Commensurate-incommensurate transitions of quantum Hall stripe states in double-quantum-well systems

In higher Landau levels (N>0) and around filling factors nu =4N+1, a two-dimensional electron gas in a double-quantum-well system supports a stripe groundstate in which the electron density in each well is spatially modulated. When a parallel magnetic field is added in the plane of the wells, tunneling between the wells acts as a spatially rotating effective Zeeman field coupled to the ``pseudospins'' describing the well index of the electron states. For small parallel fields, these pseudospins follow this rotation, but at larger fields they do not, and a commensurate-incommensurate transition results. Working in the Hartree-Fock approximation, we show that the combination of stripes and commensuration in this system leads to a very rich phase diagram. The parallel magnetic field is responsible for oscillations in the tunneling matrix element that induce a complex sequence of transitions between commensurate and incommensurate liquid or stripe states. The homogeneous and stripe states we find can be distinguished by their collective excitations and tunneling I-V, which we compute within the time-dependent Hartree-Fock approximation.Comment: 23 pages including 8 eps figure

Skyrme Crystal In A Two-Dimensional Electron Gas

The ground state of a two-dimensional electron gas at Landau level filling factors near $\nu =1$ is a Skyrme crystal with long range order in the positions and orientations of the topologically and electrically charged elementary excitations of the $\nu=1$ ferromagnetic ground state. The lowest energy Skyrme crystal is a square lattice with opposing postures for topological excitations on opposite sublattices. The filling factor dependence of the electron spin-polarization, calculated for the square lattice Skyrme crystal, is in excellent agreement with recent experiments.Comment: 3 pages, latex, 3 figures available upon request from [email protected]

Dynamics of electrons in the quantum Hall bubble phases

In Landau levels N > 1, the ground state of the two-dimensional electron gas (2DEG) in a perpendicular magnetic field evolves from a Wigner crystal for small filling of the partially filled Landau level, into a succession of bubble states with increasing number of guiding centers per bubble as the filling increases, to a modulated stripe state near half filling. In this work, we show that these first-order phase transitions between the bubble states lead to measurable discontinuities in several physical quantities such as the density of states and the magnetization of the 2DEG. We discuss in detail the behavior of the collective excitations of the bubble states and show that their spectra have higher-energy modes besides the pinned phonon mode. The frequencies of these modes, at small wavevector k, have a discontinuous evolution as a function of filling factor that should be measurable in, for example, microwave absorption experiments.Comment: 13 pages, 7 figures. Corrected typos in eqs. (38),(39),(40

Sympathetic and swap cooling of trapped ions by cold atoms in a MOT

A mixed system of cooled and trapped, ions and atoms, paves the way for ion assisted cold chemistry and novel many body studies. Due to the different individual trapping mechanisms, trapped atoms are significantly colder than trapped ions, therefore in the combined system, the strong binary ion$-$atom interaction results in heat flow from ions to atoms. Conversely, trapped ions can also get collisionally heated by the cold atoms, making the resulting equilibrium between ions and atoms intriguing. Here we experimentally demonstrate, Rubidium ions (Rb$^+$) cool in contact with magneto-optically trapped (MOT) Rb atoms, contrary to the general expectation of ion heating for equal ion and atom masses. The cooling mechanism is explained theoretically and substantiated with numerical simulations. The importance of resonant charge exchange (RCx) collisions, which allows swap cooling of ions with atoms, wherein a single glancing collision event brings a fast ion to rest, is discussed.Comment: 10 pages, 3 figure

Combined ion and atom trap for low temperature ion-atom physics

We report an experimental apparatus and technique which simultaneously traps ions and cold atoms with spatial overlap. Such an apparatus is motivated by the study of ion-atom processes at temperatures ranging from hot to ultra-cold. This area is a largely unexplored domain of physics with cold trapped atoms. In this article we discuss the general design considerations for combining these two traps and present our experimental setup. The ion trap and atom traps are characterized independently of each other. The simultaneous operation of both is then described and experimental signatures of the effect of the ions and cold-atoms on each other are presented. In conclusion the use of such an instrument for several problems in physics and chemistry is briefly discussed.Comment: 24 pages, 13 figures. Figures Fixe

Répercussions biopsychosociales du Sida et pistes d’intervention et de recherche

Le syndrome d'immunodéficience acquise (SIDA) constitue un problème mondial de santé publique d'une ampleur exceptionnelle. Son impact est considérable sur plusieurs plans : individuel, social, économique, démographique et culturel. Les écrits relatifs au SIDA démontrent bien l'impact émotionnel des pertes occasionnées par cette maladie fatale sur les personnes atteintes. Celles-ci éprouvent des sentiments intenses d'anxiété, de peur, de colère, d'impuissance et de désespoir. À cette détresse psychologique s'ajoute l'isolement des victimes. La réaction négative entretenue au sein de la société à l'égard du SIDA contribue à discriminer et stigmatiser les victimes de cette maladie. Le SIDA est en quelque sorte un drame personnel et un effondrement de la vie familiale et sociale. Les répercussions psychosociales de cette maladie exigent des stratégies d'intervention et de recherche appropriées aux divers problèmes soulevés par le syndrome.The acquired immunodeficiency syndrome (AIDS) is a world wide health problem of exceptional magnitude with considerable effect in many areas : individual, social, economic, demographic and cultural. Current literature clearly indicates the emotional impact of the losses occasionnée by those suffering from this fatal disease. There include strong anxiety, fear, anger powerlessness and hopelessness. To this psychological distress is added isolation of AIDS victims. The negative reaction of society contributes to discrimination and stigmatization of these victims. AIDS may be seen as a personnal tragedy which often spells the collapse of family life. The psychosocial repercussions of this disease require intervention and further research with respect to the many unresolved problems generated by this syndrome

