@article{vonReppertWilligPudelletal.2018,
  author    = {von Reppert, Alexander and Willig, Lisa and Pudell, Jan-Etienne and Roessle, M. and Leitenberger, Wolfram and Herzog, Marc and Ganss, F. and Hellwig, O. and Bargheer, Matias},
  title     = {Ultrafast laser generated strain in granular and continuous FePt thin films},
  series = {Applied physics letters},
  volume    = {113},
  journal   = {Applied physics letters},
  number    = {12},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/1.5050234},
  pages     = {5},
  year      = {2018},
  abstract  = {We employ ultrafast X-ray diffraction to compare the lattice dynamics of laser-excited continuous and granular FePt films on MgO (100) substrates. Contrary to recent results on free-standing granular films, we observe in both cases a pronounced and long-lasting out-of-plane expansion. We attribute this discrepancy to the in-plane expansion, which is suppressed by symmetry in continuous films. Granular films on substrates are less constrained and already show a reduced out-of-plane contraction. Via the Poisson effect, out-of-plane contractions drive in-plane expansion and vice versa. Consistently, the granular film exhibits a short-lived out-of-plane contraction driven by ultrafast demagnetization which is followed by a reduced and delayed expansion. From the acoustic reflections of the observed strain waves at the film-substrate interface, we extract a 13\% reduction of the elastic constants in thin 10 nm FePt films compared to bulk-like samples. (C) 2018 Author(s).},
  language  = {en}
}
@article{vonReppertPuddellKocetal.2016,
  author    = {von Reppert, Alexander and Puddell, J. and Koc, A. and Reinhardt, M. and Leitenberger, Wolfram and Dumesnil, K. and Zamponi, Flavio and Bargheer, Matias},
  title     = {Persistent nonequilibrium dynamics of the thermal energies in the spin and phonon systems of an antiferromagnet},
  series = {Structural dynamics},
  volume    = {3},
  journal   = {Structural dynamics},
  publisher = {AIP Publishing LLC},
  address   = {Melville, NY},
  issn      = {2329-7778},
  doi       = {10.1063/1.4961253},
  year      = {2016},
  abstract  = {We present a temperature and fluence dependent Ultrafast X-Ray Diffraction study of a laser-heated antiferromagnetic dysprosium thin film. The loss of antiferromagnetic order is evidenced by a pronounced lattice contraction. We devise a method to determine the energy flow between the phonon and spin system from calibrated Bragg peak positions in thermal equilibrium. Reestablishing the magnetic order is much slower than the cooling of the lattice, especially around the N{\´e}el temperature. Despite the pronounced magnetostriction, the transfer of energy from the spin system to the phonons in Dy is slow after the spin-order is lost.},
  language  = {en}
}
@misc{vonReppertPuddellKocetal.2016,
  author    = {von Reppert, Alexander and Puddell, J. and Koc, A. and Reinhardt, M. and Leitenberger, Wolfram and Dumesnil, K. and Zamponi, Flavio and Bargheer, Matias},
  title     = {Persistent nonequilibrium dynamics of the thermal energies in the spin and phonon systems of an antiferromagnet},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-98710},
  pages     = {11},
  year      = {2016},
  abstract  = {We present a temperature and fluence dependent Ultrafast X-Ray Diffraction study of a laser-heated antiferromagnetic dysprosium thin film. The loss of antiferromagnetic order is evidenced by a pronounced lattice contraction. We devise a method to determine the energy flow between the phonon and spin system from calibrated Bragg peak positions in thermal equilibrium. Reestablishing the magnetic order is much slower than the cooling of the lattice, especially around the N{\´e}el temperature. Despite the pronounced magnetostriction, the transfer of energy from the spin system to the phonons in Dy is slow after the spin-order is lost.},
  language  = {en}
}
@article{BojahrGohlkeLeitenbergeretal.2015,
  author    = {Bojahr, Andre and Gohlke, Matthias and Leitenberger, Wolfram and Pudell, Jan-Etienne and Reinhardt, Matthias and von Reppert, Alexander and R{\"o}ssle, Matthias and Sander, Mathias and Gaal, Peter and Bargheer, Matias},
  title     = {Second Harmonic Generation of Nanoscale Phonon Wave Packets},
  series = {Physical review letters},
  volume    = {115},
  journal   = {Physical review letters},
  number    = {19},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {0031-9007},
  doi       = {10.1103/PhysRevLett.115.195502},
  pages     = {5},
  year      = {2015},
  abstract  = {Phonons are often regarded as delocalized quasiparticles with certain energy and momentum. The anharmonic interaction of phonons determines macroscopic properties of the solid, such as thermal expansion or thermal conductivity, and a detailed understanding becomes increasingly important for functional nanostructures. Although phonon-phonon scattering processes depicted in simple wave-vector diagrams are the basis of theories describing these macroscopic phenomena, experiments directly accessing these coupling channels are scarce. We synthesize monochromatic acoustic phonon wave packets with only a few cycles to introduce nonlinear phononics as the acoustic counterpart to nonlinear optics. Control of the wave vector, bandwidth, and consequently spatial extent of the phonon wave packets allows us to observe nonlinear phonon interaction, in particular, second harmonic generation, in real time by wave-vector-sensitive Brillouin scattering with x-rays and optical photons.},
