293 research outputs found
Spin orbit interaction in nanotubes
In recent years, silicene, germanene, and stanene have received considerable
attention due to their possibilities to show a spin Hall effect. Nanoribbons
made of these materials are expected to have topologically protected states.
In this work, we study the electronic properties of nanotubes made of Si, Ge,
Sn, and functionalized Sn. The main difference between these materials and
graphene is the relevance of spin-orbit interaction. The lack of edge states in
a seamless tube eliminates the possibility to find a topological edge state.
The spin-orbit interaction breaks the degeneracy of Dirac's cones and
eliminates the chances of finding a metal nanotube. As a consequence, this
transforms all nanotubes with spin-orbit interaction in trivial band
insulators.
We focus our attention on two features. First, we study the energy band gap
as a function of the diameter of the nanotubes. Then, we concentrate on
controlling the band gap of a nanotube by applying an external radial electric
field.Comment: 8 pages with 8 figure
New Numerical Results Indicate a Half-Filling SU(4) Kondo State in Carbon Nanotubes
Numerical calculations simulate transport experiments in carbon nanotube
quantum dots (P. Jarillo-Herrero et al., Nature 434, 484 (2005)), where a
strongly enhanced Kondo temperature T_K ~ 8K was associated with the SU(4)
symmetry of the Hamiltonian at quarter-filling for an orbitally
double-degenerate single-occupied electronic shell. Our results clearly suggest
that the Kondo conductance measured for an adjacent shell with T_K ~ 16K,
interpreted as a singlet-triplet Kondo effect, can be associated instead to an
SU(4) Kondo effect at half-filling. Besides presenting spin-charge Kondo
screening similar to the quarter-filling SU(4), the half-filling SU(4) has been
recently associated to very rich physical behavior, including a
non-Fermi-liquid state (M. R. Galpin et al., Phys. Rev. Lett. 94, 186406
(2005)).Comment: 7 pages, 7 figure
A Novel Approach to Study Highly Correlated Nanostructures: The Logarithmic Discretization Embedded Cluster Approximation
This work proposes a new approach to study transport properties of highly
correlated local structures. The method, dubbed the Logarithmic Discretization
Embedded Cluster Approximation (LDECA), consists of diagonalizing a finite
cluster containing the many-body terms of the Hamiltonian and embedding it into
the rest of the system, combined with Wilson's idea of a logarithmic
discretization of the representation of the Hamiltonian. The physics associated
with both one embedded dot and a double-dot side-coupled to leads is discussed
in detail. In the former case, the results perfectly agree with Bethe ansatz
data, while in the latter, the physics obtained is framed in the conceptual
background of a two-stage Kondo problem. A many-body formalism provides a solid
theoretical foundation to the method. We argue that LDECA is well suited to
study complicated problems such as transport through molecules or quantum dot
structures with complex ground states.Comment: 17 pages, 13 figure
Full electrostatic control over polarized currents through spin-orbital Kondo effect
Numerical calculations indicate that by suitably controlling the individual
gate voltages of a capacitively coupled parallel double quantum dot, with each
quantum dot coupled to one of two independent non-magnetic channels, this
system can be set into a spin-orbital Kondo state by applying a magnetic field.
This Kondo regime, closely related to the SU(4) Kondo, flips spin from one to
zero through cotunneling processes that generate almost totally spin-polarized
currents with opposite spin orientation along the two channels. Moreover, by
appropriately changing the gate voltages of both quantum dots, one can
simultaneously flip the spin polarization of the currents in each channel. As a
similar zero magnetic field Kondo effect has been recently observed by Y.
Okazaki {\it et al.} [Phys. Rev. B {\bf 84}, (R)161305 (2011)], we analyze a
range of magnetic field values where this polarization effect seems robust,
suggesting that the setup may be used as an efficient bipolar spin filter,
which can generate electrostatically reversible spatially separated spin
currents with opposite polarizations.Comment: 6 pages, 8 figures (including supplemental material
Kondo regime in triangular arrangements of quantum dots: Molecular orbitals, interference and contact effects
Transport properties of an interacting triple quantum dot system coupled to
three leads in a triangular geometry has been studied in the Kondo regime.
Applying mean-field finite-U slave boson and embedded cluster approximations to
the calculation of transport properties unveils a set of rich features
associated to the high symmetry of this system. Results using both calculation
techniques yield excellent overall agreement and provide additional insights
into the physical behavior of this interesting geometry. In the case when just
two current leads are connected to the three-dot system, interference effects
between degenerate molecular orbitals are found to strongly affect the overall
conductance. An S=1 Kondo effect is also shown to appear for the perfect
equilateral triangle symmetry. The introduction of a third current lead results
in an `amplitude leakage' phenomenon, akin to that appearing in beam splitters,
which alters the interference effects and the overall conductance through the
system.Comment: 14 pages, 9 figures, submitted to PR
Interference Effects in the Conductance of Multi-Level Quantum Dots
Using exact-diagonalization techniques supplemented by a Dyson equation
embedding procedure, the transport properties of multilevel quantum dots are
investigated in the Kondo regime. The conductance can be decomposed into the
contributions of each level. It is shown that these channels can carry a
different phase, and destructive interference processes are observed when the
phase difference between them is . This effect is very different from
those observed in bulk metals with magnetic impurities, where the phase
differences play no significant role. The effect is also different from other
recent studies of interference processes in dots, as discussed in the text. In
particular, no external magnetic field is here introduced, and the hopping
amplitudes dot-leads for all levels are the same. However, conductance
cancellations induced by interactions are still observed. Another interesting
effect reported here is the formation of localized states that do not
participate in the transport. When one of these states crosses the Fermi level,
the electronic occupation of the quantum dot changes, modifying the many-body
physics of the system and indirectly affecting the transport properties. Novel
discontinuities between two finite conductance values can occur as the gate
voltage is varied, as discussed here
Determination of the phase shifts for interacting electrons connected to reservoirs
We describe a formulation to deduce the phase shifts, which determine the
ground-state properties of interacting quantum-dot systems with the inversion
symmetry, from the fixed-point eigenvalues of the numerical renormalization
group (NRG). Our approach does not assume the specific form of the Hamiltonian
nor the electron-hole symmetry, and it is applicable to a wide class of quantum
impurities connected to noninteracting leads. We apply the method to a triple
dot which is described by a three-site Hubbard chain connected to two
noninteracting leads, and calculate the dc conductance away from half-filling.
The conductance shows the typical Kondo plateaus of Unitary limit in some
regions of the gate voltages, at which the total number of electrons N_el in
the three dots is odd, i.e., N_el =1, 3 and 5. In contrast, the conductance
shows a wide minimum in the gate voltages corresponding to even number of
electrons, N_el = 2 and 4.
We also discuss the parallel conductance of the triple dot connected
transversely to four leads, and show that it can be deduced from the two phase
shifts defined in the two-lead case.Comment: 9 pages, 12 figures: Fig. 12 has been added to discuss T_
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