quantum dot based photodetectors. Embedding the self organized InAs quantum dots into an AlAs/GaAs superlattice enables us to tune the optical transition energy by changing the superlattice period as well as by changing the growth conditions of the dots. Using a one band envelope function framework we are able, in a fully three dimensional calculation, to predict the photocurrent spectra of these devices as well as their polarization properties. The calculations further predict a strong impact of the dots on the superlattices minibands. The impact of vertical dot alignment or misalignment on the absorption properties of this dot/superlattice structure is investigated. The observed photocurrent spectra of vertically coupled quantum dot stacks show very good agreement with the calculations.In these experiments, vertically coupled quantum dot stacks show the best performance in the desired photodetector application., Comment: 8 pages, 10 figures, submitted to PRB

We study combinatorial indicators related to the characteristic phase transitions associated with coloring a graph optimally and finding a maximum independent set. In particular, we investigate the role of the acyclic orientations of the graph in the hardness of finding the graph's chromatic number and independence number. We provide empirical evidence that, along a sequence of increasingly denser random graphs, the fraction of acyclic orientations that are `shortest' peaks when the chromatic number increases, and that such maxima tend to coincide with locally easiest instances of the problem. Similar evidence is provided concerning the `widest' acyclic orientations and the independence number.

We present a model to take into account the interface defects contribution on the binding energy of charged exciton in GaAs/Al$_{0.3}$Ga$_{0.7}$As quantum wells. The dependence of the binding energy gain and of the trion size on the quantum well width are variationally calculated. We show that the trion is more sensitive to interface defects than the exciton., Comment: 9 pages, 3 figures - OECS7 Proceeding

White light tunable phosphors are obtained from a sol-gel derived siliceous network to which oxyethylene units are covalently grafted by means of urea or urethane cross-linkages. The bright photoluminescence is ascribed to an unusual convolution of distinct emissions originated in the NH groups of the urea or urethane bridges and in the siliceous nanodomains. The lifetimes of these emissions show a remarkable temperature dependence of approximately six orders of magnitude between $\approx 220$ and 300 K associated with the hybrids glass transition temperature. This set of xerogels is one of the most surprising classes of full color emitters lacking metal activator ions., Comment: 9 pages, 4 figures, submitted to Chem. Phys. Letters