Monday, January 12, 2015

Unstoppable Magnetoresistance

Mazhar Ali, a fifth-year graduate student in the laboratory of Bob Cava, the Russell Wellman Moore Professor of Chemistry at Princeton University, has spent his academic career discovering new superconductors, materials coveted for their ability to let electrons flow without resistance. While testing his latest candidate, the semimetal tungsten ditelluride (WTe2), he noticed a peculiar result.
Ali applied a magnetic field to a sample of WTe2, one way to kill superconductivity if present, and saw that its resistance doubled. Intrigued, Ali worked with Jun Xiong, a student in the laboratory of Nai Phuan Ong, the Eugene Higgins Professor of Physics at Princeton, to re-measure the material’s magnetoresistance, which is the change in resistance as a material is exposed to stronger magnetic fields.
“He noticed the magnetoresistance kept going up and up and up—that never happens.” said Cava. The researchers then exposed WTe2 to a 60-tesla magnetic field, close to the strongest magnetic field mankind can create, and observed a magnetoresistance of 13 million percent. The material’s magnetoresistance displayed unlimited growth, making it the only known material without a saturation point. The results were published on September 14 in the journal Nature.
Electronic information storage is dependent on the use of magnetic fields to switch between distinct resistivity values that correlate to either a one or a zero. The larger the magnetoresistance, the smaller the magnetic field needed to change from one state to another, Ali said. Today’s devices use layered materials with so-called “giant magnetoresistance,” with changes in resistance of 20,000 to 30,000 percent when a magnetic field is applied. “Colossal magnetoresistance” is close to 100,000 percent, so for a magnetoresistance percentage in the millions, the researchers hoped to coin a new term.
Their original choice was “ludicrous” magnetoresistance, which was inspired by “ludicrous speed,” the fictional form of fast-travel used in the comedy “Spaceballs.” They even included an acknowledgement to director Mel Brooks. After other lab members vetoed “ludicrous,” the researchers considered “titanic” before Nature editors ultimately steered them towards the term “large magnetoresistance.”
Terminology aside, the fact remained that the magnetoresistance values were extraordinarily high, a phenomenon that might be understood through the structure of WTe2. To look at the structure with an electron microscope, the research team turned to Jing Tao, a researcher at Brookhaven National Laboratory.
“Jing is a great microscopist. They have unique capabilities at Brookhaven,” Cava said. “One is that they can measure diffraction at 10 Kelvin (-441 °F). Not too many people on Earth can do that, but Jing can.”
Electron microscopy experiments revealed the presence of tungsten dimers, paired tungsten atoms, arranged in chains responsible for the key distortion from the classic octahedral structure type. The research team proposed that WTe2 owes its lack of saturation to the nearly perfect balance of electrons and electron holes, which are empty docks for traveling electrons. Because of its structure, WTe2 only exhibits magnetoresistance when the magnetic field is applied in a certain direction. This could be very useful in scanners, where multiple WTe2 devices could be used to detect the position of magnetic fields, Ali said.
“Aside from making devices from WTe2, the question to ask yourself as a scientist is: How can it be perfectly balanced, is there something more profound,” Cava said.

