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Yi Shi

Publications and source records attributed to Yi Shi.

At least 73 records · Page 4Linked to original sources
arXiv2024

An inevitably aging world -- Analysis on the evolutionary pattern of age structure in 200 countries

Ignoring the differences between countries, human reproductive and dispersal behaviors can be described by some standardized models, so whether there is a universal law of population growth hidden in the abundant and unstructured data from various countries remains unclear. The age-specific population data constitute a three-dimensional tensor containing more comprehensive information. The existing literature often describes the characteristics of global or regional population evolution by subregion aggregation and statistical analysis, which makes it challenging to identify the underlying rules by ignoring national or structural details. Statistical physics can be used to summarize the macro characteristics and evolution laws of complex systems based on the attributes and motions of masses of individuals by decomposing high-dimensional tensors. Specifically, it can be used to assess the evolution of age structure in various countries over the past approximately 70 years, rather than simply focusing on the regions where aging has become apparent. It provides a universal scheme for the growing elderly and working age populations, indicating that the demographics on all continents are inevitably moving towards an aging population, including the current "young" continents of Africa, and Asia, South America with a recent "demographic dividend". It is a force derived from the "life cycle", and most countries have been unable to avoid this universal evolutionary path in the foreseeable future.

physics.soc-ph↗
arXiv2023

Securing NextG Systems against Poisoning Attacks on Federated Learning: A Game-Theoretic Solution

This paper studies the poisoning attack and defense interactions in a federated learning (FL) system, specifically in the context of wireless signal classification using deep learning for next-generation (NextG) communications. FL collectively trains a global model without the need for clients to exchange their data samples. By leveraging geographically dispersed clients, the trained global model can be used for incumbent user identification, facilitating spectrum sharing. However, in this distributed learning system, the presence of malicious clients introduces the risk of poisoning the training data to manipulate the global model through falsified local model exchanges. To address this challenge, a proactive defense mechanism is employed in this paper to make informed decisions regarding the admission or rejection of clients participating in FL systems. Consequently, the attack-defense interactions are modeled as a game, centered around the underlying admission and poisoning decisions. First, performance bounds are established, encompassing the best and worst strategies for attackers and defenders. Subsequently, the attack and defense utilities are characterized within the Nash equilibrium, where no player can unilaterally improve its performance given the fixed strategies of others. The results offer insights into novel operational modes that safeguard FL systems against poisoning attacks by quantifying the performance of both attacks and defenses in the context of NextG communications.

cs.NI↗
arXiv2023

On the density of Birkhoff sums for Anosov diffeomorphisms

Let f be an Anosov diffeomorphism on a nilmanifold. We consider Birkhoff sums for a Holder continuous observation along periodic orbits. We show that if there are two Birkhoff sums distributed at both sides of zero, then the set of Birkhoff sums of all periodic points is dense in the whole set of real numbers.

math.DS↗
arXiv2023

Strong Electron Correlation from Partition Density Functional Theory

Standard approximations for the exchange-correlation (XC) functional in Kohn-Sham density functional theory (KS-DFT) typically lead to unacceptably large errors when applied to strongly-correlated electronic systems. Partition-DFT (PDFT) is a formally exact reformulation of KS-DFT in which the ground-state density and energy of a system are obtained through self-consistent calculations on isolated fragments, with a partition energy representing the \textit{inter}-fragment interactions. Here we show how typical errors of the local density approximation (LDA) in KS-DFT can be largely suppressed through a simple approximation, the generalized overlap approximation (GOA), for the partition energy in PDFT. Our method is illustrated on simple models of one-dimensional strongly-correlated linear hydrogen chains. The GOA, when used in combination with the LDA for the fragments, improves the LDA dissociation curves of hydrogen chains and produces results that are comparable to those of spin-unrestricted LDA, but without breaking the spin symmetry. GOA also induces a correction to the LDA electron density that partially captures the correct density dimerization in strongly-correlated hydrogen chains. Moreover, with an additional correction to the partition energy, the approximation is shown to produce dissociation energies in quantitative agreement to calculations based on the Density Matrix Renormalization Group method.

