arXiv subjects

Ming Hu

Publications and source records attributed to Ming Hu.

176 records · Page 10Linked to original sources

arXiv2018

The role of resonant bonding in governing the thermal transport properties of two-dimensional black phosphorus

Fundamental insight into lattice dynamics and phonon transport is critical to the efficient manipulation of heat flow, which is one of the appealing thermophysical problems with enormous practical implications. Phosphorene, a novel elemental two-dimensional (2D) semiconductor with high carrier mobility and intrinsically large direct band gap, possesses fascinating chemical and physical properties distinctively different from other 2D materials. The rapidly growing applications of phosphorene in nano-/opto-electronics and thermoelectrics call for fundamental understanding of the thermal transport properties. In this study, based on the analysis of electronic structure and lattice dynamics, we demonstrate the formation of resonant bonding in phosphorene. Fundamental insight into the thermal transport in phosphorene is provided by discussing the role of resonant bonding in driving long-range interactions and strong phonon anharmonicity. We reveal that the strong phonon anharmonicity is associated with the soft transverse optical (TO) phonon modes and arises from the long-range interactions driven by the orbital governed resonant bonding. Our study highlights the physical origin of the phonon anharmonicity in phosphorene, and also provides new insights into phonon transport from the view of orbital states, which would be of great significance to the design and development of high-performance phosphorene based nano-devices.

cond-mat.mtrl-sci↗

arXiv2017

Modeling Coefficient Alpha for Measurement of Individualized Test Score Internal Consistency

A method for measuring individualized reliability of several tests on subjects with heterogenecity is proposed. A regression model is developed based on three sets of generalized estimating equations (GEE). The first set of GEE models the expectation of the responses, the second set of GEE models the response's variance, and the third set is proposed to estimate the individualized coefficient alpha, defined and used to measure individualized internal consistency of the responses. We also extend our method to handle missing data in the covariates. Asymptotic property of the estimators is discussed, based on which interval estimation of the coefficient alpha and significance detection are derived. Performance of our method is evaluated through simulation study and real data analysis. The real data application is from a health literacy study in Hunan province of China.

stat.ME↗

arXiv2017

Probing the phonon surface interaction by wave packet simulation: effect of roughness and morphology

One way to reduce the lattice thermal conductivity of solids is to induce additional phonon surface scattering through nanostructures. However, how phonons interact with boundaries, especially at the atomic level, is not well understood. In this work, we performed two-dimensional atomistic wave packet simulations to investigate the phonon surface interaction. Emphasis has been given to the angular-resolved phonon reflection at smooth, periodically rough, and amorphous surfaces. We found that the acoustic phonon reflection at a smooth surface is not simply specular. Mode conversion can occur after reflection, and the detailed energy distribution after reflection will dependent on surface condition and polarization of incident phonon. At periodically rough surfaces, the reflected wave packet distribution does not follow the well-known Ziman's model, but shows a nonmonotonic dependence on the depth of surface roughness. When an amorphous layer is attached to the surface, the incident wave packet will be absorbed by the amorphous region, and results in quite diffusive reflection. Our results clearly show that the commonly used specular-diffusive model is not enough to describe the phonon reflection at a periodically rough surface, while an amorphous layer can induce strong diffusive reflection. This work provides a careful analysis of phonon reflection at a surface with different morphology, which is important to a better understanding of thermal transport in various nanostructures.

cond-mat.mes-hall↗

arXiv2017

DIMM-SC: A Dirichlet mixture model for clustering droplet-based single cell transcriptomic data

Motivation: Single cell transcriptome sequencing (scRNA-Seq) has become a revolutionary tool to study cellular and molecular processes at single cell resolution. Among existing technologies, the recently developed droplet-based platform enables efficient parallel processing of thousands of single cells with direct counting of transcript copies using Unique Molecular Identifier (UMI). Despite the technology advances, statistical methods and computational tools are still lacking for analyzing droplet-based scRNA-Seq data. Particularly, model-based approaches for clustering large-scale single cell transcriptomic data are still under-explored. Methods: We developed DIMM-SC, a Dirichlet Mixture Model for clustering droplet-based Single Cell transcriptomic data. This approach explicitly models UMI count data from scRNA-Seq experiments and characterizes variations across different cell clusters via a Dirichlet mixture prior. An expectation-maximization algorithm is used for parameter inference. Results: We performed comprehensive simulations to evaluate DIMM-SC and compared it with existing clustering methods such as K-means, CellTree and Seurat. In addition, we analyzed public scRNA-Seq datasets with known cluster labels and in-house scRNA-Seq datasets from a study of systemic sclerosis with prior biological knowledge to benchmark and validate DIMM-SC. Both simulation studies and real data applications demonstrated that overall, DIMM-SC achieves substantially improved clustering accuracy and much lower clustering variability compared to other existing clustering methods. More importantly, as a model-based approach, DIMM-SC is able to quantify the clustering uncertainty for each single cell, facilitating rigorous statistical inference and biological interpretations, which are typically unavailable from existing clustering methods.

