Ralph Ernstorfer (SIMES Seminar)

Date/Time
Date(s) - Feb 16 2016
11:30 AM - 12:30 PM

Location
Sycamore Room, Building 40, room 195

Category(ies)


Electronic and Structural Dynamics in Simple Metals, Phase Change Materials and Transition Metal Dichalcogenides

 Ralph Ernstorfer

Max Planck Research Group for Structural & Electronic Surface Dynamics

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

http://w0.rz-berlin.mpg.de/pc/ernstorfer

 

The coupling and mutual dependence of electronic and vibrational degrees of freedom is at the heart of microscopic as well as macroscopic phenomena in condensed matter. Ultrafast pump-probe techniques provide experimental access to these coupling and correlation effects through the microscopic response to specific excitation of the material. We employ femtosecond electron diffraction (FED) [1] and imaging [2], time- and angle-resolved photoelectron spectroscopy (trARPES) and optical spectroscopy for probing the dynamics of phonons, electrons and the dielectric function subsequent to optical excitation of the electrons.

I will discuss electron-lattice interaction and electron dynamics in three classes of materials. First, the electron-phonon coupling in the free-electron metal aluminium is quantitatively investigated with FED and first-principle calculations. Our results challenge the validity of the two-temperature approximation and we propose a refined non-thermal lattice model [3]. Second, we investigate the photo-induced structural dynamics in Ge2Sb2Te5 (GST), a popular phase change material exhibiting two metastable crystalline states which can be switched by light or current pulses. We observe distinct differences between the dynamics of optical properties and lattice, which we explain in terms of the resonant bonding present in these phase change materials [4]. Third, I will briefly discuss recent results on electron dynamics in the semiconducting transition metal dichalcogenide WSe2. Parts of these results are obtained with a new 500 kHz XUV source for trARPES. Based on an optical parametric chirped pulse amplifier [5], we achieve a monochromatized photon flux exceeding 1011 photons/s at the sample position for a high harmonic with 22 eV photon energy. This light source bridges the technology gap between low-repetition rate XUV sources and high-repetition rate UV sources and provides access to ultrafast excited state electron dynamics in the entire Brillouin zone.

 

[1]        L. Waldecker et al., J. Appl. Phys. 117, 044903 (2015).

[2]        M. Müller et al., Nature Communications 5, 5292 (2014). M. Müller et al., arXiv:1512.07037.

[3]        L. Waldecker et al., arXiv:1507.03743.

[4]        L. Waldecker et al., Nature Materials 14, 991 (2015).

[5]        M. Puppin et al., Opt. Exp. 23 1491 (2015).