A new strategy for making and manipulating superconductors at higher temperatures

Superconductors have intrigued physicists for decades. But these materials, which allow a perfect and lossless flow of electrons, generally only present this particularity of quantum mechanics at temperatures so low – a few degrees above absolute zero – that it makes them impractical.

A research team led by Philip Kim, professor of physics and applied physics at Harvard, has demonstrated a new strategy for making and manipulating a widely studied class of high-temperature superconductors called cuprates, paving the way for engineering new unusual forms of superconductivity in previously inaccessible materials. .

Using a unique method of manufacturing low-temperature devices, Kim and his team report in the journal Science a promising candidate for the world’s first high-temperature superconducting diode – essentially a switch that causes current to flow in one direction – made from thin cuprate crystals.

Such a device could theoretically power nascent industries like quantum computing, which rely on mechanical phenomena that are ephemeral and difficult to maintain.

“High-temperature superconducting diodes are, in fact, possible without the application of magnetic fields, and open new doors to the study of exotic materials,” Kim said.

Cuprates are copper oxides that decades ago shook the world of physics by showing that they become superconductors at temperatures much higher than theorists thought possible, “higher” being a relative term (the current record for a cuprate superconductor is -225 degrees Fahrenheit). However, manipulating these materials without destroying their superconducting phases is very complex due to their complex electronic and structural characteristics.

The team’s experiments were led by SY Frank Zhao, a former student at the Griffin Graduate School of Arts and Sciences and now a postdoctoral researcher at MIT. Using an airless method of manipulating cryogenic crystals in ultrapure argon, Zhao designed a clean interface between two extremely thin layers of copper oxide, cuprate, bismuth, strontium, calcium, nicknamed BSCCO (“bisco”) .

BSCCO is considered a “high temperature” superconductor because it begins to superconduct at around -288 Fahrenheit (-177 C) – very cold by practical standards but surprisingly high among superconductors, which typically need to be cooled to around – 400 Fahrenheit (-240 C). ).

Zhao first divided the BSCCO into two layers, each one thousandth the width of a human hair. Then, at -130 F (-90 C), he stacked the two layers in a 45-degree twist, like an ice cream sandwich with wafers askew, retaining superconductivity at the fragile interface.

The team discovered that the maximum supercurrent that can pass through the interface without resistance is different depending on the direction of the current. Importantly, the team also demonstrated electronic control of the interfacial quantum state by reversing this polarity.

This control effectively allowed them to make a high-temperature switchable superconducting diode, a demonstration of fundamental physics that could one day be incorporated into computer technology, such as a quantum bit.

“It is a starting point for studying topological phases, featuring quantum states protected from imperfections,” Zhao said.

The Harvard team worked with colleagues Marcel Franz of the University of British Columbia and Jed Pixley of Rutgers University, whose teams have already performed theoretical calculations accurately predicting the behavior of the cuprate superconductor in a wide range. range of twist angles. Reconciling the experimental observations also required new theoretical developments made by Pavel A. Volkov of the University of Connecticut.

Correction note (12/182023): Degrees Celsius have been added to the article to complement the associated Fahrenheit measurements.