Unexpected Magnetic Order Among Titanium Atoms Discovered

The results, published inNature Communications, have special significance for the design of future electronic devices for computations and telecommunications, according to co-author Satoshi Okamoto of ORNL's Materials Science and Technology Division. The work was performed at Universidad Complutense de Madrid, synchrotron radiation facilities in France and Japan, University of Bristol and University of Warwick.

"What the team found was an unexpected magnetic order among the titanium atoms at an interface between strontium titanate and lanthanum manganite, which are both insulators in bulk," Okamoto said.

With today's nano-fabrication tools, scientists can develop artificial materials with controlled precision -- almost atom by atom -- of alternating very thin crystalline layers of different materials. The properties of these materials are determined by the structure of interfaces of the different materials and how atoms interact through the interfaces.

Such an interface has traditionally been considered a source of disorder, but in the case of materials such as complex oxides used for this study, the result was something that does not exist naturally in any other material. In order to clarify the electronic properties of such interfaces, the research team made detailed synchrotron X-ray measurements.

"The result was even more surprising as we observed a new type of magnetism in titanium atoms, which are non-magnetic in bulk strontium titanate," Okamoto said.

Furthermore, the researchers were able to manipulate the structure of spin, or magnetism, at atomic scale. The theoretical work by Okamoto provided the key to understand the origin of this novel form of interfacial magnetism and is of particular importance for the development of new spintronic devices such as tunneling magneto-resistance junction, which can be used as a head of a hard-disc drive.

While today's electronic devices are based on the transfer of electrical charge between two materials, a potential alternative, spintronic devices, would also use the magnetic moment, or spin, of electrons in addition to their charge and would therefore be more efficient for sending or storing information as an electric signal.

The research, published Sept. 21, was led by Jacobo Santamaria of Universidad Complutense de Madrid. Funding was provided by the Spanish Ministry of Science and Innovation. Work at ORNL was supported by DOE's Office of Basic Energy Sciences.

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Trapping Charged Particles With Laser Light

Dr. Tobias Schätz, leader of the Emmy-Noether-Research Group Quantum Simulations at the Max Planck Institute of Quantum Optics, and his team have now demonstrated the possibility of combining both methods as well as both kinds of particles: they succeeded to store an ion in an optical trap for the first time. Their experiment opens new perspectives, for example for using controllable quantum systems in the simulation of condensed matter properties. At the same time completely new experimental possibilities may arise in the field of ultracold chemistry.

Experimental quantum simulations are based on the principle of modeling a complex many-body system (e.g. a metallic solid state), whose quantum properties are neither understood nor controllable, by yet another system which allows the study of analogue properties under precisely defined conditions. These model systems can be realized in different ways. Most promising are systems based on ions sitting in radiofrequency traps, and systems made of neutral atoms stored in light fields. A special case of the latter is an optical lattice which is created by overlapping laser waves in a way that a periodic pattern of bright and dark areas emerges. For about three decades these"crystals of light" have shown to be a very useful tool for manipulating and controlling ultracold neutral atoms.

The decision to exploit which kind of particles strongly depends on the question under investigation. One of the topics the group of Dr. Schätz is very interested in is quantum properties of magnetic matter. Magnetism of a solid state can occur when the individual elemental atoms carry an angular momentum, a so-call spin. Depending on external conditions the interaction of each two spins makes them to align either parallel or antiparallel, thus eventually leading to ferromagnetic or antiferromagnetic (in the latter for uneven spin numbers even"frustrated") states. The investigation of the quantum dynamics of these states may make a contribution to a better understanding of high-temperature superconductivity. For the analogue simulation of spin-spin-interaction and its consequences ions are their preferred candidates because the Coulomb-force between neighbouring ions is much stronger than the interaction of neighbouring atoms in an optical lattice. Experimental quantum simulations with ions therefore could take a much shorter time than in the case of atoms whereby the influence of external fields would get largely reduced.

Because of their electric charge ions are also most easily to influence by external electromagnetic fields. Hence physicists have used the method of trapping ions with alternating radiofrequency fields by now for more than sixty years. Meanwhile storage times of up to several months are achieved. However, these systems suffer from a severe drawback: it is very difficult to scale them to larger architectures, which limits the possibilities of performing quantum simulations with sufficiently many ions. So, what is the reason that up to now optical lattices have not been used as an alternative for storing ions?

"Optical fields are disfavoured because they don't allow for potential wells nearly as deep as they are guaranteed by radiofrequency fields," Dr. Schätz explains."At the same time ions react in a very sensitive way to external stray fields. This has caused the widely believed prejudice that optical potentials are too shallow and therefore unable to trap ions. But, as a matter of fact, we were able to experimentally demonstrate that ions can indeed be trapped by the interaction with light.

The scientists start their experiment with cooling a single magnesium ion down to a few thousandths of a degree above absolute zero temperature. In the next step external stray fields are compensated for by appropriate"counter fields." Then a strongly collimated laser beam is turned on while the radiofrequency field gets switched off. According to a series of measurements the ion was kept in place for several milliseconds. This corresponds to a couple of hundred oscillations of the ion in the potential well, despite its relatively shallow shape.

