Interplay between Ferrolectricity And Superconductivity

在具有超导基态的掺杂半导体中，SrTiO3的特点是其超导性的早熟。由于每个公式单位（f.u）只有10-5个电子，该系统变成了超导系统。当载流子密度超过0.02/f.u时。，就不再是这样了。这个超导圆顶提出了两个截然不同且仍未得到解答的问题：尽管费米温度和德拜温度之间存在层次反转，但超导性如何在稀极限下持续存在？为什么尽管态的电子密度稳步增加，但它在过掺杂侧却消失了？这种非凡的超导体的绝缘母体是一个量子对电体，它通过用钙取代锶而成为一个真正的铁电体。这两个伙伴最近发现，在ca取代的缺氧钛酸锶中，这两个阶共存。乍一看，这种共存似乎令人惊讶。铁电性是物质的一种状态，在这种状态下，固体具有宏观可逆的静电偶极子。由于金属的移动电子有望屏蔽这样的偶极子，因此只有缺乏反转对称性的离子绝缘体才有望成为真正的铁电体。超导性是金属中电子的不稳定性，由吸引相互作用引起，克服了电子之间的库仑排斥，产生了宏观上凝聚的库珀对。这两种物质状态没有什么共同之处。它们的互斥性被认为比超导性和磁性之间的敌意更强烈。然而，我们发现当移动电子和电偶极子都被稀释时，它们可以共存，并且电子的超导不稳定性和偶极子的铁电排列并不相互阻碍。这一实验观察提出了一系列新问题：筛选一个偶极子需要多少个移动电子？在稀释极限下，筛选过程的动力学方面必须如何考虑？费米海中的偶极子是如何相互交流的？库珀对和偶极子是如何相互作用的？如何比较铁电量子临界与磁性量子临界？本提案旨在记录不同掺杂剂（铌、镧、钙、钡、氧空位……）的SrTiO3及其姊妹化合物EuTiO3中铁电、超导和反铁扭曲序之间接近的后果。这两个合作伙伴拥有互补的专业知识。德国合作伙伴一直积极探索广泛频率范围内的复杂电导，并在稀土钛酸盐单晶生长和量子相变热力学研究方面拥有强大的专业知识。法国合作伙伴一直在研究极端温度和磁场条件下零频率的电、热电和热输运。

Among doped semiconductors with a superconducting ground state, SrTiO3 is distinguished by the precocity of its superconductivity. With only 10-5 electrons per formula unit(f.u.), the system becomes superconducting. When carrier density exceeds 0.02/f.u., it ceases to be so. This superconducting dome raises two distinct and still unanswered questions: how does superconductivity persist in the dilute limit despite the hierarchy inversion between Fermi and Debye temperatures? Why does it disappear on the overdoped side in spite of the steady increase in the electronic density of states? The insulating parent of this remarkable superconductor is a quantum paraelectric, which becomes a true ferroelectric by replacing strontium with calcium. The two partners have recently discovered that the two orders coexist in Ca-substituted-oxygen-deficient strontium titanate. At first sight, such a coexistence appears surprising. Ferroelectricity is a state of matter in which the solid hosts a macroscopic reversible static electric dipole. Since mobile electrons of a metal are expected to screen such a dipole, only ionic insulators which lack inversion symmetry are expected to qualify as true ferroelectrics. Superconductivity is an electron instability in a metal triggered by an attractive interaction overcoming the Coulomb repulsion among electrons and creating Cooper pairs which condensate macroscopically. These two states of matter have little in common. Their mutual exclusiveness was considered even stronger than the documented animosity between superconductivity and magnetism. However, we have found that when mobile electrons and electric dipoles are both dilute, they can coexist and the superconducting instability of electrons and ferroelectric alignment of dipoles do not impede each other. This experimental observation raises a host of new questions: How many mobile electrons does it take to screen a dipole and how do the dynamical aspects of the screening process have to be considered in the dilute limit? How do dipoles inside a Fermi sea communicate with each other? How do Cooper pairs and aligned dipoles interact? How to compare ferroelectric quantum criticality with its magnetic counterpart?This proposal aims to document the consequences of the proximity between ferroelectric, superconducting and antiferrodistortive orders in SrTiO3 subject to various dopants (niobium, lanthanum, calcium, barium, oxygen vacancies…) as well as in its sister compound EuTiO3. The two partners possess complementary expertise. The German partner has been active in exploring the complex conductance in a broad range of frequency and has strong expertise in single-crystal growth of rare-earth titanates and thermodynamic studies of quantum phase transitions. The French partner has been studying electric, thermoelectric and thermal transport at zero frequency in extreme conditions of temperature and magnetic field.