| 1. |
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| 2. |
- Lloyd-Hughes, J., et al.
(author)
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The 2021 ultrafast spectroscopic probes of condensed matter roadmap
- 2021
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In: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 33:35
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Research review (peer-reviewed)abstract
- In the 60 years since the invention of the laser, the scientific community has developed numerous fields of research based on these bright, coherent light sources, including the areas of imaging, spectroscopy, materials processing and communications. Ultrafast spectroscopy and imaging techniques are at the forefront of research into the light-matter interaction at the shortest times accessible to experiments, ranging from a few attoseconds to nanoseconds. Light pulses provide a crucial probe of the dynamical motion of charges, spins, and atoms on picosecond, femtosecond, and down to attosecond timescales, none of which are accessible even with the fastest electronic devices. Furthermore, strong light pulses can drive materials into unusual phases, with exotic properties. In this roadmap we describe the current state-of-the-art in experimental and theoretical studies of condensed matter using ultrafast probes. In each contribution, the authors also use their extensive knowledge to highlight challenges and predict future trends.
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| 3. |
- Ranitovic, Predrag, et al.
(author)
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IR-assisted ionization of helium by attosecond extreme ultraviolet radiation
- 2010
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In: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 12, s. 13008-
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Journal article (peer-reviewed)abstract
- Attosecond science has opened up the possibility of manipulating electrons on their fundamental timescales. Here, we use both theory and experiment to investigate ionization dynamics in helium on the attosecond timescale by simultaneously irradiating the atom with a soft x-ray attosecond pulse train (APT) and an ultrafast laser pulse. Because the APT has resolution in both energy and time, we observe processes that could not be observed without resolution in both domains simultaneously. We show that resonant absorption is important in the excitation of helium and that small changes in energies of harmonics that comprise the APT can result in large changes in the ionization process. With the help of theory, ionization pathways for the infrared-assisted excitation and ionization of helium by extreme ultraviolet (XUV) attosecond pulses have been identified and simple model interpretations have been developed that should be of general applicability to more complex systems (Zewail A 2000 J. Phys. Chem. A 104 5660-94).
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| 4. |
- Reis, D. A, et al.
(author)
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Probing impulsive strain propagation with x-ray pulses
- 2001
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In: Physical Review Letters. - 1079-7114. ; 86:14, s. 3072-3075
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Journal article (peer-reviewed)abstract
- Pump-probe time-resolved x-ray diffraction of allowed and nearly forbidden reflections in InSb is used to follow the propagation of a coherent acoustic pulse generated by ultrafast laser excitation. The surface and bulk components of the strain could be simultaneously measured due to the large x-ray penetration depth. Comparison of the experimental data with dynamical diffraction simulations suggests that the conventional model for impulsively generated strain underestimates the partitioning of energy into coherent modes.
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| 5. |
- Rudolf, Dennis, et al.
(author)
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Element Selective Investigation of Spin Dynamics in Magnetic Multilayers
- 2015
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In: Ultrafast Magnetism I. - Cham : Springer International Publishing. - 9783319077437 - 9783319077420 ; , s. 307-309
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Conference paper (peer-reviewed)abstract
- Our understanding of ultrafast switching processes in novel spin-based electronics depends on our detailed knowledge of interactions between spin, charge and phonons in magnetic structures. We present element-selective studies, using extreme ultraviolet (XUV) light, to gain insight into spin dynamics in exchange coupled magnetic multilayers on the femtosecond time scale.
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| 6. |
- Rudolf, Dennis, et al.
(author)
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Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current
- 2012
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In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 3, s. 1037-
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Journal article (peer-reviewed)abstract
- Uncovering the physical mechanisms that govern ultrafast charge and spin dynamics is crucial for understanding correlated matter as well as the fundamental limits of ultrafast spin-based electronics. Spin dynamics in magnetic materials can be driven by ultrashort light pulses, resulting in a transient drop in magnetization within a few hundred femtoseconds. However, a full understanding of femtosecond spin dynamics remains elusive. Here we spatially separate the spin dynamics using Ni/Ru/Fe magnetic trilayers, where the Ni and Fe layers can be ferroor antiferromagnetically coupled. By exciting the layers with a laser pulse and probing the magnetization response simultaneously but separately in Ni and Fe, we surprisingly find that optically induced demagnetization of the Ni layer transiently enhances the magnetization of the Fe layer when the two layer magnetizations are initially aligned parallel. Our observations are explained by a laser-generated superdiffusive spin current between the layers.
