No abstract

It is shown that in graphite the spectral density of phonons polarized along the c axis has a V shaped feature similar to the so-called Dirac singularity characteristic of the electron density of states in graphene. The formation of quasilocal states, which increase the occupation of the quasiparticle levels near this feature, is analyzed from a unified standpoint for the phonon spectrum of metal-intercalated graphite and the electronic spectrum of graphene with vacancies. It is determined that in the electronic spectrum of graphene with an isolated vacancy quasilocal states are characteristic only of atoms belonging to the sublattice that does not contain this vacancy.

Low-temperature heat capacity of cryocrystals, which contain impurity clusters has been investigated theoretically and experimentally. Such defects might essentially enrich low-frequency part of the phonon spectrum by introducing both localized and delocalized vibrations. The effect of both types of the vibrations on the temperature dependence of the heat capacity is analyzed. Heat capacity of the disordered solid solution Kr-Ar (Ar concentration is~25%) is studied as an example of the effect of the light weakly connected impurities on the low-temperature thermodynamic characteristics of the system. The mass defect of such an impurity induces «phonon pumping» from the low-frequency part of the spectrum into the high-frequency part and decreasing the low-temperature heat capacity, while the weakened interaction between the impurity and the host atoms combined with even weaker interaction between the impurities leads to the formation of the low-temperature maximum on the heat capacity temperature dependence. The analysis performed shows that at rather high Ar concentrations, the nonmonotonous temperature dependence of the relative change in the heat capacity of solid Kr Ar 1-p p solutions is determined by excitation of delocalized high-dispersion low-frequency phonons. PACS: 63.20.-e Phonons in crystal lattice; 63.20.Mt Phonon-defect interaction; 63.50.+x Vibrational states of disordered systems; 63.70.+h Statistical mechanics of lattice vibrations and displacive phase transitions.