arXiv subjects

Luqing Wang

Publications and source records attributed to Luqing Wang.

6 recordsLinked to original sources

arXiv2025

A Proximal Gradient Method With Probabilistic Multi-Gossip Communications for Decentralized Composite Optimization

Decentralized optimization methods with local updates have recently gained attention for their provable ability to communication acceleration. In these methods, nodes perform several iterations of local computations between the communication rounds. Nevertheless, this capability is effective only when the network is sufficiently well-connected and the loss function is smooth. In this paper, we propose a communication-efficient method MG-Skip with probabilistic local updates and multi-gossip communications for decentralized composite (smooth + nonsmooth) optimization, whose stepsize is independent of the number of local updates and the network topology. For any undirected and connected networks, MG-Skip allows for the multi-gossip communications to be skipped in most iterations in the strongly convex setting, while its computation complexity is $\mathcal{O}\left(κ\log \frac{1}ε\right)$ and communication complexity is only $\mathcal{O}\left(\sqrt{\fracκ{(1-ρ)}} \log \frac{1}ε\right)$, where $κ$ is the condition number of the loss function, $ρ$ reflects the connectivity of the network topology, and $ε$ is the target accuracy. The theoretical results indicate that MG-Skip achieves provable communication acceleration, thereby validating the advantages of local updates in the nonsmooth setting.

math.OC↗

arXiv2024

Differentially Private Decentralized Optimization with Relay Communication

Security concerns in large-scale networked environments are becoming increasingly critical. To further improve the algorithm security from the design perspective of decentralized optimization algorithms, we introduce a new measure: Privacy Leakage Frequency (PLF), which reveals the relationship between communication and privacy leakage of algorithms, showing that lower PLF corresponds to lower privacy budgets. Based on such assertion, a novel differentially private decentralized primal--dual algorithm named DP-RECAL is proposed to take advantage of operator splitting method and relay communication mechanism to experience less PLF so as to reduce the overall privacy budget. To the best of our knowledge, compared with existing differentially private algorithms, DP-RECAL presents superior privacy performance and communication complexity. In addition, with uncoordinated network-independent stepsizes, we prove the convergence of DP-RECAL for general convex problems and establish a linear convergence rate under the metric subregularity. Evaluation analysis on least squares problem and numerical experiments on real-world datasets verify our theoretical results and demonstrate that DP-RECAL can defend some classical gradient leakage attacks.

math.OC↗

arXiv2021

Step-edge epitaxy for borophene growth on insulators

Borophene, a monoatomic layer of boron atoms, stands out among two-dimensional (2D) materials, with its versatile properties of polymorphism, metallicity, plasmonics, superconductivity, tantalizing for physics exploration and next-generation devices. Yet its phases are all synthesized on and stay bound to metal substrates, hampering both characterization and use. The growth on the inert insulator would allow post-synthesis exfoliation of borophene, but its weak adhesion to such substrate results in a very high 2D-nucleation barrier preventing clean borophene growth. This challenge can be circumvented in a devised and demonstrated here, with ab initio calculations, strategy. Naturally present 1D-defects, the step-edges on h-BN substrate surface, enable boron epitaxial assembly, reduce the nucleation dimensionality and lower the barrier by an order of magnitude (to 1.1 eV or less), yielding v1/9 phase. Weak borophene adhesion to the insulator makes it readily accessible for comprehensive property tests or transfer into the device setting.

cond-mat.mtrl-sci↗

arXiv2020

Nanoscale probing of image-potential states and electron transfer doping in borophene polymorphs

Using field-emission resonance spectroscopy with an ultrahigh vacuum scanning tunneling microscope, we reveal Stark-shifted image-potential states of the v_1/6 and v_1/5 borophene polymorphs on Ag(111) with long lifetimes, suggesting high borophene lattice and interface quality. These image-potential states allow the local work function and interfacial charge transfer of borophene to be probed at the nanoscale and test the widely employed self-doping model of borophene. Supported by apparent barrier height measurements and density functional theory calculations, electron transfer doping occurs for both borophene phases from the Ag(111) substrate. In contradiction with the self-doping model, a higher electron transfer doping level occurs for denser v_1/6 borophene compared to v_1/5 borophene, thus revealing the importance of substrate effects on borophene electron transfer.

cond-mat.mtrl-sci↗

arXiv2019

Defining shapes of 2D-crystals with undefinable edge-energies

The equilibrium shape of crystals is a fundamental property of both aesthetic appeal and practical import. It is also a visible macro-manifestation of the underlying atomic-scale forces and chemical makeup, most conspicuous in two-dimensional (2D) materials of keen current interest. If the crystal surface/edge energy is known for different directions, its shape can be obtained by geometric Wulff construction, a tenet of crystal physics. However, if symmetry is lacking, the crystal edge energy cannot be defined or calculated, so its shape becomes elusive, presenting an insurmountable problem for theory. Here we show how, in fact, one can proceed with "latent edge energies" towards constructive prediction of a unique crystal shape, and demonstrate it for challenging material-examples, like ${\rm SnS}$ of ${\rm C_{2v}}$ symmetry and even ${\rm AgNO_2}$ of ${\rm C_1}$-no symmetry at all.

cond-mat.mtrl-sci↗

arXiv2014

Many-body and spin-orbit effects on direct-indirect band gap transition of strained monolayer MoS$_2$ and WS$_2$

Monolayer transition metal dichalcogenides are promising materials for photoelectronic devices. Among them, molybdenum disulphide (MoS$_2$) and tungsten disulphide (WS$_2$) are some of the best candidates due to their favorable band gap values and band edge alignments. Here we consider various perturbative corrections to the DFT electronic structure, e.g. GW, spin-orbit coupling, as well as many-body excitonic and trionic effects, and calculate accurate band gaps as a function of homogeneous strain in these materials. We show that all of these corrections are of comparable magnitudes and need to be included in order to obtain an accurate electronic structure. We calculate the strain at which the direct-to-indirect gap transition occurs. After considering all contributions, the direct to indirect gap transition strain is found to be at 2.7% in MoS$_2$ and 3.9% in WS$_2$. These values are generally higher than the previously reported theoretical values.

cond-mat.mtrl-sci↗