Getting at the many-particle problem—"Urbana style"

Rick Kubetz, Engineering at Illinois
3/14/2016 11:55 AM

Professor David Ceperley
Professor David Ceperley
Professor Brian DeMarco
Professor Brian DeMarco
One of the grandest and most impactful frontiers of physics is the quantum many-particle problem. Scientists today still don’t well understand what happens when many quantum particles—like electrons, protons, and neutrons—come together and interact with each other. This problem spans some of the largest scales in the universe, like understanding the nuclear matter in neutron stars, to the smallest, such as electron transport in photosynthesis and the quarks and gluons inside a proton. 

Now, two teams at the University of Illinois at Urbana Champaign, working together and attacking the problem from different physics disciplines, have shed new light on our understanding of disordered quantum materials. Professor Brian DeMarco and his group perform innovative experiments in atomic, molecular, and optical physics using ultracold atoms trapped in an optical lattice to simulate phenomena in solid materials. Professor David Ceperley and his group work in theoretical condensed matter physics; they perform supercomputing simulations to model phenomena in solid materials.

The two groups collaborated across physics disciplines to understand how disorder in a quantum material gives rise to an exotic quantum state called a Bose glass. The results are published in Nature Physics in the article, “Probing the Bose glass–superfluid transition using quantum quenches of disorder.”

Recently graduated doctoral students Carolyn Meldgin from DeMarco’s group and Ushnish Ray from Ceperley’s group are the lead authors on the paper.

“A Bose glass is a strange and poorly understood insulator that can occur when disorder is added to a superfluid or superconductor,” Meldgin explains.

This figure illustrates puddles of localized quasi-condensates found using a quantum Monte Carlo simulation of trapped atoms in a disordered lattice.  The individual puddles, consisting of 10-20 particles each, are incoherent relative to each other.  The Bose glass is composed of these puddle-like structures.Image courtesy of Ushnish Ray, University of Illinois at Urbana-Champaign
This figure illustrates puddles of localized quasi-condensates found using a quantum Monte Carlo simulation of trapped atoms in a disordered lattice. The individual puddles, consisting of 10-20 particles each, are incoherent relative to each other. The Bose glass is composed of these puddle-like structures.Image courtesy of Ushnish Ray, University of Illinois at Urbana-Champaign
In her experiments, Meldgin was able to use optical disorder to induce a Bose glass, and Ray exactly simulated the experiment using the Titan supercomputer. Ray’s simulation is the largest ever carried out for a disordered quantum material that exactly describes an experiment.

“In both cases, the same amount of disorder is required to turn a superfluid into a Bose glass,” Ray states. “This is critically important to our understanding of disordered quantum materials, which are ubiquitous, since disorder is difficult to avoid.  It also has important implications for quantum annealers, like the D-Wave Systems device.”

In DeMarco’s group’s experiments, the atoms in a gas are cooled to just billionths of a degree above absolute zero temperature in order to experimentally simulate models of materials such as high-temperature superconductors. The atoms play the role of electrons in a material, and the analog of material parameters (like disorder) are completely controlled and known and can be changed every 90-second experimental cycle. Measurements on the atoms are used to expose new physics and test theories.

Demarco comments, “This result was a fantastic collaboration between theory and experiment. Meldgin and Ray were able to show something startling: that a dynamic probe in the experiment connects to the equilibrium computer simulations.  In both cases, the same amount of disorder is required to turn a superconductor into a Bose glass.”

In their work, Ceperley’s group achieved the largest-scale computer simulations possible of a disordered quantum many-particle system on the biggest supercomputers in existence. These computer-generated models are able to simulate relatively large numbers of particles, such as the 30,000 atoms used in DeMarco’s experiments.

 “In most cases, we lack predictive power, because these problems are not readily computable—a classical computer requires exponentially costly resources to simulate many quantum systems” explains Ceperley. “A key example of this problem with practical challenges lies with materials such as high-temperature superconductors. Even armed with the chemical composition and structure of these materials, it is almost impossible to predict today at what temperature they will super-conduct. This new finding is an important step forward.”

Meldgin and Ray are featured in a video (https://www.youtube.com/watch?v=c3XE0X-Mh0k), produced by the American Physical Society (APS), highlighting this interdisciplinary “Urbana style” of approaching complex physics problems using tightly knit theory and experiment.

Watch the 2015 APS TV video featuring Carolyn Melgdin (left) and Ushnish Ray's work.
Watch the 2015 APS TV video featuring Carolyn Melgdin (left) and Ushnish Ray's work.

Ray and DeMarco will speak about how their calculations and measurements match up in paired invited talks at the APS March Meeting on Tuesday, March 15.

 

Recent News

  • Accolades
  • Alumni News

Congratulations to Physics Illinois alumnus M. George Craford on being presented today with the IEEE Edison Medal of the Institute of Electrical and Electronics Engineers. The medal is awarded annually in recognition of a career of meritorious achievement in electrical science, electrical engineering, or the electrical arts. The citation reads, “for a lifetime of pioneering contributions to the development and commercialization of visible LED materials and devices.”

 

Craford is best known for his invention of the first yellow light emitting diode (LED). During his career, he developed and commercialized the technologies yielding the highest-brightness yellow, amber, and red LEDs as well as world-class blue LEDs. He is a pioneer whose contributions to his field are lasting.

  • Research

While heritable genetic mutations can alter phenotypic traits and enable populations to adapt to their environment, adaptation is frequently limited by trade-offs: a mutation advantageous to one trait might be detrimental to another.

Because of the interplay between the selection pressures present in complex environments and the trade-offs constraining phenotypes, predicting evolutionary dynamics is difficult.

Researchers at the University of Illinois at Urbana-Champaign have shown how evolutionary dynamics proceed when selection acts on two traits governed by a trade-off. The results move the life sciences a step closer to understanding the full complexity of evolution at the cellular level.

  • Research
  • Condensed Matter Physics

Since the discovery two decades ago of the unconventional topological superconductor Sr2RuO4, scientists have extensively investigated its properties at temperatures below its 1 K critical temperature (Tc), at which a phase transition from a metal to a superconducting state occurs. Now experiments done at the University of Illinois at Urbana-Champaign in the Madhavan and Abbamonte laboratories, in collaboration with researchers at six institutions in the U.S., Canada, United Kingdom, and Japan, have shed new light on the electronic properties of this material at temperatures 4 K above Tc. The team’s findings may elucidate yet-unresolved questions about Sr2RuO4’s emergent properties in the superconducting state.

  • Research
  • AMO/Quantum Physics

Using an atomic quantum simulator, scientists at the University of Illinois at Urbana-Champaign have achieved the first-ever direct observation of chiral currents in the model topological insulator, the 2-D integer quantum Hall system.

Topological Insulators (TIs) are arguably the most promising class of materials discovered in recent years, with many potential applications theorized. That’s because TIs exhibit a special quality: the surface of the material conducts electricity, while the bulk acts as an insulator. Over the last decade, scientists have extensively probed the microscopic properties of TIs, to better understand the fundamental physics that govern their peculiar behavior.

Atomic quantum simulation has proven an important tool for probing the characteristics of TIs, because it allows researchers greater control and greater possibilities for exploring regimes not currently accessible in real materials. Finely tuned laser beams are used to trap ultracold rubidium atoms (about a billion times colder than room temperature) in a lattice structure that precisely simulates the structure of ideal materials.