Electron transport lifetimes in InSb/Al1-xInxSb quantum well 2DEGs

We report magnetotransport measurements of InSb/Al1-xInxSb modulation doped quantum well (QW) structures and the extracted transport ( ) tt and quantum (tq) lifetime of carriers at low temperature (<2K.) We consider conventional transport lifetimes over a range of samples with different doping levels and carrier densities, and deduce different transport regimes dependent on QW state filling calculated from self-consistent Schrödinger–Poisson modelling. For samples where only the lowest QW subband is occupied at electron densities of 2.13 10 ´ 11 cm−2 and 2.54 10 ´ 11 cm−2 quantum lifetimes of tq » 0.107 ps, and tq » 0.103 ps are extracted from Shubnikov–de Haas oscillations below a magnetic field of 0.8 T. The extracted ratios of transport to quantum lifetimes, t t t q » 17 and t t t q » 20 are similar to values reported in other binary QW two-dimensional electron gas systems, but are inconsistent with predictions from transport modelling which assumes that remote ionized donors are the dominant scattering mechanism. We find the low t t t q ratio and the variation in transport mobility with carrier density cannot be explained by reasonable levels of background impurities or well width fluctuations. Thus, there is at least one additional scattering mechanism unaccounted for, most likely arising from structural defects

VEZF1 elements mediate protection from DNA methylation

There is growing consensus that genome organization and long-range gene regulation involves partitioning of the genome into domains of distinct epigenetic chromatin states. Chromatin insulator or barrier elements are key components of these processes as they can establish boundaries between chromatin states. The ability of elements such as the paradigm &#946;-globin HS4 insulator to block the range of enhancers or the spread of repressive histone modifications is well established. Here we have addressed the hypothesis that a barrier element in vertebrates should be capable of defending a gene from silencing by DNA methylation. Using an established stable reporter gene system, we find that HS4 acts specifically to protect a gene promoter from de novo DNA methylation. Notably, protection from methylation can occur in the absence of histone acetylation or transcription. There is a division of labor at HS4; the sequences that mediate protection from methylation are separable from those that mediate CTCF-dependent enhancer blocking and USF-dependent histone modification recruitment. The zinc finger protein VEZF1 was purified as the factor that specifically interacts with the methylation protection elements. VEZF1 is a candidate CpG island protection factor as the G-rich sequences bound by VEZF1 are frequently found at CpG island promoters. Indeed, we show that VEZF1 elements are sufficient to mediate demethylation and protection of the APRT CpG island promoter from DNA methylation. We propose that many barrier elements in vertebrates will prevent DNA methylation in addition to blocking the propagation of repressive histone modifications, as either process is sufficient to direct the establishment of an epigenetically stable silent chromatin stat

The prevalence, patterns of usage and people's attitude towards complementary and alternative medicine (CAM) among the Indian community in Chatsworth, South Africa

BACKGROUND: The purpose of this study was to determine, among the Indian community of Chatsworth, South Africa, the prevalence and utilisation patterns of complementary and alternative medicine (CAM), attitudes associated with CAM use and communication patterns of CAM users with their primary care doctors. METHODS: Face-to-face structured interviews were conducted in Chatsworth, a suburb of Durban in which South Africans of Indian origin predominantly reside. Participants were 200 randomly selected adult English-speaking Indian residents. RESULTS: The prevalence of CAM usage for period 2000/2001 was 38.5% (95% confidence interval 31.7% to 45.6%). Spiritual healing and herbal/natural medicines, including vitamins were the most common types of CAM used, accounting for 42.8% and 48.1% respectively of overall CAM usage. People used CAM to treat conditions including diabetes mellitus, headaches, arthritis and joint pains, stress, skin disorders, backaches, hypertension and nasal disorders. Half of the CAM users used allopathic medicines concurrently. The cost of CAM utilization over this 1-year period, incurred by 80.5% of users for the duration of therapy for their most troublesome condition was below R500 (approximately US$50). Age, sex, marital status, religion, level of education and income were shown not to influence the use of CAM. Greater than half (51.9%) of CAM users did so either upon the advice of someone they knew, or after noticing a CAM advertisement in the local press. Seventy-nine percent of CAM users indicated that they had positive outcomes with their treatments. Fifty four percent of CAM users (excluding those using spiritual healing only) failed to inform their doctors that they used CAM. The main reason given by half of this group was that informing their doctors did not seem necessary. CONCLUSION: The prevalence of CAM in Chatsworth is similar to findings in other parts of the world. Although CAM was used to treat many different ailments, this practice could not be attributed to any particular demographic profile. The majority of CAM users were satisfied with the effects of CAM. Findings support a need for greater integration of allopathic medicine and CAM, as well as improved communication between patients and caregivers regarding CAM usage

Crystal, Solution and In silico Structural Studies of Dihydrodipicolinate Synthase from the Common Grapevine