stat.ML↗

arXiv2017

Decouple Electronic and Phononic Transport in Nanotwinned Structure: A New Strategy for Enhancing the Figure-of-merit of Thermoelectrics

Thermoelectrics (TE) materials manifest themselves in direct conversion of temperature differences to electric power and vice versa. Despite remarkable advances have been achieved in the past decades for various TE systems, the energy conversion efficiencies of TE devices, which is characterized by a dimensionless figure-of-merit (ZT ), remain a generally poor factor that severely limits their competitiveness and range of employment. The bottleneck for substantially boosting ZT coefficient lies in the strong interdependence of the physical parameters involved in electronic and phononic transport. Here, we propose a new strategy of incorporating nanotwinned structures to decouple the electronic and phononic transport. Combining the new concept of nanotwin with the previously widely used nanocrystalline approach, the power factor of the Si nanotwin-nanocrystalline heterostructures is enhanced by 120% compared to bulk crystalline Si, while the lattice thermal conductivity is reduced to a level well below the amorphous limit, yielding a theoretical limit of 0.43 for ZT coefficient at room temperature. This value is almost two orders of magnitude larger than that for bulk Si and twice of the polycrystalline Si. Even for the experimentally existing nanotwin-nanocrystalline heterostructures (e.g. grain size of 5 nm), the ZT coefficient can be as high as 0.2 at room temperature, which is the highest ZT value among all the Si based bulk nanostructures so far. Such substantial improvement stems from two aspects: (1) the improvement of the power factor is caused by the increase of Seebeck coefficient (degeneracy of the band valley) and the enhancement of electrical conductivity (the reduction of the effective band mass); (2) the significant reduction of the lattice thermal conductivity is mainly caused by the extremely strong phonon-grain boundary and phonon-twin boundary scattering.

cond-mat.mtrl-sci↗

arXiv2016

On the mechanism of hydrophilicity of graphene

It is generally accepted that the hydrophilic property of graphene can be affected by the underlying substrate. However, the role of intrinsic vs. substrate contributions and the related mechanisms are vividly debated. Here we show that the intrinsic hydrophilicity of graphene can be intimately connected to the position of its Fermi level, which affects the interaction between graphene and water molecules. The underlying substrate, or dopants, can tune hydrophilicity by modulating the Fermi level of graphene. By shifting the Fermi level of graphene away from its Dirac point, via either chemical or electrical voltage doping, we show enhanced hydrophilicity with experiments and first principle simulations. Increased vapor condensation on graphene, induced by a simple shifting of its Fermi level, exemplifies applications in the area of interfacial transport phenomena.

cond-mat.mtrl-sci↗

arXiv2016

Methodology for determining the electronic thermal conductivity of metals via direct non-equilibrium ab initio molecular dynamics

Many physical properties of metals can be understood in terms of the free electron model, as proven by the Wiedemann-Franz law. According to this model, electronic thermal conductivity ($κ_{el}$) can be inferred from the Boltzmann transport equation (BTE). However, the BTE does not perform well for some complex metals, such as Cu. Moreover, the BTE cannot clearly describe the origin of the thermal energy carried by electrons or how this energy is transported in metals. The charge distribution of conduction electrons in metals is known to reflect the electrostatic potential (EP) of the ion cores. Based on this premise, we develop a new methodology for evaluating $κ_{el}$ by combining the free electron model and non-equilibrium ab initio molecular dynamics (NEAIMD) simulations. We demonstrate that the kinetic energy of thermally excited electrons originates from the energy of the spatial electrostatic potential oscillation (EPO), which is induced by the thermal motion of ion cores. This method directly predicts the $κ_{el}$ of pure metals with a high degree of accuracy.

cond-mat.mtrl-sci↗

arXiv2016

Diverse anisotropy of phonon transport in two-dimensional IV-VI compounds: A comparative study

