27 research outputs found
Influence of Magnetic Moment Formation on the Conductance of Coupled Quantum Wires
In this report, we develop a model for the resonant interaction between a
pair of coupled quantum wires, under conditions where self-consistent effects
lead to the formation of a local magnetic moment in one of the wires. Our
analysis is motivated by the experimental results of Morimoto et al. [Appl.
Phys. Lett. \bf{82}, 3952 (2003)], who showed that the conductance of one of
the quantum wires exhibits a resonant peak at low temperatures, whenever the
other wire is swept into the regime where local-moment formation is expected.
In order to account for these observations, we develop a theoretical model for
the inter-wire interaction that calculated the transmission properties of one
(the fixed) wire when the device potential is modified by the presence of an
extra scattering term, arising from the presence of the local moment in the
swept wire. To determine the transmission coefficients in this system, we
derive equations describing the dynamics of electrons in the swept and fixed
wires of the coupled-wire geometry. Our analysis clearly shows that the
observation of a resonant peak in the conductance of the fixed wire is
correlated to the appearance of additional structure (near or
) in the conductance of the swept wire, in agreement with the
experimental results of Morimoto et al
Electron Dynamics in a Coupled Quantum Point Contact Structure with a Local Magnetic Moment
We develop a theoretical model for the description of electron dynamics in
coupled quantum wires when the local magnetic moment is formed in one of the
wires. We employ a single-particle Hamiltonian that takes account of the
specific geometry of potentials defining the structure as well as electron
scattering on the local magnetic moment. The equations for the wave functions
in both wires are derived and the approach for their solution is discussed. We
determine the transmission coefficient and conductance of the wire having the
local magnetic moment and show that our description reproduces the
experimentally observed features.Comment: Based on work presented at 2004 IEEE NTC Quantum Device Technology
Worksho
Detection of local-moment formation using the resonant interaction between coupled quantum wires
We study the influence of many-body interactions on the transport
characteristics of a novel device structure, consisting of a pair of quantum
wires that are coupled to each other by means of a quantum dot. Under
conditions where a local magnetic moment is formed in one of the wires, we show
that tunnel coupling to the other gives rise to an associated peak in its
density of states, which can be detected directly in a conductance measurement.
Our theory is therefore able to account for the key observations in the recent
study of T. Morimoto et al. [Appl. Phys. Lett. {\bf 82}, 3952 (2003)], and
demonstrates that coupled quantum wires may be used as a system for the
detection of local magnetic-moment formation
Reversible Metal-Semiconductor Transition of ssDNA-Decorated Single-Walled Carbon Nanotubes
A field effect transistor (FET) measurement of a SWNT shows a transition from
a metallic one to a p-type semiconductor after helical wrapping of DNA. Water
is found to be critical to activate this metal-semiconductor transition in the
SWNT-ssDNA hybrid. Raman spectroscopy confirms the same change in electrical
behavior. According to our ab initio calculations, a band gap can open up in a
metallic SWNT with wrapped ssDNA in the presence of water molecules due to
charge transfer.Comment: 13 pages, 6 figure
Spintronics: Fundamentals and applications
Spintronics, or spin electronics, involves the study of active control and
manipulation of spin degrees of freedom in solid-state systems. This article
reviews the current status of this subject, including both recent advances and
well-established results. The primary focus is on the basic physical principles
underlying the generation of carrier spin polarization, spin dynamics, and
spin-polarized transport in semiconductors and metals. Spin transport differs
from charge transport in that spin is a nonconserved quantity in solids due to
spin-orbit and hyperfine coupling. The authors discuss in detail spin
decoherence mechanisms in metals and semiconductors. Various theories of spin
injection and spin-polarized transport are applied to hybrid structures
relevant to spin-based devices and fundamental studies of materials properties.
Experimental work is reviewed with the emphasis on projected applications, in
which external electric and magnetic fields and illumination by light will be
used to control spin and charge dynamics to create new functionalities not
feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes
from the published versio
Helicity and broken symmetry of DNA-nanotube hybrids
We study breaking of the “supersymmetry" of an intrinsically achiral armchair carbon nanotube by means of a
helical perturbation. Lowering of the symmetry results in the appearance of a non-zero effective mass for nanotube low-energy
excitations, which otherwise are massless Dirac fermions. Other important consequences of the symmetry breaking are opening of
gaps in the energy spectrum and shifting of the Fermi points, which we classify according to their functional dependence on the
nanotube and helix parameters. Within each class the gaps are proportional to the inverse of the nanotube radius, and appear to
be sensitive to the exact position of the helix in a unit cell. These results are of immediate importance for the study of
DNA-nanotube complexes, and can be verified by means of optical/electron spectroscopy or tunneling microscopy
Scaling laws for the bifurcation-escape rate in a nanomechanical resonator
We report on experimental and theoretical studies of the fluctuation-induced escape time from a metastable state of a nanomechanical Duffing resonator in cryogenic environment. By tuning in situ the non-linear coefficient we could explore a wide range of the parameter space around the bifurcation point, where the metastable state becomes unstable. We measured in a relaxation process the distribution of the escape times. We have been able to verify its exponential distribution and extract the escape rate . We investigated the scaling of with respect to the distance to the bifurcation point and , finding an unprecedented quantitative agreement with the theoretical description of the stochastic problem. Simple power scaling laws turn out to hold in a large region of the parameter's space, as anticipated by recent theoretical predictions. These unique findings, implemented in a model dynamical system, are relevant to all systems experiencing under-damped saddle-node bifurcation.Nano Mécanique QuantiqueEUROPEAN MICROKELVIN COLLABORATIO