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1. Speeding up Monte Carlo simulations for the adaptive sum of powered score test with importance sampling

   https://doi.org/10.1111/biom.13407  

   Deng, Yangqing; He, Yinqiu; Xu, Gongjun; Pan, Wei (December 2020, Biometrics)

   Abstract A central but challenging problem in genetic studies is to test for (usually weak) associations between a complex trait (e.g., a disease status) and sets of multiple genetic variants. Due to the lack of a uniformly most powerful test, data‐adaptive tests, such as the adaptive sum of powered score (aSPU) test, are advantageous in maintaining high power against a wide range of alternatives. However, there is often no closed‐form to accurately and analytically calculate thep‐values of many adaptive tests like aSPU, thus Monte Carlo (MC) simulations are often used, which can be time consuming to achieve a stringent significance level (e.g., 5e‐8) used in genome‐wide association studies (GWAS). To estimate such a smallp‐value, we need a huge number of MC simulations (e.g., 1e+10). As an alternative, we propose using importance sampling to speed up such calculations. We develop some theory to motivate a proposed algorithm for the aSPU test, and show that the proposed method is computationally more efficient than the standard MC simulations. Using both simulated and real data, we demonstrate the superior performance of the new method over the standard MC simulations. 

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2. Ferroelectric nonlinear anomalous Hall effect in few-layer WTe2

   https://doi.org/10.1038/s41524-019-0257-1  

   Wang, Hua; Qian, Xiaofeng (December 2019, npj Computational Materials)

   Abstract Under broken time reversal symmetry such as in the presence of external magnetic field or internal magnetization, a transverse voltage can be established in materials perpendicular to both longitudinal current and applied magnetic field, known as classical Hall effect. However, this symmetry constraint can be relaxed in the nonlinear regime, thereby enabling nonlinear anomalous Hall current in time-reversal invariant materials – an underexplored realm with exciting new opportunities beyond classical linear Hall effect. Here, using group theory and first-principles theory, we demonstrate a remarkable ferroelectric nonlinear anomalous Hall effect in time-reversal invariant few-layer WTe₂where nonlinear anomalous Hall current switches in odd-layer WTe₂except 1T′ monolayer while remaining invariant in even-layer WTe₂upon ferroelectric transition. This even-odd oscillation of ferroelectric nonlinear anomalous Hall effect was found to originate from the absence and presence of Berry curvature dipole reversal and shift dipole reversal due to distinct ferroelectric transformation in even and odd-layer WTe₂. Our work not only treats Berry curvature dipole and shift dipole on an equal footing to account for intraband and interband contributions to nonlinear anomalous Hall effect, but also establishes Berry curvature dipole and shift dipole as new order parameters for noncentrosymmetric materials. The present findings suggest that ferroelectric metals and Weyl semimetals may offer unprecedented opportunities for the development of nonlinear quantum electronics. 

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3. Carbazolylene‐Ethynylene Macrocycle based Conductive Covalent Organic Frameworks

   https://doi.org/10.1002/anie.202303538  

   Huang, Shaofeng; Choi, Ji Yong; Xu, Qiucheng; Jin, Yinghua; Park, Jihye; Zhang, Wei (April 2023, Angewandte Chemie International Edition)

   Abstract Two covalent organic frameworks consisting of carbazolylene‐ethynylene shape‐persistent macrocycles with azine (MC‐COF‐1) or imine (MC‐COF‐2) linkages were synthesized via imine condensation. The obtained 2D frameworks are fully conjugated which imparts semiconducting properties. In addition, the frameworks showed high porosity with aligned accessible porous channels along the z axis, serving as an ideal platform for post‐synthetic incorporation of I₂into the channels to enable electrical conductivity. The resulting MC‐COF‐1 showed an electrical conductivity up to 7.8×10⁻⁴ S cm⁻¹at room temperature upon I₂doping with the activation energy as low as 0.09 eV. Furthermore, we demonstrated that the electrical properties of both MC‐COFs are switchable between electron‐conducting and insulating states by simply implementing doping‐regenerating cycles. The knowledge gained in this study opens new possibilities for the future development of tunable conductive 2D organic materials. 

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4. (MC)^2: Lazy MemCopy at the Memory Controller

   Kamath, Aditya K; Peter, Simon (June 2024, ACM)

   (MC)^2 is a lazy memory copy mechanism which can be used within memcpy-like functions to significantly reduce the CPU overhead for copies that are sparsely accessed. It can also hide copy latencies by enhancing the CPU’s ability to execute them asynchronously. (MC)^2’s lazy memcpy avoids copying data at the time of invocation. Instead, (MC)^2 tracks prospective copies. If copied data is later accessed by a CPU or the cache, (MC)^2 uses the tracking information to lazily execute a copy, when necessary. Placing (MC)^2 at the memory controller puts it at the perfect vantage point to eliminate the largest source of memcpy overhead—CPU stalls due to cache misses in the critical path—while imposing minimal overhead itself. (MC)^2 consists of three main components: memory controller extensions that implement a lazy memcpy operation, a new instruction exposing the lazy memcpy, and a flexible software wrapper with semantics identical to memcpy. We implement and evaluate (MC)^2 in the gem5 simulator using a variety of microbenchmarks and workloads, including Google’s Protobuf, where (MC)^2 provides a 43% speedup and Linux huge page copy-on-write faults, where (MC)^2 provides 250× lower latency. 

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5. Large magnetocapacitance beyond 420% in epitaxial magnetic tunnel junctions with an MgAl2O4 barrier

   https://doi.org/10.1038/s41598-022-11545-6  

   Sato, Kenta; Sukegawa, Hiroaki; Ogata, Kentaro; Xiao, Gang; Kaiju, Hideo (May 2022, Scientific Reports)

   Abstract Magnetocapacitance (MC) effect has been observed in systems where both symmetries of time-reversal and space-inversion are broken, for examples, in multiferroic materials and spintronic devices. The effect has received increasing attention due to its interesting physics and the prospect of applications. Recently, a large tunnel magnetocapacitance (TMC) of 332% at room temperature was reported using MgO-based (001)-textured magnetic tunnel junctions (MTJs). Here, we report further enhancement in TMC beyond 420% at room temperature using epitaxial MTJs with an MgAl₂O₄(001) barrier with a cation-disordered spinel structure. This large TMC is partially caused by the high effective tunneling spin polarization, resulted from the excellent lattice matching between the Fe electrodes and the MgAl₂O₄barrier. The epitaxial nature of this MTJ system sports an enhanced spin-dependent coherent tunneling effect. Among other factors leading to the large TMC are the appearance of the spin capacitance, the large barrier height, and the suppression of spin flipping through the MgAl₂O₄barrier. We explain the observed TMC by the Debye-Fröhlich modelled calculation incorporating Zhang-sigmoid formula, parabolic barrier approximation, and spin-dependent drift diffusion model. Furthermore, we predict a 1000% TMC in MTJs with a spin polarization of 0.8. These experimental and theoretical findings provide a deeper understanding on the intrinsic mechanism of the TMC effect. New applications based on large TMC may become possible in spintronics, such as multi-value memories, spin logic devices, magnetic sensors, and neuromorphic computing. 

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