New Research Reveals How Energy Dissipates Outside Earth’s Magnetic Field

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Published: Thursday, May 10 2018 10:46

Earth’s magnetic field provides an invisible but crucial barrier that protects Earth from the solar wind—a stream of charged particles launched from the sun’s outer layers. The protective properties of the magnetic field can fail due to a process known as magnetic reconnection, which occurs when two opposing magnetic field lines break and reconnect with each other, dissipating massive amounts of energy and accelerating particles that threaten air traffic and satellite communication systems.

In this visualization, as the supersonic solar wind (yellow haze) flows around the Earth's magnetic field (blue wavy lines), it forms a highly turbulent boundary layer called the “magnetosheath” (yellow swirling area). A new research paper describes observations of small-scale magnetic reconnection within the magnetosheath, revealing important clues about heating in the sun's outer layers and elsewhere in the universe. Image credit: NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-Keith (Click image to download hi-res version.)
In this visualization, as the supersonic solar wind (yellow haze) flows around the Earth's magnetic field (blue wavy lines), it forms a highly turbulent boundary layer called the “magnetosheath” (yellow swirling area). A new research paper describes observations of small-scale magnetic reconnection within the magnetosheath, revealing important clues about heating in the sun's outer layers and elsewhere in the universe. Image credit: NASA’s Goddard Space Flight Center/Mary Pat Hrybyk-Keith (Click image to download hi-res version.)

Just outside of Earth’s magnetic field, the solar wind’s onslaught of electrons and ionized gases creates a turbulent maelstrom of magnetic energy known as the magnetosheath. While magnetic reconnection has been well documented closer to Earth, physicists have sought to determine whether reconnection also happens in this turbulent zone.

A new research paper co-authored by University of Maryland Physics Professor James Drake suggests that the answer to this question is yes. The observation 

A new research paper co-authored by James Drake, a University of Maryland Distinguished University Professor of Physics, suggests that the answer to this question is yes. The observations, published in the May 10, 2018 issue of the journal Nature, provide the first evidence of magnetic reconnection occurring at very small spatial scales in the turbulent magnetosheath. However, unlike the reconnection that occurs with the Earth’s magnetic field, which involves electrons as well as ions, turbulent reconnection in the magnetosheath involves electrons alone. 

“We know that magnetic energy in churning, turbulent systems cascades to smaller and smaller scales. At some point that energy is fully dissipated. The big question is how that happens, and what role magnetic reconnection plays at such small scales,” Drake said. “This study shows that reconnection indeed can happen at the electron scale, with no ions involved at all, suggesting that reconnection may help dissipate magnetic energy at very small scales.” 

By drawing a clearer picture of the physics of magnetic reconnection, the discovery promises to advance scientists’ understanding of several open questions in solar physics. For example, electron-scale magnetic reconnection may play a role in heating of the solar corona—an expansive layer of charged particles that surrounds the sun and reaches temperatures hundreds of times higher than the sun’s visible surface. This in turn could help explain the physics of the solar wind, as well as the nature of turbulent magnetic systems elsewhere in space.

NASA’s Magnetospheric Multiscale (MMS) mission gathered the data for the analysis. Flying in a pyramid formation with as little as 4.5 miles’ distance between four identical spacecraft, MMS imaged electrons within the pyramid once every 30 milliseconds. These highly precise measurements enabled the researchers to capture turbulent, electron-only magnetic reconnection, a phenomenon not previously observed. 

“MMS discovered electron magnetic reconnection, a new process much different from the standard magnetic reconnection that happens in calmer areas around Earth,” said Tai Phan, a senior fellow in the Space Sciences Laboratory at the University of California, Berkeley and the lead author of the paper. “This finding helps scientists understand how turbulent magnetic fields dissipate energy throughout the cosmos.”

Because turbulent reconnection involves only electrons, it remained hidden from scientists looking for the telltale signature of standard magnetic reconnection: ion jets. Compared with standard reconnection, in which broad jets of ions stream out tens of thousands of miles from the site of reconnection, turbulent reconnection ejects narrow jets of electrons only a couple miles wide.

But MMS scientists were able to leverage the design of one instrument, the Fast Plasma Investigation, to create a technique that allowed them to read between the lines and gather extra data points to resolve the jets.

“The key event of the paper happens in 45 milliseconds. This would be one data point with the regular data,” said Amy Rager, a graduate student at the Catholic University of America in Washington, D.C., who worked at NASA’s Goddard Space Flight Center to develop the technique. “But instead we can get six to seven data points in that region with this method, allowing us to understand what is happening.”

