Paul Fons (SIMES Friday Seminar)

Date(s) - Apr 25 2014
11:00 AM - 12:00 PM

Shasta Room, Bldg. 40, Room 361


Controlling Electron-Phonon Coupling: Free Electron Laser Probing of Ultrafast
Laser Induced Atomic Rearrangements in Ge2Sb2Te5

Paul Fons

National Institutes of Advanced Industrial Science and Technology (NIAIST ), Japan

Phase-change materials as symbolized by the prototypical phase-change alloy Ge2Sb2Te5 (GST), experience large changes in physical properties in response to structural change. Typically this has utilized for memory applications via the switching of GST between the metastable crystalline and amorphous phases. The dramatic property differences that occur from the transition are due to a large change in bonding properties. In particular, the metastable crystalline form for GST exhibits octahedrally coordinated bonding which is dominated by px, py, and pz-like orbitals leaving bonds unsaturated with slightly more than three electrons/atom instead of the six electrons required for saturation. The bonds are easily polarizable and lead to large changes in optical properties from the (N-8) coordinated amorphous phase; this type of bonding has been characterized as resonant bonding. The harnessing of such changes has enabled the development of optical storage (DVDRAM) as well as the next generation of nonvolatile storage, phase-change random access memory (PC-RAM). Information storage via structure as opposed to charge is predicted to allow the future scaling of memory devices far beyond the limits of charge-based devices. The presence of a bonding hierarchy of long and short bonds and p-orbital based bonding in GST has been suggested theoretically to offer an ultrafast pathway to structural rearrangement via selective excitation of particular phonon modes[1]. In particular, such studies suggest that there may be energetic pathways between the two bonding states that do not require quenching from the melt, but are attainable due to excited state effects and strictly local rearrangement of bonds.
Experimentally, it has been shown that light can enhance structural switching in GST [2,3] and that the use of ultrafast lasers to selectively generate coherent phonons can give rise to non-thermal structure changes. Interestingly, it has also been demonstrated that by making fine-grained superlattices of GeTe and Sb2Te3, it is possible to make electrical phase-change devices with resistance changes comparable to those of GST alloy devices using switching powers more than an order of magnitude less than identical composition alloy films. In this case, it has been suggested that electric fields can drive local structural changes across interfaces in a pseudo 1D system bypassing the need for the melt-quench process of more traditional phase-change memory designs again suggesting that non-equilibrium switching is possible by manipulation of local structure.[4]
While most experimental results on coherent phonon generation have been based upon all optical pump-probe experiments, the advent of the free electron laser allows the direct probing of atomic
rearrangement due to ultrafast optical pulses. Using the free-electron laser SACLA at SPring-8, we have observed the atomic rearrangement, in particular, transient excitation-induced disorder, in epitaxial films of GST due to coherent phonon generation in GST in response to a 800 nm 30 fs CPA laser pump using a 10 fs, 8 keV ultrafast hard x-ray free-electron laser probe pulse. Upon excitation with a single 30 fs pulse for larger fluences, the diffraction peak intensity is found to fall more than 90% of its initial value, fully recovering its intensity in several hundred picoseconds in a reversible process. The initial approximately twelve picosecond delay of the onset of the transient disorder after the pump pulse was found to be controllable over the range from a couple to twelve picoseconds by the use of multiple pulses whose temporal spacing is resonant with a selected
optical phonon mode. The resulting temperature increase as was about 100 C. We discuss the observations based upon a model of potential softening induced reduction of the electron-phonon coupling constant and the “trapping” of electrons in an excited (high-temperature) state.
[1] A. V. Kolobov et al., Nature Chem., 3, 311 (2011).
[2] P. Fons et al., Phys. Rev. B, 82, 041203 (2010).
[3] K. Makino et al., Opt. Express, 19, 1260 (2011).
[4] R. E. Simpson et al., Nature Nanotech., 6, 501–505 (2011).
ACKNOWLEDGEMENT: The authors would like to acknowledge the support from the X-ray Free Electron Laser Priority Strategy Program of MEXT, Japan.