Affordable Fuel Cells May Get Boost From Artificial Diamonds

Affordable fuel cells could reduce the need for imported oil. However, solid oxide fuel cells currently don't fit the budget of most homeowners. The cost is tied to the internal temperature of the cell, around 1000 degrees Celsius. This temperature means the cell must be built using very durable, very expensive ceramics. Lower temperatures mean the cells could be built from stainless steel and other less expensive materials. The trick to dropping the temperature, and thus the cost, is the membrane or solid electrolyte that quickly passes oxygen from one side of the cell to the other.

In this study, the team investigated why some materials are better than others at passing oxygen along. "We could take an Edisonian approach—trying 10,000 materials, but it would be expensive, and we'd be here forever," said Dr. Ram Devanathan, a materials scientist at PNNL. "So, we are using all of the tools we have in EMSL—experimental, computational, and theoretical—to look into the materials."

Using oxygen-plasma-assisted molecular beam epitaxy, the researchers grew scandia-stabilized zirconia films on sapphire substrates. The films were examined using x-ray diffraction, electron spectroscopy, and microscopy.

However, experimental data alone was not enough, Devanathan explained. Imagine taking photos at the beginning and end of a raucous party. The photos, like the experiments, show you where you began and where you ended. However, theory shows what happened and why. Theory also allows predictions about what will happen next and what would happen under different circumstances.

So, the team applied theoretical calculations and models to the experimental data. They determined that the nanoscale, nanosecond interactions occurring in the scandia-doped cubic film conducted oxygen faster than the yttrium doping in current electrolytes.

This study provides a fundamental understanding of how ions move in scandia-doped zirconia, and shows the material is very stable. "Our integrated approach takes the science to the next level," said Dr. Theva Thevuthasan, who worked on the project and currently oversees the deposition and microfabrication capability at EMSL.

The scientists are using resources at EMSL and PNNL to provide a more detailed understanding of the atomic interactions in another promising material for fuel cells: nano layers of zirconia and ceria.

Oxygen (red spheres) migrates from one vacancy to another inside the scandia-doped cubic zirconia. The cations the oxygen must brush past are marked by the letter E

Nombre: Franklin J. Quintero C.
Asignatura: CRF
Dirección: http://www.physorg.com/news191576465.html
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A Straightforward Solution for Increasing Solar Cell Performance

"The energy that the earth receives from one hour of sunlight is enough to accommodate the world's energy needs for an entire year," said Ioana Gearba, the lead author of the study, formerly a researcher at Brookhaven's Center for Functional Nanomaterials (CFN) and presently at the University of Texas at Austin. "Efficient methods for converting sunlight to usable energy, such as solar cells, can contribute significantly to society's future energy needs. But commercial solar cells made with silicon do not produce cost-competitive electricity due, in part, to high manufacturing costs. An exciting development in the field has been the discovery of organic semiconductors, which can, in principle, lower the manufacturing costs of large-area solar devices."

But solar cells made from semiconducting organic materials, based on carbon, have their own drawbacks. Although this class of solar cells may provide a more cost-effective manufacturing route, they also are less efficient.

"Solar cells made from organic plastic materials are both attractive and unattractive at the same time," said Chuck Black, the Electronic Materials Group Leader at the CFN. "For example, they may deform a little bit when put out in the hot sun, and their electrical properties may change as they move. For solar cells made out of such materials to really work, they will need to withstand significant changes in conditions.

In the October 26, 2009, edition of Applied Physics Letters, the Brookhaven researchers report that one way to increase the stability of organic solar cells is "locking" the semiconducting base layer, the first layer upon which the solar cell is built, in place. To do this, a chemical crosslinker, which interconnects the polymer chains in the material's base layer, was added to the solution-based starting materials. This high degree of interconnection immobilizes the polymer structure and helps preserve its properties during changes in temperature, for example.

"We wanted to find a chemical way to freeze, or immobilize, the organic polymer in order to make it more stable," said Black. "We found a straightforward and elegant way to do that, and the method has an added benefit of making the material convert sunlight to electricity slightly more efficiently."

"Adding the crosslinker only takes an extra 10 minutes," said Gearba. "Our work is the first time that anyone has used a commercially available crosslinker to interconnect solution-based semiconducting polymers. Other groups have done something similar, but used an approach that may not work for all polymers."

