96 research outputs found
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
Unoccupied electronic structure and momentum-dependent scattering dynamics in Pb/Si(557) nanowire arrays
The unoccupied electronic structure of quasi-one-dimensional reconstructions of Pb atoms on a Si(557) surface is investigated by means of femtosecond time- and angle-resolved two-photon photoemission. Two distinct unoccupied electronic states are observed at E-EF=3.55 and 3.30 eV, respectively. Density functional theory calculations reveal that these states are spatially located predominantly on the lead wires and that they are energetically degenerated with an energy window of reduced electronic density of states in Si. We further find momentum-averaged lifetimes of 24 and 35 fs of these two states, respectively. The photoemission yield and the population dynamics depend on the electron momentum component perpendicular to the steps of the Si substrate, and the momentum-dependent dynamics cannot be described by means of rate equations. We conclude that momentum- and direction-dependent dephasing of the electronic excitations, likely caused by elastic scattering at the step edges on the vicinal surface, modifies the excited-state population dynamics in this system. © 2015 American Physical Society.DFG/FOR/170
Doping dependence and electron–boson coupling in the ultrafast relaxation of hot electron populations in Ba(Fe₁₋ₓCoₓ)₂As₂
Using femtosecond time- and angle-resolved photoemission spectroscopy we investigate the effect of
electron doping on the electron dynamics in Ba(Fe1-xCox)2As2 in a range of x< 0.15 at
temperatures slightly above the Néel temperature. By analyzing the time-dependent photoemission
intensity of the pump laser excited population as a function of energy, we found that the relaxation
times at 0 < E - E < 0.2 eV are doping dependent and about 100 fs shorter at optimal doping than
for overdoped and parent compounds. Analysis of the relaxation rates also reveals the presence of a
pump fluence dependent step in the relaxation time at E - E = 200 meV whichwe explain by
coupling of the excited electronic system to a boson of this energy.Wecompare our results with static
ARPES and transport measurements and find disagreement and agreement concerning the dopingdependence,
respectively.Wediscuss the effect of the electron–boson coupling on the energydependent
relaxation and assign the origin of the boson to a magnetic excitation
Ultrafast doublon dynamics in photoexcited -
Strongly correlated materials exhibit intriguing properties caused by intertwined microscopic interactions that are hard to disentangle in equilibrium. Employing nonequilibrium time-resolved photoemission spectroscopy on the quasi-two- dimensional transition-metal dichalcogenide 1T-TaS2, we identify a spectroscopic signature of doubly occupied sites (doublons) that reflects fundamental Mott physics. Doublon-hole recombination is estimated to occur on timescales of electronic hopping ℏ/J≈14 fs. Despite strong electron-phonon coupling, the dynamics can be explained by purely electronic effects captured by the single-band Hubbard model under the assumption of weak hole doping, in agreement with our static sample characterization. This sensitive interplay of static doping and vicinity to the metal- insulator transition suggests a way to modify doublon relaxation on the few- femtosecond timescale
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
The Sloan Great Wall. Rich clusters
We present the results of the study of the substructure and galaxy content of
ten rich clusters of galaxies in three different superclusters of the Sloan
Great Wall. We determine the substructure in clusters using the 'Mclust'
package from the 'R' statistical environment and analyse their galaxy content.
We analyse the distribution of the peculiar velocities of galaxies in clusters
and calculate the peculiar velocity of the first ranked galaxy. We show that
clusters in our sample have more than one component; in some clusters different
components also have different galaxy content. We find that in some clusters
with substructure the peculiar velocities of the first ranked galaxies are
large. All clusters in our sample host luminous red galaxies. They can be found
both in the central areas of clusters as well as in the outskirts, some of them
have large peculiar velocities. About 1/3 of red galaxies in clusters are
spirals. The scatter of colours of red ellipticals is in most clusters larger
than that of red spirals. The presence of substructure in rich clusters, signs
of possible mergers and infall, as well as the large peculiar velocities of the
first ranked galaxies suggest that the clusters in our sample are not yet
virialized. We present merger trees of dark matter haloes in an N-body
simulation to demonstrate the formation of present-day dark matter haloes via
multiple mergers during their evolution. In simulated dark matter haloes we
find a substructure similar to that in observed clusters.Comment: 19 pages, 44 figures, accepted for publication in Astronomy and
Astrophysic
Population genetic analysis of brazilian peach breeding germplasm.
ABSTRACT Peach has great economic and social importance in Brazil. Diverse sources of germplasm were used to introduce desirable traits in the Brazilian peach breeding pool, composed mainly by local selections and accessions selected from populations developed by the national breeding programs, adapted to subtropical climate, with low chill requirement, as well as accessions introduced from several countries. In this research, we used SSR markers, selected by their high level of polymorphism, to access genetic diversity and population structure of a set composed by 204 peach selected genotypes, based on contrasting phenotypes for valuable traits in peach breeding. A total of 80 alleles were obtained, giving an average of eight alleles per locus. In general, the average value of observed heterozygosity (0.46) was lower than the expected heterozygosity (0.63). STRUCTURE analysis assigned 162 accessions splitted into two subpopulations based mainly on their flesh type: melting (96) and non-melting (66) flesh cultivars. The remaining accessions (42) could not be assigned under the 80% membership coefficient criteria. Genetic variability was greater in melting subpopulation compared to non-melting. Additionally, 55% of the alleles present in the breeding varieties were also present in the founder varieties, indicating that founding clones are well represented in current peach cultivars and advanced selections developed. Overall, this study gives a first insight of the peach genetic variability available and evidence for population differentiation (structure) in this peach panel to be exploited and provides the basis for genome-wide association studies
Sound localization and word discrimination in reverberant environment in children with developmental dyslexia
In-situ bioconjugation in stationary media and in liquid flow by femtosecond laser ablation
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