Publications 2015

1.        Abeykoon, A. M. M.;  H. Hu, L. Wu, Y. Zhu and S. J. L. Billinge, “Calibration and data collection protocols for reliable lattice parameter values in electron pair distribution function studies,” J. of Appl. Crystallography 48, 244-251 (2015)

2.        Ciston, J.;  H. G. Brown, A. J. D'Alfonso, P. Koirala, C. Ophus, Y. Lin, Y. Suzuki, H. Inada, Y. Zhu, L. J. Allen, L. D. Marks, “Surface determination through atomically resolved secondary-electron imaging”, Nat. Comm., 6, 7358 (2015)

3.        Egerton, R.F.; Konstantinova, T.;  Zhu, Y.; “Analysis of Beam-Sensitive Materials by Electrons and X-rays”, in: M. Berz, P.M. Duxbury, K. Makino, C.-Y. Ruan (Eds.), Femtosecond Electron Imaging and Spectroscopy 2013, Advances in Imaging and Electron Physics 191, pp. 70–80, Elsevier, (2015).

4.        Jasion, D., Barforoush, J.M., Qiao, Q., Zhu, Y., Ren, S., and Leonard, KC.,  "Low-Dimensional Hyperthin FeS2 Nanostructures for Efficient and Stable Hydrogen Evolution Electrocatalysis" ACS Catalysis, 5, 6653 (2015).

5.        Koirala, N.;  Brahlek, M. J.; Salehi, M.; Wu, L.; Dai, J.; Waugh, J.; Nummy, T.; Han, M-G.; Moon, J.; Zhu, Y.; Dessau, D.; Wu, W.; Armitage, N. P.; and Oh, S., “Record Surface State Mobility and Quantum Hall Effect in Topological Insulator Thin Films via Interface Engineering”, Nano Lett., 15, 8245−8249 (2015)

6.        Li, M.;  C-Z. Chang, L. Wu, J. Tao, W. Zhao, MHW. Chan, JS. Moodera, J. Li, and Y. Zhu, “Experimental Verification of the Van Vleck Nature of Long-Range Ferromagnetic Order in the Vanadium-Doped Three-Dimensional Topological Insulator Sb2Te3”, Phys. Rev. Lett., 114, 146802 (2015).

7.        Li, M.; C-Z. Chang, BJ. Kirby, M. Jamer, W. Cui, L. Wu, P. Wei, Y. Zhu, D. Heiman, J. Li, and JS. Moodera, "Proximity-driven enhanced magnetic order at ferromagnetic-insulator--magnetic-topological-insulator interface", Phys. Rev. Lett. 115, 087201 (2015).

8.        Li, M., W. Cui, L. Wu, Q. Meng, Y. Zhu, Y. Zhang, W. Liu, and Z. Ren, "Topological effect of surface plasmon excitation in gapped isotropic topological insulator nanowires", Can. J. Phys. 93: 591-598 (2015).

9.        Luo, H. X.;  Xie, W. W. Xie, J. Tao, H. Inoue, A. Gyenis, J. W. Krizan, A. Yazdani, Y. M. Zhu, R. J. Cava, “Polytypism, polymorphism, and superconductivity in TaSe2-xTex,” Proc. of Nat’l. Acad. Sci. 112, E1174-1180, (2015)

10.     Luo, H. X.; Krizan, J. W.; Seibel, E. M.; Xie, W. W.; Sahasrabudhe, G. S.; Bergman, S. L.; Phelan, B. F.; Tao, J.; Wang, Z.; Zhang, J. D.; Cava, R. J., "Cr-Doped TiSe2 - A Layered Dichalcogenide Spin Glass"Chemistry of Materials, 27, 6810, (2015)

11.     Ma, J.; S-H Bo, L. Wu, Y. Zhu, CP. Grey, and PG. Khalifah, “Ordered and Disordered Polymorphs of Na(Ni2/3Sb1/3)O2: Honeycomb-Ordered Cathodes for Na-Ion Batteries,” Chemistry of Materials 27 (7), 2387–2399 (2015).

12.     Meng, Q.;  M. G. Han, J. Tao, G. Y. Xu, D. O. Welch, Y. M. Zhu, “Velocity of domain-wall motion during polarization reversal in ferroelectric thin films: Beyond Merz's Law,” Phys. Rev. B 91, 054104 (2015)

13.     Meng, Q.; L. Wu, DO. Welch, and Y. Zhu, “Lattice vibrations in the Frenkel-Kontorova model. I. Phonon dispersion, number density, and energy”, Phys. Rev. B 91, 224305 (2015).

