Physical Properties of Metallic Antiferromagnetic CaCo{1.86}As2 Single Crystals

We report studies of CaCo{1.86}As2 single crystals. The electronic structure is probed by angle-resolved photoemission spectroscopy (ARPES) measurements of CaCo{1.86}As2 and by full-potential linearized augmented-plane-wave calculations for the supercell Ca8Co15As16 (CaCo{1.88}As2). Our XRD crystal structure refinement is consistent with the previous combined refinement of x-ray and neutron powder diffraction data showing a collapsed-tetragonal ThCr2Si2-type structure with 7(1)% vacancies on the Co sites corresponding to the composition CaCo{1.86}As2 [D. G. Quirinale et al., Phys. Rev. B 88, 174420 (2013)]. The anisotropic magnetic susceptibility chi(T) data are consistent with the magnetic neutron diffraction data of Quirianale et al. that demonstrate the presence of A-type collinear antiferromagnetic order below the Neel temperature TN = 52(1) K with the easy axis being the tetragonal c axis. However, no clear evidence from the resistivity rho(T) and heat capacity Cp(T) data for a magnetic transition at TN is observed. A metallic ground state is demonstrated from band calculations and the rho(T), Cp(T) and ARPES data, and spin-polarized calculations indicate a competition between the A-type AFM and FM ground states. The Cp(T) data exhibit a large Sommerfield electronic coefficient reflecting a large density of states at the Fermi energy D(EF), consistent with the band structure calculations which also indicate a large D(EF) arising from Co 3d bands. At 1.8 K the M(H) data for H|| c exhibit a well-defined first-order spin-flop transition at an applied field of 3.5 T. The small ordered moment of 0.3 muB/Co obtained from the M(H) data at low T, the large exchange enhancement of chi and the lack of a self-consistent interpretation of the chi(T) and M(H,T) data in terms of a local moment Heisenberg model together indicate that the magnetism of CaCo{1.86}As2 is itinerant.Comment: 18 pages, 15 figures, 4 tables, 61 references; v2: extended the fits of experimental data by additional electronic structure calculations; published versio

Hyperfine interaction induced decoherence of electron spins in quantum dots

We investigate in detail, using both analytical and numerical tools, the decoherence of electron spins in quantum dots (QDs) coupled to a bath of nuclear spins in magnetic fields or with various initial bath polarizations, focusing on the longitudinal relaxation in low and moderate field/polarization regimes. An increase of the initial polarization of nuclear spin bath has the same effect on the decoherence process as an increase of the external magnetic field, namely, the decoherence dynamics changes from smooth decay to damped oscillations. This change can be observed experimentally for a single QD and for a double-QD setup. Our results indicate that substantial increase of the decoherence time requires very large bath polarizations, and the use of other methods (dynamical decoupling or control of the nuclear spins distribution) may be more practical for suppressing decoherence of QD-based qubits.Comment: Rev. Tex, 5 pages, 3 eps color figures, submitted to Phys. Rev.

Rapid Adaptation to Mammalian Sociality via Sexually Selected Traits

Background: Laboratory studies show that the sexual selection via mate choice and intrasexual competition can profoundly affect the development and fitness of offspring. Less is known, however, about the total effects of sexual selection on offspring in normal social conditions. For many animals, opportunity for mating success is determined by complex social interactions, such as dominance hierarchies. Social selection is an extended view of sexual selection that incorporates competition during sexual and nonsexual interactions, and predicts complex evolutionary dynamics. Whether social selection improves or constrains offspring fitness is controversial. Results: To characterize the consequences of social selection, we introduced wild-derived mice to seminatural competition for three consecutive generations (promiscuous lineage). In parallel, we bred a control lineage in cages using random mate assignment (monogamous lineage). A direct competition experiment using second-generation animals revealed that promiscuous line males had greater reproductive success than monogamous line males (particularly during extrapair matings), in spite of higher mortality and equivalent success in social dominance and sperm competition. There were no major female fitness effects, though promiscuous line females had fewer litters than monogamous line females. We confirmed a behavioral sexual attraction mechanism by showing that, when given a choice, females had both odor and mating preferences for promiscuous line over monogamous line males. Conclusions: Our study demonstrates novel evidence for the strength of sexual selection under normal social conditions, and shows rapid male adaptation to sociality driven largely by sexual trait expression, with tradeoffs in survivorship and female fecundity. The speed at which these phenotypes emerged suggests the possibility of transgenerational inheritance. Mouse population densities fluctuate dramatically in nature, and we propose that sexually selected phenotypes arise dynamically during periods of social competition.Molecular and Cellular Biolog

Carbon flux from decomposing wood and its dependency on temperature, wood N2 fixation rate, moisture and fungal composition in a Norway spruce forest

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Theory of Electron Spin Relaxation in ZnO

Doped ZnO is a promising material for spintronics applications. For such applications, it is important to understand the spin dynamics and particularly the spin coherence of this II-VI semiconductor. The spin lifetime $\tau_{s}$ has been measured by optical orientation experiments, and it shows a surprising non-monotonic behavior with temperature. We explain this behavior by invoking spin exchange between localized and extended states. Interestingly, the effects of spin-orbit coupling are by no means negligible, in spite of the relatively small valence band splitting. This is due to the wurtzite crystal structure of ZnO. Detailed analysis allows us to characterize the impurity binding energies and densities, showing that optical orientation experiments can be used as a characterization tool for semiconductor samples.Comment: 7 pages, 1 figure: minor changes Accepted by Phys. Rev.

Ba{1-x}KxMn2As2: An Antiferromagnetic Local-Moment Metal

The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a local-moment antiferromagnetic insulator with a Neel temperature TN = 625 K and a large ordered moment mu = 3.9 mu_B/Mn. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K, while retaining the same crystal and antiferromagnetic structures together with nearly the same high TN and large mu. Ba_{1-x}K_xMn2As2 is thus the first metallic ThCr2Si2-type MAs-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba_{1-x}K_xMn2As2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high Tc superconductivity.Comment: 5 two-column typeset pages, 5 figures, 20 references; v2: minor revisions, 4 new references, published versio

Detection of Large Grains in the Coma of Comet C/2001 A2 (LINEAR) from Arecibo Radar Observations

Arecibo S-band (lambda=13cm) radar observations of Comet C/2001 A2 (LINEAR) on 2001 July 7-9 showed a strong echo from large coma grains. This echo was significantly depolarized. This is the first firm detection of depolarization in a grain-coma radar echo and indicates that the largest grains are at least lambda / 2 or 2 cm in radius. The grains are moving at tens of m/s with respect to the nucleus. The non-detection of the nucleus places an upper limit of 3 km on its diameter. The broad, asymmetric echo power spectrum suggests a fan of grains that have a steep (differential number ~ a^-4) size distribution at cm-scales, though the observed fragmentation of this comet complicates that picture.Comment: 20 pages, 3 tables, 5 figures Submitted to Icaru