Oscillating paramagnetic Meissner effect and Berezinskii–Kosterlitz–Thouless transition in Bi2Sr2CaCu2O8+δ monolayer

Abstract

Monolayers of a prototypical cuprate high transition-temperature (TC) superconductor Bi2Sr2CaCu2O8+δ (Bi2212) was recently found to show TC and other electronic properties similar to those of the bulk. The robustness of superconductivity in an ideal two-dimensional (2D) system was an intriguing fact that defied the Mermin-Wagner theorem. Here, we took advantage of the high sensitivity of scanning SQUID susceptometry to image the phase stiffness throughout the phase transition of Bi2212 in the 2D limit. We found susceptibility oscillated with flux between diamagnetism and paramagnetism in a Fraunhofer-like pattern up till TC. The temperature and sample size-dependence of the modulation period agreed well with our Coulomb gas analogy of a finite 2D system based on Berezinskii–Kosterlitz–Thouless (BKT) transition. In the multilayers, the susceptibility oscillation differed in a small temperature regime below TC in consistent with a dimensional-crossover led by interlayer coupling. Serving as strong evidence of BKT transition in the bulk, there appeared a sharp superfluid density jump at zero-field and paramagnetism at small fields just below TC. These results unified the phase transitions from the monolayer Bi2212 to the bulk as BKT transition with finite interlayer coupling. This elucidating picture favored the pre-formed pairs scenario for the underdoped cuprates regardless of lattice dimensionality.

Suppression and Revival of Superconducting Phase Coherence in Monolayer FeSe/SrTiO3

“…However, the superfluid density does not exhibit a sharp jump at T c (Figure a), as is expected for BKT of a large and homogeneous 2D superconductor . Noting that the jump of superfluid density for a BKT transition can be smeared out due to a finite-size effect , or disorder, we can infer that the revived interfacial phase coherence by the FeTe layers is spatially inhomogeneous.…”

“…To definitively show the phase nematicity, an imaging technique that is directly sensitive to the superconducting phase or flux would be very helpful. Scanning SQUID susceptometry and Josephson tunneling microscopy, for example, may be attempted toward this goal if the phases are not fluctuating in time.…”

“…In high- T c superconductors represented by the cuprate- and iron-based families, ubiquitous emergent electronic and spin ordering often intertwine with superconducting phase coherence . High- T c superconductivity in the 2D limit is even more enigmatic, given the enhanced phase fluctuations. , What is more, the interfacial coupling from a substrate or capping layer is capable of shifting the balance of any competition among charge, spin, and phase.…”

Flux focusing with a superconducting nanoneedle for scanning SQUID susceptometry

“…This is important for studies of samples with small volumes, especially two-dimensional quantum materials and quantum devices fabricated from these materials 8 . While the direct flux sensitivities of nano-SQUIDs are useful in imaging physical quantities such as the magnetization of a ferromagnet, the vortex in a superconductor 9 , 10 or the edge current in a quantum spin Hall insulator 11 , susceptometry 12 – 15 is indispensable for probing spin correlations 16 or superfluid densities 17 – 19 , which are not observable with magnetometry.…”