LOW-TEMPERATURE THERMAL AND OPTICAL-PROPERTIES OF SINGLE-GRAINED DECAGONAL AL-NI-CO QUASI-CRYSTALS

We report measurements of the specific heat C-p(T) and of the thermal conductivities lambda(p)(T) along the periodic direction and lambda(q) (T) along a direction in the quasiperiodic plane for decagonal Al-Ni-C o at low temperatures. The coefficient gamma=0.63 mJ mol(-1) K-2 of th e linear term to C-p(T) indicates a low density of electronic states a t E-F. The magnitude of the cubic-in-T term to the low-temperature spe cific heal is in fair agreement with the acoustic contribution to C-p( T) calculated from the results of low-temperature measurements of the elastic stiffness constants c(ij). The phonon contribution to lambda(p )(T) shows a distinct maximum at 25 K, typical for periodic crystals. The dominant feature in the phonon contribution to lambda(q)(T) is an extended plateau between 30 and 70 K, in agreement with the concept of generalized Umklapp processes in quasicrystals. This distinct differe nce between the phonon contributions to lambda(p)(T) and lambda(q)( T) is similar to that previously observed for Al65CU20Co15 The optical c onductivity sigma(1)(omega) is obtained from reflectivity data in the frequency range between 1 meV and 12 eV. At low frequencies the optica l conductivity sigma(1)(omega) is characterized by an anisotropic Drud e contribution. Al high frequencies an additional absorption for the p eriodic and the quasiperiodic direction is observed centered at approx imately 1 and 2 eV, respectively, which can be associated with electro nic excitations across a pseudogap.