Harvard Presents Novel Method for High-Temperature Superconductors

The fabrication process may make it easier to find materials.

A team from Harvard led by Philip Kim uses cuprates to innovate in high-temperature superconductors.

advanced quantum computing by creating the first superconducting diode in history.

Control over quantum states and directional supercurrent in BSCCO demonstrated.

For many years, physicists have been fascinated by superconductors. However, the perfect, lossless flow of electrons that these materials permit typically only manifests at temperatures a few degrees above absolute zero, which makes them unusable.

The creation and manipulation of cuprates, a class of higher-temperature superconductors that have been extensively studied, has been made easier by a research team led by Harvard Professor of Physics and Applied Physics Philip Kim. This discovery paves the way for the engineering of novel and unusual forms of superconductivity in hitherto unachievable materials.

Kim and his team report in the journal Science a promising candidate for the world’s first high-temperature, superconducting diode—basically, a switch that makes current flow in one direction—made out of thin cuprate crystals. They did this by using a specially low-temperature device fabrication method. Theoretically, a gadget like this could support emerging sectors like quantum computing, which depend on ephemeral mechanical phenomena that are hard to maintain.

Without the use of magnetic fields, Kim stated, “high-temperature superconducting diodes are in fact possible and open new doors toward exotic materials study.”

A few decades ago, cuprates—copper oxides—upended the world of physics by demonstrating that they could become superconducting at temperatures far higher than previously thought. The word “higher” is relative; the record for a cuprate superconductor is currently -225 degrees Fahrenheit. But because of their complex electronic and structural features, handling these materials without destroying their superconducting phases is very difficult.

S. Y. Frank Zhao, a postdoctoral researcher at MIT and a graduate of the Griffin Graduate School of Arts and Sciences, oversaw the team’s experiments. In ultrapure argon, Zhao created a clean interface between two incredibly thin layers of cuprate bismuth strontium calcium copper oxide, or BSCCO (“bisco”), by using an air-free, cryogenic crystal manipulation technique. Because BSCCO begins superconducting at approximately -288 degrees Fahrenheit, which is extremely cold by practical standards but remarkably high among superconductors, which normally need to be cooled to about -400 degrees, it is referred to as a “high-temperature” superconductor.

First, Zhao divided the BSCCO into two layers, each one thousandth of a human hair’s width. Subsequently, at -130, he twisted the two layers at a 45-degree angle, resembling an ice cream sandwich with crooked wafers, and managed to preserve superconductivity at the delicate junction.

The group found that depending on the direction of the current, there are differences in the maximum supercurrent that can flow through the interface without encountering resistance. Most importantly, by flipping this polarity, the group also showed electronic control over the interfacial quantum state. Because of this control, they were able to create a high-temperature, switchable superconducting diode, which served as an example of fundamental physics and may eventually be used in a quantum bit or other computing device.

Zhao stated, “This is a beginning point in exploring topological phases, featuring quantum states shielded from imperfections.”

Reference: S. Y. Frank Zhao, Xiaomeng Cui, Pavel A. Volkov, Hyobin Yoo, Sangmin Lee, Jules A. Gardener, Austin J. Akey, Rebecca Engelke, Yuval Ronen, Ruidan Zhong, Genda Gu, Stephan Plugge, Tarun Tummuru, Miyoung Kim, Marcel Franz, Jedediah H. Pixley, Nicola Poccia, and Philip Kim, “Time-reversal symmetry breaking superconductivity between twisted cuprate superconductors”