ALBERT

Description: Electrons in a single sheet of graphene behave quite differently from those in traditional two-dimensional electron systems. Like massless relativistic particles, they have linear dispersion and chiral eigenstates. Furthermore, two sets of electrons centred at different points in reciprocal space ('valleys') have this dispersion, giving rise to valley degeneracy. The symmetry between valleys, together with spin symmetry, leads to a fourfold quartet degeneracy of the Landau levels, observed as peaks in the density of states produced by an applied magnetic field. Recent electron transport measurements have observed the lifting of the fourfold degeneracy in very large applied magnetic fields, separating the quartet into integer and, more recently, fractional levels. The exact nature of the broken-symmetry states that form within the Landau levels and lift these degeneracies is unclear at present and is a topic of intense theoretical debate. Here we study the detailed features of the four quantum states that make up a degenerate graphene Landau level. We use high-resolution scanning tunnelling spectroscopy at temperatures as low as 10 mK in an applied magnetic field to study the top layer of multilayer epitaxial graphene. When the Fermi level lies inside the fourfold Landau manifold, significant electron correlation effects result in an enhanced valley splitting for even filling factors, and an enhanced electron spin splitting for odd filling factors. Most unexpectedly, we observe states with Landau level filling factors of 7/2, 9/2 and 11/2, suggestive of new many-body states in graphene.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Song, Young Jae -- Otte, Alexander F -- Kuk, Young -- Hu, Yike -- Torrance, David B -- First, Phillip N -- de Heer, Walt A -- Min, Hongki -- Adam, Shaffique -- Stiles, Mark D -- MacDonald, Allan H -- Stroscio, Joseph A -- England -- Nature. 2010 Sep 9;467(7312):185-9. doi: 10.1038/nature09330.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Center for Nanoscale Science and Technology, NIST, Gaithersburg, Maryland 20899, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/20829790" target="_blank"〉PubMed〈/a〉

Description: Ultrathin epitaxial graphite was grown on single-crystal silicon carbide by vacuum graphitization. The material can be patterned using standard nanolithography methods. The transport properties, which are closely related to those of carbon nanotubes, are dominated by the single epitaxial graphene layer at the silicon carbide interface and reveal the Dirac nature of the charge carriers. Patterned structures show quantum confinement of electrons and phase coherence lengths beyond 1 micrometer at 4 kelvin, with mobilities exceeding 2.5 square meters per volt-second. All-graphene electronically coherent devices and device architectures are envisaged.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Berger, Claire -- Song, Zhimin -- Li, Xuebin -- Wu, Xiaosong -- Brown, Nate -- Naud, Cecile -- Mayou, Didier -- Li, Tianbo -- Hass, Joanna -- Marchenkov, Alexei N -- Conrad, Edward H -- First, Phillip N -- de Heer, Walt A -- New York, N.Y. -- Science. 2006 May 26;312(5777):1191-6. Epub 2006 Apr 13.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/16614173" target="_blank"〉PubMed〈/a〉

Description: Application of a magnetic field to conductors causes the charge carriers to circulate in cyclotron orbits with quantized energies called Landau levels (LLs). These are equally spaced in normal metals and two-dimensional electron gases. In graphene, however, the charge carrier velocity is independent of their energy (like massless photons). Consequently, the LL energies are not equally spaced and include a characteristic zero-energy state (the n = 0 LL). With the use of scanning tunneling spectroscopy of graphene grown on silicon carbide, we directly observed the discrete, non-equally-spaced energy-level spectrum of LLs, including the hallmark zero-energy state of graphene. We also detected characteristic magneto-oscillations in the tunneling conductance and mapped the electrostatic potential of graphene by measuring spatial variations in the energy of the n = 0 LL.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Miller, David L -- Kubista, Kevin D -- Rutter, Gregory M -- Ruan, Ming -- de Heer, Walt A -- First, Phillip N -- Stroscio, Joseph A -- New York, N.Y. -- Science. 2009 May 15;324(5929):924-7. doi: 10.1126/science.1171810.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/19443780" target="_blank"〉PubMed〈/a〉

