Chiral Magnetic Effect Generates Quantum Current

Separating left- and right-handed particles in a semi-metallic material produces anomalously high conductivity

 

Scientists at the U.S Department of Energy's (DOE) Brookhaven National Laboratory and Stony Brook University have discovered a new way to generate very low-resistance electric current in a new class of materials. The discovery, which relies on the separation of right- and left-"handed" particles, points to a range of potential applications in energy, quantum computing, and medical imaging, and possibly even a new mechanism for inducing superconductivity—the ability of some materials to carry current with no energy loss. 

The material the scientists worked with, zirconium pentatelluride, has a surprising trait: When placed in parallel electric and magnetic fields, it responds with an imbalance in the number of right- and left-handed particles—a chiral imbalance. That imbalance pushes oppositely charged particles in opposite directions to create a powerful electric current. 

This "chiral magnetic effect" had long been predicted theoretically, but never observed definitively in a materials science laboratory at the time this work was done. 

"The resistance of this material drops as the magnetic field strength increases, which could open up a completely different route toward achieving something like superconductivity." — Brookhaven Lab/Stony Brook University nuclear physics theorist Dmitri Kharzeev

In fact, when physicists in Brookhaven's Condensed Matter Physics & Materials Science Department (CMP&MS) first measured the significant drop in electrical resistance, and the accompanying dramatic increase in conductivity, they were quite surprised. "We didn't know this large magnitude of 'negative magnetoresistance' was possible," said Qiang Li, a physicist and head of the advanced energy materials group in the department and a co-author on a paper describing these results just published in the journal Nature Physics. But after teaming up with Dmitri Kharzeev, the head of the RIKEN-BNL theory group at Brookhaven and a professor at Stony Brook, the scientists had an explanation. 

Kharzeev had explored similar behavior of subatomic particles in the magnetic fields created in collisions at the Lab's Relativistic Heavy Ion Collider (RHIC), a DOE Office of Science User Facility where nuclear physicists explore the fundamental building blocks of matter. He suggested that in both the RHIC collisions and zirconium pentatelluride, the separation of charges could be triggered by a chiral imbalance.

To test the idea, they compared their measurements with the mathematical predictions of how powerful the increase in conductivity should be with increasing magnetic field strength. 

"We looked at the data and we said, 'Gee, that's it!' We tested six different samples and confirmed that no matter how you do it, it's there as long as the magnetic field is parallel to the electrical current. That's the smoking gun," Li said. 

Going Chiral

 

Right- or left-handed chirality is determined by whether a particle's spin is aligned with or against its direction of motion. In order for chirality to be definitively established, particles have to behave as if they are nearly massless and able to move as such in all three spatial directions. 

While free-flowing nearly massless particles are commonly found in the quark-gluon plasma created at RHIC, this was not expected to occur in condensed matter. However, in some recently discovered materials, including "Dirac semimetals"—named for the physicist who wrote the equations to describe fast-moving electrons—nearly massless "quasiparticle" versions of electrons (and positively charged "holes") propagate through the crystal in this free manner. 

Some aspects of this phenomenon, namely the linear dependence of the particles' energy on their momentum, can be directly measured and visualized using angle-resolved photoemission spectroscopy (ARPES).

"On first sight, zirconium pentatelluride did not even look like a 3D material," said Brookhaven physicist Tonica Valla, who performed the measurements with collaborators at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory and at Brookhaven's National Synchrotron Light Source (NSLS)—two additional DOE Office of Science User Facilities. "It is layered, similar to graphite, so a quasi-2D electronic structure would be more expected. However, as soon as we did the first ARPES measurements, it was clear that the material is a 3D Dirac semimetal."

These results agreed nicely with the ones on conductivity and explained why the chiral magnetic effect was observed in this material.