The crosslinker mechanically stabilizes the polymer and increases its conductivity by as much as five times. The efficiency of model solar cells made from the crosslinked polymer also increased, up to three fold. The solution-based process is cost-effective and allows for large-scale uses, such as in spray-on or roll-to-roll manufacturing methods.

 

Nombre: Franklin J. Quintero C.
Asignatura: CRF
Dirección: http://www.physorg.com/news191778902.html
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New way to extract light from semiconductors could lead to ultra-high efficiency LEDs

As the researchers explain, the key to improving an LED’s energy-conversion efficiency lies in extracting the light generated in the semiconductor with the highest efficiency possible. However, the strong internal reflection in the semiconductor makes efficient light extraction very difficult, since light tends to remain inside the semiconductor. Most techniques to improve the light extraction efficiency have high production costs, but finding a highly efficient, low-cost light extraction technique is essential for popularizing LED lighting.

The AIST researchers, XueLun Wang and Mutsuo Ogura, were able to design a semiconductor to take advantage of the effects of evanescent waves for improving light extraction efficiency. As the scientists explain, evanescent waves are a special kind of light existing only near a reflection interface. When two evanescent waves meet, they are efficiently transformed into light.

In their experiments, the researchers fabricated a GaAs/AlGaAs nanostructure with V-shaped grooves and even smaller ridges between the grooves. They then deposited a 150-nm-thick layer of SiO2 onto the nanostructure. This design enabled evanescent waves to form and couple at the semiconductor-SiO2 and SiO2-air interfaces on flat planes at the tops of the ridges, resulting in an increase of the amount of light that could be extracted.

Photoluminescence studies revealed that the SiO2-coated semiconductor’s light-emitting layer at the ridges was enhanced by a factor of 1.7. According to a press release, the light-extraction efficiency builds upon and exceeds the 50% efficiency of a similar technique, although the exact efficiency of the current design is not mentioned. In contrast, uncoated light-emitting semiconductor materials deposited on flat substrates only enable a few percent of the light generated in the semiconductor to be extracted; for example, GaAs has only a 2% efficiency.

One advantage of the new design is that it doesn’t require any significant changes to the conventional LED fabrication process, which should keep fabrication costs low. The method could also be used with other materials, such as indium tin oxide or zinc oxide as the coating, and AlGaInP-based and GaN-based semiconductor materials, which can be used to develop visible LEDs with ultra-high light extraction efficiency

The illustration in (a) shows evanescent waves coupling at two interfaces on the flat planes of a ridge. Figure (b) shows the simulated electromagnetic field intensity of the structure. Image credit: AIST

Nombre: Franklin J. Quintero C.
Asignatura: CRF
Dirección: http://www.physorg.com/news193061367.html
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El superconductor de alta temperatura más delgado: una capa monoatómica de un cuprato

Los cupratos están formados por capas alternas. ¿Cuántas capas planas son necesarias para observar la superconductividad? Sólo una. Un nuevo estudio experimental publicado en Science ha observado la superconductividad con una Tc de 32 K en una película “bicapa” con una sola capa metálica, dopada con zinc, LCZO, y una sola capa aislante, LCO. El dopado con zinc de toda la película de cuprato, elimina completamente la superconductividad. Cuando sólo se dopan ciertos planos, la temperatura crítica se reduce de 32 K a solo 18 K. Logvenov y sus colegas han dopado con zinc un solo plano de una “bicapa” y han observado que la Tc se mantiene en 32 K. Interpretan su experimento como que el origen de las superconductividad se encuentra en la capa monoatómica que hace de interface entre ambas capas de la bicapa, la metálica y la aislante. Han fabricado esta estructura utilizando la técnica de epitaxia por haces moleculares (MBE). El trabajo es un gran avance experimental que no sólo aporta gran información para los teóricos sino que además tendrá múltiples aplicaciones que requieren capas superconductoras ultradelgadas. El artículo técnico es G. Logvenov, A. Gozar, I. Bozovic, “High-Temperature Superconductivity in a Single Copper-Oxygen Plane,” Science 5953: 699-702, 30 October 2009. Se han hecho eco de este artículo en ”High-temperature superconductor goes super thin,” Physics Today, Nov 2, 2009.

