Chiral magnetic effect generates quantum current

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

http://www.sciencedaily.com/releases/2016/02/160208123826.htm

This photos shows nuclear theorist Dmitri Kharzeev of Stony Brook University and Brookhaven Lab with Brookhaven Lab materials scientists Qiang Li, Genda Gu, and Tonica Valla in a lab where the team measured the unusual high conductivity of zirconium pentatelluride.

Credit: Brookhaven National Laboratory

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.

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 journalNature 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,https://www.bnl.gov/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, https://www.bnl.gov/ps/nsls/about-NSLS.asp) -- 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.

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Story Source:

The above post is reprinted from materials provided by DOE/Brookhaven National Laboratory. Note: Materials may be edited for content and length.

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Journal Reference:

1. Qiang Li et al. Observation of the chiral magnetic effect in ZrTe5.Nature Physics, February 2016 DOI: 10.1038/nphys3648

Mastering microbunching for linac-based light sources

An electron bunch, as seen on the spectrometer screen. By controlling the microbunching instability, this bunch is completely split into about 10 sub–bunches spaced 25 micrometers apart. The ability to control the number of sub-bunches, the distance between them, and their intensity could be beneficial for a number of light sources.

by Boris Podobedov, Sergei Seletskiy, & Xi Yang
http://phys.org/news/2013-08-mastering-microbunching-linac-based-sources.html#jCp
(Phys.org) —Designing accelerators requires years of research and development. Throughout the Lab's history, scientists and engineers at Brookhaven have helped lead the way in designing accelerator technologies for cutting-edge facilities here on site and at institutions around the world.

Our team in the Photon Sciences Directorate and Yuzhen Shen, who is now with the U.S. Patent and Trademark Office, recently tackled a significant problem in accelerator design, a phenomenon called "microbunching instability" that has been identified as one of the most serious challenges to the performance of advanced linear accelerator (linac)-based light sources.

A bit about light sources and microbunching
Light sources, including the National Synchrotron Light Source (NSLS) and the future NSLS-II at Brookhaven, are important tools for producing ultra-bright light that scientists can use to analyze the atomic and molecular structures for advances in different areas of science, ranging from biology and physics to chemistry and geophysics, as well as medicine and materials science. Some light sources are linac-based and others, such as NSLS and NSLS-II, are based on storage rings. In both kinds, the bright light—large quantities of photons—is produced from electron beams that travel in bunches through certain accelerator components, for instance bending magnets or undulators.
Electrons are never distributed completely evenly inside a bunch and any small "noise" in an unstable bunch can cause the electrons to form microstructures as they clump closer or spread further apart. This effect, the microbunching instability, results in chaotic changes of beam density distribution. In extreme cases, especially for more intense bunches that provide higher brightness at light sources, bunches simply split apart. This effect degrades the quality of electron beams and the photon beams they produce.
The microbunching instability is rather common and especially important in the fourth generation light sources, such as the Linear Coherent Light Source at SLAC National Accelerator Laboratory in California. They rely on short-wavelength free electron lasers (FELs) that use short, dense bunches of electrons traveling at nearly the speed of light to make short photon bunches.

Semi-OT A Happy Accident and Subsequent Insight: A New X-Ray Imaging Technique Yields Unprecedented View of Nanoworld

Physicists Kevin Yager (left) and Ben Ocko reviewing their paper on the cover of the Journal of Applied Crystallography [Photo Courtesy: Brookhaven National Laboratory] http://www.2physics.com/2013/03/a-happy-accident-and-subsequent-insight.html

Photographers rely on precision lenses to generate well-focused and crystal-clear images. These high-quality optics—readily available and produced in huge quantities—are often taken for granted. But as scientists explore the details of materials spanning just billionths of a meter, engineering the nanoscale equivalent of a camera lens becomes notoriously difficult.

Xinhui Lu, lead author of the GTSAXS study [Photo Courtesy: Brookhaven National Laboratory] 

Instead of working with polished glass, physicists must use ingenious tricks, including shooting concentrated beams of x-rays directly into materials. These samples then act as light-bending lenses, and the x-ray deflections can be used to deduce the material's nanostructures. Unfortunately, the multilayered internal structures of real materials bend light in extremely complex and unexpected ways. When scientists grapple with this kind of warped imagery, they use elaborate computer calculations to correct for the optical obstacles found on the nanoscale and create detailed visual models.

Now, owing to a happy accident and subsequent insight, researchers at the US Department of Energy's (DOE) Brookhaven National Laboratory have developed a new and strikingly simple x-ray scattering technique—detailed in their paper in the Journal of Applied Crystallography — to help draw nanomaterials ranging from catalysts to proteins into greater focus.

"During an experiment, we noticed that one of the samples was misaligned," said physicist Kevin Yager, a coauthor on the new study. "Our x-ray beam was hitting the edge, not the center as is typically desired. But when we saw how clean and undistorted the data was, we immediately realized that this could be a huge advantage in measuring nanostructures."

