265 research outputs found
Lagrangian coherent structures and plasma transport processes
A dynamical system framework is used to describe transport processes in
plasmas embedded in a magnetic field. For periodic systems with one degree of
freedom the Poincar\'e map provides a splitting of the phase space into regions
where particles have different kinds of motion: periodic, quasi-periodic or
chaotic. The boundaries of these regions are transport barriers; i.e., a
trajectory cannot cross such boundaries during the whole evolution of the
system. Lagrangian Coherent Structure (LCS) generalize this method to systems
with the most general time dependence, splitting the phase space into regions
with different qualitative behaviours. This leads to the definition of
finite-time transport barriers, i.e. trajectories cannot cross the barrier for
a finite amount of time. This methodology can be used to identify fast
recirculating regions in the dynamical system and to characterize the transport
between them
Coherent transport structures in magnetized plasmas II: Numerical results
In a pair of linked articles (called Article I and II respectively) we apply
the concept of Lagrangian Coherent Structures borrowed from the study of
Dynamical Systems to magnetic field configurations in order to separate regions
where field lines have different kind of behavior. In the present article,
article II, by means of a numerical procedure we investigate the Lagrangian
Coherent Structures in the case of a two-dimensional magnetic configuration
with two island chains that are generated by magnetic reconnection and evolve
nonlinearly in time. The comparison with previous results, obtained by assuming
a fixed magnetic field configuration, allows us to explore the dependence of
transport barriers on the particle velocity
Coherent transport structures in magnetized plasmas, I : Theory
In a pair of linked articles (called Article I and II respectively) we apply
the concept of Lagrangian Coherent Structures (LCSs) borrowed from the study of
Dynamical Systems to magnetic field configurations in order to separate regions
where field lines have different kind of behaviour. In the present article,
article I, after recalling the definition and the properties of the LCSs, we
show how this conceptual framework can be applied to the study of particle
transport in a magnetized plasma. Futhermore we introduce a simplified model
that allows us to consider explicitly the case where the magnetic configuration
evolves in time on timescales comparable to the particle transit time through
the configuration. In contrast with previous works on this topic, this analysis
requires that a system that is aperiodic in time be investigated. In this case
the Poincar\'e map technique cannot be applied and LCSs remain the only viable
tool
Normative data for the lower extremity functional scale (LEFS)
Background and purpose — The lower extremity functional scale (LEFS) is a well-known and validated instrument for measurement of lower extremity function. The LEFS was developed in a group of patients with various musculoskeletal disorders, and no reference data for the healthy population are available. Here we provide normative data for the LEFS. Methods — Healthy visitors and staff at 4 hospitals were requested to participate. A minimum of 250 volunteers had to be included at each hospital. Participants were excluded if they had undergone lower extremity surgery within 1 year of filling out the questionnaire, or were scheduled for lower extremity surgery. Normative values for the LEFS for the population as a whole were calculated. Furthermore, the influence of sex, age, type of employment, socioeconomic status, and history o
Ideal Spin Filters: Theoretical Study of Electron Transmission Through Ordered and Disordered Interfaces Between Ferromagnetic Metals and Semiconductors
It is predicted that certain atomically ordered interfaces between some
ferromagnetic metals (F) and semiconductors (S) should act as ideal spin
filters that transmit electrons only from the majority spin bands or only from
the minority spin bands of the F to the S at the Fermi energy, even for F with
both majority and minority bands at the Fermi level. Criteria for determining
which combinations of F, S and interface should be ideal spin filters are
formulated. The criteria depend only on the bulk band structures of the S and F
and on the translational symmetries of the S, F and interface. Several examples
of systems that meet these criteria to a high degree of precision are
identified. Disordered interfaces between F and S are also studied and it is
found that intermixing between the S and F can result in interfaces with spin
anti-filtering properties, the transmitted electrons being much less spin
polarized than those in the ferromagnetic metal at the Fermi energy. A patent
application based on this work has been commenced by Simon Fraser University.Comment: RevTeX, 12 pages, 5 figure
Transcript-indexed ATAC-seq for precision immune profiling.
T cells create vast amounts of diversity in the genes that encode their T cell receptors (TCRs), which enables individual clones to recognize specific peptide-major histocompatibility complex (MHC) ligands. Here we combined sequencing of the TCR-encoding genes with assay for transposase-accessible chromatin with sequencing (ATAC-seq) analysis at the single-cell level to provide information on the TCR specificity and epigenomic state of individual T cells. By using this approach, termed transcript-indexed ATAC-seq (T-ATAC-seq), we identified epigenomic signatures in immortalized leukemic T cells, primary human T cells from healthy volunteers and primary leukemic T cells from patient samples. In peripheral blood CD4+ T cells from healthy individuals, we identified cis and trans regulators of naive and memory T cell states and found substantial heterogeneity in surface-marker-defined T cell populations. In patients with a leukemic form of cutaneous T cell lymphoma, T-ATAC-seq enabled identification of leukemic and nonleukemic regulatory pathways in T cells from the same individual by allowing separation of the signals that arose from the malignant clone from the background T cell noise. Thus, T-ATAC-seq is a new tool that enables analysis of epigenomic landscapes in clonal T cells and should be valuable for studies of T cell malignancy, immunity and immunotherapy
Spin-accumulation and Andreev-reflection in a mesoscopic ferromagnetic wire
The electron transport though ferromagnetic metal-superconducting hybrid
devices is considered in the non-equilibrium Green's function formalism in the
quasiclassical approximation. Attention if focused on the limit in which the
exchange splitting in the ferromagnet is much larger than the superconducting
energy gap. Transport properties are then governed by an interplay between
spin-accumulation close to the interface and Andreev reflection at the
interface. We find that the resistance can either be enhanced or lowered in
comparison to the normal case and can have a non-monotonic temperature and
voltage dependence. In the non-linear voltage regime electron heating effects
may govern the transport properties, leading to qualitative different behaviour
than in the absence of heating effects. Recent experimental results on the
effect of the superconductor on the conductance of the ferromagnet can be
understood by our results for the energy-dependent interface resistance
together with effects of spin- accumulation without invoking long range pairing
correlations in the ferromagnet.Comment: 15 pages, 12 figures included, submitted to PR
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