Estudiar si una sola capa de un cuprato puede ser superconductora es difícil porque una capa ultradelgada presenta defectos superficiales que reducen la temperatura crítica como la rugosidad superficial o la interdifusión de cationes con el substrato. En superconductores de la familia de los La-Sr-Cu-O, la temperatura crítica más alta observada en películas delgadas era de unos 10 K en películas formadas por 4 planos de óxido de cobre superconductores. En capas biatómicas de plomo se observó la superconductividad convencional (BCS) este año (“Superconductividad observada en capas biatómicas de plomo,” 7 Mayo 2009).

Descubrir el secreto de la superconductividad de alta temperatura crítica le quita el sueño a muchos investigadores. Un fenómeno polifacético del que cada día descubrimos nuevas caras. Los cupratos están formados por capas alternas (los pnicturos también). Muchos teóricos piensan que el origen de la superconductividad de alta Tc está en dicha estructura en capas planas. Han dopado una a una las capas de una película ultradelgada de cuprato para obtener la estructura de la figura: con 6 capas metálicas (LSCO), una capa aislante LCO, una metálica LCZO y 4 aislantes (LCO). La adición de zinc a una capa de óxido de cobre reduce la Tc en dicha capa a sólo 18 K (dopar todas las capas, la destruye). El nuevo estudio ha mostrado la estructura superconductora más delgada conocida, con una sola capa con un grosor de 3 celdas unidad de la estructura cristalina que es superconductora con una temperatura de transición de 32 K. Un trabajo espectacular y necesario desde que se descubrió que algunos pnicturos son superconductores tridimensionales, a diferencia de los cupratos y el resto de los pnicturos. Cada día el secreto de los superconductores de alta Tc está más próximo.

 
 

Nombre: Franklin J. Quintero C.
Asignatura: CRF
Dirección: francisthemulenews.wordpress.com/
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Observado el efecto Hall cuántico fraccionario en grafeno

El grafeno (una lámina monoatómica de grafito, átomos de carbono) sigue sorprendiendo a los físicos por sus asombrosas propiedades electrónicas. Dos artículos publicados en Nature han observado el efecto Hall cuántico fraccionario en grafeno, por el que los electrones se comportan como si tuvieran carga fraccionaria, como cuasipartículas que fueran “trozos” de electrones. La interacción entre electrones en un sólido produce un campo efectivo que se interpreta como cuasipartículas con propiedades exóticas. Los electrones en un medio bidimensional plano al que se le aplica un campo magnético fuerte, con cierto ángulo respecto a dicho plano, se comportan como cuasipartículas con una carga fraccionaria, el llamado efecto Hall cuántico fraccionario, observado en experimentos en 1982 por Daniel Tsui y Horst Störmer en heteroestructuras semiconductoras ultrapuras (estructuras formadas por capas alternas en forma de sandwich). Nos lo cuenta Alberto F. Morpurgo, “Condensed-matter physics: Dirac electrons broken to pieces,” News & Views, Nature 462: 170-171, 12 Nov. 2009, que se hace eco de los artículos técnicos de Xu Du, Ivan Skachko, Fabian Duerr, Adina Luican, Eva Y. Andrei, “Fractional quantum Hall effect and insulating phase of Dirac electrons in graphene,” Nature 462: 192-195, 12 Nov. 2009, y Kirill I. Bolotin, Fereshte Ghahari, Michael D. Shulman, Horst L. Stormer, Philip Kim, “Observation of the fractional quantum Hall effect in graphene,” Nature 462: 196-199, 12 Nov. 2009.

En la presencia de un campo magnético los electrones están sometidos a la fuerza de Lorentz que curva su trayectoria en la dirección perpendicular a las del campo aplicado y su velocidad. Estos electrones son desviados y se acumulan en los bordes del material, generando un campo eléctrico que compensa exactamente la fuerza de Lorentz. El voltaje que resulta genera una resistencia eléctrica llamada de Hall, descubierta en 1879, que crece conforme crece el campo magnético aplicado. Un siglo después se descubrió que en un conductor plano (bidimensional) la dependencia de la resistencia con el campo aplicado es más complicada, presenta una serie de escalones (plateaux), debidos al comportamiento cuántico de los electrones en el campo magnético, los niveles de energía de Landau, el llamado efecto Hall cuántico (Premio Nobel de Física de 1985 para el alemán Klaus von Klitzing). En el centro del conductor, los niveles de Landau están separados por bandas prohibidas, pero en los bordes están curvados de forma que definen un canal por el cual los electrones se pueden propagar en una única dirección. Los lectores de Investigación y Ciencia pueden recurrir a Klaus von Klitzing, “El efecto Hall cuántico,” IyC 116, mayo 1986, o la recopilación de artículos de Física del Estado Sólido editada en 1993 por la misma editorial, Prensa Científica.