physics.chem-ph↗
arXiv2023

$C^r$-Chain closing lemma for certain partially hyperbolic diffeomorphisms

For every $r\in\mathbb{N}_{\geq 2}\cup\{\infty\}$, we prove a $C^r$-orbit connecting lemma for dynamically coherent and plaque expansive partially hyperbolic diffeomorphisms with 1-dimensional orientation preserving center bundle. To be precise, for such a diffeomorphism $f$, if a point $y$ is chain attainable from $x$ through pseudo-orbits, then for any neighborhood $U$ of $x$ and any neighborhood $V$ of $y$, there exist true orbits from $U$ to $V$ by arbitrarily $C^r$-small perturbations. As a consequence, we prove that for $C^r$-generic diffeomorphisms in this class, periodic points are dense in the chain recurrent set, and chain transitivity implies transitivity.

math.DS↗
arXiv2023

Oxidation kinetics and non-Marcusian charge transfer in dimensionally confined semiconductors

Electrochemical reactions represent essential processes in fundamental chemistry that foster a wide range of applications. Although most electrochemical reactions in bulk substances can be well described by the classical Marcus-Gerischer charge transfer theory, the realistic reaction character and mechanism in dimensionally confined systems remain unknown. Here, we report the multiparametric survey on the kinetics of lateral photooxidation in structurally identical WS2 and MoS2 monolayers, where electrochemical oxidation occurs at the atomically thin monolayer edges. The oxidation rate is correlated quantitatively with various crystallographic and environmental parameters, including the density of reactive sites, humidity, temperature, and illumination fluence. In particular, we observe distinctive reaction barriers of 1.4 and 0.9 eV for the two structurally identical semiconductors and uncover an unusual non-Marcusian charge transfer mechanism in these dimensionally confined monolayers due to the limit in reactant supplies. A scenario of band bending is proposed to explain the discrepancy in reaction barriers. These results add important knowledge into the fundamental electrochemical reaction theory in low-dimensional systems.

cond-mat.mtrl-sci↗
arXiv2023

On multifractal analysis and large deviations of singular-hyperbolic attractors

In this paper we study the multifractal analysis and large derivations for singular hyperbolic attractors, including the geometric Lorenz attractors. For each singular hyperbolic homoclinic class whose periodic orbits are all homoclinically related and such that the space of ergodic probability measures is connected, we prove that: (i) level sets associated to continuous observables are dense in the homoclinic class and satisfy a variational principle; (ii) irregular sets are either empty or are Baire generic and carry full topological entropy. The assumptions are satisfied by $C^1$-generic singular hyperbolic attractors and $C^r$-generic geometric Lorenz attractors $(r\ge 2)$. Finally we prove level-2 large deviations bounds for weak Gibbs measures, which provide a large deviations principle in the special case of Gibbs measures. The main technique we apply is the horseshoe approximation property.

math.DS↗
arXiv2023

Non-invasive digital etching of van der Waals semiconductors

The capability to finely tailor material thickness with simultaneous atomic precision and non-invasivity would be useful for constructing quantum platforms and post-Moore microelectronics. However, it remains challenging to attain synchronized controls over tailoring selectivity and precision. Here we report a protocol that allows for non-invasive and atomically digital etching of van der Waals transition-metal dichalcogenides through selective alloying via low-temperature thermal diffusion and subsequent wet etching. The mechanism of selective alloying between sacrifice metal atoms and defective or pristine dichalcogenides is analyzed with high-resolution scanning transmission electron microscopy. Also, the non-invasive nature and atomic level precision of our etching technique are corroborated by consistent spectral, crystallographic and electrical characterization measurements. The low-temperature charge mobility of as-etched MoS$_2$ reaches up to $1200\,$cm$^{2}\cdot$V$^{-1}\cdot$s$^{-1}$, comparable to that of exfoliated pristine counterparts. The entire protocol represents a highly precise and non-invasive tailoring route for material manipulation.

