Physicists develop a device that could provide conclusive evidence for the existence (or not) of non-Abelian anyons

Capacitance measurement of bilayer graphene at a high magnetic field. The vertical dark blue to orange lines are signatures of fractional quantum Hall states that are shared between the two layers of the bilayer graphene sheet. The vertical line going through the center is believed to host an intriguing type of particles: non-Abelian anyons. Credit: University of California - Santa Barbara

What kinds of 'particles' are allowed by nature? The answer lies in the theory of quantum mechanics, which describes the microscopic world.

In a bid to stretch the boundaries of our understanding of the quantum world, UC Santa Barbara researchers have developed a device that could prove the existence of non-Abelian anyons, a quantum particle that has been mathematically predicted to exist in two-dimensional space, but so far not conclusively shown. The existence of these particles would pave the way toward major advances in topological quantum computing.

In a study that appears in the journal Nature, physicist Andrea Young, his graduate student Sasha Zibrov and their colleagues have taken a leap toward finding conclusive evidence for non-Abelian anyons. Using graphene, an atomically thin material derived from graphite (a form of carbon), they developed an extremely low-defect, highly tunable device in which non-Abelian anyons should be much more accessible. First, a little background: In our three-dimensional universe, elementary particles can be either fermions or bosons: think electrons (fermions) or the Higgs (a boson).

"The difference between these two types of 'quantum statistics' is fundamental to how matter behaves," Young said. For example, fermions cannot occupy the same quantum state, allowing us to push electrons around in semiconductors and preventing neutron stars from collapsing. Bosons can occupy the same state, leading to spectacular phenomena such as Bose-Einstein condensation and superconductivity, he explained. Combine a few fermions, such as the protons, neutrons, and electrons that make up atoms and you can get either type, but never evade the dichotomy.

In a two-dimensional universe, however, the laws of physics allow for a third possibility. Known as "anyons," this type of quantum particle is neither a boson nor a fermion, but rather something completely different—and some kinds of anyons, known as non-Abelian anyons, retain a memory of their past states, encoding quantum information across long distances and forming the theoretical building blocks for topological quantum computers.

Although we don't live in a two dimensional universe, when confined to a very thin sheet or slab of material, electrons do. In this case, anyons can emerge as "quasiparticles" from correlated states of many electrons. Perturbing such a system, say with an electrical potential, leads to the entire system rearranging just as if an nayon had moved.

The hunt for non-Abelian anyons begins by identifying the collective states that host them. "In fractional quantum Hall states—a type of collective electron state observed only in two dimensional samples at very high magnetic fields—the quasiparticles are known to have precisely a rational fraction of the electron charge, implying that they are anyons," Young said.

"Mathematically, sure, non-Abelian statistics are allowed and even predicted for some fractional quantum Hall states." he continued. However, scientists in this field have been limited by the fragility of the host states in the semiconductor material where they are typically studied. In these structures, the collective states themselves appear only at exceptionally low temperatures, rendering it doubly difficult to explore the unique quantum properties of individual anyons.

Graphene proves to be an ideal material to build devices to search for the elusive anyons. But, while scientists had been building graphene-based devices, other materials surrounding the graphene sheet—such as glass substrates and metallic gates—introduced enough disorder to destroy any signatures of non-Abelian states, Zibrov explained. The graphene is fine, it's the environment that is the problem, he said.

The solution? More atomically thin material.

"We've finally reached a point where everything in the device is made out of two-dimensional single crystals," said Young. "So not only the graphene itself, but the dielectrics are single crystals of hexagonal boron nitride that are flat and perfect and the gates are single crystals of graphite which are flat and perfect." By aligning and stacking these flat and perfect crystals of material on top of each other, the team achieved not only a very low-disorder system, but one that is also extremely tunable.

"Besides realizing these states, we can tune microscopic parameters in a very well controlled way and understand what makes these states stable and what destabilizes them," Young said. The fine degree of experimental control—and elimination of many unknowns— allowed the team to theoretically model the system with high accuracy, building confidence in their conclusions.

The materials advance gives these fragile excitations a certain amount of robustness, with the required temperatures nearly ten times higher than needed in other material systems. Bringing non-Abelian statistics into a more convenient temperature range proves an opportunity for not only for investigations of fundamental physics, but reignites hope for developing a topological quantum bit, which could form the basis for a new kind of quantum computer. Non-Abelian anyons are special in that they are thought to be able to process and store quantum information independent of many environmental effects, a major challenge in realizing quantum computers with traditional means.

