Dynamical Quantum Properties of Phonons in Crystals
Final Report Abstract
In the joint (DFG/FNR) project we investigated both experimentally and theoretically the quantum properties of phonons in crystals which are beyond the physical picture presented in textbooks. The most interesting experimental discovery of this project is the large contribution of electronic degrees of freedom to low-frequency phonons in semiconductors, insulators, and molecular crystals. The origin of the large contribution of electronic degrees of freedom in the phonon quasiparticle is dipole-dipole coupling among all spatially disjunct dipoles in non-metallic condensed matter. Thus, both dipoles created by ionic displacements and those due to the time-integrated electronic currents (typically located in the interatomic bonds) are participants in this coupling. In contrast to the adiabatic or Born-Oppenheimer approximation (BOA), in which the nuclear degrees of freedom are considered as a sort of a separate ”world”, dipole-dipole interactions couple already for the linear electromagnetic response of the crystal nuclear and electronic degrees of freedom resulting in hybrid phonon quasiparticles. Almost all experimental results of this project are incompatible with any theoretical description based on the BOA. Another experimental discovery was the extraordinarily long dephasing time of purely longitudinal low-frequency optical phonons. Two experimental key concepts were applied to unravel the quantum properties of the hybrid phonon quasiparticles: (i) femtosecond x-ray powder diffraction and (ii) nonlinear 2D-THz spectroscopy. Thus, the main topic of the theoretical research was the development of methods beyond the Born- Oppenheimer approximation (BO) for the collective dynamics of electrons and nuclear vibrations. The two main proposed research directions (RD) were: (i) benchmark calculations of nucleoelectronic properties beyond BO and (ii) development of model Hamiltonians for beyond-BO treatment based on coupled quantum Drude oscillators (QDOs). A brand new Quantum Monte Carlo (QMC) code is being completed at this moment and will be used in the future to our joint (experimetal-theoretical) projects. First joint publications are expected in 2024.
Publications
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Frequency Upshift of the Transverse Optical Phonon Resonance in GaAs by Femtosecond Electron-Hole Excitation. Physical Review Letters, 125(2).
Ghalgaoui, Ahmed; Reimann, Klaus; Woerner, Michael; Elsaesser, Thomas; Flytzanis, Christos & Biermann, Klaus
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Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites. Physical Review X, 11(2).
Pal, Shovon; Strkalj, Nives; Yang, Chia-Jung; Weber, Mads C.; Trassin, Morgan; Woerner, Michael & Fiebig, Manfred
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Correlated Wave Functions for Electron–Positron Interactions in Atoms and Molecules. Journal of Chemical Theory and Computation, 18(4), 2267-2280.
Charry, Martinez Jorge Alfonso; Barborini, Matteo & Tkatchenko, Alexandre
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Phonon-Induced Relocation of Valence Charge in Boron Nitride Observed by Ultrafast X-Ray Diffraction. Physical Review Letters, 128(13).
Priyadarshi, Shekhar; Gonzalez-Vallejo, Isabel; Hauf, Christoph; Reimann, Klaus; Woerner, Michael & Elsaesser, Thomas
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The three-center two-positron bond. Chemical Science, 13(46), 13795-13802.
Charry, Jorge; Moncada, Félix; Barborini, Matteo; Pedraza-González, Laura; Varella, Márcio T. do N.; Tkatchenko, Alexandre & Reyes, Andrés
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Underdamped longitudinal soft modes in ionic crystallites—lattice and charge motions observed by ultrafast x-ray diffraction. Structural Dynamics, 9(2).
Gonzalez-Vallejo, Isabel; Koç, Azize; Reimann, Klaus; Woerner, Michael & Elsaesser, Thomas
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Quantum pathways of carrier and coherent phonon excitation in bismuth. Physical Review B, 107(18).
Koç, Azize; Gonzalez-Vallejo, Isabel; Runge, Matthias; Ghalgaoui, Ahmed; Reimann, Klaus; Kremeyer, Laurenz; Thiemann, Fabian; Hoegen, Michael Horn-von; Sokolowski-Tinten, Klaus; Woerner, Michael & Elsaesser, Thomas
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Ultrafast carrier dynamics and symmetry reduction in bismuth by nonperturbative optical excitation in the terahertz range. Physical Review B, 107(24).
Runge, Matthias; Ghalgaoui, Ahmed; Gonzalez-Vallejo, Isabel; Thiemann, Fabian; Horn-von, Hoegen Michael; Reimann, Klaus; Woerner, Michael & Elsaesser, Thomas
