Probing non-equilibrium first order phase transitions of long-range order with resonant and non-resonant sub-picosecond X-ray diffraction

by Gerhard Ingold (Laboratory for Synchrotron Radiation, Paul Scherrer Institut, Switzerland)

Friday, November 18, 2011 from to (Europe/Berlin)
at AER19 Room 3.11
There are several classes of materials in condensed matter that exhibit coupling between electronic and structural degrees of freedom of long range order. A prominent class are strongly correlated electron systems where competing charge-, orbital-, spin- and structural order determine their physical properties. In the presence of a first order phase transition, stimulated phase competition is a promising route to study the coupled dynamics of competing orders in such materials. Pump-probe resonant and non-resonant X-ray diffraction have emerged as a suitable technique to study phase competition on the sub-picosecond time scale. Two examples are given where a coherent process causes an ordered phase to change into a different ordered phase of different symmetry on a time scale below 200 - 300 fs. The first example is a metal-to-insulator transition via coherent phonon excitation in a manganite measured with non-resonant hard X-ray diffraction at a slicing source. The second example is a magnetic phase transition via coherent spin wave excitation in a multiferroic measured with resonant soft X-ray diffraction at a free electron laser. In this context our latest results on the dynamics of phase-coexistence in magneto-structural phase transitions will be presented. Another aspect is the reversibility of phase transformations in crystalline multiferroic solids to have profound technological implications. Finally I will discuss technical issues regarding a dedicated endstation to pursue such kinds of experiments in the future. We propose to construct an UHV cryo-diffraction station that allows flexibility concerning the laser based pumping schemes, the sample environment, the polarization of pump and probe beams, and the exploitation of coherence properties. At free electron lasers such as SwissFEL and European XFEL, timing diagnostics as part of the endstation will mostly likely be needed to perform pump-probe experiments with time resolution 10 fs in a repeatable and reliable manner.