Rattling and Superconducting Properties of the Cage Compound Ga_xV_2Al_20
Rattling and Superconducting Properties of the Cage Compound Ga_xV_2Al_20
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笼状化合物Ga_xV_2Al_20的振动特性和超导特性
DOI：
10.1143/jpsj.81.124707
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发表时间：
2012
期刊：
影响因子：
--
通讯作者：
H. Harima
中科院分区：
文献类型：
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作者：
Z. Hiroi;A. Onosaka;Y. Okamoto;J. Yamaura;H. Harima
研究了笼状化合物 GaxV2Al20 中的低能震颤模式及其对超导性的影响。通过电阻率、磁化率和热容测量来检查一系列 0 <x≤0.6 的多晶样品。所有样品均在 Tc= 1.4–1.7 K 以下观察到弱耦合 BCS 超导性。对于低于 0.20 的小 Ga 含量，大约 30% 的笼子被爱因斯坦温度 TE 为 23 K 的振动 Al 原子占据，可能大多数 Ga 原子取代了形成笼子的 Al 原子。对于较高的 Ga 含量，大约 0.05 Ga 和 0.25-0.35 Al 原子在统计上共存于笼内，并且分别在 TE∼8 和 23 K 下表现为响尾蛇。当 0.05 个 Ga 原子引入壳体时，观察到 Tc 急剧上升 8% atx= 0.20，这清楚地证明了 Ga 震颤对超导性的显着影响。电子-声子相互作用可能因 Ga 原子极低能量振动模式对声子态密度的额外贡献而显着增强。此外，当 x≥0.20 时，观察到声波模式的大幅软化，这表明在低能 Ga 震颤模式存在的情况下，笼本身变得异常柔软。
Low-energy rattling modes and their effects on superconductivity are studied in the cage compound GaxV2Al20. A series of polycrystalline samples of 0 <x≤0.6 are examined through resistivity, magnetic susceptibility, and heat capacity measurements. A weak-coupling BCS superconductivity is observed belowTc= 1.4–1.7 K for all the samples. For small Ga contents below 0.20, approximately 30% of the cages are occupied by rattling Al atoms having an Einstein temperatureTEof 23 K, probably with most Ga atoms substituting for the cage-forming Al atoms. For higher Ga contents, approximately 0.05 Ga and 0.25–0.35 Al atoms coexist statistically inside the cages and behave as rattlers withTE∼8 and 23 K, respectively. A significant effect of Ga rattling on the superconductivity is clearly evidenced by the observation of a sharp rise inTcby 8% atx= 0.20 when 0.05 Ga atoms are introduced into the case. Probably, the electron–phonon interaction is significantly enhanced by an additional contribution to the phonon density of states from the extremely low energy rattling modes of Ga atoms. In addition, a large softening of the acoustic modes is observed forx≥0.20, suggesting that the cage itself becomes anomalously soft in the presence of low-energy Ga rattling modes.