| 1. |
|
|
| 2. |
- Henighan, T., et al.
(author)
-
Generation mechanism of terahertz coherent acoustic phonons in Fe
- 2016
-
In: PHYSICAL REVIEW B. - 2469-9950. ; 93:22
-
Journal article (peer-reviewed)abstract
- We use femtosecond time-resolved hard x-ray scattering to detect coherent acoustic phonons generated during ultrafast laser excitation of ferromagnetic bcc Fe films grown on MgO(001). We observe the coherent longitudinal-acoustic phonons as a function of wave vector through analysis of the temporal oscillations in the x-ray scattering signal. The width of the extracted strain wave front associated with this coherent motion is similar to 100 fs. An effective electronic Gruneisen parameter is extracted within a two-temperature model. However, ab initio calculations show that the phonons are nonthermal on the time scale of the experiment, which calls into question the validity of extracting physical constants by fitting such a two-temperature model.
|
|
| 3. |
- Chernenkaya, A., et al.
(author)
-
Microscopic origin of the charge transfer in single crystals based on thiophene derivatives : A combined NEXAFS and density functional theory approach
- 2016
-
In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 145:3
-
Journal article (peer-reviewed)abstract
- We have investigated the charge transfer mechanism in single crystals of DTBDT-TCNQ and DTBDT-F4TCNQ (where DTBDT is dithieno[2,3-d;2′,3′-d′] benzo[1,2-b;4,5-b′]dithiophene) using a combination of near-edge X-ray absorption spectroscopy (NEXAFS) and density functional theory calculations (DFT) including final state effects beyond the sudden state approximation. In particular, we find that a description that considers the partial screening of the electron-hole Coulomb correlation on a static level as well as the rearrangement of electronic density shows excellent agreement with experiment and allows to uncover the details of the charge transfer mechanism in DTBDT-TCNQ and DTBDT-F4 TCNQ, as well as a reinterpretation of previous NEXAFS data on pure TCNQ. Finally, we further show that almost the same quality of agreement between theoretical results and experiment is obtained by the much faster Z+1/2 approximation, where the core hole effects are simulated by replacing N or F with atomic number Z with the neighboring atom with atomic number Z+1/2.
|
|
| 4. |
- Trigo, M., et al.
(author)
-
Fourier-transform inelastic X-ray scattering from time- and momentum-dependent phonon-phonon correlations
- 2013
-
In: Nature Physics. - 1745-2473. ; 9:12, s. 790-794
-
Journal article (peer-reviewed)abstract
- The macroscopic characteristics of a material are determined by its elementary excitations, which dictate the response of the system to external stimuli. The spectrum of excitations is related to fluctuations in the density-density correlations and is typically measured through frequency-domain neutron(1) or X-ray(2-4) scattering. Time-domain measurements of these correlations could yield a more direct way to investigate the excitations of solids and their couplings both near to and far from equilibrium. Here we show that we can access large portions of the phonon dispersion of germanium by measuring the diffuse scattering from femtosecond X-ray free-electron laser pulses. A femtosecond optical laser pulse slightly quenches the vibrational frequencies, producing pairs of high-wavevector phonons with opposite momenta. These phonons manifest themselves as time-dependent coherences in the displacement correlations(5) probed by the X-ray scattering. As the coherences are preferentially created in regions of strong electron-phonon coupling, the time-resolved approach is a natural spectroscopic tool for probing low-energy collective excitations in solids, and their microscopic interactions.
|
|
| 5. |
- Fuchs, M.a b, et al.
(author)
-
Nonlinear X-ray compton scattering
- 2014
-
Conference paper (other academic/artistic)abstract
- We use XFEL pulses to observe the most fundamental nonlinear X-ray-matter interaction: nonlinear Compton scattering. In contrast to theoretical predictions, we measure an anonymous and yet to be explained red-shift in the observed photon energy.
|
|
| 6. |
- Medjanik, K., et al.
(author)
-
Near-Edge x‑ray absorption fine structure investigation of the quasi-One-Dimensional organic conductor (TMTSF)2PF6
- 2016
-
In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 120:43, s. 8574-8583
-
Journal article (peer-reviewed)abstract
- We present high-resolution near-edge X-ray absorption fine structure (NEXAFS) measurements at the P L2/3 edges, F K edge, C K edge, and Se M2/3 edges of the quasi-one-dimensional (1D) conductor and superconductor (TMTSF)2PF6. NEXAFS allows probing the donor and acceptor moieties separately; spectra were recorded between room temperature (RT) and 30 K at normal incidence. Spectra taken around RT were also studied as a function of the angle (θ) between the electric field of the X-ray beam and the 1D conducting direction. In contrast with a previous study of the S L2/3-edges spectra in (TMTTF)2AsF6, the Se M2/3 edges of (TMTSF)2PF6 do not exhibit a well-resolved spectrum. Surprisingly, the C K-edge spectra contain three well-defined peaks exhibiting strong and nontrivial θ and temperature dependence. The nature of these peaks as well as those of the F K-edge spectra could be rationalized on the basis of first-principles DFT calculations. Despite the structural similarity, the NEXAFS spectra of (TMTSF)2PF6 and (TMTTF)2AsF6 exhibit important differences. In contrast with the case of (TMTTF)2AsF6, the F K-edge spectra of (TMTSF)2PF6 do not change with temperature despite stronger donor−anion interactions. All these features reveal subtle differences in the electronic structure of the TMTSF and TMTTF families of salts.
|
|
| 7. |
- Shen, L., et al.
