Interplay between Ferrolectricity And Superconductivity

在具有超导基态的掺杂半导体中，钛酸锶以其超导电性的早熟而著称。当每个公式单位(F.U)只有10-5个电子时，该系统就变成了超导。当载流子密度超过0.02个/平方英尺时，就不再是这样了。这个超导穹顶提出了两个截然不同但仍然悬而未决的问题：尽管费米温度和德拜温度之间存在等级反转，但超导电性如何保持在稀释极限？为什么在电子态密度稳步增加的情况下，它在过度掺杂的一侧消失了？这种非凡的超导体的绝缘母体是量子顺电材料，它通过用钙取代锶而成为真正的铁电材料。这两个合作伙伴最近发现，这两种有序共存于钙取代的缺氧钛酸锶中。乍一看，这种共存似乎令人惊讶。铁电性是一种物质状态，在这种状态下，固体中存在宏观上可逆的静电偶极子。由于金属的可移动电子预计会屏蔽这样的偶极，因此只有缺乏反转对称性的离子绝缘体才有资格成为真正的铁电体。超导性是金属中的一种电子不稳定性，这种不稳定性是由一种吸引的相互作用引起的，它克服了电子之间的库仑排斥，并产生了宏观上凝聚的库珀对。这两种物质状态几乎没有什么共同之处。它们的相互排他性被认为比文献中记载的超导和磁性之间的敌意更强。然而，我们发现，当运动电子和电偶极子都是稀薄的时，它们可以共存，电子的超导不稳定性和偶极子的铁电排列不会相互阻碍。这一实验观察提出了一系列新的问题：屏蔽偶极子需要多少个移动电子，屏蔽过程的动力学方面必须如何在稀释限内考虑？费米海中的偶极子是如何相互交流的？库珀对和排列的偶极子是如何相互作用的？如何将铁电量子临界性与其磁相对应的量子临界性进行比较？这一建议旨在证明在不同掺杂(Nb、La、钙、钡、氧空位…)作用下，钛酸锶中铁电序、超导序和反铁扭曲序之间的接近所产生的后果以及在其姊妹化合物EuTiO3中。这两个伙伴拥有互补的专业知识。这家德国合作伙伴在广泛的频率范围内积极探索复杂的电导，并在稀土钛酸盐单晶生长和量子相变的热力学研究方面拥有丰富的专业知识。这位法国合作伙伴一直在研究在极端温度和磁场条件下零频率的电、热电和热传输。

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.