New Measurements Reveal Evidence of Elusive Particles in a Newly Discovered Superconductor

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Published: Friday, April 03 2020 11:31

New measurements show evidence for the presence of exotic Majorana particles on the surface of an unconventional superconductor, Uranium ditelluride. Graphic provided by Dr. E. Edwards, Managing Director of Illinois Quantum Information Science and Technology Center (IQUIST).

Now a team of researchers at the University of Illinois, led by physicist Vidya Madhavan, in collaboration with researchers from the National Institute of Standards and Technology (NIST), the University of Maryland (UMD), Boston College, and ETH Zurich, have used high-resolution microscopy tools to peer into the inner-workings of an unusual type of superconductor, uranium ditelluride (UTe₂). Their measurements reveal strong evidence that this material may be a natural home to an exotic quasiparticle that’s been hiding from physicists for decades. The study was published March 26, 2020 in the journal Nature.

The particles in question were theorized back in 1937 by an Italian physicist named Ettore Majorana. Since then, physicists have been trying to prove that they exist. Scientists think a particular class of materials called chiral unconventional superconductors may naturally host Majoranas. UTe₂ may have all of the right properties to spawn these elusive quasiparticles.

“We know the physics of conventional superconductors and understand how they can conduct electricity or transport electrons from one end of a wire to the other with no resistance,” said Madhavan. “Chiral unconventional superconductors are much rarer, and the physics is less well known. Understanding them is important for fundamental physics and has potential applications in quantum computing.”

Inside of a normal superconductor, the electrons pair up in a way that enables lossless and persistent currents. This is in contrast to a normal conductor, like copper wire, which heats up as current passes through it. Part of the theory behind superconductivity was formulated decades ago by three scientists at the University of Illinois who earned a Nobel prize in physics for their work. For this conventional kind of superconductivity, magnetic fields are the enemy and break up the pairs, returning the material back to normal. Over the last year, researchers showed that uranium ditelluride behaves differently.

In 2019, Sheng Ran and Nicholas Butch (both co-authors on this study affiliated with the UMD Quantum Materials Center and NIST) and their collaborators announced that UTe₂ remains superconducting in the presence of magnetic fields up to 65 Tesla, which is about 10,000 times stronger than a refrigerator magnet. This unconventional behavior, combined with other measurements, led the authors of that paper to surmise that the electrons were pairing up in an unusual way that enabled them to resist break-ups. The pairing is important because superconductors with this property could very likely have Majorana particles on the surface. The new study from Madhavan and collaborators now strengthens the case that UTe₂ is one of these superconductors.

The team used a high-resolution scanning tunneling microscope to look for evidence of the unusual electron pairing and Majorana particles. This microscope not only maps out the surface of uranium ditelluride down to the level of atoms but also probes what’s happening with the electrons. The material itself is silvery, with steps jutting up from the surface. These step features are where evidence for Majorana quasiparticles is best seen. They provide a clean edge that, if predictions are correct, should show signatures of a continuous current that moves in one direction, even without the application of a voltage. The team scanned opposite sides of the step and saw a signal with a peak. But the peak was different, depending on which side of the step was scanned.

“Looking at both sides of the step, you see a signal that is a mirror image of each other. In a normal superconductor, you cannot find that,” said Madhavan. “The best explanation for seeing the mirror images is that we are directly measuring the presence of moving Majorana particles.” The team says that the measurements indicate that free-moving Majorana quasiparticles are circulating together in one direction, giving rise to mirrored, or chiral, signals.

Madhavan says the next step is to make measurements that would confirm that the material has broken time-reversal symmetry. This means that the particles should move differently if the arrow of time were theoretically reversed. Such a study would provide additional evidence for the chiral nature of UTe₂.

If confirmed, uranium ditelluride would be the only material, other than superfluid He-3, proven to be a chiral unconventional superconductor. “This is a huge discovery that will allow us to understand this rare kind of superconductivity, and maybe, in time, we could even manipulate Majorana quasiparticles in a useful way for quantum information science,” Madhavan said.

This story was originally published by the Grainger College of Engineering at the University of Illinois. It has been adapted with minor changes here.

