@article{DeckerBornRuotsalainenetal.2021,
  author    = {Decker, Regis and Born, Artur and Ruotsalainen, Kari and Bauer, Karl and Haverkamp, Robert and B{\"u}chner, Robby and Pietzsch, Annette and F{\"o}hlisch, Alexander},
  title     = {Spin-lattice angular momentum transfer of localized and valence electrons in the demagnetization transient state of gadolinium},
  series = {Applied physics letters},
  volume    = {119},
  journal   = {Applied physics letters},
  number    = {15},
  publisher = {AIP Publishing},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/5.0063404},
  pages     = {5},
  year      = {2021},
  abstract  = {The electron-phonon scattering is one of the main microscopic mechanisms responsible for the spin-flip in the transient state of ultrafast demagnetization. Here, we present an experimental determination of the temperature-dependent electron-phonon scattering rate in Gd. Using a static x-ray emission spectroscopy method, where the reduction of the decay peak intensities when increasing the temperature is quantified, we measure independently the electron-phonon scattering rate for the 5d and the 4f electrons. We deduce the temperature dependence of scattering for the 5d electrons, while no effect on the phonon population is observed for the 4f electrons. Our results suggest that the ultrafast magnetization dynamics in Gd is triggered by the spin-flip in the 5d electrons. We also evidence the existence of a temperature threshold, above which spin-flip scattering of the 5d electrons takes place. We deduce that during the transient state of ultrafast demagnetization, the exchange energy between 5d electrons has to be overcome before the microscopic electron-phonon scattering process can occur.},
  language  = {en}
}
@article{BornDeckerHaverkampetal.2021,
  author    = {Born, Artur and Decker, Regis and Haverkamp, Robert and Ruotsalainen, Kari and Bauer, Karl and Pietzsch, Annette and F{\"o}hlisch, Alexander and B{\"u}chner, Robby},
  title     = {Thresholding of the Elliott-Yafet spin-flip scattering in multi-sublattice magnets by the respective exchange energies},
  series = {Scientific reports},
  volume    = {11},
  journal   = {Scientific reports},
  number    = {1},
  publisher = {Springer Nature},
  address   = {Berlin},
  issn      = {2045-2322},
  doi       = {10.1038/s41598-021-81177-9},
  pages     = {7},
  year      = {2021},
  abstract  = {How different microscopic mechanisms of ultrafast spin dynamics coexist and interplay is not only relevant for the development of spintronics but also for the thorough description of physical systems out-of-equilibrium. In pure crystalline ferromagnets, one of the main microscopic mechanism of spin relaxation is the electron-phonon (el-ph) driven spin-flip, or Elliott-Yafet, scattering. Unexpectedly, recent experiments with ferro- and ferrimagnetic alloys have shown different dynamics for the different sublattices. These distinct sublattice dynamics are contradictory to the Elliott-Yafet scenario. In order to rationalize this discrepancy, it has been proposed that the intra- and intersublattice exchange interaction energies must be considered in the microscopic demagnetization mechanism, too. Here, using a temperature-dependent x-ray emission spectroscopy (XES) method, we address experimentally the element specific el-ph angular momentum transfer rates, responsible for the spin-flips in the respective (sub)lattices of Fe20Ni80, Fe50Ni50 and pure nickel single crystals. We establish how the deduced rate evolution with the temperature is linked to the exchange coupling constants reported for different alloy stoichiometries and how sublattice exchange energies threshold the related el-ph spin-flip channels. Thus, these results evidence that the Elliott-Yafet spin-flip scattering, thresholded by sublattice exchange energies, is the relevant microscopic process to describe sublattice dynamics in alloys and elemental magnetic systems.},
  language  = {en}
}
@article{BuechnerFondellHaverkampetal.2021,
  author    = {B{\"u}chner, Robby and Fondell, Mattis and Haverkamp, Robert and Pietzsch, Annette and Vaz da Cruz, Vin{\´i}cius and F{\"o}hlisch, Alexander},
  title     = {The porphyrin center as a regulator for metal-ligand covalency and pi hybridization in the entire molecule},
  series = {Physical chemistry, chemical physics : PCCP ; a journal of European Chemical Societies},
  volume    = {23},
  journal   = {Physical chemistry, chemical physics : PCCP ; a journal of European Chemical Societies},
  number    = {43},
  publisher = {Royal Society of Chemistry},
  address   = {Cambridge},
  issn      = {1463-9076},
  doi       = {10.1039/d1cp03944j},
  pages     = {24765 -- 24772},
  year      = {2021},
  abstract  = {The central moiety of porphyrins is shown to control the charge state of the inner complex and links it by covalent interaction to the peripheral substituents. This link, which enables the versatile functions of porphyrins, is not picked up in the established, reduced four orbital picture [Gouterman, J. Mol. Spectrosc., 1961, 6, 138]. X-ray absorption spectroscopy at the N K-edge with density functional theory approaches gives access to the full electronic structure, in particular the pi* manifold beyond the Gouterman orbitals. Systematic variation of the central moiety highlights two linked, governing trends: The ionicity of the porphyrin center increases from the aminic N-H to N-Cu to N-Zn to N-Mg to the iminic N:. At the same time covalency with peripheral substituents increases and compensates the buildup of high charge density at the coordinated nitrogen sites.},
  language  = {en}
}
@article{BuechnerFondellMascarenhasetal.2021,
  author    = {B{\"u}chner, Robby and Fondell, Mattis and Mascarenhas, Eric Johnn and Pietzsch, Annette and Vaz da Cruz, Vin{\´i}cius and F{\"o}hlisch, Alexander},
  title     = {How hydrogen bonding amplifies isomeric differences in pyridones toward strong changes in acidity and tautomerism},
  series = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry},
  volume    = {125},
  journal   = {The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces \& biophysical chemistry},
  number    = {9},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {1520-6106},
  doi       = {10.1021/acs.jpcb.0c10873},
  pages     = {2372 -- 2379},
  year      = {2021},
  abstract  = {Steric hindrance of hydration and hydrogen bond enhancement by localized charges have been identified as key factors for the massive chemical differences between the hydroxypyridine/pyridone isomers in aqueous solution. While all isomers occur mainly in the hydroxypyridine form in the gas phase, they differ by more than 3 orders of magnitude both in their acidity and tautomeric equilibrium constants upon hydration. By monitoring the electronic and solvation structures as a function of the protonation state and the O- substitution position on the pyridine ring, the amplification of the isomeric differences in aqueous solution has been investigated. Near-edge X-ray absorption fine structure (NEXAFS) measurements at the N K-edge served as the probe of the chemical state. The combination of molecular dynamics simulations, complete active space self-consistent field (CASSCF), and time-dependent density functional theory (TD-DFT) spectral calculations contributes to unraveling the principles of tautomerism and acidity in multiple biochemical systems based on tautomerism.},
  language  = {en}
}