Superconductivity in Orbit: Scientists Find New Path to Loss-Free Electricity

Armed with just the right atomic arrangements, superconductors allow electricity to flow without loss and radically enhance energy generation, delivery, and storage. Scientists tweak these superconductor recipes by swapping out elements or manipulating the valence electrons in an atom's outermost orbital shell to strike the perfect conductive balance. Most high-temperature superconductors contain atoms with only one orbital impacting performance—but what about mixing those elements with more complex configurations?
"For the first time, we obtained direct experimental evidence of the subtle changes in electron orbitals by comparing an unaltered, non-superconducting material with its doped, superconducting twin," said Brookhaven Lab physicist and project leader Yimei Zhu. Now, researchers at the U.S. Department of Energy's Brookhaven National Laboratory have combined atoms with multiple orbitals and precisely pinned down their electron distributions. Using advanced electron diffraction techniques, the scientists discovered that orbital fluctuations in iron-based compounds induce strongly coupled polarizations that can enhance electron pairing—the essential mechanism behind superconductivity. The study, set to publish soon in the journal Physical Review Letters, provides a breakthrough method for exploring and improving superconductivity in a wide range of new materials.
"For the first time, we obtained direct experimental evidence of the subtle changes in electron orbitals by comparing an unaltered, non-superconducting material with its doped, superconducting twin," said Brookhaven Lab physicist and project leader Yimei Zhu. 
While the effect of doping the multi-orbital barium iron arsenic—customizing its crucial outer electron count by adding cobalt—mirrors the emergence of high-temperature superconductivity in simpler systems, the mechanism itself may be entirely different. 
"Now superconductor theory can incorporate proof of strong coupling between iron and arsenic in these dense electron cloud interactions," said Brookhaven Lab physicist and study coauthor Weiguo Yin. "This unexpected discovery brings together both orbital fluctuation theory and the 50-year-old 'excitonic' theory for high-temperature superconductivity, opening a new frontier for condensed matter physics."
The experimental work at Brookhaven Lab was supported by DOE's Office of Science. The materials synthesis was carried out at the Chinese Academy of Sciences' Institute of Physics. Brookhaven Lab coauthors of the study also include Chao Ma, Lijun Wu, and Chris Homes.
DOE's Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit the Office of Science website at science.energy.gov.

Harnessing Magnetic Vortices for Making Nanoscale Antennas


UPTON, NY—Scientists at the U.S. Department of Energy's Brookhaven National Laboratory are seeking ways to synchronize the magnetic spins in nanoscale devices to build tiny yet more powerful signal-generating or receiving antennas and other electronics. Their latest work, published in Nature Communications, shows that stacked nanoscale magnetic vortices separated by an extremely thin layer of copper can be driven to operate in unison, potentially producing a powerful signal that could be put to work in a new generation of cell phones, computers, and other applications.
The aim of this "spintronic" technology revolution is to harness the power of an electron's "spin," the property responsible for magnetism, rather than its negative charge.
"Almost all of today's electronic technology, from the light bulb to the smartphone, involves the movement of charge," said Brookhaven physicist Javier Pulecio, lead author on the new study. "But harnessing spin could open the door for much more compact and novel types of antennas that act as spin wave emitters, signal generators—such as the clocks that synchronize everything that goes on inside a computer—as well as memory and logic devices." 
The secret to harnessing spin is to control its evolution and spin configuration.
"If you grab a circular refrigerator magnet and place it under a microscope that could image electron spins, you would see the magnet has several regions called domains, where within each domain all the spins point in the same direction," explained group leader Yimei Zhu. "If you were to shrink that magnet down to a size smaller than a red blood cell, the spins inside the magnet will begin to align themselves into unique spin textures."
In the Nature Communications paper, Pulecio, Zhu, and their collaborators at the Swiss Light Source, Brookhaven's National Synchrotron Light Source, and Stony Brook University explored expanding the device in three dimensions by stacking one vortex on top of another, with the individual discs separated by a thin non-magnetic layer. They investigated how changing the thickness of the non-magnetic layer affected the fundamental interactions at the nanoscale, and how those, in turn, affected the coupled dynamics of the vortices. They directly imaged how the vortices responded to high-frequency stimulation using high-resolution Lorentz transmission electron microscopy imaging. 
The results: A thicker separating layer resulted in somewhat unordered motion of the coupled vortices in the two discs. The thinner the separating layer, the stronger the vortices were linked, synching up in space into coherent circular motion. This could help to overcome the power limitations of current vortex-based spintronic antennas by creating arrays of synchronized tiny oscillators through coupled 3D stacks.

