摘要:
运用基于密度泛函理论的第一性原理计算方法,研究两种β型烧绿石氧化物超导体AOs_2O_6（A=K,Rb）的结构稳定性,声子软化以及与超导电性的关系.通过计算发现,AOs_2O_6中碱金属原子A（=K,Rb）沿〈111〉晶向具有不稳定性,且以K原子的不稳定性更为突出.同时,计算得到的KOs2_O_6在布里渊区中心的卢子频率普遍比RbOs_2O_6的低,使得KOs_2O_6的电声子耦合常数比RbOs_2O_6的大.本文计算结果表明,较小的碱金属原子K位于较大的氧笼子中,活动性较强,导致声子的软化,是引起KOs_2O_6具有较强的电声子耦合及较高的超导转变温度的根本原因.这些结果对解释两种β型烧绿石氧化物超导体AOs_2O_6（A=K,Rb）的超导电性具有重要意义.
Abstract:
Using the first-principles calculational method based on the density functional theory, we study the structural instabilities, phonon softenings, and their relation to the superconductivities of twoβ-pyrochlore oxide superconductors AOs206 （A=K, Rb）. It is found that there are structural instabilities of alkali ions along the （111） direction in the two/3-pyrochlore oxide superconductors AOs206 （A=K, Rb）, especially in KOs206. Meanwhile, a comparison of the pbonon frequency at zone-center between KOs206 and RbOs206 shows that the frequency of KOs206 is lower in general than that of RbOs206, leading to the stronger electron-phonon coupling. We conclude that K atom located in a large oxygen cage has an unusual large atomic displacement parameter and strong activity, thereby resulting in strong phonon softening. This is the foundamental cause for stronger electron-phonon coupling and higher superconducting transition temperature of KOs206. These are of significance for explaining the superconductivities inβ-pyrochlore oxide superconductors AOs2O6（A=K, Rb）.
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