Evidence for Dirac flat band superconductivity enabled by quantum geometry
Evidence for Dirac flat band superconductivity enabled by quantum geometry
复制标题
DOI：
10.1038/s41586-022-05576-2
复制
发表时间：
2023-02
期刊：
影响因子：
64.8
通讯作者：
Haidong Tian;Xue-Jian Gao;Yuxin Zhang;S. Che;Tianyi Xu;Patrick Cheung;Kenji Watanabe;T. Taniguchi;M. Randeria;Fan Zhang;C. N. Lau;M. Bockrath
中科院分区：
文献类型：
--
作者：
Haidong Tian;Xue-Jian Gao;Yuxin Zhang;S. Che;Tianyi Xu;Patrick Cheung;Kenji Watanabe;T. Taniguchi;M. Randeria;Fan Zhang;C. N. Lau;M. Bockrath
在平带超导体中，载流子的群速度vF非常慢。其中的超导电性尤其耐人寻味，因为它与高温超导体和重费米子系统长期存在的谜团有关。然而，平带中超导电性的出现似乎是自相矛盾的，因为在传统的Bardeen-Cooper-Schrieffer理论中，一个小的vF意味着相干长度、超流硬度和临界电流为零。在这里，我们利用扭曲的双层石墨烯，，，-，研究了超导Dirac平带系统中极小速度的深刻影响。利用Schwinger受限的非线性输运研究，我们证明了在莫尔超晶格的≈ 1/2和−3/4之间填充分数 的ν存在一个极慢的正常态漂移速度vn−1,000 mν−-1。在超导状态下，相同的速度极限构成了临界电流的一种新的限制机制，类似于相对论超流体。重要的是，我们对控制超导体电动力学响应的超流硬度的测量表明，它不是由动能主导的，而是由相互作用驱动的超导能隙主导的，这与最近关于量子几何贡献的理论是一致的。我们发现了小库珀对的证据，这是Bardeen-Cooper-Schrieffer到玻色-爱因斯坦凝聚交叉的特征，具有前所未有的超导转变温度与费米温度之比超过1的证据，并讨论了超平Dirac带中超强耦合超导电性是如何产生的。
In a flat band superconductor, the charge carriers’ group velocityvFis extremely slow. Superconductivity therein is particularly intriguing, being related to the long-standing mysteries of high-temperature superconductors and heavy-fermion systems. Yet the emergence of superconductivity in flat bands would appear paradoxical, as a smallvFin the conventional Bardeen–Cooper–Schrieffer theory implies vanishing coherence length, superfluid stiffness and critical current. Here, using twisted bilayer graphene, , , –, we explore the profound effect of vanishingly small velocity in a superconducting Dirac flat band system, , , , –. Using Schwinger-limited non-linear transport studies,, we demonstrate an extremely slow normal state drift velocityvn≈ 1,000 m s–1for filling fractionνbetween −1/2 and −3/4 of the moiré superlattice. In the superconducting state, the same velocity limit constitutes a new limiting mechanism for the critical current, analogous to a relativistic superfluid. Importantly, our measurement of superfluid stiffness, which controls the superconductor’s electrodynamic response, shows that it is not dominated by the kinetic energy but instead by the interaction-driven superconducting gap, consistent with recent theories on a quantum geometric contribution, , , –. We find evidence for small Cooper pairs, characteristic of the Bardeen–Cooper–Schrieffer to Bose–Einstein condensation crossover, –, with an unprecedented ratio of the superconducting transition temperature to the Fermi temperature exceeding unity and discuss how this arises for ultra-strong coupling superconductivity in ultra-flat Dirac bands.