@article{ZaldenQuirinSchumacheretal.2019,
  author    = {Zalden, Peter and Quirin, Florian and Schumacher, Mathias and Siegel, Jan and Wei, Shuai and Koc, Azize and Nicoul, Matthieu and Trigo, Mariano and Andreasson, Pererik and Enquist, Henrik and Shu, Michael J. and Pardini, Tommaso and Chollet, Matthieu and Zhu, Diling and Lemke, Henrik and Ronneberger, Ider and Larsson, J{\"o}rgen and Lindenberg, Aaron M. and Fischer, Henry E. and Hau-Riege, Stefan and Reis, David A. and Mazzarello, Riccardo and Wuttig, Matthias and Sokolowski-Tinten, Klaus},
  title     = {Femtosecond x-ray diffraction reveals a liquid-liquid phase transition in phase-change materials},
  series = {Science},
  volume    = {364},
  journal   = {Science},
  number    = {6445},
  publisher = {American Assoc. for the Advancement of Science},
  address   = {Washington, DC},
  issn      = {0036-8075},
  doi       = {10.1126/science.aaw1773},
  pages     = {1062 -- 1067},
  year      = {2019},
  abstract  = {In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid-liquid phase transition in the phase-change materials Ag4In3Sb67Te26 and Ge15Sb85 at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics.},
  language  = {en}
}
@article{IurchukSchickBranetal.2016,
  author    = {Iurchuk, V. and Schick, D. and Bran, J. and Colson, D. and Forget, A. and Halley, D. and Koc, Azize and Reinhardt, Mathias and Kwamen, C. and Morley, N. A. and Bargheer, Matias and Viret, M. and Gumeniuk, R. and Schmerber, G. and Doudin, B. and Kundys, B.},
  title     = {Optical Writing of Magnetic Properties by Remanent Photostriction},
  series = {Physical review letters},
  volume    = {117},
  journal   = {Physical review letters},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {0031-9007},
  doi       = {10.1103/PhysRevLett.117.107403},
  pages     = {5},
  year      = {2016},
  abstract  = {We present an optically induced remanent photostriction in BiFeO3, resulting from the photovoltaic effect, which is used to modify the ferromagnetism of Ni film in a hybrid BiFeO3/Ni structure. The 75\% change in coercivity in the Ni film is achieved via optical and nonvolatile control. This photoferromagnetic effect can be reversed by static or ac electric depolarization of BiFeO3. Hence, the strain dependent changes in magnetic properties are written optically, and erased electrically. Light-mediated straintronics is therefore a possible approach for low-power multistate control of magnetic elements relevant for memory and spintronic applications.},
  language  = {en}
}
@article{ReinhardtKocLeitenbergeretal.2016,
  author    = {Reinhardt, Matthias and Koc, Azize and Leitenberger, Wolfram and Gaal, Peter and Bargheer, Matias},
  title     = {Optimized spatial overlap in optical pump-X-ray probe experiments with high repetition rate using laser-induced surface distortions},
  series = {Journal of synchrotron radiation},
  volume    = {23},
  journal   = {Journal of synchrotron radiation},
  publisher = {International Union of Crystallography},
  address   = {Chester},
  issn      = {1600-5775},
  doi       = {10.1107/S1600577515024443},
  pages     = {474 -- 479},
  year      = {2016},
  abstract  = {Ultrafast X-ray diffraction experiments require careful adjustment of the spatial overlap between the optical excitation and the X-ray probe pulse. This is especially challenging at high laser repetition rates. Sample distortions caused by the large heat load on the sample and the relatively low optical energy per pulse lead to only tiny signal changes. In consequence, this results in small footprints of the optical excitation on the sample, which turns the adjustment of the overlap difficult. Here a method for reliable overlap adjustment based on reciprocal space mapping of a laser excited thin film is presented.},
  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}
}