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| 7. |
- Zusin, Dmitriy, et al.
(author)
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Direct measurement of the static and transient magneto-optical permittivity of cobalt across the entire M-edge in reflection geometry by use of polarization scanning
- 2018
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In: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 97:2
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Journal article (peer-reviewed)abstract
- The microscopic state of amagnetic material is characterized by its resonant magneto-optical response through the off-diagonal dielectric tensor component epsilon(xy). However, the measurement of the full complex epsilon(xy) in the extreme ultraviolet spectral region covering the M absorption edges of 3d ferromagnets is challenging due to the need for either a careful polarization analysis, which is complicated by a lack of efficient polarization analyzers, or scanning the angle of incidence in fine steps. Here, we propose and demonstrate a technique to extract the complex resonant permittivity epsilon(xy) simply by scanning the polarization angle of linearly polarized high harmonics to measure the magneto-optical asymmetry in reflection geometry. Because this technique is more practical and faster to experimentally implement than previous approaches, we can directly measure the full time evolution of epsilon(xy)(t) during laser-induced demagnetization across the entire M-2,M-3 absorption edge of cobalt with femtosecond time resolution. We find that for polycrystalline Co films on an insulating substrate, the changes in epsilon(xy) are uniform throughout the spectrum, to within our experimental precision. This result suggests that, in the regime of strong demagnetization, the ultrafast demagnetization response is primarily dominated by magnon generation. We estimate the contribution of exchange-splitting reduction to the ultrafast demagnetization process to be no more than 25%.
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| 8. |
- Fan, Tingting, et al.
(author)
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Bright circularly polarized soft X-ray high harmonics for X-ray magnetic circular dichroism
- 2015
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In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 112:46, s. 14206-14211
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Journal article (peer-reviewed)abstract
- We demonstrate, to our knowledge, the first bright circularly polarized high-harmonic beams in the soft X-ray region of the electromagnetic spectrum, and use them to implement X-ray magnetic circular dichroism measurements in a tabletop-scale setup. Using counterrotating circularly polarized laser fields at 1.3 and 0.79 mu m, we generate circularly polarized harmonics with photon energies exceeding 160 eV. The harmonic spectra emerge as a sequence of closely spaced pairs of left and right circularly polarized peaks, with energies determined by conservation of energy and spin angular momentum. We explain the single-atom and macroscopic physics by identifying the dominant electron quantum trajectories and optimal phase-matching conditions. The first advanced phase-matched propagation simulations for circularly polarized harmonics reveal the influence of the finite phase-matching temporal window on the spectrum, as well as the unique polarization-shaped attosecond pulse train. Finally, we use, to our knowledge, the first tabletop X-ray magnetic circular dichroism measurements at the N-4,N-5 absorption edges of Gd to validate the high degree of circularity, brightness, and stability of this light source. These results demonstrate the feasibility of manipulating the polarization, spectrum, and temporal shape of high harmonics in the soft X-ray region by manipulating the driving laser waveform.
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| 9. |
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| 10. |
- Hickstein, Daniel D., et al.
(author)
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Non-collinear generation of angularly isolated circularly polarized high harmonics
- 2015
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In: Nature Photonics. - 1749-4885 .- 1749-4893. ; 9:11, s. 743-750
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Journal article (peer-reviewed)abstract
- We generate angularly isolated beams of circularly polarized extreme ultraviolet light through the first implementation of non-collinear high harmonic generation with circularly polarized driving lasers. This non-collinear technique offers numerous advantages over previous methods, including the generation of higher photon energies, the separation of the harmonics from the pump beam, the production of both left and right circularly polarized harmonics at the same wavelength and the capability of separating the harmonics without using a spectrometer. To confirm the circular polarization of the beams and to demonstrate the practicality of this new light source, we measure the magnetic circular dichroism of a 20 nm iron film. Furthermore, we explain the mechanisms of non-collinear high harmonic generation using analytical descriptions in both the photon and wave models. Advanced numerical simulations indicate that this non-collinear mixing enables the generation of isolated attosecond pulses with circular polarization.
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| 11. |
- Zusin, Dmitriy, et al.
