148 research outputs found
Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe
We investigate the interactions of photoexcited carriers with lattice
vibrations in thin films of the layered transition metal dichalcogenide (TMDC)
WSe. Employing femtosecond electron diffraction with monocrystalline
samples and first principle density functional theory calculations, we obtain a
momentum-resolved picture of the energy-transfer from excited electrons to
phonons. The measured momentum-dependent phonon population dynamics are
compared to first principle calculations of the phonon linewidth and can be
rationalized in terms of electronic phase-space arguments. The relaxation of
excited states in the conduction band is dominated by intervalley scattering
between valleys and the emission of zone-boundary phonons.
Transiently, the momentum-dependent electron-phonon coupling leads to a
non-thermal phonon distribution, which, on longer timescales, relaxes to a
thermal distribution via electron-phonon and phonon-phonon collisions. Our
results constitute a basis for monitoring and predicting out of equilibrium
electrical and thermal transport properties for nanoscale applications of
TMDCs
Photoinduced ultrafast transition of the local correlated structure in chalcogenide phase-change materials
Revealing the bonding and time-evolving atomic dynamics in functional
materials with complex lattice structures can update the fundamental knowledge
on rich physics therein, and also help to manipulate the material properties as
desired. As the most prototypical chalcogenide phase change material, Ge2Sb2Te5
has been widely used in optical data storage and non-volatile electric memory
due to the fast switching speed and the low energy consumption. However, the
basic understanding of the structural dynamics on the atomic scale is still not
clear. Using femtosecond electron diffraction, structure factor calculation and
TDDFT-MD simulation, we reveal the photoinduced ultrafast transition of the
local correlated structure in the averaged rock-salt phase of Ge2Sb2Te5. The
randomly oriented Peierls distortion among unit cells in the averaged rock-salt
phase of Ge2Sb2Te5 is termed as local correlated structures. The ultrafast
suppression of the local Peierls distortions in individual unit cell gives rise
to a local structure change from the rhombohedral to the cubic geometry within
~ 0.3 ps. In addition, the impact of the carrier relaxation and the large
amount of vacancies to the ultrafast structural response is quantified and
discussed. Our work provides new microscopic insights into contributions of the
local correlated structure to the transient structural and optical responses in
phase change materials. Moreover, we stress the significance of femtosecond
electron diffraction in revealing the local correlated structure in the subunit
cell and the link between the local correlated structure and physical
properties in functional materials with complex microstructures
Chaperone activity and structure of monomeric polypeptide binding domains of GroEL
The chaperonin GroEL is a large complex composed of 14 identical 57-kDa subunits that requires ATP and GroES for some of its activities. We find that a monomeric polypeptide corresponding to residues 191 to 345 has the activity of the tetradecamer both in facilitating the refolding of rhodanese and cyclophilin A in the absence of ATP and in catalyzing the unfolding of native barnase. Its crystal structure, solved at 2.5 A resolution, shows a well-ordered domain with the same fold as in intact GroEL. We have thus isolated the active site of the complex allosteric molecular chaperone, which functions as a "minichaperone." This has mechanistic implications: the presence of a central cavity in the GroEL complex is not essential for those representative activities in vitro, and neither are the allosteric properties. The function of the allosteric behavior on the binding of GroES and ATP must be to regulate the affinity of the protein for its various substrates in vivo, where the cavity may also be required for special functions
Traversing double-well potential energy surfaces: photoinduced concurrent intralayer and interlayer structural transitions in XTe2 (X=Mo, W)
Manipulating crystal structure and the corresponding electronic properties in
quantum materials provides opportunities for the exploration of exotic physics
and practical applications. Here, by ultrafast electron diffraction, structure
factor calculation and TDDFT-MD simulations, we report the photoinduced
concurrent intralayer and interlayer structural transitions in the Td and 1T'
phase of XTe2 (X=Mo, W). Concomitant with the interlayer structural transition
by shear displacement, the ultrafast suppression of the intralayer Peierls
distortion within 0.3 ps is demonstrated and attributed to Mo-Mo (W-W) bond
stretching. We discuss the modification of multiple quantum electronic states
associated with the intralayer and interlayer structural transitions, such as
the topological band inversion and the higher-order topological state. The twin
structure and the stacking fault in XTe2 are identified by the ultrafast
structural response. Our work elucidates the pathway of the photoinduced
intralayer and interlayer structural transitions in atomic and femtosecond
spatiotemporal scale. Moreover, the concurrent intralayer and interlayer
structural transitions reveals the traversal of all double-well potential
energy surfaces (DWPES) by laser excitation in material system, which may be an
intrinsic mechanism in the field of photoexcitation-driven symmetry
engineering, beyond the single DWPES transition model and the order-disorder
transition model
Intrinsic energy flow in laser-excited 3d ferromagnets
