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

Rick Kubetz, Engineering at Illinois
3/14/2016

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

  • Research
  • Condensed Matter Theory

We analyze the interplay between a d-wave uniform superconducting and a pair-density-wave (PDW) order parameter in the neighborhood of a vortex. We develop a phenomenological nonlinear sigma model, solve the saddle-point equation for the order-parameter configuration, and compute the resulting local density of states in the vortex halo. The intertwining of the two superconducting orders leads to a charge density modulation with the same periodicity as the PDW, which is twice the period of the charge density wave that arises as a second harmonic of the PDW itself. We discuss key features of the charge density modulation that can be directly compared with recent results from scanning tunneling microscopy and speculate on the role PDW order may play in the global phase diagram of the hole-doped cuprates.

  • Research
  • Condensed Matter Physics

Now, a novel sample-growing technique developed at the U. of I. has overcome these obstacles. Developed by physics professor James Eckstein in collaboration with physics professor Tai-Chang Chiang, the new “flip-chip” TI/SC sample-growing technique allowed the scientists to produce layered thin-films of the well-studied TI bismuth selenide on top of the prototypical SC niobium—despite their incompatible crystalline lattice structures and the highly reactive nature of niobium.

These two materials taken together are ideal for probing fundamental aspects of the TI/SC physics, according to Chiang: “This is arguably the simplest example of a TI/SC in terms of the electronic and chemical structures. And the SC we used has the highest transition temperature among all elements in the periodic table, which makes the physics more accessible. This is really ideal; it provides a simpler, more accessible basis for exploring the basics of topological superconductivity,” Chiang comments.