(author)
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Ultrafast perturbation of magnetic domains by optical pumping in a ferromagnetic multilayer
- 2022
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 106:14
-
Journal article (peer-reviewed)abstract
- Ultrafast optical pumping of spatially nonuniform magnetic textures is known to induce far-from-equilibrium spin transport effects. Here, we use ultrafast x-ray diffraction with unprecedented dynamic range to study the laser-induced dynamics of labyrinth domain networks in ferromagnetic CoFe/Ni multilayers. We detected azimuthally isotropic, odd order, magnetic diffraction rings up to fifth order. The amplitudes of all three diffraction rings quench to different degrees within 1.6 ps. In addition, all three of the detected diffraction rings both broaden by 15% and radially contract by 6% during the quench process. We are able to rigorously quantify a 31% ultrafast broadening of the domain walls via Fourier analysis of the order-dependent quenching of the three detected diffraction rings. The broadening of the diffraction rings is interpreted as a reduction in the domain coherence length, but the shift in the ring radius, while unambiguous in its occurrence, remains unexplained. In particular, we demonstrate that a radial shift explained by domain-wall broadening can be ruled out. With the unprecedented dynamic range of our data, our results provide convincing evidence that labyrinth domain structures are spatially perturbed at ultrafast speeds under far-from-equilibrium conditions, albeit the mechanism inducing the perturbations remains yet to be clarified.
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| 12. |
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| 13. |
- Barrenäs, Fredrik, et al.
(author)
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Deep Transcriptional Sequencing of Mucosal Challenge Compartment from Rhesus Macaques Acutely Infected with Simian Immunodeficiency Virus Implicates Loss of Cell Adhesion Preceding Immune Activation
- 2014
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In: Journal of Virology. - 0022-538X .- 1098-5514. ; 88:14, s. 7962-7972
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Journal article (peer-reviewed)abstract
- Pathology resulting from human immunodeficiency virus (HIV) infection is driven by protracted inflammation; the primary loss of CD4(+) T cells is caused by activation-driven apoptosis. Recent studies of nonhuman primates (NHPs) have suggested that during the acute phase of infection, antiviral mucosal immunity restricts viral replication in the primary infection compartment. These studies imply that HIV achieves systemic infection as a consequence of a failure in host antiviral immunity. Here, we used high-dose intrarectal inoculation of rhesus macaques with simian immunodeficiency virus (SIV) SIVmac251 to examine how the mucosal immune system is overcome by SIV during acute infection. The host response in rectal mucosa was characterized by deep mRNA sequencing (mRNA-seq) at 3 and 12 days postinoculation (dpi) in 4 animals for each time point. While we observed a strong host transcriptional response at 3 dpi, functions relating to antiviral immunity were absent. Instead, we observed a significant number of differentially expressed genes relating to cell adhesion and reorganization of the cytoskeleton. We also observed downregulation of genes encoding members of the claudin family of cell adhesion molecules, which are coexpressed with genes associated with pathology in the colorectal mucosa, and a large number of noncoding transcripts. In contrast, at 12 dpi the differentially expressed genes were enriched in those involved with immune system functions, in particular, functions relating to T cells, B cells, and NK cells. Our findings indicate that host responses that negatively affect mucosal integrity occur before inflammation. Consequently, when inflammation is activated at peak viremia, mucosal integrity is already compromised, potentially enabling rapid tissue damage, driving further inflammation. IMPORTANCE The HIV pandemic is one of the major threats to human health, causing over a million deaths per year. Recent studies have suggested that mucosal antiviral immune responses play an important role in preventing systemic infection after exposure to the virus. Yet, despite their potential role in decreasing transmission rates between individuals, these antiviral mechanisms are poorly understood. Here, we carried out the first deep mRNA sequencing analysis of mucosal host responses in the primary infection compartment during acute SIV infection. We found that during acute infection, a significant host response was mounted in the mucosa before inflammation was triggered. Our analysis indicated that the response has a detrimental effect on tissue integrity, causing increased permeability, tissue damage, and recruitment of SIV target cells. These results emphasize the importance of mucosal host responses preceding immune activation in preventing systemic SIV infection.
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| 14. |
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| 15. |
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| 16. |
- Singh, K. P., et al.
