MSGID: 1:317/3 6476cdb4   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Symmetry breaking by ultrashort light pulses opens new quantum pathways   
   for coherent phonons    
      
     Date:   
         May 30, 2023   
     Source:   
         Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy   
         (MBI)   
     Summary:   
         Researchers have demonstrated a novel concept for exciting and   
         probing coherent phonons in crystals of a transiently broken   
         symmetry. The key of this concept lies in reducing the symmetry   
         of a crystal by appropriate optical excitation, as has been shown   
         with the prototypical crystalline semimetal bismuth (Bi).   
      
      
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   FULL STORY   
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   Atoms in a crystal form a regular lattice, in which they can move   
   over small distances from their equilibrium positions. Such phonon   
   excitations are represented by quantum states. A superposition of phonon   
   states defines a so- called phonon wavepacket, which is connected with   
   collective coherent oscillations of the atoms in the crystal. Coherent   
   phonons can be generated by excitation of the crystal with a femtosecond   
   light pulse and their motions in space and time be followed by scattering   
   an ultrashort x-ray pulse from the excited material. The pattern of   
   scattered x-rays gives direct insight in the momentary position of and   
   distances between the atoms. A sequence of such patterns provides a   
   'movie' of the atomic motions.   
      
   The physical properties of coherent phonons are determined by the   
   symmetry of the crystal, which represents a periodic arrangement of   
   identical unit cells.   
      
   Weak optical excitation does not change the symmetry properties of   
   the crystal.   
      
   In this case, coherent phonons with identical atomic motions in all unit   
   cells are excited . In contrast, strong optical excitation can break the   
   symmetry of the crystal and make atoms in adjacent unit cells oscillate   
   differently. While this mechanism holds potential for accessing other   
   phonons, it has not been explored so far.   
      
   In the journal Physical Review B, researchers from the Max-Born-Institute   
   in Berlin in collaboration with researchers from the University of   
   Duisburg-Essen have demonstrated a novel concept for exciting and probing   
   coherent phonons in crystals of a transiently broken symmetry. The key of   
   this concept lies in reducing the symmetry of a crystal by appropriate   
   optical excitation, as has been shown with the prototypical crystalline   
   semimetal bismuth (Bi).   
      
   Ultrafast mid-infrared excitation of electrons in Bi modifies the   
   spatial charge distribution and, thus, reduces the crystal symmetry   
   transiently. In the reduced symmetry, new quantum pathways for the   
   excitation of coherent phonons open up. The symmetry reduction causes a   
   doubling of the unit-cell size from the red framework with two Bi atoms   
   to the blue framework with four Bi atoms.   
      
   In addition to the unidirectional atomic motion, the unit cell with   
   4 Bi atoms allows for coherent phonon wave packets with bidirectional   
   atomic motions.   
      
   Probing the transient crystal structure directly by femtosecond x-ray   
   diffraction reveals oscillations of diffracted intensity, which persist   
   on a picosecond time scale. The oscillations arise from coherent wave   
   packet motions along phonon coordinates in the crystal of reduced   
   symmetry. Their frequency of 2.6 THz is different from that of phonon   
   oscillations at low excitation level.   
      
   Interestingly, this behavior occurs only above a threshold of the optical   
   pump fluence and reflects the highly nonlinear, so-called non-perturbative   
   character of the optical excitation process.   
      
   In summary, optically induced symmetry breaking allows for modifying the   
   excitation spectrum of a crystal on ultrashort time scales. These results   
   may pave the way for steering material properties transiently and, thus,   
   implementing new functions in optoacoustics and optical switching.   
      
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   Story Source: Materials provided by   
   Max_Born_Institute_for_Nonlinear_Optics_and_Short_Pulse   
   Spectroscopy_(MBI). Note: Content may be edited for style and length.   
      
      
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   Related Multimedia:   
       * Figures_showing_coherent_phonon_oscillations   
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   Journal Reference:   
      1. Azize Koc,, Isabel Gonzalez-Vallejo, Matthias Runge, Ahmed   
      Ghalgaoui,   
         Klaus Reimann, Laurenz Kremeyer, Fabian Thiemann, Michael Horn-von   
         Hoegen, Klaus Sokolowski-Tinten, Michael Woerner, Thomas Elsaesser.   
      
         Quantum pathways of carrier and coherent phonon excitation in   
         bismuth.   
      
         Physical Review B, 2023; 107 (18) DOI: 10.1103/PhysRevB.107.L180303   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/05/230530125440.htm   
      
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