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

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

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

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

Essais de quantification du débit des résurgences sous-marines autour du Piton de la Fournaise (la Réunion, Océan Indien)

On étudie, par des mesures en plongée sous-marine, les émergences en mer des écoulements souterrains dans le Piton de la Fournaise (île de la Réunion) dans le cadre de recherches sur les relations entre circulations souterraines, flux de chaleur et phénomènes magmatiques. On établit:- que l'ensemble des émergences se situe au niveau du rivage, et non en profondeur- que grâce à des mesures approchées des phénomènes de mélange des eaux, on peut estimer le débit moyen des résurgences en mer à 0,4 m3s-1 par kilomètre de côte sur la façade sud de l'île. Ce chiffre est cohérent avec des estimations indirectes obtenues à partir du bilan hydrique.The « Piton de la Fournaise » on the island of la Réunion receives as much as 6 to 10 m y-l of rainfall, which almost immediately infiltrates due to the highly pervious nature of the basalt (microfissures, scoria). Very few springs or rivers drain the systeml however, numerous marine outlets have been observed by infrared thermographic surveys. Some of these outlets can also be observed on SPOT images, although no infrared charmel is available on SPOT.In an attempt to study the role of underground flow in the thermal balance of the volcano and its relation with magmatic phenomena, we have tried to observe the nature of these outlets by underwater exploration and to evaluate their flow rate by salinity and temperature mesurements.Longitudinal and transverse salinity profiles were measured by divers using a salinity-conductivity meter with automatic correction of temperature, transported in a waterproof container, on those plumes that had been observed on airborne images or by helicopter surveys.All the observed plumes are indeed mixing zones of seawater and freshwater, with salinity ranging from 29.9 to 35.1 %o (ocean salinity is35.2%o at la Réunion). Their temperature is in general lower than that of the sea, which explains why they are visible on infrared irnages.It was found, however, that all outlets are systematically situated very close to the shore line, with an elevation between + 1 and - 2 m of sea level. In none of the observations were outlets found that could originate deeper down in the sea. Most observable oulets are located at the base of basalt flows, in natural « tunnels » under such lava flows. This is consistent with the existence of an abrupt interface between seawater and freshwater on the island, which has been observed in a fewboreholes, and which forces the freshwater flow upwards towards the shore line, even if the nature of the flow is very discontinuous in the basalt.Based on the salinity profiles, we have attempted to estimate the flow rate. We focus here on a particular outlet at Vincendo. We liken the development and mixing of the plume to what occurs in an estuary with low flow. It has been observed that three mechanisms control the mixing in an estuary: wind which creates currents and pushes freshwater toward the edges; tidal effects and waves creating currents and turbulent mixing due to rugosity of the sides and bottom; density difrerences between seawater and freshwater, the latter floating on top of the former. Three cases are generally considered:a - Seas without tide: the interface is stratified;b - Seas with small tides: stratification and mixing occur sirnultaneously;c - Seas with large tides: no stratification and regular mixing in the vertical dimension.The observed salinity profiles at Vincendo clearly indicate that we are in the second case at la Réunion, where the tide amplitude is small (0.7 m).Two dimensionless numbers are used in estuaries: the Richardson and Froude numbers (see expression in text) (FISHER et al., 1979). It has been observed that the transition from case a to c corresponds to Richardson numbers in the range 0.08 to 0.8. Assuming that salinity profiles are consistent with case b, i.e. a Richadson number in the range 0.25 - 0.80, we find that the freshwater flowrate should be in the range 0.020 - 0.260 m3. s-1.A second independent estimation can be obtained by observing that the average concentration gradient in the 7,000 m2 of the obseved mixing zone (30 x 40 m) is on the order of 0.3 kg. m-3. m-1. Selecting (from FISCHER et al., 1979) a turbulent dispersion coefficient for coastal watcrs over the scale of several thousand m2 of 2 to 5 x 10-3 m2. s-1, we can estimate the vertical dispersive flux over the mixing zone and, by mass balance, we obtain another estimate of the flux in the range 0.150 - 0.400 m2. s-1.We conclude that the flowrate is on the order of 0.150 m3.s-1, with a plausible range of 0.100 - 0.300 m3.s-1.These results were extrapolated to the entire South and East shores ofthe island by assuming that the flow rate of an outlet was proportional to its area as observed on infrared surveys. We obtain an average flux of 0,4 m3.s-1 km-l for the southern flank of the volcano. This ligure is consistent with a global estimate (0,6 m3.s-1km-l) obtained by a surface hydrologic balance over this part of the volcano. The difference can represent diffuse outlets into the sea