But, say the physicists, first things first. Directly measuring the quantum properties of the emergent quasiparticles is very challenging, Zibrov explained. While some properties—such as fractional charge—have been definitively demonstrated, definitive proof of non-Abelian statistics—much less harnessing nonabelian anyons for quantum computation—has remained far out of the reach of experiments. "We don't really know yet experimentally if non-Abelian anyons exist," Zibrov said.

"Our experiments so far are consistent with theory, which tells us that some of the states we observed should be non-Abelian, but we still don't have an experimental smoking gun."

"We'd like an experiment that actually demonstrates a phenomenon unique to non-Abelian statistics," said Young, who has won numerous awards for his work, including the National Science Foundation's CAREER Award. "Now that we have a material that we understand really well, there are many ways to do this—we'll see if nature cooperates!"

Source:PHYS

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A novel in situ method for simultaneous growth of smart material single crystals and thin films

Development of a novel in situ method for simultaneous growth of single crystals and thin films of a smart material spinel is achieved. Material to be grown as metal-incorporated single crystal and thin film was taken as a precursor and put into a bath containing acid as a reaction speed-up reagent (catalyst) as well as a solvent with a metal foil as cation scavenger. By this novel method, zinc aluminate crystals having hexagonal facets and thin films having single crystalline orientation were prepared from a single optimized bath. Properties of both crystals and thin films were studied using an x-ray diffractometer and EDAX. ZnAl2O4 is a well-known wide bandgap compound semiconductor (Eg = 3.8 eV), ceramic, opto-mechanical and anti-thermal coating in aerospace vehicles. Thus a space_ gmr technique was found to be a new low cost and advantageous method for in situ and simultaneous growth of single crystals and thin films of a smart material.

Source:IOPscience

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Scientists study the powers of tiny crystals

When it comes to the way scientists react to their discoveries, "That's interesting" falls somewhere between "Eureka!" and "Uh-oh."

"Interesting" is just what Dr. Jeremiah Gassensmith and his graduate student Madushani Dharmarwardana thought when they noticed unusual behavior in a sample of crystals they were working with in Gassensmith's chemistry lab at The University of Texas at Dallas.

As part of her doctoral research, Dharmarwardana was investigating how the material, from a family of organic semiconducting materials called naphthalene diimides, changes color from orange to yellow as it is heated.

"We were looking at this material as a thermochromic semiconductor," said Gassensmith, an assistant professor in the Department of Chemistry and Biochemistry in the School of Natural Sciences and Mathematics. "These types of semiconducting materials change color as temperature changes. Think of beer cans that change color when they're cold or color-changing thermometer strips you put on your forehead to check for fever."

As Dharmarwardana heated the tiny crystals—the samples were only about an eighth of an inch, or a couple of millimeters, in size—she noticed they would move, which was unexpected.

"The crystals would bend, coil, flex or jump, they would do all sorts of things," Gassensmith said. "That was … interesting."

Although such thermosalient behavior—also known as the jumping crystal effect—has been observed in other types of crystals, it had not been observed in this particular class of organic semiconducting crystals, Gassensmith said. Such behavior is of interest to researchers because it might be exploited for applications such as micromachines, sensors, or tiny actuators for medical devices and artificial muscles.

Dharmarwardana conducted a new set of experiments in which she glued down one end of the crystal to a glass cover slip and placed the slip on a hot plate.

"As the plate heated up, the crystal always tried to bend away from the heat," she said. "The explanation for this is that, once the crystal reaches a certain temperature, the arrangement of molecules within the crystal changes. Those changes move sequentially through the material, starting at the hot part that's stuck to the surface and propagating out. This causes the crystal to change shape."

"We see colossal expansion in these materials, almost 20 percent in size," Gassensmith said. "That's among the largest percentage change seen in an organic material."

In her next set of experiments, Dharmarwardana glued tiny stainless steel balls to the anchored crystals to see how much weight the crystal cantilevers could lift as they were heated. Because the crystals are brittle, she expected them to break under the load.

"It amazed me when I saw it was actually lifting the ball because the crystal is very small compared to the weight, which was almost 100 times heavier than the crystal," Dharmarwardana said. "When I designed the experiment, I never thought it would lift up. I thought it would break the crystal."

