Researchers demonstrate existence of new form of electronic matter

Lois Yoksoulian for the Illinois News Bureau
3/14/2018

Researchers Kitt Peterson, left, Taylor Hughes, Wladimir Benalcazar and Gaurav Bahl are the first to demonstrate a new phase of matter called quadrupole topological insulators that was recently predicted using theoretical physics.
Photo by L. Brian Stauffer
Researchers Kitt Peterson, left, Taylor Hughes, Wladimir Benalcazar and Gaurav Bahl are the first to demonstrate a new phase of matter called quadrupole topological insulators that was recently predicted using theoretical physics. Photo by L. Brian Stauffer
Researchers have produced a “human scale” demonstration of a new phase of matter called quadrupole topological insulators that was recently predicted using theoretical physics. These are the first experimental findings to validate this theory.

The researchers report their findings in the journal Nature.

The team’s work with QTIs was born out of the decade-old understanding of the properties of a class of materials called topological insulators. “TIs are electrical insulators on the inside and conductors along their boundaries, and may hold great potential for helping build low-power, robust computers and devices, all defined at the atomic scale,” said mechanical science and engineering professor and senior investigator Gaurav Bahl.

The uncommon properties of TIs make them a special form of electronic matter. “Collections of electrons can form their own phases within materials. These can be familiar solid, liquid and gas phases like water, but they can also sometimes form more unusual phases like a TI,” said co-author and physics professor Taylor Hughes.

TIs typically exist in crystalline materials and other studies confirm TI phases present in naturally occurring crystals, but there are still many theoretical predictions that need to be confirmed, Hughes said.

A single circuit board, foreground, that when joined with others forms the experimental array of the quadrupole topological insulator.

Photo by L. Brian Stauffer
A single circuit board, foreground, that when joined with others forms the experimental array of the quadrupole topological insulator. Photo by L. Brian Stauffer
One such prediction was the existence of a new type of TI having an electrical property known as a quadrupole moment. “Electrons are single particles that carry charge in a material,” said physics graduate student Wladimir Benalcazar. “We found that electrons in crystals can collectively arrange to give rise not only to charge dipole units – that is, pairings of positive and negative charges – but also high-order multipoles in which four or eight charges are brought together into a unit. The simplest member of these higher-order classes are quadrupoles in which two positive and two negative charges are coupled.”
It is not currently feasible to engineer a material atom by atom, let alone control the quadrupolar behavior of electrons. Instead, the team built a workable-scale analogue of a QTI using a material created from printed circuit boards. Each circuit board holds a square of four identical resonators – devices that absorb electromagnetic radiation at a specific frequency. The boards are arranged in a grid pattern to create the full crystal analogue.

“Each resonator behaves as an atom, and the connections between them behave as bonds between atoms,” said Kitt Peterson, the lead author and an electrical engineering graduate student. “We apply microwave radiation to the system and measure how much is absorbed by each resonator, which tells us about how electrons would behave in an analogous crystal. The more microwave radiation is absorbed by a resonator, the more likely it is to find an electron on the corresponding atom.”

The detail that makes this a QTI and not a TI is a result of the specifics of the connections between resonators, the researchers said.

“The edges of a QTI are not conductive like you would see in a typical TI,” Bahl said, “Instead only the corners are active, that is, the edges of the edges, and are analogous to the four localized point charges that would form what is known as a quadrupole moment.  Exactly as Taylor and Wladimir predicted.”

“We measured how much microwave radiation each resonator within our QTI absorbed, confirming the resonant states in a precise frequency range and located precisely in the corners,” Peterson said. “This pointed to the existence of predicted protected states that would be filled by electrons to form four corner charges.”

Those corner charges of this new phase of electronic matter may be capable of storing data for communications and computing. “That may not seem realistic using our ‘human scale’ model,” Hughes said. “However, when we think of QTIs on the atomic scale, tremendous possibilities become apparent for devices that perform computation and information processing, possibly even at scales below that we can achieve today.”   

The researchers said the agreement between experiment and prediction offered promise that scientists are beginning to understand the physics of QTIs well enough for practical use.

