Researchers at the Lawrence Berkeley National Laboratory (Berkeley Lab) of U.S. Department of Energy have performed research to study how undoped graphene functions close to the "Dirac point”,which is present only in graphene. David Siegel, the key author of a paper reporting the team's research findings in the Proceedings of the National Academy of Sciences (PNAS) stated that graphene is not an insulator, semiconductor or a metal but a unique kind of semimetal with interesting electronic properties.
Using the ALS beamline 12.0.1, Siegel and his co-workers inspected a sample of graphene prepared with angle-resolved photoemission spectroscopy (ARPES) to determine how graphene which does not have any charge carriers behaves close to the “Dirac point”. The “Dirac point is a special feature of the band structure of graphene.
Graphene has no energy gap between the vacant conduction band and the electron-filled valence band. These bands are symbolized by Dirac cones whose points come in contact, and intersect linearly at the Dirac point. Graphene exhibits a set of unique properties when the conduction band is vacant and the valence band of graphene is filled.
An ARPES experiment measures a portion in between the cones by plotting directly the angle of electrons and kinetic energy obtained from the graphene sample when the ALS emits an X-ray on the sample to cause excitation. When the emitted electrons come in contact with the detector screen, a spectrum is formed and slowly develops into a cone.
Electrons interact in a unique way in undoped graphene when compared to a metal. The sides of the cone form an inward curvature, showing that electronic interactions can take place even at distances up to 790Å apart and contribute to higher electron velocities. These are extraordinary properties arising due to a common phenomenon known as "renormalization."
So Siegel and his coworkers conducted studies on “quasi-freestanding” graphene, with a silicon carbide substrate. At high temperatures, the silicon is pushed out of the silicon carbide and carbon collects as a thick layer of graphite on the surface. But successive graphene layers present in the thick sample of graphite are rotated in such a way that an each layer acts like an individual isolated layer in the stack. He added that undoped graphene is very much different from a normal Fermi liquid, and their results are in-line with theoretical computations.
Siegel stated that unscreened, long-range interactions take place in graphene, which alters the behavior of graphene in a basic way.
2011年7月18日星期一
2011年4月21日星期四
Physicist Seeks Nanomaterials with Rationally Designed Properties
A University of Arkansas physicist has received the largest award granted to an individual researcher from the Army Research Laboratory to search for a novel class of nanomaterials with rationally designed properties.
Physicist Jak Chakhalian seeks to create a new class of materials – so-called topological insulators combined with magnetic and superconductivity properties within just a few atomic layers. From the practical perspective, having all of these properties in one material could lead to building never-before realized topological quantum computers, which could be used to break complex encryption codes and compute things beyond the power of today’s supercomputers.
“If you have that, it will revolutionize the way we think about electrons moving in conventional insulators and metals even at the nanoscale,” Chakhalian said. He has funding from the Army Research Laboratory of $1.2 million over five years.
Recently Chakhalian, associate professor of physics in the J. William Fulbright College of Arts and Sciences, and colleagues found a novel way to “look” at atomic orbitals and found that they change substantially at the interface between a ferromagnet and a high-temperature superconductor. This finding opens up a new way of designing nanoscale superconducting materials. It also fundamentally changes scientific convention, which suggests that only electron spin and atomic charge – not atomic orbitals – influence the properties of nanostructures. It also has profound implications for interfaces between many other complex oxide materials.
This research was cited by Science magazine as one of the top 10 research breakthroughs of 2007.
Until recently, researchers only recognized three fundamental types of materials: metals such as iron and gold, insulators and semiconductors. In 2006, theoretical physicists suggested that another completely unknown class of insulating materials might exist. This class, called topological insulators, would not conduct electricity inside the crystal but permits the perfect conduction on the surface within a single atomic layer. This happens because geometry protects the surface electrons. In 2007, scientists looked at the alloy bismuth telluride and found the properties that this theory predicted. They had discovered a new class of material.
“On the inside, bismuth telluride is an insulator, but on the surface, within one atomic layer, it’s a perfect conductor,” Chakhalian said. “It will conduct within the single atomic layer no matter how disordered the crystal on the inside. This is a whole new class of materials very similar to the Nobel prize-winning material, graphene, with many other interesting twists.”
Chakhalian wants to create a topological insulator as a nanostructure with magnetic and superconducting properties in a few atomic layers at the interface. He admits that his goal is ambitious, but he likens the research to going to the moon in the 1960s – no one thought it could be done, but it happened.
“We need scientists to be courageous, to jump into the unknown,” he said. Chakhalian will use the grant from the Army Research Laboratory to build new equipment to create and test atomically thin superlattices by combining novel materials and using the interface as a tool.
Chakhalian is a member of the University of Arkansas Institute for Nanoscience and Engineering. He holds the Charles E. and Clydene Scharlau Endowed Professorship in Chemistry.
Physicist Jak Chakhalian seeks to create a new class of materials – so-called topological insulators combined with magnetic and superconductivity properties within just a few atomic layers. From the practical perspective, having all of these properties in one material could lead to building never-before realized topological quantum computers, which could be used to break complex encryption codes and compute things beyond the power of today’s supercomputers.
“If you have that, it will revolutionize the way we think about electrons moving in conventional insulators and metals even at the nanoscale,” Chakhalian said. He has funding from the Army Research Laboratory of $1.2 million over five years.
Recently Chakhalian, associate professor of physics in the J. William Fulbright College of Arts and Sciences, and colleagues found a novel way to “look” at atomic orbitals and found that they change substantially at the interface between a ferromagnet and a high-temperature superconductor. This finding opens up a new way of designing nanoscale superconducting materials. It also fundamentally changes scientific convention, which suggests that only electron spin and atomic charge – not atomic orbitals – influence the properties of nanostructures. It also has profound implications for interfaces between many other complex oxide materials.
This research was cited by Science magazine as one of the top 10 research breakthroughs of 2007.
Until recently, researchers only recognized three fundamental types of materials: metals such as iron and gold, insulators and semiconductors. In 2006, theoretical physicists suggested that another completely unknown class of insulating materials might exist. This class, called topological insulators, would not conduct electricity inside the crystal but permits the perfect conduction on the surface within a single atomic layer. This happens because geometry protects the surface electrons. In 2007, scientists looked at the alloy bismuth telluride and found the properties that this theory predicted. They had discovered a new class of material.
“On the inside, bismuth telluride is an insulator, but on the surface, within one atomic layer, it’s a perfect conductor,” Chakhalian said. “It will conduct within the single atomic layer no matter how disordered the crystal on the inside. This is a whole new class of materials very similar to the Nobel prize-winning material, graphene, with many other interesting twists.”
Chakhalian wants to create a topological insulator as a nanostructure with magnetic and superconducting properties in a few atomic layers at the interface. He admits that his goal is ambitious, but he likens the research to going to the moon in the 1960s – no one thought it could be done, but it happened.
“We need scientists to be courageous, to jump into the unknown,” he said. Chakhalian will use the grant from the Army Research Laboratory to build new equipment to create and test atomically thin superlattices by combining novel materials and using the interface as a tool.
Chakhalian is a member of the University of Arkansas Institute for Nanoscience and Engineering. He holds the Charles E. and Clydene Scharlau Endowed Professorship in Chemistry.
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