This research was supported by the Core-Research Programs within Basic Energy Science, DOE Office of Science. Fabrication of the devices was supported in part by the Center for Functional Nanomaterials at Brookhaven National Laboratory.
DOE's Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Thursday, December 11, 2014

Unusual Electronic State Found in New Class of Unconventional Superconductors

chain of atoms
A team of scientists from the U.S. Department of Energy's (DOE) Brookhaven National Laboratory, Columbia Engineering, Columbia Physics and Kyoto University has discovered an unusual form of electronic order in a new family of unconventional superconductors. The finding, described in the journal Nature Communications, establishes an unexpected connection between this new group of titanium-oxypnictide superconductors and the more familiar cuprates and iron-pnictides, providing scientists with a whole new family of materials from which they can gain deeper insights into the mysteries of high-temperature superconductivity.
"Finding this new material is a bit like an archeologist finding a new Egyptian pharaoh's tomb," said Simon Billinge, a physicist at Brookhaven Lab and Columbia University's School of Engineering and Applied Science, who led the research team. "As we try and solve the mysteries behind unconventional superconductivity, we need to discover different but related systems to give us a more complete picture of what is going on—just as a new tomb will turn up treasures not found before, giving a more complete picture of ancient Egyptian society."
Harnessing the power of superconductivity, or the ability of certain materials to conduct electricity with zero energy loss, is one of the most exciting possibilities for creating a more energy-efficient future. But because most superconductors only work at very low temperatures—just a few degrees above absolute zero, or -273 degrees Celsius—they are not yet useful for everyday life. The discovery in the 1980s of "high-temperature" superconductors that work at warmer temperatures (though still not room temperature) was a giant step forward, offering scientists the hope that a complete understanding of what enables these materials to carry loss-free current would help them design new materials for everyday applications. Each new discovery of a common theme among these materials is helping scientists unlock pieces of the puzzle.
This work was supported by the DOE Office of Science, the U.S. National Science Foundation (NSF, OISE-0968226), the Japan Society of the Promotion of Science, the Japan Atomic Energy Agency, and the Friends of Todai Inc. 
Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the UnitedStates, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.




Wednesday, February 19, 2014

Superconductivity in Orbit: Scientists Find New Path to Loss-Free Electricity

Armed with just the right atomic arrangements, superconductors allow electricity to flow without loss and radically enhance energy generation, delivery, and storage. Scientists tweak these superconductor recipes by swapping out elements or manipulating the valence electrons in an atom's outermost orbital shell to strike the perfect conductive balance. Most high-temperature superconductors contain atoms with only one orbital impacting performance—but what about mixing those elements with more complex configurations? 

Link: http://www.bnl.gov/newsroom/news.php?a=11609





Sunday, December 1, 2013

2013

1.  Hirai, D.; Bremholm, M.; Allred, J. M.; Krizan, J.; Schoop, L. M.; Huang, Q.; Tao, J.; and Cava, R. J., “Spontaneous formation of zigzag chains at the metal-insulator transition in the beta -pyrochlore CsW2O6”, Phys. Rev. Lett., 110, 166402 (2013).

2.  Krejci, A.J.; Thomas, C.G.W; Mandal, J.; Gonzalo-Juan, I.; He, W.; Stillwell, R.L.; Park, J.-H.; Prasai, D.; Volkov, V.; Bolotin, K.I.; and Dickerson J.H. “Using Voronoi Tessellations to Assess Nanoparticle-Nanoparticle Interactions and Ordering in Monolayer Films Formed through Electrophoretic Deposition”, J. of Phys. Chem. B 117, 1664-1669 (2013).

3.  Chen, S., Jenkins, S., Tao, J., Zhu Y., and Chen, J., “Anisotropic Seeded Growth of Cu-M (M=Au, Pt, or Pd) “Bimetallic Nanorods with Tunable Optical and Catalytic Properties”, J. Phys. Chem. C, 117, 8924 (2013).

