Real-time experiments on phonon-phonon interaction

• Applicant: Professor Dr. Matias Bargheer
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2014 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 264132744

Phonons are the vibrational quanta of crystal lattices, representing a set of harmonic oscillators. Their quantum character is evident from specific heat measurements of any solid. Coupling of phonons is induced by anharmonic interaction between the atoms, giving rise to thermal expansion and explaining the observed high temperature dependence of heat conductivity. The theory of phonon-phonon interaction is well developed and it can in explain these macroscopic phenomena. This coupling is also manifest as a finite lifetime of phonon modes in the bandwidth of inelastic neutron scattering.In this project, we will monitor the nonlinear interaction of transverse and longitudinal optical or acoustic phonons in real time on a timescale of femtoseconds to picoseconds. We will selectively excite phonon modes with femtosecond laser pulses and selectively detect in a frequency and wavevector-resolved manner both the decay and the nonlinear generation of phonons in a three-phonon scattering processes. E.g. two phonons are annihilated and a new phonon is generated according to energy and momentum conservation. We aim at disentangling the contributions of coherent phonons (hypersound) and incoherent phonons (heat). As the ultimate challenge, we will tackle the problem of observing umklapp-scattering processes as opposed to normal scattering processes. The phonon momentum is not conserved but changes by an integer multiple of a reciprocal lattice vector. As a result, we shall develop methods that improve our control of complex lattice dynamics, opening the way for a whole range of new experiments on the role of phonons in various quantum-transitions of solids. Our approach complements the activities of many groups that currently investigate the important problem of thermal transport on the nanoscale by measuring macroscopic parameters. We want to develop a new perspective on the experimentally underexplored problem of nonlinear phonon-phonon interaction.

• DFG Programme: Research Grants