Phonon Bloch oscillations in acoustic-cavity structures

We describe a semiconductor multilayer structure based in acoustic phonon cavities and achievable with MBE technology, designed to display acoustic phonon Bloch oscillations. We show that forward and backscattering Raman spectra give a direct measure of the created phononic Wannier-Stark ladder. We also discuss the use of femtosecond laser impulsions for the generation and direct probe of the induced phonon Bloch oscillations. We propose a gedanken experiment based in an integrated phonon source-structure-detector device, and we present calculations of pump and probe time dependent optical reflectivity that evidence temporal beatings in agreement with the Wannier-Stark ladder energy splitting.Comment: PDF file including 4 figure

Dynamical Corrections to Spin Wave Excitations in Quantum Wells due to Coulomb Interactions and Magnetic Ions

We have measured dispersions of spin-flip waves and spin-flip single-particle excitations of a spin polarized two-dimensional electron gas in a CdMnTe quantum well using resonant Raman scattering. We find the energy of the spin-flip wave to be below the spin-flip single particle excitation continuum, a contradiction to the theory of spin waves in diluted magnetic semiconductors put forth in [Phys. Rev. B 70, 045205 (2004)]. We show that the inclusion of terms accounting for the Coulomb interaction between carriers in the spin wave propagator leads to an agreement with our experimental results. The dominant Coulomb contribution leads to an overall red shift of the mixed electron-Mn spin modes while the dynamical coupling between Mn ions results in a small blue shift. We provide a simulated model system which shows the reverse situation but at an extremely large magnetic field.Comment: 6 pages, 7 figure

Cavity polariton optomechanics: Polariton path to fully resonant dispersive coupling in optomechanical resonators

Resonant photoelastic coupling in semiconductor nanostructures opens new perspectives for strongly enhanced light-sound interaction in optomechanical resonators. One potential problem, however, is the reduction of the cavity Q-factor induced by dissipation when the resonance is approached. We show in this letter that cavity-polariton mediation in the light-matter process overcomes this limitation allowing for a strongly enhanced photon-phonon coupling without significant lifetime reduction in the strongly-coupled regime. Huge optomechanical coupling factors in the PetaHz/nm range are envisaged, three orders of magnitude larger than the backaction produced by the mechanical displacement of the cavity mirrors.Comment: 6 pages, 4 figure

Sub-Terahertz Monochromatic Transduction with Semiconductor Acoustic Nanodevices

We demonstrate semiconductor superlattices or nanocavities as narrow band acoustic transducers in the sub-terahertz range. Using picosecond ultrasonics experiments in the transmission geometry with pump and probe incident on opposite sides of the thick substrate, phonon generation and detection processes are fully decoupled. Generating with the semiconductor device and probing on the metal, we show that both superlattices and nanocavities generate spectrally narrow wavepackets of coherent phonons with frequencies in the vicinity of the zone center and time durations in the nanosecond range, qualitatively different from picosecond broadband pulses usually involved in picosecond acoustics with metal generators. Generating in the metal and probing on the nanoacoustic device, we furthermore evidence that both nanostructured semiconductor devices may be used as very sensitive and spectrally selective detectors

Dispersion and damping of multi-quantum well polaritons from resonant Brillouin scattering by folded acoustic modes

We report on confined exciton resonances of acoustic and folded acoustic phonon light scattering in a GaAs/AlAs multi-quantum-well. Significant variations of the line shifts and widths are observed across the resonance and quantitatively reproduced in terms of the polariton dispersion. This high resolution Brillouin study brings new unexpectedly detailed informations on the polariton dynamics in confined systems

Décomposition chimique et isotopique d'un hydrogramme de crue d'un torrent méditerranéen - Réflexions méthodologiques

La décomposition chimique et isotopique d'un hydrogramme de crue d'un petit torrent méditerranéen (Cannone, Corse, France), a été tentée dans un cas simple, en confrontant les résultats des deux méthodes et en discutant leurs limites. L'outil isotopique permet de bien différencier l'eau préexistante et l'eau nouvelle. L'analyse de la variation des éléments chimiques met en évidence la participation d'eau du soi, sans qu'il soit possible de quantifier sa contribution; elle démontre que la composition de la pluie n'est pas conservative; le rôle du pluviolessivage est mis en évidence, conjointement à celui du transfert dans la zone non-saturée. Ainsi les décompositions chimiques des hydrogrammes (en particulier celles réalisées avec la conductivité) reposent le plus souvent sur des approximations théoriquement contestables et conduisent à des erreurs notables, à l'exception toutefois de celles réalisées avec la silice.Problems of environmental water quality such as transfer of pollutants and ecosystem acidification call for a new insight to the path and the contact time of water in the different subsurface reservoirs. An isotopic and chemical hydrograph separation is carried out for a flood in a small upland watershed in the Mediterranean region (Cannone, Corsica Island, France) with special attention to a comparison between the two methods and discussion of their limits. Here, old and new water are clearly distinguished by isotopic composition. The pattern of the dissolved constituents of stream water shows the contribution of ground water, often assumed to be negligible. The non-conservative behaviour of rain-water chemistry during its path to the stream channel due to enrichment by throughfall and leaching of soils, is the main obstacle to chemical separation. Chemical separation, especially through specific conductance seems generally to be hazardous, except perhaps for dissolved silica. Checking the variation of all dissolved constituents would prevent questionable approximations

