Anisotropic supercurrent through twisted Bi2Sr2CaCu2O8+x van der Waals stacks: a step towards explaining high-temperature superconductivity

A material that can conduct current without resistance at low temperatures is called a superconductor. Superconductors can transmit a large amount of electricity without energy loss, rendering it useful for many applications such as electrical power transmission, MRIs, magnetic levitation trains, and others. Unlike low-temperature superconductors that require the costly liquid helium, high-temperature superconductors only require the cost-effective liquid nitrogen – But their operational principle is still under wraps. To this, a Korean research team has recently demonstrated the anisotropic superconductivity of a high-temperature superconductor that might support to reveal the mechanism behind high-temperature superconductors by stacking them in various twisted angles.

A POSTECH research team led by Professor Emeritus Hu-Jong Lee, Professor Gil-Ho Lee, and Ph.D. candidate Jongyun Lee (Department of Physics) in collaboration with Brookhaven National Laboratory has demonstrated the anisotropic superconductivity of the copper oxide-based high-temperature superconductor by stacking twisted pieces of Bi₂Sr₂CaCu₂O_8+x (Bi-2212 hereafter) using the newly developed microcleave-and-stack technique.

Completely novel properties can appear even if the same materials are stacked with a twisted angle. One good example is twisted bilayer graphene – not a superconductor – which exhibits superconductivity when stacked at a twisted angle of approximately 1.1 degrees. Graphene is an isotropic layered material that exhibits uniform properties regardless of the direction of the crystal, but in anisotropic crystal layers whose properties vary depending on the direction, the physical properties would change more dramatically according to the twist angle.

In particular, the anisotropic superconductivity resulting from the anisotropic crystal structure is known to be closely related to the mechanism of high-temperature superconductors. So far, the anisotropic superconductivity in high-temperature superconductors has been verified using the scanning tunneling microscopy or the angle-resolved photoemission spectroscopy.

Other studies to verify the anisotropic superconductivity using transport measurement has been attempted within the last 20 years. However, findings have been inconclusive as the crystal structure of the junction interface becomes deformed as the temperature reaches the temperature close to melting point or as the flakes attach and detach repeatedly.

To this, Professor Gil-Ho Lee’s team aimed to investigate the superconductivity of Bi-2212 with the transport property in twisted van der Waals Josephson junctions. In order to prevent the deformation of the crystal structure at the junction interface, the exerted force is minimized by stacking the Bi-2212 crystal layers using the van der Waals force. At this time, one single crystal is cleaved into two – top and bottom layers – with air blocked to prevent any contamination or oxidation, and the two layers are twisted and stacked.

As a result, the Josephson coupling shows a twist angle dependence, which well corresponds to the theoretical expectation with anisotropic superconductivity in Bi-2212. This study not only confirmed the material properties of high-temperature superconductors using the transport measurement, but also has significance in that it developed a promising new fabrication method for nanomaterials. The microcleave-and-stack technique used in this study is anticipated to be applicable for studying interfaces of air-sensitive materials.

“Twistronics, which controls the twist angle to produce new material properties, has recently been attracting much attention and the twist angle has become a novel control parameter,” explained Professor Gil-Ho Lee who led the study. He added, “So far in twistronics, graphene has been mostly studied, but our research team has expanded into superconductors and has pioneered the new field of superconductor-based twistronics.”

Recently published in the leading international journal Nano Letters, this study was supported by the National Research Foundation of Korea and the Samsung Science and Technology Foundation.