El efecto Hall cuántico se observa en materiales a muy baja temperatura. Sin embargo, en el grafeno dicho efecto también se observa a temperatura ambiente (descubierto en 2005) siendo el responsable de sus propiedades como semiconductor y dando lugar a las aplicaciones electrónicas del grafeno. Más aún, los niveles de Landau están indexados por un número entero, pero en ciertos materiales se observa que aparecen niveles indexados por un número no entero, se trata del efecto Hall cuántico fraccionario. En estos materiales la unidad de carga más pequeña no es el electrón, sino una fracción del electrón. Se ha observado que los electrones en el material se rompen en trozos, unas cuasipartículas de carga fraccionaria. Estas cuasipartículas exóticas tienen propiedades cuánticas muy curiosas que han sido demostradas experimentalmente. El material ideal para observar dichas propiedades y utilizarlas en aplicaciones es el grafeno.

El descubrimiento de que en el grafeno también se puede observar el efecto Hall cuántico fraccionario a temperaturas altas (aunque todavía no a temperatura ambiente, ya que se ha podido observar sólo a 20 K), es un gran avance (es una temperatura 100 veces superior a la de otros materiales). El dispositivo utilizado requiere suspender una tira de grafeno de unas pocas micras entre dos contactos de forma que los efectos del substrato no impidan observar el efecto Hall cuántico fraccionario. El resultado ha sido la observación de cuasipartículas con una carga de 1/3 la carga del electrón. Ahora los investigadores tendrán que caracterizar las funciones de onda de estas cuasipartículas y comprobar si corresponden a lo predicho para la teoría en función de la ecuación de Dirac para los electrones. Una posibilidad es aprovechar que el grafeno es plano para utilizar el microscopio de efecto túnel y visualizar dichas funciones de onda de carga fraccionaria directamente.

Por ahora las aplicaciones de este descubrimiento se limitarán al campo teórico, donde muchas cosas quedan aún por corroborar y descubrir en este interesante campo científico. Las aplicaciones prácticas que podrán tener estos descubrimientos son ahora difíciles de imaginar, pero haberlas las habrá.

Nombre: Franklin J. Quintero C.
Asignatura: CRF
Dirección: francisthemulenews.wordpress.com/
Ver Blog: http://franklinqcrf2.blogspot.com/

Condensed matter physics covers a wide variety of different problems

Condensed matter physics covers a wide variety of different problems, which involve systems as different as polymers, electrons constrained to move in two dimensions, exotic magnets, multi-component fluids, superconductors, and even populations of infectious or economic agents. We study systems as hard to manufacture as ultraclean semiconductor `sandwiches', or as commonplace as water ice.

Despite this large variety, there is a common thread to our research: we seek to understand the collective behaviour of systems which consist of large numbers of constituent elements. Such collective behaviour often turns out to be very rich and complex.

Equally varied as the research topics are the methods employed to study them, which range from elaborate quantum field theories to computer-based studies, for the most intensive of which we have dedicated computer clusters.

These methods are not peculiar to condensed matter physics; rather, they provide the common language of theoretical physics, which enables the interchange of ideas with other disciplines such as high energy or astrophysics.

We are also in a good position to maintain strong links to experimental physics: the Dresden area also houses a strong experimental condensed matter and materials research effort. Notable research institutions are the Max Planck Institute for Chemical Physics of Solids, the Institut fuer Werkstoffwissenschaften and of course the Technical University, all of which are located in the immediate vicinity of our institute. We are also lucky to have the the Forschungszentrum Dresden-Rossendorf with its free electron laser and high mangnetic field laboratory just outside Dresden.

Our department and the institute organises regular seminars and workshops, many with visiting speakers and others showcasing the recent work of its own members. Graduate students can also benefit greatly from the International Max Planck Research School.

