topology

Matter—all the stuff we see around us—can be classified into familiar phases: our chairs are solid, our coffee is liquid, and the oxygen we breathe is a gas. This grouping obscures the nitty gritty details of what each molecule or atom is up to and reduces all that complexity down to a few main features that are most salient in our everyday lives.

On the cover of the Pink Floyd album Dark Side of the Moon, a prism splits a ray of light into all the colors of the rainbow. This multicolored medley, which owes its emergence to the fact that light travels as a wave, is almost always hiding in plain sight; a prism simply reveals that it was there.

When you brush your hair in the morning, there’s a pretty good chance you’re not thinking about quantum physics. But the way your hair swirls as you brush is actually related to some features of the quantum world. Important properties of quantum particles are described by topology—a field of mathematics that classifies objects according to how many holes they have. This is not merely a question of fashion. In physical materials, topology can determine many interesting properties. Now, a team of JQI researchers has engineered a new kind of topological matter—one with a single whorl—by breaking free from the constraints of crystalline solids. They managed to do this by grooming their atomic states into a whorl situated in an abstract, infinite plane, rather than a coconut or donut shape.

In this episode of Relatively Certain, Dina Genkina sits down with JQI Fellow Jay Sau, an associate professor of physics at UMD, and Johnpierre Paglione, a professor of physics at UMD and the director of the Quantum Materials Center.

Deep within solids, individual electrons zip around on a nanoscale highway paved with atoms. For the most part, these electrons avoid one another, kept in separate lanes by their mutual repulsion. But vibrations in the atomic road can blur their lanes and sometimes allow the tiny particles to pair up. The result is smooth and lossless travel, and it’s one way to create superconductivity.But there are other, less common ways to achieve this effect. Scientists from the University of Maryland (UMD), the University of California, Irvine (UCI) and Fudan University have now shown that tiny magnetic tremors lead to superconductivity in a material made from metallic nano-layers. And, beyond that, the resulting electron pairs shatter a fundamental symmetry between past and future. Although the material is a known superconductor, these researchers provide a theoretical model and measurement, which, for the first time, unambiguously reveals the material’s exotic nature.

The 2015 discovery of a Weyl semimetal—and the Weyl fermions it harbored—provoked a flurry of activity from researchers around the globe. A quick glance at a recent physics journal or the online arXiv preprint server testifies to the topic’s popularity. The arXiv alone has had more than 200 papers on Weyl semimetals posted in 2016.Researchers at JQI and the Condensed Matter Theory Center (CMTC) at the University of Maryland have been interested in Weyl physics since before last summer’s discovery, publishing 18 papers on the topic over the past two years. In all, more than a dozen scientists at Maryland have been working to understand the fundamental properties of these curious new materials.

For decades, particle accelerators have grabbed headlines while smashing matter together at faster and faster speeds. But in recent years, alongside the progress in high-energy experiments, another realm of physics has been taking its own exciting strides forward.That realm, which researchers call condensed matter physics, studies chunks of matter moving decidedly slower than the protons in the LHC. In fact, the materials under study—typically solids or liquids—are usually sitting still. That doesn't make them boring, though. Their calm appearance can often hide exotic physics that arises from their microscopic activity."In condensed matter physics, the energy scales are much lower," says Pallab Goswami, a postdoctoral researcher at JQI and the Condensed Matter Theory Center (CMTC) at the University of Maryland. "We want to go to lower energies and find new phenomena, which is exactly the opposite of what is done in particle physics."

A trio of researchers who laid the foundation for understanding numerous exotic phases of matter have split the 2016 Nobel Prize in Physics.The Royal Swedish Academy of Sciences awarded the prize "for theoretical discoveries of topological phase transitions and topological phases of matter" to three laureates: David Thouless of the University of Washington, Duncan Haldane of Princeton University and Michael Kosterlitz of Brown University."It is a very, very well-deserved prize," says JQI Fellow and CMTC Director Sankar Das Sarma. "This work led to an extremely exciting area and that excitement is still growing."The research behind the prize "illustrates, in a very nice way, the interplay between physics and mathematics," Thors Hans Hansson, a physicist at Stockholm Univsersity, said in an interview following the announcement ceremony. He explained the mathematics behind the prize during the ceremony using a cinnamon bun, a bagel and a pretzel.

When it comes to quantum physics, light and matter are not so different. Under certain circumstances, negatively charged electrons can fall into a coordinated dance that allows them to carry a current through a material laced with imperfections. That motion, which can only occur if electrons are confined to a two-dimensional plane, arises due to a phenomenon known as the quantum Hall effect.Researchers, led by Mohammad Hafezi, a JQI Fellow and assistant professor in the Department of Electrical and Computer Engineering at the University of Maryland, have made the first direct measurement that characterizes this exotic physics in a photonic platform. The research was published online Feb. 22 in Nature Photonics. These techniques may be extended to more complex systems, such as one in which strong interactions and long-range quantum correlations play a role.

The quantum Hall effect, discovered in the early 1980s, is a phenomenon that was observed in a two-dimensional gas of electrons existing at the interface between two semiconductor layers. Subject to the severe criteria of very high material purity and very low temperatures, the electrons, when under the influence of a large magnetic field, will organize themselves into an ensemble state featuring remarkable properties.
Many physicists believe that quantum Hall physics is not unique to electrons, and thus it should be possible to observe this behavior elsewhere, such as in a collection of trapped ultracold atoms. Experiments at JQI and elsewhere are being planned to do just that. On the theoretical front, scientists at JQI and University of Maryland have also made progress, which they describe in the journal Physical Review Letters. The result, to be summarized here, proposes using quantum matter made from a neutral atomic gas, instead of electrons. In this new design, elusive exotic states that are predicted to occur in certain quantum Hall systems should emerge. These states, known as parafermionic zero modes, may be useful in building robust quantum gates.