Quasiparticle lifetimes of rare-earth elements: ab initio theory and time-resolved two-photon photoelectron spectroscopy

• Applicant: Professor Dr. Martin Aeschlimann
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2008 to 2011
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 54145860

• Final Report Year: 2012

Final Report Abstract

The objective of the proposed experimental program was to resolve fundamental questions regarding single electron relaxation dynamics selected rare-earth metals (La, Gd, Tb). We carried out real-time experiments by means of time-resolved two-photon photoemission spectroscopy, which enables us to investigate the electron dynamics in a broad energy range by using different photon excitation energies. The experimental results have been compared with ab initio theory done by V. P. Zhukov, San Sebastian. The structurally and chemically very similar 4f metals are selected as an ideal case study for the impact of very different decay channels, which are expected to give rise to a strong variation in the quasiparticle lifetimes across the lanthanide series. Of particular interest was the influence of the changing occupancy of the 4f orbitals on the decay dynamics as well as the role of spin-flip transitions due to magnon excitations in the magnetic elements Gd and Tb. The main result of our studies is that both experimental TR-2PPE and theoretical lifetime data of the rare-earth elements Gd, Tb and La deviate distinctly from the quadratic energy dependence. To our surprise, the influence of the 4f orbitals on the hot electron dynamics is of less importance than has been anticipated before. The lifetimes for the lanthanide series are more or less of the same order; the effect of the exact position of the 4f-states is only marginal. Compared to other metals, the lanthanides exhibit lifetimes comparable to those of transition metals, i.e. Gd and Ta have similar lifetimes. Hence the relaxation dynamics of the rare-earth metals might be determined by the d-bands as in case of transition metals. Furthermore, by comparing the TR- 2PPE results for Eph = 1.55 eV and Eph = 3.1 eV we can estimate the influence of secondary electrons, which is much less than expected for a polycrystalline film. By means of temperature dependent experiments we are able to prove that electron-electron scattering dominates over electron-phonon and electron-magnon scattering in excited low-energy electronic states.

Publications

• Energy and spin relaxation of optically excited electrons in metals IFF-Seminar, IVW Dresden, 6. Juli 2007
  Martin Aeschlimann
  
• Hot electron lifetimes in metals probed by two-photon photoemission. Seminar at the Donostia International Physics Center (DICP), San Sebastian 26.04.2008
  Alexander Marienfeld
  
• PhD thesis “Hot electron lifetimes in metals probed by two-photon photoemission” TU Kaiserslautern, 2008
  Alexander Marienfeld
  
• Zeit- und ortsaufgelöste Photoelektronen-Spektroskopie Physikalisches Kolloquium, Max-Born-Institut Berlin, 25.11.2008
  Martin Aeschlimann
  
• Energy and spin relaxation of optically excited electrons in metals and organic semiconductors. Physikalisches Kolloquium, Osnabrück, 09.07.2009
  Martin Aeschlimann
  
• Lifetime of optically excited electrons. Passion for knowledge – Celebrating 10 years of the DIPC, San Sebastian, Spanien, 27.09.2010
  Martin Aeschlimann
  
• Linewidth versus real time measurements International Workshop and Winter School: Photoemission, Dijon, France, 22.02.2010
  Martin Aeschlimann
  
• Zeitaufgelöste Photoelektronenspektroskopie AOFA, 16. Arbeitstagung Angewandte Oberflächenanalytik, IFOS Kaiserslautern, 27.09.2010
  Martin Aeschlimann