Localized Spins on Graphene

The problem of a magnetic impurity, atomic or molecular, absorbed on top of a carbon atom in otherwise clean graphene is studied using the numerical renormalization group. The spectral, thermodynamic, and scattering properties of the impurity are described in detail. In the presence of a small magnetic field, the low energy electronic features of graphene make possible to inject spin polarized currents through the impurity using a scanning tunneling microscope (STM). Furthermore, the impurity scattering becomes strongly spin dependent and for a finite impurity concentration it leads to spin polarized bulk currents and a large magnetoresistance. In gated graphene the impurity spin is Kondo screened at low temperatures. However, at temperatures larger than the Kondo temperature, the anomalous magnetotransport properties are recovered.Comment: 4+ pages, 4 figures. Added reference

Partial preservation of chiral symmetry and colossal magnetoresistance in adatom doped graphene

We analyze the electronic properties of adatom doped graphene in the low impurity concentration regime. We focus on the Anderson localized regime and calculate the localization length ($\xi$) as a function of the electron doping and an external magnetic field. The impurity states hybridize with carbon's $p_z$ states and form a partially filled band close to the Dirac point. Near the impurity band center, the chiral symmetry of the system's effective Hamiltonian is partially preserved which leads to a large enhancement of $\xi$. The sensitivity of transport properties, namely Mott's variable range hopping scale $T_0$, to an external magnetic field perpendicular to the graphene sheet leads to a colossal magnetoresistance effect, as observed in recent experiments.Comment: 5 pages, 4 figs. Few comments and references added. To appear in PR

Transport properties and structures of vortex matter in layered superconductors

In this paper we analyze the structure, phase transitions and some transport properties of the vortex system when the external magnetic field lies parallel to the planes in layered superconductors. We show that experimental results for resistivity are qualitatively consistent with numerical simulations that describe the melting of a commensurate rotated lattice. However for some magnetic fields, the structure factor indicates the occurrence of smectic peaks at an intermediate temperature regime.Comment: 8 pages, 8 eps figure

Transport through quantum dots in mesoscopic circuits

We study the transport through a quantum dot, in the Kondo Coulomb blockade valley, embedded in a mesoscopic device with finite wires. The quantization of states in the circuit that hosts the quantum dot gives rise to finite size effects. These effects make the conductance sensitive to the ratio of the Kondo screening length to the wires length and provide a way of measuring the Kondo cloud. We present results obtained with the numerical renormalization group for a wide range of physically accessible parameters.Comment: 4 pages, 5 figure