2,113 research outputs found
Nonsteady-Flow Thrust Augmenting Ejectors
Ejector augmenters in which the transfer of mechanical energy from the primary to the secondary flow takes place through the work of interface pressure forces are investigated. Nonsteady flow processes are analyzed from the standpoint of energy transfer efficiency and a comparison of a rotary jet augmenter to an ejector is presented
Floquet topological transitions in a driven one-dimensional topological insulator
The Su-Schrieffer-Heeger model of polyacetylene is a paradigmatic Hamiltonian
exhibiting non-trivial edge states. By using Floquet theory we study how the
spectrum of this one-dimensional topological insulator is affected by a
time-dependent potential. In particular, we evidence the competition among
different photon-assisted processes and the native topology of the unperturbed
Hamiltonian to settle the resulting topology at different driving frequencies.
While some regions of the quasienergy spectrum develop new gaps hosting Floquet
edge states, the native gap can be dramatically reduced and the original edge
states may be destroyed or replaced by new Floquet edge states. Our study is
complemented by an analysis of Zak phase applied to the Floquet bands. Besides
serving as a simple example for understanding the physics of driven topological
phases, our results could find a promising test-ground in cold matter
experiments
Crafting zero-bias one-way transport of charge and spin
We explore the electronic structure and transport properties of a metal on
top of a (weakly coupled) two-dimensional topological insulator. Unlike the
widely studied junctions between topological non-trivial materials, the systems
studied here allow for a unique bandstructure and transport steering. First,
states on the topological insulator layer may coexist with the gapless bulk
and, second, the edge states on one edge can be selectively switched-off,
thereby leading to nearly perfect directional transport of charge and spin even
in the zero bias limit. We illustrate these phenomena for Bernal stacked
bilayer graphene with Haldane or intrinsic spin-orbit terms and a perpendicular
bias voltage. This opens a path for realizing directed transport in materials
such as van der Waals heterostructures, monolayer and ultrathin topological
insulators.Comment: 7 pages, 7 figure
Inelastic Quantum Transport and Peierls-like Mechanism in Carbon Nanotubes
We report on a theoretical study of inelastic quantum transport in
carbon nanotubes. By using a many-body description of the electron-phonon
interaction in Fock space, a novel mechanism involving optical phonon emission
(absorption) is shown to induce an unprecedented energy gap opening at half the
phonon energy, , above (below) the charge neutrality point.
This mechanism, which is prevented by Pauli blocking at low bias voltages, is
activated at bias voltages in the order of .Comment: 4 pages, 4 figure
Non-Hermitian robust edge states in one-dimension: Anomalous localization and eigenspace condensation at exceptional points
Capital to topological insulators, the bulk-boundary correspondence ties a
topological invariant computed from the bulk (extended) states with those at
the boundary, which are hence robust to disorder. Here we put forward an
ordering unique to non-Hermitian lattices, whereby a pristine system becomes
devoid of extended states, a property which turns out to be robust to disorder.
This is enabled by a peculiar type of non-Hermitian degeneracy where a
macroscopic fraction of the states coalesce at a single point with geometrical
multiplicity of , that we call a phenomenal point.Comment: 6 pages, 4 figure
Mono-parametric quantum charge pumping: interplay between spatial interference and photon-assisted tunneling
We analyze quantum charge pumping in an open ring with a dot embedded in one
of its arms. We show that cyclic driving of the dot levels by a \textit{single}
parameter leads to a pumped current when a static magnetic flux is
simultaneously applied to the ring. Based on the computation of the
Floquet-Green's functions, we show that for low driving frequencies ,
the interplay between the spatial interference through the ring plus
photon-assisted tunneling gives an average direct current (dc) which is
proportional to . The direction of the pumped current can be
reversed by changing the applied magnetic field.Comment: 7 pages, 4 figures. To appear in Phys. Rev.
Directional photoelectric current across the bilayer graphene junction
A directional photon-assisted resonant chiral tunneling through a bilayer
graphene barrier is considered. An external electromagnetic field applied to
the barrier switches the transparency in the longitudinal direction from
its steady state value T=0 to the ideal T=1 at no energy costs. The switch
happens because the a.c. field affects the phase correlation between the
electrons and holes inside the graphene barrier changing the whole angular
dependence of the chiral tunneling (directional photoelectric effect). The
suggested phenomena can be implemented in relevant experiments and in various
sub-millimeter and far-infrared optical electronic devices.Comment: 7 pages 5 figure
Topological states of non-Hermitian systems
Recently, the search for topological states of matter has turned to
non-Hermitian systems, which exhibit a rich variety of unique properties
without Hermitian counterparts. Lattices modeled through non-Hermitian
Hamiltonians appear in the context of photonic systems, where one needs to
account for gain and loss, circuits of resonators, and also when modeling the
lifetime due to interactions in condensed matter systems. Here we provide a
brief overview of this rapidly growing subject, the search for topological
states and a bulk-boundary correspondence in non-Hermitian systems.Comment: Invited short review for the special issue "Topological States of
Matter: Theory and Applications
First-line therapy for post-traumatic stress disorder : a systematic review of cognitive behavioural therapy and psychodynamic approaches
Background: Despite evidence supporting cognitive behavioural therapy (CBT) based interventions as the most effective approach for treating posttraumatic stress disorder (PTSD) in randomised control trials, alternative treatment interventions are often used in clinical practice. Psychodynamic (PDT) based interventions are one
example of such preferred approaches, this is despite comparatively limited available evidence supporting their effectiveness for treating PTSD. Aims: Existing research
exploring effective therapeutic interventions for PTSD includes trauma-focused CBT involving exposure techniques. The present review sought to establish the treatment efficacy of CBT and PDT approaches, and considers the potential impact of selecting
PDT-based techniques over CBT-based techniques for the treatment of PTSD.Results: The evidence reviewed provided examples supporting PDT-based therapy as an effective treatment for PTSD, but confirmed CBT as more effective in the treatment of this particular disorder. Comparable dropout rates were reported for
both treatment approaches, suggesting that relative dropout rate should not be a pivotal factor in the selection of a PDT approach over CBT for treatment of PTSD.Conclusion/Implications: The need to routinely observe evidence-based recommendations for effective treatment of PTSD is highlighted and factors
undermining practitioner engagement with CBT-based interventions for the treatment of PTSD are identified
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