5 research outputs found
Defining and controlling double quantum dots in single-walled carbon nanotubes
We report the experimental realization of double quantum dots in
single-walled carbon nanotubes. The device consists of a nanotube with source
and drain contact, and three additional top-gate electrodes in between. We show
that, by energizing these top-gates, it is possible to locally gate a nanotube,
to create a barrier, or to tune the chemical potential of a part of the
nanotube. At low temperatures we find (for three different devices) that in
certain ranges of top-gate voltages our device acts as a double quantum dot,
evidenced by the typical honeycomb charge stability pattern.Comment: 9 pages, 3 figure
Linear Kondo conductance in a quantum dot
In a tunneling experiment across a quantum dot it is possible to change the
coupling between the dot and the contacts at will, by properly tuning the
trasparency of the barriers and the temperature. Gate voltages allow for
changes of the relative position of the dot addition energies and the Fermi
level of the leads. Here we discuss the two limiting cases: weak and strong
coupling in the tunneling Hamiltonian. In the latter case Kondo resonant
conductance can emerge at low temperature in a Coulomb blockade valley. We give
a pedagogical approach to the single-channel Kondo physics at equilibrium and
review the Nozieres scattering picture of the correlated fixed point. We
emphasize the effect of an applied magnetic field and show how an orbital Kondo
effect can take place in vertical quantum dots tuned both to an even and to an
odd number of electrons at a level crossing. We extend the approach to the
two-channel overscreened Kondo case and discuss recent proposals for detecting
the non-Fermi liquid fixed point which could be reached at strong coupling.Comment: 31 pages, invited review articl
Spin effects in single electron tunneling
An important consequence of the discovery of giant magnetoresistance in
metallic magnetic multilayers is a broad interest in spin dependent effects in
electronic transport through magnetic nanostructures. An example of such
systems are tunnel junctions -- single-barrier planar junctions or more complex
ones. In this review we present and discuss recent theoretical results on
electron and spin transport through ferromagnetic mesoscopic junctions
including two or more barriers. Such systems are also called ferromagnetic
single-electron transistors. We start from the situation when the central part
of a device has the form of a magnetic (or nonmagnetic) metallic nanoparticle.
Transport characteristics reveal then single-electron charging effects,
including the Coulomb staircase, Coulomb blockade, and Coulomb oscillations.
Single-electron ferromagnetic transistors based on semiconductor quantum dots
and large molecules (especially carbon nanotubes) are also considered. The main
emphasis is placed on the spin effects due to spin-dependent tunnelling through
the barriers, which gives rise to spin accumulation and tunnel
magnetoresistance. Spin effects also occur in the current-voltage
characteristics, (differential) conductance, shot noise, and others. Transport
characteristics in the two limiting situations of weak and strong coupling are
of particular interest. In the former case we distinguish between the
sequential tunnelling and cotunneling regimes. In the strong coupling regime we
concentrate on the Kondo phenomenon, which in the case of transport through
quantum dots or molecules leads to an enhanced conductance and to a pronounced
zero-bias Kondo peak in the differential conductance.Comment: topical review (36 figures, 65 pages), to be published in J. Phys.:
Condens. Matte
Recipes for spin-based quantum computing
Technological growth in the electronics industry has historically been
measured by the number of transistors that can be crammed onto a single
microchip. Unfortunately, all good things must come to an end; spectacular
growth in the number of transistors on a chip requires spectacular reduction of
the transistor size. For electrons in semiconductors, the laws of quantum
mechanics take over at the nanometre scale, and the conventional wisdom for
progress (transistor cramming) must be abandoned. This realization has
stimulated extensive research on ways to exploit the spin (in addition to the
orbital) degree of freedom of the electron, giving birth to the field of
spintronics. Perhaps the most ambitious goal of spintronics is to realize
complete control over the quantum mechanical nature of the relevant spins. This
prospect has motivated a race to design and build a spintronic device capable
of complete control over its quantum mechanical state, and ultimately,
performing computations: a quantum computer.
In this tutorial we summarize past and very recent developments which point
the way to spin-based quantum computing in the solid-state. After introducing a
set of basic requirements for any quantum computer proposal, we offer a brief
summary of some of the many theoretical proposals for solid-state quantum
computers. We then focus on the Loss-DiVincenzo proposal for quantum computing
with the spins of electrons confined to quantum dots. There are many obstacles
to building such a quantum device. We address these, and survey recent
theoretical, and then experimental progress in the field. To conclude the
tutorial, we list some as-yet unrealized experiments, which would be crucial
for the development of a quantum-dot quantum computer.Comment: 45 pages, 12 figures (low-res in preprint, high-res in journal)
tutorial review for Nanotechnology; v2: references added and updated, final
version to appear in journa
Quantum-to-classical correspondence in open chaotic systems.
We review properties of open chaotic mesoscopic systems with a finite Ehrenfest time �E. The Ehrenfest time separates a short-time regime of the quantum dynamics, where wave packets closely follow the deterministic classical motion, from a long-time regime of fully-developed wave chaos. For a vanishing Ehrenfest time the quantum systems display a degree of universality which is well described by random-matrix theory. In the semiclassical limit, �E becomes parametrically larger than the scattering time off the boundaries and the dwell time in the system. This results in the emergence of an increasing number of deterministic transport and escape modes, which induce strong deviations from random-matrix universality. We discuss these deviations for a variety of physical phenomena, including shot noise, conductance fluctuations, decay of quasi-bound states and the mesoscopic proximity effect in Andreev billiards