Magnetic Moment Formation in Graphene Detected by Scattering of Pure Spin Currents

Hydrogen adatoms are shown to generate magnetic moments inside single layer graphene. Spin transport measurements on graphene spin valves exhibit a dip in the non-local spin signal as a function of applied magnetic field, which is due to scattering (relaxation) of pure spin currents by exchange coupling to the magnetic moments. Furthermore, Hanle spin precession measurements indicate the presence of an exchange field generated by the magnetic moments. The entire experiment including spin transport is performed in an ultrahigh vacuum chamber, and the characteristic signatures of magnetic moment formation appear only after hydrogen adatoms are introduced. Lattice vacancies also demonstrate similar behavior indicating that the magnetic moment formation originates from pz-orbital defects.Comment: accepted to Phys. Rev. Let

Precision Electron-Beam Polarimetry using Compton Scattering at 1 GeV

We report on the highest precision yet achieved in the measurement of the polarization of a low energy, $\mathcal{O}$(1 GeV), electron beam, accomplished using a new polarimeter based on electron-photon scattering, in Hall~C at Jefferson Lab. A number of technical innovations were necessary, including a novel method for precise control of the laser polarization in a cavity and a novel diamond micro-strip detector which was able to capture most of the spectrum of scattered electrons. The data analysis technique exploited track finding, the high granularity of the detector and its large acceptance. The polarization of the $180~\mu$A, $1.16$~GeV electron beam was measured with a statistical precision of $<$~1\% per hour and a systematic uncertainty of 0.59\%. This exceeds the level of precision required by the \qweak experiment, a measurement of the vector weak charge of the proton. Proposed future low-energy experiments require polarization uncertainty $<$~0.4\%, and this result represents an important demonstration of that possibility. This measurement is also the first use of diamond detectors for particle tracking in an experiment.Comment: 9 pages, 7 figures, published in PR

Equatorial adjusment in the Eastern Atlantic

Observations suggest that the annual upwelling event in the Gulf of Guinea is not associated with changes in the local winds. A possible explanation is that a strong upwelling signal, generated by increased westward wind stress in the Western Atlantic as an equatorially trapped Kelvin wave. This explanation is analogous to current theories of El Nino in the Pacific Ocean. (Résumé d'auteur

Spin Relaxation in Single Layer Graphene with Tunable Mobility

Graphene is an attractive material for spintronics due to theoretical predictions of long spin lifetimes arising from low spin-orbit and hyperfine couplings. In experiments, however, spin lifetimes in single layer graphene (SLG) measured via Hanle effects are much shorter than expected theoretically. Thus, the origin of spin relaxation in SLG is a major issue for graphene spintronics. Despite extensive theoretical and experimental work addressing this question, there is still little clarity on the microscopic origin of spin relaxation. By using organic ligand-bound nanoparticles as charge reservoirs to tune mobility between 2700 and 12000 cm2/Vs, we successfully isolate the effect of charged impurity scattering on spin relaxation in SLG. Our results demonstrate that while charged impurities can greatly affect mobility, the spin lifetimes are not affected by charged impurity scattering.Comment: 13 pages, 5 figure

Local density of states and scanning tunneling currents in graphene

We present exact analytical calculations of scanning tunneling currents in locally disordered graphene using a multimode description of the microscope tip. Analytical expressions for the local density of states (LDOS) are given for energies beyond the Dirac cone approximation. We show that the LDOS at the $A$ and $B$ sublattices of graphene are out of phase by $\pi$ implying that the averaged LDOS, as one moves away from the impurity, shows no trace of the $2q_F$ (with $q_F$ the Fermi momentum) Friedel modulation. This means that a STM experiment lacking atomic resolution at the sublattice level will not be able of detecting the presence of the Friedel oscillations [this seems to be the case in the experiments reported in Phys. Rev. Lett. {\bf 101}, 206802 (2008)]. The momentum maps of the LDOS for different types of impurities are given. In the case of the vacancy, $2q_F$ features are seen in these maps. In all momentum space maps, $K$ and $K+K^\prime$ features are seen. The $K+K^\prime$ features are different from what is seen around zero momentum. An interpretation for these features is given. The calculations reported here are valid for chemical substitution impurities, such as boron and nitrogen atoms, as well as for vacancies. It is shown that the density of states close to the impurity is very sensitive to type of disorder: diagonal, non-diagonal, or vacancies. In the case of weakly coupled (to the carbon atoms) impurities, the local density of states presents strong resonances at finite energies, which leads to steps in the scanning tunneling currents and to suppression of the Fano factor.Comment: 21 pages. Figures 6 and 7 are correctly displayed in this new versio

Measurement of the Neutron Radius of 208Pb Through Parity-Violation in Electron Scattering

We report the first measurement of the parity-violating asymmetry A_PV in the elastic scattering of polarized electrons from 208Pb. A_PV is sensitive to the radius of the neutron distribution (Rn). The result A_PV = 0.656 \pm 0.060 (stat) \pm 0.014 (syst) ppm corresponds to a difference between the radii of the neutron and proton distributions Rn - Rp = 0.33 +0.16 -0.18 fm and provides the first electroweak observation of the neutron skin which is expected in a heavy, neutron-rich nucleus.Comment: 6 pages, 1 figur