With the new method, MMS scientists are hopeful they can comb through existing data sets to find more of these events and other unexpected discoveries as well. 

“There were some surprises in the data,” Drake said. “Magnetic reconnection occurs when you have two magnetic fields pointing in opposite directions and they annihilate each other. In the present case a large ambient magnetic field survived after annihilation occurred. Honestly, we were surprised that turbulent reconnection at very small scales could occur with this background magnetic field present.”

Magnetic reconnection occurs throughout the universe, so whatever scientists learn about it near Earth can be applied to other phenomena. For example, the discovery of turbulent electron reconnection may help scientists understand the role that magnetic reconnection plays in heating the inexplicably hot solar corona—the sun’s outer atmosphere—and accelerating the supersonic solar wind. NASA’s upcoming Parker Solar Probe mission will travel directly toward the sun in the summer of 2018 to investigate these questions—armed with a new understanding of magnetic reconnection near Earth.

VIDEO: NASA Spacecraft Discovers New Magnetic Process in Turbulent Space:

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This release was adapted from text provided by the University of California, Berkeley, and NASA. 

The research paper, “Electron Magnetic Reconnection Without Ion Coupling in Earth’s Turbulent Magnetosheath,” Tai Phan et al., was published in the journal Nature on May 10, 2018.

This work was supported by NASA (Award Nos. NNG04EB99C and NNX08AO83G), the UK Science and Technology Facilities Council (Award No. ST/N000692/1), the French Centre National d'Études Spatiales and the French Centre National de la Recherche Scientifique. The content of this article does not necessarily reflect the views of these organizations.

Original story.

Atoms May Hum a Tune from Grand Cosmic Symphony

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Published: Thursday, April 19 2018 13:54

Researchers playing with a cloud of ultracold atoms uncovered behavior that bears a striking resemblance to the universe in microcosm. Their work, which forges new connections between atomic physics and the sudden expansion of the early universe, will be published in Physical Review X and highlighted by Physics.

"From the atomic physics perspective, the experiment is beautifully described by existing theory," says Stephen Eckel, an atomic physicist at the National Institute of Standards and Technology (NIST) and the lead author of the new paper. "But even more striking is how that theory connects with cosmology."

In several sets of experiments, Eckel and his colleagues rapidly expanded the size of a doughnut-shaped cloud of atoms, taking snapshots during the process. The growth happens so fast that the cloud is left humming, and a related hum may have appeared on cosmic scales during the rapid expansion of the early universe—an epoch that cosmologists refer to as the period of inflation.

The work brought together experts in atomic physics and gravity, and the authors say it is a testament to the versatility of the Bose-Einstein condensate (BEC)—an ultracold cloud of atoms that can be described as a single quantum object—as a platform for testing ideas from other areas of physics.

"Maybe this will one day inform future models of cosmology," Eckel says. "Or vice versa. Maybe there will be a model of cosmology that’s difficult to solve but that you could simulate using a cold atomic gas."

It’s not the first time that researchers have connected BECs and cosmology. Prior studies mimicked black holes and searched for analogs of the radiation predicted to pour forth from their shadowy boundaries. The new experiments focus instead on the BEC’s response to a rapid expansion, a process that suggests several analogies to what may have happened during the period of inflation.

The first and most direct analogy involves the way that waves travel through an expanding medium. Such a situation doesn’t arise often in physics, but it happened during inflation on a grand scale. During that expansion, space itself stretched any waves to much larger sizes and stole energy from them through a process known as Hubble friction.

In one set of experiments, researchers spotted analogous features in their cloud of atoms. They imprinted a sound wave onto their cloud—alternating regions of more atoms and fewer atoms around the ring, like a wave in the early universe—and watched it disperse during expansion. Unsurprisingly, the sound wave stretched out, but its amplitude also decreased. The math revealed that this damping looked just like Hubble friction, and the behavior was captured well by calculations and numerical simulations.

"It's like we're hitting the BEC with a hammer," says Gretchen Campbell, the NIST co-director of the Joint Quantum Institute (JQI) and a coauthor of the paper, "and it’s sort of shocking to me that these simulations so nicely replicate what's going on."

In a second set of experiments, the team uncovered another, more speculative analogy. For these tests they left the BEC free of any sound waves but provoked the same expansion, watching the BEC slosh back and forth until it relaxed.

In a way, that relaxation also resembled inflation. Some of the energy that drove the expansion of the universe ultimately ended up creating all of the matter and light around us. And although there are many theories for how this happened, cosmologists aren’t exactly sure how that leftover energy got converted into all the stuff we see today.