14.     Meng, Q.; L. Wu, DO. Welch, and Y. Zhu, “Lattice vibrations in the Frenkel-Kontorova Model. II. Thermal conductivity”, Phys. Rev. B 91, 224306 (2015).

15.     Mildner, S.; Beleggia, M.; Mierwaldt, D.; Hansen, T.W.; Wagner, J.B.; Yazdi, S.; Kasama, T.; Ciston, J.; Zhu, Y.; Jooss, Ch.; "Environmental TEM Study of Electron Beam Induced Electrochemistry of Pr0.64Ca0.36MnO3 Catalysts for Oxygen Evolution", J. of Phys. Chem. C, 1195301-5310 (2015).

16.     Que, Y.; Y. Zhang, Y. L. Wang, L. Huang, W. Y. Xu, J. Tao, L. J. Wu, Y. M. Zhu, K. Kim, M. Weinl, M. Schreck, C. M. Shen, S. X. Du, Y. Q. Liu and H.-J. Gao, “Graphene-Silicon Layered Structures on Single-Crystalline Ir(111) Thin Films,” Advanced Materials Interfaces 2, 1400543, (2015)

17.     Rodriguez, J. A.;  R. Si, J. Evans, W. Q. Xu, J. C. Hanson, J. Tao, Y. M. Zhu, “Active gold-ceria and gold-ceria/titania catalysts for CO oxidation: From single-crystal model catalysts to powder catalysts,” Catalysis Today 240, 229-235, (2015).

18.     Shi, X.; J. Yang, L. Wu, J. R. Salvador, W.L. Villaire, D. Haddad, J. Yang, Y. Zhu, and Q. Li, "Band Structure Engineering and Thermoelectric Properties of Charge-Compensated Filled Skutterudites", Scientific Report,  5 14641 (2015).

19.     Ugur, A.; F. Katmis, M. Li, L. Wu, Y. Zhu, KK. Varanasi, and KK. Gleason, “Low-Dimensional Conduction Mechanisms in Highly Conductive and Transparent Conjugated Polymers”, Adv. Mater., DOI: 10.1002/adma.201502340 (2015).

20.     Wei, Z.; W. Zhang, F. Wang, Q. Zhang, B. Qiu, S. Han, Y. Xia, Y. Zhu, Z. Liu, “Eliminating Voltage Decay of Lithium-Rich Li1.14Mn0.54Ni0.14Co0.14O2 Cathodes by Controlling the Electrochemical Process”, Chemistry - A European J. 21, 7503-7510 (2015).

21.     Wu, D.; Zhao, L-D.; Tong, X.; Li, W.; Wu, L.; Tan, Q.; Pei, Y.; Huang, L.; Li, J-F.; Zhu, Y.; Kanatzidis, MG.; He, J.; “Superior thermoelectric performance in PbTe-PbS pseudo-binary: extremely low thermal conductivity and modulated carrier concentration”, Energy & Environmental Science, 8 2056-2068 (2015).

22.     Wu, L.; Feng X., Zhu, Y., AB. Brady, J. Huang, JL. Durham, E. Dooryhee, AC. Marschilok, ES. Takeuchi, and KJ. Takeuchi, “Structural Defects of Silver Hollandite, AgxMn8Oy, Nanorods: Dramatic Impact on Electrochemistry”, ACS Nano, ACS Nano 9 8430-8439, (2015).   

23.     Wei, Z.; Zhang, W.; Wang, F.; Zhang, Q.; Qiu, B.; Han, S.; Xia, Y.; Zhu, Y.; Liu, Z.;          “Eliminating Voltage Decay of Lithium-Rich Li1.14Mn0.54Ni0.14Co0.14O2 Cathodes by Controlling the Electrochemical Process”, Chem.- A European Journal 21, 7503-7510 (2015).

24.     Yan, D. H.; Tao, J.; Kisslinger, K.; Cen, J. J.; Wu, Q. Y.; Orlov, A.; Liu, M. Z., “The role of the domain size and titanium dopant in nanocrystalline hematite thin films for water photolysis”, Nanoscale, 7, 18515-18523 (2015)

25.     Yin, Y-W.; Huang, W-C.; Liu, Y-K.; Yang, S-W.; Dong, S-N.; Tao, J.; Zhu, Y-M.; Li, Q.; Li, X.-G.;  “Octonary Resistance States in La0.7Sr0.3MnO3/BaTiO3/La0.7Sr0.3MnO3 Multiferroic Tunnel Junctions”, Adv. Electronic Mater. Nov.  1 1500183 (2015).