Datta-Das transistor: Significance of channel direction, size-dependence of source contacts, and boundary effects

We analyze the spin expectation values for injected spin-polarized electrons (spin vectors) in a [001]-grown Rashba-Dresselhaus two-dimensional electron gas (2DEG). We generalize the calculation for point spin injection in semi-infinite 2DEGs to finite-size spin injection in bounded 2DEGs. Using the obtained spin vector formula, significance of the channel direction for the Datta-Das transistor is illustrated. Numerical results indicate that the influence due to the finite-size injection is moderate, while the channel boundary reflection may bring unexpected changes. Both effects are concluded to decrease when the spin-orbit coupling strength is strong. Hence [110] is a robust channel direction and is therefore the best candidate for the design of the Datta-Das transistor.Comment: 5 pages, 4 figures, accepted for publication in Physical Review

Resonance-like piezoelectric electron-phonon interaction in layered structures

We show that mismatch of the piezoelectric parameters between layers of multiple-quantum well structures leads to modification of the electron-phonon interaction. In particular, short-wavelength phonons propagating perpendicular to the layers with wavevector close to $2\pi n/d$, where $d$ is the period of the structure, induce a strong smoothly-varying component of the piezo-potential. As a result, they interact efficiently with 2D electrons. It is shown, that this property leads to emission of collimated quasi-monochromatic beams of high-frequency acoustic phonons from hot electrons in multiple-quantum well structures. We argue that this effect is responsible for the recently reported monochromatic transverse phonon emission from optically excited GaAs/AlAs superlattices, and provide additional experimental evidences of this.Comment: 6 pages, 7 figure

Teneurs en 18O et concentration saline d'eaux paraliques et continentales égyptiennes