  language  = {en}
}
@article{vonReppertPudellKocetal.2016,
  author    = {von Reppert, Alexander and Pudell, Jan-Etienne and Koc, A. and Reinhardt, M. and Leitenberger, Wolfram and Dumesnil, K. and Zamponi, Flavio and Bargheer, Matias},
  title     = {Persistent nonequilibrium dynamics of the thermal energies in the spin and phonon systems of an antiferromagnet},
  series = {Structural dynamics},
  volume    = {3},
  journal   = {Structural dynamics},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {2329-7778},
  doi       = {10.1063/1.4961253},
  pages     = {11},
  year      = {2016},
  abstract  = {We present a temperature and fluence dependent Ultrafast X-Ray Diffraction study of a laser-heated antiferromagnetic dysprosium thin film. The loss of antiferromagnetic order is evidenced by a pronounced lattice contraction. We devise a method to determine the energy flow between the phonon and spin system from calibrated Bragg peak positions in thermal equilibrium. Reestablishing the magnetic order is much slower than the cooling of the lattice, especially around the Neel temperature. Despite the pronounced magnetostriction, the transfer of energy from the spin system to the phonons in Dy is slow after the spin-order is lost. (C) 2016 Author(s).},
  language  = {en}
}
@article{KocReinhardtvonReppertetal.2017,
  author    = {Koc, Azize and Reinhardt, M. and von Reppert, Alexander and Roessle, Matthias and Leitenberger, Wolfram and Dumesnil, K. and Gaal, Peter and Zamponi, Flavio and Bargheer, Matias},
  title     = {Ultrafast x-ray diffraction thermometry measures the influence of spin excitations on the heat transport through nanolayers},
  series = {Physical review : B, Condensed matter and materials physics},
  volume    = {96},
  journal   = {Physical review : B, Condensed matter and materials physics},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {2469-9950},
  doi       = {10.1103/PhysRevB.96.014306},
  pages     = {7},
  year      = {2017},
  abstract  = {We investigate the heat transport through a rare earth multilayer system composed of yttrium (Y), dysprosium (Dy), and niobium (Nb) by ultrafast x-ray diffraction. This is an example of a complex heat flow problem on the nanoscale, where several different quasiparticles carry the heat and conserve a nonequilibrium for more than 10 ns. The Bragg peak positions of each layer represent layer-specific thermometers that measure the energy flow through the sample after excitation of the Y top layer with fs-laser pulses. In an experiment-based analytic solution to the nonequilibrium heat transport problem, we derive the individual contributions of the spins and the coupled electron-lattice system to the heat conduction. The full characterization of the spatiotemporal energy flow at different starting temperatures reveals that the spin excitations of antiferromagnetic Dy speed up the heat transport into the Dy layer at low temperatures, whereas the heat transport through this layer and further into the Y and Nb layers underneath is slowed down. The experimental findings are compared to the solution of the heat equation using macroscopic temperature-dependent material parameters without separation of spin and phonon contributions to the heat. We explain why the simulated energy density matches our experiment-based derivation of the heat transport, although the simulated thermoelastic strain in this simulation is not even in qualitative agreement.},
  language  = {en}
}
@article{KocReinhardtvonReppertetal.2017,
  author    = {Koc, A. and Reinhardt, M. and von Reppert, Alexander and R{\"o}ssle, Matthias and Leitenberger, Wolfram and Gleich, M. and Weinelt, M. and Zamponi, Flavio and Bargheer, Matias},
  title     = {Grueneisen-approach for the experimental determination of transient spin and phonon energies from ultrafast x-ray diffraction data: gadolinium},
  series = {Journal of physics : Condensed matter},
  volume    = {29},
  journal   = {Journal of physics : Condensed matter},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0953-8984},
  doi       = {10.1088/1361-648X/aa7187},
  pages     = {5884 -- 5891},
  year      = {2017},
  abstract  = {We study gadolinium thin films as a model system for ferromagnets with negative thermal expansion. Ultrashort laser pulses heat up the electronic subsystem and we follow the transient strain via ultrafast x-ray diffraction. In terms of a simple Grueneisen approach, the strain is decomposed into two contributions proportional to the thermal energy of spin and phonon subsystems. Our analysis reveals that upon femtosecond laser excitation, phonons and spins can be driven out of thermal equilibrium for several nanoseconds.},
  language  = {en}
}