In the absence of magnetic and electric fields, zirconium pentatelluride has an even split of right- and left- handed quasiparticles. But adding parallel magnetic and electric fields introduces a chiral preference: The magnetic field aligns the spins of the positive and negative particles in opposite directions, and the electric field starts the oppositely charged particles moving—positive particles move with the electric field, negative ones against it. If the two fields are pointing in the same direction, this creates a preference for positive and negative particles that are each moving in a direction aligned with their spin orientation—right-handed chiral particles—but with positive and negative particles moving away from one another. (If the magnetic field orientation is flipped relative to the electric field, the preference would be for left-handed particles, but still with opposite charges separating.) 

"This chiral imbalance gives a big boost to the separation of the oppositely charged particles, which can be connected through an external circuit," Kharzeev said. And once the chiral state is set it's hard to alter, "so very little energy is lost in this chiral current." 

Potential applications

 

The dramatic conductivity and low electrical resistance of Dirac semimetals may be key to potential applications, including "quantum electricity generators" and quantum computing, Li said.

"In a classic generator, the current increases linearly with increasing magnetic field strength, which needs to be changing dynamically. In these materials, current increases much more dramatically in a static magnetic field. You could pull current out of the 'sea' of available quasiparticles continuously. It's a pure quantum behavior," Li said.

Separating the two chiral states could also give a new way of encoding information—analogous to the zeros and ones of computing. And because the chiral state is very stable compared with other electrical states, it's much less prone to interference from external influences, including defects in the material. It could therefore be a more reliable material for quantum computing, Li said.

Kharzeev has some other ideas: "The resistance of this material drops as the magnetic field strength increases, which could open up a completely different route toward achieving something like superconductivity—zero resistance," he said. Right now the materials show at least some reduction in resistance at temperatures as high as 100 Kelvin—in the realm of the best high-temperature superconductors. But there are many different types of Dirac semimetals to experiment with to explore the possibility of higher temperatures or even more dramatic effects. Such low-resistance materials could help overcome a major limit in the speed of microprocessors by reducing the dissipation of current, Kharzeev added. 

"In zirconium pentatelluride and other materials that have since been discovered to have the chiral magnetic effect, an external magnetic field is required to start reducing resistivity," Valla said.

"However, we envision that in some magnetic materials, the electrical current could flow with little or no resistance in a direction parallel with the material's internal magnetic field. That would eliminate the need for external magnetic fields and would offer another avenue for dissipationless transport of electrical current."

Kharzeev and Li are also interested in exploring unusual optical properties in chiral materials. "These materials possess collective excitations in the terahertz frequency range, which could be important for wireless communications and also in imaging techniques that could improve the diagnosis of cancer," Kharzeev said.

Getting back to his nuclear physics roots, Kharzeev added, "The existence of massless quasiparticles that strongly interact makes this material quite similar to the quark-gluon plasma created in collisions at RHIC, where nearly massless quarks strongly interact through the exchange of gluons. So this makes Dirac semimetals an interesting arena for testing some of the ideas proposed in nuclear physics."

"This research illustrates a deep connection between two seemingly unrelated fields, and required contributions from an interdisciplinary team of condensed matter and nuclear physicists," said James Misewich, the Associate Laboratory Director for Energy Science at Brookhaven Lab and a professor of physics at Stony Brook University, who played the central role of introducing the members of this research team to one another. "We're fortunate to have scientists with expertise in these fields here at Brookhaven and nearby Stony Brook University, and the kind of collaborative spirit to make such a project come to fruition," he said.

Brookhaven National Laboratory 

Laser spectroscopy of ultrathin semiconductor reveals rise of ‘trion’ quasiparticles

Insight into dynamics may spur improved materials for solar energy, quantum computing

Quasiparticles—excitations that behave collectively like particles—are central to energy applications but can be difficult to detect. Recently, however, researchers have seen evidence of quasiparticles called negative trions forming and fading in a layer of semiconducting material that is 100,000 times thinner than a human hair. Scientists at the Department of Energy’s Oak Ridge National Laboratory used ultrafast laser spectroscopy at the Center for Nanophase Materials Sciences (CNMS) to demystify the dynamics of the negative trions. They explored the behavior of the charged quasiparticle in a two-dimensional (2D) semiconductor that is an excellent absorber of sunlight.