26.     Zhang, B., C.-J. Sun, W. Lu, T. Venkatesan, M.-G. Han, Y. Zhu, J. Chen, and G. M. Chow, “Electric-field-induced strain effects on the magnetization of a Pr0.6Sr0.33MnO3 film”, Phys. Rev. B 91, 174431 (2015).

27.     Zheng, H.; Meng, Y. S.; and Zhu, Y.; “Frontiers of in situ electron microscopy”, MRS BULLETIN, 40, 12-18, (2015).

28.     Zhu, C.; Min, H.; Xu, F.; Chen, J.; Dong, H.; Tong, L.; Zhu, Y.; Sun, L.; “Ultrafast electrochemical preparation of graphene/CoS nanosheet counter electrodes for efficient dye-sensitized solar cells”, RSC Adv., 5, 85822–85830 (2015).

29.     Zhu, P.; Y. Zhu, Y. Hidaka, L. Wu, J. Cao, H. Berger, J. Geck, R. Kraus, S. Pjerov, Y. Shen, R. Tobey, J.P. Hill, and X.J. Wang, “Femtosecond time-resolved MeV electron diffraction”, New J. of Physics, 17 063004 (2015).

30.     Zhu, Y. and H. Durr, “The future of electron microscopy”, Physics Today, 68 32-38 (2015).

A New Look at Surface Chemistry

Most materials correlate with other materials by their surfaces, that are mostly opposite in both structure and chemistry from a bulk of a material. Many critical processes take place during surfaces, trimming from a catalysts used for a era of energy-dense fuels from object and CO dioxide, to how bridges and airplanes rust.

“In essence, a aspect of each element can act as a possess nanomaterial cloaking that can severely change a chemistry and behavior,” Ciston says. “To know these processes and urge element opening it is critical to know how a atoms are organised during surfaces. While there are now many good methods for receiving this information for rather prosaic surfaces, when a surfaces are severe many now accessible collection are singular in what they can reveal.”

“The beauty of this technique is that we can picture aspect atoms and bulk atoms simultaneously,” says co-author Zhu, a scientist during Brookhaven National Laboratory. “Currently nothing of any existent methods can grasp this.”

Scanning nucleus microscopy (SEM) is an glorious technique for investigate surfaces yet typically provides information customarily about topology during nanoscale resolution. A rarely earnest new chronicle of scanning nucleus microscopy, called “high-resolution scanning nucleus microscopy,” or HRSEM, extends this fortitude to a atomic scale and provides information on both aspect and bulk atoms simultaneously, maintaining most of a aspect attraction of normal SEM by delegate electrons.

Secondary electrons are a outcome of a rarely energized lamp of electrons distinguished a element and causing atoms in a element to evacuate appetite in a form of electrons rather than photons. As a vast apportionment of delegate electrons are issued from a aspect of a element in further to a bulk they are good resources for receiving information about atomic aspect structure. However, a aspect selectivity of HRSEM has never been entirely exploited.

(Read More)

Ciston is a lead and analogous author of a paper describing this new methodical process in a biography Nature Communications. The essay is patrician “Surface Determination by Atomically Resolved Secondary Electron Imaging.” Other co-authors are Hamish Brown, Adrian D’Alfonso, Pratik Koirala, Colin Ophus, Yuyuan Lin, Yuya Suzuki, Hiromi Inada, Yimei Zhu, Les Allen, and Laurence Marks.

Long-Sought Magnetic Mechanism Observed in Exotic Hybrid Materials

Scientists have measured the subatomic intricacies of an exotic phenomenon first predicted more than 60 years ago. This so-called van Vleck magnetism is the key to harnessing the quantum quirks of topological insulators—hybrid materials that are both conducting and insulating—and could lead to unprecedented electronics. 

“Our experiment is the first to show conclusive evidence of van Vleck magnetism, which mediates the magnetic properties of topological insulators,” said MIT and Brookhaven Lab Ph.D. student Mingda Li, lead author on the study. “Synthesis and characterization techniques have finally caught up to seminal theoretical work, and we are thrilled to have performed this groundbreaking research.” The collaboration—including the U.S. Department of Energy’s Brookhaven National Laboratory, MIT, and Pennsylvania State University—used cutting-edge electron microscopy facilities at Brookhaven Lab to pinpoint this never-before-seen behavior. The results were published online April 9, 2015, in the journal Physical Review Letters.