In the BEC, the energy of the expansion was quickly transferred to things like sound waves traveling around the ring. Some early guesses for why this was happening looked promising, but they fell short of predicting the energy transfer accurately. So the team turned to numerical simulations that could capture a more complete picture of the physics.

What emerged was a complicated account of the energy conversion: After the expansion stopped, atoms at the outer edge of the ring hit their new, expanded boundary and got reflected back toward the center of the cloud. There, they interfered with atoms still traveling outward, creating a zone in the middle where almost no atoms could live. Atoms on either side of this inhospitable area had mismatched quantum properties, like two neighboring clocks that are out of sync.

The situation was highly unstable and eventually collapsed, leading to the creation of vortices throughout the cloud. These vortices, or little quantum whirlpools, would break apart and generate sound waves that ran around the ring, like the particles and radiation left over after inflation. Some vortices even escaped from the edge of the BEC, creating an imbalance that left the cloud rotating.

Unlike the analogy to Hubble friction, the complicated story of how sloshing atoms can create dozens of quantum whirlpools may bear no resemblance to what goes on during and after inflation. But Ted Jacobson, a coauthor of the new paper and a physics professor at the University of Maryland specializing in black holes, says that his interaction with atomic physicists yielded benefits outside these technical results.

"What I learned from them, and from thinking so much about an experiment like that, are new ways to think about the cosmology problem," Jacobson says. "And they learned to think about aspects of the BEC that they would never have thought about before. Whether those are useful or important remains to be seen, but it was certainly stimulating."

Eckel echoes the same thought. "Ted got me to think about the processes in BECs differently," he says, "and any time you approach a problem and you can see it from a different perspective, it gives you a better chance of actually solving that problem."

Future experiments may study the complicated transfer of energy during expansion more closely, or even search for further cosmological analogies. "The nice thing is that from these results, we now know how to design experiments in the future to target the different effects that we hope to see," Campbell says. "And as theorists come up with models, it does give us a testbed where we could actually study those models and see what happens."

The new paper included contributions from two coauthors not mentioned in the text: Avinash Kumar, a graduate student at JQI; and Ian Spielman, a JQI Fellow and NIST physicist.

Story by Chris Cesare

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Machine Learning’s ‘Amazing’ Ability to Predict Chaos

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Published: Thursday, April 19 2018 13:13

The findings come from Professors Michelle Girvan and Edward Ott along with two other UMD collaborators. "They employed a machine-learning algorithm called reservoir computing to “learn” the dynamics of an archetypal chaotic system called the Kuramoto-Sivashinsky equation. The evolving solution to this equation behaves like a flame front, flickering as it advances through a combustible medium."

Read the story in Quanta magazine.

A Different Spin on Superconductivity: Unusual Particle Interactions Open up new Possibilities in Exotic Materials

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Published: Friday, April 06 2018 14:36

When you plug in an appliance or flip on a light switch, electricity seems to flow instantly through wires in the wall. But in fact, the electricity is carried by tiny particles called electrons that slowly drift through the wires. On their journey, electrons occasionally bump into the material’s atoms, giving up some energy with every collision.

The degree to which electrons travel unhindered determines how well a material can conduct electricity. Environmental changes can enhance conductivity, in some cases drastically. For example, when certain materials are cooled to frigid temperatures, electrons team up so they can flow uninhibited, without losing any energy at all—a phenomenon called superconductivity.

Now a team* of researchers from the University of Maryland (UMD) Department of Physics together with collaborators has seen exotic superconductivity that relies on highly unusual electron interactions. While predicted to occur in other non-material systems, this type of behavior has remained elusive. The team’s research, published in the April 6 issue of Science Advances, reveals effects that are profoundly different from anything that has been seen before with superconductivity.

Electron interactions in superconductors are dictated by a quantum property called spin. In an ordinary superconductor, electrons, which carry a spin of ½, pair up and flow uninhibited with the help of vibrations in the atomic structure. This theory is well-tested and can describe the behavior of most superconductors. In this new research, the team uncovers evidence for a new type of superconductivity in the material YPtBi, one that seems to arise from spin-3/2 particles.

“No one had really thought that this was possible in solid materials,” explains Johnpierre Paglione, a UMD physics professor and senior author on the study. “High-spin states in individual atoms are possible but once you put the atoms together in a solid, these states usually break apart and you end up with spin one-half. “

Finding that YPtBi was a superconductor surprised the researchers in the first place. Most superconductors start out as reasonably good conductors, with a lot of mobile electrons—an ingredient that YPtBi is lacking. According to the conventional theory, YPtBi would need about a thousand times more mobile electrons in order to become superconducting at temperatures below 0.8 Kelvin. And yet, upon cooling the material to this temperature, the team saw superconductivity happen anyway. This was a first sign that something exotic was going on inside this material.

