1,537 research outputs found
Site-centered impurities in quantum spin chains
The magnetic behavior of antiferromagnetic spin 1/2 chains with site-centered
impurities in a magnetic field is investigated. The effect of impurities is
implemented by considering different situations of both diagonal and
off-diagonal disorder. The resulting magnetization curves present a wide
variety of plateaux, whose position strongly depends on the kind of disorder
considered. The relevance of these results to experimental situations is also
discussed.Comment: 6 pages, 6 figure
Current and noise correlations in a double dot Cooper pair beam splitter
We consider a double quantum dot coupled to two normal leads and one
superconducting lead, modeling the Cooper pair beam splitter studied in two
recent experiments. Starting from a microscopic Hamiltonian we derive a general
expression for the branching current and the noise crossed correlations in
terms of single and two-particle Green's function of the dot electrons. We then
study numerically how these quantities depend on the energy configuration of
the dots and the presence of direct tunneling between them, isolating the
various processes which come into play. In absence of direct tunneling, the
antisymmetric case (the two levels have opposite energies with respect to the
superconducting chemical potential) optimizes the Crossed Andreev Reflection
(CAR) process while the symmetric case (the two levels have the same energies)
favors the Elastic Cotunneling (EC) process. Switching on the direct tunneling
tends to suppress the CAR process, leading to negative noise crossed
correlations over the whole voltage range for large enough direct tunneling
Real-time simulation of finite frequency noise from a single electron emitter
We study the real-time emission of single electrons from a quantum dot
coupled to a one dimensional conductor, using exact diagonalization on a
discrete tight-binding chain. We show that from the calculation of the
time-evolution of the one electron states, we have a simple access to all the
relevant physical quantities in the system. In particular, we are able to
compute accurately the finite frequency current autocorrelation noise. The
method which we use is general and versatile, allowing to study the impact of
many different parameters like the dot transparency or level position. Our
results can be directly compared with existing experiments, and can also serve
as a basis for future calculations including electronic interactions using the
time dependent density-matrix renormalisation group and other techniques based
on tight-binding models.Comment: 10 page
Current correlations in the interacting Cooper-pair beam-splitter
We propose an approach allowing the computation of currents and their
correlations in interacting multiterminal mesoscopic systems involving quantum
dots coupled to normal and/or superconducting leads. The formalism relies on
the expression of branching currents and noise crossed correlations in terms of
one- and two-particle Green's functions for the dots electrons, which are then
evaluated self-consistently within a conserving approximation. We then apply
this to the Cooper-pair beam-splitter setup recently proposed [L. Hofstetter et
al. Nature (London) 461 960 (2009); Phys. Rev. Lett. 107 136801 (2011); L. G.
Herrmann et al. Phys. Rev. Lett. 104 026801 (2010)], which we model as a double
quantum dot with weak interactions, connected to a superconducting lead and two
normal ones. Our method not only enables us to take into account a local
repulsive interaction on the dots, but also to study its competition with the
direct tunneling between dots. Our results suggest that even a weak Coulomb
repulsion tends to favor positive current cross correlations in the
antisymmetric regime (where the dots have opposite energies with respect to the
superconducting chemical potential)
Cooper pair splitting in a nanoSQUID geometry at high transparency
We describe a Josephson device composed of two superconductors separated by
two interacting quantum dots in parallel, as a probe for Cooper pair splitting.
In addition to sequential tunneling of electrons through each dot, an
additional transport channel exists in this system: crossed Andreev reflection,
where a Cooper pair from the source is split between the two dots and
recombined in the drain superconductor. Unlike non-equilibrium scenarios for
Cooper pair splitting which involves superconducting/normal metal "forks", our
proposal relies on an Aharonov-Bohm measurement of the DC Josephson current
when a flux is inserted between the two dots. We provide a path integral
approach to treat arbitrary transparencies, and we explore all contributions
for the individual phases ( or ) of the quantum dots. We propose a
definition of the Cooper pair splitting efficiency for arbitrary
transparencies, which allows us to find the phase associations which favor the
crossed Andreev process. Possible applications to experiments using nanowires
as quantum dots are discussed.Comment: 12 pages, 13 figure
Chaining of welding and finish turning simulations for austenitic stainless steel components
The chaining of manufacturing processes is a major issue for industrials who want to understand and control the quality of their products in order to ensure their in-service integrity (surface integrity, residual stresses, microstructure, metallurgical changes, distortions,…). Historically, welding and machining are among the most studied processes and dedicated approaches of simulation have been developed to provide reliable and relevant results in an industrial context with safety requirements. As the simulation of these two processes seems to be at an operationnal level, the virtual chaining of both must now be applied with a lifetime prediction prospect. This paper will first present a robust method to simulate multipass welding processes that has been validated through an international round robin. Then the dedicated “hybrid method”, specifically set up to simulate finish turning, will be subsequently applied to the welding simulation so as to reproduce the final state of the pipe manufacturing and its interaction with previous operations. Final residual stress fields will be presented and compared to intermediary results obtained after welding. The influence of each step on the final results will be highlighted regarding surface integrity and finally ongoing validation works and numerical modeling enhancements will be discussed
Proposal for the observation of nonlocal multipair production: the biSQUID
We propose an all-superconducting three-terminal setup consisting in a carbon
nanotube (or semiconducting nanowire) contacted to three superconducting leads.
The resulting device, referred to as a "biSQUID", is made of four quantum dots
arranged in two loops of different surface area. We show how this biSQUID can
prove a useful tool to probe nonlocal quantum phenomena in an interferometry
setup. We study the measured critical current as a function of the applied
magnetic field, which shows peaks in its Fourier spectrum, providing clear
signatures of multipair Josephson processes. The device does not require any
specific fine-tuning as these features are observed for a wide range of
microscopic parameters -- albeit with a non-trivial dependence. Competing
effects which may play a significant role in actual experimental realizations
are also explored.Comment: 13 pages, 9 figure
Multipair DC-Josephson Resonances in a biased all-superconducting Bijunction
An all-superconducting bijunction consists of a central superconductor
contacted to two lateral superconductors, such that non-local crossed Andreev
reflection is operating. Then new correlated transport channels for the Cooper
pairs appear in addition to those of separated conventional Joseph- son
junctions. We study this system in a configuration where the superconductors
are connected through gate-controllable quantum dots. Multipair phase-coherent
resonances and phase-dependent multiple Andreev reflections are both obtained
when the voltages of the lateral superconductors are commensurate, and they add
to the usual local dissipative transport due to quasiparticles. The two-pair
resonance (quartets) as well as some other higher order multipair resonances
are {\pi}-shifted at low voltage. Dot control can be used to dramatically
enhance the multipair current when the voltages are resonant with the dot
levels.Comment: 6 page
Giant shot noise from Majorana zero modes in topological trijunctions
The clear-cut experimental identification of Majorana bound states in
transport measurements still poses experimental challenges. We here show that
the zero-energy Majorana state formed at a junction of three topological
superconductor wires is directly responsible for giant shot noise amplitudes,
in particular at low voltages and for small contact transparency. The only
intrinsic noise limitation comes from the current-induced dephasing rate due to
multiple Andreev reflection processes
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