Tuesday, August 16, 2011
As the sizes of electronic components shrink, soon down to the size of single atoms or molecules, quantum interactions become ever more important. Consequently, enhanced knowledge and exploitation of quantum effects is essential. Researchers at the Joint Quantum Institute (JQI) in College Park, Maryland, operated by the University of Maryland and the National Institute of Standards and Technology (NIST), and at Georgetown University have uncovered evidence for a long-sought-after quantum state of matter, a spin liquid (more on quantum spin-liquid see this article).
The research was performed by JQI postdoctoral scientists Christopher Varney and Kai Sun, JQI Fellow Victor Galitski, and Marcos Rigol of Georgetown University. The results appear in an editor-recommended article in the 12 August issue of the journalPhysical Review Letters.
You can't pour a spin liquid into a glass. It's not a material at all, at least not a material you can touch. It is more like a kind of magnetic disorder within an ordered array of atoms. Nevertheless, it has many physicists excited.
To understand this exotic state of matter, first consider the concept of spin, which is at the heart of all magnetic phenomena. For instance, a refrigerator magnet, at the microscopic level, consists of trillions of trillions of iron atoms all lined up. Each of these atoms can be thought of loosely as a tiny spinning ball. The orientation of that spin is what makes the atom into a tiny magnet. The refrigerator magnet is an example of a ferromagnet, the ferro part coming from the Latin word for iron. In a ferromagnet, all the atomic spins are lined up in the same way, producing a large cooperative magnetic effect.
Important though they may be, ferromagnets aren't the only kind of material where magnetic interactions between spins are critical. In anti-ferromagnets, for instance, the neighboring spins are driven to be anti-aligned. That is, the orientations of the spins alternate up and down (see top picture in figure). The accumulative magnetic effect of all these up and down spins is that the material has no net magnetism. The high-temperature superconducting materials discovered in the 1980s are an important example of an anti-ferromagnetic structure.
More complicated and potentially interesting magnetic arrangements are possible, which may lead to a quantum spin liquid. Imagine an equilateral triangle, with an atom (spin) at each corner. Anti-ferromagnetism in such a geometry would meet with difficulties. Suppose that one spin points up while a second spin points down. So far, so good. But what spin orientation can the third atom take? It can't simultaneously anti-align with both of the other atoms in the triangle. Physicists employ the word "frustration" to describe this baffling condition where all demands cannot be satisfied. Read more to learn how physicist address this 'frustration' in this review from which this post is created.
Thursday, August 11, 2011
|In the early hours of Tuesday morning, our nearest star put on a show that won't be forgotten for a long, long time. Under the ever-watchful eyes of an armada of solar observatories, the sun unleashed an M2-class solar flare|
|This spectacular high definition image of the sun was captured by NASA's Solar Dynamics Observatory (SDO) just as sunspot 1087 was rotating out of view. Although it will soon disappear behind the sun, this sunspot region isn't going quietly|
|On April 2, NASA's Solar Dynamics Observatory (SDO) captured this rare view of the sun. Only twice a year, SDO enters an "eclipse season" when the Earth blocks its otherwise uninterrupted view of our nearest star. For up to 72 minutes a day, an ominous shadow can be seen to obscure the otherwise high-definition view of the solar surface|
|Highest resolution photograph of the sun available to date, part of a brand new series of NASA Solar Dynamics Observatory (SDO) observations.|
Monday, August 8, 2011
Monday, August 1, 2011
|Apparatus from the original 1853 paper in which the Wiedemann-Franz Law was first established. (Credit: Image courtesy of University of Bristol)|
A violation of one of the oldest empirical laws of physics has been observed by scientists at the University of Bristol. Their experiments on purple bronze, a metal with unique one-dimensional electronic properties, indicate that it breaks the Wiedemann-Franz Law.
In 1996, American physicists C. L. Kane and Matthew Fisher made a theoretical prediction that if you confine electrons to individual atomic chains, the Wiedemann-Franz law could be strongly violated. In this one-dimensional world, the electrons split into two distinct components or excitations, one carrying spin but not charge (the spinon), the other carrying charge but not spin (the holon). When the holon encounters an impurity in the chain of atoms it has no choice but for its motion to be reflected. The spinon, on the other hand, has the ability to tunnel through the impurity and then continue along the chain. This means that heat is conducted easily along the chain but charge is not. This gives rise to a violation of the Wiedemann-Franz law that grows with decreasing temperature.
The experimental group, led by Professor Nigel Hussey of the Correlated Electron Systems Group at the University of Bristol, tested this prediction on a purple bronze material comprising atomic chains along which the electrons prefer to travel.
Not only does this remarkable capability of this compound to conduct heat have potential from a technological perspective, such unprecedented violation of the Wiedemann-Franz law provides striking evidence for this unusual separation of the spin and charge of an electron in the one-dimensional world.
Professor Hussey said: "One can create purely one-dimensional atomic chains on substrates, or free-standing two-dimensional sheets, like graphene, but in a three-dimensional complex solid, there will always be some residual coupling between individual chains of atoms within the complex that allow the electrons to move in three-dimensional space.
|Graphene is a two-dimensional crystal consisting of a single layer of carbon atoms arranged hexagonally. (Credit: Berkeley Lab/U.S. Department of Energy)|
The Nobel prize winning scientists Professor Andre Geim and Professor Kostya Novoselov who discovered world's thinnest material graphene are now at the process of using it to produce fastest electronics. Graphene is a novel two-dimensional material which can be seen as a monolayer of carbon atoms arranged in a hexagonal lattice. It possesses a number of unique properties, such as extremely high electron and thermal conductivities due to very high velocities of electrons and high quality of the crystals, as well as mechanical strength.
Well, the opportunities for a faster electronics with devices like touch-screens, ultra-fast transistors and photodetectors are accelerated with such discoveries like graphenes and one-dimensional material like purple bronze!