To Tune Up Your Quantum Computer, Better Call an AI Mechanic

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Published: Friday, April 03 2020 09:31

UMD Scientists Help Discover the Highest-Energy Light Coming from the Sun

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Published: Tuesday, March 31 2020 14:31

Sometimes, the best place to hide a secret is in broad daylight. Just ask the sun.

A new paper in Physical Review Letters details the discovery of the highest-energy light ever observed from the sun. The international team behind the discovery also found that this type of light, known as gamma rays, is surprisingly bright. That is, there’s more of it than scientists had previously anticipated.

Watching like a HAWCWhat an excess of solar gamma rays looks like to the High-Altitude Water Cherenkov Observatory Collaboration, which includes researchers from the University of Maryland. Credit: Courtesy of the HAWC Collaboration

Although the high-energy light doesn’t reach the Earth’s surface (and thus is no threat to life), these gamma rays create cascades of particles that move at near the speed of light through the atmosphere. They were detected by an international group of scientists, including University of Maryland astrophysicists, using the High-Altitude Water Cherenkov Observatory, or HAWC.  

HAWC is an important part of the story. Unlike other observatories, it works around the clock observing more than 2/3 of the entire sky every day.

“HAWC has made numerous discoveries about very high energy gamma rays from exotic objects like supernova remnants, microquasars, and active galaxies, but this discovery comes from much closer to home – our own Sun,” says Jordan Goodman, UMD Distinguished University Professor and Principal Investigator for the HAWC project, which is funded by the National Science Foundation, the National Council of Humanities Science and Technology (CONACyT) of México, the U.S. Department of Energy, Los Alamos National Lab and the Max Planck Institute for Nuclear Physics in Heidelberg, 

“We now have observational techniques that weren’t possible a few years ago,” said Mehr Un Nisa, a postdoctoral research associate at Michigan State University and corresponding author of the new paper. 

“In this particular energy regime, other ground-based telescopes couldn’t look at the sun because they only work at night,” she said. “Ours operates 24/7.”

In addition to working differently from conventional telescopes, HAWC looks a lot different from the typical telescope.

Rather than a tube outfitted with glass lenses, HAWC uses a network of 300 large water tanks, each filled with about 200 metric tons of water. The network is nestled between two dormant volcano peaks in Mexico, more than 13,000 feet above sea level.

From this vantage point, it can observe the aftermath of gamma rays striking air in the atmosphere. Such collisions create what are called air showers, which are a bit like particle explosions that are imperceptible to the naked eye.

The energy of the original gamma ray is liberated and redistributed amongst new fragments consisting of lower energy particles and light. It’s these particles — and the new particles they create on their way down — that HAWC can “see.”

When the shower particles interact with water in HAWC’s tanks, they create what’s known as Cherenkov radiation that can be detected with the observatory’s instruments.

Nisa and her colleagues began collecting data in 2015. In 2021, the team had accrued enough data to start examining the sun’s gamma rays with sufficient scrutiny. The gamma rays lose energy in Earth’s atmosphere, meaning they don’t present a concern to life.

“After looking at six years’ worth of data, out popped this excess of gamma rays,” Nisa said. “When we first saw it, we were like, ‘We definitely messed this up. The sun cannot be this bright at these energies.’”

Making history

The sun gives off a lot of light spanning a range of energies, but some energies are more abundant than others.

For example, through its nuclear reactions, the sun provides a ton of visible light — that is, the light we see. This form of light carries an energy of about 1 electron volt, which is a handy unit of measure in physics.

The gamma rays that HAWC observed had about 1 trillion electron volts, or 1 tera electron volt, abbreviated 1 TeV. Not only was this energy level surprising, but so was the fact that they were seeing so much of it.

In the 1990s, scientists predicted that the sun could produce gamma rays when high-energy cosmic rays — particles accelerated by a cosmic powerhouse like a black hole or supernova — smash into protons in the sun. But, based on what was known about cosmic rays and the sun, the researchers also hypothesized it would be rare to see these gamma rays reach Earth.

At the time, though, there wasn’t an instrument capable of detecting such high-energy gamma rays and there wouldn’t be for a while. The first observation of gamma rays with energies of more than a billion electron volts came from NASA’s Fermi Gamma-ray Space Telescope in 2011.