(author)
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Control of Electron Localization in Deuterium Molecular Ions using an Attosecond Pulse Train and a Many-Cycle Infrared Pulse
- 2010
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 104:2, s. 23001-
-
Journal article (peer-reviewed)abstract
- We demonstrate an experimental control of electron localization in deuterium molecular ions created and dissociated by the combined action of an attosecond pulse train and a many-cycle infrared (IR) pulse. The attosecond pulse train is synthesized using both even and odd high order harmonics of the driving IR frequency so that it can strobe the IR field once per IR cycle. An asymmetric ejection of the deuterium ions oscillates with the full IR period when the APT-IR time-delay is scanned. The observed control is due to the creation of a coherent superposition of 1s sigma(g) and 2p sigma(u) states via interference between one-photon and two-photon dissociation channels.
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| 17. |
- Tao, Zhensheng, et al.
(author)
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The nature of non-equilibrium ultrafast demagnetization in ferromagnetic nickel
- 2019
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In: Eleventh International Conference On Information Optics And Photonics (CIOP 2019). - : SPIE. - 9781510631748
-
Conference paper (peer-reviewed)abstract
- It has long been known that ferromagnets undergo a phase transition from ferromagnetic to paramagnetic at the Curie temperature, associated with critical phenomena such as a divergence in the heat capacity. A ferromagnet can also be transiently demagnetized by heating it with an ultrafast laser pulse. However, to date the connection between out-of-equilibrium and equilibrium phase transitions was not known, nor how fast the out-of-equilibrium phase transitions can proceed. In this work, by combining time- and angle-resolved photoemission (Tr-ARPES) with time-resolved transverse magneto-optical Kerr (Tr-TMOKE) spectroscopies, we show that the same critical behavior also governs the ultrafast magnetic phase transition in nickel. This is evidenced by several observations. First, we observe a divergence of the transient heat capacity of the electron spin system preceding material demagnetization. Second, when the electron temperature is transiently driven above the Curie temperature, we observe an extremely rapid change in the material response: the spin system absorbs sufficient energy within the first 20 fs to subsequently proceed through the phase transition, while demagnetization and the collapse of the exchange splitting occur on much longer timescales. Third, we find that the transient electron temperature alone dictates the magnetic response. By comparing results obtained from different methods, we show that the critical behaviors are essential for fully explaining the fluence-dependent magnetization dynamics measured using magneto-optical spectroscopy.
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| 18. |
- Tengdin, Phoebe, et al.
(author)
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Critical behavior within 20 fs drives the out-of-equilibrium laser-induced magnetic phase transition in nickel
- 2018
-
In: Science Advances. - : AMER ASSOC ADVANCEMENT SCIENCE. - 2375-2548. ; 4:3
-
Journal article (peer-reviewed)abstract
- It has long been known that ferromagnets undergo a phase transition from ferromagnetic to paramagnetic at the Curie temperature, associated with critical phenomena such as a divergence in the heat capacity. A ferromagnet can also be transiently demagnetized by heating it with an ultrafast laser pulse. However, to date, the connection between out-of-equilibrium and equilibrium phase transitions, or how fast the out-of-equilibrium phase transitions can proceed, was not known. By combining time-and angle-resolved photoemission with time-resolved transverse magneto-optical Kerr spectroscopies, we show that the same critical behavior also governs the ultrafast magnetic phase transition in nickel. This is evidenced by several observations. First, we observe a divergence of the transient heat capacity of the electron spin system preceding material demagnetization. Second, when the electron temperature is transiently driven above the Curie temperature, we observe an extremely rapid change in the material response: The spin system absorbs sufficient energy within the first 20 fs to subsequently proceed through the phase transition, whereas demagnetization and the collapse of the exchange splitting occur on much longer, fluence-independent time scales of similar to 176 fs. Third, we find that the transient electron temperature alone dictates the magnetic response. Our results are important because they connect the out-of-equilibrium material behavior to the strongly coupled equilibrium behavior and uncover a new time scale in the process of ultrafast demagnetization.
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| 19. |
- Turgut, Emrah, et al.
(author)
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Stoner versus Heisenberg : Ultrafast exchange reduction and magnon generation during laser-induced demagnetization
- 2016
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In: PHYSICAL REVIEW B. - 2469-9950. ; 94:22
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Journal article (peer-reviewed)abstract
- Understanding how the electronic band structure of a ferromagnetic material is modified during laser-induced demagnetization on femtosecond time scales has been a long-standing question in condensed matter physics. Here, we use ultrafast high harmonics to measure time-, energy-, and angle-resolved M-edge magnetic asymmetry spectra for Co films after optical pumping to induce ultrafast demagnetization. This provides a complete data set that we can compare with advanced ab initio magneto-optical calculations. Our analysis identifies that the dominant mechanisms contributing to ultrafast demagnetization on time scales up to several picoseconds are a transient reduction in the exchange splitting and the excitation of ultrafast magnons. Surprisingly, we find that the magnon contribution to ultrafast demagnetization is already strong on subpicosecond time scales, while the reduction in exchange splitting persists to several picoseconds.
