168 Hits in 0.56 sec

Oblivious Online Contention Resolution Schemes [article]

Hu Fu, Pinyan Lu, Zhihao Gavin Tang, Abner Turkieltaub, Hongxun Wu, Jinzhao Wu, Qianfan Zhang
2021 arXiv   pre-print
Contention resolution schemes (CRSs) are powerful tools for obtaining "ex post feasible" solutions from candidates that are drawn from "ex ante feasible" distributions. Online contention resolution schemes (OCRSs), the online version, have found myriad applications in Bayesian and stochastic problems, such as prophet inequalities and stochastic probing. When the ex ante distribution is unknown, it was unknown whether good CRSs/OCRSs exist with no sample (in which case the scheme is oblivious)
more » ... eme is oblivious) or few samples from the distribution. In this work, we give a simple 1/e-selectable oblivious single item OCRS by mixing two simple schemes evenly, and show, via a Ramsey theory argument, that it is optimal. On the negative side, we show that no CRS or OCRS with O(1) samples can be Ω(1)-balanced/selectable (i.e., preserve every active candidate with a constant probability) for graphic or transversal matroids.
arXiv:2111.10607v1 fatcat:fay7xfsq4zgark2wivvw23wa6e

Modulating Super-Exchange Strength to Achieve Robust Ferromagnetic Couplings in Two-Dimensional Semiconductors [article]

Jiewen Xiao, Dominik Legut, Weidong Luo, Xiaopeng Liu, Ruifeng Zhang,, Qianfan Zhang
2019 arXiv   pre-print
Low-dimensional semiconducting ferromagnets have attracted considerable attention due to their promising applications as nano-size spintronics. However, realizing robust ferromagnetic couplings that can survive at high temperature is restrained by two decisive factors: super-exchange couplings and anisotropy. Despite widely explored low-dimensional anisotropy, strengthening super-exchange couplings has rarely been investigated. Here, we found that ligands with lower electronegativity can
more » ... gativity can strengthen ferromagnetic super-exchange couplings and further proposed the ligand modulation strategy to enhance the Curie temperature of low-dimensional ferromagnets. Based on the metallic CrX2 (X = S, Se, Te) family, substituting ligand atoms by halides can form stable semiconducting phase as CrSeCl, CrSeBr and CrTeBr. It is interesting to discover that, the nearest ferromagnetic super-exchange couplings can be strengthened when substituting ligands from S to Se and Te. Such evolution originates from the enhanced electron hopping integral and reduced energy intervals between d and p orbits. While the second nearest anti-ferromagnetic couplings are also benefitted due to delocalized p-p interactions. Finally, ligand modulation strategy is applied in other ferromagnetic monolayers, further verifying our theory and providing a fundamental understanding on controlling super-exchange couplings in low-dimension.
arXiv:1911.11333v1 fatcat:xrfflhtndzfzhddkxo3a2d6fbu

Wavelength tracking with thermally controlled silicon resonators

Ciyuan Qiu, Jie Shu, Zheng Li, Xuezhi Zhang, Qianfan Xu
2011 Optics Express  
We experimentally demonstrate feedback controlling of the resonant wavelength of a silicon dual-ring resonator. The feedback signal is the difference in optical scattering from the two coupled microring resonators, and the control mechanism is based on thermo-optic tuning with micro-heaters. This control scheme keeps the central wavelength of the resonator aligning with the input wavelength, which can be used to compensate fabrication variations, environmental temperature shift and the drift of
more » ... ft and the drift of laser wavelength. This feedback control scheme allows microringbased electro-optic modulators to be used in a dynamic environment.
doi:10.1364/oe.19.005143 pmid:21445149 fatcat:cgyjsekhj5bjpg3ag7xvxthfoa

Designing high-energy lithium–sulfur batteries

Zhi Wei Seh, Yongming Sun, Qianfan Zhang, Yi Cui
2016 Chemical Society Reviews  
Inspired by this study, the Zhang group decided to use nitrogen-doped carbon nanotubes as conductive hosts to achieve stable cycling in lithium-sulfur batteries. 32 Conductivity measurements showed that  ... 
doi:10.1039/c5cs00410a pmid:27460222 fatcat:3hndxfhqmnaxjltgkn5zh2zw7q

Analysis and Control of Current Ripples of Z-Source Inverters

Dong Shuai, Zhang Qianfan
2020 IEEE Access  
Load current ripple is an important issue in the design and control of inverters since it influences the torque ripple and noise amplitude of a motor drive. The current ripple of Z-source inverter (ZSI) is analyzed in this paper. The expression of current ripple is derived and the current ripples of ZSI and voltage-source inverter (VSI) are compared. A variable DC-link voltage and switching frequency method was adopted to reduce conduction and switching losses without increasing the predicted
more » ... ing the predicted peak current ripple. These theoretical findings were further verified by simulations and experiments.
doi:10.1109/access.2020.2976811 fatcat:kxvtk4vwvvetrlfiayjmcecqka