We very much welcome interest in our group! If you need information you cannot find on these webpages, please do not hesitate to get in touch with the group secretary (Regine Schuppe) or the member of our group who interests you most.

Nombre: Franklin J. Quintero C.
Asignatura: CRF
Dirección:http://www.mpipks-dresden.mpg.de/mpi-doc/CondensedMatter//content/OrdAndDisord.shtml
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Superconductivity Spintronics

Superconductivity SpintronicsSuperconductivity and ferromagnetism are generally considered competing phenomena - the former associated with electron attraction in an anti-parallel spin orientation to form Cooper pairs, while the latter encourages the parallel alignment of spins. The combination, therefore, of ferromagnetic and superconducting materials in close proximity has led to a range of novel and interesting phenomena, including "π-shift" Josephson Junctions in which the phase of the supercurrent undergoes a sign change due to the oscillatory order parameter in the ferromagnetic barrier leads to re-entrant superconducting critical currents with temperature and ferromagnetic layer thickness and spin-polarisation dependent proximity effects. More generally, the combination of superconducting and ferromagnetic materials can potentially create artificial analogues of the rather rare materials that exhibit both superconductivity and ferromagnetism simultaneously, such as RuSr2GdCu2O8 or heavy fermion materials such as UGe2.

Key to understanding the interactions between superconductors and non-superconductors, including ferromagnets, is the process of Andreev reflection. In conventional Andreev reflection, an electron incident on a normal metal/superconductor interface at energy close to the Fermi energy and below the superconducting gap energy is reflected as a hole with opposite momentum and spin. Furthermore the reflected hole carries information both on the phase of the electron state and the macroscopic phase of the superconductor. In this fashion, the effect of the superconducting condensate can 'leak' into the non-superconducting material over a length scale that can differ from the superconductor's coherence length. In a spin polarised material, the spin dependency of the density of states results in suppression of the Andreev reflection. Thus, information on the local properties of ferromagnetic materials, such as the spin-polarisation and spin-flip scattering, are obtainable through measurements of Andreev reflection at superconductor-ferromagnet interfaces.

We are investigating interaction of superconductivity with artificial magnetic structures, developing a new breed of hybrid superconducting-magnetic devices in which the devices are actively controllable through the magnetic or electron-spin configuration. This represents a convergence of superconductivity and spintronics and so depends upon, and contribute to current spintronics research.

Nombre: Franklin J. Quintero C.
Asignatura: CRF
Dirección: http://www.stoner.leeds.ac.uk/
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Condensed matter physics

Condensed matter physics is the field of physics that deals with the macroscopic physical properties of matter. In particular, it is concerned with the "condensed" phases that appear whenever the number of constituents in a system is extremely large and the interactions between the constituents are strong. The most familiar examples of condensed phases are solids and liquids, which arise from the bonding and electromagnetic force between atoms. More exotic condensed phases include the superfluid and the Bose-Einstein condensate found in certain atomic systems at very low temperature, the superconducting phase exhibited by conduction electrons in certain materials, and the ferromagnetic and antiferromagnetic phases of spins on atomic lattices. Condensed matter physics is the field of physics that deals with the macroscopic physical properties of matter. ... Bose-Einstein condensate from http://www. ... Bose-Einstein condensate from http://www. ... General Name, Symbol, Number rubidium, Rb, 37 Chemical series alkali metals Group, Period, Block 1, 5, s Appearance grey white Standard atomic weight 85. ... A Bose–Einstein condensate (BEC) is a state of matter formed by a system of bosons confined in an external potential and cooled to temperatures very near to absolute zero (0 kelvin or −273. ... Condensed matter physics is the field of physics that deals with the macroscopic physical properties of matter. ... This article is about matter in physics and chemistry. ... In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ... Solid-state physics, the largest branch of condensed matter physics, is the study of rigid matter, or solids. ... For other uses, see Liquid (disambiguation). ... In physics, the electromagnetic force is the force that the electromagnetic field exerts on electrically charged particles. ... For other uses, see Atom (disambiguation). ... Helium II will creep along surfaces in order to find its own level - after a short while, the levels in the two containers will equalize. ... A Bose–Einstein condensate is a phase of matter formed by bosons cooled to temperatures very near to absolute zero (0 kelvins or -273. ... For other uses, see Temperature (disambiguation). ... A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. ... Electrical conduction is the current (movement of charged particles) through a material in response to an electric field. ... A ferromagnet is a piece of ferromagnetic material, in which the microscopic magnetized regions, called domains, have been aligned by an external magnetic field (e. ... In materials that exhibit antiferromagnetism, the spins of magnetic electrons align in a regular pattern with neighboring spins pointing in opposite directions. ... In physics, spin refers to the angular momentum intrinsic to a body, as opposed to orbital angular momentum, which is the motion of its center of mass about an external point. ... In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ...

