Wang Shiyuan, Yu Yijun, Hao Jinxiang, Liang Keyi, Xiang Bingke, Zhu Jinjiang, Lin Yishi, Pan Yinping, Gu Genda, Watanabe Kenji, Taniguchi Takashi, Qi Yang, Zhang Yuanbo, Wang Yihua
State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai 200433, China.
Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA.
Natl Sci Rev. 2023 Sep 18;11(5):nwad249. doi: 10.1093/nsr/nwad249. eCollection 2024 May.
Superconducting phase transitions in two dimensions lie beyond the description of the Ginzburg-Landau symmetry-breaking paradigm for three-dimensional superconductors. They are Berezinskii-Kosterlitz-Thouless (BKT) transitions of paired-electron condensate driven by the unbinding of topological excitations, i.e. vortices. The recently discovered monolayers of layered high-transition-temperature ([Formula: see text]) cuprate superconductor BiSrCaCuO (Bi2212) meant that this 2D superconductor promised to be ideal for the study of unconventional superconductivity. But inhomogeneity posed challenges for distinguishing BKT physics from charge correlations in this material. Here, we utilize the phase sensitivity of scanning superconducting quantum interference device microscopy susceptometry to image the local magnetic response of underdoped Bi2212 from the monolayer to the bulk throughout its phase transition. The monolayer segregates into domains with independent phases at elevated temperatures below [Formula: see text]. Within a single domain, we find that the susceptibility oscillates with flux between diamagnetism and paramagnetism in a Fraunhofer-like pattern up to [Formula: see text]. The finite modulation period, as well as the broadening of the peaks when approaching [Formula: see text] from below, suggests well-defined vortices that are increasingly screened by the dissociation of vortex-antivortex plasma through a BKT transition. In the multilayers, the susceptibility oscillation differs in a small temperature regime below [Formula: see text], consistent with a dimensional crossover led by interlayer coupling. Serving as strong evidence for BKT transition in the bulk, we observe a sharp jump in phase stiffness and paramagnetism at small fields just below [Formula: see text]. These results unify the superconducting phase transitions from the monolayer to the bulk underdoped Bi2212, and can be collectively referred to as the BKT transition with interlayer coupling.
二维超导相变超出了三维超导体的金兹堡 - 朗道对称性破缺范式的描述范围。它们是由拓扑激发（即涡旋）的解束缚驱动的配对电子凝聚态的贝雷津斯基 - 科斯特利茨 - Thouless（BKT）相变。最近发现的层状高温（[公式：见正文]）铜酸盐超导体BiSrCaCuO（Bi2212）的单层意味着这种二维超导体有望成为研究非常规超导性的理想材料。但不均匀性给区分该材料中BKT物理与电荷相关性带来了挑战。在此，我们利用扫描超导量子干涉器件显微镜磁化率测量的相位敏感性，对欠掺杂Bi2212从单层到体相在其相变过程中的局部磁响应进行成像。在低于[公式：见正文]的高温下，单层分离成具有独立相位的畴。在单个畴内，我们发现磁化率在高达[公式：见正文]时以类似夫琅禾费的模式在抗磁性和顺磁性之间随磁通量振荡。有限的调制周期以及从下方接近[公式：见正文]时峰值的展宽表明存在明确的涡旋，这些涡旋通过BKT相变由涡旋 - 反涡旋等离子体的解离而逐渐被屏蔽。在多层中，磁化率振荡在低于[公式：见正文]的小温度范围内有所不同，这与层间耦合导致的维度交叉一致。作为体相中BKT相变的有力证据，我们观察到在略低于[公式：见正文]的小磁场下，相位刚度和顺磁性出现急剧跃升。这些结果统一了欠掺杂Bi2212从单层到体相的超导相变，并且可以统称为具有层间耦合的BKT相变。