Suspended Si ring resonator for mid-IR application

Yang Xia, Ciyuan Qiu, Xuezhi Zhang, Weilu Gao, Jie Shu, Qianfan Xu
2013 Optics Letters  
In this Letter we fabricate and characterize suspended silicon (Si) waveguides and ring resonators for mid-infrared wavelength range. Both tunable laser and thermal tuning are used to cover wavelength ranges near 5.2 and 3.4 μm. The loaded quality factors are estimated at 2700 at 5.2 μm and 7900 at 3.4 μm. Heat induced spectrum distortion is observed. Absorption loss is analyzed based on the spectrum distortion, simulation, and all-optical modulation. It is shown that main loss comes from
more » ... ss comes from scattering at surface and should be possible to reduce by improving fabrication process.
doi:10.1364/ol.38.001122 pmid:23546264 fatcat:mphtztruc5civeayqerfoaz5hi

Path Integral Treatment of Proton Transport Processes inBaZrO3

Qianfan Zhang, Göran Wahnström, Mårten E. Björketun, Shiwu Gao, Enge Wang
2008 Physical Review Letters  
Nuclear quantum effects on proton transfer and reorientation in BaZrO 3 is investigated theoretically using the ab initio path-integral molecular-dynamics simulation technique. The result demonstrates that adding quantum fluctuations has a large effect on, in particular, the transfer barrier. The corresponding rates and diffusion coefficient are evaluated using the path-centroid transition state theory. In contrast with what is found assuming classical mechanics for the nuclear motion, the
more » ... ar motion, the reorientation step becomes rate limiting below 600 K.
doi:10.1103/physrevlett.101.215902 pmid:19113425 fatcat:jrd7ekuq6bf2xpmrmphskziqme

Efficient coupler between chip-level and board-level optical waveguides

Jie Shu, Ciyuan Qiu, Xuezhi Zhang, Qianfan Xu
2011 Optics Letters  
We propose an efficient optical coupler between a submicrometer-sized silicon waveguide on a silicon photonic chip and a multi-micrometer wide polymer waveguide on an optical printed circuit board for interchip optical networks. We show low coupling loss <0:4 dB with high lateral and angular tolerance to misalignment so that coupling can be done by automatic pick-and-place equipments with high throughput and low cost. The coupler has a wide optical bandwidth from 1470 to 1650 nm.
doi:10.1364/ol.36.003614 pmid:21931408 fatcat:ze7vzh2tjzcsdaxv4x7pi66zmi

Theoretical analysis of actively mode-locked fiber ring laser with semiconductor optical amplifier

Can Peng, Minyu Yao, Janfeng Zhang, Hongming Zhang, Qianfan Xu, Yizhi Gao
2002 Optics Communications  
A theoretical study of active mode-locking fiber loop laser using semiconductor optical amplifier (SOA) was presented in this paper. The effects of linewidth broadening factor of SOA on active mode-locking were investigated originally. The relations between parameters of output pulse series and the critical parameters of SOA and other elements in fiber loop were obtained. Ó
doi:10.1016/s0030-4018(02)01620-6 fatcat:eyk6sscgcffypmirdtsnry6vkm

Conducted EMI Investigation of a SiC-Based Multiplexing Converter for EV/PHEV

Jianzhen Qu, Qianfan Zhang, Yang Wang, Shumei Cui
2021 IEEE Access  
doi:10.1109/access.2021.3072976 fatcat:pobli6b5ljf2xicmbbftyj3q2e

Interferometric method of suppressing the pattern effect in a semiconductor optical amplifier

Qianfan Xu, Minyu Yao, Yi Dong, Jianfeng Zhang
2000 Optics Letters  
A new idea of using change in index of refraction to suppress gain variation in a saturated semiconductor optical amplifier (SOA) is presented. This kind of gain compensation has the advantage of high speed because it involves two phenomena that always accompany each other. This compensation can be achieved with a nonsymmetrical Mach -Zehnder interferometer structure. Calculated results show that with this structure the input and output power of the SOA can be extended to nearly 10 dB from the
more » ... rly 10 dB from the former small-signal limit when less than 1-dB gain variation is permitted. Numerical simulations with an advanced dynamic model of the SOA agree with the calculated results.
doi:10.1364/ol.25.001597 pmid:18066288 fatcat:3eptyvsjsjgs3nyl57v7dxzcwa

