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Superconductivity in the quantum Hall regime

Abstract:

One of the promising routes towards creating novel topological states and excitations is to combine superconductivity with quantum Hall (QH) effect. However, signatures of superconductivity in the QH regime remain scarce, and a superconducting current through a QH weak link has until recently eluded experimental observation. Here, we explore high mobility graphene/boron nitride heterostructures contacted by type II superconducting electrodes that could withstand magnetic fields of a few Tesla.

“Nonstandard Finite Difference (NSFD) Schemes for Differential Equations: Methodology and Applications”

Abstract

NSFD schemes are based on a methodology not centered on the a priori satisfaction of particular mathematical requirements. A central and critical feature is that the discretization equations be dynamical consistent with the differential equations with regard to specific properties of the (physical) original system. Major consequences include modification of the step-size function and the non-local discrete representations of function. The general procedures will be illustrated by means of several explicit examples.

Biographical Summary

More Workers Working Might Not Get More Work Done, Ants (and Robots) Show

Thursday, August 16, 2018

For ants and robots operating in confined spaces like tunnels, having more workers does not necessarily mean getting more work done. Just as too many cooks in a kitchen get in each other’s way, having too many robots in tunnels creates clogs that can bring the work to a grinding halt.

A study published August 17 in the journal Science shows that in fire ant colonies, a small number of workers does most of the digging, leaving the other ants to look somewhat less than industrious. For digging nest tunnels, this less busy approach gets the job done without ant traffic jams – ensuring smooth excavation flow. Researchers found that applying the ant optimization strategy to autonomous robots avoids mechanized clogs and gets the work done with the least amount of energy.

Optimizing the activity of autonomous underground robots could be useful for tasks such as disaster recovery, mining or even digging underground shelters for future planetary explorers. The research was supported by the National Science Foundation’s Physics of Living Systems program, the Army Research Office and the Dunn Family Professorship.

“We noticed that if you have 150 ants in a container, only 10 or 15 of them will actually be digging in the tunnels at any given time,” said Daniel Goldman, a professor in the School of Physics at the Georgia Institute of Technology. “We wanted to know why, and to understand how basic laws of physics might be at work. We found a functional, community benefit to this seeming inequality in the work environment. Without it, digging just doesn’t get done.”

By monitoring the activities of 30 ants that had been painted to identify each individual, Goldman and colleagues, including former postdoctoral fellow Daria Monaenkova and Ph.D. student Bahnisikha Dutta, discovered that just 30 percent of the ants were doing 70 percent of the work – an inequality that seems to keep the work humming right along. However, that is apparently not because the busiest ants are the most qualified. When the researchers removed the five hardest working ants from the nest container, they saw no productivity decline as the remaining 25 continued to dig.

Having a nest is essential to fire ants, and if a colony is displaced – by a flood, for instance – the first thing the ants will do upon reaching dry land is start digging. Their tunnels are narrow, barely wide enough for two ants to pass, a design feature hypothesized to give locomotion advantages in the developing vertical tunnels. Still, the ants know how to avoid creating clogs by retreating from tunnels already occupied by other workers – and sometimes by not doing anything much at all. 

To avoid clogs and maximize digging in the absence of a leader, robots built by Goldman’s master’s degree student Vadim Linevich were programmed to capture aspects of the dawdling and retreating ants. The researchers found that as many as three robots could work effectively in a narrow horizontal tunnel digging 3D printed magnetic plastic balls that simulated sticky soil. If a fourth robot entered the tunnel, however, that produced a clog that stopped the work entirely.

“When we put four robots into a confined environment and tried to get them to dig, they immediately jammed up,” said Goldman, who is the Dunn Family Professor in the School of Physics. “While observing the ants, we were surprised to see that individuals would sometimes go to the tunnel and if they encountered even a small amount of clog, they’d just turn around and retreat. When we put those rules into combinations with the robots, that created a good strategy for digging rapidly with low amounts of energy use per robot.”

