Quantum Liquids on Crystals Could Lead to Future Electronics

Tuesday, October 25, 2016

Quantum Liquids on Crystals Could Lead to Future Electronics

Future Technology 

Using a microscope capable of detecting electrons, researchers have imaged strange elliptical orbits of electrons on the surface of a crystal of bismuth. The unusual collective behavior in electrons that suggests new ways of manipulating the charged particles, and may provide the basis for faster and more efficient electronic technologies.

A new experiment has directly imaged electron orbits in a high-magnetic field for the first time, illuminating an unusual collective behavior in electrons and suggesting new ways of manipulating the charged particles.

The study, conducted by researchers at Princeton University and the University of Texas-Austin was published recently 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 Princeton and other universities.

"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."
"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, the leader of 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," Yazdani said. "In fact, it is our ability to image these orbits that allowed us to detect the formation of this strange quantum liquid."

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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.

bismuth crystalThe research team 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. 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, which makes it ideal for watching what happens to a flow of electrons subjected to a high magnetic field. Despite its purity, the crystal the team grew contained some defects. About 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. 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 the researchers 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.

"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," he said. "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."

A nematic liquid crystal is a transparent or translucent liquid that causes the polarizationof light waves to change as the waves pass through the liquid. The extent of the change in polarization depends on the intensity of an applied electric field . Nematic comes from a Greek prefix nemato meaning threadlike. It is used in this case because the molecules in the liquid align themselves into a threadlike shape. Nematic liquid crystals are used in twisted nematic displays, the most common form of liquid crystal display.

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.

SOURCE  Princeton University

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