Tunable topological insulators could lay the foundation for new generations of spintronics, quantum computers, and ultra-efficient semiconductor devices (see sidebar).

Van Vleck’s volleyball

Classical materials tend to conduct electricity or insulate against it—think rubber versus copper. Topological insulators, however, live in both worlds: the bulk is insulating, but the surface is highly conductive. The relationship between these competing qualities introduces strange phenomena, especially in the surface electrons.

“The surface electrons—called Dirac electrons—exhibit the light-like mobility and extreme stability that enables so many exciting potential applications,” Li said. “But these electrons cannot be controlled directly. That’s where van Vleck magnetism comes in, to induce and harness Dirac electrons.”

Imagine an endless game of volleyball between perfectly matched opponents. Now replace the players with magnetic ions and the ball with a free electron—that interplay mirrors magnetism in traditional semiconductors. Interrupting the game or shifting the behavior of that free electron, which is key to semiconductor applications, is a relatively simple task.

In topological insulators, however, that volleyball game never gets going. The magnetic action is contained within a single crystal structure—no back-and-forth and no free electrons. This subtle, intra-atomic magnetism behaves like a lone player engaging in a virtual volley. In fact, a rogue volleyball (free electron) would ruin the game. 

“Those all-important outer electrons can only be influenced through the topological insulator’s core electrons,” Li said. “The outer electrons can ‘feel’ the effect of energy or magnetic fields on the core. That conversation between core and shell is mediated by van Vleck magnetism.” 

John Hasbrouck van Vleck, considered the father of modern magnetism, won the 1977 Nobel Prize in Physics for his quantum revisions of magnetism theory. His groundbreaking work included predicting this internal magnetism, which has been notoriously difficult to detect—until now.

Congratualtions Mingda!!

Full article: http://www.bnl.gov/newsroom/news.php?a=25598

In addition, these findings have been featured on a number of other science news media websites:

- http://www.sciencedaily.com/releases/2015/04/150413091641.htm

- http://phys.org/news/2015-04-long-sought-magnetic-mechanism-exotic-hybrid.html

- http://esciencenews.com/sources/science.daily/2015/04/13/long.sought.magnetic.mechanism.observed.exotic.hybrid.materials

- http://www.microscopy-analysis.com/editorials/editorial-listings/eels-confirms-elusive-magnetism

New Path to Loss-Free Electricity

The Science

Electric current flows without any resistance in a superconducting state thanks to a surprising redistribution of bonding electrons and the associated electronic and atomic behavior after substitution of some cobalt atoms for iron in barium iron arsenide.

The Impact

This discovery of substitution-induced charge redistribution demonstrates the prominent role of bonding (and the associated electron fluctuations) in the emergence of superconductivity in iron-based alloys. It suggests a new route for finding higher performance superconductors through engineering and optimization of the electron density among the atoms in the material.

Summary

The flow of current in ordinary metals and other materials that conduct electricity is composed of electrons; however, the charge carriers are scattered when they conduct electricity, resulting in dissipation and energy loss, typically in the form of heat. In a superconductor, the electrons form into pairs that allow them to move through the material without resistance, eliminating the energy lost and thus increasing the efficient use of electricity. The challenge in creating such electron pairs is overcoming the natural tendency for electrons to repel each other. One solution is to utilize electronic polarizability that can yield an attractive interaction between electrons, thus allowing pair formation and the potential for loss-free current flow. Such a mechanism was proposed almost five decades ago, but it was never experimentally verified. Using electron diffraction with subatomic precision, scientists at Brookhaven National Laboratory have mapped out the redistribution of orbital electrons in barium iron arsenide, with and without cobalt substitution. The results reveal a remarkable increase in charge distribution around the iron and arsenic atoms as cobalt is incorporated into the material. Electron energy loss spectroscopy was carried out to determine the charge carrier-injection effect of cobalt substitution, while density functional theory was used to model electronic and atomic behavior. The induced charge redistribution around the iron and arsenic atoms after cobalt substitution suggests that the strongly coupled bond-electron fluctuation and charge separation may provide a new mechanism for high-temperature superconductivity. These results may guide the design of new superconductors.

link: http://science.energy.gov/bes/highlights/2015/bes-2015-02-o/

Meet Joe Garlow

When Joseph Garlow graduated from Ward Melville High School he was fairly certain what career path he wanted to follow. So, he headed off to the State University of New York (SUNY) at Cortland to work toward a degree in sports medicine and orthopedics. Shortly after his arrival, though, he became interested in biomedical engineering and nanotechnology. Through a series of transformations, including a stint as a student intern at Brookhaven Lab, he’s spun those interests in to a possible future career potential with a big impact on energy.