Evaluating the quality of Douglas-fir (Pseudotsuga menziessii (Mirb.) Franco) seedlings : a comparison of some current and proposed evaluation methods

The physiological condition of tree seedlings at the time they are planted can have a profound impact on their subsequent field performance. Damaged or low vigor seedlings have a much greater chance of dying in the field, or at best, of growing slowly during their initial establishment period. Several methods of evaluating the physiological quality of seedlings are currently used to try to predict how seedlings will perform after outplanting. The root growth potential (RGP) approach measures the ability of seedlings to initiate and elongate roots when placed in an environment favorable to root growth. The vigor method, or test, monitors the survival and bud burst of potted seedlings in a growthstimulating environment. While both methods reportedly provide a general indication of seedling health or vigor, the effectiveness of these techniques in actually predicting field survival and height growth of Douglas-fir [Pseudotsuqa menziesii (Mirb.) Franca] seedlings has either not been sufficiently documented, or there have been inconsistent results. A primary goal of this research was to evaluate and compare how well these methods predict growth and survival of Douglasfir seedlings. Another goal was to examine two new, more rapid, methods of evaluating seedling quality and determine how effective they are. The first of these, called the PMS method, monitors changes in plant moisture stress (PMS) of potted seedlings during a one-week interval in the growth room. The second method measures the root respiration rate of seedlings to determine if changes in seedling quality are reflected in altered root respiratory activities. Results from these studies indicated that measurements from both the RGP and vigor method were significantly correlated with first- and second-year survival and height growth for two Douglas-fir seed sources planted on a variety of sites. For RGP, the best predictor of field performance was the number of new roots greater than 0.5 cm produced during 28 days in the growth room. For the vigor method, the survival of both stressed and unstressed seedlings after 6 weeks in the growth room were the best predictors. Seedlings from lots that produced more new roots, or had higher growth room survival, also had higher field survival and greater growth. The RGP procedure was the best predictor of seedling terminal height growth, while the vigor method was the best predictor of field survival. When these procedures were used in conjunction, correlations with field performance variables improved slightly, but not enough to justify the time and expense of using both. Measurements of plant moisture stress were found to reliably predict damage to seedlings caused by accidental freezing. In addition, this pressure chamber method predicted reductions in field survival and growth caused from several other types of injury. In contrast, root respiration measurements were not consistently related to growth-room survival or growth and this approach does not appear promising as an effective evaluation procedure

Photoperiodic responses of Douglas-fir (Pseudotsuga Menziesii (Mirb.) Franco) and Ponderosa pine (Pinus ponderosa) seedlings

An important cultural tool in the greenhouse production of tree seedlings in the Pacific Northwest is the control of photoperiod. By artificially lengthening the period of daily exposure to light, it is possible to increase both the magnitude and duration of seedling growth. By shortening the photoperiod, one can induce dormancy and hasten the development of frost hardiness, A variety of supplemental lighting and light blocking techniques are currently used in the production of Douglas-fir and ponderosa pine seedlings. However, there is little information on the relative advantages of alternative lighting and light blocking schemes in terms of biological benefits or production costs. Three experiments were therefore conducted to investigate the effects of a variety of photoperiodic regimes on controlling the growth, dormancy and frost hardiness of seedlings of these two species. The objectives of these experiments were to determine the following: 1) the minimum light intensity, of an eight-hour period of supplemental light, capable of delaying dormancy and increasing the vegetative growth of newly germinated seedlings in the fall; 2) the effects of a variety of nighttime lighting treatments on maintaining seedlings in an actively growing condition in the early spring; and 3) the effects of varying daylengths and varying intensities of light leakage on the development of frost hardiness of seedlings in the late summer and early fall. In all three experiments there were pronounced differences between the responses of Douglas-fir seedlings and those of ponderosa pine seedlings. For Douglas-fir seedlings, an eight-hour period of supplemental lighting delayed dormancy and increased both stem elongation and dry weight when the intensity of the light provided was ten or more foot candles. Intensities of one foot candle or less did not cause any increase in the magnitude or duration of seedling growth. A variety of nighttime lighting regimes, including several intermittent lighting treatments, a two-hour night break, and 16- and 22-hour photoperiods, successfully extended the period of active growth of newly germinated winter-sown seedlings. Finally, shortening the photoperiod in the late summer and early fall substantially increased the frost hardiness of seedlings, but relatively low intensities of light leakage reduced seedling hardiness. For ponderosa pine seedlings, there was no obvious threshold supplemental light intensity above which dormancy was delayed and below which it was induced. Ten foot candles or more resulted in an increase in height growth, but there was no consistent relationship between light intensity and dry weight, bud set or basal diameter. For pine seedlings in the second experiment, growth was greatest for those exposed to only a natural photoperiod. Lastly, shortening the photoperiod in the late summer and early fall resulted in a general decrease in the level of frost hardiness developed in ponderosa pine seedlings. These results suggest that the control of photoperiod can be a highly effective tool for regulating the annual growth cycle of Douglas-fir seedlings, but is relatively ineffective for ponderosa pine seedlings