Performance de la nanofiltration pour l'élimination de la matière organique naturelle: essais sur l'usine de Méry/Oise

L'intérêt croissant que les traiteurs d'eaux portent à l'élimination de la matière organique naturelle (MON) a abouti au développement de nouvelles technologies de traitement. Dans ce but, un prototype de nanofiltration à l'échelle industrielle (2 x 1400 m 3 j-¹) est installé à l'usine de Méry sur Oise depuis juillet 1992. Utilisé en traitement de finition après clarification et filtration sur sable, il alimente depuis février 1993 un réseau test de la commune d'Auvers sur Oise (6 000 hbts) en région parisienne.L'objectif de cette publication est de présenter quelques uns des résultats de caractérisation de la MON obtenus pendant 9 mois d'expérimentation (octobre 1992 à juillet 1993), et en particulier ceux concernant les rendements d'élimination de la matière organique naturelle et par voie de conséquence de la demande en chlore.Ces rendements sont généralement supérieurs à 90 % en termes de COD,CODB et d'absorbance UV à 254 et 270 nm éliminés. L'analyse des potentiels de réactivité avec le chlore (taux de chloration: 2,5 mg Cl2/mg C, temps de réaction: 72 heures, pH = 7,5, 20 °C) montre que le perméat est peu consommateur de chlore (demande en chlore < 0,2 mg l-¹ Cl2) et peu précurseur de chloroforme et de trihalométhanes (PFCHC13 < 3 µg l-¹, PFTHM < 11 µg l-¹). Les rendements d'élimination des PFTHM et PFTOX sont généralement supérieurs à 90 %.L'analyse spécifique des constituants majoritaires du perméat montre que les acides aminés totaux (hydrolyse acide puis dérivation à l'OPA/HPLC) constituent une proportion importante du COD (25 à 60 % selon les saisons). Ces composés représentent la quasi totalité de la demande en chlore du perméat si l'on se réfère aux données bibliographiques.Compte tenu de ces résultats, la nanofiltration apparaît comme un excellent procédé de traitement de finition des eaux à potabiliser. En effet, bien qu'elle constitue une barrière de sécurité contre les germes pathogènes, la très faible charge organique du perméat obtenu par nanofiltration (COD~0,15 à 0,3 mg l-¹ C, CODB<0,1 mg l-¹ C) rend plus aisée la maîtrise du résiduel de chlore (lorsqu'une chloration est nécessaire pour maintenir la qualité de l'eau dans les réseaux) et constitue une limitation importante de la formation des sous-produits de chloration.Increasing interest in removing natural organic matter (NOM) has lead to the development of new drinking water treatment technologies. Since July 1992, a nanofiltration demonstration plant (2 x 1400 m3 d-1) has been used to treat sandfiltered water from the Oise river. The permeate has been distributed since February 1993 to the 6000 inhabitants of Auvers/Oise in the Paris suburb. The purpose of this paper is to present and discuss some ofthe results obtained over nine months of operation of this full scale plant, particularly yields of NOM removal and consequently the decreasing of chlorine reactivity (chlorine demand, TTIM and TOX formation potentials).Dissolved organic carbon (DOC) and UV-absorbance were determined using DOC analyser and a spectrophotometer. Biodegradable dissolved organic carbon (BDOC), which represents the biologically assimilable portion of DOC, was determined using the method of JORET et LEVI (1986). Chlorine demand, trihalomethane and total organohalide formation potentials (THMFP and TOXFP) were carried out under the following experimental conditions: applied chlorine dose of 2.5 mg Cl2/mg DOC, pH = 7,5 72 h-contact time and 20°C. Ultrafiltration experiments involved the use ofa laboratory ultrafiltration cell, Total amino-acids were analysed by HPLC after hydrolysis and orthophtaldialdehyde (OPA) derivatization. Aldehyde and ketone determination was based on the method developedby GLAZE et al. (1989) involving pentafluorobenzyl hydroxyl amine (PFBHA) derivatization.Characterization of sand-filtered water (SFW): The sand-frltered water (SFW) upstream of the nanofiItraton membranes has a DOC between 2.4 and 4.l mg l-1, depending on the season (table 1). Its BDOC ranges from 0.7 to l.l mg l-1 C. In fact, a BDOC value higher than 0.3 mg l-1 C has been mentioned by several authors as the limit above which possible bacterial regrowth can take place in the distribution network.The chlorine consumption curves, shown in figure 2 for five sampling campaigns, indicate that the chlorine demand of the SFW can reach 3.4 to 5.2 mg l-1 depending on the season (table 2). The THMFP and the TOXFP are 108-149 ug l-1 and 344-446 ug l-1 Cl- respectively. Note that the ratio of chlorine demand over DOC varies from 1.0 to 1.7 mg Cl2/mg DOC while the THMFP/DOC and TOXFP/DOC ratios present average values of 47.5 ug/mg DOC and 160 ugCl-/mg DOC respectively.The distribution of the SFW (table 3) indicates that the fraction with apparent mo lecular weight Iess than 3 kilodatons contains the major compounds at this stage of the water treatment. This fraction presents the highest chlorine consumption. Specific total amino acids (TAA) analyses demonstrate that TAA represent 3 to 8% of the DOC of the sand-filtered water. The most abundant arnino acids are glycine, aspartic acid, glutamic acid, serine and alanine. The chlorine consumption attributed to these amino acids is evaluated as 1 mg l-1 Cl2, that is to say 1/5 to 1/3 of the SFW chlorine demand. Formaldehyde and acetaldehyde seem to be the major aldehydes present in the SFW with a level of 7 ug l-1 and 20 ug l-1 of formaldehyde and acetaldehyde respectively. They represent only about 0.5 to 0.6 % of the SFW DOC.Characterization of the permeate: The nanofiltration permeate presents a very low NOM level in terms of DOC, BDOC and UV absorbance at 270 nm, that is to say 0.14 to 0.34 mg l-1 C, < 0.1 mg l-1 C and < 0.006 cm-1-l respectively (table 6).The chlorine consumption curves, showt in figure 4 for five sampling campaigns, demonstrate the low permeate reactivity with chlorine. The chlorine demands (table 7) after 72 hours are between 0.12 and 0.32 mg l-1. Moreover chlorine demand/DOC ratios have a value from 0.46 to 0.93 mg Cl2/mg DOC, i.e. half the values measured for SFW. The THMFP and TOXFP (72hours) range from 7 to 11 ug l-1 and 26 to 31 ug l-1 Cl- respectively.Total amino acid (TAA) analyses showed that TAA represent 35 to 60% of the permeate DOC and can account almost entirely for the chlorine consumption. Formaldehyde and acetaldehyde (the major aldehydes analysed) represent 7 to 8% of the permeate DOC.According to the results presented in this paper, nanofiltration appears to be an excellent technolory as a polishing step in surface water treatment. Whereas the level of sand-fïltered water (SFW) DOC varies from 2.4 to 4.1 mg l-1 C (depending on the season), the permeate DOC is consistently lower than 0.3 mg l-1 C. The efficiency of nanofiltration is about 90% for DOC, BDOC and consequently for chlorine demand, THMFP and TOXFP. The high retention of NOM is probably in relation with the percentage (75%) of compounds with apparent molecular weight above 500 daltons in the SFW. In fact the low values of BDOC and chlorine demand justify the use of nanofiltration for the production of a water which represents a very low risk of bacterial regrowth and a low risk of formation of disinfection by product in the network when distributed with a low concentration of residual chlorine

