48 research outputs found
Infrared magneto-optical properties of (III,Mn)V ferromagetic semiconductors
We present a theoretical study of the infrared magneto-optical properties of
ferromagnetic (III,Mn)V semiconductors. Our analysis combines the kinetic
exchange model for (III,Mn)V ferromagnetism with Kubo linear response theory
and Born approximation estimates for the effect of disorder on the valence band
quasiparticles. We predict a prominent feature in the ac-Hall conductivity at a
frequency that varies over the range from 200 to 400 meV, depending on Mn and
carrier densities, and is associated with transitions between heavy-hole and
light-hole bands. In its zero frequency limit, our Hall conductivity reduces to
the -space Berry's phase value predicted by a recent theory of the
anomalous Hall effect that is able to account quantitatively for experiment. We
compute theoretical estimates for magnetic circular dichroism, Faraday
rotation, and Kerr effect parameters as a function of Mn concentration and free
carrier density. The mid-infrared response feature is present in each of these
magneto-optical effects.Comment: 11 pages, 5 figure
Durations of asymptomatic, symptomatic, and care-seeking phases of tuberculosis disease with a Bayesian analysis of prevalence survey and notification data
Background
Ratios of bacteriologically positive tuberculosis (TB) prevalence to notification rates are used to characterise typical durations of TB disease. However, this ignores the clinical spectrum of tuberculosis disease and potentially long infectious periods with minimal or no symptoms prior to care-seeking.
Methods
We developed novel statistical models to estimate progression from initial bacteriological positivity including smear conversion, symptom onset and initial care-seeking. Case-detection ratios, TB incidence, durations, and other parameters were estimated by fitting the model to tuberculosis prevalence survey and notification data (one subnational and 11 national datasets) within a Bayesian framework using Markov chain Monte Carlo methods.
Results
Analysis across 11 national datasets found asymptomatic tuberculosis durations in the range 4–8 months for African countries; three countries in Asia (Cambodia, Lao PDR, and Philippines) showed longer durations of > 1 year. For the six countries with relevant data, care-seeking typically began half-way between symptom onset and notification. For Kenya and Blantyre, Malawi, individual-level data were available. The sex-specific durations of asymptomatic bacteriologically-positive tuberculosis were 9.0 months (95% credible interval [CrI]: 7.2–11.2) for men and 8.1 months (95% CrI: 6.2–10.3) for women in Kenya, and 4.9 months (95% CrI: 2.6–7.9) for men and 3.5 months (95% CrI: 1.3–6.2) for women in Blantyre. Age-stratified analysis of data for Kenya showed no strong age-dependence in durations. For Blantyre, HIV-stratified analysis estimated an asymptomatic duration of 1.3 months (95% CrI: 0.3–3.0) for HIV-positive people, shorter than the 8.5 months (95% CrI: 5.0–12.7) for HIV-negative people. Additionally, case-detection ratios were higher for people living with HIV than HIV-negative people (93% vs 71%).
Conclusion
Asymptomatic TB disease typically lasts around 6 months. We found no evidence of age-dependence, but much shorter durations among people living with HIV, and longer durations in some Asian settings. To eradicate TB transmission, greater gains may be achieved by proactively screening people without symptoms through active case finding intervention
On-site Coulomb interaction and the magnetism of (GaMn)N and (GaMn)As
We use the local density approximation (LDA) and LDA+U schemes to study the
magnetism of (GaMn)As and (GaMn)N for a number of Mn concentrations and varying
number of holes. We show that for both systems and both calculational schemes
the presence of holes is crucial for establishing ferromagnetism. For both
systems, the introduction of increases delocalization of the holes and,
simultaneously, decreases the p-d interaction. Since these two trends exert
opposite influences on the Mn-Mn exchange interaction the character of the
variation of the Curie temperature (T) cannot be predicted without direct
calculation. We show that the variation of T is different for two systems.