cond-mat.mtrl-sci↗
arXiv2023

Antiferromagnetic $α$-MnTe: Molten-Salt-Assisted Chemical Vapor Deposition Growth and Magneto-Transport Properties

Antiferromagnetic (AF) materials are attracting increasing interest of research in magnetic physics and spintronics. Here, we report controllable synthesis of room-temperature AF $α$-MnTe nanocrystals (Néel temperature ~ 307 K) via molten-salt-assisted chemical vapor deposition method. The growth kinetics are investigated regarding the dependence of flake dimension and macroscopic shape on growth time and temperature. The high crystalline quality and atomic structure are confirmed by various crystallographic characterization means. Cryogenic magneto-transport measurements reveal anisotropic magnetoresistance (MR) response and a complicated dependence of MR on temperature, owing to the subtle competition among multiple scattering mechanisms of thermally excited magnetic disorders (magnon drag), magnetic transition and thermally populated lattice phonons. Overall positive MR behavior with twice transitions in magnitude is observed when out-of-plane external magnetic field ($B$) is applied, while a transition from negative to positive MR response is recorded when in-plane $B$ is applied. The rich magnetic transport properties render $α$-MnTe a promising material for exploiting functional components in magnetic devices.

cond-mat.mtrl-sci↗
arXiv2023

Unraveling Intertwined Impacts between Lattice Vacancy and Substrate on Photonic Quasiparticles in Monolayer MoS$_2$

Lattice defects and interfacial absorbates represent two extrinsic but ubiquitous factors that exert profound impacts on the luminescent properties of semiconductors. However, their impacts are normally tangled and remain to be separately elucidated. Here, we clarify the individual roles of each factor by tracking the weight evolution of photonic quasiparticles by modulating the densities of sulfur vacancies in monolayer MoS$_2$ via ad hoc defect engineering. In particular, we perform atomistic analyses on the densities of sulfur vacancies by employing atomically resolved scanning transmission electron microscopy to quantitatively characterize the generation rates of sulfur vacancies in MoS$_2$ on two types of substrates with opposite surface hydrophobicity natures. We reveal that the generation rate of sulfur vacancies can even double on the hydrophilic substrates. More importantly, the impact of sulfur vacancies over the weight of trionic emission is closely associated with the substrate hydrophobicity, which is manifested noticeably in the hydrophobic substrates, but insignificant in the hydrophilic ones. The results represent an in-depth understanding on the roles of the extrinsic factors on the luminescent properties in atomically thin semiconductors.

cond-mat.mtrl-sci↗
arXiv2023

Intrinsic Electronic Properties of BN Encapsulated, van der Waals Contacted MoSe$_2$ FETs

Two-dimensional (2D) semiconductors have attracted considerable interest for their unique physical properties. Here, we report the intrinsic cryogenic electronic transport properties in few-layer MoSe$_2$ field-effect transistors (FETs) that are simultaneously van der Waals contacted with gold electrodes and are fully encapsulated in ultraclean hexagonal boron nitride dielectrics. The FETs exhibit electronically favorable channel/dielectric interfaces with low densities of interfacial traps ($10^{10}\,$cm$^{-2}$), which lead to outstanding device characteristics at room temperature, including a near-Boltzmann-limit subthreshold swings ($\lt65\,$mV/dec), a high carrier mobility ($68\,$cm$^{2}\cdot$V$^{-1}\cdot$s$^{-1}$), and a negligible scanning hysteresis ($\lt15\,$mV). The dependence of various contact-related quantities on temperature and carrier density are also systematically characterized to understand the van der Waals contacts between gold and MoSe$_2$. The results provide insightful information on the device physics in van der Waals contacted and encapsulated 2D FETs.

