LOW-TEMPERATURE PHONON THERMAL-CONDUCTIVITY OF THE QUASI-ONE-DIMENSIONAL SINGLE-CRYSTALS (TA1-XNBXSE4)(2)I

We report on thermal conductivity measurements of the quasi-onedimensi onal single crystals (Ta1-xNbxSe4)(2)I (x = 0, 0.008, 0.01) between 50 mK and 6 K. The thermal conductivity of a (TaSe4)(2)I sample obeys a cubic variation below 0.8 K, in good agreement with the boundary scatt ering regime. For two Nb doped samples, the thermal conductivity betwe en 0.1 K and 1 K obeys a quasiquadratic regime, which is ascribed to e xtended ''linear or planar defects,'' specific to this one-dimensional structure, and introduced either by doping or by successive thermal c ycling, while it obeys a cubic variation below 0.1 K, as in the case o f pure (TaSe4)(2)I. In the vicinity of 1 K and 2 K for pure and 1 at.% Nb doped samples, thermal conductivity shows a very sharp peak, which exceeds the low-temperature cubic boundary Casimir regime, or the qua si quadratic regime, respectively. The presence of the peak in these s ystems is explained by a phonon Poiseuille flow, originating from thei r strong anharmonicity, amplified by the lattice anisotropy.