The study, conducted by researchers at Princeton University and the University of Texas at Austin was published Oct. 21, in the journal Science. The study demonstrates that the electrons, when kept at very low temperatures where their quantum behaviors emerge, can spontaneously begin to travel in identical elliptical paths on the surface of a crystal of bismuth, forming a quantum fluid state. This behavior was anticipated theoretically during the past two decades by researchers from several institutions, including UT Austin physics professor Allan MacDonald.
"This is the first visualization of a quantum fluid of electrons in which interactions between the electrons make them collectively choose orbits with these unusual shapes," said Ali Yazdani, a physics professor at Princeton, who led the research.
Fundamental explorations of materials may provide the basis for faster and more efficient electronic technologies. Today's electronic devices, from computers to cellphones, use processors made from silicon. With silicon reaching its maximum capacity for information processing, researchers are looking to other materials and mechanisms.
One area of progress has been in two-dimensional materials, which allow control of electron motion by breaking the particles away from the constraints of the underlying crystal lattice. This involves moving electrons among "pockets" or "valleys" of possible states created by the crystal. Some researchers are working on ways to apply this process in an emerging field of research known as "valleytronics."
In the current work, the strange elliptical orbits correspond to the electrons being in different "valleys" of states. This experiment demonstrates one of the rare situations where electrons spontaneously occupy one valley or another, the researchers said.
The team at Princeton used a scanning tunneling microscope to visualize electrons on the surface of a bismuth crystal at extremely low temperatures where quantum behaviors can be observed.
Bismuth has relatively few electrons, which makes it ideal for watching what happens to a flow of electrons subjected to a high magnetic field. Despite its purity, the crystal that the team grew contained some defects. Roughly one atom was slightly out of place for every tens of thousands of atoms.
Normally, in the absence of the magnetic field, electrons in a crystal will flit from atom to atom. Applying a strong magnetic field perpendicular to the flow of electrons forces the electrons' paths to curve into orbit around a nearby defect in the crystal, like planets going around the sun.
Due to the crystal's lattice structure, the researchers expected to see three differently shaped elliptical orbits. Instead they found that all the electron orbits spontaneously lined up in the same direction, or "nematic" order. The researchers determined that this behavior occurred because the strong magnetic field caused electrons to interact with each other in ways that disrupted the symmetry of the underlying lattice.
"It is as if spontaneously the electrons decided, 'It would lower our energy if we all picked one particular direction in the crystal and deformed our motion in that direction,'" Yazdani said.
Spontaneous broken symmetries are an active area of study thought to underlie physical properties such as high-temperature superconductivity, which enables electrons to flow without resistance.
Prior to directly imaging the behavior of these electrons in magnetic fields, researchers had hints of this behavior, which they call a nematic quantum Hall liquid, from other types of experiments, but the study is the first direct measurement.
"People have been looking at these states in a bunch of different contexts and this experiment represents a new way of observing them," said UT Austin's Allan MacDonald, who contributed theoretical understanding to the study along with graduate student Fengcheng Wu, who is now at Argonne National Laboratory.
"I'd done some work on a similar system together with former graduate students," said MacDonald. "When Yazdani's group showed me what they saw, I immediately recognized that they had identified a state that we had predicted, but in a completely unexpected way. It was quite a happy surprise."
Funding for the study was provided by the Gordon and Betty Moore Foundation, the U.S. Department of Energy, the National Science Foundation through the Princeton Center for Complex Materials, the U.S. Army Research Office, and the Eric and Wendy Schmidt Transformative Technology Fund at Princeton.
The study, "Observation of a nematic quantum Hall liquid on the surface of bismuth," was published Oct. 21 in the journal Science.
This post was adapted from a press release by Princeton University.
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