Experimentally, the research team tested three potential behaviors for the robots, which they termed “eager,” “reversal” or “lazy.” Using the eager strategy, all four robots plunged into the work – and quickly jammed up. In the reversal behavior, robots gave up and turned around when they encountered delays reaching the work site. In the lazy strategy, dawdling was encouraged.

“Eager is the best strategy if you only have three robots, but if you add a fourth, that behavior tanks because they get in each other’s way,” said Goldman. “Reversal produces relatively sane and sensible digging. It is not the fastest strategy, but there are no jams. If you look at energy consumed, lazy is the best course.” Analysis techniques based on glassy and supercooled fluids, led by former Ph.D. student Jeffrey Aguilar, gave insight into how the different strategies mitigated and prevented clog-forming clusters.

To understand what was going on and experiment with the parameters, Goldman and colleagues – including Will Savoie, a Georgia Tech Ph.D. student, Research Assistant Hui-Shun Kuan and Professor Meredith Betterton from the Department of Physics at the University of Colorado Boulder – used computer modeling known as cellular automata that has similarities to the way in which traffic engineers model the movement of cars and trucks on a highway.

“On highways, too few cars don’t provide much flow, while too many cars create a jam,” Goldman said. “There is an intermediate level where things are best, and that is called the fundamental diagram. From our modeling, we learned that the ants are working right at the peak of the diagram. The right mix of unequal work distributions and reversal behaviors has the benefit of keeping them moving at maximum efficiency without jamming.”

The ability to avoid clumping seems to meet a need that many systems have, Betterton noted. “The ants work in a sweet spot where they can dig quickly without too many clogs. We see the same physics in ant digging, simulation models, and digging by robots, which suggests that for groups of animals that need to excavate, avoiding clogs is crucial.”

The researchers used robots designed and built for the research, but they were no match for the capabilities of the ants. The ants are flexible and robust, able to squeeze past each other in confines that would cause the inflexible robots to jam. In some cases, the robots in Goldman’s lab even damaged each other while jostling into position for digging.

The research findings could be useful for space exploration where tunnels might be needed to quickly shield humans from approaching dust storms or other threats. “If you were a robot swarm on Mars and needed to dig deeply in a hurry to get away from dust storms, this strategy might help provide shelter without having perfect information about what everybody was doing,” Goldman explained. 

Beyond the potential robotics applications, the work provides insights into the complex social skills of ants and adds to the understanding of active matter. 

“Ants that live in complex subterranean environments have to develop sophisticated social rules to avoid the bad things that can happen when you have a lot of individuals in a crowded environment,” Goldman said. “We are also contributing to understanding the physics of task-oriented active matter, putting more experimental knowledge into phenomenon such as swarms.”

In addition to those already mentioned, the research included Michael Goodisman, associate professor in Georgia Tech’s School of Biological Sciences.

This research was supported by the National Science Foundation through grant numbers PoLS-0957659, PHY-1205878 and DMR-1551095 as well as a grant W911NF-13-1-0347 from the Army Research Office, and the National Academies Keck Futures Initiative. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or Army Research Office.

CITATION: J. Aguilar, et. al., “Collective clog control: optimizing traffic flow in confined biological and robophysical excavation,” (Science 2018).

Research News
Georgia Institute of Technology
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Media Relations Contact: John Toon (404-894-6986) (jtoon@gatech.edu).

Writer: John Toon

Media Contact: 

John Toon

Research News

(404) 894-6986

Summary: 

For ants and robots operating in confined spaces like tunnels, having more workers does not necessarily mean getting more work done. Just as too many cooks in a kitchen get in each other’s way, having too many robots in tunnels creates clogs that can bring the work to a grinding halt.

Intro: 

For ants and robots operating in confined spaces like tunnels, having more workers does not necessarily mean getting more work done. Just as too many cooks in a kitchen get in each other’s way, having too many robots in tunnels creates clogs that can bring the work to a grinding halt.

Alumni: 

State of the School Address

School Chair will give an overview of the School of Physics last year and some upcoming news and events.

Public Lecture: When Will We Find E.T. and What Happens If We Do?

