Manfred Sigrist

Electronic and magnetic properties of nanographite ribbons

Electronic and magnetic properties of ribbon-shaped nanographite systems with zigzag and armchair edges in a magnetic field are investigated by using a tight-binding model. One of the most remarkable features of these systems is the appearance of edge states, strongly localized near zigzag edges. The edge state in a magnetic field, generating a rational fraction of the magnetic flux ( f5 p/q) in each hexagonal plaquette of the graphite plane, behaves like a zero-field edge state with q internal degrees of freedom. The orbital diamagnetic susceptibility strongly depends on the edge shapes. The reason is found in the analysis of the ring currents, which are very sensitive to the lattice topology near the edge. Moreover, the orbital diamagnetic susceptibility is scaled as a function of the temperature, Fermi energy, and ribbon width. Because the edge states lead to a sharp peak in the density of states at the Fermi level, the graphite ribbons with zigzag edges show Curie-like temperature dependence of the Pauli paramagnetic susceptibility. Hence, there is a crossover from hightemperature diamagnetic to low-temperature paramagnetic behavior in the magnetic susceptibility of nanographite ribbons with zigzag edges. @S0163-1829~99!02111-6#

Superconductivity without inversion symmetry: MnSi versus CePt3Si.

Superconductivity in materials without spatial inversion symmetry is studied. We show that in contrast to common belief, spin-triplet pairing is not entirely excluded in such systems. Moreover, paramagnetic limiting is analyzed for both spin-singlet and -triplet pairing. The lack of inversion symmetry reduces the effect of the paramagnetic limiting for spin-singlet pairing. These results are applied to MnSi and CePt3Si.

Paramagnetic Effect in High T c Superconductors -A Hint for d-Wave Superconductivity

The paramagnetic effect found experimentally in some granular high temperature superconductors is explained by a model of a Josephson network of d -wave superconductors. The unusual microwave absorption characteristics of these systems is also explained in a consistent way. Finally, a new experiment is proposed to give a decisive test for d -wave superconductivity in high T c superconductors.

Superconductivity, Spin Gaps and Luttinger Liquids in a Class of Cuprates

A homologous series of cuprates, Srn -1 Cun + 1 O2n, formed by introducing a parallel array of planar defects into the infinite-layer cuprate, SrCuO2, have been reported by Takano et al. In each CuO2 plane line defects consisting of CuO double chains result. An analysis of the electronic properties of such planes demonstrates that the stoichiometric compounds with n = 3, 7, 11,... will be frustrated quantum antiferromagnets and spin liquids. When lightly doped with holes the spin gap will remain and singlet superconductivity should occur on a separate but high temperature scale. This prediction may shed new light on the origin of the separate energy scales for the spin gap and superconductivity in other lightly doped cuprates.

The intriguing superconductivity of strontium ruthenate

Structurally, this metal oxide resembles the high-Tc cuprates, but its superconductivity is more like the superfluidity of helium-3.

Spin wave mode of edge-localized magnetic states in nanographite zigzag ribbons

We consider the low-energy magnetic excitations of nanographite ribbons with zigzag edges. The zigzag ribbons possess almost flat bands at the Fermi level which cause a ferrimagnetic spin polarization localized at the edge sites. The spin wave mode of this magnetic state is investigated by a random phase approximation of the corresponding Hubbard model. This result is used to derive an effective Heisenberg model with ladder structure. Although this system has a spin gap (Haldane type), our analysis shows that the gap is small and the tendency towards ferrimagnetic correlation at the edges is strong.

SPIN LADDERS WITH SPIN GAPS : A DESCRIPTION OF A CLASS OF CUPRATES

We investigate the magnetic properties of the Cu-O planes in stoichiometric Sr

Coupled Superconducting and Magnetic Order in CeCoIn5

_{n-1}

Zero-conductance resonances due to flux states in nanographite ribbon junctions

Cu

PERFECTLY CONDUCTING CHANNEL AND UNIVERSALITY CROSSOVER IN DISORDERED GRAPHENE NANORIBBONS

_{n+1}