Interaction Between Superconducting and Ferromagnetic Order Parameters in Graphite-Sulfur Composites

The superconductivity of graphite-sulfur composites is highly anisotropic and associated with the graphite planes. The superconducting state coexists with the ferromagnetism of pure graphite, and a continuous crossover from superconducting to ferromagnetic-like behavior could be achieved by increasing the magnetic field or the temperature. The angular dependence of the magnetic moment m(alpha) provides evidence for an interaction between the ferromagnetic and the superconducting order parameters.Comment: 11 pages, 4 figures, to be published in Phys. Rev.

Magnetic field-induced insulating behavior in highly oriented pyrolitic graphite

We propose an explanation for the apparent semimetal-insulator transition observed in highly oriented pyrolitic graphite in the presence of magnetic field perpendicular to the layers. We show that the magnetic field opens an excitonic gap in the linear spectrum of the Coulomb interacting quasiparticles, in a close analogy with the phenomenon of dynamical chiral symmetry breaking in the relativistic theories of the 2+1-dimensional Dirac fermions. Our strong-coupling appoach allows for a non-perturbative description of the corresponding critical behavior

Evolving properties of two dimensional materials, from graphene to graphite

We have studied theoretically, using density functional theory, several materials properties when going from one C layer in graphene to two and three g raphene layers and on to graphite. The properties we have focused on are the elastic constants, electronic structure (energy bands and density of state s), and the dielectric properties. For any of the properties we have investigated the modification due to an increase in the number of graphene layers is within a few percent. Our results are in agreement with the analysis presented recently by Kopelevich and Esquinazi (unpublished)

Local Ferromagnetism in Microporous Carbon with the Structural Regularity of Zeolite Y

Magnetization M(H,T) measurements have been performed on microporous carbon (MC) with a three-dimensional nano-array structure corresponding to that of a zeolite Y supercage. The obtained results unambiguously demonstrate the occurrence of high-temperature ferromagnetism in MC, probably originating from a topological disorder associated with curved graphene sheets. The results provide evidence that the ferromagnetic behavior of MC is governed by isolated clusters in a broad temperature range, and suggest the occurrence of percolative-type transition with the temperature lowering. A comparative analysis of the results obtained on MC and related materials is given.Comment: To be published in Physical Review B (2003

Critical role of dynamic flexibility in ge-containing zeolites: impact on diffusion

Incorporation of germanium in zeolites is well known to confer static flexibility to their framework, by stabilizing the formation of small rings. In this work, we show that the flexibility associated to Ge atoms in zeolites goes beyond this static effect, manifesting also a clear dynamic nature, in the sense that it leads to enhanced molecular diffusion. Our study combines experimental and theoretical methods providing evidence for this effect, which has not been described previously, as well as a rationalization for it, based on atomistic grounds. We have used both pure-silica and silico-germanate ITQ-29 (LTA topology) zeolites as a case study. Based on our simulations, we identify the flexibility associated to the pore breathing-like behavior induced by the Ge atoms, as the key factor leading to the enhanced diffusion observed experimentally in Ge-containing zeolites

Pairing symmetry of superconducting graphene

The possibility of intrinsic superconductivity in alkali-coated graphene monolayers has been recently suggested theoretically. Here, we derive the possible pairing symmetries of a carbon honeycomb lattice and discuss their phase diagram. We also evaluate the superconducting local density of states (LDOS) around an isolated impurity. This is directly related to scanning tunneling microscopy experiments, and may evidence the occurrence of unconventional superconductivity in graphene.Comment: Eur. Phys. J. B, to appea

Magnetic field driven metal-insulator phase transition in planar systems

A theory of the magnetic field driven (semi-)metal-insulator phase transition is developed for planar systems with a low density of carriers and a linear (i.e., relativistic like) dispersion relation for low energy quasiparticles. The general structure of the phase diagram of the theory with respect to the coupling constant, the chemical potential and temperature is derived in two cases, with and without an external magnetic field. The conductivity and resistivity as functions of temperature and magnetic field are studied in detail. An exact relation for the value of the "offset" magnetic field $B_c$, determining the threshold for the realization of the phase transition at zero temperature, is established. The theory is applied to the description of a recently observed phase transition induced by a magnetic field in highly oriented pyrolytic graphite.Comment: 22 pages, REVTeX, 16 figures. The version corresponding to that published in Phys.Rev.

Experiments in vortex avalanches

Avalanche dynamics is found in many phenomena spanning from earthquakes to the evolution of species. It can be also found in vortex matter when a type II superconductor is externally driven, for example, by increasing the magnetic field. Vortex avalanches associated with thermal instabilities can be an undesirable effect for applications, but "dynamically driven" avalanches emerging from the competition between intervortex interactions and quenched disorder constitute an interesting scenario to test theoretical ideas related with non-equilibrium dynamics. However, differently from the equilibrium phases of vortex matter in type II superconductors, the study of the corresponding dynamical phases - in which avalanches can play a role - is still in its infancy. In this paper we critically review relevant experiments performed in the last decade or so, emphasizing the ability of different experimental techniques to establish the nature and statistical properties of the observed avalanche behavior.Comment: To be published in Reviews of Modern Physics April 2004. 17 page

The Flux-Line Lattice in Superconductors

Magnetic flux can penetrate a type-II superconductor in form of Abrikosov vortices. These tend to arrange in a triangular flux-line lattice (FLL) which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-$T_c$ supercon- ductors (HTSC's) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft mainly because of the large magnetic penetration depth: The shear modulus of the FLL is thus small and the tilt modulus is dispersive and becomes very small for short distortion wavelength. This softness of the FLL is enhanced further by the pronounced anisotropy and layered structure of HTSC's, which strongly increases the penetration depth for currents along the c-axis of these uniaxial crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause thermally activated depinning of the flux lines or of the 2D pancake vortices in the layers. Various phase transitions are predicted for the FLL in layered HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to interesting effects which strongly depend on the geometry of the experiment.Comment: Review paper for Rep.Prog.Phys., 124 narrow pages. The 30 figures do not exist as postscript file