“I had a strong background in biology, and I come from a family of engineers, so I applied to the biomedical engineering program at Stony Brook University,” said Garlow. “I was really happy to be accepted to the Stony Brook program.” 

In addition to his regular Stony Brook classes, Garlow found a job as a research assistant in Professor Balaji Sitharaman’s multi-functional nano and supramolecular biosystems lab. There, he focused on two successful projects involving carbon nanotubes, graphene nanoribbons, graphene nanoparticles, and graphene-anode microbial fuel cells. The project results indicated that microbial fuel cells produce 10 times the power output of commercially available fuel-cell anodes, which may lead to more efficient fuel-cell technologies.

But Garlow didn’t only spend time in the classroom or lab. Soon after joining the student ranks at Stony Brook, he recognized the importance of helping underprivileged teenagers become interested in science and technology. He joined the school’s Science and Technology Entry Program (STEP) and worked as a mentor preparing and guiding students to pursue careers in science and engineering. His outreach efforts also included helping students prepare for science competitions. Garlow also served as a judiciary working with the dean of students, faculty, and other student leaders on best practices for allocations of funds for student organizations. 

Garlow’s introduction to Brookhaven started in 2012 when he came to the Lab as an undergraduate intern with post-doc Javier Pulecio in Yimei Zhu’s group in the Lab’s Condensed Matter Physics and Materials Science Department (CMPMS). 

“From then on, I was definitely hooked on science,” said Garlow.

In 2013, Garlow stayed on as a member of Zhu’s team through the U.S. Department of Energy’s Science Undergraduate Laboratory Internship (SULI) program. Zhu’s work suited Garlow’s research interests perfectly. 

“Joe had his research goals all lined up when we first met to discuss a mentorship,” said Zhu, who is also an adjunct professor at Stony Brook. “He is extremely motivated and it took very little convincing for me to welcome him to our lab. He is a valued member of our team.”

Along with Zhu and Pulecio, Garlow uses transmission electron microscopy and raman spectroscopy to explore the atomic structures and properties of precisely aligned graphene layers grown on nickel films. This research landed Garlow a spot as first author on a paper submitted to Scientific Reports.   

“Doing hands-on research at the Lab has surpassed all of my expectations and has given me a solid background not just in the actual science, but how it feels to work on a team of scientists who are seeking knowledge and discoveries,” said Garlow. “The experience is nothing short of amazing. I am grateful for this opportunity from Brookhaven’s Office of Educational Programs and scientists at the Lab.”

Currently working toward his Ph.D. at Stony Brook under the leadership of Zhu, Garlow says his current research focuses on charge, spin, and lattice correlations in layered materials such as perovskite oxides. These types of materials can display intriguing (and potentially useful) electrical and magnetic properties, including superconductivity, the flow of electrical current with no resistance; giant magnetoresistance, which renders a significant difference in electrical resistance depending on the alignment of magnetization within materials; and multiferroics, where intriguing, electronic and magnetic properties can be manipulated. These unusual properties may have significant implications in the emerging field of spintronics (a new type of technology which may revolutionize the future of computing), and lead to applications that will create faster and more efficient computers and new electronic and energy technologies.  

“Being mentored by Dr. Zhu and the many other Lab researchers has given me an opportunity to fully discover how fascinating nanotechnology and physics can be, and how the work we do today impacts our future,” said Garlow. “Brookhaven Lab’s brilliant scientists and state-of-the-art equipment afforded me the opportunity to accomplish much more than I anticipated. I am already looking forward to more scientific adventures and discoveries that will make a difference in our world now and to future generations.”

“For now, I am using most of my energy to focus on my studies and research,” added Garlow. “But, perhaps in the future I will again have the chance to join my rugby friends on the field. Hey, a guy still needs to have some fun!” he said.

link: http://www.bnl.gov/newsroom/news.php?a=25497

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.

(full article)

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.

(full text)

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.

(full link)