This serendipitous discovery at Brookhaven's National Synchrotron Light Source (NSLS) led to the development of a breakthrough imaging technique called Grazing-Transmission Small Angle X-ray Scattering (GTSAXS). The new method requires considerably less correction and a much simpler analysis, resulting in superior images with profound implications for future advances in materials science.

"Conventional scattering produces images that are 'distorted'—the data you want is there, but it's stretched, compressed, and multiply scattered in complicated ways as the x-rays enter and exit the sample," said physicist and coauthor Ben Ocko. "Our insight was that undistorted scattering rays were emitted inside the sample—but they usually get absorbed as they travel through the substrate. By moving the sample and beam near the edge of the substrate, we allow this undistorted scattering to escape and reach the detector."

The Brookhaven Lab collaboration was not the first group to encounter the diffraction that occurs along a material's edge, but it was the first to reconsider and harness the unexpected error.

This rendering shows the high-intensity x-ray beam striking and then traveling through the gray sample material. In this new technique, the x-ray scattering—the blue and white ripples—is considerably less distorted than in other methods, producing superior images with less complex analysis [Image Courtesy: Brookhaven National Laboratory]. 

"Until now, no one bothered to dig into the details, and figure out how to use it as a measurement technique, rather than as a misalignment to be corrected," added Xinhui Lu, the lead author of the study.

GTSAXS, like other scattering techniques, offers a complement to other imaging processes because it can measure the average structure throughout a sample, rather than just pinpointing selected areas. Scattering also offers an ideal method for the real-time studies of nanoscale changes and reactions such as the propagation of water through soft nanomaterials.

"This technique is broadly applicable to any nanostructure sitting on a flat substrate," said study coauthor Chuck Black. "Lithographic patterns, catalytic nanoparticles, self-assembled polymers, etc.—they can all be studied. This technique should be particularly powerful for very thin films with complicated three-dimensional structures, which to date have been difficult to study."

Brookhaven's NSLS supplies the intense x-ray beams essential to this technique, which requires extremely short wavelengths to interact with nanoscale materials. At NSLS, accelerated electrons emit these high-energy photons, which are then channeled down a beamline and focused to precisely strike the target material. When the next generation light source, NSLS-II, opens in 2014, GTSAXS will offer even greater experimental potential.

"We look forward to implementing this technique at NSLS-II," Yager said, with Ocko adding: "The excellent beam focusing should enable us to probe the near-edge region more effectively, making GTSAXS even more robust."

Reference:
[1] Xinhui Lu, Kevin G. Yager, Danvers Johnston, Charles T. Black and Benjamin M. Ocko, "Grazing-incidence transmission X-ray scattering: surface scattering in the Born approximation", Journal of Applied Crystallography, 46, 165-172 (2013). Abstract.

[This report is written by Justin Eure of Brookhaven National Laboratory, USA]

Shaken, not heated: the ideal recipe for manipulating magnetism

http://www.nanowerk.com/news/newsid=24130.php Nanowerk News) Scientists have found a way to distort the atomic arrangement and change the magnetic properties of an important class of electronic materials with ultra-short pulses of terahertz (mid-infrared) laser light without heating the material up. While the achievement is currently of purely scientific interest, the researchers say this new approach control could ultimately lead to extremely fast, low-energy, non-volatile computer memory chips or data-switching devices.

Working at the SLAC National Accelerator Laboratory's Linear Coherent Light Source (LCLS), the scientists aimed intense, 130-femtosecond-long pulses of terahertz light at samples of manganite, a class of complex manganese-oxide compounds that has many desirable electronic and magnetic properties.

With each flash, the material's atoms shimmied and shifted positions, although the overall temperature of the solid barely changed. The scientists then used X-ray laser pulses from the LCLS's Soft X-ray Materials Science (SXR) instrument to measure the material's altered magnetism.

This graphic depicts an ultrashort pulse of terahertz light (yellow arrow) distorting a manganite crystal lattice. Around where the light hits, the diamond-like shaped arrangement of manganese (blue) and oxygen (red) atoms changes to become more square-like, a distortion that also alters the material's magnetism, which is indicated by the direction of red and blue arrows shown over the manganese atoms. The extremely brief duration and high intensity of the LCLS X-ray laser pulses allowed the team to take stop-action images used to measure the material's altered magnetism. (Image courtesy Jörg Harms, Max-Planck Department for Structural Dynamics, Center for Free Electron Laser Science)

Rapid light-induced switching of manganite magnetism has been known for many years, said physicist Michael Först of the Max Planck Department of Structural Dynamics (MPSD) in Hamburg, Germany, one of the leaders of the international research group. However, earlier efforts to trigger this switch with higher-energy, near-visible lasers heated the materials, which greatly limits potential applications.

