1,797 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)
Hanbury Brown and Twiss noise correlations in a topological superconductor beam splitter
We study Hanbury-Brown and Twiss current cross-correlations in a
three-terminal junction where a central topological superconductor (TS)
nanowire, bearing Majorana bound states at its ends, is connected to two normal
leads. Relying on a non-perturbative Green function formalism, our calculations
allow us to provide analytical expressions for the currents and their
correlations at subgap voltages, while also giving exact numerical results
valid for arbitrary external bias. We show that when the normal leads are
biased at voltages and smaller than the gap, the sign of the
current cross-correlations is given by -\mbox{sgn}(V_1 \, V_2). In
particular, this leads to positive cross-correlations for opposite voltages, a
behavior in stark contrast with the one of a standard superconductor, which
provides a direct evidence of the presence of the Majorana zero-mode at the
edge of the TS. We further extend our results, varying the length of the TS
(leading to an overlap of the Majorana bound states) as well as its chemical
potential (driving it away from half-filling), generalizing the boundary TS
Green function to those cases. In the case of opposite bias voltages,
\mbox{sgn}(V_1 \, V_2)=-1, driving the TS wire through the topological
transition leads to a sign change of the current cross-correlations, providing
yet another signature of the physics of the Majorana bound state.Comment: 14 pages, 8 figure
Electronic Hong-Ou-Mandel interferometry in two-dimensional topological insulators
The edge states of a two-dimensional topological insulator are characterized
by their helicity, a very remarkable property which is related to the
time-reversal symmetry and the topology of the underlying system. We
theoretically investigate a Hong-Ou-Mandel like setup as a tool to probe it.
Collisions of two electrons with the same spin show a Pauli dip, analogous to
the one obtained in the integer quantum Hall case. Moreover, the collisions
between electrons of opposite spin also lead to a dip, known as
dip, which is a direct consequence of the constraints imposed
by time-reversal symmetry. In contrast to the integer quantum Hall case, the
visibility of these dips is reduced by the presence of the additional edge
channels, and crucially depends on the properties of the quantum point contact.
As a unique feature of this system, we show the possibility of three-electron
interference, which leads to a total suppression of the noise independently of
the point contact configuration. This is assured by the peculiar interplay
between Fermi statistics and topology. This work intends to extend the domain
of applicability of electron quantum optics.Comment: 12 pages, 7 figure
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
Determinants Of Adaptive Immunity In Cancer
Immune checkpoint blockade results in T cell antitumor responses but most patients fail to respond. This raises fundamental questions about mechanisms of tumor immune recognition and resistance. Here, I first report tumor regressions in a subset of patients with metastatic melanoma treated with anti-CTLA4 antibody and radiation (RT) and reproduced this effect in mouse models. Although combined treatment improved responses in irradiated and unirradiated tumors, resistance was common due to upregulation of PD-L1 on tumor cells and corresponding T cell exhaustion. Accordingly, optimal response in melanoma and other cancer types required RT, anti-CTLA4, and anti-PD-L1/PD1. When I investigated determinants of improved responses to combination therapy, I found that RT enhanced the antigenic diversity of intratumoral T cells, anti-CTLA4 predominantly inhibited regulatory T cells, and anti-PD-L1 reversed T cell exhaustion. I next extended my investigation of this combination therapy to pancreatic ductal adenocarcinoma (PDA), finding that optimal responses required addition of agonist CD40 monoclonal antibody
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
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