Physics Faculty Promotions

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Category: Department News

Published: Monday, July 01 2019 07:54

Vladimir Manucharyan, who was promoted to the rank of Associate Professor, is an experimentalist researching solid state physics with a focus on fundamental aspects of superconductivity and quantum technology applications. He is a Fellow of the Joint Quantum Institute and a member of the Physics Frontier Center. While at UMD he has received a Sloan Research Fellowship, a DARPA Young Faculty Award, a National Science Foundation CAREER Award and a Google Faculty Research Award. He earned his Ph.D. at Yale University in 2012, researching quantum information with superconducting qubits, and was elected a Junior Fellow of the Harvard Society of Fellows for the term 2010-2013 before joining UMD in 2014.

Arpita Upadhyaya, who was promoted to the rank of Professor, is a biophysicist studying how physical properties of living cells are regulated to guide mechanical behaviors such as cell shape changes and force generation and how these guide physical regulation of cell function. Upadhyaya has received a Pappalardo Fellowship in Physics at the Massachusetts Institute of Technology, an Alfred P. Sloan Research Fellowship, and the Richard A. Ferrell Distinguished Faculty Fellowship from the UMD Department of Physics. She holds a doctorate from the University of Notre Dame, and in addition to her work at MIT, was a researcher at UNC Chapel Hill before joining the Department of Physics and the Institute for Physical Science and Technology (IPST) in 2006.

Jay Sau, who was promoted to the rank of Associate Professor, is a condensed matter theorist studying topological principles to create protected solid-state and cold-atomic systems for quantum information processing. He is a member of the Joint Quantum Institute and Condensed Matter Theory Center, and has won an Alfred P. Sloan Research Fellowship and a National Science Foundation CAREER Award. He also received the Richard A. Ferrell Distinguished Faculty Fellowship from the UMD Department of Physics. Sau holds a doctorate from the University of California at Berkeley, and held postdoctoral appointments at Berkeley, the University of Maryland and Harvard University before his appointment as a UMD Assistant Professor in 2013.

Alberto Belloni, who was promoted to the rank of Associate Professor, is an experimental particle physicist who works on the Compact Muon Solenoid (CMS) experiment at CERN’s Large Hadron Collider. He has held several posts within the collaboration, and in September 2018 became convener of the CMS Standard Model Physics Vector-Bosons subgroup of about 100 physicists, dedicated to inclusive electroweak boson studies. In August 2018 he received the Richard A. Ferrell Distinguished Faculty Fellowship from the UMD Department of Physics. Belloni holds a doctorate from the Massachusetts Institute of Technology, and was a post-doctoral associate and research associate at Harvard University before his appointment as a UMD Assistant Professor in 2013. 

Dan Lathrop Named Distinguished Scholar-Teacher

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Category: Department News

Published: Friday, June 28 2019 10:37

Professor Daniel Lathrop has been named a University of Maryland Distinguished Scholar-Teacher.  He will be recognized during the university’s annual Convocation ceremony on Wed., September 18 at 3 p.m. in the Memorial Chapel. A reception will follow in the Chapel Garden.

The Distinguished Scholar-Teacher Program, established in 1978, honors a small number of faculty members each year who have demonstrated notable success in both scholarship and teaching. Distinguished Scholar-Teachers receive an honorarium of $5,000 to support their professional activities, and make public presentations on a topic within their scholarly discipline. Lathrop will give his lecture on October 29 at 4 p.m. in the PSC lobby.  

An interdisciplinary researcher with additional joint appointments in UMD’s Department of Geology, Institute for Research in Electronics and Applied Physics, and Institute for Physical Science and Technology, Lathrop studies turbulence—the chaotic motion of fluids such as air or water. Understanding turbulence is crucial to studying phenomena such as the flow of air over an airplane wing, water flowing down a drain or the behavior of Earth’s molten outer core.

Lathrop is particularly interested in Earth’s core, which generates a magnetic field that shields Earth from the sun’s radiation, allowing life to exist. Geological records show that the Earth’s magnetic field has reversed polarity numerous times in the past. When it does so, the magnetic field weakens, leaving the planet unprotected. However, scientists do not understand how Earth’s magnetic field is generated and what causes it to reverse. To investigate this question, Lathrop constructed a 3-meter spinning sphere filled with molten liquid sodium to model Earth’s core.

Lathrop was elected a fellow of the American Association for the Advancement of Science in 2011 and a fellow of the American Physical Society in 2005. He also received a Presidential Early Career Award from the National Science Foundation in 1997.

In addition to his research career, Lathrop is an enthusiastic mentor. Since joining UMD in 1997, he has mentored nine postdoctoral scholars, 19 Ph.D. students, 10 M.S. students, and more than 60 undergraduate and high school students.