cond-mat.mtrl-sci↗
arXiv2023

Noninvasive Photodelamination of van der Waals Semiconductors for High-Performance Electronics

Atomically thin two-dimensional (2D) van der Waals semiconductors are promising candidate materials for post-silicon electronics. However, it remains challenging to attain completely uniform monolayer semiconductor wafers free of over-grown islands. Here, we report the observation of the energy funneling effect and ambient photodelamination phenomenon in inhomogeneous few-layer WS$_2$ flakes under low illumination fluencies down to several nW/$μ$m$^{2}$ and its potential as a non-invasive post-etching strategy for selectively stripping the local excessive overlying islands. Photoluminescent tracking on the photoetching traces reveals relatively fast etching rates around $0.3-0.8\,μ$m/min at varied temperatures and an activation energy of $1.7\,$eV. By using crystallographic and electronic characterization, we also confirm the non-invasive nature of the low-power photodelamination and the highly preserved lattice quality in the as-etched monolayer products, featuring a comparable average density of atomic defects (ca.$4.2\times 10^{13}\,$cm$^{-2}$) to pristine flakes and a high electron mobility up to $80\,$cm$^{2}\cdot$V$^{-1}\cdot$s$^{-1}$) at room temperature. This approach opens a non-invasive photoetching route for thickness uniformity management in 2D van der Waals semiconductor wafers for electronic applications.

cond-mat.mtrl-sci↗
arXiv2023

Coulomb screening and scattering in atomically thin transistors across dimensional crossover

Layered two-dimensional dichalcogenides are potential candidates for post-silicon electronics. Here, we report insightfully experimental and theoretical studies on the fundamental Coulomb screening and scattering effects in these correlated systems, in response to the changes of three crucial Coulomb factors, including electric permittivity, interaction length, and density of Coulomb impurities. We systematically collect and analyze the trends of electron mobility with respect to the above factors, realized by synergic modulations on channel thicknesses and gating modes in dual-gated MoS2 transistors with asymmetric dielectric cleanliness. Strict configurative form factors are developed to capture the subtle parametric changes across dimensional crossover. A full diagram of the carrier scattering mechanisms, in particular on the pronounced Coulomb scattering, is unfolded. Moreover, we clarify the presence of up to 40% discrepancy in mobility by considering the permittivity modification across dimensional crossover. The understanding is useful for exploiting atomically thin body transistors for advanced electronics.

cond-mat.mes-hall↗
arXiv2023

Clean BN encapsulated 2D FETs with lithography compatible contacts

Device passivation through ultraclean hexagonal BN encapsulation is proven one of the most effective ways for constructing high-quality devices with atomically thin semiconductors that preserves the ultraclean interface quality and intrinsic charge transport behavior. However, it remains challenging to integrate lithography compatible contact electrodes with flexible distributions and patterns. Here, we report the feasibility in straightforwardly integrating lithography defined contacts into BN encapsulated 2D FETs, giving rise to overall device quality comparable to the state-of-the-art results from the painstaking pure dry transfer processing. Electronic characterization on FETs consisting of WSe$_2$ and MoS$_2$ channels reveals an extremely low scanning hysteresis of ca. 2 mV on average, a low density of interfacial charged impurity of ca. $10^{11}\,$cm$^{-2}$, and generally high charge mobilities over $1000\,$cm$^{2}\cdot$V$^{-1}\cdot$s$^{-1}$ at low temperatures. The overall high device qualities verify the viability in directly integrating lithography defined contacts into BN encapsulated devices to exploit their intrinsic charge transport properties for advanced electronics.

