arXiv subjects

Yifei Yu

Publications and source records attributed to Yifei Yu.

51 records · Page 3Linked to original sources

arXiv2021

Principal Decomposition of Velocity Gradient Tensor in the Cartesian Coordinates

Traditional Cauchy-Stokes decomposition of velocity gradient tensor gives a symmetric and an anti-symmetric subtensors which are called the strain-rate and vorticity tensors. There are two problems with Cauchy-Stokes decomposition. The first one is that the anti-symmetric or vorticity tensor cannot represent the fluid rotation or vortex. The second one is that the symmetric (strain-rate) tensor cannot distinguish the stretching (compression) and shear. The stretching and shear are dependent on the coordinate or are not Galilean invariant. Since vorticity cannot distinguish between the non-rotational shear and the rigid rotation, vorticity has been decomposed to a rigid rotation called Liutex and anti-symmetric shear in our previous work. A Liutex-based principal coordinate was developed and the velocity gradient tensor was decomposed in the principal coordinate as a rigid rotation (Liutex tensor), a pure shear tensor and a stretching (compression) tensor, which is called the principal decomposition. However, the principal decomposition is made at each point which has own principal coordinate different from other points. This paper derives the principal decomposition in the original xyz coordinate system, and, therefore, provides a new tool for fluid kinematics to conduct the velocity gradient tensor decomposition to rigid rotation, pure shear, and stretching (compression) which is Galilean invariant and has clear physical meanings. The new velocity gradient tensor decomposition could become a foundation for new fluid kinematics.

physics.flu-dyn↗

arXiv2021

A mathematical study on the local fluid rotation axis

As widely recognized, vortex represents flow rotation. Vortex should have a local rotation axis as its direction and angular speed as its strength. Vorticity vector has been considered the rotation axis, and vorticity magnitude the rotational strength for a long time in classical fluid kinematics. However, this concept cannot stand in viscous flow. This study demonstrates by rigorous mathematical proof that the vorticity vector is not the fluid rotation axis, and vorticity is not the rotation strength. On the other hand, the Liutex vector is mathematically proved as the fluid rotation axis, and the Liutex magnitude is twice the fluid angular speed.

physics.flu-dyn↗

arXiv2020

Stretching and Shearing Contamination Analysis for Liutex/Rortex and Other Vortex Identification Methods

Although traditional vortex identification methods such as Q, Delta, Lambda2, Lambdaci remain popular in the identification and visualization of vortices, these methods count on shearing and stretching as a part of vortex strength. However, shearing and stretching do not contribute to fluid rotation. In this paper, the contamination effects of stretching and shearing of these methods are investigated and compared with Liutex method. From our investigation, the Liutex is an exact definition of fluid rotation or vortex, while other vortex identification methods are contaminated by stretching and shearing at different levels. The decomposition of the velocity gradient tensor can only be conducted in a so-called Principal Coordinate for uniqueness. The mathematical relation between Liutex and other vortex identification function are derived in this paper and then the effects of shearing and stretching on different vortex identification methods are studied. The mathematical formula and computation of the stretching and shearing effects on different schemes clearly show that the Liutex method has superiority over the other vortex identification methods as it only counts on local fluid rigid rotation while other methods count on stretching/compression and shearing as a part of the fluid rotation or vortex.

physics.flu-dyn↗

arXiv2020

Liutex-based Modified Navier-Stokes Equation

The Navier-Stokes (NS) partial differential equations, as the governing equation of fluid dynamics, are based on particles with zero volume. In such a condition, conservation law of moment of momentum is automatically satisfied, and thus NS equations only contain conservation of mass, momentum and energy. Because of the difficulty to get an analytical solution of NS equations, scientists develop finite element method (FEM) and finite volume method (FVM) to obtain approximate solutions. However, these methods are dependent on the size of finite volumes, which is not zero. Considering the size of the finite volume, conservation law of moment of momentum is no longer automatically satisfied and it is necessary to develop a new control equation to take effect of momentum into account. In this paper, new relations of reciprocal shear stresses are derived from conservation law of moment of momentum, and so are new constitutive relations and modified NS equations. The Liutex based NS equation may be the universal governing equations for both laminar and turbulent flow which is dominated by vortices and conservation of moment of momentum must be satisfied.

physics.flu-dyn↗

arXiv2019

Room-Temperature Electron-Hole Liquid in Monolayer MoS2

Excitons in semiconductors are usually non interacting and behave like an ideal gas, but may condense to a strongly correlated liquid like state, i.e. electron hole liquid (EHL), at high density and appropriate temperature. EHL is a macroscopic quantum state with exotic properties and represents the ultimate attainable charge excitation density in steady states. It bears great promise for a variety of fields such as ultrahigh power photonics and quantum science and technology. However, the condensation of gas like excitons to EHL has often been restricted to cryogenic temperatures, which significantly limits the prospect of EHL for use in practical applications. Herein we demonstrate the formation of EHL at room temperature in monolayer MoS2 by taking advantage of the monolayer's extraordinarily strong exciton binding energy. This work demonstrates the potential for the liquid like state of charge excitations to be a useful platform for the studies of macroscopic quantum phenomena and the development of optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2019

