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Site: http://phys.org/physics-news/

Artist's impression of a high critical temperature superconductor immersed in a magnetic field. The magnetic field generates whirls of current called vortices. These allow to better perceive an ordered electronic structure that coexists with the superconducting state. Credit: © UNIGE - Xavier Ravinet Superconducting materials have the characteristic of letting an electric current flow without resistance. The study of superconductors with a high critical temperature discovered in the 1980s remains a very attractive research subject for physicists. Indeed, many experimental observations still lack an adequate theoretical description. Researchers from the University of Geneva (UNIGE) in Switzerland and the Technical University Munich in Germany have lifted the veil on the electronic characteristics of high-temperature superconductors. Their research, published in Nature Communications, shows that the electronic densities measured in these superconductors are a combination of two separate effects. As a result, they propose a new model that suggests the existence of two coexisting states rather than competing ones postulated for the past thirty years, a small revolution in the world of superconductivity. Below a certain temperature, a superconducting material loses all electrical resistance (equal to zero). When immersed in a magnetic field, high-temperature superconductors (high-Tc) allow this field to penetrate in the form of filamentary regions, called vortices, a condition in which the material is no longer superconducting. Each vortex is a whirl of electronic currents generating their own magnetic fields and in which the electronic structure is different from the rest of the material. Some theoretical models describe high-Tc superconductors as a competition between two fundamental states, each developing its own spectral signature. The first is characterized by an ordered spatial arrangement of electrons. The second, corresponding to the superconducting phase, is characterized by electrons assembled in pairs. "However, by measuring the density of electronic states with local tunneling spectroscopy, we discovered that the spectra that were attributed solely to the core of a vortex, where the material is not in the superconducting state, are also present elsewhere—that is to say, in areas where the superconducting state exists. This implies that these spectroscopic signatures do not originate in the vortex cores and cannot be in competition with the superconducting state," explains Christoph Renner, professor in the Department of Quantum Matter Physics of the Faculty of Science at UNIGE. "This study therefore questions the view that these two states are in competition, as largely assumed until now. Instead, they turn out to be two coexisting states that together contribute to the measured spectra," professor Renner says. Indeed, physicists from UNIGE using theoretical simulation tools have shown that the experimental spectra can be reproduced perfectly by considering the superposition of the spectroscopic signature of a superconductor and this other electronic signature, brought to light through this new research. This discovery is a breakthrough toward understanding the nature of the high-temperature superconducting state. It challenges some theoretical models based on the competition of the two states mentioned above. It also sheds new light on the electronic nature of the vortex cores, which potentially has an impact on their dynamics. Mastery of these dynamics, and particularly of the anchoring of vortices that depend on their electronic nature, is critical for many applications such as high-field electromagnets. Explore further: Insights into the stages of high-temperature superconductivity


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Site: http://www.nanotech-now.com/

