Oscillating paramagnetic Meissner effect and Berezinskii-Kosterlitz-Thouless transition in underdoped Bi2Sr2CaCu2O8+δ

Abstract

Abstract Superconducting phase transitions in 2-dimension (2D) lie beyond the description of Ginzburg-Landau symmetry-breaking paradigm for 3-dimensional superconductors. It is a Berezinskii-Kosterlitz-Thouless (BKT) transition of paired-electron condensate driven by unbinding of topological excitations, i.e., vortices. The recently discovered monolayers of layered cuprate high transition-temperature (${{\rm{T}}}_{{\bf C}}$) superconductor Bi2Sr2CaCu2O8+δ (Bi2212) promised to be a 2D superconductor 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 SQUID 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 ${{\rm{T}}}_{{\bf C}}$. Within a single domain, we find the susceptibility oscillates with flux between diamagnetism and paramagnetism in a Fraunhofer-like pattern up till ${{\rm{T}}}_{{\bf C}}$. The finite modulation period as well as broadening of the peaks when approaching ${{\rm{T}}}_{{\bf C}}$ from below suggests well-defined vortices which 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 ${{\rm{T}}}_{{\bf C}}$ in consistence 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 ${{\rm{T}}}_{{\bf C}}$. These results unify the superconducting phase transitions from the monolayer to the bulk underdoped Bi2212 as BKT transition with interlayer coupling.