Evidence for unconventional superconductivity in twisted bilayer graphene
Evidence for unconventional superconductivity in twisted bilayer graphene
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DOI：
10.1038/s41586-021-04121-x
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发表时间：
2021-10-20
期刊：
影响因子：
64.8
通讯作者：
Yazdani, Ali
中科院分区：
文献类型：
--
作者：
Oh, Myungchul;Nuckolls, Kevin P.;Yazdani, Ali
魔角扭曲双层石墨烯(MATBG)中超导电性和相关绝缘体的出现提出了一种有趣的可能性，即它的配对机制不同于Bardeen-Cooper-Schrieffer(BCS)理论所描述的传统超导体(1-4)。然而，最近的研究表明，即使部分屏蔽了库仑相互作用，超导电性仍然存在(5，6)。这表明在MATBG中的配对在本质上可能是常规的，并且是其平坦带的大密度态的结果。在这里，我们将隧道效应和安德列夫反射光谱与扫描隧道显微镜相结合，观察了MATBG中非常规超导电性的几个关键实验特征。我们发现，在转变温度T-c以下的隧道谱与传统的S波超导体的隧道谱不一致，而与具有各向异性配对机制的节点超导体的隧道谱相似。我们观察到隧道能隙Delta(T)远远超过平均场BCS比(2 Delta(T)/k(B)T(C)类似于25)和由Andreev反射光谱提取的能隙Delta(AR)(2 Delta(AR)/k(B)T(C)类似于6)之间的巨大差异。即使在超导电性被抑制的情况下，隧道能隙仍然存在，这表明它是从伪隙相中出现的。此外，当MATBG与六方氮化硼对准时，赝隙和超导电性都不存在。这些发现和这里报告的其他观察结果为MATBG中超导电性的非BCS机制提供了优势证据。
The emergence of superconductivity and correlated insulators in magic-angle twisted bilayer graphene (MATBG) has raised the intriguing possibility that its pairing mechanism is distinct from that of conventional superconductors(1-4), as described by the Bardeen-Cooper-Schrieffer (BCS) theory. However, recent studies have shown that superconductivity persists even when Coulomb interactions are partially screened(5,6). This suggests that pairing in MATBG might be conventional in nature and a consequence of the large density of states of its flat bands. Here we combine tunnelling and Andreev reflection spectroscopy with a scanning tunnelling microscope to observe several key experimental signatures of unconventional superconductivity in MATBG. We show that the tunnelling spectra below the transition temperature T-c are inconsistent with those of a conventional s-wave superconductor, but rather resemble those of a nodal superconductor with an anisotropic pairing mechanism. We observe a large discrepancy between the tunnelling gap Delta(T), which far exceeds the mean-field BCS ratio (with 2 Delta(T)/k(B)T(c) similar to 25), and the gap Delta(AR) extracted from Andreev reflection spectroscopy (2 Delta(AR)/k(B)T(c) similar to 6). The tunnelling gap persists even when superconductivity is suppressed, indicating its emergence from a pseudogap phase. Moreover, the pseudogap and superconductivity are both absent when MATBG is aligned with hexagonal boron nitride. These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.