Boundary-obstructed topological high-Tc_c 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 Ca$_{1-x}$La$_x$FeAs$_2$, 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 $N$ equipped at the IRS becomes sufficiently large, the maximum sensing SNR increases proportionally to $N^2$ for the semi-passive-IRS sensing system, but proportionally to $N^4$ 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 $N$ grows, the minimum CRB decreases inversely proportionally to $N^4$ and $N^6$ 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 $N$ 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 $N$.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