Origin of the quasi-universality of the graphene minimal conductivity

It is a fact that the minimal conductivity $\sigma_0$ of most graphene samples is larger than the well-established universal value for ideal graphene $4e^2/\pi h$; in particular, larger by a factor $\gtrsim\pi$. Despite intense theoretical activity, this fundamental issue has eluded an explanation so far. Here we present fully atomistic quantum mechanical estimates of the graphene minimal conductivity where electron-electron interactions are considered in the framework of density functional theory. We show the first conclusive evidence of the dominant role on the minimal conductivity of charged impurities over ripples, which have no visible effect. Furthermore, in combination with the logarithmic scaling law for diffusive metallic graphene, we ellucidate the origin of the ubiquitously observed minimal conductivity in the range $8e^2/h > \sigma_0 \gtrsim 4e^2/h$.Comment: 6 pages, expanded version to appear in PR

Transport in magnetically ordered Pt nanocontacts

Pt nanocontacts, like those formed in mechanically controlled break junctions, are shown to develop spontaneous local magnetic order. Our density functional calculations predict that a robust local magnetic order exists in the atoms presenting low coordination, i. e., those forming the atom-sized neck. In contrast to previous work, we thus find that the electronic transport can be spin-polarized, although the net value of the conductance still agrees with available experimental information. Experimental implications of the formation of this new type of nanomagnet are discussed.Comment: 4 pages, 3 figure

Critical comparison of electrode models in density functional theory based quantum transport calculations

We study the performance of two different electrode models in quantum transport calculations based on density functional theory: Parametrized Bethe lattices and quasi-one dimensional wires or nanowires. A detailed account of implementation details in both cases is given. From the systematic study of nanocontacts made of representative metallic elements, we can conclude that parametrized electrode models represent an excellent compromise between computational cost and electronic structure definition as long as the aim is to compare with experiments where the precise atomic structure of the electrodes is not relevant or defined with precision. The results obtained using parametrized Bethe lattices are essentially similar to the ones obtained with quasi one dimensional electrodes for large enough sections of these, adding a natural smearing to the transmission curves that mimics the true nature of polycrystalline electrodes. The latter are more demanding from the computational point of view, but present the advantage of expanding the range of applicability of transport calculations to situations where the electrodes have a well-defined atomic structure, as is case for carbon nanotubes, graphene nanoribbons or semiconducting nanowires. All the analysis is done with the help of codes developed by the authors which can be found in the quantum transport toolbox Alacant and are publicly available.Comment: 17 pages, 12 figure

Kondo effect and spin quenching in high-spin molecules on metal substrates

Using a state-of-the art combination of density functional theory and impurity solver techniques we present a complete and parameter-free picture of the Kondo effect in the high-spin ($S=3/2$) coordination complex known as Manganese Phthalocyanine adsorbed on the Pb(111) surface. We calculate the correlated electronic structure and corresponding tunnel spectrum and find an asymmetric Kondo resonance, as recently observed in experiments. Contrary to previous claims, the Kondo resonance stems from only one of three possible Kondo channels with origin in the Mn 3d-orbitals, its peculiar asymmetric shape arising from the modulation of the hybridization due to strong coupling to the organic ligand. The spectral signature of the second Kondo channel is strongly suppressed as the screening occurs via the formation of a many-body singlet with the organic part of the molecule. Finally, a spin-1/2 in the 3d-shell remains completely unscreened due to the lack of hybridization of the corresponding orbital with the substrate, hence leading to a spin-3/2 underscreened Kondo effect.Comment: 5 pages, 2 figure

Spin-transfer torque on a single magnetic adatom

We theoretically show how the spin orientation of a single magnetic adatom can be controlled by spin polarized electrons in a scanning tunneling microscope configuration. The underlying physical mechanism is spin assisted inelastic tunneling. By changing the direction of the applied current, the orientation of the magnetic adatom can be completely reversed on a time scale that ranges from a few nanoseconds to microseconds, depending on bias and temperature. The changes in the adatom magnetization direction are, in turn, reflected in the tunneling conductance.Comment: 5 pages, 3 figure

A critical analysis of vacancy-induced magnetism in mono and bilayer graphene

The observation of intrinsic magnetic order in graphene and graphene-based materials relies on the formation of magnetic moments and a sufficiently strong mutual interaction. Vacancies are arguably considered the primary source of magnetic moments. Here we present an in-depth density functional theory study of the spin-resolved electronic structure of (monoatomic) vacancies in graphene and bilayer graphene. We use two different methodologies: supercell calculations with the SIESTA code and cluster-embedded calculations with the ALACANT package. Our results are conclusive: The vacancy-induced extended $\pi$ magnetic moments, which present long-range interactions and are capable of magnetic ordering, vanish at any experimentally relevant vacancy concentration. This holds for $\sigma$-bond passivated and un-passivated reconstructed vacancies, although, for the un-passivated ones, the disappearance of the $\pi$ magnetic moments is accompanied by a very large magnetic susceptibility. Only for the unlikely case of a full $\sigma$-bond passivation, preventing the reconstruction of the vacancy, a full value of 1$\mu_B$ for the $\pi$ extended magnetic moment is recovered for both mono and bilayer cases. Our results put on hold claims of vacancy-induced ferromagnetic or antiferromagnetic order in graphene-based systems, while still leaving the door open to $\sigma$-type paramagnetism.Comment: Submitted to Phys. Rev B, 9 page

Entanglement in a second order quantum phase transition

We consider a system of mutually interacting spin 1/2 embedded in a transverse magnetic field which undergo a second order quantum phase transition. We analyze the entanglement properties and the spin squeezing of the ground state and show that, contrarily to the one-dimensional case, a cusp-like singularity appears at the critical point $\lambda_c$, in the thermodynamic limit. We also show that there exists a value $\lambda_0 \geq \lambda_c$ above which the ground state is not spin squeezed despite a nonvanishing concurrence.Comment: 4 pages, 4 EPS figures, minor corrections added and title change

Entanglement dynamics in the Lipkin-Meshkov-Glick model

The dynamics of the one-tangle and the concurrence is analyzed in the Lipkin-Meshkov-Glick model which describes many physical systems such as the two-mode Bose-Einstein condensates. We consider two different initial states which are physically relevant and show that their entanglement dynamics are very different. A semiclassical analysis is used to compute the one-tangle which measures the entanglement of one spin with all the others, whereas the frozen-spin approximation allows us to compute the concurrence using its mapping onto the spin squeezing parameter.Comment: 11 pages, 11 EPS figures, published versio