Sr1-xCaxTiO3体系 “铁电”超导电性研究

Ferroelectricity (FE) is a state of matter with parallel aligned reversible electric dipoles and mobile electrons in metals are expected to screen the Coulomb interaction between the dipoles. In this sense, only ionic insulators lacking inversion symmetry are qualified as true ferroelectrics. Superconductivity (SC) is an instability in metals，arising from dissipationless movement of electrons. These two states of matter have little in common. ..Our recent paper reported that the FE is suppressed by electron doping in Sr1-xCaxTiO3, concomitant with the emergence of SC and both phases coexist in an intermediate doping range (Nat. Phys. 13, 643 (2017)). The result raises several new questions: How do dipoles communicate with each other inside a Fermi sea? What is the link between FE and SC? Do the FE fluctuations have positive impacts on SC electron pairing？..The pre-experiment has found a trend of re-emergence of FE instability in the deeper metallic phase, which indicates that the interaction between mobile electrons and dipoles is not the classical Thomas-Fermi screening and one may invoke quantum mechanisms for explanation. This proposal plans to study Sr1-xCaxTiO3 samples with various Ca contents by utilizing electric transport, thermal transport, thermal expansion and ultrasonic velocity measurements, in order to determine the mechanism of dipole-dipole interaction, find the evidence of SC phase impacted by FE instability and unveil the symmetry of SC order parameter. The research may provide an in-depth interpretation for the possible interaction between FE and SC and promote the exploration of new SC mechanisms.

根据常识，铁电相拥有可翻转的宏观电偶极矩，只存在于中心反演对称破缺的离子型绝缘体中；超导态源自电子库伯对的无损运动，铁电与超导毫不相关。我们最近报道在铁电体Sr1-xCaxTiO3中引入少量电子可抑制铁电相，诱导超导相变，且在一定电子浓度范围存在超导与铁电相共存现象。该发现引出一系列问题，如电偶极矩在费米面中如何相互作用，铁电和超导相的相互关系是什么，铁电涨落是否有可能提供超导电子配对所需的元激发等。我们前期实验研究表明铁电相在金属相中出现再入迹象，运动电子与局域电偶极矩的相互作用不是经典的托马斯-费米屏蔽，需要寻求量子机制解释。本项目计划利用电输运、热输运、热膨胀系数，声速等手段对不同Ca含量的Sr1-xCaxTiO3体系进行详细测量，明确电偶极矩相互作用机制，寻找铁电涨落影响超导相的直接证据，揭示超导序参量对称性。本研究有助于深入理解铁电-超导相互作用，探索新型超导机制。

铁电相拥有可翻转的宏观电偶极矩，只存在于中心反演对称破缺的绝缘体中。金属中的自由电子会屏蔽电偶极矩间的长程库伦相互作用，因此在一般情况下铁电相与金属相无法共存。近些年，人们发现当自由电子浓度极低，库伦屏蔽作用无法完全破坏铁电相时，存在铁电/金属共存情况，并伴随奇异物性的产生。本项目在相关方面做了一系列研究，成果如下： 1. 验证了极化金属的铁电屏蔽RKKY模型，在潜在的铁电金属Bi2O2Se体系发现低温电阻率的温度平方关系不能由费米液体电子-电子散射机制解释，挑战了教课书中的知识；2. 合成了绝缘相Bi2O2Se块材和薄膜样品，研究其热电、电学输运性质；3. 发现了金属掺杂的过渡金属二硫化物（TMDC）奇异的公度平移堆栈相变，丰富了人们对TMDC结构相变的理解。以上工作探索了新型铁电金属材料，拓展了人们对铁电金属物理现象的认知。

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