Tobias Schätz does not seem to be awfully surprised by this change of paradigm."In principle, both traps, the radio frequency as well as the optical, work the same way: they capture the particle by a fast changing electromagnetic field." At present the lifetime of the ion in the optical trap is only limited by heating which is caused by scattering light of the optical field. It could be largely improved with state-of-the art techniques.

Once it was possible to extend the principle of optical trapping as demonstrated in this experimental approach to a large number of ions in an optical lattice a completely new class of experiments could be carried out. Besides simulating complex spin-systems, hybrid quantum systems could be developed which combine ions and atoms in a common optical lattice with the quantum particles"sharing" the excess charges.

There are also intriguing possibilities for investigating chemical reactions at extremely low temperatures. If, for example, a single ion was embedded in a cold atomic quantum gas (a so-called Bose-Einstein condensate) in a common optical trap, the particles would -- due to their very low kinetic energy -- spend so much time together, that novel chemical reactions caused by quantum mechanical tunneling might evolve. Hence this experiment is both the beginning of a new generation of quantum simulations and of a new era of ultracold chemistry.

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Water Could Hold Answer to Graphene Nanoelectronics

By exposing a graphene film to humidity, Rensselaer Professor Nikhil Koratkar and his research team were able to create a band gap in graphene -- a critical prerequisite to creating graphene transistors. At the heart of modern electronics, transistors are devices that can be switched"on" or"off" to alter an electrical signal. Computer microprocessors are comprised of millions of transistors made from the semiconducting material silicon, for which the industry is actively seeking a successor.

Graphene, an atom-thick sheet of carbon atoms arranged like a nanoscale chain-link fence, has no band gap. Koratkar's team demonstrated how to open a band gap in graphene based on the amount of water they adsorbed to one side of the material, precisely tuning the band gap to any value from 0 to 0.2 electron volts. This effect was fully reversible and the band gap reduced back to zero under vacuum. The technique does not involve any complicated engineering or modification of the graphene, but requires an enclosure where humidity can be precisely controlled.

"Graphene is prized for its unique and attractive mechanical properties. But if you were to build a transistor using graphene, it simply wouldn't work as graphene acts like a semi-metal and has zero band gap," said Koratkar, a professor in the Department of Mechanical, Aerospace, and Nuclear Engineering at Rensselaer."In this study, we demonstrated a relatively easy method for giving graphene a band gap. This could open the door to using graphene for a new generation of transistors, diodes, nanoelectronics, nanophotonics, and other applications."

Results of the study were detailed in an article recently published by the journalSmall. 

In its natural state, graphene has a peculiar structure but no band gap. It behaves as a metal and is known as a good conductor. This is compared to rubber or most plastics, which are insulators and do not conduct electricity. Insulators have a large band gap -- an energy gap between the valence and conduction bands -- which prevents electrons from conducting freely in the material.

Between the two are semiconductors, which can function as both a conductor and an insulator. Semiconductors have a narrow band gap, and application of an electric field can provoke electrons to jump across the gap. The ability to quickly switch between the two states --"on" and"off" -- is why semiconductors are so valuable in microelectronics.

"At the heart of any semiconductor device is a material with a band gap," Koratkar said."If you look at the chips and microprocessors in today's cell phones, mobile devices, and computers, each contains a multitude of transistors made from semiconductors with band gaps. Graphene is a zero band gap material, which limits its utility. So it is critical to develop methods to induce a band gap in graphene to make it a relevant semiconducting material."

The symmetry of graphene's lattice structure has been identified as a reason for the material's lack of band gap. Koratkar explored the idea of breaking this symmetry by binding molecules to only one side of the graphene. To do this, he fabricated graphene on a surface of silicon and silicon dioxide, and then exposed the graphene to an environmental chamber with controlled humidity. In the chamber, water molecules adsorbed to the exposed side of the graphene, but not on the side facing the silicon dioxide. With the symmetry broken, the band gap of graphene did, indeed, open up, Koratkar said. Also contributing to the effect is the moisture interacting with defects in the silicon dioxide substrate.

"Others have shown how to create a band gap in graphene by adsorbing different gasses to its surface, but this is the first time it has been done with water," he said."The advantage of water adsorption, compared to gasses, is that it is inexpensive, nontoxic, and much easier to control in a chip application. For example, with advances in micro-packaging technologies it is relatively straightforward to construct a small enclosure around certain parts or the entirety of a computer chip in which it would be quite easy to control the level of humidity."

Based on the humidity level in the enclosure, chip makers could reversibly tune the band gap of graphene to any value from 0 to 0.2 electron volts, Korarkar said.

This study was supported by the Advanced Energy Consortium (AEC), National Institute of Standards and Technology (NIST) Nanoelectronics Research Initiative, and the U.S. Department of Energy Office of Basic Energy Sciences (BES).