For low Mn concentrations we obtain the tendency to increasing T in the
case of (GaMn)N whereas an opposite tendency to decreasing T is obtained
for (GaMn)As. We reveal the origin of this difference by inspecting the
properties of the densities of states and holes for both systems. The main body
of calculations is performed within a supercell approach. The Curie
temperatures calculated within the coherent potential approximation to atomic
disorder are reported for comparison. Both approaches give similar qualitative
behavior. The results of calculations are related to the experimental data.Comment: to appear in Physical Review
Single-Band Model for Diluted Magnetic Semiconductors: Dynamical and Transport Properties and Relevance of Clustered States
Dynamical and transport properties of a simple single-band spin-fermion
lattice model for (III,Mn)V diluted magnetic semiconductors (DMS) is here
discussed using Monte Carlo simulations. This effort is a continuation of
previous work (G. Alvarez, Phys. Rev. Lett. 89, 277202 (2002)) where the static
properties of the model were studied. The present results support the view that
the relevant regime of J/t (standard notation) is that of intermediate
coupling, where carriers are only partially trapped near Mn spins, and locally
ordered regions (clusters) are present above the Curie temperature T_C. This
conclusion is based on the calculation of the resistivity vs. temperature, that
shows a soft metal to insulator transition near T_C, as well on the analysis of
the density-of-states and optical conductivity. In addition, in the clustered
regime a large magnetoresistance is observed in simulations. Formal analogies
between DMS and manganites are also discussed.Comment: Revtex4, 20 figures. References updated, minor changes to figures and
tex
Exchange anisotropy, disorder and frustration in diluted, predominantly ferromagnetic, Heisenberg spin systems
Motivated by the recent suggestion of anisotropic effective exchange
interactions between Mn spins in GaMnAs (arising as a result of
spin-orbit coupling), we study their effects in diluted Heisenberg spin
systems. We perform Monte Carlo simulations on several phenomenological model
spin Hamiltonians, and investigate the extent to which frustration induced by
anisotropic exchanges can reduce the low temperature magnetization in these
models and the interplay of this effect with disorder in the exchange. In a
model with low coordination number and purely ferromagnetic (FM) exchanges, we
find that the low temperature magnetization is gradually reduced as exchange
anisotropy is turned on. However, as the connectivity of the model is
increased, the effect of small-to-moderate anisotropy is suppressed, and the
magnetization regains its maximum saturation value at low temperatures unless
the distribution of exchanges is very wide. To obtain significant suppression
of the low temperature magnetization in a model with high connectivity, as is
found for long-range interactions, we find it necessary to have both
ferromagnetic and antiferromagnetic (AFM) exchanges (e.g. as in the RKKY
interaction). This implies that disorder in the sign of the exchange
interaction is much more effective in suppressing magnetization at low
temperatures than exchange anisotropy.Comment: 9 pages, 8 figure
Carrier-mediated ferromagnetic ordering in Mn ion-implanted p+GaAs:C
Highly p-type GaAs:C was ion-implanted with Mn at differing doses to produce
Mn concentrations in the 1 - 5 at.% range. In comparison to LT-GaAs and
n+GaAs:Si samples implanted under the same conditions, transport and magnetic
properties show marked differences. Transport measurements show anomalies,
consistent with observed magnetic properties and with epi- LT-(Ga,Mn)As, as
well as the extraordinary Hall Effect up to the observed magnetic ordering
temperature (T_C). Mn ion-implanted p+GaAs:C with as-grown carrier
concentrations > 10^20 cm^-3 show remanent magnetization up to 280 K
Magnetic susceptibilities of diluted magnetic semiconductors and anomalous Hall-voltage noise
The carrier spin and impurity spin densities in diluted magnetic
semiconductors are considered using a semiclassical approach. Equations of
motions for the spin densities and the carrier spin current density in the
paramagnetic phase are derived, exhibiting their coupled diffusive dynamics.
The dynamical spin susceptibilities are obtained from these equations. The
theory holds for p-type and n-type semiconductors doped with magnetic ions of
arbitrary spin quantum number. Spin-orbit coupling in the valence band is shown
to lead to anisotropic spin diffusion and to a suppression of the Curie
temperature in p-type materials. As an application we derive the Hall-voltage
noise in the paramagnetic phase. This quantity is critically enhanced close to
the Curie temperature due to the contribution from the anomalous Hall effect.Comment: 18 pages, 1 figure include
Exact asymptotic form of the exchange interactions between shallow centers in doped semiconductors
The method developed in [L. P. Gor'kov and L. P. Pitaevskii, Sov. Phys. Dokl.
8, 788 (1964); C. Herring and M. Flicker, Phys. Rev. 134, A362 (1964)] to
calculate the asymptotic form of exchange interactions between hydrogen atoms
in the ground state is extended to excited states. The approach is then applied
to shallow centers in semiconductors. The problem of the asymptotic dependence
of the exchange interactions in semiconductors is complicated by the multiple
degeneracy of the ground state of an impurity (donor or acceptor) center in
valley or band indices, crystalline anisotropy and strong spin-orbital
interactions, especially for acceptor centers in III-V and II-VI groups
semiconductors. Properties of two coupled centers in the dilute limit can be
accessed experimentally, and the knowledge of the exact asymptotic expressions,
in addition to being of fundamental interest, must be very helpful for
numerical calculations and for interpolation of exchange forces in the case of
intermediate concentrations. Our main conclusion concerns the sign of the
magnetic interaction -- the ground state of a pair is always non-magnetic.
Behavior of the exchange interactions in applied magnetic fields is also
discussed
Phenotypic, cytogenetic and spike fertility characterization of a population of male-sterile triticale
Triticale (X Triticosecale Wittmack) is a good cereal for production of flour and feed. A segregating population of triticale was developed from a male-sterile (MS) plant. To determine whether this new source of male sterility in triticale is appropriate for use in breeding programs the expression of the male sterility phenotype was characterized through spike fertility, meiotic behavior, and pollen. Controlled crosses between male-sterile plants and control varieties male-fertile (MF) of triticale were also conducted, and cytological analyses were performed in the F2 and backcross plants. Plants with male-sterile phenotypes displayed reduced spike fertility when compared to plants with male-fertile phenotypes. Compared to male-fertile plants, male-sterile plants exhibited a lower percentage of normal meiotic cells, a reduced meiotic index and reduced pollen viability. The F2 plants had improved pollen fertility when compared to the male-sterile population; however there were no corresponding improvements in the percentage of normal meiotic cells or in the meiotic index. A single generation of backcrosses resulted in an improved meiotic index and increased pollen viability. However, no changes in the percentage of normal meiotic cells were observed. Meiotic instability, which was shown to be inheritable, was the likely cause of male sterility. Therefore, the use of this population in triticale breeding was considered to be inappropriate because it could promote or contribute to the maintenance of meiotic instability, which is commonly observed in this species