“It is rare—and wonderful—to find a teacher and mentor as fully engaged and enthusiastic as is Professor Lathrop,” wrote Steven Rolston, professor and chair of the UMD Department of Physics, in a letter nominating Lathrop for the award. “He provides not only scientific knowledge but his deep-down love of the lab and of learning.”

Lathrop earned his B.A. in physics from the University of California, Berkeley, in 1987 and his Ph.D. in physics from the University of Texas at Austin in 1991.

Distinguished Scholar Teachers in the Department of Physics

EPS to Honor DØ, CDF Collaborations for Top Quark Detection

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Category: Department News

Published: Tuesday, June 25 2019 11:25

The European Physical Society has announced that the DØ and CDF collaborations will receive the 2019 High Energy and Particle Physics Prize for "the discovery of the top quark and the detailed measurement of its properties".

In 1973, Makoto Kobayashi and Toshihide Maskawa predicted a top and bottom quark to explain CP violation. Four years later, Leon Lederman’s team at Fermilab discovered the bottom quark. But because of the complexity required to find the massive, short-lived (5x10-25 second) top quark, nearly two decades ensued before its confirmation at Fermilab’s Tevatron proton-antiproton collider, home to both DØ and CDF. The Tevatron was the world’s most powerful accelerator before the Large Hadron Collider at CERN opened in 2009.

UMD physicists played important roles in the top quark discovery. In the DØ collaboration, Nick Hadley was co-convenor of the Top Group at the time of detection. Sarah Eno’s precise measurement of the decay width and mass of the electroweak W boson helped predict the mass of the top quark.  Drew Baden devised an analysis technique, called HT, which allowed the experiment to separate the top quark signal for the much more numerous background events. Before coming to Maryland, Greg Sullivan was physics co-convener of the dilepton group for the CDF collaboration, and Kara Hoffman was also a member of the CDF collaboration.

“The top quark remains one of nature’s most interesting particles. It has the mass of a gold atom, but behaves like an elementary particle with a volume at least a billion times smaller than a proton,” said Hadley.

The EPS prize will be formally awarded at the EPS Conference on July 15 in Ghent.

Perfect Quantum Portal Emerges at Exotic Interface

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Category: Research News

Published: Wednesday, June 19 2019 14:08

In Klein tunneling, an electron can transit perfectly through a barrier. In a new experiment, researchers observed the Klein tunneling of electrons into a special kind of superconductor. (Credit: E. Edwards/JQI)

A junction between an ordinary metal and a special kind of superconductor has provided a robust platform to observe Klein tunneling.

Researchers at the University of Maryland have captured the most direct evidence to date of a quantum quirk that allows particles to tunnel through a barrier like it’s not even there. The result, featured on the cover of the June 20, 2019 issue of the journal Nature, may enable engineers to design more uniform components for future quantum computers, quantum sensors and other devices.

The new experiment is an observation of Klein tunneling, a special case of a more ordinary quantum phenomenon. In the quantum world, tunneling allows particles like electrons to pass through a barrier even if they don’t have enough energy to actually climb over it. A taller barrier usually makes this harder and lets fewer particles through.

Klein tunneling occurs when the barrier becomes completely transparent, opening up a portal that particles can traverse regardless of the barrier’s height. Scientists and engineers from UMD’s Center for Nanophysics and Advanced Materials (CNAM), the Joint Quantum Institute (JQI) and the Condensed Matter Theory Center (CMTC), with appointments in UMD’s Department of Materials Science and Engineering and Department of Physics, have made the most compelling measurements yet of the effect.

“Klein tunneling was originally a relativistic effect, first predicted almost a hundred years ago,” says Ichiro Takeuchi, a professor of materials science and engineering (MSE) at UMD and the senior author of the new study. “Until recently, though, you could not observe it.”

It was nearly impossible to collect evidence for Klein tunneling where it was first predicted—the world of high-energy quantum particles moving close to the speed of light. But in the past several decades, scientists have discovered that some of the rules governing fast-moving quantum particles also apply to the comparatively sluggish particles traveling near the surface of some unusual materials.

One such material—which researchers used in the new study—is samarium hexaboride (SmB6), a substance that becomes a topological insulator at low temperatures. In a normal insulator like wood, rubber or air, electrons are trapped, unable to move even when voltage is applied. Thus, unlike their free-roaming comrades in a metal wire, electrons in an insulator can’t conduct a current.

Topological insulators such as SmB6 behave like hybrid materials. At low enough temperatures, the interior of SmB6 is an insulator, but the surface is metallic and allows electrons some freedom to move around. Additionally, the direction that the electrons move becomes locked to an intrinsic quantum property called spin that can be oriented up or down. Electrons moving to the right will always have their spin pointing up, for example, and electrons moving left will have their spin pointing down.

