Examination of phase transitions and electron-phonon coupling by near IR pump-probe spectroscopy
Damir Dominko (Institut für Physik)

Crystal structure of (NbSe4)3I is organized in a chainlike manner with niobium ions of mixed, but spatially ordered, valence. Such a structure yields two phase transitions (first at 274 K and the second between 90-160 K) featuring in-chain displacements of niobium ions. Electron-phonon coupling is shown to drive the first transition [1-3], while the mechanism and the crystal symmetry of the second one couldn’t be clearly revealed from XRD study. Following reduction of the crystal symmetry leads to appearance of new Raman modes below 274 K, some of which exhibit substantial softening [3-5].
We have used temperature evolution of these modes as probes for examining the two phase transitions by static technique- Raman spectroscopy and dynamic technique- pump-probe time resolved spectroscopy. In addition, time traces of lattice vibrations have been investigated with pump fluencies high enough to drive the system through the both phase transitions (from 4 K). Complex vibrational response and an unexpected phonon thermalization have been revealed.

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