4,507 research outputs found
Hamiltonian decomposition for bulk and surface states
We demonstrate that a tight-binding Hamiltonian with nearest- and
next-nearest-neighbor hopping integrals can be decomposed into bulk and
boundary parts in a general lattice system. The Hamiltonian decomposition
reveals that next nearest-neighbor hopping causes sizable changes in the energy
spectrum of surface states even if the correction to the energy spectrum of
bulk states is negligible. By applying the Hamiltonian decomposition to edge
states in graphene systems, we show that the next nearest-neighbor hopping
stabilizes the edge states.Comment: 5 pages, 4 figure
Instabilities at [110] Surfaces of d_{x^2-y^2} Superconductors
We compare different scenarios for the low temperature splitting of the
zero-energy peak in the local density of states at (110) surfaces of
d_{x^2-y^2}-wave superconductors, observed by Covington et al.
(Phys.Rev.Lett.79 (1997), 277). Using a tight binding model in the
Bogolyubov-de Gennes treatment we find a surface phase transition towards a
time-reversal symmetry breaking surface state carrying spontaneous currents and
an s+id-wave state. Alternatively, we show that electron correlation leads to a
surface phase transition towards a magnetic state corresponding to a local spin
density wave state.Comment: 4 pages, 5 figure
Numerical study of the lattice vacancy effects on the single-channel electron transport of graphite ribbons
Lattice vacancy effects on electrical conductance of nanographite ribbon are
investigated by means of the Landauer approach using a tight binding model. In
the low-energy regime ribbons with zigzag boundary provide a single conducting
channel whose origin is connected with the presence of edge states. It is found
that the chemical potential dependence of conductance strongly depends on the
difference () of the number of removed A and B sublattice sites. The
large lattice vacancy with shows zero-conductance dips
in the single-channel region, however, the large lattice vacancy with
has no dip structure in this region. The connection between this
conductance rule and the Longuet-Higgins conjecture is also discussed
Nonuniversal Shot Noise in Disordered Quantum Wires with Channel-Number Imbalance
The number of conducting channels for one propagating direction is equal to
that for the other direction in ordinary quantum wires. However, they can be
imbalanced in graphene nanoribbons with zigzag edges. Employing the model
system in which a degree of channel-number imbalance can be controlled, we
calculate the shot-noise power at zero frequency by using the
Boltzmann-Langevin approach. The shot-noise power in an ordinary diffusive
conductor is one-third of the Poisson value. We show that with increasing the
degree of channel-number imbalance, the universal one-third suppression breaks
down and a highly nonuniversal behavior of shot noise appears.Comment: 10 pages, 3 figure
Correlation effects of carbon nanotubes at boundaries: Spin polarization induced by zero-energy boundary states
When a carbon nanotube is truncated with a certain type of edges, boundary
states localized near the edges appear at the fermi level. Starting from
lattice models, low energy effective theories are constructed which describe
electron correlation effects on the boundary states. We then focus on a thin
metallic carbon nanotube which supports one or two boundary states, and discuss
physical consequences of the interaction between the boundary states and bulk
collective excitations. By the renormalization group analyses together with the
open boundary bosonization, we show that the repulsive bulk interactions
suppress the charge fluctuations at boundaries, and assist the spin
polarization.Comment: 8 pages, 1 figur
Anomalous Enhancement of the Boltzmann Conductivity in Disordered Zigzag Graphene Nanoribbons
We study the conductivity of disordered zigzag graphene nanoribbons in the
incoherent regime by using the Boltzmann equation approach. The band structure
of zigzag nanoribbons contains two energy valleys, and each valley has an
excess one-way channel. The crucial point is that the numbers of conducting
channels for two propagating directions are imbalanced in each valley due to
the presence of an excess one-way channel. It was pointed out that as a
consequence of this imbalance, a perfectly conducting channel is stabilized in
the coherent regime if intervalley scattering is absent. We show that even in
the incoherent regime, the conductivity is anomalously enhanced if intervalley
scattering is very weak. Particularly, in the limit of no intervalley
scattering, the dimensionless conductance approaches to unity with increasing
ribbon length as if there exists a perfectly conducting channel. We also show
that anomalous valley polarization of electron density appears in the presence
of an electric field.Comment: 10 pages, 3 figure
Conductance Distribution in Disordered Quantum Wires with a Perfectly Conducting Channel
We study the conductance of phase-coherent disordered quantum wires focusing
on the case in which the number of conducting channels is imbalanced between
two propagating directions. If the number of channels in one direction is by
one greater than that in the opposite direction, one perfectly conducting
channel without backscattering is stabilized regardless of wire length.
Consequently, the dimensionless conductance does not vanish but converges to
unity in the long-wire limit, indicating the absence of Anderson localization.
To observe the influence of a perfectly conducting channel, we numerically
obtain the distribution of conductance in both cases with and without a
perfectly conducting channel. We show that the characteristic form of the
distribution is notably modified in the presence of a perfectly conducting
channel.Comment: 7 pages, 16 figure
Valley filter and valley valve in graphene
It is known that the lowest propagating mode in a narrow ballistic ribbon of
graphene may lack the twofold valley degeneracy of higher modes. Depending on
the crystallographic orientation of the ribbon axis, the lowest mode mixes both
valleys or lies predominantly in a single valley (chosen by the direction of
propagation). We show, using a tight-binding model calculation, that a
nonequilibrium valley polarization can be realized in a sheet of graphene, upon
injection of current through a ballistic point contact with zigzag edges. The
polarity can be inverted by local application of a gate voltage to the point
contact region. Two valley filters in series may function as an
electrostatically controlled ``valley valve'', representing a
zero-magnetic-field counterpart to the familiar spin valve.Comment: RevTeX, 4 pages, 5 figure
Low-frequency modes in the Raman spectrum of sp-sp2 nanostructured carbon
A novel form of amorphous carbon with sp-sp2 hybridization has been recently
produced by supersonic cluster beam deposition showing the presence in the film
of both polyynic and cumulenic species [L. Ravagnan et al. Phys. Rev. Lett. 98,
216103 (2007)]. Here we present a in situ Raman characterization of the low
frequency vibrational region (400-800 cm-1) of sp-sp2 films at different
temperatures. We report the presence of two peaks at 450 cm-1 and 720 cm-1. The
lower frequency peak shows an evolution with the variation of the sp content
and it can be attributed, with the support of density functional theory (DFT)
simulations, to bending modes of sp linear structures. The peak at 720 cm-1
does not vary with the sp content and it can be attributed to a feature in the
vibrational density of states activated by the disorder of the sp2 phase.Comment: 15 pages, 5 figures, 1 tabl
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