Using new technique, scientists uncover a delicate magnetic balance for superconductivity

 A new imaging technology is giving scientists unprecedented views of the processes that affect the flow of electrons through materials.

By modifying a familiar tool in nanoscience -- the scanning tunneling microscope -- a team at Cornell University's Laboratory for Atomic and Solid State Physics have been able to visualize what happens when they change the electronic structure of a "heavy fermion" compound made of uranium, ruthenium and silicon. What they found sheds light on superconductivity -- the movement of electrons without resistance -which typically occurs at extremely low temperatures and that researchers hope one day to achieve at something close to room temperature, which would revolutionize electronics.

What they found was that, while at higher-temperatures magnetism is detrimental to superconductivity, at low temperatures in heavy fermion materials, magnetic atoms are a necessity. "We found that removing the magnetic atoms proved detrimental to the flow [of electrons]," said researcher Mohammad Hamidian. This is important, Hamidian explains, because "if we can resolve how superconductivity can co-exist with magnetism, then we have a whole new understanding of superconductivity, which could be applied toward creating high-temperature superconductors. In fact, magnetism at the atomic scale could become a new tuning parameter of how you can change the behavior of new superconducting materials that we make."

To make things finding, the researchers modified a scanning microscope that lets you pull or push electrons into a material. With the modification, the microscope could also measure how hard it was to push and pull -- a development that Hamidian explains is also significant. "By doing this, we actually learn a lot about the material's electronic structure. Then by mapping that structure out over a wide area, we can start seeing variations in those electronic states, which come about for quantum-mechanical reasons. Our newest advance, crucial to this paper, was the ability to see at each atom the strength of the interactions that make the electrons 'heavy.'"

The Cornell experiment and its results are presented this week by the Proceedings of the National Academy of Sciences. The research team included J.C. Séamus Davis, a member of the Kavli Institute at Cornell for Nanoscale Science and developer of the SI-STM technique. Working with synthesized samples created by Graeme Luke from McMaster University (Canada), the experiment was designed by Hamidian, a post-doctoral fellow in Davis' research group, along with Andrew R. Schmidt, a former student of Davis at Cornell and now a post-doctoral fellow in physics at UC Berkeley. This research was supported by the DOE's Office of Science, the Natural Sciences and Engineering Research Council of Canada, and the Canadian Institute for Advanced Research. Additional collaborators included Ines Firmo of Brookhaven Lab and Cornell, and Andy Schmidt now at the University of California, Berkeley.

For the complete interview with Hamidian, visit: http://www.kavlifoundation.org/science-spotlights/Cornell-disturbing-nanosphere-superconductivity

Story Source:

The above story is reprinted from materials provided by The Kavli Foundation.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

M. H. Hamidian, A. R. Schmidt, I. A. Firmo, M. P. Allan, P. Bradley, J. D. Garrett, T. J. Williams, G. M. Luke, Y. Dubi, A. V. Balatsky, J. C. Davis. How Kondo-holes create intense nanoscale heavy-fermion hybridization disorder. Proceedings of the National Academy of Sciences, 2011; DOI: 10.1073/pnas.1115027108

'Impossible' feat: Certain materials can exhibit ferromagnetism and superconductivity at same time

 It actually seems impossible: Scientists from the Helmholtz-Zentrum Dresden-Rossendorf and the TU Dresden were able to verify with an intermetallic compound of bismuth and nickel that certain materials actually exhibit the two contrary properties of superconductivity and ferromagnetism at the same time. A phenomenon that had only been demonstrated around the globe on a small number of materials and which might provide highly interesting technological opportunities in future.

Just in time for the 100th anniversary to commemorate the discovery of superconductivity by the Dutch physicist Heike Kamerlingh Onnes on April 8, 1911, scientists from the Helmholtz-Zentrum Dresden-Rossendorf and the TU Dresden published their research results in the journal Physical Review B.

Headed by Dr. Thomas Herrmannsdörfer, the team from the HZDR's High Magnetic Field Laboratory (HLD) examined a material consisting of the elements bismuth and nickel (Bi3Ni) with a diameter of only a few nanometers -- which is unique since it has not been achieved elsewhere so far. This was made possible through a new chemical synthesis procedure at low temperatures which had been developed at the TU Dresden under the leadership of Prof. Michael Ruck. The nano scale size and the special form of the intermetallic compound -- namely, tiny fibers -- caused the physical properties of the material, which is non-magnetic under normal conditions, to change so dramatically. This is a particularly impressive example of the excellent opportunities modern nanotechnology can provide today, emphasizes Dr. Thomas Herrmannsdörfer. "It's really surprising to which extend the properties of a substance can vary if one manages to reduce their size to the nanometer scale."

There are numerous materials which become superconducting at ultralow temperatures. However, this property competes with ferromagnetism which normally suppresses superconductivity. This does not happen with the analyzed compound: Here, the Dresden researchers discovered with their experiments in high magnetic fields and at ultralow temperatures that the nanostructured material exhibits completely different properties than larger-sized samples of the same material. What's most surprising: The compound is both ferromagnetic and superconducting at the same time. It is, thus, one of those rarely known materials which exhibit this unusual and physically not yet completely understood combination. Perhaps bismuth-3-nickel features a special type of superconductivity, says Dr. Herrmannsdörfer. The physicist and doctoral candidate Richard Skrotzki, who has just turned 25, is making a vital contribution to the research results and describes the phenomenon as "the bundling of contrary properties in a single strand."

The TU Dresden and the HZDR are partners in the research alliance DRESDEN-concept which pursues the objective of making visible the excellence of Dresden research.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Helmholtz Association of German Research Centres, via EurekAlert!, a service of AAAS.