The metallic surface of SmB6 would not have been enough to spot Klein tunneling, though. It turned out that Takeuchi and colleagues needed to transform the surface of SmB6 into a superconductor—a material that can conduct electrical current without any resistance.

To turn SmB6 into a superconductor, they put a thin film of it atop a layer of yttrium hexaboride (YB6). When the whole assembly was cooled to just a few degrees above absolute zero, the YB6 became a superconductor and, due to its proximity, the metallic surface of SmB6 became a superconductor, too.

It was a “piece of serendipity” that SmB6 and its yttrium-swapped relative shared the same crystal structure, says Johnpierre Paglione, a professor of physics at UMD, the director of CNAM and a co-author of the research paper. “However, the multidisciplinary team we have was one of the keys to this success. Having experts on topological physics, thin-film synthesis, spectroscopy and theoretical understanding really got us to this point,” Paglione adds.

The combination proved the right mix to observe Klein tunneling. By bringing a tiny metal tip into contact with the top of the SmB6, the team measured the transport of electrons from the tip into the superconductor. They observed a perfectly doubled conductance—a measure of how the current through a material changes as the voltage across it is varied.

“When we first observed the doubling, I didn’t believe it,” Takeuchi says. “After all, it is an unusual observation, so I asked my postdoc Seunghun Lee and research scientist Xiaohang Zhang to go back and do the experiment again.”

When Takeuchi and his experimental colleagues convinced themselves that the measurements were accurate, they didn’t initially understand the source of the doubled conductance. So they started searching for an explanation. UMD’s Victor Galitski, a JQI Fellow, a professor of physics and a member of CMTC, suggested that Klein tunneling might be involved.

“At first, it was just a hunch,” Galitski says. “But over time we grew more convinced that the Klein scenario may actually be the underlying cause of the observations.”

Valentin Stanev, an associate research scientist in MSE and a research scientist at JQI, took Galitski’s hunch and worked out a careful theory of how Klein tunneling could emerge in the SmB6 system—ultimately making predictions that matched the experimental data well.

The theory suggested that Klein tunneling manifests itself in this system as a perfect form of Andreev reflection, an effect present at every boundary between a metal and a superconductor. Andreev reflection can occur whenever an electron from the metal hops onto a superconductor. Inside the superconductor, electrons are forced to live in pairs, so when an electron hops on, it picks up a buddy.

In order to balance the electric charge before and after the hop, a particle with the opposite charge—which scientists call a hole—must reflect back into the metal. This is the hallmark of Andreev reflection: an electron goes in, a hole comes back out. And since a hole moving in one direction carries the same current as an electron moving in the opposite direction, this whole process doubles the overall conductance—the signature of Klein tunneling through a junction of a metal and a topological superconductor.

In conventional junctions between a metal and a superconductor, there are always some electrons that don’t make the hop. They scatter off the boundary, reducing the amount of Andreev reflection and preventing an exact doubling of the conductance.

But because the electrons in the surface of SmB6 have their direction of motion tied to their spin, electrons near the boundary can’t bounce back—meaning that they will always transit straight into the superconductor.

“Klein tunneling had been seen in graphene as well,” Takeuchi says. “But here, because it’s a superconductor, I would say the effect is more spectacular. You get this exact doubling and a complete cancellation of the scattering, and there is no analog of that in the graphene experiment.”

Junctions between superconductors and other materials are ingredients in some proposed quantum computer architectures, as well as in precision sensing devices. The bane of these components has always been that each junction is slightly different, Takeuchi says, requiring endless tuning and calibration to reach the best performance. But with Klein tunneling in SmB6, researchers might finally have an antidote to that irregularity.

“In electronics, device-to-device spread is the number one enemy,” Takeuchi says. “Here is a phenomenon that gets rid of the variability.”

Story by Chris Cesare

In addition to Takeuchi, Paglione, Lee, Zhang, Galitski and Stanev, co-authors of the research paper include Drew Stasak, a former research assistant in MSE; Jack Flowers, a former graduate student in MSE; Joshua S. Higgins, a research scientist in CNAM and the Department of Physics; Sheng Dai, a research fellow in the department of chemical engineering and materials science at the University of California, Irvine (UCI); Thomas Blum, a graduate student in physics and astronomy at UCI; Xiaoqing Pan, a professor of chemical engineering and materials science and of physics and astronomy at UCI; Victor M. Yakovenko, a JQI Fellow, professor of physics at UMD and a member of CMTC; and Richard L. Greene, a professor of physics at UMD and a member of CNAM.