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End date: 2019 × Start date: 2018 × DDC: 500 × Type: Text × Publisher: Washington, DC [u.a.] : Assoc. × WGL Institute: MBI ×

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    Dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation
    (Washington, DC [u.a.] : Assoc., 2018) Maehrlein, Sebastian F.; Radu, Ilie; Maldonado, Pablo; Paarmann, Alexander; Gensch, Michael; Kalashnikova, Alexandra M.; Pisarev, Roman V.; Wolf, Martin; Oppeneer, Peter M.; Barker, Joseph; Kampfrath, Tobias
    To gain control over magnetic order on ultrafast time scales, a fundamental understanding of the way electron spins interact with the surrounding crystal lattice is required. However, measurement and analysis even of basic collective processes such as spin-phonon equilibration have remained challenging. Here, we directly probe the flow of energy and angular momentum in the model insulating ferrimagnet yttrium iron garnet. After ultrafast resonant lattice excitation, we observe that magnetic order reduces on distinct time scales of 1 ps and 100 ns. Temperature-dependent measurements, a spin-coupling analysis, and simulations show that the two dynamics directly reflect two stages of spin-lattice equilibration. On the 1-ps scale, spins and phonons reach quasi-equilibrium in terms of energy through phonon-induced modulation of the exchange interaction. This mechanism leads to identical demagnetization of the ferrimagnet’s two spin sublattices and to a previously inaccessible ferrimagnetic state of increased temperature yet unchanged total magnetization. Finally, on the much slower, 100-ns scale, the excess of spin angular momentum is released to the crystal lattice, resulting in full equilibrium. Our findings are relevant for all insulating ferrimagnets and indicate that spin manipulation by phonons, including the spin Seebeck effect, can be extended to antiferromagnets and into the terahertz frequency range.
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    Molecular orbital imprint in laser-driven electron recollision
    (Washington, DC [u.a.] : Assoc., 2018) Schell, Felix; Bredtmann, Timm; Schulz, Claus Peter; Patchkovskii, Serguei; Vrakking, Marc J. J.; Mikosch, Jochen
    Electrons released by strong-field ionization from atoms and molecules or in solids can be accelerated in the oscillating laser field and driven back to their ion core. The ensuing interaction, phase-locked to the optical cycle, initiates the central processes underlying attosecond science. A common assumption assigns a single, welldefined return direction to the recolliding electron. We study laser-induced electron rescattering associated with two different ionization continua in the same, spatially aligned, polyatomic molecule. We show by experiment and theory that the electron return probability is molecular frame-dependent and carries structural information on the ionized orbital. The returning wave packet structure has to be accounted for in analyzing strong-field spectroscopy experiments that critically depend on the interaction of the laser-driven continuum electron, such as laser-induced electron diffraction.
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    Reconstruction of the time-dependent electronic wave packet arising from molecular autoionization
    (Washington, DC [u.a.] : Assoc., 2018) Bello, Roger Y.; Canton, Sophie E.; Jelovina, Denis; Bozek, John D.; Rude, Bruce; Smirnova, Olga; Ivanov, Mikhail Y.; Palacios, Alicia; Martín, Fernando
    Autoionizing resonances are paradigmatic examples of two-path wave interferences between direct photoionization, which takes a few attoseconds, and ionization via quasi-bound states, which takes much longer. Time-resolving the evolution of these interferences has been a long-standing goal, achieved recently in the helium atom owing to progress in attosecond technologies. However, already for the hydrogen molecule, similar time imaging has remained beyond reach due to the complex interplay between fast nuclear and electronic motions. We show how vibrationally resolved photoelectron spectra of H2 allow one to reconstruct the associated subfemtosecond autoionization dynamics by using the ultrafast nuclear dynamics as an internal clock, thus forgoing ultrashort pulses. Our procedure should be general for autoionization dynamics in molecules containing light nuclei, which are ubiquitous in chemistry and biology.