Effect of initial spin polarization on spin dephasing and electron g factor in a high-mobility two-dimensional electron system

We have investigated the spin dynamics of a high-mobility two-dimensional electron system (2DES) in a GaAs--Al$_{0.3}$Ga$_{0.7}$As single quantum well by time-resolved Faraday rotation (TRFR) in dependence on the initial degree of spin polarization, $P$, of the 2DES. From $P\sim 0$ to $P\sim 30$ %, we observe an increase of the spin dephasing time, $T_2^\ast$, by an order of magnitude, from about 20 ps to 200 ps, in good agreement with theoretical predictions by Weng and Wu [Phys. Rev. B {\bf 68}, 075312 (2003)]. Furthermore, by applying an external magnetic field in the Voigt configuration, also the electron $g$ factor is found to decrease for increasing $P$. Fully microscopic calculations, by numerically solving the kinetic spin Bloch equations considering the D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms, reproduce the most salient features of the experiments, {\em i.e}., a dramatic decrease of spin dephasing and a moderate decrease of the electron $g$ factor with increasing $P$. We show that both results are determined dominantly by the Hartree-Fock contribution of the Coulomb interaction.Comment: 4 pages, 4 figures, to be published in PR

Clearance of endosymbionts in the tick vector Rhipicephalus sanguineus

Abstract only availableVector-borne diseases reduce livestock production in areas whose human populations are starving, such as the Tsetse belt in Africa. In the United States, people are infected every year with Lyme disease, Rocky Mountain Spotted Fever, and Human Monocytic Ehrlichiosis. A method of controlling vectors, such as ticks, would have many uses. Previous studies have indicated that all ticks of the species Amblyomma americanum are infected with a Coxiella sp. bacterial symbiont. After treatment with the antibiotics rifampin and tetracycline, nymphs and adults were shown to have lower numbers of this symbiont and treatment of engorged adult females was shown to result in decreased fecundity (Zhong et al. 2007). It was speculated that this Coxiella symbiont plays a vital role in tick fitness and female fecundity. The goals of the current experiment are to: demonstrate the presence of the same or similar symbiont in Rhipicephalus sanguineus adults and larvae, demonstrate clearance of the symbiont in larvae after treatment with antibiotics, observe any negative effects on the fecundity of the treated females, and test the acquisition rate of larvae that feed on a carrier of Ehrlichia canis, a pathogen of which ticks are a vector. A Coxiella sp. symbiont was shown in adult and larvae R. sanguineus using PCR. Adult females were then fed with males on dogs to repletion, with several groups receiving different treatments of antibiotics prior to mating. Three of the treatment groups suffered significantly lower fecundity than the control. DNA from the resulting larvae was PCR assayed for symbionts, and the Coxiella symbiont has been found in lower numbers in the larvae of these treated groups. The next phase of the experiment is to feed symbiont cleared or reduced larvae on an E. canis carrier and compare their rate of acquisition to untreated larvae.Dr. Roger W. Stich, Veterinary Pathobiolog

Experimentelle Überprüfung einer evolutionstheoretischen Erklärung von Schlafplatzpräferenzen des Menschen

Obwohl Menschen einen großen Teil des Tages an ihrem Schlafplatz verbringen, gibt es kaum experimentelle und theoriegeleitete Studien zur Schlafplatzwahl des Menschen. Da der Mensch im Schlaf relativ schutzlos gegenüber Angreifern ist und eine passende Wahl der Schlafumgebung Schutz und damit höhere Überlebenschancen bietet, wird die Schlafplatzwahl aus evolutionärer Perspektive untersucht. Es wird ein Evolutionsbedingter Psychologischer Mechanismus (EPM) zur Schlafplatzwahl postuliert und daraus abgeleitete Hypothesen überprüft: Ein Schlafplatz sollte so gewählt werden, dass er schnelle Reaktionen auf Angreifer ermöglicht. Daher sollte (1) vom Bett aus die Tür im Blickfeld sein, (2) das Bett in deutlicher Weise von der Tür abgerückt sein und (3) bevorzugt in die Zimmerhälfte gestellt werden, in die sich die Tür öffnet. Eine Studie mit 138 Probanden, die Möbel auf experimentell manipulierten Grundrissen anrichten, kann diese Hypothesen bestätigen

Atomic layering at the liquid silicon surface: a first- principles simulation

We simulate the liquid silicon surface with first-principles molecular dynamics in a slab geometry. We find that the atom-density profile presents a pronounced layering, similar to those observed in low-temperature liquid metals like Ga and Hg. The depth-dependent pair correlation function shows that the effect originates from directional bonding of Si atoms at the surface, and propagates into the bulk. The layering has no major effects in the electronic and dynamical properties of the system, that are very similar to those of bulk liquid Si. To our knowledge, this is the first study of a liquid surface by first-principles molecular dynamics.Comment: 4 pages, 4 figures, submitted to PR

Dependence of spin dephasing on initial spin polarization in a high-mobility two-dimensional electron system

We have studied the spin dynamics of a high-mobility two-dimensional electron system in a GaAs/Al_{0.3}Ga_{0.7}As single quantum well by time-resolved Faraday rotation and time-resolved Kerr rotation in dependence on the initial degree of spin polarization, P, of the electrons. By increasing the initial spin polarization from the low-P regime to a significant P of several percent, we find that the spin dephasing time, $T_2^\ast$, increases from about 20 ps to 200 ps; Moreover, $T_2^\ast$ increases with temperature at small spin polarization but decreases with temperature at large spin polarization. All these features are in good agreement with theoretical predictions by Weng and Wu [Phys. Rev. B {\bf 68}, 075312 (2003)]. Measurements as a function of spin polarization at fixed electron density are performed to further confirm the theory. A fully microscopic calculation is performed by setting up and numerically solving the kinetic spin Bloch equations, including the D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms, with {\em all} the scattering explicitly included. We reproduce all principal features of the experiments, i.e., a dramatic decrease of spin dephasing with increasing $P$ and the temperature dependences at different spin polarizations.Comment: 8 pages, 8 figures, to be published in PR