Dihydrodipicolinate synthase (DHDPS) catalyzes the rate limiting step in lysine biosynthesis in bacteria and plants. The structure of DHDPS has been determined from several bacterial species and shown in most cases to form a homotetramer or dimer of dimers. However, only one plant DHDPS structure has been determined to date from the wild tobacco species, Nicotiana sylvestris (Blickling et al. (1997) J. Mol. Biol. 274, 608–621). Whilst N. sylvestris DHDPS also forms a homotetramer, the plant enzyme adopts a ‘back-to-back’ dimer of dimers compared to the ‘head-to-head’ architecture observed for bacterial DHDPS tetramers. This raises the question of whether the alternative quaternary architecture observed for N. sylvestris DHDPS is common to all plant DHDPS enzymes. Here, we describe the structure of DHDPS from the grapevine plant, Vitis vinifera, and show using analytical ultracentrifugation, small-angle X-ray scattering and X-ray crystallography that V. vinifera DHDPS forms a ‘back-to-back’ homotetramer, consistent with N. sylvestris DHDPS. This study is the first to demonstrate using both crystal and solution state measurements that DHDPS from the grapevine plant adopts an alternative tetrameric architecture to the bacterial form, which is important for optimizing protein dynamics as suggested by molecular dynamics simulations reported in this study

Magnetoabsorption in InSb quantum-well heterostructures

Spin-resolved cyclotron resonance is observed in InSb/AlInSb quantum-well heterostructures. Spin-resolved transitions are identified for fields greater than 2.2 T and confirmed by k⋅p modeling. The cyclotron effective mass is calculated for a range of fields up to 11.5 T and is indicative of the strong nonparabolicity in this material. Resonant coupling is observed between the ground and first occupied subbands for normal-incidence excitation. This is explained in the context of conduction band-valance band mixing due to the narrow bandgap. Further resonant coupling is observed via interaction with a hybrid intersubband plasmon-LO phonon mode. This allows a quantitative estimation of the subband spacing and of the electron-phonon interactions in the quantum well

Gate dependence of spin-splitting in an InSb/InAlSb quantum well

A high mobility single subband occupancy InSb/InAlSb quantum well was grown by molecular beam epitaxy. The low-temperature, high-field magnetotransport properties are measured as a function of gate bias. Spin-resolved Shubnikov-de Haas oscillations are observed. A preliminary analysis of the Shubnikov-de Haas oscillations indicates a strong gate bias dependence of the Rashba spin-orbit term

High-current breakdown of the quantum Hall effect and electron heating in InSb/AlInSb

We report measurements of the temperature and electric field dependent breakdown of the quantum Hall effect in two-dimensional InSb/AlInSb heterostructures. The electron temperature Te is studied as a function of electric field and it is shown that the energy loss rates of electrons to the lattice follow a (Te3−TL3) dependence for 2 K<Te<22 K at a lattice temperature TL=1.5 K. The high-current induced breakdown of the quantum Hall effect (QHE) is linearly proportional to sample width as deduced from the Hall resistivity and shows breakdown at lower current densities as deduced from the resistivity (ρxx) due to nonuniformity in carrier density. Temperature dependent studies show that the quantum Hall effect persists to considerably higher temperatures than the conventional GaAs/AlGaAs system. Using the energy loss rates, we describe the QHE breakdown in terms of bootstrap-type electron heating

Zero-field spin splitting and spin lifetime in n-InSb/In1-xAlxSb asymmetric quantum well heterostructures

The spin-orbit (SO) coupling parameters for the lowest conduction subband due to structural inversion asymmetry (SIA) and bulk inversion asymmetry (BIA) are calculated for a range of carrier densities in [001]-grown delta-doped n-type InSb/In1-xAlxSb quantum wells using the established eight-band k center dot p formalism [J. Deng , Phys. Rev. B 59, R5312 (1999)]. We present calculations for conditions of zero bias at 10 K. It is shown that both the SIA and BIA parameters scale approximately linearly with carrier density, and exhibit a marked dependence on well width when alloy composition is adjusted to allow maximum upper barrier height for a given well width. In contrast to other material systems, the BIA contribution to spin splitting is found to be of significant and comparable value to the SIA mechanism in these structures. We calculate the spin lifetime tau(s[1 (1) over bar0]) for spins oriented along [1 (1) over bar0] based on D'yakonov-Perel' mechanism using both the theory of Averkiev [J. Phys.: Condens. Matter 14, R271 (2002)] and also directly the rate of precession of spins about the effective magnetic field, taking into account all three SO couplings, which show good agreement. tau(s[1 (1) over bar0]) is largest in the narrowest wells over the range of moderate carrier densities considered, which is attributed to the reduced magnitude of the k-cubic BIA parameter in narrow wells. The inherently large BIA induced SO coupling in these systems is shown to have considerable effect on tau(s[1 (1) over bar0]), which exhibits significant reduction in the maximum spin lifetime compared to previous studies that consider systems with relatively weak BIA induced SO coupling. The relaxation rate of spins oriented in the [001] direction is found to be dominated by the k-linear SIA and BIA coupling parameters and at least an order of magnitude greater than in the [1 (1) over bar0] direction.</p