Mysterious class of ‘strange metals’ deliver quantum secrets

Physicists at Rutgers University have offered theoretical perspectives on an experiment involving a “strange metal” that could play a crucial role in the development of future quantum technologies.

Researchers studying a compound dubbed the “Y-Ball,” which belongs to a mysterious class of “strange metals” believed to be crucial to the development of advanced quantum materials, have discovered new ways to study and understand its behavior.

The results of the experiments can contribute to the development of disruptive technologies and devices.

“Most likely, quantum materials will drive the next generation of technology, and strange metals will be part of that story,” said Piers Coleman, distinguished professor at the Rutgers Center for Materials Theory in the Department of Physics and Astronomy at Rutgers School of Arts and Sciences and one of the researchers on the study involved theorists. “We know that strange metals like Y-Ball have properties that need to be understood in order to develop these future applications. We’re pretty sure that understanding this strange metal will give us new ideas and help us design and discover new materials.”

Reporting in the diary Science, an international team of researchers from Rutgers, the University of Hyogo and the University of Tokyo in Japan, the University of Cincinnati and Johns Hopkins University, described details of electron motion that provide new insights into Y-Ball’s unusual electrical properties. Technically known as the compound YbAlB4, the material contains the elements ytterbium, aluminum and boron. The late Elihu Abrahams, founding director of the Rutgers Center for Materials Theory, nicknamed it “Y-Ball”.

The experiment revealed unusual fluctuations in the electric charge of the strange metal. The work is groundbreaking, the researchers said, because the experimenters studied Y-Ball in a novel way by firing gamma rays at it using a synchrotron, a type of particle accelerator.

The Rutgers team — including Coleman, physics professor Premala Chandra, and former postdoc Yashar Komijani (now an assistant professor at the University of Cincinnati) — have spent years exploring the mysteries of strange metals. They do so within the framework of quantum mechanics, the physical laws that govern the realm of the ultra-small, home to nature’s building blocks like electrons.

The scientists analyzed the material using a technique known as Mössbauer spectroscopy, and examined Y-Ball with gamma rays to measure the rate at which the strange metal’s electrical charge fluctuates. In a conventional metal, electrons jump in and out of atoms as they move, causing their electrical charge to fluctuate, but at a rate a thousand times too fast to be seen with Mössbauer spectroscopy. In this case, the change happened in a nanosecond, a billionth of a second.

“In the quantum world, a nanosecond is an eternity,” said Komijani. “We’ve long wondered why these fluctuations are actually so slow.” “We thought,” Chandra continued, “that an electron jumps into an ytterbium every time[{” attribute=””>atom, it stays there long enough to attract the surrounding atoms, causing them to move in and out. This synchronized dance of the electrons and atoms slows the whole process so that it can be seen by the Mossbauer.”

They moved to the next step. “We asked the experimentalists to look for these vibrations,” said Komijani, “and to our delight, they detected them.”

Coleman explained that when an electrical current flows through conventional metals, such as copper, random atomic motion scatters the electrons causing friction called resistance. As the temperature is raised, the resistance increases in a complex fashion and at some point, it reaches a plateau.

In strange metals such as Y-ball, however, resistance increases linearly with temperature, a much simpler behavior. In addition, further contributing to their “strangeness,” when Y-ball and other strange metals are cooled to low temperatures, they often become superconductors, exhibiting no resistance at all.

The materials with the highest superconducting temperatures fall into this strange family. These metals are thus very important because they provide the canvas for new forms of electronic matter – especially exotic and high-temperature superconductivity.

Superconducting materials are expected to be central to the next generation of quantum technologies because, in eliminating all electrical resistance, they allow an electric current to flow in a quantum mechanically synchronized fashion. The researchers see their work as opening a door to future, perhaps unimaginable possibilities.

“In the 19th century, when people were trying to figure out electricity and magnetism, they couldn’t have imagined the next century, which was entirely driven by that understanding,” Coleman said. “And so, it’s also true today, that when we use the vague phrase ‘quantum materials,’ we can’t really envisage how it will transform the lives of our grandchildren.”

Reference: “Observation of a critical charge mode in a strange metal” by Hisao Kobayashi, Yui Sakaguchi, Hayato Kitagawa, Momoko Oura, Shugo Ikeda, Kentaro Kuga, Shintaro Suzuki, Satoru Nakatsuji, Ryo Masuda, Yasuhiro Kobayashi, Makoto Seto, Yoshitaka Yoda, Kenji Tamasaku, Yashar Komijani, Premala Chandra and Piers Coleman, 2 March 2023, Science.
DOI: 10.1126/science.abc4787

The study was funded by the National Science Foundation, the U.S. Department of Energy, Japan Science and Technology Agency, the Ministry of Education, Culture, Sports, Science, and Technology of Japan, Japan Synchrotron Radiation Research Institute, and RIKEN.