878 research outputs found
Observation of antiphase coherent phonons in the warped Dirac cone of Bi2Te3
In this Rapid Communication we investigate the coupling between excited electrons and phonons in the highly anisotropic electronic structure of the prototypical topological insulator Bi2Te3. Using time and angle resolved photoemission spectroscopy we are able to identify the emergence and ultrafast temporal evolution of the longitudinal optical A1g coherent phonon mode in Bi2Te3. We observe an antiphase behavior in the onset of the coherent phonon oscillations between the amp; 915;K and the amp; 915;M high symmetry directions that is consistent with warping. The qualitative agreement between our density functional theory calculations and the experimental results reveals the critical role of the anisotropic coupling between Dirac fermions and phonon modes in the topological insulator Bi2Te
Band Renormalization of Blue Phosphorus on Au 111
Most recently, theoretical calculations predicted the stability of a novel two dimensional phosphorus honeycomb lattice named blue phosphorus. Here, we report on the growth of blue phosphorus on Au 111 and unravel its structural details using diffraction, microscopy and theoretical calculations. Most importantly, by utilizing angle resolved photoemission spectroscopy we identify its momentum resolved electronic structure. We find that Au 111 breaks the sublattice symmetry of blue phosphorus leading to an orbital dependent band renormalization upon the formation of a 4 4 superstructure. Notably, the semiconducting two dimensional phosphorus realizes its valence band maximum at 0.9 eV binding energy, however, shifted in momentum space due to the substrate induced band renormalizatio
Morphological Transformations of Galaxies in the A901/02 Supercluster from STAGES
We present a study of galaxies in the Abell 901/902 Supercluster at z~0.165,
based on HST ACS F606W, COMBO-17, Spitzer 24um, XMM-Newton X-ray, and
gravitational lensing maps, as part of the STAGES survey. We characterize
galaxies with strong externally-triggered morphological distortions and normal
relatively undisturbed galaxies, using visual classification and quantitative
CAS parameters. We compare normal and distorted galaxies in terms of their
frequency, distribution within the cluster, star formation properties, and
relationship to dark matter (DM) or surface mass density, and intra-cluster
medium (ICM) density. We revisit the morphology density relation, which
postulates a higher fraction of early type galaxies in dense environments, by
considering separately galaxies with a low bulge-to-disk (B/D) ratio and a low
gas content as these two parameters may not be correlated in clusters. We
report here on our preliminary analysis.Comment: To appear in the ASP conference proceedings of the "Frank N. Bash
Symposium 2007: New Horizons in Astronomy", Eds. A. Frebel, J. Maund, J.
Shen, M. Siegel. 4 pages, 4 figure
Physiological Effects of Tapering and Detraining in World-Class Kayakers
. Purpose: This study analyzed changes in neuromuscular, body composition, and endurance markers during 4 wk of tapering and subsequent 5 wk of reduced training (RT) or training cessation (TC). Methods: Fourteen world-class kayakers were randomly assigned to either a TC (n = 7) or an RT group (n = 7). One-repetition maximum (1RM) strength, mean concentric velocity with 45% 1RM (V 45% ) in the bench press (BP) and prone bench pull (PBP) exercises, and body composition assessments were conducted at the start (T0) and end (T1) of a 43-wk training program, after tapering for the world championships (T2) and after TC or RT (T3). A graded exercise test on a kayak ergometer for determination of maximal oxygen uptake at T0, T1, and T3 was also performed. Results: After tapering, no significant changes were observed in 1RM or V 45% . TC resulted in significantly greater declines in 1RM strength (j8.9% and j7.8%, P G 0.05, respectively, for BP and PBP) than those observed for RT (j3.9% and j3.4%). Decreases in V 45% in BP and PBP were larger for TC (j12.6% and j10.0%) than for RT (j9.0% and j6.7%). Increases in sum of eight skinfolds were observed after both TC and RT, whereas declines in maximal aerobic power were lower for RT (j5.6%) than for TC (j11.3%). Conclusions: Short-term TC results in large decreases in maximal strength and especially V 45% in highly trained athletes. These results suggest the need of performing a minimal maintenance program to avoid excessive declines in neuromuscular function in cases where a prolonged break from training is required. Key Words: TRAINING CESSATION, REDUCED TRAINING, MAXIMAL STRENGTH, MUSCLE POWER, CANOEING A well-known and proven effective coaching strategy for improving sports performance before main competition events is to incorporate a tapering phase of significantly reduced training (RT) volume while the intensity is kept high The incorporation of periods of 3-6 wk of training cessation (TC) after the conclusion of the main event of the season to allow physical and mental recovery before the start of a new training cycle is a common training practice in many sports. In these situations, training reduction is generally preferred over complete exercise stoppage because it seems to be more effective to avoid the negative impact of insufficient training stimuli on athletic performance (21). The magnitude of performance declines observed after detraining periods appears to be related to the chosen recovery strategy (i.e., reduced training (RT) or complete training cessation (TC)), initial fitness level, and total time under reduced or absence of training stimuli Current research seems to indicate that neuromuscular performance is more susceptible to decline because of detraining in highly trained athletes compared with recently or moderately trained individual