4.  Han, M.G., Zhu, Y., Wu, L., Aoki, T., Volkov V., Wang, X., Chae, C., and Cheong, S.-W., “Ferroelectric switching dynamics of topological vortex domains in a hexagonal manganite”, Advanced Materials, 25, 2415-2421 (2013).

5.  Hockel, J. L., Pollard, S. D.,  Wetzlar, K. P., Wu, T.,  Zhu, Y.,  Carman, G. P., "Electrically Controlled Reversible and Hysteretic Domain Wall Evolution in Nickel Thin Film/Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32 (011) Heterostructure Observed by Lorentz TEM," Applied Physics Letters 102, 242901 (2013).

6.  Hsieh, Y.-C., Zhang, Y., Su, D., Volkov, V.V., Si, R., Wu, L., Zhu, Y., An, W., Liu, P., He, P., Ye, S., Adzic, R.R., Wang, J.X., “Ordered bilayer ruthenium-platinum core-shell nanoparticles as carbon monoxide-tolerant fuel cell catalysts”, Nature Comm., 4, 2466 (2013).

7.  Inada, H., and Zhu, Y., “Secondary electron microscopy in STEM”, chapter in   N. Tanaka Ed., 39 pages, in Scanning Transmission Electron Microscopy, Springer, in press (2013).

8.  Jiang, L., Choi, W.S., Jeen, H., Dong, S., Kim, Y., Han, M.-G., Zhu, Y., Kalinin, S., Dagotto, E., Egami, T., Lee, and Ho, N., "Tunneling Electroresistance Induced by Interfacial Phase Transitions in Ultrathin Oxide Heterostructures", Nano Letters, in press (2013).

9.  Liu, X., Liu, S., Han, M-G., Zhao, L., Deng, H., Li, J., Zhu, Y., Krusin-Elbaum, L., and O'Brien, S., “Magnetoelectricity in CoFe2O4 nanocrystal-P(VDF-HFP) thin films”, Nanoscale Res. Lett., 8, 374 (2013).

10. Meng, Q., Wu, L., and Zhu, Y., “Phonon scattering of interfacial strain field between dissimilar lattices”. Phy. Rev. B 87, 064102 (2013).

11. Nam, K.-W., Bak, S.-M., Hu, E., Yu, X., Zhou, Y., Wang, X., Wu, L., Zhu, Y., Chung, K.-Y., Yang, X.-Q., "Combining In Situ Synchrotron X-Ray Diffraction and Absorption Techniques with Transmission Electron Microscopy to Study the Origin of Thermal Instability in Overcharged Cathode Materials for Lithium-Ion Batteries", Adv. Funct. Mater., 23, 1047-1063 (2013).

12. Pan, C., H. Li, B. Akgun,  S. K. Satijia, Y. Zhu, D. Xu, J. Ortiz, D. Gersappe, and M. H. Rafailovich, "Enhancing the Efficiency of Bulk Heterojunction Solar Cells via Templated Self-Assembly", ACS Macromolecules, 46 1812-1819 (2013).

13. Piazza, L., Mann, A., Carbone, F., Ma, C., Yang, H., Li, L., and Zhu, Y., “Ultrafast structural and electronic dynamics of the metallic phase in a layered manganite”, Structure Dynamics, in press (2013).

14. Pollard, S. D., Malec, M., Bellagia, M., Kawasaki, M., and Zhu, Y., "Magnetic imaging with a Zernike-type phase plate in a transmission electron microscope," Appl. Phys. Lett., 102, 192401 (2013) (Cover article).

15. Pollard, S. D., and Zhu, Y., "The Aharonov-Bohm effect, magnetic monopoles, and reversal in spin-ice lattices," Microscopy 62 (suppl 1): S55-S64, (2013).

