Electron–phonon coupling strength in hydrogen-bonded network crystals in the THz frequency range

Advances in terahertz (THz) technology over the last few decades have enabled the precise determination of the coupling strength of electron–phonon interactions in crystals by analyzing temperature-dependent frequency shifts in the THz energy region. In this study, we estimate the electron–phonon coupling strength in the THz energy region for several hydrogen-bonded network materials using THz spectroscopy. This study reveals that the electron–phonon interaction strength is discrete and expressed as multiples of a common unit strength, regardless of the materials and vibrational transitions. The common unit strength is estimated to be approximately 5 × 10 –3 cm –1 K –1 , which is nearly equal to k B /137, or αk B , where k B is the Boltzmann constant (0.695 cm –1 K –1 ) and α is the dimensionless fine-structure constant. The fine-structure constant α (∼ 1/137) is a fundamental measure of coupling strength limited to the electromagnetic force, one of the four fundamental forces or interactions. The electromagnetic force works between the electric current generated by moving charged particles such as electrons and the electromagnetic field induced by spin, light, etc. This finding indicates that the coupling-constant ( α ) scheme is applicable to electron–phonon interactions in the THz energy region