189 research outputs found
Origin of High-Temperature Superconductivity in Compressed LaH
Room-temperature superconductivity has been one of the most challenging
subjects in modern physics. Recent experiments reported that lanthanum hydride
LaH (1) raises a superconducting transition temperature
up to 260 (or 215) K at high pressures around 190 (150)
GPa. Here, based on first-principles calculations, we reveal the existence of
topological Dirac-nodal-line (DNL) states in compressed LaH. Remarkably,
the DNLs protected by the combined inversion and time-reversal symmetry and the
rotation symmetry create a van Hove singularity (vHs) near the Fermi energy,
giving rise to large electronic density of states. Contrasting with other La
hydrides containing cationic La and anionic H atoms, LaH shows a
peculiar characteristic of electrical charges with anionic La and both cationic
and anionic H species, caused by a strong hybridization of the La and H
orbitals. We find that a large number of electronic states at the vHs are
strongly coupled to the H-derived high-frequency phonon modes that are induced
via the unusual, intricate bonding network of LaH, thereby yielding a
high . Our findings not only elucidate the microscopic origin of the
observed high- BCS-type superconductivity in LaH, but also
pave the route for achieving room-temperature topological superconductors in
compressed hydrogen-rich compounds.Comment: 9 pages, 11 figure
Towards Optimal Decoding for Polar Codes
In the conventional successive cancellation (SC) decoder for polar codes, all
the future bits to be estimated later are treated as random variables. However,
polar codes inevitably involve frozen bits, and their concatenated coding
schemes also include parity bits causally generated from the past bits
estimated earlier. We refer to the frozen and parity bits located behind a
target decoding bit as its future constraints (FCs). Although the values of FCs
are deterministic given the past estimates, they have not been exploited in the
conventional SC-based decoders, not leading to optimality. In this paper, we
propose SC-check (SCC) and belief-propagation SCC (BP-SCC) decoding algorithms
in order to leverage FCs in decoding.We further devise a tree search technique
based on stack-based backjumping (SBJ) to solve dynamic constraint satisfaction
problems (CSPs) formulated by FCs. Over the binary erasure channel (BEC),
numerical results show that a combination of the BP-SCC algorithm and the SBJ
tree search technique achieves the erasure recovery performance close to the
dependence testing (DT) bound, a bound of achievable finite-length performance
Combining Non-probability and Probability Survey Samples Through Mass Imputation
This paper presents theoretical results on combining non-probability and
probability survey samples through mass imputation, an approach originally
proposed by Rivers (2007) as sample matching without rigorous theoretical
justification. Under suitable regularity conditions, we establish the
consistency of the mass imputation estimator and derive its asymptotic variance
formula. Variance estimators are developed using either linearization or
bootstrap. Finite sample performances of the mass imputation estimator are
investigated through simulation studies and an application to analyzing a
non-probability sample collected by the Pew Research Centre.Comment: Submitted to Journal of the Royal Statistical Society: Series
Competing edge structures of Sb and Bi bilayers by trivial and nontrivial band topologies
One-dimensional (1D) edge states formed at the boundaries of 2D normal and
topological insulators have shown intriguing quantum phases such as charge
density wave and quantum spin Hall effect. Based on first-principles
density-functional theory calculations including spin-orbit coupling (SOC), we
show that the edge states of zigzag Sb(111) and Bi(111) nanoribbons drastically
change the stability of their edge structures. For zigzag Sb(111) nanoribbon,
the Peierls-distorted or reconstructed edge structure is stabilized by a
band-gap opening. However, for zigzag Bi(111) nanoribbon, such two insulating
structures are destabilized due to the presence of topologically protected
gapless edge states, resulting in the stabilization of a metallic,
shear-distorted edge structure. We also show that the edge states of the
Bi(111) nanoribbon exhibit a larger Rashba-type spin splitting at the boundary
of Brillouin zone, compared to those of the Sb(111) nanoribbon. Interestingly,
the spin textures of edge states in the Peierls-distorted Sb edge structure and
the shear-distorted Bi edge structure have all three spin components
perpendicular and parallel to the edges, due to their broken mirror-plane
symmetry. The present findings demonstrate that the topologically trivial and
nontrivial edge states play crucial roles in determining the edge structures of
normal and topological insulators.Comment: 7 pages, 8 figure
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