Their insights, published in the journal Physical Review B, may prove important for advancing technologies for solar energy and quantum computing.

“We observed negative trions in a two-dimensional tungsten disulfide monolayer excited by a laser beam,” said ultrafast laser spectroscopist Abdelaziz Boulesbaa, who co-led the study with theorist Bing Huang and consulted with laser spectroscopy expert Alex Puretzky. “This discovery may open new opportunities to optoelectronic applications, including information technology, as well as fundamental research in the physics of low-dimensional materials.”

When a semiconductor absorbs light, electrons can be knocked loose and can participate in an electrical current. However, typically two charges form—one negative (an electron) and one positive (a hole)—and are bound to each other for a short time, traveling through the crystal as a quasiparticle called an “exciton.” When an exciton binds to an additional electron, the complex formed is a negative trion, or if it binds to an additional hole, the resulting quasiparticle is a positive trion.

Quasiparticles like excitons may sound exotic, but getting electrons and holes together is the basis for everyday light-emitting diodes (LEDs). When an electron and hole recombine in an LED, a photon is emitted. That’s the light we see in applications from traffic lights and electronic signage to camera flashes and vehicle headlights.

Whereas LEDs emit light, solar cells absorb light and convert its energy into electricity. To make solar cells work, scientists try to separate the electrons from the holes and collect those charges before they have a chance to recombine. Future materials may make use of negative trions to improve charge collection in solar cells, according to Boulesbaa.

Pump–probe experiment

To harness negative trions for improving solar cells and other optoelectronic technologies, scientists need answers to basic questions: How do negative trions form? How long do they live? Why do they form so efficiently in an ultrathin semiconductor?

To answer these questions, the ORNL scientists needed a “camera” of sorts that could make a super-slow-motion movie to reveal quasiparticle dynamics, akin to the camera techniques photographers employ to capture speeding bullets obliterating apples—only a billion times faster. A split laser beam created that camera.

Employing half the laser beam, they fired laser pulses lasting a mere 40 femtoseconds (million-billionths of a second) to excite an ultrathin crystal of tungsten disulfide. Then, for their super slow-motion movie, they fashioned a strobe using the other half of the laser beam—an ultrafast flash of white light—and passed it through the crystal at different delayed times. By measuring the photon energy wavelengths (colors) the crystals absorbed at each time, the scientists built, frame by frame, a slow-motion “movie” of how trions form and fade.  They probably skipped the popcorn, as their movie lasted only a nanosecond (one billionth of a second).

Their movie revealed trions form only after electron–hole pairs form. Then the holes get trapped, most likely by the substrate in contact with the crystal, leaving extra electrons.  

These extra electrons allow the crystal to absorb another photon to form a negative trion.  Because the ultrathin crystals are all “surface,” they have a lot of opportunity to interact with surroundings and to separate charges that are created, making them great trion generators.

Because the researchers used white light, a mixture of all frequencies of light in the visible spectrum, their observation of light of different colors revealed that two different trions had formed, which had not been seen previously.

Next the scientists will study the role of the substrate in defining optical and electrical properties of 2D semiconducting materials. Some substrates trap electrons, leaving excess holes to carry charges, whereas others trap holes, leaving excess electrons to carry charges. Furthermore, the researchers will isolate the 2D semiconductor from the substrate by introducing, in between, an insulator to prevent holes and electrons from reaching the substrate, allowing excitons to live longer and emit light for a greater duration.

The title of the paper is “Observation of two distinct negative trions in tungsten disulfide monolayers.” This research was conducted at the Center for Nanophase Materials Sciences at ORNL. Computations were performed at the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory. Both are DOE Office of Science User Facilities.

ORNL

Nanotechnology World Association

Beyond Majorana: Ultracold gases as a platform for observing exotic robust quantum states

The quantum Hall effect, discovered in the early 1980s, is a phenomenon that was observed in a two-dimensional gas of electrons existing at the interface between two semiconductor layers. Subject to the severe criteria of very high material purity and very low temperatures, the electrons, when under the influence of a large magnetic field, will organize themselves into an ensemble state featuring remarkable properties.

