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.

Physics at the Edge of the World

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Published: Friday, March 09 2018 14:10

A view of Amundsen-Scott Station at the South Pole. (Credit: Dwight Bohnet/NSF)

Deep within the ice covering the South Pole, thousands of sensitive cameras strain their digital eyes in search of a faint blue glow—light that betrays the presence of high-energy neutrinos.

For this episode, Chris sat down with UMD graduate student Liz Friedman and physics professor Kara Hoffman to learn more about IceCube, the massive underground neutrino observatory located in one of the most desolate spots on Earth. It turns out that IceCube is blind to the highest-energy neutrinos, and Friedman is heading down to the South Pole to help install stations for a new observatory—the Askaryan Radio Array—which uses radio waves instead of blue light to tune into the whispers of these ghostly visitors.

This episode of Relatively Certain was produced by Chris Cesare and Emily Edwards. It features music by Dave Depper and Podington Bear. Relatively Certain is a production of the Joint Quantum Institute, a research partnership between the University of Maryland and the National Institute of Standards and Technology, and you can find it on iTunes, Google Play or Soundcloud.

New Hole-Punched Crystal Clears a Path for Quantum Light

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Published: Monday, February 12 2018 15:28

Optical highways for light are at the heart of modern communications. But when it comes to guiding individual blips of light called photons, reliable transit is far less common. Now, a collaboration of researchers from the Joint Quantum Institute (JQI), led by Mohammad Hafezi and Edo Waks, has created a photonic chip that both generates single photons, and steers them around. The device, described in the Feb. 9 issue of Science, features a way for the quantum light to seamlessly move, unaffected by certain obstacles.

"This design incorporates well-known ideas that protect the flow of current in certain electrical devices," says Hafezi. "Here, we create an analogous environment for photons, one that protects the integrity of quantum light, even in the presence of certain defects."

The chip starts with a photonic crystal, which is an established, versatile technology used to create roadways for light. They are made by punching holes through a sheet of semiconductor. For photons, the repeated hole pattern looks very much like a real crystal made from a grid of atoms. Researchers use different hole patterns to change the way that light bends and bounces through the crystal. For instance, they can modify the hole sizes and separations to make restricted lanes of travel that allow certain light colors to pass, while prohibiting others.

Sometimes, even in these carefully fabricated devices, there are flaws that alter the light’s intended route, causing it to detour into an unexpected direction. But rather than ridding their chips of every flaw, the JQI team mitigates this issue by rethinking the crystal’s hole shapes and crystal pattern. In the new chip, they etch out thousands of triangular holes in an array that resembles a bee’s honeycomb. Along the center of the device they shift the spacing of the holes, which opens a different kind of travel lane for the light. Previously, these researchers predicted that photons moving along that line of shifted holes should be impervious to certain defects because of the overall crystal structure, or topology. Whether the lane is a switchback road or a straight shot, the light’s path from origin to destination should be assured, regardless of the details of the road.

The light comes from small flecks of semiconductor—dubbed quantum emitters—embedded into the photonic crystal. Researchers can use lasers to prod this material into releasing single photons. Each emitter can gain energy by absorbing laser photons and lose energy by later spitting out those photons, one at time. Photons coming from the two most energetic states of a single emitter are different colors and rotate in opposite directions. For this experiment, the team uses photons from an emitter found near the chip’s center.

The team tested the capabilities of the chip by first changing a quantum emitter from its lowest energy state to one of its two higher energy states. Upon relaxing back down, the emitter pops out a photon into the nearby travel lane. They continued this process many times, using photons from the two higher energy states. They saw that photons emitted from the two states preferred to travel in opposite directions, which was evidence of the underlying crystal topology.

To confirm that the design could indeed offer protected lanes of traffic for single photons, the team created a 60 degree turn in the hole pattern. In typical photonic crystals, without built-in protective features, such a kink would likely cause some of the light to reflect backwards or scatter elsewhere. In this new chip, topology protected the photons and allowed them to continue on their way unhindered.

“On the internet, information moves around in packets of light containing many photons, and losing a few doesn’t hurt you too much”, says co-author Sabyasachi Barik, a graduate student at JQI. “In quantum information processing, we need to protect each individual photon and make sure it doesn't get lost along the way. Our work can alleviate some forms of loss, even when the device is not completely perfect.”

The design is flexible, and could allow researchers to systematically assemble pathways for single photons, says Waks. "Such a modular approach may lead to new types of optical devices and enable tailored interactions between quantum light emitters or other kinds of matter."

Written by E. Edwards

*Mohammad Hafezi is an Associate Professor in the University of Maryland (UMD) Departments of Electrical and Computer Engineering and Physics. Edo Waks is a Professor in the UMD Department of Electrical and Computer Engineering.

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Rare Decays Provide Hints of Particle Mischief

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Published: Friday, February 09 2018 13:54

Scientists cataloguing the disintegration of an ethereal particle may have spotted new signs of a subtle discrepancy in the Standard Model—the theory that wraps up all of particle physics in a single equation.

The new measurement, performed at the Large Hadron Collider beauty experiment (LHCb) and led by a team at UMD, adds to a growing body of evidence that appears to contradict a basic property of the Standard Model. That property, called lepton flavor universality, seems to emerge directly from the underlying mathematics, and it imposes a democratic order on three fundamental particles: Electrons, together with their heavier cousins —the muon and the tau, should behave identically during certain kinds of particle interactions.

But for several years physicists have been finding cracks in this egalitarian image. The latest result, which makes detailed use of the different ways that very rare particles decay, strengthens the case against lepton universality and may point the way toward new physics. As more rare decays are recorded, researchers could begin to mount an even stronger case and investigate other aspects of the violation.

The paper has been accepted for publication in Physical Review Letters, and a preprint is available for download on the arXiv. The CERN Courier highlighted the result this past October.

The UMD team that devised and led the measurement includes Professor Hassan Jawahery, graduate student Jack Wimberley and postdoctoral researcher Brian Hamilton.