321 research outputs found
Quantum wire networks with local Z2 symmetry
For a large class of networks made of connected loops, in the presence of an
external magnetic field of half flux quantum per loop, we show the existence of
a large local symmetry group, generated by simultaneous flips of the electronic
current in all the loops adjacent to a given node. Using an ultra-localized
single particle basis adapted to this local Z_2 symmetry, we show that it is
preserved by a large class of interaction potentials. As a main physical
consequence, the only allowed tunneling processes in such networks are induced
by electron-electron interactions and involve a simultaneous hop of two
electrons. Using a mean-field picture and then a more systematic
renormalization-group treatment, we show that these pair hopping processes do
not generate a superconducting instability, but they destroy the Luttinger
liquid behavior in the links, giving rise at low energy to a strongly
correlated spin-density-wave state.Comment: 16 pages, 9 figures, v.2 section IV D added,accepted for publication
in PR
Regular networks of Luttinger liquids
We consider arrays of Luttinger liquids, where each node is described by a
unitary scattering matrix. In the limit of small electron-electron interaction,
we study the evolution of these scattering matrices as the high-energy single
particle states are gradually integrated out. Interestingly, we obtain the same
renormalization group equations as those derived by Lal, Rao, and Sen, for a
system composed of a single node coupled to several semi-infinite 1D wires. The
main difference between the single node geometry and a regular lattice is that
in the latter case, the single particle spectrum is organized into periodic
energy bands, so that the renormalization procedure has to stop when the last
totally occupied band has been eliminated. We therefore predict a strongly
renormalized Luttinger liquid behavior for generic filling factors, which
should exhibit power-law suppression of the conductivity at low temperatures
E_{F}/(k_{F}a) >
1. Some fully insulating ground-states are expected only for a discrete set of
integer filling factors for the electronic system. A detailed discussion of the
scattering matrix flow and its implication for the low energy band structure is
given on the example of a square lattice.Comment: 16 pages, 7 figure
Large transconductance oscillations in a single-well vertical Aharonov-Bohm interferometer
Aharonov-Bohm (AB) interference is reported for the first time in the
conductance of a vertical nanostructure based on a single GaAs/AlGaAs quantum
well (QW). The two lowest subbands of the well are spatially separated by the
Hartree barrier originating from electronic repulsion in the modulation-doped
QW and provide AB two-path geometry. Split-gates control the in-plane
electronic momentum dispersion. In our system, we have clearly demonstrated AB
interference in both electrostatic and magnetic modes. In the latter case the
magnetic field was applied parallel to the QW plane, and perpendicular to the
0.02 um^2 AB loop. In the electrostatic mode of operation the single-QW scheme
adopted led to large transconductance oscillations with relative amplitudes
exceeding 30 %. The relevance of the present design strategy for the
implementation of coherent nanoelectronic devices is underlined.Comment: Accepted for publication on Physical Review B Rapid Communication
Theory of Incompressible States in a Narrow Channel
We report on the properties of a system of interacting electrons in a narrow
channel in the quantum Hall effect regime. It is shown that an increase in the
strength of the Coulomb interaction causes abrupt changes in the width of the
charge-density profile of translationally invariant states. We derive a phase
diagram which includes many of the stable odd-denominator states as well as a
novel fractional quantum Hall state at lowest half-filled Landau level. The
collective mode evaluated at the half-filled case is strikingly similar to that
for an odd-denominator fractional quantum Hall state.Comment: 4 pages, REVTEX, and 4 .ps file
Stretching and unzipping nucleic acid hairpins using a synthetic nanopore
We have explored the electromechanical properties of DNA by using an electric field to force single hairpin molecules to translocate through a synthetic pore in a silicon nitride membrane. We observe a threshold voltage for translocation of the hairpin through the pore that depends sensitively on the diameter and the secondary structure of the DNA. The threshold for a diameter 1.5 < d < 2.3 nm is V > 1.5 V, which corresponds to the force required to stretch the stem of the hairpin, according to molecular dynamics simulations. On the other hand, for 1.0 < d < 1.5 nm, the threshold voltage collapses to V < 0.5 V because the stem unzips with a lower force than required for stretching. The data indicate that a synthetic nanopore can be used like a molecular gate to discriminate between the secondary structures in DNA
Observation of Quantum Asymmetry in an Aharonov-Bohm Ring
We have investigated the Aharonov-Bohm effect in a one-dimensional
GaAs/GaAlAs ring at low magnetic fields. The oscillatory magnetoconductance of
these systems are for the first time systematically studied as a function of
density. We observe phase-shifts of in the magnetoconductance
oscillations, and halving of the fundamental period, as the density is
varied. Theoretically we find agreement with the experiment, by introducing an
asymmetry between the two arms of the ring.Comment: 4 pages RevTex including 3 figures, submitted to Phys. Rev.
