112 research outputs found
Boundary-obstructed topological high-T superconductivity in iron pnictides
Non-trivial topology and unconventional pairing are two central guiding
principles in the contemporary search for and analysis of superconducting
materials and heterostructure compounds. Previously, a topological
superconductor has been predominantly conceived to result from a topologically
non-trivial band subject to intrinsic or external superconducting proximity
effect. Here, we propose a new class of topological superconductors which are
uniquely induced by unconventional pairing. They exhibit a boundary-obstructed
higher-order topological character and, depending on their dimensionality,
feature unprecedently robust Majorana bound states or hinge modes protected by
chiral symmetry. We predict the 112-family of iron pnictides, such as
CaLaFeAs, to be a highly suited material candidate for our
proposal, which can be tested by edge spectroscopy. Because of the
boundary-obstruction, the topologically non-trivial feature of the 112
pnictides does not reveal itself for a bulk-only torus band analysis without
boundaries, and as such had evaded previous investigations. Our proposal not
only opens a new arena for highly stable Majorana modes in high-temperature
superconductors, but also provides the smoking gun evidence for extended s-wave
order in the iron pnictides.Comment: 8 pages, 4 figures + supplementary material
Fully-Passive versus Semi-Passive IRS-Enabled Sensing: SNR and CRB Comparison
This paper investigates the sensing performance of two intelligent reflecting
surface (IRS)-enabled non-line-of-sight (NLoS) sensing systems with
fully-passive and semi-passive IRSs, respectively. In particular, we consider a
fundamental setup with one base station (BS), one uniform linear array (ULA)
IRS, and one point target in the NLoS region of the BS. Accordingly, we analyze
the sensing signal-to-noise ratio (SNR) performance for a target detection
scenario and the estimation Cram\'er-Rao bound (CRB) performance for a target's
direction-of-arrival (DoA) estimation scenario, in cases where the transmit
beamforming at the BS and the reflective beamforming at the IRS are jointly
optimized. First, for the target detection scenario, we characterize the
maximum sensing SNR when the BS-IRS channels are line-of-sight (LoS) and
Rayleigh fading, respectively. It is revealed that when the number of
reflecting elements equipped at the IRS becomes sufficiently large, the
maximum sensing SNR increases proportionally to for the semi-passive-IRS
sensing system, but proportionally to for the fully-passive-IRS
counterpart. Then, for the target's DoA estimation scenario, we analyze the
minimum CRB performance when the BS-IRS channel follows Rayleigh fading.
Specifically, when grows, the minimum CRB decreases inversely
proportionally to and for the semi-passive and fully-passive-IRS
sensing systems, respectively. Finally, numerical results are presented to
corroborate our analysis across various transmit and reflective beamforming
design schemes under general channel setups. It is shown that the
fully-passive-IRS sensing system outperforms the semi-passive counterpart when
exceeds a certain threshold. This advantage is attributed to the additional
reflective beamforming gain in the IRS-BS path, which efficiently compensates
for the path loss for a large .Comment: 13 pages,7 figure
Visualizing Band Offsets and Edge States in Bilayer-Monolayer Transition Metal Dichalcogenides Lateral Heterojunction
Semiconductor heterostructures are fundamental building blocks for many
important device applications. The emergence of two-dimensional semiconductors
opens up a new realm for creating heterostructures. As the bandgaps of
transition metal dichalcogenides thin films have sensitive layer dependence, it
is natural to create lateral heterojunctions using the same materials with
different thicknesses. Using scanning tunneling microscopy and spectroscopy,
here we show the real space image of electronic structures across the
bilayer-monolayer interface in MoSe2 and WSe2. Most bilayer-monolayer
heterojunctions are found to have a zigzag-orientated interface, and the band
alignment of such atomically sharp heterojunctions is of type-I with a
well-defined interface mode which acts as a narrower-gap quantum wire. The
ability to utilize such commonly existing thickness terrace as lateral
heterojunctions is a crucial addition to the tool set for device applications
based on atomically thin transition metal dichalcogenides, with the advantage
of easy and flexible implementation.Comment: 19 pages in total, 4 figures and 1 tabl
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