Ultrafast magnetization dynamics are governed by energy flow between electronic, magnetic, and lattice degrees of freedom. A quantitative understanding of these dynamics must be based on a model that agrees with experimental results for all three subsystems. However, ultrafast dynamics of the lattice remain largely unexplored experimentally. Here we combine femtosecond electron diffraction experiments of the lattice dynamics with energy-conserving atomistic spin dynamics (ASD) simulations and ab initio calculations to study the intrinsic energy flow in the 3d ferromagnets cobalt (Co) and iron (Fe). The simulations yield a good description of experimental data, in particular an excellent description of our experimental results for the lattice dynamics. We find that the lattice dynamics are influenced significantly by the magnetization dynamics due to the energy cost of demagnetization. Our results highlight the role of the spin system as the dominant heat sink in the first hundreds of femtoseconds. Together with previous findings for nickel [Zahn et al., Phys. Rev. Research 3, 023032 (2021)], our work demonstrates that energy-conserving ASD simulations provide a general and consistent description of the laser-induced dynamics in all three elemental 3d ferromagnets
Prion Protein in Milk
BACKGROUND: Prions are known to cause transmissible spongiform encephalopathies (TSE) after accumulation in the central nervous system. There is increasing evidence that prions are also present in body fluids and that prion infection by blood transmission is possible. The low concentration of the proteinaceous agent in body fluids and its long incubation time complicate epidemiologic analysis and estimation of spreading and thus the risk of human infection. This situation is particularly unsatisfactory for food and pharmaceutical industries, given the lack of sensitive tools for monitoring the infectious agent. METHODOLOGY/PRINCIPAL FINDINGS: We have developed an adsorption matrix, Alicon PrioTrap®, which binds with high affinity and specificity to prion proteins. Thus we were able to identify prion protein (PrP(C))–the precursor of prions (PrP(Sc))–in milk from humans, cows, sheep, and goats. The absolute amount of PrP(C) differs between the species (from µg/l range in sheep to ng/l range in human milk). PrP(C) is also found in homogenised and pasteurised off-the-shelf milk, and even ultrahigh temperature treatment only partially diminishes endogenous PrP(C) concentration. CONCLUSIONS/SIGNIFICANCE: In view of a recent study showing evidence of prion replication occurring in the mammary gland of scrapie infected sheep suffering from mastitis, the appearance of PrP(C) in milk implies the possibility that milk of TSE-infected animals serves as source for PrP(Sc)
Subgenual activation and the finger of blame: individual differences and depression vulnerability.
BACKGROUND: Subgenual cingulate cortex (SCC) responses to self-blaming emotion-evoking stimuli were previously found in individuals prone to self-blame with and without a history of major depressive disorder (MDD). This suggested SCC activation reflects self-blaming emotions such as guilt, which are central to models of MDD vulnerability. METHOD: Here, we re-examined these hypotheses in an independent larger sample. A total of 109 medication-free participants (70 with remitted MDD and 39 healthy controls) underwent fMRI whilst judging self- and other-blaming emotion-evoking statements. They also completed validated questionnaires of proneness to self-blaming emotions including those related to internal (autonomy) and external (sociotropy) evaluation, which were subjected to factor analysis. RESULTS: An interaction between group (remitted MDD v. Control) and condition (self- v. other-blame) was observed in the right SCC (BA24). This was due to higher SCC signal for self-blame in remitted MDD and higher other-blame-selective activation in Control participants. Across the whole sample, extracted SCC activation cluster averages for self- v. other-blame were predicted by a regression model which included the reliable components derived from our factor analysis of measures of proneness to self-blaming emotions. Interestingly, this prediction was solely driven by autonomy/self-criticism, and adaptive guilt factors, with no effect of sociotropy/dependency. CONCLUSIONS: Despite confirming the prediction of SCC activation in self-blame-prone individuals and those vulnerable to MDD, our results suggest that SCC activation reflects blame irrespective of where it is directed rather than selective for self. We speculate that self-critical individuals have more extended SCC representations for blame in the context of self-agency
Lattice dynamics and ultrafast energy flow between electrons, spins, and phonons in a 3d ferromagnet
The ultrafast dynamics of magnetic order in a ferromagnet are governed by the interplay between electronic, magnetic, and lattice degrees of freedom. In order to obtain a microscopic understanding of ultrafast demagnetization, information on the response of all three subsystems is required. A consistent description of demagnetization and microscopic energy flow, however, is still missing. Here, we combine a femtosecond electron diffraction study of the ultrafast lattice response of nickel to laser excitation with ab initio calculations of the electron-phonon interaction and energy-conserving atomistic spin dynamics simulations. Our model is in agreement with the observed lattice dynamics and previously reported electron and magnetization dynamics. Our approach reveals that the spin system is the dominating heat sink in the initial few hundred femtoseconds and implies a transient nonthermal state of the spins. Our results provide a clear picture of the microscopic energy flow between electronic, magnetic, and lattice degrees of freedom on ultrafast timescales and constitute a foundation for theoretical descriptions of demagnetization that are consistent with the dynamics of all three subsystems
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