Renormalization group study of the conductances of interacting quantum wire systems with different geometries
We examine the effect of interactions between the electrons on the
conductances of some systems of quantum wires with different geometries. The
systems include a wire with a stub in the middle, a wire containing a ring
which can enclose a magnetic flux, and a system of four wires which are
connected in the middle through a fifth wire. Each of the wires is taken to be
a weakly interacting Tomonaga-Luttinger liquid, and scattering matrices are
introduced at all the junctions. Using a renormalization group method developed
recently for studying the flow of scattering matrices for interacting systems
in one dimension, we compute the conductances of these systems as functions of
the temperature and the wire lengths. We present results for all three regimes
of interest, namely, high, intermediate and low temperature. These correspond
respectively to the thermal coherence length being smaller than, comparable to
and larger than the smallest wire length in the different systems, i.e., the
length of the stub or each arm of the ring or the fifth wire. The
renormalization group procedure and the formulae used to compute the
conductances are different in the three regimes. We present a
phenomenologically motivated formalism for studying the conductances in the
intermediate regime where there is only partial coherence. At low temperatures,
we study the line shapes of the conductances versus the electron energy near
some of the resonances; the widths of the resonances go to zero with decreasing
temperature. Our results show that the conductances of various systems of
experimental interest depend on the temperature and lengths in a non-trivial
way when interactions are taken into account.Comment: Revtex, 17 pages including 15 figure
Diffraction of complex molecules by structures made of light
We demonstrate that structures made of light can be used to coherently
control the motion of complex molecules. In particular, we show diffraction of
the fullerenes C60 and C70 at a thin grating based on a standing light wave. We
prove experimentally that the principles of this effect, well known from atom
optics, can be successfully extended to massive and large molecules which are
internally in a thermodynamic mixed state and which do not exhibit narrow
optical resonances. Our results will be important for the observation of
quantum interference with even larger and more complex objects.Comment: 4 pages, 3 figure
Energy spectra of quantum rings
Ring geometries have fascinated experimental and theoretical physicists over
many years. Open rings connected to leads allow the observation of the
Aharonov-Bohm effect, a paradigm of quantum mechanical phase coherence. The
phase coherence of transport through a quantum dot embedded in one arm of an
open ring has been demonstrated. The energy spectrum of closed rings has only
recently been analysed by optical experiments and is the basis for the
prediction of persistent currents and related experiments. Here we report
magnetotransport experiments on a ring-shaped semiconductor quantum dot in the
Coulomb blockade regime. The measurements allow us to extract the discrete
energy levels of a realistic ring, which are found to agree well with
theoretical expectations. Such an agreement, so far only found for few-electron
quantum dots, is here extended to a many-electron system. In a semiclassical
language our results indicate that electron motion is governed by regular
rather than chaotic motion, an unexplored regime in many-electron quantum dots.Comment: 10 pages, 4 figure
Analytical method for parameterizing the random profile components of nanosurfaces imaged by atomic force microscopy
The functional properties of many technological surfaces in biotechnology,
electronics, and mechanical engineering depend to a large degree on the
individual features of their nanoscale surface texture, which in turn are a
function of the surface manufacturing process. Among these features, the
surface irregularities and self-similarity structures at different spatial
scales, especially in the range of 1 to 100 nm, are of high importance because
they greatly affect the surface interaction forces acting at a nanoscale
distance. An analytical method for parameterizing the surface irregularities
and their correlations in nanosurfaces imaged by atomic force microscopy (AFM)
is proposed. In this method, flicker noise spectroscopy - a statistical physics
approach - is used to develop six nanometrological parameters characterizing
the high-frequency contributions of jump- and spike-like irregularities into
the surface texture. These contributions reflect the stochastic processes of
anomalous diffusion and inertial effects, respectively, in the process of
surface manufacturing. The AFM images of the texture of corrosion-resistant
magnetite coatings formed on low-carbon steel in hot nitrate solutions with
coating growth promoters at different temperatures are analyzed. It is shown
that the parameters characterizing surface spikiness are able to quantify the
effect of process temperature on the corrosion resistance of the coatings. It
is suggested that these parameters can be used for predicting and
characterizing the corrosion-resistant properties of magnetite coatings.Comment: 7 pages, 3 figures, 2 tables; to be published in Analys
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