 “As theoretical physicists, Wladimir and I could predict the existence of this new form of matter, but no material has been found to have these properties so far,” Hughes said. “Collaborating with engineers helped turn our prediction into reality.”

The National Science Foundation and U.S. Office of Naval Research supported this study. The conclusions presented are those of the researchers and not necessarily those of the funding agencies.

 

Recent News

  • Research
  • Condensed Matter Physics
  • Condensed Matter Experiment
  • Condensed Matter Theory

One of the greatest mysteries in condensed matter physics is the exact relationship between charge order and superconductivity in cuprate superconductors. In superconductors, electrons move freely through the material—there is zero resistance when it’s cooled below its critical temperature. However, the cuprates simultaneously exhibit superconductivity and charge order in patterns of alternating stripes. This is paradoxical in that charge order describes areas of confined electrons. How can superconductivity and charge order coexist?  

Now researchers at the University of Illinois at Urbana-Champaign, collaborating with scientists at the SLAC National Accelerator Laboratory, have shed new light on how these disparate states can exist adjacent to one another. Illinois Physics post-doctoral researcher Matteo Mitrano, Professor Peter Abbamonte, and their team applied a new x-ray scattering technique, time-resolved resonant soft x-ray scattering, taking advantage of the state-of-the-art equipment at SLAC. This method enabled the scientists to probe the striped charge order phase with an unprecedented energy resolution. This is the first time this has been done at an energy scale relevant to superconductivity.

  • Alumni News
  • In the Media

Will Hubin was one of those kids whose wallpaper and bed sheets were covered in airplanes and who loved building model airplanes. By the time he took his first flight in the late 1940s, he was hooked.

Now, he shares his passion for planes with children by taking them for their first flight, at no charge, in his four-seat 2008 Diamond DA-40 aircraft through the local Experimental Aircraft Association’s Young Eagles program.

“It’s a lot of fun and pretty rewarding. Anyone who loves flying likes to introduce others to it. It’s true of anything, any hobbyist. Some will talk constantly but they’re ecstatic,” said Hubin, a retired Kent State University physics professor.

Hubin learned to fly in 1962 when he was earning a doctorate in physics at the University of Illinois and has been flying ever since, adding commercial, instrument, instructor, multi-engine and seaplane ratings.

  • Research
  • Theoretical Biological Physics
  • Biological Physics
  • Biophysics

While watching the production of porous membranes used for DNA sorting and sequencing, University of Illinois researchers wondered how tiny steplike defects formed during fabrication could be used to improve molecule transport. They found that the defects – formed by overlapping layers of membrane – make a big difference in how molecules move along a membrane surface. Instead of trying to fix these flaws, the team set out to use them to help direct molecules into the membrane pores.

Their findings are published in the journal Nature Nanotechnology.

Nanopore membranes have generated interest in biomedical research because they help researchers investigate individual molecules – atom by atom – by pulling them through pores for physical and chemical characterization. This technology could ultimately lead to devices that can quickly sequence DNA, RNA or proteins for personalized medicine.

  • In Memoriam

We are saddened to report that John Robert Schrieffer, Nobel laureate and alumnus of the Department of Physics at the University of Illinois at Urbana-Champaign, passed away on July 27, 2019, in Tallahassee, Florida. He was 88 years old.

Schrieffer was the “S” in the famous BCS theory of superconductivity, one of the towering achievements of 20th century theoretical physics, which he co-developed with his Ph.D advisor Professor John Bardeen and postdoctoral colleague Dr. Leon N. Cooper. At the time that Schrieffer began working with Bardeen and Cooper, superconductivity was regarded as one of the major challenges in physics. Since the discovery of the hallmark feature of superconductivity in 1911—the zero resistance apparently experienced by a current in a metal at temperatures near absolute zero—a long list of famous theoretical physicists had attempted to understand the phenomenon, including Albert Einstein, Niels Bohr, Richard Feynman, Lev Landau, Felix Bloch, Werner Heisenberg and John Bardeen himself (who was awarded the Nobel Prize for his co-invention of the transistor at around the time that Schrieffer began working with him in 1956).