PHONON-SPECTRA AND LATTICE SPECIFIC-HEATS OF THE THALLIUM-BASED HIGH-TEMPERATURE SUPERCONDUCTORS

We present the phonon-dispersion curves, the one-phonon densities of state, the lattice specific heat c(v)(T), and the Debye temperatures THETA(T) of the thallium-based high-temperature superconductors, based on full lattice dynamical calculations. Repelling effects due to hybridization of the lowlying transverse acoustic and optical modes lead to small Debye regions of the specific heat (less-than-or-equal-to 4 K), while the occurrence of low-lying dispersionless transverse modes in the outer parts of the Brillouin zone causes large drops in the Debye temperatures THETA(T) in the range 0 to 20 K. These effects are extremely pronounced in Tl2Ca2Ba2Cu3O10 because of the appearance of very-low-frequency modes.