Anomaly in the dielectric response at the charge orbital ordering transition of crystalline Pr0.67Ca0.33MnO3

The complex impedance of a Pr0.67Ca0.33MnO3 crystal has been measured. The frequency dependence is studied for a wide range of temperatures (50K-403K) and is found to be characteristic of relaxation process with a single Debye time relaxation constant, which is interpreted as a dielectric constant of the material. A strong peak is observed in this dielectric constant (up to two millions) at the charge ordering transition suggesting an interpretation in terms of ordering of electric dipoles at TCO or in term of phase separation. Comparison with Pr0.63Ca0.37MnO3 - in which the phase separation is much smaller and the peak in the dielectric constant is absent - suggests an interpretation in term of phase separation between insulating and metallic states.Comment: pdf fil

Disruption of Yarrowia lipolytica TPS1 Gene Encoding Trehalose-6-P Synthase Does Not Affect Growth in Glucose but Impairs Growth at High Temperature

We have cloned the Yarrowia lipolytica TPS1 gene encoding trehalose-6-P synthase by complementation of the lack of growth in glucose of a Saccharomyces cerevisiae tps1 mutant. Disruption of YlTPS1 could only be achieved with a cassette placed in the 3′half of its coding region due to the overlap of its sequence with the promoter of the essential gene YlTFC1. The Yltps1 mutant grew in glucose although the Y. lipolytica hexokinase is extremely sensitive to inhibition by trehalose-6-P. The presence of a glucokinase, insensitive to trehalose-6-P, that constitutes about 80% of the glucose phosphorylating capacity during growth in glucose may account for the growth phenotype. Trehalose content was below 1 nmol/mg dry weight in Y. lipolytica, but it increased in strains expressing YlTPS1 under the control of the YlTEF1promoter or with a disruption of YALI0D15598 encoding a putative trehalase. mRNA levels of YlTPS1 were low and did not respond to thermal stresses, but that of YlTPS2 (YALI0D14476) and YlTPS3 (YALI0E31086) increased 4 and 6 times, repectively, by heat treatment. Disruption of YlTPS1 drastically slowed growth at 35°C. Homozygous Yltps1 diploids showed a decreased sporulation frequency that was ascribed to the low level of YALI0D20966 mRNA an homolog of the S. cerevisiae MCK1 which encodes a protein kinase that activates early meiotic gene expression

Observation of two transient episodes in an Arctic Coastal Polynya in late winter: Supercooled water and sea ice desalination

International audienceCoastal Arctic polynyas play an important role in forming dense water and contribute to the deep ocean ventilation. Dense water formation results from brine rejection associated with sea-ice production. In March 2007, a 60m-long ice tethered mooring, recording temperature and salinity at high frequency with a relatively fine vertical resolution and a prototype float (Ice-T) measuring ice-thickness and thermal profiles in sea ice were deployed in Storfjorden in the Svalbard archipelago. This fjord hosts a latent heat polynya which is maintained open under northeasterly wind conditions. The water masses analysis reveals that the main pool of the fjord is occupied by Brine-enriched Shelf Water (BSW) over the whole water column, while the southwest part of the fjord is occupied by Arctic Water (ArW). The BSW recorded at the bottom of the mooring has a salinity of 35.2 (density of 28.31 kg m-3), which is among the densest values of BSW reported in this region in the literature. Measurements indicate that the water column is slightly supercooled down to 30m for the first day of the record until a warm storm breaks the ice. Maximum supercooling values are 0.007±0.002°C below the in-situ freezing point with a mean value of 0.005±0.002°C at 10m. The origin of this supercooled water is investigated by analyzing the ice cover and the ocean-atmosphere heat flux during the days that preceded these observations. Although the amount of supercooling is relatively low compared with values recently reported in Storfjorden, these observations show that supercooled water can still be present under the ice several days after the cooling and freezing period. A sudden and brief salinity anomaly reaching 0.2 ppt is observed at the base of the ice. Concomitant temperature anomalies are recorded in the sea ice. The origin of this signal, which occurs during a warm storm, is analyzed. As this salinity anomaly cannot be associated with ice formation, different sea ice desalination processes are investigated, in particular flushing by meltwater and gravity drainage