Dynamic Structure Factor of Liquid and Amorphous Ge From Ab Initio Simulations

We calculate the dynamic structure factor S(k,omega) of liquid Ge (l-Ge) at temperature T = 1250 K, and of amorphous Ge (a-Ge) at T = 300 K, using ab initio molecular dynamics. The electronic energy is computed using density-functional theory, primarily in the generalized gradient approximation, together with a plane wave representation of the wave functions and ultra-soft pseudopotentials. We use a 64-atom cell with periodic boundary conditions, and calculate averages over runs of up to 16 ps. The calculated liquid S(k,omega) agrees qualitatively with that obtained by Hosokawa et al, using inelastic X-ray scattering. In a-Ge, we find that the calculated S(k,omega) is in qualitative agreement with that obtained experimentally by Maley et al. Our results suggest that the ab initio approach is sufficient to allow approximate calculations of S(k,omega) in both liquid and amorphous materials.Comment: 31 pages and 8 figures. Accepted for Phys. Rev.

Large Scale Electronic Structure Calculations with Multigrid Acceleration

We have developed a set of techniques for performing large scale ab initio calculations using multigrid accelerations and a real-space grid as a basis. The multigrid methods permit efficient calculations on ill-conditioned systems with long length scales or high energy cutoffs. The technique has been applied to systems containing up to 100 atoms, including a highly elongated diamond cell, an isolated C$_{60}$ molecule, and a 32-atom cell of GaN with the Ga d-states in valence. The method is well suited for implementation on both vector and massively parallel architectures.Comment: 4 pages, 1 postscript figur

Detection of large magneto-anisotropy of electron spin dephasing in a high-mobility two-dimensional electron system in a [001][001] GaAs/AlGaAs quantum well

In time-resolved Faraday rotation experiments we have detected an inplane anisotropy of the electron spin-dephasing time (SDT) in an $n$--modulation-doped GaAs/Al$_{0.3}$Ga$_{0.7}$As single quantum well. The SDT was measured with magnetic fields of $B\le 1$ T, applied in the $[110]$ and $[1\bar{1}0]$ inplane crystal directions of the GaAs quantum well. For fields along $[1\bar{1}0]$, we have found an up to a factor of about 2 larger SDT than in the perpendicular direction. Fully microscopic calculations, by numerically solving the kinetic spin Bloch equations considering the D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms, reproduce the experimental findings quantitatively. This quantitative analysis of the data allowed us to determine the relative strengths of Rashba and Dresselhaus terms in our sample. Moreover, we could estimate the SDT for spins aligned in the $[110]$ {\em inplane} direction to be on the order of several nanoseconds, which is up to two orders of magnitude larger than that in the perpendicular {\em inplane} direction.Comment: 4 pages, 4 figures, to be published in PR

Thermodynamic Behavior of a Model Covalent Material Described by the Environment-Dependent Interatomic Potential

Using molecular dynamics simulations we study the thermodynamic behavior of a single-component covalent material described by the recently proposed Environment-Dependent Interatomic Potential (EDIP). The parameterization of EDIP for silicon exhibits a range of unusual properties typically found in more complex materials, such as the existence of two structurally distinct disordered phases, a density decrease upon melting of the low-temperature amorphous phase, and negative thermal expansion coefficients for both the crystal (at high temperatures) and the amorphous phase (at all temperatures). Structural differences between the two disordered phases also lead to a first-order transition between them, which suggests the existence of a second critical point, as is believed to exist for amorphous forms of frozen water. For EDIP-Si, however, the unusual behavior is associated not only with the open nature of tetrahedral bonding but also with a competition between four-fold (covalent) and five-fold (metallic) coordination. The unusual behavior of the model and its unique ability to simulation the liquid/amorphous transition on molecular-dynamics time scales make it a suitable prototype for fundamental studies of anomalous thermodynamics in disordeered systems.Comment: 48 pages (double-spaced), 13 figure

Challenges and pitfalls of experimental bariatric procedures in rats

Introduction: The impact of Roux-en-Y gastric bypass (RYGB) and sleeve gastrectomy (SG) on obesity and obesity-related diseases is unquestionable. Up to now, the technical descriptions of these techniques in animals/rats have not been very comprehensive. Methods: For SG and RYGB, operating time, learning curve, and intraoperative mortality in relation to weight of the rat and type of anesthesia were recorded. Furthermore, a review of the literature on experimental approaches towards SG and RYGB in rats was carried out, merging in a detailed technical description for both procedures. Results: The data presented here revealed that the mean operating time for SG (69.4 +/- 22.2 min (SD)) was shorter than for RYGB (123.0 +/- 20.7 min). There is a learning curve for both procedures, resulting in a reduced operating time of up to 60% in SG and 35% in RYGB (p < 0.05; t-test). However, with increased weight, operating time increases to about 80 min for SG and about 120 min for RYGB. Obese rats have an increased intraoperative mortality rate of up to 50%. After gaseous anesthesia the mortality can be even higher. The literature search revealed 40 papers dealing with SG and RYGB in rats. 18 articles (45%) contained neither photographs nor illustrations; 14 articles (35%) did not mention the applied type of anesthesia. The mortality rate was described in 15 papers (37.5%). Conclusion: Experimental obesity surgery in rats is challenging. Because of the high mortality in obese rats operated under gaseous anesthesia, exercises to establish the techniques should be performed in small rats using intraperitoneal anesthesia. Copyright (C) 2012 S. Karger GmbH, Freibur