9,002 research outputs found
Quantum transport in carbon nanotubes
Carbon nanotubes are a versatile material in which many aspects of condensed
matter physics come together. Recent discoveries, enabled by sophisticated
fabrication, have uncovered new phenomena that completely change our
understanding of transport in these devices, especially the role of the spin
and valley degrees of freedom. This review describes the modern understanding
of transport through nanotube devices.
Unlike conventional semiconductors, electrons in nanotubes have two angular
momentum quantum numbers, arising from spin and from valley freedom. We focus
on the interplay between the two. In single quantum dots defined in short
lengths of nanotube, the energy levels associated with each degree of freedom,
and the spin-orbit coupling between them, are revealed by Coulomb blockade
spectroscopy. In double quantum dots, the combination of quantum numbers
modifies the selection rules of Pauli blockade. This can be exploited to read
out spin and valley qubits, and to measure the decay of these states through
coupling to nuclear spins and phonons. A second unique property of carbon
nanotubes is that the combination of valley freedom and electron-electron
interactions in one dimension strongly modifies their transport behaviour.
Interaction between electrons inside and outside a quantum dot is manifested in
SU(4) Kondo behavior and level renormalization. Interaction within a dot leads
to Wigner molecules and more complex correlated states.
This review takes an experimental perspective informed by recent advances in
theory. As well as the well-understood overall picture, we also state clearly
open questions for the field. These advances position nanotubes as a leading
system for the study of spin and valley physics in one dimension where
electronic disorder and hyperfine interaction can both be reduced to a very low
level.Comment: In press at Reviews of Modern Physics. 68 pages, 55 figure
Buffalo National River Ecosystems - Part II
The priorities were established for the Buffalo National River Ecosystem Studies through meetings and correspondence with Mr. Roland Wauer and other personnel of the Office of Natural Sciences, Southwest Region of the National Park Service. These priorities were set forth in the appendix of contract no. CX 700050443 dated May 21, 1975
Space station automation of common module power management and distribution
The purpose is to automate a breadboard level Power Management and Distribution (PMAD) system which possesses many functional characteristics of a specified Space Station power system. The automation system was built upon 20 kHz ac source with redundancy of the power buses. There are two power distribution control units which furnish power to six load centers which in turn enable load circuits based upon a system generated schedule. The progress in building this specified autonomous system is described. Automation of Space Station Module PMAD was accomplished by segmenting the complete task in the following four independent tasks: (1) develop a detailed approach for PMAD automation; (2) define the software and hardware elements of automation; (3) develop the automation system for the PMAD breadboard; and (4) select an appropriate host processing environment
Novel Methods for Determining Effective Interactions for the Nuclear Shell Model
The Contractor Renormalization (CORE) method is applied in combination with
modern effective-theory techniques to the nuclear many-body problem. A
one-dimensional--yet ``realistic''--nucleon-nucleon potential is introduced to
test these novel ideas. It is found that the magnitude of ``model-space''
(CORE) corrections diminishes considerably when an effective potential that
eliminates the hard-momentum components of the potential is first introduced.
As a result, accurate predictions for the ground-state energy of the there-body
system are made with relatively little computational effort when both
techniques are used in a complementary fashion.Comment: 14 pages, 5 figures and 2 tabl
A transit timing analysis of nine RISE light curves of the exoplanet system TrES-3
We present nine newly observed transits of TrES-3, taken as part of a transit
timing program using the RISE instrument on the Liverpool Telescope. A
Markov-Chain Monte-Carlo analysis was used to determine the planet-star radius
ratio and inclination of the system, which were found to be
Rp/Rstar=0.1664^{+0.0011}_{-0.0018} and i = 81.73^{+0.13}_{-0.04} respectively,
consistent with previous results. The central transit times and uncertainties
were also calculated, using a residual-permutation algorithm as an independent
check on the errors. A re-analysis of eight previously published TrES-3 light
curves was conducted to determine the transit times and uncertainties using
consistent techniques. Whilst the transit times were not found to be in
agreement with a linear ephemeris, giving chi^2 = 35.07 for 15 degrees of
freedom, we interpret this to be the result of systematics in the light curves
rather than a real transit timing variation. This is because the light curves
that show the largest deviation from a constant period either have relatively
little out-of-transit coverage, or have clear systematics. A new ephemeris was
calculated using the transit times, and was found to be T_c(0) = 2454632.62610
+- 0.00006 HJD and P = 1.3061864 +- 0.0000005 days. The transit times were then
used to place upper mass limits as a function of the period ratio of a
potential perturbing planet, showing that our data are sufficiently sensitive
to have probed for sub-Earth mass planets in both interior and exterior 2:1
resonances, assuming the additional planet is in an initially circular orbit.Comment: 21 pages, 4 figures, Accepted for publication in Ap
Fermion Condensates of massless at Finite Density in non-trivial Topological Sectors
Vacuum expectation values of products of local bilinears are
computed in massless at finite density. It is shown that chiral
condensates exhibit an oscillatory inhomogeneous behaviour depending on the
chemical potential. The use of a path-integral approach clarifies the
connection of this phenomenon with the topological structure of the theory.Comment: 16 pages, no figures, To be published in Phys.Rev.
A gauge invariant and string independent fermion correlator in the Schwinger model
We introduce a gauge invariant and string independent two-point fermion
correlator which is analyzed in the context of the Schwinger model (QED_2). We
also derive an effective infrared worldline action for this correlator, thus
enabling the computation of its infrared behavior. Finally, we briefly discuss
possible perspectives for the string independent correlator in the QED_3
effective models for the normal state of HTc superconductors.Comment: 14 pages, LaTe
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