Electron-Phonon Coupling in Group V Semimetals

Semimetals bismuth (Bi) and antimony (Sb) have been model systems for coherent phonon studies. They both have an A7 crystalline structure and sustain two Raman active optical phonon modes of A g and Eg symmetries (Fig. 2.4). Their pump-induced reflectivity change, shown in Fig. 2.7, consists of oscillatory (ARosc) and non-oscillatory (ARnonosc) components. ARosc is dominated by the coherent nuclear motion of the A g and Eg symmetries, while Af nonosc is attributed to the modification in the electronic and the lattice temperatures. 

The coherent oscillation of the A g mode was a cosine function of time (Fig. 2.8) [21,24-26]. Recent X-ray measurements demonstrated a clear shift in the equilibrium position at photoexcitation [35], as we will see in the next chapter. These results confirmed the displacive generation of coherent A g phonons, as discussed in Sect. 2.2.2. In contrast, the coherent oscillation of the Eg mode was a sine function of time, and its amplitude exhibited a cos 2ip dependence on the pump polarization angle ip. Both features indicated the ISRS generation of the coherent Eg phonons [21,25], 

Under moderate (pJ/cm2) photoexcitation, where the photoexcited carrier density is comparable or less than the intrinsic density, time evolution of coherent A g and Eg phonons is respectively described by a damped harmonic oscillation 

Under even more intense photoexcitation ( 10mJ/cm2), the coherent A g and Eg phonons of Bi and Sb exhibit a collapse-revival in their amplitudes (Fig. 2.10) [42,43], This phenomenon has a clear threshold in the pump density, which is common for the two phonon modes but depends on temperature and the crystal (Bi or Sb). At first glance, the amplitude collapse-revival appears to be analogous to the fractional revival in nuclear wavepackets in molecules [44,45]. However, the pump power dependence may be an indication of a polarization, not quantum, beating between different spatial components of the coherent response within the laser spot [46], 

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