Description: The reduced form of graphene oxide (GO) is an attractive alternative to graphene for producing large-scale flexible conductors and for creating devices that require an electronic gap. We report on a means to tune the topographical and electrical properties of reduced GO (rGO) with nanoscopic resolution by local thermal reduction of GO with a heated atomic force microscope tip. The rGO regions are up to four orders of magnitude more conductive than pristine GO. No sign of tip wear or sample tearing was observed. Variably conductive nanoribbons with dimensions down to 12 nanometers could be produced in oxidized epitaxial graphene films in a single step that is clean, rapid, and reliable.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Wei, Zhongqing -- Wang, Debin -- Kim, Suenne -- Kim, Soo-Young -- Hu, Yike -- Yakes, Michael K -- Laracuente, Arnaldo R -- Dai, Zhenting -- Marder, Seth R -- Berger, Claire -- King, William P -- de Heer, Walter A -- Sheehan, Paul E -- Riedo, Elisa -- New York, N.Y. -- Science. 2010 Jun 11;328(5984):1373-6. doi: 10.1126/science.1188119.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Chemistry Division, U.S. Naval Research Laboratory, Code 6177, Washington, DC 20375, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/20538944" target="_blank"〉PubMed〈/a〉

Description: Electric deflections of gas-phase, cryogenically cooled, neutral niobium clusters [NbN; number of atoms (N) = 2 to 150, temperature (T) = 20to 300kelvin], measured in molecular beams, show that cold clusters may attain an anomalous component with very large electric dipole moments. In contrast, room-temperature measurements show normal metallic polarizabilities. Characteristic energies kBTG(N) [Boltzmann constant kB times a transition temperature TG(N)] are identified, below which the ferroelectric-like state develops. Generally, TG decreases [110 〉 TG(N) 〉 10K] as N increases, with pronounced even-odd alternations for N 〉 38. This new state of metallic matter may be related to bulk superconductivity.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Moro, Ramiro -- Xu, Xiaoshan -- Yin, Shuangye -- de Heer, Walt A -- New York, N.Y. -- Science. 2003 May 23;300(5623):1265-9.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉School of Physics, Georgia Institute of Technology, Atlanta GA, 30332-0430, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/12764191" target="_blank"〉PubMed〈/a〉

Description: The formation of carbon nanotubes in a pure carbon arc in a helium atmosphere is found to involve liquid carbon. Electron microscopy shows a viscous liquid-like amorphous carbon layer covering the surfaces of nanotube-containing millimeter-sized columnar structures from which the cathode deposit is composed. Regularly spaced, submicrometer-sized spherical beads of amorphous carbon are often found on the nanotubes at the surfaces of these columns. Apparently, at the anode, liquid-carbon drops form, which acquire a carbon-glass surface due to rapid evaporative cooling. Nanotubes crystallize inside the supercooled, glass-coated liquid-carbon drops. The carbon-glass layer ultimately coats and beads on the nanotubes near the surface.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉de Heer, Walt A -- Poncharal, Philippe -- Berger, Claire -- Gezo, Joseph -- Song, Zhimin -- Bettini, Jefferson -- Ugarte, Daniel -- New York, N.Y. -- Science. 2005 Feb 11;307(5711):907-10.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA. walt.deheer@physics.gatech.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/15705847" target="_blank"〉PubMed〈/a〉

Description: Many potential applications have been proposed for carbon nanotubes, including conductive and high-strength composites; energy storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and nanometer-sized semiconductor devices, probes, and interconnects. Some of these applications are now realized in products. Others are demonstrated in early to advanced devices, and one, hydrogen storage, is clouded by controversy. Nanotube cost, polydispersity in nanotube type, and limitations in processing and assembly methods are important barriers for some applications of single-walled nanotubes.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Baughman, Ray H -- Zakhidov, Anvar A -- de Heer, Walt A -- New York, N.Y. -- Science. 2002 Aug 2;297(5582):787-92.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉NanoTech Institute, University of Texas at Dallas, Richardson, TX 75083-0688, USA. ray.baughman@utdallas.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/12161643" target="_blank"〉PubMed〈/a〉