First principles study of lithium insertion in bulk silicon

Wenhui Wan, Qianfan Zhang, Yi Cui, Enge Wang
2010 Journal of Physics: Condensed Matter  
Si is an important anode material for the next generation of Li ion batteries. Here the energetics and dynamics of Li atoms in bulk Si have been studied at different Li concentrations on the basis of first principles calculations. It is found that Li prefers to occupy an interstitial site as a shallow donor rather than a substitutional site. The most stable position is the tetrahedral (T d ) site. The diffusion of a Li atom in the Si lattice is through a T d -Hex-T d trajectory, where the Hex
more » ... ry, where the Hex site is the hexagonal transition site with an energy barrier of 0.58 eV. We have also systematically studied the local structural transition of a Li x Si alloy with x varying from 0 to 0.25. At low doping concentration (x = 0-0.125), Li atoms prefer to be separated from each other, resulting in a homogeneous doping distribution. Starting from x = 0.125, Li atoms tend to form clusters induced by a lattice distortion with frequent breaking and reforming of Si-Si bonds. When x 0.1875, Li atoms will break some Si-Si bonds permanently, which results in dangling bonds. These dangling bonds create negatively charged zones, which is the main driving force for Li atom clustering at high doping concentration.
doi:10.1088/0953-8984/22/41/415501 pmid:21386598 fatcat:u6hx6oshwrbc3csiscqbuh7fv4

Anisotropic Bi2O2Se(Te) Monolayer: Realizing Ultra-High Carrier Mobility and Giant Electric Polarization in Two-Dimension [article]

Jiewen Xiao, Yuxin Wang, Xiang Feng, Dominik Legut, Tianshuai Wang, Yanchen Fan, Tonghui Su, Rushi Gong, Ruifeng Zhang, Qianfan Zhang
2019 arXiv   pre-print
Here, we have identified the monolayer phase of Bi2O2Se as a promising two-dimensional semiconductor with ultra-high carrier mobility and giant electric polarization. Due to the strong reconstruction originated from the interlayer electrostatic force, we have applied structure prediction algorithms to explore the crystalline geometry of Bi2O2Se monolayer with the lowest total energy. Considering Se and Te belong to the same group, Bi2O2Te monolayer is also investigated based on a similar
more » ... on a similar scheme. Further calculations suggest that the high carrier mobility is maintained in the monolayer phase and the moderate band gap will lead to the strong optical absorption in the visible light region. In particular, the electron mobility in Bi2O2Te can reach as high as 3610 cm2V-1s-1 at room temperature, which is almost ten times of conventional transition metal dichalcogenides (TMD) family. Because of the strong structural anisotropy, a remarkable spontaneous in-plane and out-of-plane electric polarization is additionally revealed along with significant piezoelectric properties, endowing them as promising candidates in the area of photovoltaic solar cells, optoelectronic materials and field effect transistors.
arXiv:1911.11327v1 fatcat:cpqjyv6bzfbznl44h3uz6fubdq

Nature of proton transport in a water-filled carbon nanotube and in liquid water [article]

Ji Chen, Xin-Zheng Li, Qianfan Zhang, Angelos Michaelides, Enge Wang
2014 arXiv   pre-print
Proton transport (PT) in bulk liquid water and within a thin water-filled carbon nanotube has been examined with ab initio pathintegral molecular dynamics (PIMD). Barrierless proton transfer is observed in each case when quantum nuclear effects (QNEs) are accounted for. The key difference between the two systems is that in the nanotube facile PT is facilitated by a favorable prealignment of water molecules, whereas in bulk liquid water solvent reorganization is required prior to PT.
more » ... to PT. Configurations where the quantum excess proton is delocalized over several adjacent water molecules along with continuous interconversion between different hydration states reveals that, as in liquid water, the hydrated proton under confinement is best described as a fluxional defect, rather than any individual idealized hydration state such as Zundel, Eigen, or the so-called linear H7O3+ complex along the water chain. These findings highlight the importance of QNEs in intermediate strength hydrogen bonds (HBs) and explain why H+ diffusion through nanochannels is impeded much less than other cations.
arXiv:1404.7280v1 fatcat:cb6vw7su4ffn5knxohloi5gtme

Application of Computational Simulation on the Study of Lithium Metal Anodes

Guangbin Hua, Yanchen Fan, Qianfan Zhang
2020 Wuli huaxue xuebao  
Zhang 等 123 利用氧化锌和碳纳米管构建出对锂的亲疏性 La 3 Zr 1.5 Ta 0.5 O 12 (LLZTO)的表 面,两者紧密接触,界面阻碍降至11 Ω·cm 2 。  ...  Zhang等 10 对富含 LiF表层的形成过程进行动力学研究(图3a-d),发 现相比于电解液EC/DEC,添加剂FEC具有更低能 量的LUMO,能更好地在锂电极上分解,促进形成 致密表层,利于锂的均匀沉积(图3e),从而使锂金 属电池在长循环下依然高效稳定工作。  ... 
doi:10.3866/pku.whxb202008089 fatcat:7tawddxgtvef5ikrxq3t2a4hgi
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