The maximum load lifted with a 3.5 millimeter-long crystal cantilever was about 4 milligrams, to a height of 0.24 millimeters.

As the crystal cooled, it lowered and became straight once more. While reheating the material did not result in another shape change, the material continued to change color with repeated temperature changes.

"This isn't a reversible transformation," Gassensmith said. "Basically, the crystal starts out loaded with the potential energy to change shape and carry out the motion, but it holds on to that energy until the material reaches a phase transition temperature. At that point, the crystal wants to release this energy. If it is not bound to anything, the crystal will just pop or curl, but by fixing it on one end, we can direct how that energy is released.

"It's still a single crystal, but its molecules are now in a different packing arrangement that has a lower energy."

Gassensmith said the next step is to further investigate different variations of the material, including whether the bending behavior of the materials can be incorporated into color-changing sensors or act as a mechanical breaker inside organic electronics.

"It will be interesting to see whether we can induce curling in these crystals electronically," he said. "In principle, we should be able to apply an electric current to lift things, instead of using a bunch of heaters."

Source:PHYS

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Scientists manage to observe the inner structure of photonic crystals

The scheme of analysis of photonic crystals' inner structure with the help of ptychography. Credit: ©NUST MISIS

With the help of electronic microscopy, scientists have tracked defects in the surface of two-dimensional photonic crystals. But there are difficulties with bulk photonic crystals. There is no way for scientists to research the interiors of these unusual crystals. So scientists have been searching for a method to better measure these crystals for some time.

Ilya Besedin, an engineer from the NUST MISIS Laboratory of Superconducting Metamaterials, jointly with a group of scientists from Germany, the Netherlands, and Russia has demonstrated that there is a method of non-destructive analysis of the inner structure of the substance, which cannot be seen with the use of conventional X-rays. The new system will help to create microprocessors for optical computers. The work was published in Small.

The research group, led by Professor Ivan Vartanyants from MEPhI, has applied the recently developed method of ptychographic to photonic crystals. The method's essence is that the substance is illuminated by X-ray radiation of an exactly defined wave. Sources of such radiation are called synchrotrons, and the experiments were conducted at DESY in Germany.

"With conventional X-rays you can scan either macroscopic or very ordered structures. In our case, for structures of polystyrene spheres of nearly micron size, the accuracy of the image will be even worse than in fluoroscopy. At least, it won't be possible to discern a single object [smaller] than a micron," said Ilya Besedin.

Thanks to such a high quality X-ray, Ilya Besedin and his colleagues have managed to observe the structures of crystals ordered at a scale of tens and hundreds of nanometers. Most importantly, scientists have managed to identify internal defects of mesoscopic structures.

Photonic crystal received with the help of the method of ptychography. Credit: ©NUST MISIS

As Ilya Besedin explained, if the crystal is perfect, the beam can pass through or be reflected. However, because of defects, the beam might deviate from a straight line. "By knowing information about packaging defects, we can understand the logic through which the beam changes its direction. This means we can try to collect logical designs based on photonic crystals. Another thing is that we are not able to control the formation of these defects, we can only try to reduce [the defects] at the macro level," explained Besedin.

"A photonic crystal is like a waveguide for the light, only better. The waveguide is almost impossible to bend, and it's impossible to create photonic microchips on waveguides. A photonic crystal is most suitable for the creation of integral optical microchips where the light can spread where the developers need it to," noted Ilya Besedin. This is why the main value of this work is in the analysis of photonic crystals' inner structure with the help of ptychography.

As Ilya Besedin explained, if the crystal is perfect, the beam can pass through or be reflected. However, because of defects, the beam might deviate from a straight line. "By knowing information about packaging defects, we can understand the logic through which the beam changes its direction. This means we can try to collect logical designs based on photonic crystals. Another thing is that we are not able to control the formation of these defects, we can only try to reduce [the defects] at the macro level," explained Besedin.

"A photonic crystal is like a waveguide for the light, only better. The waveguide is almost impossible to bend, and it's impossible to create photonic microchips on waveguides. A photonic crystal is most suitable for the creation of integral optical microchips where the light can spread where the developers need it to," noted Ilya Besedin. This is why the main value of this work is in the analysis of photonic crystals' inner structure with the help of ptychography.

soource:HPY

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