cond-mat.mes-hall↗
arXiv2023

Electrical contact properties between Yb and few-layer WS$_2$

Charge injection mechanism from contact electrodes into two-dimensional (2D) dichalcogenides is an essential topic for exploiting electronics based on 2D channels, but remains not well understood. Here, low-work-function metal ytterbium (Yb) was employed as contacts for tungsten disulfide (WS$_2$) to understand the realistic injection mechanism. The contact properties in WS$_2$ with variable temperature (T) and channel thickness (tch) were synergetically characterized. It is found that the Yb/WS$_2$ interfaces exhibit a strong pinning effect between energy levels and a low contact resistance ($R_\rm{C}$) value down to $5\,kΩ\cdotμ$m. Cryogenic electrical measurements reveal that $R_\rm{C}$ exhibits weakly positive dependence on T till 77 K, as well as a weakly negative correlation with tch. In contrast to the non-negligible $R_\rm{C}$ values extracted, an unexpectedly low effective thermal injection barrier of 36 meV is estimated, indicating the presence of significant tunneling injection in subthreshold regime and the inapplicability of the pure thermionic emission model to estimate the height of injection barrier.

cond-mat.mtrl-sci↗
arXiv2023

A nanogapped hysteresis-free field-effect transistor

We propose a semi-suspended device structure and construct nanogapped, hysteresis-free field-effect transistors (FETs), based on the van der Waals stacking technique. The structure, which features a semi-suspended channel above a submicron-long wedge-like nanogap, is fulfilled by transferring ultraclean BN-supported MoS$_2$ channels directly onto dielectric-spaced vertical source/drain stacks. Electronic characterization and analyses reveal a high overall device quality, including ultraclean channel interfaces, negligible electrical scanning hysteresis, and Ohmic contacts in the structures. The unique hollow FET structure holds the potential for exploiting reliable electronics, as well as nanofluid and pressure sensors.

cond-mat.mes-hall↗
arXiv2023

Enhancement of Carrier Mobility in Semiconductor Nanostructures by Carrier Distribution Engineering

Two-dimensional (2D) van der Waals semiconductors are appealing for low-power transistors. Here, we show the feasibility in enhancing carrier mobility in 2D semiconductors through engineering the vertical distribution of carriers confined inside the ultrathin channels via symmetrizing gate configuration or increasing channel thickness. Through self-consistently solving the Schrödinger-Poisson equations, the shapes of electron envelope functions are extensively investigated by clarifying their relationship with gate configuration, channel thickness, dielectric permittivity, and electron density. The impacts of electron distribution variation on various carrier scattering matrix elements and overall carrier mobility are insightfully clarified. It is found that the carrier mobility can be generally enhanced in the dual-gated configuration due to the centralization of carrier redistribution in the nanometer-thick semiconductor channels and the rate of increase reaches up to 23% in HfO$_2$ dual-gated 10-layer MoS$_2$ channels. This finding represents a viable strategy for performance optimization in transistors consisting of 2D semiconductors.

cond-mat.mtrl-sci↗
arXiv2023

Stretching bonds in Density Functional Theory without artificial symmetry breaking

Accurate first-principles calculations for the energies, charge distributions, and spin symmetries of many-electron systems are essential to understand and predict the electronic and structural properties of molecules and materials. Kohn-Sham density functional theory (KS-DFT) stands out among electronic-structure methods due to its balance of accuracy and computational efficiency. It is now extensively used in fields ranging from materials engineering to rational drug design. However, to achieve chemically accurate energies, standard density functional approximations in KS-DFT often need to break underlying symmetries, a long-standing "symmetry dilemma". By employing fragment spin densities as the main variables in calculations (rather than total molecular densities as in KS-DFT), we present an embedding framework in which this symmetry dilemma is resolved for the case of stretched molecules. The spatial overlap between fragment densities is used as the main ingredient to construct a simple, physically-motivated approximation to a universal functional of the fragment densities. This 'overlap approximation' is shown to significantly improve semi-local KS-DFT binding energies of molecules without artificial symmetry breaking.

physics.chem-ph↗