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| 20. |
- Chen, Cong, et al.
(author)
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Distinguishing attosecond electron-electron scattering and screening in transition metals
- 2017
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In: Proceedings of the National Academy of Sciences of the United States of America. - : National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 114:27, s. E5300-E5307
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Journal article (peer-reviewed)abstract
- Electron-electron interactions are the fastest processes in materials, occurring on femtosecond to attosecond timescales, depending on the electronic band structure of the material and the excitation energy. Such interactions can play a dominant role in light-induced processes such as nano-enhanced plasmonics and catalysis, light harvesting, or phase transitions. However, to date it has not been possible to experimentally distinguish fundamental electron interactions such as scattering and screening. Here, we use sequences of attosecond pulses to directly measure electron-electron interactions in different bands of different materials with both simple and complex Fermi surfaces. By extracting the time delays associated with photoemission we show that the lifetime of photoelectrons from the d band of Cu are longer by similar to 100 as compared with those from the same band of Ni. We attribute this to the enhanced electron-electron scattering in the unfilled d band of Ni. Using theoretical modeling, we can extract the contributions of electron-electron scattering and screening in different bands of different materials with both simple and complex Fermi surfaces. Our results also show that screening influences high-energy photoelectrons (approximate to 20 eV) significantly less than low-energy photoelectrons. As a result, high-energy photoelectrons can serve as a direct probe of spin-dependent electron-electron scattering by neglecting screening. This can then be applied to quantifying the contribution of electron interactions and screening to low-energy excitations near the Fermi level. The information derived here provides valuable and unique information for a host of quantum materials.
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| 21. |
- Hickstein, Daniel D., et al.
(author)
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Direct Visualization of Laser-Driven Electron Multiple Scattering and Tunneling Distance in Strong-Field Ionization
- 2012
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In: Physical Review Letters. - 1079-7114. ; 109:7
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Journal article (peer-reviewed)abstract
- Using a simple model of strong-field ionization of atoms that generalizes the well-known 3-step model from 1D to 3D, we show that the experimental photoelectron angular distributions resulting from laser ionization of xenon and argon display prominent structures that correspond to electrons that pass by their parent ion more than once before strongly scattering. The shape of these structures can be associated with the specific number of times the electron is driven past its parent ion in the laser field before scattering. Furthermore, a careful analysis of the cutoff energy of the structures allows us to experimentally measure the distance between the electron and ion at the moment of tunnel ionization. This work provides new physical insight into how atoms ionize in strong laser fields and has implications for further efforts to extract atomic and molecular dynamics from strong-field physics.
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| 22. |
- Imbesi, S, et al.
(author)
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Functional decline and quality of life in the inpatient oncology setting.
- 2013
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In: APA Conference 2013. - : Australian Physiotherapy Association. ; , s. 3-3
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Conference paper (other academic/artistic)abstract
- Question: What effect does inpatient oncology treatment have on patient quality of life and physical functioning? Does recent weight loss associated with inferior physical functioning in patients admitted for inpatient oncology treatment? What are the implications for physiotherapy practice?Design: Prospective observational cohort studyParticipants: Thirty-two patients receiving inpatient oncology treatment.Outcome Measures: EORTC-30, SF8, isometric muscle strength, 30 second arm curl, sit to stand and timed up and go.Results: The EORTC-30 and SF-8 physical functioning and fatigue scales as well as timed up and go and sit to stand functional measures showed a trend of weekly decline in performance during inpatient oncology treatment but changes were not statistically different from baseline. Emotional and cognitive functioning and the 30 second arm curl however improved compared to baseline (p = <0.05). Social functioning showed a decline at two weeks compared to baseline (p = <0.05). Bivariate correlation analysis of baseline data showed sit to stand (r = -0.52), isometric knee extension (r = -0.39) and foot dorsiflexion (r = -0.42) strength to be significantly negatively associated with weight loss (p = <0.05).Conclusion: Despite non-significant decline in physical functioning during inpatient oncology treatment, comparison to healthy aged matched normative values showed obvious inferiority in quality of life and physical functioning at time of admission to hospital.Key Practice Points:• Physiotherapy management of patients in the inpatient oncology setting should focus on preventing decline in physical and social functioning.• Patients with greater reported weight loss may be more prone to larger declines in physical functioning in the inpatient oncology setting.