The latest LCLS experiments confirm that terahertz light only distorts the lattice enough to rearrange the electronic and magnetic properties while not generating extra heat.

The research team was led by scientists from the MPSD (Först and Andrea Cavalleri) and Brookhaven National Laboratory (Ron Tobey and John Hill), and included researchers from England and SLAC. They published their results last month in Physical Review B ("Driving magnetic order in a manganite by ultrafast lattice excitation"). SLAC co-authors are Bill Schlotter and Josh Turner, SXR instrument scientists; Wei-Sheng Lee and Rob Moore of the Stanford Institute for Materials & Energy Science (SIMES), and Mariano Trigo of Photon Ultrafast Laser Science and Engineering (PULSE).

"We will come back to LCLS later this year to use the X-ray Pump Probe instrument to measure terahertz-light-driven atomic displacements directly," said Först. In recent years, his same group of MPSD scientists has also used terahertz light pulses to shake materials into a superconducting state and to change insulators into metals.

Future research aims to explore the phenomenon more deeply and start developing capabilities essential for applications, such as reverse-switching techniques, materials that can switch magnetically at room temperature or higher, and a laser source suitable for using on chips.

Source: By Mike Ross, SLAC National Accelerator Laboratory

Terahertz Spectroscopy used to measure fluctuations in superconductivity across a wide range of temperatures

Scientists at Johns Hopkins University and the U.S. Department of Energy’s Brookhaven National Laboratory report new uses for terahertz spectroscopy.

http://www.azonano.com/news.asp?NewsID=21590

The new technique, allows scientists to see fluctuations lasting mere billionths of a billionth of a second revealing that these fleeting fluctuations disappear 10-15 Kelvin (K) above the transition temperature (T_c) at which superconductivity sets in.

“Our findings suggest that in cuprate superconductors, the transition to the non-superconducting state is driven by a loss of coherence among the electron pairs,” said Brookhaven physicist Ivan Bozovic, a co-author on a paper describing the results in Nature Physics online, February 13, 2011.

Scientists have been searching for an explanation of high-T_c superconductivity in cuprates since these materials were discovered some 25 years ago. Because they can operate at temperatures much warmer than conventional superconductors, which must be cooled to near absolute zero (0 K or -273 degrees Celsius), high- T_c superconductors have the potential for real world applications. If scientists can unravel the current-carrying mechanism, they may even be able to discover or design versions that operate at room temperature for applications such as zero-loss power transmission lines. For this reason, many researchers believe that understanding how this transition to superconductivity occurs in cuprates is one of the most important open questions in physics today.

In conventional superconductors, electron pairs form at the transition temperature and condense into a collective, coherent state to carry current with no resistance. In high- T_cvarieties, which can operate at temperatures as high as 165 K, there are some indications that electron pairs might form at temperatures 100-200 K higher, but only condense to become coherent when cooled to the transition temperature.

To explore the phase transition, the Johns Hopkins-BNL team sought evidence for superconducting fluctuations above T_c.

“These fluctuations are something like small islands or droplets of superconductivity, within which the electron pairs are coherent, which pop up here and there and live for a while and then evaporate to pop up again elsewhere,” Bozovic said. “Such fluctuations occur in every superconductor,” he explained, “but in conventional ones only very, very close to T_c — the transition is in fact very sharp.”

Some scientists have speculated that in cuprates, on the contrary, superconducting fluctuations might exist in an extremely broad region, all the way up to the temperature at which the electron pairs form. In the present study, the scientists tackle this question head-on, by measuring the conductivity as a function of temperature and frequency up to the terahertz range.

“With this technique, one can see superconducting fluctuations as short-lived as one billionth of one billionth of a second — the shortest possible — and over the entire phase diagram,” Bozovic said.

The scientists studied a superconductor containing variable amounts of lanthanum and strontium layered with copper oxide. The samples were fabricated at Brookhaven, using a unique atomic-layer molecular beam epitaxy system that allows for digital synthesis of atomically smooth and perfect thin films. Terahertz spectroscopy measurements were performed at Johns Hopkins.

The central finding was somewhat surprising: The scientists clearly observed superconducting fluctuations, but these fluctuations faded out relatively quickly, within about 10-15 K above T_c, regardless of the lanthanum/strontium ratio.

This implies that in cuprates at the transition temperature, electron pairs lose their coherence. This is in contrast to what happens in conventional superconductors, where the electron pairs break apart at the transition temperature.

“So, unlike in conventional superconductors, the transition in cuprates is not driven by electron (de)pairing but rather by loss of coherence between pairs — that is, by phase fluctuations,” Bozovic said. “The hope is that understanding this process in full detail may bring us one step closer towards cracking the enigma of high-temperature superconductivity.”

This research was supported by DOE’s Office of Science.

Source: http://www.bnl.gov/