Apports des outils chiniques et isotopiques à l'identification des origines de la salinisation des eaux : Cas de la nappe de La Chaouia côtière (Maroc)

L'étude hydrochimique des eaux de la nappe libre de la Chaouia côtière montre une teneur excessive en sels dans ces eaux, spécialement des chlorures (jusqu'à 3 g.l-1).Cette salinité élevée peut, à terme, nuire gravement à l'économie de la région qui se consacre essentiellement à la culture maraîchère sous irrigation.Des mesures isotopiques (18O/16O, 2H/1H) couplées aux éléments chimiques, notamment Br-/Cl-, excluent l'hypothèse d'une intrusion marine généralisée. L'existence d'une paléosalinité (SHIVANNA andal.,1993) n'est pas soutenue par les mesures des teneurs en isotopes radioactifs (3H, 14C). Il en est de même pour le lessivage de dépôts chlorurés préexistants dans les sédiments. L'apport essentiel en chlorures provient de l'altération de la roche mère, des fertilisants et des embruns marins lessivés par suite de l'irrigation et entraînés vers la nappe par percolation. Le recyclage de l'eau souterraine depuis au moins trente ans, par l'irrigation, n'a fait qu'augmenter cette salinité.Toutefois en bordure de l'océan quelques puits semblent montrer la présence d'eau marine, en effet les débits d'exhaure des puits sont à la limite d'exploitation de la nappe et toute augmentation de débits dans ces puits engendrera fatalement une invasion marine généralisée.In West of Morocco, the coastal plain Chaouia is located between Casablanca and Azemmour (south-west of Casablanca) over a distance of 65km (Figure 1). This plain aquifer spreads over 1100 km2 in semi-arid climatic conditions with about 370 mm.year-1 of mean precipitation and 17°C in mean air temperature.Irrigated agricultural farming is the main economic resource of the region. The only source of water is provided by highly mineralized groundwaters, which are harmful for the rural population and agricultural irrigation. The chloride content represents the main contribution of the salinity of ground waters. Most of the previous authors and authorities have attributed the high mineralization to seawater intrusion (YOUNSI, 1994). The objective of the present study is to determine the cause of salinity, considering three hypothesis: seawater intrusion, leaching of salts and evaporation of water. The environmental isotopes (2H,18O, 3H, 14C) have been used with hydrogeology together with major and minor ion chemistry to identify the source of salinity and to estimate the residence time of ground water. Moreover an environmental isotope study was carried out in order to identify the origin of the salinization (MERLIVAT et al., 1970; COTECCHIA et al., 1974; GASPARINI, 1989; CABRAL et al., 1991; GOMEZ-MARTOS et al., 1993; HASHASH et al., 1995). Both isotopic studies and Br-/Cl- ratios have been used to identify the salinity origin. The potential sources for this salinization are: seawater encroachment as a consequence of intense exploitation of the aquifer, dissolution of the halite from the several diapiric structures intruded in the aquifers. The isotopic waters sampling were carried out in August 1993 on several wells (Figure 2), in order to study the variations of the oxygen-18, deuterium, tritium and carbon-14 isotopes. Table 1 shows the results of the isotope and chemical analyses carried out. As oxygen-18 data for local precipitation are not available in Morocco, the oxygen-18 and deuterium mean contents in precipitation were taken from stations of the International Atomic Energy Agency (IAEA) network near the studied area (Gibraltar, Faro, Ponta Delgada) and/or deduced from published isotopic composition of shallow groundwaters in Morocco (KABBAJ et al., 1978; LOUVA and BICHARA, 1990; BOUCHAOU et al., 1995); the mean values for the precipitation are -4.0 ± 0.5‰ and -23.0 ± 2 ‰ (vs. V-SMOW) respectively. The oxygen-18 and deuterium mean contents in ground waters are -3.7 ± 0.3 ‰ and -20.1 ± 1.8 ‰ (vs.V-SMOW) respectively. The relationship between the oxygen-18 and deuterium contents of some groundwater samples indicates a good fitting along the world meteoric line (CRAIG, 1961) (Figure 3), suggesting that the ground waters are mainly of meteoric origin. There is no indication of possible mixing with seawater because there is no indication of a relation between the oxygen-18 content of the wells and their distance from the sea (Figure 4). On the other hand, no isotopic enrichment is noted during the infiltration of water from precipitation and/or irrigation.The diagram d18O-Cl- (Figure 5) shows that all the data do not lie along a theoretical mixing line with seawater and/or an evaporation line. Every 18O data point is very close to the isotopic values of precipitation [-4.0± 0.5 ‰ (vs. V-SMOW)] but with a large scatter in Cl- content. In this zone, an increase in the salt concentration is not accompanied by an isotopic effect. The sources of the chloride must be found among the products of rock alteration, fertilizers and marine airborne salts. The recycling of salt water by irrigation increases the salinity of soil and ground waters. Halite is characterized by a very low Br-/Cl- ratio(0.183 x 10-3). Water in some wells presents Br-/Cl- ratios similar to those of halite, but most of the groundwater samples have a greater ratio. These results argue against the hypothesis of dissolution of halite in the aquifer formation. But the Br-/Cl- ratio alone is not sufficient for detecting a possible seawater intrusion.During August 1994, 20 drilled wells were sampled over the study area for determining tritium concentrations. Carbon-14 activities were measured only on some samples. The relatively high tritium content in the majority of the sampled waters [between 2 and 9 tritium units (TU)] indicates a recent groundwater recharge (Table 1). The use of natural radioactive isotopes (3H, 14C) in the ground waters gives an estimate of mean residence time of 50 ± 20 years (mixing model). Some waters show a mean residence time greater than 100 years because they are isolated from the general circulation and limited to local spots. The recent 14C activity of the groundwater (_100 pcm) allows us to reject the hypothesis of a mixing with paleoconnate waters as demonstrated elsewhere (SHIVANNA et al., 1993).In the case of the coastal Chaouia (semi-arid environment), the use of both chemical tracers (Cl-, Br-) and isotopes (18O, 2H,3H, 14C) has shown that rainfall constitutes the essential origin of ground water in the area. The high salinity caused by seawater intrusion is probably limited in space to some coastal wells; neither evaporation of water before and during percolation, nor dissolution of evaporitic deposits (halite), has been found. The past marine transgression salt origin has also been discarded. The main source of the salinity is the washout of the rock alteration salts, fertilizers, marine airborne salts, and the unusable salts rejected by plants in the soil. Thus the increase of salinity is well explained by the recycling of salty ground waters by irrigation during at least thirty years