Journal Reference:

T. Herrmannsdörfer, R. Skrotzki, J. Wosnitza, D. Köhler, R. Boldt, M. Ruck. Structure-induced coexistence of ferromagnetic and superconducting states of single-phase Bi_{3}Ni seen via magnetization and resistance measurements. Physical Review B, 2011; 83 (14) DOI: 10.1103/PhysRevB.83.140501

New kinds of superconductivity? Physicists demonstrate coveted 'spin-orbit coupling' in atomic gases

Physicists at the Joint Quantum Institute (JQI), a collaboration of the National Institute of Standards and Technology (NIST) and the University of Maryland-College Park, have for the first time caused a gas of atoms to exhibit an important quantum phenomenon known as spin-orbit coupling. Their technique opens new possibilities for studying and better understanding fundamental physics and has potential applications to quantum computing, next-generation "spintronics" devices and even "atomtronic" devices built from ultracold atoms.

In the researchers' demonstration of spin-orbit coupling, two lasers allow an atom's motion to flip it between a pair of energy states. The new work, published in Nature, demonstrates this effect for the first time in bosons, which make up one of the two major classes of particles. The same technique could be applied to fermions, the other major class of particles, according to the researchers. The special properties of fermions would make them ideal for studying new kinds of interactions between two particles -- for example those leading to novel "p-wave" superconductivity, which may enable a long-sought form of quantum computing known as topological quantum computation.

In an unexpected development, the team also discovered that the lasers modified how the atoms interacted with each other and caused atoms in one energy state to separate in space from atoms in the other energy state.

One of the most important phenomena in quantum physics, spin-orbit coupling describes the interplay that can occur between a particle's internal properties and its external properties. In atoms, it usually describes interactions that only occur within an atom: how an electron's orbit around an atom's core (nucleus) affects the orientation of the electron's internal bar-magnet-like "spin." In semiconductor materials such as gallium arsenide, spin-orbit coupling is an interaction between an electron's spin and its linear motion in a material.

"Spin-orbit coupling is often a bad thing," said JQI's Ian Spielman, senior author of the paper. "Researchers make 'spintronic' devices out of gallium arsenide, and if you've prepared a spin in some desired orientation, the last thing you'd want it to do is to flip to some other spin when it's moving."

"But from the point of view of fundamental physics, spin-orbit coupling is really interesting," he said. "It's what drives these new kinds of materials called 'topological insulators.'"

One of the hottest topics in physics right now, topological insulators are special materials in which location is everything: the ability of electrons to flow depends on where they are located within the material. Most regions of such a material are insulating, and electric current does not flow freely. But in a flat, two-dimensional topological insulator, current can flow freely along the edge in one direction for one type of spin, and the opposite direction for the opposite kind of spin. In 3-D topological insulators, electrons would flow freely on the surface but be inhibited inside the material. While researchers have been making higher and higher quality versions of this special class of material in solids, spin-orbit coupling in trapped ultracold gases of atoms could help realize topological insulators in their purest, most pristine form, as gases are free of impurity atoms and the other complexities of solid materials.

Usually, atoms do not exhibit the same kind of spin-orbit coupling as electrons exhibit in gallium-arsenide crystals. While each individual atom has its own spin-orbit coupling going on between its internal components (electrons and nucleus), the atom's overall motion generally is not affected by its internal energy state.

But the researchers were able to change that. In their experiment, researchers trapped and cooled a gas of about 200,000 rubidium-87 atoms down to 100 nanokelvins, 3 billion times colder than room temperature. The researchers selected a pair of energy states, analogous to the "spin-up" and "spin-down" states in an electron, from the available atomic energy levels. An atom could occupy either of these "pseudospin" states. Then researchers shined a pair of lasers on the atoms so as to change the relationship between the atom's energy and its momentum (its mass times velocity), and therefore its motion. This created spin-orbit coupling in the atom: the moving atom flipped between its two "spin" states at a rate that depended upon its velocity.

"This demonstrates that the idea of using laser light to create spin-orbit coupling in atoms works. This is all we expected to see," Spielman said. "But something else really neat happened."

They turned up the intensity of their lasers, and atoms of one spin state began to repel the atoms in the other spin state, causing them to separate.

"We changed fundamentally how these atoms interacted with one another," Spielman said. "We hadn't anticipated that and got lucky."

The rubidium atoms in the researchers' experiment were bosons, sociable particles that can all crowd into the same space even if they possess identical values in their properties including spin. But Spielman's calculations show that they could also create this same effect in ultracold gases of fermions. Fermions, the more antisocial type of atoms, cannot occupy the same space when they are in an identical state. And compared to other methods for creating new interactions between fermions, the spin states would be easier to control and longer lived.

A spin-orbit-coupled Fermi gas could interact with itself because the lasers effectively split each atom into two distinct components, each with its own spin state, and two such atoms with different velocities could then interact and pair up with one other. This kind of pairing opens up possibilities, Spielman said, for studying novel forms of superconductivity, particularly "p-wave" superconductivity, in which two paired atoms have a quantum-mechanical phase that depends on their relative orientation. Such p-wave superconductors may enable a form of quantum computing known as topological quantum computation.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by National Institute of Standards and Technology (NIST).

Journal Reference:

Y.-J. Lin, K. Jiménez-García, I. B. Spielman. Spin-orbit-coupled Bose-Einstein condensates. Nature, 471, 83-86 (2 March 2011) DOI: 10.1038/nature09887

Note: If no author is given, the source is cited instead.