BERTSel: Answer Selection with Pre-trained Models

Recently, pre-trained models have been the dominant paradigm in natural language processing. They achieved remarkable state-of-the-art performance across a wide range of related tasks, such as textual entailment, natural language inference, question answering, etc. BERT, proposed by Devlin et.al., has achieved a better marked result in GLUE leaderboard with a deep transformer architecture. Despite its soaring popularity, however, BERT has not yet been applied to answer selection. This task is different from others with a few nuances: first, modeling the relevance and correctness of candidates matters compared to semantic relatedness and syntactic structure; second, the length of an answer may be different from other candidates and questions. In this paper. we are the first to explore the performance of fine-tuning BERT for answer selection. We achieved STOA results across five popular datasets, demonstrating the success of pre-trained models in this task.

cs.CL↗

arXiv2018

Immunity to Scaling in MoS2 Transistors Using Edge Contacts

Atomically thin two-dimensional (2D) materials are promising candidates for sub-10 nm transistor channels due to their ultrathin body thickness, which results in strong electrostatic gate control. Properly scaling a transistor technology requires reducing both the channel length (distance from source to drain) and the contact length (distance that source and drain interface with semiconducting channel). Contact length scaling remains an unresolved epidemic for transistor scaling, affecting devices from all semiconductors, from silicon to 2D materials. Here, we show that clean edge contacts to 2D MoS2 provide immunity to the contact-scaling problem, with performance that is independent of contact length down to the 20 nm regime. Using a directional ion beam, in situ edge contacts of various metal-MoS2 interfaces are studied. Characterization of the intricate edge interface using cross-sectional electron microscopy reveals distinct morphological effects on the MoS2 depending on its thickness, from monolayer to few-layer films. Chromium is found to outperform other metals in the edge contact scheme, which is attributed to the shorter Cr-MoS2 bond length. Compared to scaled top contacts with 20 nm contact length, in situ edge contacts yield better performance with an effective contact length of ~ 1 nm and 18 times higher carrier injection efficiency. The in situ edge contacts also exhibit ~8 times higher performance compared to the best-reported edge contacts. Our work provides experimental evidence for a solution to contact scaling in transistors, using 2D materials with clean edge contact interfaces, opening a new way of designing devices with 2D materials.

cond-mat.mtrl-sci↗

arXiv2017

Giant Gating Tunability of Optical Refractive Index in Transition Metal Dichalcogenide Monolayers

We report that the refractive index of transition metal dichacolgenide (TMDC) monolayers, such as MoS2, WS2, and WSe2, can be substantially tuned by > 60% in the imaginary part and > 20% in the real part around exciton resonances using CMOS-compatible electrical gating. This giant tunablility is rooted in the dominance of excitonic effects in the refractive index of the monolayers and the strong susceptibility of the excitons to the influence of injected charge carriers. The tunability mainly results from the effects of injected charge carriers to broaden the spectral width of excitonic interband transitions and to facilitate the interconversion of neutral and charged excitons. The other effects of the injected charge carriers, such as renormalizing bandgap and changing exciton binding energy, only play negligible roles. We also demonstrate that the atomically thin monolayers, when combined with photonic structures, can enable the efficiencies of optical absorption (reflection) tuned from 40% (60%) to 80% (20%) due to the giant tunability of refractive index. This work may pave the way towards the development of field-effect photonics in which the optical functionality can be controlled with CMOS circuits.

physics.optics↗

arXiv2017

Enhancing Multifunctionalities of Transition Metal Dichalcogenide Monolayers via Intercalation of Molecules and Ions

Transition metal dichalcogenide (TMDC) monolayers present a remarkable multifunctional material with potential to enable the development of a wide range of novel devices. However, the functionalities observed often fall short of the expectation, which hinders the device development. Here we demonstrate that the optical, catalytic, and thermal functionalities of TMDC monolayers can all be substantially enhanced by up to orders of magnitude with the intercalation of water molecules or small cations (H+ and Li+) between the monolayers and underlying substrates. In contrast, the same molecules or cations adsorbed on top of the monolayers show negligible effects. We also discover two major roles of the intercalated species in the enhancement: doping the monolayers and modifying the interaction of the monolayers with the substrate. The result points out a versatile and convenient strategy of using the intercalation of molecules or ions to enhance the functionalities of TMDC monolayers.

cond-mat.mtrl-sci↗

arXiv2016

In-Situ Monitoring of Thermal Annealing Induced Evolution in Film Morphology and Film-Substrate Bonding in a Monolayer MoS2 Film

