11,320 research outputs found
Origin of the quasi-universality of the graphene minimal conductivity
It is a fact that the minimal conductivity of most graphene
samples is larger than the well-established universal value for ideal graphene
; in particular, larger by a factor . 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 .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
Magnetism in graphene nano-islands
We study the magnetic properties of nanometer-sized graphene structures with
triangular and hexagonal shapes terminated by zig-zag edges. We discuss how the
shape of the island, the imbalance in the number of atoms belonging to the two
graphene sublattices, the existence of zero-energy states, and the total and
local magnetic moment are intimately related. We consider electronic
interactions both in a mean-field approximation of the one-orbital Hubbard
model and with density functional calculations. Both descriptions yield values
for the ground state total spin, , consistent with Lieb's theorem for
bipartite lattices. Triangles have a finite for all sizes whereas hexagons
have S=0 and develop local moments above a critical size of nm.Comment: Published versio
Anomalous exchange interaction between intrinsic spins in conducting graphene systems
We address the nature and possible observable consequences of singular
one-electron states that appear when strong defects are introduced in the
metallic family of graphene, namely, metallic carbon nanotubes and nanotori. In
its simplest form, after creating two defects on the same sublattice, a state
may emerge at the Fermi energy presenting very unusual properties: It is
unique, normalizable, and features a wave function equally distributed around
both defects. As a result, the exchange coupling between the magnetic moments
generated by the two defects is anomalous. The intrinsic spins couple
ferromagnetically, as expected, but do not present an antiferromagnetic excited
state at any distance. We propose the use of metallic carbon nanotubes as a
novel electronic device based on this anomalous coupling between spins which
can be useful for the robust transmission of magnetic information at large
distances.Comment: 5 pages 5 fugure
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
magnetic moments, which present long-range interactions and are capable of
magnetic ordering, vanish at any experimentally relevant vacancy concentration.
This holds for -bond passivated and un-passivated reconstructed
vacancies, although, for the un-passivated ones, the disappearance of the
magnetic moments is accompanied by a very large magnetic susceptibility. Only
for the unlikely case of a full -bond passivation, preventing the
reconstruction of the vacancy, a full value of 1 for the 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 -type
paramagnetism.Comment: Submitted to Phys. Rev B, 9 page
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 () 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
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
Emergence of half-metallicity in suspended NiO chains
Contrary to the antiferromagnetic and insulating character of bulk NiO,
one-dimensional chains of this material can become half-metallic due to the
lower coordination of their atoms. Here we present ab initio electronic
structure and quantum transport calculations of ideal infinitely long NiO
chains and of more realistic short ones suspended between Ni electrodes. While
infinite chains are insulating, short suspended chains are half-metallic
minority-spin conductors which display very large magnetoresistance and a
spin-valve behaviour controlled by a single atom.Comment: 5 pages, 4 figures; accepted version; minor changes in introduction
and reference
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