Description: Graphene nanoribbons will be essential components in future graphene nanoelectronics. However, in typical nanoribbons produced from lithographically patterned exfoliated graphene, the charge carriers travel only about ten nanometres between scattering events, resulting in minimum sheet resistances of about one kilohm per square. Here we show that 40-nanometre-wide graphene nanoribbons epitaxially grown on silicon carbide are single-channel room-temperature ballistic conductors on a length scale greater than ten micrometres, which is similar to the performance of metallic carbon nanotubes. This is equivalent to sheet resistances below 1 ohm per square, surpassing theoretical predictions for perfect graphene by at least an order of magnitude. In neutral graphene ribbons, we show that transport is dominated by two modes. One is ballistic and temperature independent; the other is thermally activated. Transport is protected from back-scattering, possibly reflecting ground-state properties of neutral graphene. At room temperature, the resistance of both modes is found to increase abruptly at a particular length--the ballistic mode at 16 micrometres and the other at 160 nanometres. Our epitaxial graphene nanoribbons will be important not only in fundamental science, but also--because they can be readily produced in thousands--in advanced nanoelectronics, which can make use of their room-temperature ballistic transport properties.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Baringhaus, Jens -- Ruan, Ming -- Edler, Frederik -- Tejeda, Antonio -- Sicot, Muriel -- Taleb-Ibrahimi, Amina -- Li, An-Ping -- Jiang, Zhigang -- Conrad, Edward H -- Berger, Claire -- Tegenkamp, Christoph -- de Heer, Walt A -- England -- Nature. 2014 Feb 20;506(7488):349-54. doi: 10.1038/nature12952. Epub 2014 Feb 5.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉1] Institut fur Festkorperphysik, Leibniz Universitat, Hannover, Appelstrasse 2, 30167 Hannover, Germany [2]. ; 1] School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0430, USA [2]. ; Institut fur Festkorperphysik, Leibniz Universitat, Hannover, Appelstrasse 2, 30167 Hannover, Germany. ; 1] Universite de Lorraine, UMR CNRS 7198, Institut Jean Lamour, BP 70239, 54506 Vandoeuvre-les-Nancy, France [2] UR1 CNRS/Synchrotron SOLEIL, Saint-Aubin, 91192 Gif sur Yvette, France. ; Universite de Lorraine, UMR CNRS 7198, Institut Jean Lamour, BP 70239, 54506 Vandoeuvre-les-Nancy, France. ; UR1 CNRS/Synchrotron SOLEIL, Saint-Aubin, 91192 Gif sur Yvette, France. ; Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Tennessee 37831, USA. ; School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0430, USA. ; 1] School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0430, USA [2] Institut Neel, CNRS UJF-INP, 38042 Cedex 6, Grenoble, France.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/24499819" target="_blank"〉PubMed〈/a〉

Description: Open carbon nanotubes were filled with molten silver nitrate by capillary forces. Only those tubes with inner diameters of 4 nanometers or more were filled, suggesting a capillarity size dependence as a result of the lowering of the nanotube-salt interface energy with increasing curvature of the nanotube walls. Nanotube cavities should also be less chemically reactive than graphite and may serve as nanosize test tubes. This property has been illustrated by monitoring the decomposition of silver nitrate within nanotubes in situ in an electron microscope, which produced chains of silver nanobeads separated by high-pressure gas pockets.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Ugarte -- Chatelain -- de Heer WA -- New York, N.Y. -- Science. 1996 Dec 13;274(5294):1897-9.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉D. Ugarte, Laboratorio Nacional de Luz Sincrotron (CNPq/MCT), Caixa Postal 6192, 13083-970 Campinas SP, Brazil. A. Chatelain, Institut de Physique Experimentale, Departement Physique, Ecole Polytechnique Federale de Lausanne, 1015 Lausanne, Switzerland. W. A. de Heer, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/8943200" target="_blank"〉PubMed〈/a〉

Description: Molecular beam deflection measurements of small iron, cobalt, and nickel clusters show how magnetism develops as the cluster size is increased from several tens to several hundreds of atoms for temperatures between 80 and 1000 K. Ferromagnetism occurs even for the smallest sizes: for clusters with fewer than about 30 atoms the magnetic moments are atomlike; as the size is increased up to 700 atoms, the magnetic moments approach the bulk limit, with oscillations probably caused by surface-induced spin-density waves. The trends are explained in a magnetic shell model. A crystallographic phase transition from high moment to low moment in iron clusters has also been identified.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Billas, I M -- Chatelain, A -- de Heer, W A -- New York, N.Y. -- Science. 1994 Sep 16;265(5179):1682-4.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17770895" target="_blank"〉PubMed〈/a〉