'Y-ball' compound yields quantum secrets

The results of the experiments, aided by the insights of theoretical physicists at Rutgers, could play a role in the development of revolutionary technologies and devices.

"It's likely that that quantum materials will drive the next generation of technology and that strange metals will be part of that story," said Piers Coleman, a Distinguished Professor at the Rutgers Center for Materials Theory in the Department of Physics and Astronomy at the Rutgers School of Arts and Sciences and one of the theoreticians involved in the study. "We know that strange metals like Y-ball exhibit properties that need to be understood to develop these future applications. We're pretty sure that understanding this strange metal will give us new ideas and will help us design and discover new materials."

Reporting in the journal 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 insight into the unusual electrical properties of Y-ball. The material, technically known as the compound YbAlB₄, contains the elements ytterbium, aluminum and boron. It was nicknamed "Y-ball" by the late Elihu Abrahams, founding director of the Rutgers Center for Materials Theory.

The experiment revealed unusual fluctuations in the strange metal's electrical charge. The work is groundbreaking, the researchers said, because of the novel way the experimenters examined Y-ball, firing gamma rays at it using a synchrotron, a type of particle accelerator.

The Rutgers team -- including Coleman, fellow physics professor Premala Chandra and former postdoctoral fellow Yashar Komijani (now an assistant professor at the University of Cincinnati) -- have spent years exploring the mysteries of strange metals. They do so through the framework of quantum mechanics, the physical laws governing the realm of the ultra-small, home of the building blocks of nature such as electrons.

Analyzing the material using a technique known as Mossbauer spectroscopy, the scientists probed Y-ball with gamma rays, measuring the rate at which the strange metal's electrical charge fluctuates. In a conventional metal, as they move, electrons hop in and out of the atoms, causing their electrical charge to fluctuate, but at a rate that is thousands of times too fast to be seen by Mossbauer 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. "For a long time, we have been wondering why these fluctuations are actually so slow." "We reasoned," continued Chandra, "that each time an electron hops into an ytterbium 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."