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| 23. |
- Kfir, Ofer, et al.
(author)
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Generation of bright phase-matched circularly-polarized extreme ultraviolet high harmonics
- 2015
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In: Nature Photonics. - 1749-4885 .- 1749-4893. ; 9:2, s. 99-105
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Journal article (peer-reviewed)abstract
- Circularly-polarized extreme ultraviolet and X-ray radiation is useful for analysing the structural, electronic and magnetic properties of materials. To date, such radiation has only been available at large-scale X-ray facilities such as synchrotrons. Here, we demonstrate the first bright, phase-matched, extreme ultraviolet circularly-polarized high harmonics source. The harmonics are emitted when bi-chromatic counter-rotating circularly-polarized laser pulses field-ionize a gas in a hollow-core waveguide. We use this new light source for magnetic circular dichroism measurements at the M-shell absorption edges of Co. We show that phase-matching of circularly-polarized harmonics is unique and robust, producing a photon flux comparable to linearly polarized high harmonic sources. This work represents a critical advance towards the development of table-top systems for element-specific imaging and spectroscopy of multiple elements simultaneously in magnetic and other chiral media with very high spatial and temporal resolution.
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| 24. |
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| 25. |
- Ryan, Sinéad A., et al.
(author)
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Optically controlling the competition between spin flips and intersite spin transfer in a Heusler half-metal on sub-100-fs time scales
- 2023
-
In: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 9:45, s. 1428-
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Journal article (peer-reviewed)abstract
- The direct manipulation of spins via light may provide a path toward ultrafast energy-efficient devices. However, distinguishing the microscopic processes that can occur during ultrafast laser excitation in magnetic alloys is challenging. Here, we study the Heusler compound Co2MnGa, a material that exhibits very strong light-induced spin transfers across the entire M-edge. By combining the element specificity of extreme ultraviolet high-harmonic probes with time-dependent density functional theory, we disentangle the competition between three ultrafast light-induced processes that occur in Co2MnGa: same-site Co-Co spin transfer, intersite Co-Mn spin transfer, and ultrafast spin flips mediated by spin-orbit coupling. By measuring the dynamic magnetic asymmetry across the entire M-edges of the two magnetic sublattices involved, we uncover the relative dominance of these processes at different probe energy regions and times during the laser pulse. Our combined approach enables a comprehensive microscopic interpretation of laser-induced magnetization dynamics on time scales shorter than 100 femtoseconds.
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| 26. |
- Shi, Xun, et al.
(author)
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Ultrafast electron calorimetry uncovers a new long-lived metastable state in 1T-TaSe2 mediated by mode-selective electron-phonon coupling
- 2019
-
In: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 5:3
-
Journal article (peer-reviewed)abstract
- Quantum materials represent one of the most promising frontiers in the quest for faster, lightweight, energy-efficient technologies. However, their inherent complexity and rich phase landscape make them challenging to understand or manipulate. Here, we present a new ultrafast electron calorimetry technique that can systematically uncover new phases of quantum matter. Using time- and angle-resolved photoemission spectroscopy, we measure the dynamic electron temperature, band structure, and heat capacity. This approach allows us to uncover a new long-lived metastable state in the charge density wave material 1T-TaSe2, which is distinct from all the known equilibrium phases: It is characterized by a substantially reduced effective total heat capacity that is only 30% of the normal value, because of selective electron-phonon coupling to a subset of phonon modes. As a result, less energy is required to melt the charge order and transform the state of the material than under thermal equilibrium conditions.
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| 27. |
- Tao, Zhensheng, et al.
(author)
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The nature of the ultrafast magnetic phase transition in nickel revealed by correlating EUV-MOKE and ARPES spectroscopies
- 2019
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In: XXI International Conference on Ultrafast Phenomena 2018 (UP 2018). - : EDP Sciences.