Many physicists believe that quantum Hall physics is not unique to electrons, and thus it should be possible to observe this behavior elsewhere, such as in a collection of trapped ultracold atoms. Experiments at JQI and elsewhere are being planned to do just that. On the theoretical front, scientists* at JQI and University of Maryland have also made progress, which they describe in the journal Physical Review Letters. The result, to be summarized here, proposes using quantum matter made from a neutral atomic gas, instead of electrons. In this new design, elusive exotic states that are predicted to occur in certain quantum Hall systems should emerge. These states, known as parafermionic zero modes, may be useful in building robust quantum gates.

For electrons, the hallmark of quantum Hall behavior includes a free circulation of electrical charge around the edge but not in the interior of the sample. This research specifically relates to utilizing the fractional quantum Hall (FQH) effect, which is a many-body phenomenon. In this case, one should not consider just the movement of individual electrons, but rather imagine the collective action of all the electrons into particle-like “quasiparticles.” These entities appear to possess fractional charge, such as 1/3.

How does this relate to zero modes? Zero modes, as an attribute of quantum Hall systems, come into their own in the vicinity of specially tailored defects. Defects are where quasiparticles can be trapped. In previous works, physicists proposed that a superconducting nanowire serve as a defect that snags quasiparticles at either end of the wire. Perhaps the best-known example of a composite particle associated with zero-mode defects is the famed Majorana fermion.

Author David Clarke, a Postdoctoral Research Scholar from the UMD Condensed Matter Theory Center, explains, “Zero modes aren’t particles in the usual sense. They’re not even quasiparticles, but rather a place that a quasiparticle can go and not cost any energy.”

Aside from interest in them for studying fundamental physics, these zero modes might play an important role in quantum computing. This is related to what’s known as topology, which is a sort of global property that can allow for collective phenomena, such as the current of charge around the edge of the sample, to be impervious to the tiny microscopic details of a system. Here the topology endows the FQH system with multiple quantum states with exactly the same energy. The exactness and imperturbability of the energy amid imperfections in the environment makes the FQH system potentially useful for hosting quantum bits. The present report proposes a practical way to harness this predicted topological feature of the FQH system through the appearance of what are known as parafermionic zero-modes.

These strange and wonderful states, which in some ways go beyond Majoranas, first appeared on the scene only a few years ago, and have attracted significant attention. Now dubbed ‘parafermions,’ they were first proposed by Maissam Barkeshli and colleagues at Stanford University. Barkeshli is currently a postdoctoral researcher at Microsoft Station Q and will be coming soon to JQI as a Fellow. Author David Clarke was one of the early pioneers in studying how these states could emerge in a superconducting environment. Because both parafermions and Majoranas are expected to have unconventional behaviors when compared to the typical particles used as qubits, unambiguously observing and controlling them is an important research topic that spans different physics disciplines. From an application standpoint, parafermions are predicted to offer more versatility than Majorana modes when constructing quantum computing resources.

What this team does, for the first time, is to describe in detail how a parafermionic mode could be produced in a gas of cold bosonic atoms. Here the parafermion would appear at both ends of a one-dimensional trench of Bose-Einstein Condensate (BEC) atoms sitting amid a larger two-dimensional formation of cold atoms displaying FQH properties. According to first author and Postdoctoral Researcher Mohammad Maghrebi, “The BEC trench is the defect that does for atoms what the superconducting nanowire did for electrons.”

Some things are different for electrons and neutral atoms. For one thing, electrons undergo the FQH effect only if exposed to high magnetic fields. Neutral atoms have no charge and thus do not react strongly to magnetic fields; researchers must mimic this behavior by exposing the atoms to carefully designed laser pulses, which create a synthetic field environment. JQI Fellow Ian Spielman has led this area of experimental research and is currently performing atom-based studies of quantum Hall physics.

