Magnetism in Graphene Induced by Single-Atom Defects

We study from first principles the magnetism in graphene induced by single carbon atom defects. For two types of defects considered in our study, the hydrogen chemisorption defect and the vacancy defect, the itinerant magnetism due to the defect-induced extended states has been observed. Calculated magnetic moments are equal to 1 $\mu_B$ per hydrogen chemisorption defect and 1.12$-$1.53 $\mu_B$ per vacancy defect depending on the defect concentration. The coupling between the magnetic moments is either ferromagnetic or antiferromagnetic, depending on whether the defects correspond to the same or to different hexagonal sublattices of the graphene lattice, respectively. The relevance of itinerant magnetism in graphene to the high-$T_C$ magnetic ordering is discussed.Comment: 5 pages, 6 figure

Field Dependence of the Josephson Plasma Resonance in Layered Superconductors with Alternating Junctions

The Josephson plasma resonance in layered superconductors with alternating critical current densities is investigated in a low perpendicular magnetic field. In the vortex solid phase the current densities and the squared bare plasma frequencies decrease linearly with the magnetic field. Taking into account the coupling due to charge fluctuations on the layers, we extract from recent optical data for SmLa_{1-x} Sr_x CuO_{4-delta} the Josephson penetration length lambda_{ab} approximately 1100 A parallel to the layers at T=10 K.Comment: 5 pages, 6 eps-figures, final version with minor misprints correcte