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Conference paper (peer-reviewed)abstract
- By correlating time- and angle-resolved photoemission (Tr-ARPES) and time-resolved transverse- magneto-optical Kerr effect (Tr-TMOKE) measurements, both at extreme ultraviolet (EUV) wavelengths, we uncover the nature of the ultrafast photoinduced magnetic phase transition in Ni. This allows us to explain the ultrafast magnetic response of Ni at all laser fluences - from a small reduction of the magnetization at low laser fluences, to complete quenching at high laser fluences. We provide an alternative explanation to the fluence-dependent recovery timescales commonly observed in ultrafast magneto-optical spectroscopies on ferromagnets: it is due to the bulk-averaging effect and different depths of sample exhibit distinct dynamics, depending on whether a magnetic phase transition is induced. We also show evidence of two competing channels with two distinct timescales in the recovery process, that suggest the presence of coexisting phases in the material.
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| 28. |
- Tengdin, Phoebe, et al.
(author)
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Direct light–induced spin transfer between different elements in a spintronic Heusler material via femtosecond laser excitation
- 2020
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In: Science Advances. - : American Association for the Advancement of Science. - 2375-2548. ; 6:3
-
Journal article (peer-reviewed)abstract
- Heusler compounds are exciting materials for future spintronics applications because they display a wide range of tunable electronic and magnetic interactions. Here, we use a femtosecond laser to directly transfer spin polarization from one element to another in a half-metallic Heusler material, Co2MnGe. This spin transfer initiates as soon as light is incident on the material, demonstrating spatial transfer of angular momentum between neighboring atomic sites on time scales < 10 fs. Using ultrafast high harmonic pulses to simultaneously and independently probe the magnetic state of two elements during laser excitation, we find that the magnetization of Co is enhanced, while that of Mn rapidly quenches. Density functional theory calculations show that the optical excitation directly transfers spin from one magnetic sublattice to another through preferred spin-polarized excitation pathways. This direct manipulation of spins via light provides a path toward spintronic devices that can operate on few-femtosecond or faster time scales.
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| 29. |
- You, Wenjing, et al.
(author)
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Revealing the Nature of the Ultrafast Magnetic Phase Transition in Ni by Correlating Extreme Ultraviolet Magneto-Optic and Photoemission Spectroscopies
- 2018
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In: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 121:7
-
Journal article (peer-reviewed)abstract
- By correlating time-and angle-resolved photoemission and time-resolved transverse magneto-optical Kerr effect measurements, both at extreme ultraviolet wavelengths, we uncover the universal nature of the ultrafast photoinduced magnetic phase transition in Ni. This allows us to explain the ultrafast magnetic response of Ni at all laser fluences-from a small reduction of the magnetization at low laser fluences, to complete quenching at high laser fluences. Both probe methods exhibit the same demagnetization and recovery timescales. The spin system absorbs the energy required to proceed through a magnetic phase transition within 20 fs after the peak of the pump pulse. However, the spectroscopic signatures of demagnetization of the material appear only after approximate to 200 fs and the subsequent recovery of magnetization on timescales ranging from 500 fs to >70 ps. We also provide evidence of two competing channels with two distinct timescales in the recovery process that suggest the presence of coexisting phases in the material.
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| 30. |
- Zhang, Yingchao, et al.
(author)
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Coherent modulation of the electron temperature and electron-phonon couplings in a 2D material
- 2020
-
In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 117:16, s. 8788-8793
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Journal article (peer-reviewed)abstract
- Ultrashort light pulses can selectively excite charges, spins, and phonons in materials, providing a powerful approach for manipulating their properties. Here we use femtosecond laser pulses to coherently manipulate the electron and phonon distributions, and their couplings, in the charge-density wave (CDW) material 1T-TaSe2. After exciting the material with a femtosecond pulse, fast spatial smearing of the laser-excited electrons launches a coherent lattice breathing mode, which in turn modulates the electron temperature. This finding is in contrast to all previous observations in multiple materials to date, where the electron temperature decreases monotonically via electron-phonon scattering. By tuning the laser fluence, the magnitude of the electron temperature modulation changes from similar to 200 K in the case of weak excitation, to similar to 1,000 K for strong laser excitation. We also observe a phase change of pi in the electron temperature modulation at a critical fluence of 0.7 mJ/cm(2), which suggests a switching of the dominant coupling mechanism between the coherent phonon and electrons. Our approach opens up routes for coherently manipulating the interactions and properties of two-dimensional and other quantum materials using light.
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