Another author of the PRL piece, JQI Fellow Alexey Gorshkov, explains how the new research paper came about: “Motivated by recent advances in Spielman's lab and (more recently) in other cold atom labs in generating synthetic fields for ultracold neutral atoms, we show how to implement in a cold-atom system the same exotic parafermionic zero modes proposed originally in the context of condensed-matter systems.”

“We argue that these zero modes, while arguably quite difficult to observe in the condensed matter context, can be observed quite naturally in atomic experiments,” says Maghrebi. “The JQI atmosphere of close collaboration and cross-fertilization between atomic physics and condensed matter physics on the one hand and between theory and experiment on the other hand was at the heart of this work.”

“Ultracold atoms play by a slightly different set of rules from the solid state,” says JQI Fellow Jay Sau. Things which come easy in one are hard in the other. Figuring out the twists in getting a solid state idea to work for cold atoms is always fun and the JQI is one of the best places to do it.”

(*)  The PRL paper has five authors, and their affiliations illustrate the complexity of modern physics work.  Mohammad Maghrebi, Sriram Ganeshan, Alexey Gorshkov, and Jay Sau are associated with the Joint Quantum Institute, operated by the University of Maryland and the National Institute for Standards and Technology.  Three of the authors---Ganeshan, Clarke, and Sau---are also associated with the Condensed Matter Theory Center at the University of Maryland physics department.  Finally, Maghrebi and Gorshkov are associated with the Joint Center for Quantum Information and Computer Science (usually abbreviated QuICS), which is, like the JQI, a University of Maryland-NIST joint venture.

JQI

Nanotechnology World Association

Even steps to quantum computation

A rare class of quantum state that could be useful in information processes is observed in a two-dimensional oxide material system

 

Electrons are normally free to move through a solid in all three dimensions. Restricting their motion to a two-dimensional surface can, however, radically alter the properties of the material. A RIKEN-led team has now created a two-dimensional system that displays an exotic physical effect that could be useful for quantum computing.

Applying an electric potential between the two sides of a two-dimensional sheet of a semiconducting material under a magnetic field can cause charge carriers to flow sideways along the sheet. This is known as the Hall effect, and such materials display electrical resistance both in the direction of the applied voltage and perpendicular to it. The quantum Hall effect, a signature of two-dimensional systems, becomes evident when the magnetic field is increased and the perpendicular Hall resistance increases in discrete steps. 

Each of these steps corresponds to an electrical conductance equal to a fundamental constant multiplied by a fraction in which both the numerator and denominator are integers.

A team of researchers from the RIKEN Center for Emergent Matter Science, University of Tokyo, the Max Planck Institute for Solid State Research in Germany and other Japanese institutions has now observed the fractional quantum Hall effect in a two-dimensional system formed at the interface between zinc oxide and magnesium zinc oxide.

Fundamental to the team’s success in observing such an exotic quantum effect was the fabrication of high-quality material systems. The researchers created their ZnO-based structure using a method called molecular beam epitaxy, which is known for its ability to produce materials with high crystalline quality. They then attached eight electrical contacts to their sample and performed magnetoresistance measurements at ultralow temperatures.

The researchers observed a series of levels corresponding to fractional states, or filling factors, between 4/3 and 9/2. Most notably, even-denominator states were observed at 3/2 and 7/2, with some evidence for 9/2. Such a series has not been observed in any other material system.

These states are believed to arise because of the existence of quasiparticles made of pairs of electrons (Fig. 1). Such particle pairs are expected to be useful in quantum computers. “These quasiparticles are said to be topologically protected and are robust against weak perturbations,” says the study’s lead author Joseph Falson. “This is in contrast to quantum bits in say, silicon, which are very sensitive to slight changes in temperature or electric field. We now plan to probe the details of the states this work has unveiled.”

http://www.nanotechnologyworld.org/#!Even-steps-to-quantum-computation/c89r/558c7aec0cf2ef0f928ca7e5