132 research outputs found
Subband structure of II-VI modulation-doped magnetic quantum wells
Here we investigate the spin-dependent subband structure of newly-developed
Mn-based modulation-doped quantum wells. In the presence of an external
magnetic field, the s-d exchange coupling between carriers and localized d
electrons of the Mn impurities gives rise to large spin splittings resulting in
a magnetic-field dependent subband structure. Within the framework of the
effective-mass approximation, we self-consistently calculate the subband
structure at zero temperature using Density Functional Theory (DFT) with a
Local Spin Density Approximation (LSDA). We present results for the
magnetic-field dependence of the subband structure of shallow ZnSe/ZnCdMnSe
modulation doped quantum wells. Our results show a significant contribution to
the self-consistent potential due to the exchange-correlation term. These
calculations are the first step in the study of a variety of interesting
spin-dependent phenomena, e.g., spin-resolved transport and many-body effects
in polarized two-dimensional electron gases.Comment: 3 pages, 3 postscript figures, submitted to the proceedings of the
10th Brazilian Workshop on Semiconductor Physics (BWSP10
Many-body effects on the ringlike structures in two-subband wells
The longitudinal resistivity of two-dimensional electron gases
formed in wells with two subbands displays ringlike structures when plotted in
a density--magnetic-field diagram, due to the crossings of spin-split Landau
levels (LLs) from distinct subbands. Using spin density functional theory and
linear response, we investigate the shape and spin polarization of these
structures as a function of temperature and magnetic-field tilt angle. We find
that (i) some of the rings "break" at sufficiently low temperatures due to a
quantum Hall ferromagnetic phase transition, thus exhibiting a high degree of
spin polarization (%) within, consistent with the NMR data of Zhang
\textit{et al.} [Phys. Rev. Lett. {\bf 98}, 246802 (2007)], and (ii) for
increasing tilting angles the interplay between the anticrossings due to
inter-LL couplings and the exchange-correlation (XC) effects leads to a
collapse of the rings at some critical angle , in agreement with the
data of Guo \textit{et al.} [Phys. Rev. B {\bf 78}, 233305 (2008)].Comment: 4 pages, 3 figure
Hysteretic resistance spikes in quantum Hall ferromagnets without domains
We use spin-density-functional theory to study recently reported hysteretic
magnetoresistance \rho_{xx} spikes in Mn-based 2D electron gases
[Jaroszy\'{n}ski et al. Phys. Rev. Lett. (2002)]. We find hysteresis loops in
our calculated Landau fan diagrams and total energies signaling
quantum-Hall-ferromagnet phase transitions. Spin-dependent exchange-correlation
effects are crucial to stabilize the relevant magnetic phases arising from
distinct symmetry-broken excited- and ground-state solutions of the Kohn-Sham
equations. Besides hysteretic spikes in \rho_{xx}, we predict hysteretic dips
in the Hall resistance \rho_{xy}. Our theory, without domain walls,
satisfactorily explains the recent data.Comment: 4 pages, 4 figures, published version (some changes to the text; same
figures as in v1
Simple implementation of complex functionals: scaled selfconsistency
We explore and compare three approximate schemes allowing simple
implementation of complex density functionals by making use of selfconsistent
implementation of simpler functionals: (i) post-LDA evaluation of complex
functionals at the LDA densities (or those of other simple functionals); (ii)
application of a global scaling factor to the potential of the simple
functional; and (iii) application of a local scaling factor to that potential.
Option (i) is a common choice in density-functional calculations. Option (ii)
was recently proposed by Cafiero and Gonzalez. We here put their proposal on a
more rigorous basis, by deriving it, and explaining why it works, directly from
the theorems of density-functional theory. Option (iii) is proposed here for
the first time. We provide detailed comparisons of the three approaches among
each other and with fully selfconsistent implementations for Hartree,
local-density, generalized-gradient, self-interaction corrected, and
meta-generalized-gradient approximations, for atoms, ions, quantum wells and
model Hamiltonians. Scaled approaches turn out to be, on average, better than
post-approaches, and unlike these also provide corrections to eigenvalues and
orbitals. Scaled selfconsistency thus opens the possibility of efficient and
reliable implementation of density functionals of hitherto unprecedented
complexity.Comment: 12 pages, 1 figur
Investigation of Association between Susceptibility to Leprosy and SNPs inside and near the BCHE Gene of Butyrylcholinesterase
Leprosy is a chronic disease caused by Mycobacterium leprae and affects the skin and the peripheral nervous system. Butyrylcholinesterase is coded by the BCHE gene, and the atypical allele (70G; rs1799807) has been investigated as a leprosy risk factor, with conflicting results. The present study estimated the frequencies of variants of rs1799807 and of five additional SNPs at the BCHE gene or near it: rs1126680, rs1803274, rs2863381, rs4440084, and rs4387996. A total of 167 patients and 150 healthy controls were genotyped by TaqMan PCR. Significantly higher allelic (70G) and genotypic (70DG) frequencies in rs1799807 were found in the patient group, with odds ratio (OR) of 6.33 (1.40 to 28.53) for the heterozygote. This finding was replicated in a comparison of the cases against a control group of 361 blood donors. The present data suggest that the atypical BChE variant may predispose to leprosy per se
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