Screening of coulomb impurities in graphene.

We calculate exactly the vacuum polarization charge density in the field of a subcritical Coulomb impurity, Z|e|/r, in graphene. Our analysis is based on the exact electron Green's function, obtained by using the operator method, and leads to results that are exact in the parameter Zalpha, where alpha is the "fine-structure constant" of graphene. Taking into account also electron-electron interactions in the Hartree approximation, we solve the problem self-consistently in the subcritical regime, where the impurity has an effective charge Z(eff), determined by the localized induced charge.

Structure of the spin-glass state of La2-xSrxCuO4: the spiral theory.

Starting from the t-J model, we derive the effective field theory describing the spin dynamics in insulating La(2-x)Sr(x)CuO(4), x approximately < 0.055, at low temperature. The theory results in a disordered spiral ground state, in which the staggered component of the copper spins is confined in a plane determined by the spin anisotropies.