After discovering the anomalous superconducting transition, researchers made measurements that gave them insight into the underlying electron pairing.  They studied a telling feature of superconductors—their interaction with magnetic fields. As the material undergoes the transition to a superconductor, it will try to expel any added magnetic field from its interior. But the expulsion is not completely perfect. Near the surface, the magnetic field can still enter the material but then quickly decays away. How far it goes in depends on the nature of the electron pairing, and changes as the material is cooled down further and further.

To probe this effect, the researchers varied the temperature in a small sample of the material while exposing it to a magnetic field more than ten times weaker than the Earth’s. A copper coil surrounding the sample detected changes to the superconductor’s magnetic properties and allowed the team to sensitively measure tiny variations in how deep the magnetic field reached inside the superconductor.

The measurement revealed an unusual magnetic intrusion. As the material warmed from absolute zero, the field penetration depth for YPtBi increased linearly instead of exponentially as it would for a conventional superconductor. This effect, combined with other measurements and theory calculations, constrained the possible ways that electrons could pair up. The researchers concluded that the best explanation for the superconductivity was electrons disguised as particles with a higher spin—a possibility that hadn’t even been considered before in the framework of conventional superconductivity.

The discovery of this high-spin superconductor has given a new direction for this research field. “We used to be confined to pairing with spin one-half particles,” says Hyunsoo Kim, lead author and a UMD assistant research scientist. “But if we start considering higher spin, then the landscape of this superconducting research expands and just gets more interesting.”

For now, many open questions remain, including how such pairing could occur in the first place. “When you have this high-spin pairing, what’s the glue that holds these pairs together?” says Paglione. “There are some ideas of what might be happening, but fundamental questions remain–which makes it even more fascinating.”

* The research was done at UMD’s Center for Nanophysics and Advanced Materials, Condensed Matter Theory Center and the Joint Quantum Institute, in collaboration with Ames Laboratory at Iowa State University, the Lawrence Berkley National Laboratory, the University of Otago and the University of Wisconsin.

Read the original article in Science.

Written by: Nina Beier

Latest Nanowire Experiment Boosts Confidence in Majorana Sighting

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Published: Wednesday, March 28 2018 14:25

Editor's note: After further analysis of their experimental data, the authors of the paper highlighted in this story no longer claim that they measured a quantized conductance—the key experimental signature that would suggest the presence of Majorana fermions. The paper has been officially retracted. We have left this story up with this note because researchers believe the approach it describes remains a viable route to eventually detecting Majoranas.

In the latest experiment of its kind, researchers have captured the most compelling evidence to date that unusual particles lurk inside a special kind of superconductor. The result, which confirms theoretical predictions first made nearly a decade ago at the Joint Quantum Institute (JQI) and the University of Maryland (UMD), 

In the latest experiment of its kind, researchers have captured the most compelling evidence to date that unusual particles lurk inside a special kind of superconductor. The result, which confirms theoretical predictions first made nearly a decade ago at the Joint Quantum Institute (JQI) and the University of Maryland (UMD), will be published in the April 5 issue of Nature. 

A device that physicists used to spot the clearest signal yet of Majorana particles. The gray wire in the middle is the nanowire, and the green area is a strip of superconducting aluminum. (Credit: Hao Zhang/QuTech)The stowaways, dubbed Majorana quasiparticles, are different from ordinary matter like electrons or quarks—the stuff that makes up the elements of the periodic table. Unlike those particles, which as far as physicists know can’t be broken down into more basic pieces, Majorana quasiparticles arise from coordinated patterns of many atoms and electrons and only appear under special conditions. They are endowed with unique features that may allow them to form the backbone of one type of quantum computer, and researchers have been chasing after them for years.

The latest result is the most tantalizing yet for Majorana hunters, confirming many theoretical predictions and laying the groundwork for more refined experiments in the future. In the new work, researchers measured the electrical current passing through an ultra-thin semiconductor connected to a strip of superconducting aluminum—a recipe that transforms the whole combination into a special kind of superconductor.

Experiments of this type expose the nanowire to a strong magnet, which unlocks an extra way for electrons in the wire to organize themselves at low temperatures. With this additional arrangement the wire is predicted to host a Majorana quasiparticle, and experimenters can look for its presence by carefully measuring the wire’s electrical response. 