Home > Press > Superconductivity seen in a new light Abstract: Superconducting materials have the characteristic of letting an electric current flow without resistance. The study of superconductors with a high critical temperature discovered in the 1980s remains a very attractive research subject for physicists. Indeed, many experimental observations still lack an adequate theoretical description. Researchers from the University of Geneva (UNIGE) in Switzerland and the Technical University Munich in Germany have managed to lift the veil on the electronic characteristics of high-temperature superconductors. Their research, published in Nature Communications, show that the electronic densities measured in these superconductors are a combination of two separate effects. As a result, they propose a new model that suggests the existence of two coexisting states rather than competing ones as was postulated for the past thirty years. A small revolution in the world of superconductivity. A superconducting material is a material that, below a certain temperature, loses all electrical resistance (equal to zero). When immersed in a magnetic field, high-temperature superconductors (high-Tc) allow this field to penetrate in the form of filamentary regions, called vortices, in which the material is no longer superconducting. Each vortex is a whirl of electronic currents generating their own magnetic field and in which the electronic structure is different from the rest of the material. Coexistence rather than competition Some theoretical models describe high-Tc superconductors as a competition between two fundamental states, each developing its own spectral signature. The first is characterized by an ordered spatial arrangement of electrons. The second, corresponding to the superconducting phase, is characterized by electrons assembled in pairs. «However, by measuring the density of electronic states with local tunneling spectroscopy, we discovered that the spectra that were attributed solely to the core of a vortex, where the material is not in the superconducting state, are also present elsewhere, that is to say in areas where the superconducting state exists. This implies that these spectroscopic signatures do not originate in the vortex cores and cannot be in competition with the superconducting state», explains Christoph Renner, professor in the Department of Quantum Matter Physics of the Faculty of Science at UNIGE. «This study therefore questions the view that these two states are in competition, as largely assumed until now. Instead, they turn out to be two coexisting states that together contribute to the measured spectra», professor Renner says. Indeed, physicists from UNIGE have shown, using theoretical simulation tools, that the experimental spectra can be reproduced perfectly by considering the superposition of the spectroscopic signature of a superconductor and this other electronic signature, brought to light through this new research. This discovery is a breakthrough towards understanding the nature of the high temperature superconducting state. It puts some theoretical models based on the competition of the two states mentioned above in difficulty. It also sheds new light on the electronic nature of the vortex cores, which potentially has an impact on their dynamics. Mastery of this dynamics, and particularly of the anchoring of vortices that depend on their electronic nature, is critical for many applications, such as high field electromagnets. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


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Site: http://www.nrl.navy.mil/media/news-releases/

U.S. Naval Research Laboratory (NRL) research physicist, Dr. Alexander Efros, is bestowed the honor of Fellow by the Materials Research Society (MRS), recognizing a record of success in research in the field of optical science materials and technology. Most notably, Efros is recognized for his pioneering and fundamental contributions to the theory of low dimensional semiconductor structures, which have established the basic theoretical concepts that today are used by everyone for describing the electronic and optical properties of nanocrystal quantum dots, nanowires and nanoplatelets. These concepts are also commonly used in the development of novel nanocrystal-based optical devices, which are smaller, cheaper, more efficient, and consume less energy than the traditional devices. "Dr. Efros' rooted commitment to materials science theory and his profound knowledge of solid-state physics are notably recognized and respected by the scientific community worldwide," said Dr. Michael Mehl, head, NRL Center for Computational Materials Science. "His work has given us the ability to understand and control the behavior of semiconductor nanostructures, leading to new and exciting applications in lasers, light emitting diodes, and photovoltaics." In the early 1980s Efros and his colleagues, A. I. Ekimov and A. A. Onuschenko, discovered semiconductor nanocrystals while studying doped glasses. They also explained the origin of the size-dependent optical properties of nanocrystals, and established that nanocrystals act as 'artificial atoms' that opened the door to a new class of optical materials. Like atoms, nanocrystals have discrete optical energy spectra that are tunable over a wide spectral range by varying their size. At the same time these atoms can be manipulated with nanometer precision to form nanocrystal molecules and solids. Nanocrystals also can serve as dopants in nanocrystalline solids. As a result, researchers now have an unlimited number of new atomic elements available to create new materials. Based on size controlled tunable emission spectra and great structural and chemical flexibility, the nanocrystals have potential in the creation of a new class of tunable quantum dot lasers and light emitting diodes that function from the far infrared to deep ultraviolet wavelength ranges. An additional application is the labeling of biological molecules by nanocrystals, which are being developed as ultra-sensitive detectors and sensors for use in neuron and drug transport imaging, and the detection of dangerous chemical agents. The high optical stability of the nanocrystals allow for higher efficiency in quantum dot displays. In very recent developments of this area, researchers from Samsung Electronics demonstrated in 2012 a full-color quantum-dot display monitor that is brighter than liquid-crystal displays and consumes less than a fifth of the power. Receiving a Master of Science degree in physical engineering in 1973 and a Ph.D. in physics in 1978, both from the Technical University in Leningrad, USSR, Efros became senior researcher at the Ioffe Institute in Leningrad. From 1990 to 1992 he was senior researcher at the Technical University of Munich and a visiting scientist at Massachusetts Institute of Technology (MIT). In 1993 Efros came to NRL as a consultant and in 1999 become senior researcher in NRL's Materials Science and Technology Division. Efros has authored and co-authored more than 190 articles in refereed journals and holds two patents. He has given more than 80 invited talks at international meetings and more than 190 at various universities and laboratories. He is co-editor of two books on nanoscale semiconductors and co-organizer of many conferences on this topic and is a Fellow of the American Physical Society. Efros has received five Alan Berman publication awards, an NRL Patent Award (2003), the NRL Sigma Xi Award for Pure Science (2006), the E. F. Gross Medal of the D. S. Rozhdestvensky Optical Society of Russia (2013 with A. I. Ekimov and A. A. Onuschenko) the R.W. Wood Prize of the Optical Society of America (2006, with L. E. Brus and A. I. Ekimov), the Humboldt Research Award for Senior U.S. Scientists (2008), and the Dolores M. Etter Top Navy Scientists and Engineers of the Year Award (2009). About the U.S. Naval Research Laboratory The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.