(author)
-
Decoupling spin-orbital correlations in a layered manganite amidst ultrafast hybridized charge-transfer band excitation
- 2020
-
In: Physical Review B. - 2469-9950. ; 101:20
-
Journal article (peer-reviewed)abstract
- In the mixed-valence manganites, a near-infrared laser typically melts the orbital and spin order simultaneously, corresponding to the photoinduced d1d0→d0d1 excitations in the Mott-Hubbard bands of manganese. Here, we use ultrafast methods-both femtosecond resonant X-ray diffraction and optical reflectivity-to demonstrate that the orbital response in the layered manganite Nd1-xSr1+xMnO4(x=2/3) does not follow this scheme. At the photoexcitation saturation fluence, the orbital order is only diminished by a few percent in the transient state. Instead of the typical d1d0→d0d1 transition, a near-infrared pump in this compound promotes a fundamentally distinct mechanism of charge transfer, the d0→d1L, where L denotes a hole in the oxygen band. This finding may pave a different avenue for selectively manipulating specific types of order in complex materials of this class.
|
|
| 8. |
- Yang, Jie, et al.
(author)
-
Direct observation of ultrafast hydrogen bond strengthening in liquid water
- 2021
-
In: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 596:7873
-
Journal article (peer-reviewed)abstract
- Water is one of the most important, yet least understood, liquids in nature. Many anomalous properties of liquid water originate from its well-connected hydrogen bond network, including unusually efficient vibrational energy redistribution and relaxation2. An accurate description of the ultrafast vibrational motion of water molecules is essential for understanding the nature of hydrogen bonds and many solution-phase chemical reactions. Most existing knowledge of vibrational relaxation in water is built upon ultrafast spectroscopy experiments. However, these experiments cannot directly resolve the motion of the atomic positions and require difficult translation of spectral dynamics into hydrogen bond dynamics. Here, we measure the ultrafast structural response to the excitation of the OH stretching vibration in liquid water with femtosecond temporal and atomic spatial resolution using liquid ultrafast electron scattering. We observed a transient hydrogen bond contraction of roughly 0.04 Å on a timescale of 80 femtoseconds, followed by a thermalization on a timescale of approximately 1 picosecond. Molecular dynamics simulations reveal the need to treat the distribution of the shared proton in the hydrogen bond quantum mechanically to capture the structural dynamics on femtosecond timescales. Our experiment and simulations unveil the intermolecular character of the water vibration preceding the relaxation of the OH stretch.
|
|
| 9. |
- Hagberg, C. E., et al.
(author)
-
Flow Cytometry of Mouse and Human Adipocytes for the Analysis of Browning and Cellular Heterogeneity
- 2018
-
In: Cell Reports. - : Elsevier BV. - 2211-1247. ; 24:10
-
Journal article (peer-reviewed)abstract
- Adipocytes, once considered simple lipid-storing cells, are rapidly emerging as complex cells with many biologically diverse functions. A powerful high-throughput method for analyzing single cells is flow cytometry. Several groups have attempted to analyze and sort freshly isolated adipocytes; however, using an adipocyte-specific reporter mouse, we demonstrate that these studies fail to detect the majority of white adipocytes. We define critical settings required for adipocyte flow cytometry and provide a rigid strategy for analyzing and sorting white and brown adipocyte populations. The applicability of our protocol is shown by sorting mouse adipocytes based on size or UCP1 expression and demonstrating that a subset of human adipocytes lacks the beta(2)-adrenergic receptor, particularly in the insulin-resistant state. In conclusion, the present study confers key technological insights for analyzing and sorting mature adipocytes, opening up numerous downstream research applications.
|
|
| 10. |
- Kozina, M., et al.
(author)
-
Local terahertz field enhancement for time-resolved x-ray diffraction
- 2017
-
In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 110:8
-
Journal article (peer-reviewed)abstract
- We report local field strength enhancement of single-cycle terahertz (THz) pulses in an ultrafast time-resolved x-ray diffraction experiment. We show that patterning the sample with gold microstructures increases the THz field without changing the THz pulse shape or drastically affecting the quality of the x-ray diffraction pattern. We find a five-fold increase in THz-induced x-ray diffraction intensity change in the presence of microstructures on a SrTiO3 thin-film sample.
|
|