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Physicists Demonstrate a Four-Fold Quantum Memory

Their work, described in the November 18 issue of the journalNature,also demonstrated a quantum interface between the atomic memories -- which represent something akin to a computer"hard drive" for entanglement -- and four beams of light, thereby enabling the four-fold entanglement to be distributed by photons across quantum networks. The research represents an important achievement in quantum information science by extending the coherent control of entanglement from two to multiple (four) spatially separated physical systems of matter and light.

The proof-of-principle experiment, led by William L. Valentine Professor and professor of physics H. Jeff Kimble, helps to pave the way toward quantum networks. Similar to the Internet in our daily life, a quantum network is a quantum"web" composed of many interconnected quantum nodes, each of which is capable of rudimentary quantum logic operations (similar to the"AND" and"OR" gates in computers) utilizing"quantum transistors" and of storing the resulting quantum states in quantum memories. The quantum nodes are"wired" together by quantum channels that carry, for example, beams of photons to deliver quantum information from node to node. Such an interconnected quantum system could function as a quantum computer, or, as proposed by the late Caltech physicist Richard Feynman in the 1980s, as a"quantum simulator" for studying complex problems in physics.

Quantum entanglement is a quintessential feature of the quantum realm and involves correlations among components of the overall physical system that cannot be described by classical physics. Strangely, for an entangled quantum system, there exists no objective physical reality for the system's properties. Instead, an entangled system contains simultaneously multiple possibilities for its properties. Such an entangled system has been created and stored by the Caltech researchers.

Previously, Kimble's group entangled a pair of atomic quantum memories and coherently transferred the entangled photons into and out of the quantum memories. For such two-component -- or bipartite -- entanglement, the subsystems are either entangled or not. But for multi-component entanglement with more than two subsystems -- or multipartite entanglement -- there are many possible ways to entangle the subsystems. For example, with four subsystems, all of the possible pair combinations could be bipartite entangled but not be entangled over all four components; alternatively, they could share a"global" quadripartite (four-part) entanglement.

Hence, multipartite entanglement is accompanied by increased complexity in the system. While this makes the creation and characterization of these quantum states substantially more difficult, it also makes the entangled states more valuable for tasks in quantum information science.

To achieve multipartite entanglement, the Caltech team used lasers to cool four collections (or ensembles) of about one million Cesium atoms, separated by 1 millimeter and trapped in a magnetic field, to within a few hundred millionths of a degree above absolute zero. Each ensemble can have atoms with internal spins that are"up" or"down" (analogous to spinning tops) and that are collectively described by a"spin wave" for the respective ensemble. It is these spin waves that the Caltech researchers succeeded in entangling among the four atomic ensembles.

The technique employed by the Caltech team for creating quadripartite entanglement is an extension of the theoretical work of Luming Duan, Mikhail Lukin, Ignacio Cirac, and Peter Zoller in 2001 for the generation of bipartite entanglement by the act of quantum measurement. This kind of"measurement-induced" entanglement for two atomic ensembles was first achieved by the Caltech group in 2005.

In the current experiment, entanglement was"stored" in the four atomic ensembles for a variable time, and then"read out" -- essentially, transferred -- to four beams of light. To do this, the researchers shot four"read" lasers into the four, now-entangled, ensembles. The coherent arrangement of excitation amplitudes for the atoms in the ensembles, described by spin waves, enhances the matter-light interaction through a phenomenon known as superradiant emission.

"The emitted light from each atom in an ensemble constructively interferes with the light from other atoms in the forward direction, allowing us to transfer the spin wave excitations of the ensembles to single photons," says Akihisa Goban, a Caltech graduate student and coauthor of the paper. The researchers were therefore able to coherently move the quantum information from the individual sets of multipartite entangled atoms to four entangled beams of light, forming the bridge between matter and light that is necessary for quantum networks.

The Caltech team investigated the dynamics by which the multipartite entanglement decayed while stored in the atomic memories."In the zoology of entangled states, our experiment illustrates how multipartite entangled spin waves can evolve into various subsets of the entangled systems over time, and sheds light on the intricacy and fragility of quantum entanglement in open quantum systems," says Caltech graduate student Kyung Soo Choi, the lead author of the Nature paper. The researchers suggest that the theoretical tools developed for their studies of the dynamics of entanglement decay could be applied for studying the entangled spin waves in quantum magnets.

Further possibilities of their experiment include the expansion of multipartite entanglement across quantum networks and quantum metrology."Our work introduces new sets of experimental capabilities to generate, store, and transfer multipartite entanglement from matter to light in quantum networks," Choi explains."It signifies the ever-increasing degree of exquisite quantum control to study and manipulate entangled states of matter and light."

In addition to Kimble, Choi, and Goban, the other authors of the paper are Scott Papp, a former postdoctoral scholar in the Caltech Center for the Physics of Information now at the National Institute of Standards and Technology in Boulder, Colorado, and Steven van Enk, a theoretical collaborator and professor of physics at the University of Oregon, and an associate of the Institute for Quantum Information at Caltech.

This research was funded by the National Science Foundation, the National Security Science and Engineering Faculty Fellowship program at the U.S. Department of Defense (DOD), the Northrop Grumman Corporation, and the Intelligence Advanced Research Projects Activity.

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