New classes two-dimensional (2D) materials beyond graphene, including layered and non-layered, and their heterostructures, are currently attracting increasing interest due to their promising applications in nanoelectronics, optoelectronics and clean energy, where thermal transport property is one of the fundamental physical parameters. In this paper, we systematically investigated the phonon transport properties of 2D orthorhombic group IV-VI compounds of $GeS$, $GeSe$, $SnS$ and $SnSe$ by solving the Boltzmann transport equation (BTE) based on first-principles calculations. Despite the similar puckered (hinge-like) structure along the armchair direction as phosphorene, the four monolayer compounds possess diverse anisotropic properties in many aspects, such as phonon group velocity, Young's modulus and lattice thermal conductivity ($κ$), etc. Especially, the $κ$ along the zigzag and armchair directions of monolayer $GeS$ shows the strongest anisotropy while monolayer $SnS$ and $SnSe$ shows an almost isotropy in phonon transport. The origin of the diverse anisotropy is fully studied and the underlying mechanism is discussed in detail. With limited size, the $κ$ could be effectively lowered, and the anisotropy could be effectively modulated by nanostructuring, which would extend the applications in nanoscale thermoelectrics and thermal management. Our study offers fundamental understanding of the anisotropic phonon transport properties of 2D materials, and would be of significance for further study, modulation and aplications in emerging technologies.

physics.comp-ph↗

arXiv2015

Unexpectedly Large Tunability of Lattice Thermal Conductivity of Monolayer Silicene via Mechanical Strain

Strain engineering is one of the most promising and effective routes toward continuously tuning the electronic and optic properties of materials, while thermal properties are generally believed to be insensitive to mechanical strain. In this paper, the strain-dependent thermal conductivity of monolayer silicene under uniform bi-axial tension is computed by solving the phonon Boltzmann transport equation with force constants extracted from first-principles calculations. Unlike the commonly believed understanding that thermal conductivity only slightly decreases with increased tensile strain for bulk materials, it is found that the thermal conductivity of silicene first increases dramatically with strain and then slightly decreases when the applied strain increases further. At a tensile strain of 4%, the highest thermal conductivity is found to be about 7.5 times that of unstrained one. Such an unusual strain dependence is mainly attributed to the dramatic enhancement in the acoustic phonon lifetime. Such enhancement plausibly originates from the flattening of the buckling of the silicene structure upon stretching, which is unique for silicene as compared with other common two-dimensional materials. Our findings offer perspectives of modulating the thermal properties of low-dimensional structures for applications such as thermoelectrics, thermal circuits, and nanoelectronics.

cond-mat.mes-hall↗

arXiv2015

Quantitatively Analyzing Phonon Spectral Contribution of Thermal Conductivity Based on Non-Equilibrium Molecular Dynamics Simulation I: From Space Fourier Transform

Probing detailed spectral dependence of phonon transport properties in bulk materials is critical to improve the function and performance of structures and devices in a diverse spectrum of technologies. Currently, such information can only be provided by the phonon spectral energy density (SED) or equivalently time domain normal mode analysis (TDNMA) methods in the framework of equilibrium molecular dynamics simulation (EMD), but has not been realized in non-equilibrium molecular dynamics simulations (NEMD) so far. In this paper we generate a new scheme directly based on NEMD and lattice dynamics theory, called time domain direct decomposition method (TDDDM), to predict the phonon mode specific thermal conductivity. Two benchmark cases of Lennard-Jones (LJ) Argon and Stillinger-Weber (SW) Si are studied by TDDDM to characterize contributions of individual phonon modes to overall thermal conductivity and the results are compared with that predicted using SED and TDNMA. Excellent agreements are found for both cases, which confirm the validity of our TDDDM approach. The biggest advantage of TDDDM is that it can be used to investigate the size effect of individual phonon modes in NEMD simulations, which cannot be tackled by SED and TDNMA in EMD simulations currently. We found that the phonon modes with mean free path larger than the system size are truncated in NEMD and contribute little to the overall thermal conductivity. The TDDDM provides direct physical origin for the well-known strong size effects in thermal conductivity prediction by NEMD.

cond-mat.mtrl-sci↗

arXiv2015

Quantitatively Analyzing Phonon Spectral Contribution of Thermal Conductivity Based on Non-Equilibrium Molecular Dynamics Simulation II: From Time Fourier Transform