Galisova L.,Technical University | Strecka J.,Park University
Acta Physica Polonica A | Year: 2015

A hybrid spin-electron system on one-dimensional tetrahedral chain, in which the localized Ising spin regularly alternates with the mobile electron delocalized over three lattice sites, is exactly investigated using the generalized decoration-iteration transformation. The system exhibits either the ferromagnetic or antiferromagnetic ground state depending on whether the ferromagnetic or antiferromagnetic interaction between the Ising spins and mobile electrons is considered. The enhanced magnetocaloric effect during the adiabatic demagnetization suggests a potential use of the investigated system for low-temperature magnetic refrigeration. Source


News Article
Site: http://www.rdmag.com/rss-feeds/all/rss.xml/all

Superconducting materials have the characteristic of letting an electric current flow without resistance. The study of superconductors with a high critical temperature discovered in the 1980s remains a very attractive research subject for physicists. Indeed, many experimental observations still lack an adequate theoretical description. Researchers from the University of Geneva (UNIGE) in Switzerland and the Technical University Munich in Germany have lifted the veil on the electronic characteristics of high-temperature superconductors. Their research, published in Nature Communications, shows that the electronic densities measured in these superconductors are a combination of two separate effects. As a result, they propose a new model that suggests the existence of two coexisting states rather than competing ones postulated for the past thirty years, a small revolution in the world of superconductivity. Below a certain temperature, a superconducting material loses all electrical resistance (equal to zero). When immersed in a magnetic field, high-temperature superconductors (high-Tc) allow this field to penetrate in the form of filamentary regions, called vortices, a condition in which the material is no longer superconducting. Each vortex is a whirl of electronic currents generating their own magnetic fields and in which the electronic structure is different from the rest of the material. Some theoretical models describe high-Tc superconductors as a competition between two fundamental states, each developing its own spectral signature. The first is characterized by an ordered spatial arrangement of electrons. The second, corresponding to the superconducting phase, is characterized by electrons assembled in pairs. "However, by measuring the density of electronic states with local tunneling spectroscopy, we discovered that the spectra that were attributed solely to the core of a vortex, where the material is not in the superconducting state, are also present elsewhere—that is to say, in areas where the superconducting state exists. This implies that these spectroscopic signatures do not originate in the vortex cores and cannot be in competition with the superconducting state," explained Christoph Renner, professor in the Department of Quantum Matter Physics of the Faculty of Science at UNIGE. "This study therefore questions the view that these two states are in competition, as largely assumed until now. Instead, they turn out to be two coexisting states that together contribute to the measured spectra," professor Renner says. Indeed, physicists from UNIGE using theoretical simulation tools have shown that the experimental spectra can be reproduced perfectly by considering the superposition of the spectroscopic signature of a superconductor and this other electronic signature, brought to light through this new research. This discovery is a breakthrough toward understanding the nature of the high-temperature superconducting state. It challenges some theoretical models based on the competition of the two states mentioned above. It also sheds new light on the electronic nature of the vortex cores, which potentially has an impact on their dynamics. Mastery of these dynamics, and particularly of the anchoring of vortices that depend on their electronic nature, is critical for many applications such as high-field electromagnets.

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