Over the next several years, the Fermi mission showed that not only could these rays be very energetic, but also that there were about seven times more of them than scientists had originally expected. And it looked like there were gamma rays left to discover at even higher energies.

When a telescope launches into space, there’s a limit to how big and powerful its detectors can be. The Fermi telescope’s measurements of the sun’s gamma rays maxed out around 200 billion electron volts.

Theorists led by John Beacom and Annika Peter, both professors at Ohio State University, encouraged the HAWC Collaboration to take a look.

“They nudged us and said, ‘We’re not seeing a cutoff. You might be able to see something,” said Nisa.

The HAWC Collaboration includes more than 30 institutions across North America, Europe and Asia, and a sizeable portion of that is represented in the nearly 100 authors on the new paper. The University of Maryland team consists of Goodman, Research Scientist Andrew Smith, Project Engineer Michael Schneider and graduate students Kristi Engle, Elijah Wilox, Jason Fan, and Sohyoun Yun-Cárcamo.   

Now, for the first time, the team has shown that the energies of the sun’s rays extend into the TeV range, up to nearly 10 TeV, which does appear to be the maximum, Nisa said.

“This shows that HAWC is adding to our knowledge of our galaxy at the highest energies, and it’s opening up questions about our very own sun,” Nisa said. “It’s making us see things in a different light. Literally.”

 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.131.051201

 Original story courtesy of Michigan State University:https://msutoday.msu.edu/news/2023/surprising-sun-discovery

HAWC’s Measurement of the Highest Energy Photons Sets Limits on Violations of Relativity

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Published: Tuesday, March 31 2020 14:31

New measurements confirm, to the highest energies yet explored, that the laws of physics hold no matter where you are or how fast you're moving. Observations of record-breaking gamma rays prove the robustness of Lorentz Invariance—a piece of Einstein's theory of relativity that predicts the speed of light is constant everywhere in the universe. The High Altitude Water Cherenkov observatory in Puebla, Mexico detected the gamma rays coming from distant galactic sources. UMD authors on the paper were Jordan Goodman, Andy Smith, Bob Ellsworth, Kristi Engel, Israel Martinez-Castellanos, Michael Schneider and Elijah Tabachnick.

This compound graphic shows a view of the sky in ultra-high energy gamma rays. The arrows indicate the four sources of gamma rays with energies over 100 TeV from within our galaxy (courtesy of the HAWC collaboration) imposed over a photo of the HAWC Observatory’s 300 large water tanks. The tanks contain sensitive light detectors that measure showers of particles produced by the gamma rays striking the atmosphere more than 10 miles overhead. Credit: Jordan Goodman

"How relativity behaves at very high energies has real consequences for the world around us," said Pat Harding, an astrophysicist in the Neutron Science and Technology group at Los Alamos National Laboratory and a member of the HAWC scientific collaboration. "Most quantum gravity models say the behavior of relativity will break down at very high energies. Our observation of such high-energy photons at all raises the energy scale where relativity holds by more than a factor of a hundred."

Lorentz Invariance is a key part of the Standard Model of physics. However, a number of theories about physics beyond the Standard Model suggest that Lorentz Invariance may not hold at the highest energies. If Lorentz Invariance is violated, a number of exotic phenomena become possibilities. For example, gamma rays might travel faster or slower than the conventional speed of light. If faster, those high-energy photons would decay into lower-energy particles and thus never reach Earth.

The HAWC Gamma Ray Observatory has recently detected a number of astrophysical sources which produce photons above 100 TeV (a trillion times the energy of visible light), much higher energy than is available from any earthly accelerator. Because HAWC sees these gamma rays, it extends the range that Lorentz Invariance holds by a factor of 100 times.

"Detections of even higher-energy gamma rays from astronomical distances will allow more stringent the checks on relativity. As HAWC continues to take more data in the coming years and incorporate Los Alamos-led improvements to the detector and analysis techniques at the highest energies, we will be able to study this physics even further," said Harding.