Are we alone in the universe? The scientific hunt for extraterrestrial intelligence is now well into its fifth decade, and we still haven’t discovered any cosmic company. Could all this mean that finding biology beyond Earth, even if it exists, is a project for the ages – one that might take centuries or longer?

Martin Mourigal receives NSF CAREER award for quantum materials research

Wednesday, July 25, 2018

School of Physics Assistant Professor Martin Mourigal has received a National Science Foundation (NSF) Faculty Early Career Development (CAREER) award for research on so-called frustrated quantum materials. The $621,772 funding will support the research and education efforts of Mourigal’s team for the next five years.

From wood and glass to magnets and superconductors, the behavior of materials is rooted in the quantum mechanical interaction between their atomic-scale constituents: nuclei and electrons. Although the laws of the quantum world are known, predicting the macroscopic properties of materials from their atomic structure is a formidable challenge.

Entanglement is a central quantum phenomenon at the origin of chemical bonds in materials. It usually averages to zero on human length and time scales.

This is not the case with quantum materials such as superconductors, exotic metals, or frustrated quantum magnets. In such systems, the concerted behavior of many electrons propels quantum entanglement beyond the atomic-scale.

Thus, quantum materials present a unique opportunity to understand the fundamental properties of the universe while forming the building blocks to fabricate future quantum devices that may revolutionize the harvest and control of charge, light, heat, and information.

“This CAREER award will not only support my team’s efforts in the area of quantum materials; it will also support the training of the future quantum workforce by involving graduate and undergraduate students and high-school teachers.”

Mourigal’s team focuses on frustrated magnetic materials. In these materials, spins – which are atomic-scale compass needles attached to electrons – display collective quantum dynamics despite simple properties as individuals.

“By designing and studying magnetic materials in which spins occupy periodic lattices with triangular patterns, we frustrate individual spins,” Mourigal says. “Our goal is to suppress their tendency to align with their neighbors. Instead we seek to promote cooperation between spins and obtain richer collective dynamics. This is a promising route to bootstrap entanglement beyond the atomic-scale.”

Mourigal adds: “Our research is multidisciplinary, in our labs at Georgia Tech, we work on preparing and perfecting new materials and measuring their properties at very low temperatures, while simultaneously conducting a lot of theoretical calculations and analyzing large quantities of data”.

A recurring tool in Mourigal’s approach is neutron scattering, a specialized technique that can be performed only at major government-run facilities such as Oak Ridge National Laboratory, in nearby Tennessee, and the NIST Center for Neutron Research, in Maryland.

“Neutrons are an incredible tool to study magnetic materials because neutrons penetrate deep inside our samples and can tell us where spins are, where they point, and if they dance to the quantum tempo,” Mourigal says. But there is a catch, Mourigal adds: “Neutron sources tend to be relatively faint, at least compared to photon and electron sources.   Therefore our experiments require large samples, often in the form of crystals.”

Such crystals can be hard to come by. Mourigal says the NSF CAREER award will allow his team to strengthen collaborations with U.S.-based research groups with expertise and tools to grow large, high-quality crystals of quantum magnetic materials.

“I am very grateful to the NSF for its support,” Mourigal says. “This CAREER award will not only support my team’s efforts in the area of quantum materials; it will also support the training of the future quantum workforce by involving graduate and undergraduate students and high-school teachers.”

Media Contact: 

A. Maureen Rouhi, Ph.D.
Director of Communications
College of Sciences

Summary: 

School of Physics Assistant Professor Martin Mourigal has won a five-year NSF CAREER grant for research on quantum materials. The materials present a unique opportunity to understand the fundamental properties of the universe while forming the building blocks for new devices that could revolutionize how we harvest and control charge, light, heat, and information.

Intro: 

School of Physics Assistant Professor Martin Mourigal has won a five-year NSF CAREER grant for research on quantum materials. The materials present a unique opportunity to understand the fundamental properties of the universe while forming the building blocks for new devices that could revolutionize how we harvest and control charge, light, heat, and information.

Alumni: 

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