1,639 research outputs found
Functional Movement Screen Scores in High School Football Players
Please refer to the pdf version of the abstract located adjacent to the title
Energy dissipation in the time domain governed by bosons in a correlated material
In complex materials various interactions play important roles in determining
the material properties. Angle Resolved Photoelectron Spectroscopy (ARPES) has
been used to study these processes by resolving the complex single particle
self energy and quantifying how quantum interactions modify bare
electronic states. However, ambiguities in the measurement of the real part of
the self energy and an intrinsic inability to disentangle various contributions
to the imaginary part of the self energy often leave the implications of such
measurements open to debate. Here we employ a combined theoretical and
experimental treatment of femtosecond time-resolved ARPES (tr-ARPES) and show
how measuring the population dynamics using tr-ARPES can be used to separate
electron-boson interactions from electron-electron interactions. We demonstrate
the analysis of a well-defined electron-boson interaction in the unoccupied
spectrum of the cuprate BiSrCaCuO characterized by an
excited population decay time constant that maps directly to a
discrete component of the equilibrium self energy not readily isolated by
static ARPES experiments.Comment: 19 pages with 6 figure
Coherent excitations and electron phonon coupling in Ba/EuFe_2As_2 compounds investigated by femtosecond time- and angle-resolved photoemission spectroscopy
We employed femtosecond time- and angle-resolved photoelectron spectroscopy
to analyze the response of the electronic structure of the 122 Fe-pnictide
parent compounds Ba/EuFe_2As_2 and optimally doped BaFe_{1.85}Co_{0.15}As_2
near the \Gamma point to femtosecond optical excitation. We identify pronounced
changes of the electron population within several 100 meV above and below the
Fermi level, which we explain as combination of (i) coherent lattice
vibrations, (ii) a hot electron and hole distribution, and (iii) transient
modifications of the chemical potential. The response of the three different
materials is very similar. In the Fourier transformation of the time-dependent
photoemission intensity we identify three modes at 5.6, 3.3, and 2.6 THz. While
the highest frequency mode is safely assigned to the A_{1g} mode, the other two
modes require a discussion in comparison to literature. The time-dependent
evolution of the hot electron distribution follows a simplified description of
a transient three temperature model which considers two heat baths of lattice
vibrations, which are more weakly and strongly coupled to transiently excited
electron population. Still the energy transfer from electrons to the strongly
coupled phonons results in a rather weak, momentum-averaged electron-phonon
coupling quantified by values for \lambda between 30 and 70 meV^2.
The chemical potential is found to present a transient modulation induced by
the coherent phonons. This change in the chemical potential is particularly
strong in a two band system like in the 122 Fe-pnictide compounds investigated
here due to the pronounced variation of the electrons density of states close
to the equilibrium chemical potential.Comment: 10 pages, 6 figure
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Ultrafast modulation of the chemical potential in BaFe2As2 by coherent phonons
Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to study the electronic structure dynamics in BaFe2As2 around the high-symmetry points Γ and M. A global oscillation of the Fermi level at the frequency of the A1g(As) phonon mode is observed. It is argued that this behavior reflects a modulation of the effective chemical potential in the photoexcited surface region that arises from the high sensitivity of the band structure near the Fermi level to the A1g(As) phonon mode combined with a low electron diffusivity perpendicular to the layers. The results establish a novel way to tune the electronic properties of iron pnictides: coherent control of the effective chemical potential. The results further suggest that the equilibration time for the effective chemical potential needs to be considered in the ultrafast electronic structure dynamics of materials with weak interlayer coupling. © 2014 American Physical Society
Time- and momentum-resolved photoemission studies using time-of-flight momentum microscopy at a free-electron laser
Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of nonequilibrium electronic processes, transient states in chemical reactions, or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical, and structural analyses requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines free-electron laser capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in (kx, ky, E) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å−1 diameter in a binding-energy range of several eV, resolving about 2.5 × 105 data voxels simultaneously. Using the ultrafast excited state dynamics in the van der Waals semiconductor WSe2 measured at photon energies of 36.5 eV and 109.5 eV, we demonstrate an experimental energy resolution of 130 meV, a momentum resolution of 0.06 Å−1, and a system response function of 150 fs
Lo-Fi Matchmaking: A Study of Social Pairing for Backpackers
There is a new world emerging around mobile social networks and the technologies used to facilitate and mediate them. It is technically feasible for mobile social software such as pairing or matchmaking systems to introduce people to others and assist information exchange. However, little is known about the social structure of many mobile communities or why they would want pairing systems. When these systems are built, it is not clear what the social response by those communities will be or what the systems will be like to use in practice. While engaged in other work determining requirements for a mobile travel assistant we saw a potentially useful application for a pairing system to facilitate the exchange of travel information between backpackers. To explore this area, we designed two studies involving usage of a low-fidelity role prototype of a social pairing system for backpackers. Graphs of the resulting social pairings showed backpackers who were hubs in the network of travel information. It also demonstrated the effect of travel direction on information utility. Backpackers rated the utility of different pairing types, and provided feedback on the social implications of being paired based on travel histories. Practical usage of the social network pairing activity and the implications of broader societal usage are discussed
Robust Magnetic Order Upon Ultrafast Excitation of an Antiferromagnet
The ultrafast manipulation of magnetic order due to optical excitation is governed by the intricate flow of energy and momentum between the electron, lattice, and spin subsystems. While various models are commonly employed to describe these dynamics, a prominent example being the microscopic three temperature model (M3TM), systematic, quantitative comparisons to both the dynamics of energy flow and magnetic order are scarce. Here, an M3TM was applied to the ultrafast magnetic order dynamics of the layered antiferromagnet GdRh2Si2. The femtosecond dynamics of electronic temperature, surface ferromagnetic order, and bulk antiferromagnetic order were explored at various pump fluences employing time- and angle-resolved photoemission spectroscopy and time-resolved resonant magnetic soft X-ray diffraction, respectively. After optical excitation, both the surface ferromagnetic order and the bulk antiferromagnetic order dynamics exhibit two-step demagnetization behaviors with two similar timescales (<1 ps, ∼10 ps), indicating a strong exchange coupling between localized 4f and itinerant conduction electrons. Despite a good qualitative agreement, the M3TM predicts larger demagnetization than the experimental observation, which can be phenomenologically described by a transient, fluence-dependent increased Néel temperature. The results indicate that effects beyond a mean-field description have to be considered for a quantitative description of ultrafast magnetic order dynamics
Identification of distinct cytotoxic granules as the origin of supramolecular attack particles in T lymphocytes
Cytotoxic T lymphocytes (CTL) kill malignant and infected cells through the directed release of cytotoxic proteins into the immunological synapse (IS). The cytotoxic protein granzyme B (GzmB) is released in its soluble form or in supramolecular attack particles (SMAP). We utilize synaptobrevin2-mRFP knock-in mice to isolate fusogenic cytotoxic granules in an unbiased manner and visualize them alone or in degranulating CTLs. We identified two classes of fusion-competent granules, single core granules (SCG) and multi core granules (MCG), with different diameter, morphology and protein composition. Functional analyses demonstrate that both classes of granules fuse with the plasma membrane at the IS. SCG fusion releases soluble GzmB. MCGs can be labelled with the SMAP marker thrombospondin-1 and their fusion releases intact SMAPs. We propose that CTLs use SCG fusion to fill the synaptic cleft with active cytotoxic proteins instantly and parallel MCG fusion to deliver latent SMAPs for delayed killing of refractory targets
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