University professors identify novel collective state of electrons

For the most part, electrons are solitary, whirling haphazardly through space as a complex hybrid of wave and particle. An interdisciplinary team of Princeton scientists, however, has demonstrated a novel collective electron state exhibiting dynamic new properties that can be put to use in fields like computing and alternative energy development.

The researchers found that, in both bismuth and graphene, electrons subjected to a strong magnetic field at temperatures near absolute zero simultaneously moved into a lower energy configuration. The bismuth discovery is especially significant because this type of transition has never been observed before in elements in Group V of the periodic table.

Additionally, electrons normally exist in several different varieties, called “flavors,” but after this transition, the electrons in bismuth were observed to have moved into a collective state.

Both of these findings can only be explained through the framework of quantum mechanics and hint at the potential to manipulate the quantum behavior of electrons in a solid material, which would have significant implications for future developments in electronics.

Princeton Center for Complex Materials director and physics professor Nai Phuan Ong and chemistry department chair Robert Cava led the team.

“It started out as research on something completely different and ended up in one of these strange results,” Cava said of the circuitous path to their discovery.

Interest in the project was sparked a year ago when theorists began to speculate about the existence of Dirac materials, substances whose electrons behave like photons, the elementary and massless components of light.

“In ordinary particles, like a baseball, the energy of the object increases as the square of its velocity,” Ong explained. “Dirac electrons have a different relationship: Their energy is linear in momentum.”

Cava’s chemistry lab had been developing thermoelectric materials for use in cooling systems for the Air Force, and physics professor Zahid Hasan asked Cava to provide him with some of the bismuth antimony alloy for his own investigation.

The request got Ong and his students thinking about bismuth and antimony’s basic properties, which were previously thought to have been fully investigated.

“Nobody expects to be able to discover something exotic in a simple element anymore,” Cava said.

Ong first observed the electron phase transition in graphene, a single layer of covalently bonded carbon atoms. Though this discovery was significant, finding the phenomenon in a second material was important because “when you see a pattern of behavior in one material it’s special, but in two it suggests that it’s universal,” Ong explained.

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He collaborated with Cava’s lab to grow pure bismuth crystals, which Lu Li GS ’08 and Joe Checkelsky GS then took to the National High Magnetic Field Laboratory in Tallahassee, Fla., to expose it to the world’s most powerful magnet, a 34-ton behemoth that produces a 45-Tesla magnetic field — one million times that of Earth’s magnetic field.

The bismuth crystal was placed on a gold cantilever and then subjected to an increasing magnetic field. The researchers measured the degree of displacement of the cantilever and observed an abrupt jump, which identified the “critical value” at which the magnetic field forced the electrons to move cooperatively toward a lower energy state.

“Think of it like people going all directions in a train station,” Ong said. “When the whistle blows, everyone goes in the same direction toward the train.”

Collective states are not unprecedented in materials — magnetism and superconductivity are two of the most thoroughly researched instances of such states.

Superconductivity and this newly observed state are loosely related in that both “are collective states of electrons, which, under normal circumstances in normal conducting materials can be considered as non-interacting particles,” Cava said in an e-mail.

While Hasan observed the collective behavior in the bismuth antimony alloy, Ong and Cava’s team demonstrated that the electron coordination can extend throughout a crystal of a pure element.

Though Cava said he suspects some relationship between the thermoelectric properties of bismuth and the demonstrated phase transition, he noted that it is unclear to what extent the properties and discovery are related.

Practical applications

Ong said that the discovery has direct applications in the push to develop quantum computing, which would harness the properties of electrons to represent data and perform computations much faster than any current computer.

One significant hurdle in the implementation of quantum computing, however, is that the electron states that would be involved in information storage and computing are extremely fragile and can be disrupted by exposure to room temperature. Thus, though the new discovery may pave the way for significant advances in technology, the actual implementation of quantum computing may still be a long way off.

Other applications include harnessing solar energy, Ong said. In the ongoing search for efficient photovoltaic materials, the collective state behavior has pointed researchers toward new areas of the periodic table for improved performance over silicon.