Inducing transparency by kicking the atoms
The non-trivial interplay between high-energy electronic transitions, both of on-site and charge transfer character, with low-energy lattice or magnetic excitations gives rise to the rich phase diagrams observed in transition metal oxides. Unveiling the details of this reciprocal action among the different degrees of freedom is the key to achieving a better and more reliable description and design of material properties.
Time-domain studies provide a powerful platform to unveil the reciprocal action between electron, phonon and magnons. The rationale of those studies is to use ultrashort light pulses to photo-inject an excess of electronic energy in a material and inferring the strength of the coupling from the relaxation dynamics determined by the redistribution of the excess energy among all materials degrees of freedom. The commonly used multi-temperature models relies on the assumption that the different degrees of freedom (phonons, magnons, electrons) behave as independent statistical ensembles which dynamically exchange energy incoherently.
Importantly, the incoherent energy exchange rate may not provide a complete physical picture of the coupling between the electrons and the phonons (or magnons) and the coherent interaction between different excitations may be of relevance in determining the effectiveness of the interaction between the different degrees of freedom in matter. In this respect, the role played by the coherences of low-energy excitations in determining the macroscopic thermodynamics of materials properties is often elusive and addressed indirectly.
In order to study the coherent energy exchange between electrons and phonons, here we use resonant vibrational excitation which drives a coherent motion of atoms in the electronic ground and study how such excitation coherently control electronic absorption in the material. Fig.1a explains the rationale of our approach. The mid-IR excitation resonantly excites a large amplitude motion of the ions mainly along an IR-active mode while leave the system in the electronic ground state. The anharmonic coupling of the excited IR-phonon to other vibrational modes results in a dynamical contraction and expansion of the Cu–O bonds, within the octahedra, which coherently control the absorption in the visible region due to on-site optical transitions between crystal field levels. In detail, the resonant excitation of IR-active phonon modes results in a coherent vibrational motion of the apical oxygen that dynamically controls the energy and oscillator strength of the orbital transition between different crystal levels on Cu2+ ions. The experiments, part of the activities for the ERC project INCEPT, were performed in the Q4Q optical laboratories at Elettra, while the theoretical activities were the results of a an international collaboration led by the University of Trieste.
Figure 1. a) Sketch of the physical process. Mid-IR excitation resonant to a phonon mode move the atoms in the electronic ground state and visible probe measure the absorption of onsite dd crystal field phonon assisted excitations. b) Time dependence of the transmission evidencing regions of vibrationally induced transparency.
The coherent vibrational control of the electronic transition is made evident by the striking contrast between the results of time-domain experiments based on high-photon-energy pumps and mid-IR excitation. While high-photon-energy excitation results in thermal disorder that uniformly increases the absorption of crystal field levels, our experiments based on mid-IR pumps reveal a transient response characterized by regions of induced transparency (fig.1b) that can be rationalized if the electronic transitions are dynamically controlled by vibrational coherence in the electronic ground state. To disentangle the contributions to crystal field absorption that result from coherent and thermal motion of the ions, we developed a fully quantum description of dynamical phonon-mediated crystal field excitations. We use the temperature-dependent equilibrium absorption to benchmark the role of thermal fluctuations in the absorption process and extract a quantitative description of coherent versus incoherent vibrational responses. We stress that our methodology allows to distinguish the contributions to the absorption of crystal field levels that are associated with the coherent motion of the ions from those contributions driven by the incoherent thermal fluctuations.
This research was conducted by the following research team:
Alexandre Marciniak1,2,*, Stefano Marcantoni1,3,*, Francesca Giusti1,2, Filippo Glerean1,2, Giorgia Sparapassi1,2,Tobia Nova6, Andrea Cartella6, Simone Latini6, Francesco Valiera1, Angel Rubio6, Jeroen van den Brink4,5, Fabio Benatti1,3, Daniele Fausti1,2,7
1 Department of Physics, University of Trieste, Trieste, Italy
2 Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy
3 National Institute for Nuclear Physics (INFN), Trieste, Italy
4 Leibniz Institute for Solid State and Materials Research IFW, Dresden, Germany
5 Institut für Theoretische Physik and Würzburg-Dresden Cluster of Excellence ct.qmat, Technische Universität Dresden, Dresden, Germany
6 Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany
7 Department of Chemistry, Princeton University, Princeton, United States
*Those authors contributed equally to this work
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