Condensed matter physics is by far the largest field of contemporary physics. Much progress has also been made in theoretical condensed matter physics. By one estimate, one third of all American physicists identify themselves as condensed matter physicists. Historically, condensed matter physics grew out of solid-state physics, which is now considered one of its main subfields. The term condensed matter physics was apparently coined by Philip Anderson when he renamed his research group — previously solid-state theory — in 1967. In 1978, the Division of Solid State Physics at the American Physical Society was renamed as the Division of Condensed Matter Physics. Condensed matter physics has a large overlap with chemistry, materials science, nanotechnology and engineering. Not to be confused with physician, a person who practices medicine. ... Solid-state physics, the largest branch of condensed matter physics, is the study of rigid matter, or solids. ... Philip Warren Anderson (born December 13, 1923) is one of the most influential theoretical physicists of the 20th century. ... The American Physical Society was founded in 1899 and is the worlds second largest organization of physicists. ... For other uses, see Chemistry (disambiguation). ... The Materials Science Tetrahedron, which often also includes Characterization at the center Materials science or Materials Engineering is an interdisciplinary field involving the properties of matter and its applications to various areas of science and engineering. ... Buckminsterfullerene C60, also known as the buckyball, is the simplest of the carbon structures known as fullerenes. ... Engineering is the discipline of acquiring and applying knowledge of design, analysis, and/or construction of works for practical purposes.

Nombre: Franklin J. Quintero C.
Asignatura: CRF
Dirección: http://www.statemaster.com/encyclopedia/Physics
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New Quantum Phenomena and High-Quality New Materials

In order to find new quantum phenomena, we must apply the pressure or the magnetic fields, or lower the　temperature. We aim to establish the measurements methods under the multi extreme conditions such as at 30mK, in the magnetic fields of 20Tesla and under the pressure of 3GPa. Recently we have found the novel pressure-induced ferromagnetism for Ru oxides (shown below).

Condensed Matter Physics The purpose of the research of quantum condensed-matter physics is to find novel quantum phenomena and their quantum mechanisms. (1) New Quantum Phenomena and High-Quality New Materials In order to find new quantum phenomena, we must apply the pressure or the magnetic fields, or lower the　temperature. We aim to establish the measurements methods under the multi extreme conditions such as at 30mK, in the magnetic fields of 20Tesla and under the pressure of 3GPa. Recently we have found the novel pressure-induced ferromagnetism for Ru oxides (shown below). In the research of novel quantum phenomena and new materials, it is important to develop the method of crystal growth. Our group has the method to develop high-quality oxide crystals. Our group has established methods to grow high-quality crystals of oxides.

Mechanism of Superconductivity

Although the oxide high-Tc superconductors have been investigated intensively over the last decade, the mechanisms of superconductivity and anomalous metallic behaviors are not still clarified. We must resolve the following in the research of high-Tc superconductors:

What is the mechanism of superconductivity?

What is the anomalous metal? And its origin?

What is the exact phase diagram in the underdoped region?

What is the phase diagram for the electron-doped cuprates? And a symmetry between the hole-doped and electron-doped materials.

The on-site Coulomb interaction is the strongest candidate for the origin of attractive interaction between paired electrons. Although the Coulomb interaction is repulsive, the attractive interaction works for pairs of d-wave symmetry with the changes of sign across the nodes. It is, however, difficult to show that there is actually an attractive channel of d-wave symmetry for the Hubbard model or the three-band Hubbard model with d and p orbitals. In fact, it has not been established that the d-wave superconducting phase exists due to the Coulomb interaction for the Hubbard model in spite of intensive study over the last decade. One opinion concerning the theoretical study is "The superconductivity for the Hubbard model is not still established. So, we must consider the effective Hamiltonians to discuss the physical properties of high-Tc cuprates."