Plusieurs corps d'eau lagunaires et continentaux égyptiens sont comparés quant à l'abondance en oxygène-18 et la concentration saline (ici appréciée par la teneur en Cl¯) de leurs eaux superficielles.En Egypte, les eaux continentales de surface proviennent toutes du système nilotique dont les eaux, après leur longue traversée du grand Desert Oriental, se trouvent très évaporées et fortement enrichies en oxygène-18 à leur arrivée dans le delta.Dans les bassins alimentés uniquement par la mer (lagunes de Mer Rouge et du Golfe de Suez) soit par l'eau du Nil (Lac du Fayoum), les deux paramètres considérés augmentent conjointement depuis les zones d'alimentation vers les reculées marginales selon le déplacement des eaux.Dans les bassins à alimentation mixte, (Partie terminale du delta, lac Temsah sur le Canal de Suez), les eaux les plus diluées sont aussi les plus riches en isotopes lourds.Ainsi, dans le contexte climatique et géographique très particulierde l'Egypte, il est possible de reconnaître les eaux continentales, marines ou paraliques à partir des deux paramètres étudiés mais non à partir d'un seul, notamment la teneur en oxygène-18.The aim of this paper is to study some aspects of the geochemical behaviour (18O, (Cl-)) of waters of some basins in Egypt. Several types of basin are studied (figure 1).1) Basin with marine seawater input only, either wide open to the Red sea such as Zeit Bay (27°45'N, 33°25'E), relatively open such as Guesmah Lagoon (27°40'N, 33°30'E) or indirectly linked to the sea through a coastal sand bar such as the pools of sebkha Melaha (28°10'N, 33°10'E).2) Basin supplied by continental water without outflow such as Fayoum Lake (= Birket Karoum) (29°25'N, 30°30'E) which receives fresh waters from a Nile diversion.3) Water bodies with a mixed water input (continental and marine waters) such as the Nile delta and Temsah Lake (= Ismaïla Lake), (30°25'N, 31°30'E), the katter being on the Suez Canal and receiving waters from Mediterranean Sea and from the eastern branch of the Nile.Local means climatic parameter are those of Cairo, Alexandria, Queseir and Louxor (table 1).RESULTS AND DISCUSSION1 - Nile delta (figure 2 and 3)Sampling period was performed in August 1984.- At the beginning of the Nile delta the water at Beni-Suez (station 14) presents a low chloride concentration (0.1 to 0.2 g l-1) and a relatively high 18O content (+ 3 ‰) due to evaporation of the Nile water during its downstream course from the Aswan High Dam which collects waters in the upper part of the Nile watershed.- The content of both (Cl-) and 18O stightly increases in the delta region in relation to the anastomosis of branches of the Nile due to an intensive irrigation network.- In the coastal region, due to mixing with sea water or/and brackish water, the isotope content decreases while the chloride concentration increases.2 - Temsah or Ismaïla LakeTwo periods of sampling : March and August 1984. The general trend is a decrease of 18O and an increase of (Cl-) from the Nile waters to the central part of the lake, and an increase of both 18O and (Cl-) at the fringes of the lake, due to evaporation.3 - Coastal basins from the Red SeaZeit Bay and Guesmah Lagoon. Two sampling periods : March and August 1984. The isotope content and chloride concentration act in parallel and depend principally on the movement of water bodies. For the surface waters wind velocity and direction play a major rote in the spatial distribution of the chlorinity and isotope content. A slight enrichment is noted during the summer period.4 - Pools in Sebkha MelahaThe isotope composition and chloride content of the ponds water fed by sea water through the sand salt bar increased during the summer as it is the case in the first stages of evaporation of salt pans (PIERRE and FONTES, 1982).5 - Fayoun Lake : Birket KaroumTwo sampling periods : March and August 1984. Because of a lack of sufficient data (salinity and isotopic composition of the input, water flux of the incoming water, local values of the relative humidity and temperature of the atmosphere, isotopic composition of the water vapour...) a water mass balance coupled with a chloride and isotopic mass balance was not possible. Nevertheless, with our data measured (18O, (Cl-) lake levels), some estimations were made of the temperature, evaporation, wind velocities (from meteorological tables) and the isotopic composition of the input : δA in the lake (SIMPSON et al., 1987), and if we assume that the lake presents a long term dynamic equilibrium, it is possible to estimate with reasonable accuracy that : i) the total isotopic enrichment factor (ɛ) is between 8 ‰ and 24 ‰ (figure 12) : ɛ= ɛe + ɛk , with ɛe = equilibrium isotopic enrichment factor; ɛk = kinetic isotopic enrichment factor. ii) the salt content of the incoming water δA is ≈20 g l-1. The principal consequence is that the important loss of water observed between the sampling periods (1.5 m difference in water level) was not due to evaporation only, but more by seepage from the bottom of the take (≈ 560 106 m3 for a total volume of ≈800 106 m3).CONCLUSIONS1) On a diagram δ18O - CL- (figure 13 and 14) where all the data for the Egyptian basins studied are represented, the similarity in the geochemical behaviour of the basins with a single input, is noted; the relation between isotopic composition and chloride content is approximatively logarithmic. In the case of a mixed input basin (Temsah Lake) two branches on the diagram are distinct; the first branch shows the effect of mixing in the west part of the lake, between evaporated Nile water and water from the Mediterranean Sea. The shape of this branch is very similar to that of the Nile delta: the second branch represents evaporation in the east part of the take (figure 13 and 14).2) In the case of a semi-arid environment, this study shows that with two simple parameters only, such as isotopic composition and the chloride content of waters, it is possible to distinguish the marine, paralic and continental domain (figure 15)

The Two Dimensional Kondo Model with Rashba Spin-Orbit Coupling

We investigate the effect that Rashba spin-orbit coupling has on the low energy behaviour of a two dimensional magnetic impurity system. It is shown that the Kondo effect, the screening of the magnetic impurity at temperatures T < T_K, is robust against such spin-orbit coupling, despite the fact that the spin of the conduction electrons is no longer a conserved quantity. A proposal is made for how the spin-orbit coupling may change the value of the Kondo temperature T_K in such systems and the prospects of measuring this change are discussed. We conclude that many of the assumptions made in our analysis invalidate our results as applied to recent experiments in semi-conductor quantum dots but may apply to measurements made with magnetic atoms placed on metallic surfaces.Comment: 22 pages, 1 figure; reference update

Dissipation effects in spin-Hall transport of electrons and holes

We investigate the spin-Hall effect of both electrons and holes in semiconductors using the Kubo formula in the correct zero-frequency limit taking into account the finite momentum relaxation time of carriers in real semiconductors. This approach allows to analyze the range of validity of recent theoretical findings. In particular, the spin-Hall conductivity vanishes for vanishing spin-orbit coupling if the correct zero-frequency limit is performed.Comment: 5 pages, no figures, version to appear in Phys. Rev.