Electron-Phonon Interaction and Phonon Scattering Mechanisms of Electron-Doped Oxide Superconductors

测定了烧结Nd_<2-X>Ce_XCuO_4 （0<X<0.2）材料的导热系数kappa (T)、热扩散系数alpha (T)和电阻率rho (T)。kappa (T)在Ce浓度X=0.15附近达到最大值。在氩气中淬火后退火可以稳定Nd_<1.85>Ce_<0.15>CuO_4晶体的超导性。通过“淬火”过程引入氧空位似乎对超导性的稳定起着关键作用。在氩气气氛中对“淬灭”样品进行退火处理，降低了点缺陷引起的声子散射。这些“淬火+退火”样品中T_c的增强与电子局域化竞争效应的减弱是一致的。从导热系数kappa (T)分析可知，超导“淬火+退火”试样的点缺陷密度小于非超导烧结（“烧结”）试样，而“淬火+退火”试样的rho (T)值大于“烧结”试样。一些额外的散射机制，如反顺子，可能在超导样品中起作用。测定了不同Ba浓度下La_<2-X>M_XCuO_4 （M=Ba, Sr）烧结材料的热导率，并与Nd_<2-X>Ce_XCuO_4的热导率进行了比较。用少量Ba原子代替La原子，使纯La_2CuO_4的kappa值大大降低，特别是在低温下。研究发现，为了解释观测到的kappa减少现象，必须考虑一种新的声子散射中心，如双能级隧穿。

The thermal conductivity kappa (T), thermal diffusivity alpha (T) and electrical resistivity rho (T) of sintered Nd_<2-X>Ce_XCuO_4 (0<X<0.2) materials were measured. kappa (T) showed maximum nearthe Ce concentration X=0.15 at which the superconductivity occurred. A quenching process followed by annealing in Ar atmosphere was confirmed to stabilize superconductivity in Nd_<1.85>Ce_<0.15>CuO_4 crystals. Introduction of oxygen vacancies by "quench" process seems to play a key role for the stabilization of superconductivity. Annealing process in Ar atmosphere on "quench" ed samples reduces phonon scattering by point defects. The enhancement of T_c in these "quench+anneal" samples is consistent with the reduction of the competing effect of electron localization. From the analyzes of the thermal conductivity kappa (T), the point defect density is smaller in superconductive "quench+anneal" sample than non-superconductive as-sintered ("as-sinter") sample, while rho (T) values of "quench+anneal" samples are larger than those of "as-sinter" one. Some additional scattering mechanism such as due to antiparamagnons may be operative in superconducting samples. The thermal conductivity of La_<2-X>M_XCuO_4 (M=Ba, Sr) sintered materials was measured for the various concentrations of Ba and the phonon thermal conductivity was analyzed comparing with that of Nd_<2-X>Ce_XCuO_4. The pretty large kappa value for pure La_2CuO_4 was drastically diminished by substituting La by a small amount of Ba atoms especially at low temperatures. It was found that a new type of the phonon scattering center such as a two-level tunneling must be taken account of in order to explain the observed kappa reduction.