From nano-scale heat transfer point of view, currently one of the most interesting and challenging tasks is to quantitatively analyzing phonon mode specific transport properties in solid materials, which plays vital role in many emerging and diverse technological applications. It has not been so long since such information can be provided by the phonon spectral energy density (SED) or equivalently time domain normal mode analysis (TDNMA) methods in the framework of equilibrium molecular dynamics simulation (EMD). However, until now it has not been realized in non-equilibrium molecular dynamics simulations (NEMD), the other widely used computational method for calculating thermal transport of materials in addition to EMD. In this work, a computational scheme based on time Fourier transform of atomistic heat current, called frequency domain direct decomposed method (FDDDM), is proposed to analyze the contributions of frequency dependent thermal conductivity in NEMD simulations. The FDDDM results of Lennard-Jones (LJ) Argon and Stillinger-Weber (SW) Si are compared with TDNMA method from EMD simulation. Similar trends are found for both cases, which confirm the validity of our FDDDM approach. Benefiting from the inherent nature of NEMD and the theoretical formula that does not require any temperature assumption, the FDDDM can be directly used to investigate the size and temperature effect. Moreover, the unique advantage of FDDDM prior to previous methods (such as SED and TDNMA) is that it can be straightforwardly used to characterize the phonon frequency dependent thermal conductivity of disordered systems, such as amorphous materials. The FDDDM approach can also be a good candidate to be used to understand the phonon behaviors and thus provides useful guidance for designing efficient structures for advanced thermal management.

cond-mat.mes-hall↗

arXiv2015

Anisotropic intrinsic lattice thermal conductivity of phosphorene from first principles

Phosphorene, the single layer counterpart of black phosphorus, is a novel two-dimensional semiconductor with high carrier mobility and a large fundamental direct band gap, which has attracted tremendous interest recently. Its potential applications in nano-electronics and thermoelectrics call for a fundamental study of the phonon transport. Here, we calculate the intrinsic lattice thermal conductivity of phosphorene by solving the phonon Boltzmann transport equation (BTE) based on first-principles calculations. The thermal conductivity of phosphorene at $300\,\mathrm{K}$ is $30.15\,\mathrm{Wm^{-1}K^{-1}}$ (zigzag) and $13.65\,\mathrm{Wm^{-1}K^{-1}}$ (armchair), showing an obvious anisotropy along different directions. The calculated thermal conductivity fits perfectly to the inverse relation with temperature when the temperature is higher than Debye temperature ($Θ_D = 278.66\,\mathrm{K}$). In comparison to graphene, the minor contribution around $5\%$ of the ZA mode is responsible for the low thermal conductivity of phosphorene. In addition, the representative mean free path (MFP), a critical size for phonon transport, is also obtained.

cond-mat.mtrl-sci↗

arXiv2014

Mechanics of nanoscale wrinkling of graphene on a non-developable spherical surface

The configuration of graphene (GE) sheet conforming to the spherical surface substrate is studied through theoretical model and molecular simulations. Two basic configurations are observed: fully conformation and wrinkling. The final configuration of the adsorbed GE results from the competition between two energy terms: the adhesion energy between GE and substrate, the strain energy stored in the GE due to the deformations. Here, we derive theoretical solutions by accounting for two energy terms, and predict the final morphology of GE on the spherical surface (a special kind of nano-developable curved surface) substrate with using the phase diagram. A critical cone angle of the absorbed GE for an arbitrary spherical surface substrate is obtained. Fully conformation of GE is observed when the cone angle of absorbed GE is below the critical value, otherwise wrinkles appear. Molecular simulations are implemented to verify the theoretical model with results agree well with theoretical predictions. Results from our present work can offer a guide for designing new functional graphene electronical devices (such as nanoswithes) and fabricating high quality nanostructured coating (Fig. 13).

cond-mat.mes-hall↗

arXiv2007

Testing power-law cosmology with galaxy clusters

Power-law cosmologies, in which the cosmological scale factor evolves as a power law in the age, $a \propto t^α$ with $α\ga 1$, regardless of the matter content or cosmological epoch, is comfortably concordant with a host of cosmological observations.} {In this article, we use recent measurements of the X-ray gas mass fractions in clusters of galaxies to constrain the $α$ parameter with curvature $k = \pm1, 0$. We find that the best fit happens for an open scenario with the power index $α= 1.14 \pm 0.05$, though the flat and closed model can not be rule out at very high confidence level.} {Our results are in agreement with other recent analyses and show that the X-ray gas mass fraction measurements in clusters of galaxies provide a complementary test to the power law cosmology.

astro-ph↗