Distinguishing impurity concentrations in GaAs and AlGaAs, using very shallow undoped heterostructures

We demonstrate a method of making a very shallow, gateable, undoped 2-dimensional electron gas. We have developed a method of making very low resistivity contacts to these structures and systematically studied the evolution of the mobility as a function of the depth of the 2DEG (from 300nm to 30nm). We demonstrate a way of extracting quantitative information about the background impurity concentration in GaAs and AlGaAs, the interface roughness and the charge in the surface states from the data. This information is very useful from the perspective of molecular beam epitaxy (MBE) growth. It is difficult to fabricate such shallow high-mobility 2DEGs using modulation doping due to the need to have a large enough spacer layer to reduce scattering and switching noise from remote ionsied dopants.Comment: 4 pages, 5 eps figure

MicroRNA-9 represses sirtuin 1 (SIRT1) in human keratinocytes

The protein deacetylase sirtuin 1 (SIRT1) is an established regulator of diverse physiological processes and one of several promising targets for pharmacologic modulation of ageing and longevity. In normal human keratinocytes, SIRT1 has been shown to inhibit proliferation and promote differentiation. MicroRNAs (miRNAs), small non-coding RNA molecules that negatively regulate gene expression, have been shown to regulate SIRT1 expression in several cell types. Using western blotting, we show that miR-9 represses SIRT1 expression in the HaCaT human keratinocytes. The attenuation of SIRT1 levels in response to ectopic miR-9 occurred in a dose-dependent manner. As miR-9 expression is known to be under epigenetic control, the effect of the histone deacetylase (HDAC) inhibitor trichostatin A (TSA) was examined. Levels of mature miR-9 increased 8-fold following TSA treatment of HaCaT keratinocytes. Expression of the primary transcripts from which miR-9 is derived was also raised in HaCaT keratinocytes exposed to TSA, with a 7-fold elevation of pri-miR-9-1 and 4-fold increase of pri-miR-3. In contrast the DNA methyl transferase inhibitor 5-deoxy-azacytidine (DAC) had little effect on miR-9 or primary miR-9 expression. Together, our findings point to a role for chromatin remodelling in regulating miR-9 levels in human keratinocytes and in turn modulation of SIRT1 expression by miR-9

Possible effect of collective modes in zero magnetic field transport in an electron-hole bilayer

We report single layer resistivities of 2-dimensional electron and hole gases in an electron-hole bilayer with a 10nm barrier. In a regime where the interlayer interaction is stronger than the intralayer interaction, we find that an insulating state ($d\rho/dT < 0$) emerges at $T\sim1.5{\rm K}$ or lower, when both the layers are simultaneously present. This happens deep in the $"$metallic" regime, even in layers with $k_{F}l>500$, thus making conventional mechanisms of localisation due to disorder improbable. We suggest that this insulating state may be due to a charge density wave phase, as has been expected in electron-hole bilayers from the Singwi-Tosi-Land-Sj\"olander approximation based calculations of L. Liu {\it et al} [{\em Phys. Rev. B}, {\bf 53}, 7923 (1996)]. Our results are also in qualitative agreement with recent Path-Integral-Monte-Carlo simulations of a two component plasma in the low temperature regime [ P. Ludwig {\it et al}. {\em Contrib. Plasma Physics} {\bf 47}, No. 4-5, 335 (2007)]Comment: 5 pages + 3 EPS figures (replaced with published version

Improved ambient stability of thermally annealed zinc nitride thin films

Zinc nitride films are known to readily oxidize in an ambient atmosphere, forming a ZnO/Zn(OH)2 medium. We report that post-growth thermal annealing significantly improves the stability of zinc nitride with a three-order magnitude increase in degradation time from a few days in un-annealed films to several years after annealing. A degradation study was performed on samples annealed under a flow of nitrogen at 200–400 °C, which showed that the stability of the films depends strongly on the annealing temperature. We propose a mechanism for this improvement, which involves a stabilization of the native oxide layer that forms on the surface of zinc nitride films after exposure to ambient conditions. The result holds significant promise for the use of zinc nitride in devices where operational stability is a critical factor in applications

Global analysis of mutations driving microevolution of a heterozygous diploid fungal pathogen

This is the final published version, available from the National Academy of Sciences via the DOI in this recordThe sequence reported in this paper has been deposited in the NCBI Sequence Read Archive, https://www.ncbi.nlm.nih.gov/bioproject (BioProject ID PRJNA345600).Candida albicans is a heterozygous diploid yeast that is a commensal of the human gastrointestinal tract and a prevalent opportunistic pathogen. Here, whole-genome sequencing was performed on multiple C. albicans isolates passaged both in vitro and in vivo to characterize the complete spectrum of mutations arising in laboratory culture and in the mammalian host. We establish that, independent of culture niche, microevolution is primarily driven by de novo base substitutions and frequent short-tract loss-of-heterozygosity events. An average base-substitution rate of ∼1.2 × 10−10 per base pair per generation was observed in vitro, with higher rates inferred during host infection. Large-scale chromosomal changes were relatively rare, although chromosome 7 trisomies frequently emerged during passaging in a gastrointestinal model and was associated with increased fitness for this niche. Multiple chromosomal features impacted mutational patterns, with mutation rates elevated in repetitive regions, subtelomeric regions, and in gene families encoding cell surface proteins involved in host adhesion. Strikingly, de novo mutation rates were more than 800-fold higher in regions immediately adjacent to emergent loss-of-heterozygosity tracts, indicative of recombinationinduced mutagenesis. Furthermore, genomes showed biased patterns of mutations suggestive of extensive purifying selection during passaging. These results reveal how both cell-intrinsic and cell-extrinsic factors influence C. albicans microevolution, and provide a quantitative picture of genome dynamics in this heterozygous diploid species.National Institute of Health (NIH)Burroughs Wellcome FundSigma Delta Epsilon-Graduate Women in ScienceWellcome Trust/Massachusetts Institute of TechnologyNational Institute of Allergy and Infectious Diseases, NIH, Department of Health and Human Service

Temperature dependence of the band gap of zinc nitride observed in photoluminescence measurements

We report the photoluminescence properties of DC sputtered zinc nitride thin films in the temperature range of 3.7–300 K. Zinc nitride samples grown at 150 °C exhibited a narrow photoluminescence band at 1.38 eV and a broad band at 0.90 eV, which were attributed to the recombination of free carriers with a bound state and deep-level defect states, respectively. The high-energy band followed the Varshni equation with temperature and became saturated at high excitation powers. These results indicate that the high-energy band originates from shallow defect states in a narrow bandgap. Furthermore, a red-shift of the observed features with increasing excitation power suggested the presence of inhomogeneities within the samples