Development of Vegetation Communities in a Retrogressive Thaw Slump near Mayo, Yukon Territory : A 10-year Assessment

The vegetation in a retrogressive thaw slump, first surveyed and documented in 1987, was revisited 10 years later to investigate its subsequent development and to test a chronosequence-based successional model. The thaw slump stabilized in 1994, when the headwall became covered by organic and mineral debris. As a result, the meltwater supply from headwall ablation ceased. Alteration of environmental conditions due to stabilization of the headwall diverted the vegetation succession from the chronosequence determined in 1987. Areas that were marshy in 1987 dried up, and an area dominated by Polygonum alaskanum appeared close to the headwall. Much of the thaw slump was dominated by Salix spp. in 1997, rather than the Equisteum of 1987. However, the ground more than 200 m from the headwall, over a decade old in 1987, experienced less change in edaphic conditions, and the communities there continued to develop a structure approaching the surrounding undisturbed forest.La végétation située dans un décrochement de fonte régressif, étudiée et documentée pour la première fois en 1987, a fait l'objet d'une autre étude sur place dix ans plus tard pour examiner son évolution et tester un modèle de succession fondé sur la chronoséquence. Le décrochement de fonte s'est stabilisé en 1994, quand le mur de rimaye s'est couvert de débris organiques et minéraux. Il en a résulté que l'alimentation en eau de fonte venant de l'ablation du mur de rimaye a cessé. Le changement dans les conditions environnementales dû à la stabilisation du mur a détourné la succession végétale de la chronoséquence établie en 1987. Des zones qui étaient marécageuses en 1987 se sont asséchées, et une étendue où dominait Polygonum alaskanum apparue près du mur. En 1997, une grande partie de la zone du décrochement de fonte était dominée par des espèces Salix, plutôt qu'Equisteum, comme c'était le cas en 1987. Toutefois, le sol situé à plus de 200 m du mur de rimaye, sol vieux de dix ans en 1987, a subi moins de changement dans les conditions édaphiques, et les communautés y ont continué à évoluer vers une structure qui se rapproche de la forêt non perturbée située à la périphérie