16. Pollard, S. D., and Zhu, Y., Gauge theory and artificial spin ices,: imaging emergent monopoles with electron microscopy, book chapter, in “In Memory of Akira Tonomura: Physicist and Electron Microscopist”, K. Fujikawa and Y. A. Ono Eds., World Scientific Publishing Co. Pte. Ltd., p. 110-121, Exp. Pub. date: October 31, 2013, ISBN-13: 978-981-4472-88-3 

17. Pulecio, J. F., Bhanja, S., and Sarkar, S., “Parallel Energy Minimizing Computation via Dipolar Coupled Single Domain Nanomagnets”, In J. E. Morris & K. Iniewski (Eds.), Nanoelectronic Device Applications Handbook (p. 940). CRC Press (2013).

18. Santos-Ortiz, R., Volkov, V., Schmid, S., Kuo, F.-L., Kisslinger, K., Nag, S., Banerjee R., Zhu, Y., and Shepherd, N. D., “The Microstructure and Electronic Band Structure of Pulsed Laser Deposited Iron Fluoride Thin-Film for Battery Electrodes”, ACS Appl. Materials and Interfaces, 5(7), 2387-2391 (2013).  

19. Volkov, V.V., Han, M.-G., and Zhu, Y., “Double-resolution electron holography with simple Fourier transform of fringe-shifted holograms”, Ultramicroscopy, 134, 175-184 (2013).

20. Wang, F., Wu, L., Key, B., Yang, X.-Q., Grey, C.P., Zhu, Y., and Graetz, J., “Electrochemical Reaction of Lithium with Nanostructured Silicon Anodes: a Study by in-situ Synchrotron X-ray Diffraction and Electron Energy-Loss Spectroscopy”, Adv. Energy Mater., 3, 1324–1331 (2013).

21. Wang, F., Wu, L., Ma. C., Su, D., Zhu, Y., and Graetz, J., “Excess Li storage and charge compensation in nanoscale Li4+xTi5O12”, Nanotechnology, 24 424006 (2013).

22. Wu, L., Meng, Q., Jooss, Ch., Zheng, J.-C., Inada, H., Su, D., Li, Q., and Zhu, Y., “Origin of Phonon Glass –Electron Crystal behavior in Thermoelectric Layered Cobaltate”, Adv. Funct. Mater. DOI: 10.1002/adfm.201301098, published online: 13 June 2013.

23. Yu X; Pan H; Wan W; Ma C; Bai J; Meng Q; Ehrlich S; Hu Y; Yang X; “A size-dependent sodium storage mechanism in Li4Ti5O12 investigated by a novel characterization technique combining in situ x-ray diffraction and chemical sodiation”, Nano letters  13, 4721(2013).

24. Zhang, Yu., Ma, C., Zhu, Y., Si, R., Cai, Y., Wang, J.X., Adzic, R.R., "Hollow core supported Pt monolayer catalysts for oxygen reduction", Catalysis Today 202 50-54 (2013).


25. Zhu, P., J. Cao, Y. Zhu, J. Geck, Y. Hidaka, S. Pjerov, T. Ritschel, H. Berger, Y. Shen, R. Tobey, J.P. Hill and X.J. Wang, "Dynamic separation of electron excitation and lattice heating during the photoinduced melting of the periodic lattice distortion in 2H-TaSe2", Appl. Phys. Lett., 103 071914 (2013).

Tuesday, September 18, 2012

Magnetic Vortex Reveals Key to Spintronic Speed Limit


UPTON, NY — The evolution of digital electronics is a story of miniaturization – each generation of circuitry requires less space and energy to perform the same tasks. But even as high-speed processors move into handheld smart phones, current data storage technology has a functional limit: magnetically stored digital information becomes unstable when too tightly packed. The answer to maintaining the breath-taking pace of our ongoing computer revolution may be the denser, faster, and smarter technology of spintronics.
Spintronic devices use electron spin, a subtle quantum characteristic, to write and read information. But to mobilize this emerging technology, scientists must understand exactly how to manipulate spin as a reliable carrier of computer code. Now, scientists at the Department of Energy’s (DOE) Brookhaven National Laboratory have precisely measured a key parameter of electron interactions called non-adiabatic spin torque that is essential to the future development of spintronic devices... Read more