The effectively massless, relativistic behaviour of graphene’s charge carriers—known as Dirac fermions—is a result of its unique electronic structure, characterized by conical valence and conduction bands that meet at a single point in momentum space (at the Dirac crossing energy). The study of many-body interactions amongst the charge carriers in graphene and related systems such as carbon nanotubes, fullerenes and graphite is of interest owing to their contribution to superconductivity and other exotic ground states in these systems. Here we show, using angle-resolved photoemission spectroscopy, that electron–plasmon coupling plays an unusually strong role in renormalizing the bands around the Dirac crossing energy—analogous to mass renormalization by electron–boson coupling in ordinary metals. Our results show that electron–electron, electron–plasmon and electron–phonon coupling must be considered on an equal footing in attempts to understand the dynamics of quasiparticles in graphene and related systems.

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Quasiparticle dynamics in graphene

Recent advances in the design and synthesis of organic synthetic metals have yielded materials that have the highest superconducting transition temperatures ( T c ≈ 13 kelvin) reported for these systems These materials have crystal structures consisting of alternating layers of organic donor molecules and inorganic anions Organic superconductors have various electronic and magnetic properties and crystal structures that are similar to those of the inorganic copper oxide superconductors (which have high T c values); these similarities include highly anisotropic conductivities, critical fields, and short coherence lengths The largest number of organic superconductors, including those with the highest T c values, are charge-transfer salts derived from the electron donor molecule BEDT-TTF or ET [bis(ethylenedithio)-tetrathiafulvalene] The synthesis and crystal structures of these salts are discussed; their electrical, magnetic, and band electronic structure properties and their many similarities to the copper oxide superconductors are treated as well

Organic superconductors--new benchmarks.

Disclosed is a sensor for sensing the presence of an analyte component without relying on redox mediators. This sensor includes (a) a plurality of conductive polymer strands each having at least a first end and a second end and each aligned in a substantially common orientation; (b) a plurality of molecular recognition headgroups having an affinity for the analyte component and being attached to the first ends of the conductive polymer strands; and (c) an electrode substrate attached to the conductive polymer strands at the second ends. The electrode substrate is capable of reporting to an electronic circuit reception of mobile charge carriers (electrons or holes) from the conductive polymer strands. The electrode substrate may be a photovoltaic diode.

Molecular wire injection sensors

The recently discovered k -phase salts k -(BEDT-TTF) 2 ]Cu[N(CN) 2 ]X (X=Cl,Br, I) are isostructural but differ in their superconducting properties: the Br-salt is an ambient-pressure superconductor with T c =116K, the Cl-salt becomes superconducting ( T c =128K) under a slight applied pressure of 03 kbar, and the I-salt does not exhibit superconductivity under an applied-pressure of up to 5 kbar We examine the structural and electronic properties of these three salts on the basis of their crystal structures determined at 127 K by single crystal X-ray diffraction measurements The band electronic structure of the three salts are virtually the same and do not account for the differences in the conduction properties The differences in the three salts with regard to superconductivity are examined from the viewpoint of lattice softness by calculating strain indices for short intermolecular contacts The random potential arising from ethylene group conformational disorder prevents superconductivity in the iodide salt

Strain index, lattice softness and superconductivity of organic donor-molecule salts: Crystal and electronic structures of three isostructural salts k-(BEDT- TTF)2Cu[N(CN)2]X (X=Cl, Br, I)

In this Colloquium, the main features of the electron-lattice interaction are discussed and high values of the critical temperature up to room temperature could be provided. While the issue of the mechanism of superconductivity in the high ${T}_{c}$ cuprates continues to be controversial, one can state that there have been many experimental results demonstrating that the lattice makes a strong impact on the pairing of electrons. The polaronic nature of the carriers is also a manifestation of strong electron-lattice interaction. One can propose an experiment that allows an unambiguous determination of the intermediate boson (phonon, magnon, exciton, etc.) which provides the pairing. The electron-lattice interaction increases for nanosystems, and this is due to an effective increase in the density of states.

Colloquium : Electron-lattice interaction and its impact on high T c superconductivity

The two-dimensional character of the electronic structure of M-N-Cu-O alloys, with M = La,Y and N = Ba,Sm,..., is shown to favor the formation of ''acoustic'' plasmons at energies above the acoustic phonons. Providing that the Fermi energy intersects a small pocket of electrons (or holes) in addition to the expected occupation of a primary electron (or hole) band, the plasma oscillations of the secondary charge carriers may provide a mechanism for room-temperature superconductivty.

Plasmons and high-temperature superconductivity in alloys of copper oxides.

The spin-flip scattering of an electron by a pair of adjacent magnetic impurities is calculated in the presence of a magnetic field. In the limit of zero field the pair scattering yields a sharp drop in the low-temperature resistivity whose strength is inversely proportional to T. This process provides an explanation for the anomalous drop in resistivity below T\ensuremath{\le}100 K for ${\mathrm{UPt}}_{3}$ and many other heavy-fermion alloys. An external magnetic field is shown to quench this spin-flip contribution, resulting in a positive magnetoresistance for ${\mathrm{UPt}}_{3}$ at low temperatures. In conjunction with the single-impurity Kondo scattering, the pair process yields an interesting field dependence for the magnetoresistance at various temperatures and may cause a change of sign in the magnetoresistance as well as in the Hall effect.

J. Ruvalds

Papers

Plasmons and high-temperature superconductivity in alloys of copper oxides.

Magnetoresistance in heavy-fermion alloys.