Covariance of cross-correlations: towards efficient measures for large-scale structure
We study the covariance of the cross-power spectrum of different tracers for
the large-scale structure. We develop the counts-in-cells framework for the
multi-tracer approach, and use this to derive expressions for the full
non-Gaussian covariance matrix. We show, that for the usual auto-power
statistic, besides the off-diagonal covariance generated through gravitational
mode-coupling, the discreteness of the tracers and their associated sampling
distribution can generate strong off-diagonal covariance, and that this becomes
the dominant source of covariance as k>>k_f=2 pi/L. On comparison with the
derived expressions for the cross-power covariance, we show that the
off-diagonal terms can be suppressed, if one cross-correlates a high
tracer-density sample with a low one. Taking the effective estimator efficiency
to be proportional to the signal-to-noise ratio (SN), we show that, to probe
clustering as a function of physical properties of the sample, i.e. cluster
mass or galaxy luminosity, then the cross-power approach can out perform the
auto-power one by factors of a few. We confront the theory with measurements of
the mass-mass, halo-mass, and halo-halo power spectra from a large ensemble of
N-body simulations. We show that there is a significant SN advantage to be
gained from using the cross-power approach when studying the bias of rare
haloes. The analysis is repeated in configuration space and again SN
improvement is found. We estimate the covariance matrix for these samples, and
find strong off-diagonal contributions. The covariance depends on halo mass,
with higher mass samples having stronger covariance. In agreement with theory,
we show that the covariance is suppressed for the cross-power. This work points
the way towards improved estimators for clustering studies.Comment: Several significant improvements to the earlier version: for instance
it is shown more clearly how shot noise corrections may generate off-diagonal
covariance in the power spectrum. Original version submitted to MNRAS on 18th
September 2008. This version 18 pages, 7 figure
Nanostructural origin of giant Rashba effect in intercalated graphene
To enhance the spin orbit interaction in graphene by a proximity effect without compromising the quasi free standing dispersion of the Dirac cones means balancing the opposing demands for strong and weak graphene substrate interaction. So far, only the intercalation of Au under graphene Ni 1 1 1 has proven successful, which was unexpected since graphene prefers a large separation amp; 8764;3.3 A from a Au monolayer in equilibrium. Here, we investigate this system and find the solution in a nanoscale effect. We reveal that the Au largely intercalates as nanoclusters. Our density functional theory calculations show that the graphene is periodically stapled to the Ni substrate, and this attraction presses graphene and Au nanoclusters together. This, in turn, causes a Rashba effect of the giant magnitude observed in experiment. Our findings show that nanopatterning of the substrate can be efficiently used for engineering of spin orbit effects in graphen
Master equations for effective Hamiltonians
We reelaborate on a general method for obtaining effective Hamiltonians that
describe different nonlinear optical processes. The method exploits the
existence of a nonlinear deformation of the su(2) algebra that arises as the
dynamical symmetry of the original model. When some physical parameter (usually
related to the dispersive limit) becomes small, we immediately get a diagonal
effective Hamiltonian that represents correctly the dynamics for arbitrary
states and long times. We apply the same technique to obtain how the noise
terms in the original model transform under this scheme, providing a systematic
way of including damping effects in processes described in terms of effective
Hamiltonians.Comment: 10 pages, no figure
Silymarin secretion and its elicitation by methyl jasmonate in cell cultures of Silybum marianum is mediated by phospholipase D-phosphatidic acid
The flavonolignan silymarin is released to the extracellular medium of Silybum marianum cultures and its production can be stimulated by the elicitor methyljasmonate (MeJA). The sequence of the signalling processes leading to this response is unknown at present. It is reported in this work that MeJA increased the activity of the enzyme phospholipase D (PLD). Treatment with mastoparan (Mst), a PLD activity stimulator, also enhanced PLD and caused a substantial increase in silymarin production. The application of the product of PLD activity, phosphatidic acid (PA) promoted silymarin accumulation. Altering PLD activity by introducing in cultures n-butanol (nBuOH), which inhibits PA production by PLD, prevented silymarin elicitation by MeJA or Mst and also impeded its release in non-elicited cultures. Treatment with iso-, sec- or tert- butanol had no effect on silymarin production. The exogenous addition of PA reversed the inhibitory action of nBuOH, both in control and MeJA-treated cultures. These results suggest that the enzyme PLD and its product PA mediate silymarin secretion to the medium of S. marianum cultures
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