10,078 research outputs found
Universal Rashba Spin Precession of Two-Dimensional Electrons and Holes
We study spin precession due to Rashba spin splitting of electrons and holes
in semiconductor quantum wells. Based on a simple analytical expression that we
derive for the current modulation in a broad class of experimental situations
of ferromagnet/nonmagnetic semiconductor/ferromagnet hybrid structures, we
conclude that the Datta-Das spin transistor (i) is feasible with holes and (ii)
its functionality is not affected by integration over injection angles. The
current modulation shows a universal oscillation period, irrespective of the
different forms of the Rashba Hamiltonian for electrons and holes. The analytic
formulas approximate extremely well exact numerical calculations of a more
elaborate Kohn--Luttinger model.Comment: 7 pages, 2 eps figures included, minor change
Strong Tunneling in Double-Island Structures
We study the electron transport through a system of two low-capacitance metal
islands connected in series between two electrodes. The work is motivated in
part by experiments on semiconducting double-dots, which show intriguing
effects arising from coherent tunneling of electrons and mixing of the
single-electron states across tunneling barriers. In this article, we show how
coherent tunneling affects metallic systems and leads to a mixing of the
macroscopic charge states across the barriers. We apply a recently formulated
RG approach to examine the linear response of the system with high tunnel
conductances (up to 8e^2/h). In addition we calculate the (second order)
cotunneling contributions to the non-linear conductance. Our main results are
that the peaks in the linear and nonlinear conductance as a function of the
gate voltage are reduced and broadened in an asymmetric way, as well as shifted
in their positions. In the limit where the two islands are coupled weakly to
the electrodes, we compare to theoretical results obtained by Golden and
Halperin and Matveev et al. In the opposite case when the two islands are
coupled more strongly to the leads than to each other, the peaks are found to
shift, in qualitative agreement with the recent prediction of Andrei et al. for
a similar double-dot system which exhibits a phase transition.Comment: 12 page
Full Counting Statistics in Strongly Interacting Systems: Non-Markovian Effects
We present a theory of full counting statistics for electron transport
through interacting electron systems with non-Markovian dynamics. We illustrate
our approach for transport through a single-level quantum dot and a metallic
single-electron transistor to second order in the tunnel-coupling strength, and
discuss under which circumstances non-Markovian effects appear in the transport
properties.Comment: 4 pages, 2 figures, LaTeX; typos added, references adde
Cotunneling at resonance for the single-electron transistor
We study electron transport through a small metallic island in the
perturbative regime. Using a recently developed diagrammatic technique, we
calculate the occupation of the island as well as the conductance through the
transistor in forth order in the tunneling matrix elements, a process referred
to as cotunneling. Our formulation does not require the introduction of a
cut-off. At resonance we find significant modifications of previous theories
and good agreement with recent experiments.Comment: 5 pages, Revtex, 5 eps-figure
Co-tunneling current and shot noise in quantum dots
We derive general expressions for the current and shot noise, taking into
account non-Markovian memory effects. In generalization of previous approaches
our theory is valid for arbitrary Coulomb interaction and coupling strength and
is applicable to quantum dots and more complex systems like molecules. A
diagrammatic expansion up to second-order in the coupling strength, taking into
account co-tunneling processes, allows for a study of transport in a regime
relevant to many experiments. As an example, we consider a single-level quantum
dot, focusing on the Coulomb-blockade regime. We find super-Poissonian shot
noise due to spin-flip co-tunneling processes at an energy scale different from
the one expected from first-order calculations, with a sensitive dependence on
the coupling strength.Comment: 4 pages, three figures, submitted to PR
Josephson-Majorana cycle in topological single-electron hybrid transistors
Charge transport through a small topological superconducting island in
contact with a normal and a superconducting electrode occurs through a cycle
that involves coherent oscillations of Cooper pairs and tunneling in/out the
normal electrode through a Majorana bound state, the Josephson-Majorana cycle.
We illustrate this mechanism by studying the current-voltage characteristics of
a superconductor-topological superconductor-normal metal single-electron
transistor. At low bias and temperature the Josephson-Majorana cycle is the
dominant mechanism for transport. We discuss a three-terminal configuration
where the non-local character of the Majorana bound states is emergent.Comment: 6 pages, 4 figure
Topological insulator and the Dirac equation
We present a general description of topological insulators from the point of
view of Dirac equations. The Z_{2} index for the Dirac equation is always zero,
and thus the Dirac equation is topologically trivial. After the quadratic B
term in momentum is introduced to correct the mass term m or the band gap of
the Dirac equation, the Z_{2} index is modified as 1 for mB>0 and 0 for mB<0.
For a fixed B there exists a topological quantum phase transition from a
topologically trivial system to a non-trivial one system when the sign of mass
m changes. A series of solutions near the boundary in the modified Dirac
equation are obtained, which is characteristic of topological insulator. From
the solutions of the bound states and the Z_{2} index we establish a relation
between the Dirac equation and topological insulators.Comment: 9 pages, published versio
Income and distance elasticities of values of travel time savings: New Swiss results
This paper presents the findings of a study looking into the valuation of travel time savings (VTTS) in Switzerland, across modes as well as across purpose groups. The study makes several departures from the usual practice in VTTS studies, with the main one being a direct representation of the income and distance elasticity of the VTTS measures. Here, important gains in model performance and significantly different results are obtained through this approach. Additionally, the analysis shows that the estimation of robust coefficients for congested car travel time is hampered by the low share of congested time in the overall travel time, and the use of an additional rate-of-congestion coefficient, in addition to a generic car travel time coefficient, is preferable. Finally, the analysis demonstrates that the population
mean of the indicators calculated is quite different from the sample means and presents methods to calculate those, along with the associated variances. These variances are of great interest as they allow the generation of confidence intervals, which can be extremely useful in cost-benefit analyses
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