Electronic Raman scattering on under-doped Hg-1223 high-Tc superconductors:investigations on the symmetry of the order parameter

In order to obtain high quality, reliable electronic Raman spectra of a high-Tc superconductor compound, we have studied strongly under-doped HgBa2Ca2Cu3O8+d. This choice was made such as to i)minimize oxygen disorder in the Hg-plane generated by oxygen doping ii) avoid the need of phonon background subtraction from the raw data iii)eliminate traces of parasitic phases identified and monitored on the crystal surface. Under these experimental conditions we are able to present the pure electronic Raman response function in the B2g, B1g, A1g+B2g and A1g+B1g channels. The B2g spectrum exhibits a linear frequency dependence at low energy whereas the B1g one shows a cubic-like dependence. The B2g and B1g spectra display two well defined maxima at 5.6kBTc and 9kBTc respectively. In mixed A1g channels an intense electronic peak centered around 6.4 kBTc is observed. The low energy parts of the spectra correspond to the electronic response expected for a pure dx2-y2 gap symmetry and can be fitted up to the gap energy for the B1g channel. However, in the upper parts, the relative position of the B1g and B2g peaks needs expanding the B2g Raman vertex to second order Fermi surface harmonics to fit the data with the dx2-y2 model. The sharper and more intense A1g peak appears to challenge the Coulomb screening efficiency present for this channel. As compared to previous data on more optimally doped, less stoichiometric Hg-1223 compounds, this work reconciles the electronic Raman spectra of under- doped Hg-1223 crystals with the dx2-y2 model, provided that the oxygen doping is not too strong. This apparent extreme sensitivity of the electronic Raman spectra to the low lying excitations induced by oxygen doping in the superconducting state is emphasized here and remains an open question.Comment: 12 pages, 5 figure

Charge and spin density modulations in semiconductor quantum wires

We investigate static charge and spin density modulation patterns along a ferromagnet/semiconductor single junction quantum wire in the presence of spin-orbit coupling. Coherent scattering theory is used to calculate the charge and spin densities in the ballistic regime. The observed oscillatory behavior is explained in terms of the symmetry of the charge and spin distributions of eigenstates in the semiconductor quantum wire. Also, we discuss the condition that these charge and spin density oscillations can be observed experimentally.Comment: 7 pages, 8 figures (low-resolution

Tunable optical Aharonov-Bohm effect in a semiconductor quantum ring

By applying an electric field perpendicular to a semiconductor quantum ring we show that it is possible to modify the single particle wave function between quantum dot (QD)-like to ring-like. The constraints on the geometrical parameters of the quantum ring to realize such a transition are derived. With such a perpendicular electric field we are able to tune the Aharanov-Bohm (AB) effect for both single particles and for excitons. The tunability is in both the strength of the AB-effect as well as in its periodicity. We also investigate the strain induce potential inside the self assembled quantum ring and the effect of the strain on the AB effect