Then we investigate a possibility of superconductivity for the (three-band) Hubbard model in two ways. First, we consider the weak-coupling limit of the Coulomb repulsion U. In the limit of small U, the ground-state energy is calculated exactly by employing the perturbation theory in U. One must overcome, however, a numerical difficulty in order to obtain a solution for the gap equation for small U because the numerical errors are comparable to or greater than the critical temperature or the superconducting order parameter. Although for the intermediate value of U, the solution of the gap equation is derived correctly, the existence of solutions do not confirm the possibility of superconductivity since the perturbation theory is not justified for the intermediate value of U. Recently Prof. Kondo developed the method to make it possible to calculate the magnitude of superconducting gap even for a small U. He has shown that the d-wave superconducting state is stable in ground state of the Hubbard model in the limit of small U. If we suppose that the continuity principle holds for small U, the ground state is superconducting for a finite value of U unless there is some ordering. We have shown that this also holds for the three-band Hubbard model (d-p model) which is regarded as the more realistic model for high-Tc cuprates. In summary, the d-wave superconducting state is possible in the system with the on-site repulsive Coulomb interaction, and whether the superconducting state is stable or not is dependent upon a competition between superconductivity and other possible orderings such as the antiferromagnetism. Thus, as a second approach we compare the energy of several states regarded as candidates for the ground state by using the quantum variational Monte Carlo method in order to determine the phase diagram

Nombre: Franklin J. Quintero C.
Asignatura: CRF
Dirección: http://unit.aist.go.jp/nano-ele/cond-phys/en/activities_1.html
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Condensed Matter Physics

The area of major concentration is condensed matter physics, which includes the work of two experimentalists, David Carroll and Richard T. Williams, and four theorists, Natalie A. W. Holzwarth, William C. Kerr, G. Eric Matthews, and Timo Thonhauser.

The group working with Richard Williams uses ultrafast spectroscopy to investigate the immediate consequences of absorption of a photon by a crystalline solid. They use ultrashort (~80 fs) laser pulses to create an electronic excitation of the solid and monitor its evolution within the coupled electron-lattice system. Phenomena of interest include self-trapping of excitons and of charge carriers in insulators, relaxation of hot carriers and surface state dynamics in semiconductors, excitonic processes of lattice defect formation, and desorption from surfaces. Photoelectron spectroscopy and related surface science techniques are conducted both with ultrafast excitation/probe laser techniques for dynamic studies and in conventional synchrotron radiation experiments at the National Synchrotron Light Source.

Concerns with surfaces, lattice defects, and laser-solid interactions lead naturally to a parallel interest in the spatial atomic structure of surfaces and surface defects, observed directly by scanning tunneling microscopy and atomic force microscopy. The scanning tunneling microscope is used to characterize laser effects on metals and semiconductors, as well as more general atomic-scale studies.

While the focus is basic research, the department is extensively involved in laser processing of materials. Richard Williams, Eric Matthews, and their students have produced and characterized thin film high-Tc superconductors of good quality. Applied research in laser planarization and bonding of metals and in photoablation of polymers is conducted by the Williams group.

William Kerr leads a group in the study of statistical physics. The research in this group concentrates on systems in which nonlinearity is the dominating feature. In some situations nonlinearity produces highly organized behavior, which is described by solitons. In other cases nonlinearity produces seemingly random behavior, which is often characterized as chaos. This group studies these phenomena by computer simulations. Their emphasis recently has been on studying soliton mechanisms underlying the dynamics of first- and second-order phase transitions. The experimental impetus for these studies comes from structural phase transitions in solids. The major issues here include the existence or nonexistence of soft modes and of precursor fluctuations of the product phase occurring within the parent phase. Such fluctuations would be described by nonlinear partial differential equations similar to the sine-Gordon or nonlinear Schrodinger equations, perhaps with extensions which describe bond anharmonicity in addition to the site anharmonicity. In addition to these important physical questions, employing computer simulation to study these problems raises a set of methodological problems. These deal with writing computer codes that efficiently use vector supercomputer architectures and with developing methods of analysis, including visualization schemes, that effectively extract the important physical processes occurring in the system from the large volume of numbers produced by the simulations .

Nombre: Franklin J. Quintero C.
Asignatura: CRF
Dirección: http://www.wfu.edu/physics/res-cond.html
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