Electronic structure and magnetic properties of Mn and Fe impurities near the GaAs (110) surface

Combining density functional theory calculations and microscopic tight-binding models, we investigate theoretically the electronic and magnetic properties of individual substitutional transition-metal impurities (Mn and Fe) positioned in the vicinity of the (110) surface of GaAs. For the case of the [Mn2+]0 plus acceptor-hole (h) complex, the results of a tight-binding model including explicitly the impurity delectrons are in good agreement with approaches that treat the spin of the impurity as an effective classical vector. For the case of Fe, where both the neutral isoelectronic [Fe3+]0 and the ionized [Fe2+]− states are relevant to address scanning tunneling microscopy (STM) experiments, the inclusion of d orbitals is essential. We find that the in-gap electronic structure of Fe impurities is significantly modified by surface effects. For the neutral acceptor state [Fe2+,h]0, the magnetic-anisotropy dependence on the impurity sublayer resembles the case of [Mn2+,h]0. In contrast, for [Fe3+]0 electronic configuration the magnetic anisotropy behaves differently and it is considerably smaller. For this state we predict that it is possible to manipulate the Fe moment, e.g., by an external magnetic field, with detectable consequences in the local density of states probed by STM

Comparative genetic diversity studies of Shorea curtisii (Dipterocarpaceae) an assessment using SSR and DAMD markers

Genetic diversity of Shorea curtisii from different age cohorts, namely, seedlings, saplings and adult trees were determined using six SSR loci and 33 DAMD loci. To quantify genetic diversity in S. curtisii we used standard genetic diversity measures for SSR data, and both phenotypic and genotypic methods with null-allele frequency corrected for deviation from Hardy-Weinberg equilibrium (HWE) with SSR markers for DAMD data. Results showed that the genetic diversity measured using DAMD genotypic method was lower than those derived from SSR data based on the same set of samples. This suggests that DAMD allele frequencies corrected from HWE deviation using fixation index derived from SSR data may be underestimated. The genetic distance matrix generated from SSR data was significantly correlated with DAMD genotype data (r = 0.990, p < 0.05), indicating a similar genetic structure of S. curtisii being depicted by both marker types among the age cohorts tested. The relationship between sample size and genetic diversity measures demonstrated a threshold level, i.e. n = 20 and n = 30 for seedlings and saplings respectively, and n = 15 and n = 20 for adult trees revealed by SSR and DAMD markers respectively. Genetic diversity measures dropped drastically below these levels. These results further imply that a highly heterogeneous population was observed in S. curtisii from each age cohort. Collectively, both SSR and DAMD markers have good genome coverage in the S. curtisii genome

Magnetic Anisotropy of Single Mn Acceptors in GaAs in an External Magnetic Field

We investigate the effect of an external magnetic field on the physical properties of the acceptor hole states associated with single Mn acceptors placed near the (110) surface of GaAs. Crosssectional scanning tunneling microscopy images of the acceptor local density of states (LDOS) show that the strongly anisotropic hole wavefunction is not significantly affected by a magnetic field up to 6 T. These experimental results are supported by theoretical calculations based on a tightbinding model of Mn acceptors in GaAs. For Mn acceptors on the (110) surface and the subsurfaces immediately underneath, we find that an applied magnetic field modifies significantly the magnetic anisotropy landscape. However the acceptor hole wavefunction is strongly localized around the Mn and the LDOS is quite independent of the direction of the Mn magnetic moment. On the other hand, for Mn acceptors placed on deeper layers below the surface, the acceptor hole wavefunction is more delocalized and the corresponding LDOS is much more sensitive on the direction of the Mn magnetic moment. However the magnetic anisotropy energy for these magnetic impurities is large (up to 15 meV), and a magnetic field of 10 T can hardly change the landscape and rotate the direction of the Mn magnetic moment away from its easy axis. We predict that substantially larger magnetic fields are required to observe a significant field-dependence of the tunneling current for impurities located several layers below the GaAs surface.Comment: Non

Primary repair versus surgical and transcatheter palliation in infants with tetralogy of Fallot

Objectives Treatment of infants with tetralogy of Fallot (ToF) has evolved in the last two decades with increasing use of primary surgical repair (PrR) and transcatheter right ventricular outflow tract palliation (RVOTd), and fewer systemic-to-pulmonary shunts (SPS). We aim to report contemporary results using these treatment options in a comparative study. Methods This a retrospective study using data from the UK National Congenital Heart Disease Audit. All infants (n=1662, median age 181 days) with ToF and no other complex defects undergoing repair or palliation between 2000 and 2013 were considered. Matching algorithms were used to minimise confounding due to lower age and weight in those palliated. Results Patients underwent PrR (n=1244), SPS (n=311) or RVOTd (n=107). Mortality at 12 years was higher when repair or palliation was performed before the age of 60 days rather than after, most significantly for primary repair (18.7% vs 2.2%, P<0.001), less so for RVOTd (10.8% vs 0%, P=0.06) or SPS (12.4% vs 8.3%, P=0.2). In the matched groups of patients, RVOTd was associated with more right ventricular outflow tract (RVOT) reinterventions (HR=2.3, P=0.05 vs PrR, HR=7.2, P=0.001 vs SPS) and fewer pulmonary valve replacements (PVR) (HR=0.3 vs PrR, P=0.05) at 12 years, with lower mortality after complete repair (HR=0.2 versus PrR, P=0.09). Conclusions We found that RVOTd was associated with more RVOT reinterventions, fewer PVR and fewer deaths when compared with PrR in comparable, young infants, especially so in those under 60 days at the time of the first procedure