Research progress on ultrathin high-temperature superconductors of bismuth-based cuprates

High-temperature cuprate superconductors have become the testbed for state-of-the-art technologies in condensed matter physics. A great variety of advanced experimental techniques and theoretical methods have been employed to understand the mechanism of high-temperature superconductivity and to discover novel physical phenomena due to strong correlation effects. The advent of two-dimensional (2D) materials such as graphene has been accompanied by the rapid development in sample preparations, such as van der Waals mechanical exfoliation, transfer, and stacking. These emerging techniques have also benefited the study of high-temperature superconductors, particularly the bismuth-based cuprates such as Bi2Sr2CaCu2O8+δ (Bi-2212), leading to a series of important progresses and breakthroughs. They include the establishment of robust high-temperature superconductivity consistent with bulk behavior in the 2D limit and the construction of atomically flat twisted Josephson junctions. This review is organized in the following manner. First, we briefly introduce the structure of bismuth-based cuprates, showing their layered configuration of superconducting CuO2 planes alternating with insulating charge reservoir layers. The weak van der Waals bonding between BiO plane layers enables mechanical exfoliation. This structural feature provides the foundation for obtaining atomically thin flakes. Second, we provide an overview of the experimental techniques for preparing devices with ultrathin Bi-2212. They include mechanical exfoliation under inert atmospheres, protection by hexagonal boron nitride, employment of pre-patterned electrodes, and van der Waals transfer at cryogenic temperatures. These processes are necessary because Bi-2212 in the ultrathin form degrades in air and tends to lose interstitial oxygen rapidly at room temperature. The technical improvements help minimize the sample degradation, leading to a progressive enhancement in device quality and allowing for novel manipulation. These techniques eventually allow researchers to obtain monolayer Bi-2212 samples with a superconducting transition temperature that is the same as that in the bulk. It demonstrates that high-temperature superconductivity is largely captured in the 2D limit. In the third section, we summarize the diverse experimental approaches to modulate ultrathin Bi-2212 and the emergent quantum behaviors in the 2D limit. We also discuss unprecedented quantum phenomena that are found in ultrathin Bi-2212. Electrostatic gating and electrochemical gating have enabled continuous control of the carrier density, allowing systematic exploration of the superconducting phase diagram. A series of intriguing phenomena have been addressed or observed, including two-dimensional quantum phase transition between a superconductor and an insulator, enhanced regime for the sign reversal of the Hall effect, Little-Parks-like oscillations in the underdoped region, and exponential decay of the vortex entropy with the transition temperature. We also discuss the Josephson tunneling in twisted Bi-2212 bicrystals, the superconducting diode effect, and single-photon detection based on Bi-2212 thin films. In the end, we outline potential future research directions. We propose that high-pressure studies can probe the interlayer coupling within a monolayer. Angular resolved photoemission spectroscopy with micrometer spatial resolution can investigate the Fermi surface reconstruction and pseudogap evolution in the 2D limit. Nitrogen-vacancy (NV) center magnetometry can help reveal the vortex dynamics. These emerging experimental approaches, combined with vdW-based fabrication, are expected to deepen our understanding of the interplay between dimensionality and strong correlations in cuprate superconductors.