The new experiment was conducted by researchers from QuTech at the Technical University of Delft in the Netherlands and Microsoft Research, with samples of the hybrid material prepared at the University of California, Santa Barbara and Eindhoven University of Technology in the Netherlands. Experimenters compared their results to theoretical calculations by JQI Fellow Sankar Das Sarma and JQI graduate student Chun-Xiao Liu.

The same group at Delft saw hints of a Majorana in 2012, but the measured electrical effect wasn’t as big as theory had predicted. Now the full effect has been observed, and it persists even when experimenters jiggle the strength of magnetic or electric fields—a robustness that provides even stronger evidence that the experiment has captured a Majorana, as predicted in careful theoretical simulations by Liu.

"We have come a long way from the theoretical recipe in 2010 for how to create Majorana particles in semiconductor-superconductor hybrid systems," says Das Sarma, a coauthor of the paper who is also the director of the Condensed Matter Theory Center at UMD. "But there is still some way to go before we can declare total victory in our search for these strange particles."

The success comes after years of refinements in the way that researchers assemble the nanowires, leading to cleaner contact between the semiconductor wire and the aluminum strip. During the same time, theorists have gained insight into the possible experimental signatures of Majoranas—work that was pioneered by Das Sarma and several collaborators at UMD.

Theory meets experiment

The quest to find Majorana quasiparticles in thin quantum wires began in 2001, spurred by Alexei Kitaev, then a physicist then at Microsoft Research. Kitaev, who is now at the California Institute of Technology in Pasadena, concocted a relatively simple but unrealistic system that could theoretically harbor a Majorana. But this imaginary wire required a specific kind of superconductivity not available off-the-shelf from nature, and others soon began looking for ways to imitate Kitaev’s contraption by mixing and matching available materials.

One challenge was figuring out how to get superconductors, which usually go about their business with an even number of electrons—two, four, six, etc.—to also allow an odd number of electrons, a situation that is normally unstable and requires extra energy to maintain. The odd number is necessary because Majorana quasiparticles are unabashed oddballs: They only show up in the coordinated behavior of an odd number of electrons. 

In 2010, almost a decade after Kitaev’s original paper, Das Sarma, JQI Fellow Jay Deep Sau and JQI postdoctoral researcher Roman Lutchyn, along with a second group of researchers, struck upon a method to create these special superconductors, and it has driven the experimental search ever since. They suggested combining a certain kind of semiconductor with an ordinary superconductor and measuring the current through the whole thing. They predicted that the combination of the two materials, along with a strong magnetic field, would unlock the Majorana arrangement and yield Kitaev’s special material.

They also predicted that a Majorana could reveal itself in the way current flows through such a nanowire. If you connect an ordinary semiconductor to a metal wire and a battery, electrons usually have some chance of hopping off the wire onto the semiconductor and some chance of being rebuffed—the details depend on the electrons and the makeup of the material. But if you instead use one of Kitaev’s nanowires, something completely different happens. The electron always gets perfectly reflected back into the wire, but it’s no longer an electron. It becomes what scientists call a hole—basically a spot in the metal that’s missing an electron—and it carries a positive charge back in the opposite direction.

Physics demands that the current across the interface be conserved, which means that two electrons must end up in the superconductor to balance out the positive charge heading in the other direction. The strange thing is that this process, which physicists call perfect Andreev reflection, happens even when electrons in the metal receive no push toward the boundary—that is, even when they aren’t hooked up to a battery of sorts. This is related to the fact that a Majorana is its own antiparticle, meaning that it doesn’t cost any energy to create a pair of Majoranas in the nanowire. The Majorana arrangement gives the two electrons some extra room to maneuver and allows them to traverse the nanowire as a quantized pair—that is, exactly two at a time. 

"It is the existence of Majoranas that gives rise to this quantized differential conductance," says Liu, who ran numerical simulations to predict the results of the experiments on UMD’s Deepthought2 supercomputer cluster. "And such a quantization should even be robust to small changes in experimental parameters, as the real experiment shows."

Scientists refer to this style of experiment as tunneling spectroscopy because electrons are taking a quantum route through the nanowire to the other side. It has been the focus of recent efforts to capture Majoranas, but there are other tests that could more directly reveal the exotic properties of the particles—tests that would fully confirm that the Majoranas are really there. 

"This experiment is a big step forward in our search for these exotic and elusive Majorana particles, showing the great advance made in the materials improvement over the last five years," Das Sarma says. "I am convinced that these strange particles exist in these nanowires, but only a non-local measurement establishing the underlying physics can make the evidence definitive."

By Chris Cesare

 Original story.