第一原理計算による電子ドープされたSrTiOの超伝導転移温度のキャリア密度依存性; テトラへドロン法を用いた解析II

Carrier density dependence of superconducting transition temperature of electron-doped SrTiO based on the first-principles calculations; Analysis using Tetrahedron Method II

伊海田 陸*; 関川 卓也; 大野 義章*; 佐野 和博*; 枡田 佳美*

Ikaida, Riku*; Sekikawa, Takuya; Ono, Yoshiaki*; Sano, Kazuhiro*; Masuda, Yoshimi*

Nbなどを希薄に混ぜたSrTiOは、自由電子が最も希薄な超伝導体の1つとして研究されてきた。同時にSrTiOは低温で巨大な誘電率を示すが、酸素16の酸素18置換や、SrのCa置換により強誘電転移が起こる物質でもある。さらに強誘電転移が起こる付近の置換濃度で超伝導転移温度Tcの増加が観測されている。しかし、強誘電性と超伝導の関係は解明されていない。そこで本研究では第一原理計算ソフトウェアQuantum ESPRESSOを用いて超伝導転移温度の最高値を示す近傍の自由電子密度領域で強誘電性、超伝導に関する物理量を計算した。その結果、自由電子密度の減少とともに強誘電転移の兆候と見られる結晶振動の周波数低下が得られた。さらに自由電子密度の減少とともにTcが増大する結果も得られた。この結果は、SrTiOにおける強誘電性と超伝導の関係の理論解明につながる。

The SrTiO, a dilute mixture of Nb and other elements, has been studied as one of the most dilute free electron superconductors. At the same time, SrTiO has a huge dielectric constant at low temperatures, and ferroelectric transition occurs due to the substitution of O18 for O16 and Ca for Sr. Furthermore, an increase in the superconducting transition temperature Tc has been observed at substitution concentrations near where the ferroelectric transition occurs. However, the relationship between ferroelectricity and superconductivity is not clarified. In this study, we calculated the physical quantities related to ferroelectricity and superconductivity in the free electron density region near the highest superconducting transition temperature using the first-principles calculation software Quantum ESPRESSO. In the result, we found that the frequency of crystal oscillation decreases with decreasing free electron density, which is an indication of the ferroelectric transition. Furthermore, we found that Tc increases with decreasing free electron density. These results lead to a theoretical clarification of the relationship between ferroelectricity and superconductivity in SrTiO.