Story courtesy of Los Alamos National Laboratory. Article: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.131101

Lathrop Lab's Geodynamo Set for Overhaul

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Published: Friday, March 06 2020 09:53

In a hangar-sized laboratory off Paint Branch Drive, Dan Lathrop gives the signal, and what he often calls simply “the experiment” awakens. A huge, steel sphere with tubes and electrical wires snaking across its surface begins a stately, nearly silent rotation inside a towering cage-like structure.

That’s the experiment running at visitor speed, however. When only Lathrop, a Distinguished Scholar-Teacher and professor in physics, and his graduate students are present to gather data, they crank up its 350 horsepower electric motor to spin 80 times faster, until the 3-meter globe encasing 25,000 pounds of liquid sodium blurs out at four revolutions per second.Professor Dan Lathrop examines the 3-meter steel sphere he uses in simulations of the Earth's "geodynamo." Hidden inside the spinning outer sphere (diagram, below) molten sodium and an even quicker-whirling inner sphere represent the earth's liquid outer core and solid inner core, which create geomagnetism. (Photo by John T. Consoli; diagram by Kolin Behrens)

For safety reasons, in the 11 years since he first switched the experiment on, no lab guest has ever watched it run that fast. Lathrop hasn’t either, exactly. “You can’t see it at full speed,” he said.

If “the experiment” sounds pretty singular, that’s because there’s nothing else like it on the planet. Lathrop, an expert in turbulent flows, envisioned the giant apparatus and several smaller predecessors as a way to simulate and perhaps even predict changes in the Earth’s magnetic field, which originates in its core and helps protect the surface from harmful solar radiation. While the machine has fascinated the geophysics community and generated useful results about planetary magnetic fields, it has never quite fulfilled Lathrop’s hopes. So this year, supported by a recently renewed National Science Foundation grant, he and his lab members will undertake a painstaking process to drain the flammable sodium, dismantle the device, upgrade it and—if the plan works—create a better magnetic model of the Earth.

Our planet has a “geodynamo,” a self-generating, self-sustaining magnetic field created by flows in its molten outer core, a layer of mostly iron and nickel more than 3,000 kilometers beneath our feet. Swirling turbulence in the liquid metal, caused by convection and the planet’s rotation, gives rise to electrical currents and magnetic fields that feed on each other.

So far, Lathrop’s experiment needs external current to generate a magnetic field; soon he hopes that will no longer be necessary. Doctoral students Rubén Rojas and Artur Perevalov in physics, along with Heidi Myers in geology and Sarah Burnett in mathematics, have been researching ways to modify a hidden, inner sphere of the device—analogous to Earth’s solid inner core—by adding texture to create swirling, helical flows in the highly conductive liquid sodium, generating electrical currents.

It’s never been tried before, so the results are hard to predict.

“I try not to be a foolish optimist, but you know, you aren’t going to build an experiment like this without a certain amount of optimism that there are interesting things to see,” Lathrop said.

The biggest potential prize would be an ability to predict the “weather” of Earth’s magnetic field, which is constantly in flux. Geologic evidence suggests the poles have reversed hundreds of times—most recently 780,000 years ago—and indeed, the North Pole has been moving from Canada toward Russia with increasing speed in recent years. During such a flip, much of the planet’s surface could have a weaker magnetic shield from solar radiation. (For a preview of what that could be like, look at Mars, which lacks a geodynamo.)

Even now, solar storms do create problems on Earth, damaging satellites and sensitive electronics, said Sara Gibson, a solar physicist at the National Center for Atmospheric Research in Boulder, Colo. For instance, if a massive 1859 solar storm that caused aurora as far south as the tropics hit today, it could fry communications and electrical grids worldwide.

“Dan’s work is really important, because it’s vital to understand the Earth’s magnetic field, which is coupling with what’s coming from the sun, and creating these magnetic impacts,” Gibson said.

Lathrop doesn’t promote his research with disaster scenarios. A pole reversal may not be in the offing at all, and would take more than 1,000 years. But what about scientific curiosity as well as simple prudence concerning a factor that allowed life to arise on earth?

“You think you’d want a solid scientific base knowing, well, how does it work, and how did it get there?” he said. “Where’s it at now? And where’s it going?”

Original story by Chris Carroll, Maryland Today 

Watch the 3 meter experiment.