New Study Shows Ultracold Electrons Forming Quantum Fluid State

A research collaboration between Princeton University and the University of Texas-Austin has demonstrated that electrons kept at very low temperatures can spontaneously begin to travel in identical elliptical paths on the surface of a crystal of bismuth, forming a quantum fluid state. Such behavior was anticipated theoretically during the past two decades by researchers at many institutions, but this experiment, published in the journal Science, marks the first time researchers have directly imaged electron orbits in a high-magnetic field, illuminating an unusual collective behavior in electrons and suggesting new ways of manipulating the charged particles.

“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,” says Ali Yazdani, a professor of physics at Princeton who led the research. “The other big finding is that this is the first time the orbits of electrons moving in a magnetic field have been directly visualized. In fact, it is our ability to image these orbits that allowed us to detect the formation of this strange quantum liquid.”

Fundamental explorations of materials may provide the basis for faster and more efficient electronic technologies, according to the researchers. 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 with two-dimensional materials that 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.”

The Princeton experiment demonstrated a rare situation where electrons spontaneously occupy one valley or another, according to the researchers. The team used a scanning tunneling microscope to visualize electrons on the surface of a bismuth crystal cooled to extremely low temperatures where quantum behaviors can be observed. Because electrons are too small to be seen, the scanning tunneling microscope has a miniscule electrically charged needle that detects electrons as it scans the crystal surface.

Bismuth has relatively few electrons, making it ideal for watching what happens to a flow of electrons subjected to a high magnetic field. 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. The researchers found that they could measure the properties, or wave functions, of these orbits, giving them an important tool for studying the two-dimensional soup of electrons on the surface of the crystal.

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; they 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 says. “What was anticipated but never demonstrated is that we can turn the electron fluid into this nematic fluid, with a preferred orientation, by changing the interaction between electrons. By adjusting the strength of the magnetic field, you can force the electrons to interact strongly and actually see them break the symmetry of the surface of the crystal by choosing a particular orientation collectively.”

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 this study is the first direct measurement.