We perform in-situ two-cycle thermal cycling and annealing studies for a transferred CVD-grown monolayer MoS2 on a SiO2/Si substrate, using spatially resolved micro-Raman and PL spectroscopy. After the thermal cycling and being annealed at 305 deg C twice, the film morphology and film-substrate bonding are significantly modified, which together with the removal of polymer residues cause major changes in the strain and doping distribution over the film, and thus the optical properties. Before annealing, the strain associated with ripples in the transferred film dominates the spatial distributions of the PL peak position and intensity over the film; after annealing, the variation in film-substrate bonding, affecting both strain and doping, becomes the leading factor. This work reveals that the film-substrate bonding, and thus the strain and doping, is unstable under thermal stress, which is important for understanding the substrate effects on the optical and transport properties of the 2D material and their impact on device applications.

cond-mat.mtrl-sci↗

arXiv2016

Fundamental Limits of Exciton-Exciton Annihilation for Light Emission in Transition Metal Dichalcogenide Monolayers

We quantitatively illustrate the fundamental limit that exciton-exciton annihilation (EEA) may impose to the light emission of monolayer transition metal dichalcogenide (TMDC) materials. The EEA in TMDC monolayers shows dependence on the interaction with substrates as its rate increases from 0.1 cm2/s (0.05 cm2/s) to 0.3 cm2/s (0.1 cm2/s) with the substrates removed for WS2 (MoS2) monolayers. It turns to be the major pathway of exciton decay and dominates the luminescence efficiency when the exciton density is beyond 1010 cm-2 in suspended monolayers or 1011 cm-2 in supported monolayers. This sets an upper limit on the density of injected charges in light emission devices for the realization of optimal luminescence efficiency. The strong EEA rate also dictates the pumping threshold for population inversion in the monolayers to be 12-18 MW/cm2 (optically) or 2.5-4x105 A/cm2 (electrically).

physics.optics↗

arXiv2015

Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films

We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5-7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5-7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.

cond-mat.mtrl-sci↗

arXiv2015

Efficient Interlayer Relaxation and Transition of Excitons in Epitaxial and Non-epitaxial MoS2/WS2 Heterostructures

Semiconductor heterostructures provide a powerful platform for the engineering of excitons. Here we report the excitonic properties of two-dimensional (2D) heterostructures that consist of monolayer MoS2 and WS2 stacked epitaxially or non-epitaxially in the vertical direction. We find similarly efficient interlayer relaxation and transition of excitons in both the epitaxial and nonepitaxial heterostructures. This is manifested by a two orders of magnitude decrease in the photoluminescence and the appearance of an extra absorption peak at low energy region. The MoS2/WS2 heterostructures show weak interlayer coupling and can essentially act as atomicscale heterojunctions with the intrinsic bandstructures of the two monolayers largely preserved. They are particularly promising for the applications that request efficient dissociation of excitons and strong light absorption, including photovoltaics, solar fuels, photodetectors, and optical modulators. Our results also indicate that 2D heterostructures promise unprecedented capabilities to engineer excitons from the atomic level without concerns of interfacial imperfection.

cond-mat.mtrl-sci↗

arXiv2014

Temperature Dependent Valley Relaxation Dynamics in Single Layer WS2 Measured Using Ultrafast Spectroscopy

We measured the lifetime of optically created valley polarization in single layer WS2 using transient absorption spectroscopy. The electron valley relaxation is very short (< 1ps). However the hole valley lifetime is at least two orders of magnitude longer and exhibits a temperature dependence that cannot be explained by single carrier spin/valley relaxation mechanisms. Our theoretical analysis suggests that a collective contribution of two potential processes may explain the valley relaxation in single layer WS2. One process involves direct scattering of excitons from K to K' valleys with a spin flip-flop interaction. The other mechanism involves scattering through spin degenerate Gamma valley. This second process is thermally activated with an Arrhenius behavior due to the energy barrier between Gamma and K valleys.

cond-mat.mtrl-sci↗

arXiv2013

Layer-dependent Electrocatalysis of MoS2 for Hydrogen Evolution

The quantitative correlation of the catalytic activity with microscopic structure of heterogeneous catalysts is a major challenge for the field of catalysis science. It requests synergistic capabilities to tailor the structure with atomic scale precision and to control the catalytic reaction to proceed through well-defined pathways. Here we leverage on the controlled growth of MoS2 atomically thin films to demonstrate that the catalytic activity of MoS2 for the hydrogen evolution reaction decreases by a factor of ~4.47 for the addition of every one more layer. Similar layer dependence is also found in edge-riched MoS2 pyramid platelets. This layer-dependent electrocatalysis can be correlated to the hopping of electrons in the vertical direction of MoS2 layers over an interlayer potential barrier, which is found to be 0.119V and consistent with theoretical calculations. Our results point out that increasing the hopping efficiency of electrons in the vertical direction is a key for the development of high-efficiency two-dimensional material catalysts.

cond-mat.mtrl-sci↗