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Xin Luo

Publications and source records attributed to Xin Luo.

122 records · Page 7Linked to original sources
arXiv2018

A multiple-try Metropolis-Hastings algorithm with tailored proposals

We present a new multiple-try Metropolis-Hastings algorithm designed to be especially beneficial when a tailored proposal distribution is available. The algorithm is based on a given acyclic graph $G$, where one of the nodes in $G$, $k$ say, contains the current state of the Markov chain and the remaining nodes contain proposed states generated by applying the tailored proposal distribution. The Metropolis-Hastings algorithm alternates between two types of updates. The first update type is using the tailored proposal distribution to generate new states in all nodes in $G$ except in node $k$. The second update type is generating a new value for $k$, thereby changing the value of the current state. We evaluate the effectiveness of the proposed scheme in an example with previously defined target and proposal distributions.

stat.CO↗
arXiv2017

Optimization of honeycomb battery package based on space mapping algorithm

A new honeycomb battery package structure is designed and optimized in this study. It is a honeycomb structure which uses grid to reinforce the strength. To obtain the highly accurate finite element (FE) model, the material parameters of 18650 cylindrical Li-ion battery are identified by using optimization techniques based on flat compression test data. Due to the expensive cost of finite element evaluation, the space mapping (SM) algorithm is suggested to optimize the structure of the package. Compared with other space mapping algorithms, the coarse model of space mapping in this work is based on a pseudo-plane-strain model. Moreover, to guarantee the reliability, the mean and variance values of battery stress are used to be the objective function. The final optimum solution is obtained in 3 days, and it shows the magnitude of stress and the distribution of stress are improved significantly compared with initial structure. Moreover, the computational cost of optimization for the problem is decreased greatly.

physics.app-ph↗
arXiv2017

Prior specification for binary Markov mesh models

We propose prior distributions for all parts of the specification of a Markov mesh model. In the formulation we define priors for the sequential neighborhood, for the parametric form of the conditional distributions and for the parameter values. By simulating from the resulting posterior distribution when conditioning on an observed scene, we thereby obtain an automatic model selection procedure for Markov mesh models. To sample from such a posterior distribution, we construct a reversible jump Markov chain Monte Carlo algorithm (RJMCMC). We demonstrate the usefulness of our prior formulation and the limitations of our RJMCMC algorithm in two examples.

stat.ME↗
arXiv2017

Up and down grover walks on simplicial complexes

A notion of up and down Grover walks on simplicial complexes are proposed and their properties are investigated. These are abstract Szegedy walks, which is a special kind of unitary operators on a Hilbert space. The operators introduced in the present paper are usual Grover walks on graphs defined by using combinatorial structures of simplicial complexes. But the shift operators are modified so that it can contain information of orientations of each simplex in the simplicial complex. It is well-known that the spectral structures of this kind of unitary operators are completely determined by its discriminant operators. It has strong relationship with combinatorial Laplacian on simplicial complexes and geometry, even topology, of simplicial complexes. In particular, theorems on a relation between spectrum of up and down discriminants and orientability, on a relation between symmetry of spectrum of discriminants and combinatorial structure of simplicial complex are given. Some examples, both of finite and infinite simplicial complexes, are also given. Finally, some aspects of finding probability and stationary measures are discussed.

math.SP↗
arXiv2015

Tuning the Threshold Voltage of MoS2 Field-Effect Transistors via Surface Treatment

Controlling the threshold voltage (Vth) of a field-effect transistor is important for realizing robust logic circuits. Here, we report a facile approach to achieve bidirectional Vth tuning of molybdenum disulfide (MoS2) field-effect transistors. By increasing and decreasing the amount of sulfur vacancies in the MoS2 surface, the Vth of MoS2 transistors can be left- and right-shifted, respectively. Transistors fabricated on perfect MoS2 flakes are found to exhibit two-fold enhancement in mobility and a very positive Vth. More importantly, our elegant hydrogen treatment is able to tune the large Vth to a small value without any performance degradation simply by reducing the atomic ratio of S:Mo slightly; in other words, creating a certain amount of sulfur vacancies in the MoS2 surface, which generate defect states in the band gap of MoS2 that mediate conduction of a MoS2 transistor in the subthreshold regime. First-principles calculations further indicate that the edge and width of defect band can be tuned according to the vacancy density. This work not only demonstrates for the first time the ease in tuning the Vth of MoS2 transistors, but also offers a process technology solution that is critical for further development of MoS2 as a mainstream electronic material.

cond-mat.mes-hall↗
arXiv2015

Large Frequency Change with Thickness in Interlayer Breathing Mode - Significant Interlayer Interactions in Few Layer Black Phosphorus

Bulk black phosphorus (BP) consists of puckered layers of phosphorus atoms. Few-layer BP, obtained from bulk BP by exfoliation, is an emerging candidate as a channel material in post-silicon electronics. A deep understanding of its physical properties and its full range of applications are still being uncovered. In this paper, we present a theoretical and experimental investigation of phonon properties in few-layer BP, focusing on the low-frequency regime corresponding to interlayer vibrational modes. We show that the interlayer breathing mode A3g shows a large redshift with increasing thickness; the experimental and theoretical results agreeing well. This thickness dependence is two times larger than that in the chalcogenide materials such as few-layer MoS2 and WSe2, because of the significantly larger interlayer force constant and smaller atomic mass in BP. The derived interlayer out-of-plane force constant is about 50% larger than that in graphene and MoS2. We show that this large interlayer force constant arises from the sizable covalent interaction between phosphorus atoms in adjacent layers, and that interlayer interactions are not merely of the weak van der Waals type. These significant interlayer interactions are consistent with the known surface reactivity of BP, and have been shown to be important for electric-field induced formation of Dirac cones in thin film BP.

cond-mat.mtrl-sci↗
arXiv2015

Stacking sequence determines Raman intensities of observed interlayer shear modes in 2D layered materials - A general bond polarizability model

2D layered materials have recently attracted tremendous interest due to their fascinating properties and potential applications. The interlayer interactions are much weaker than the intralayer bonds, allowing the as-synthesized materials to exhibit different stacking sequences (e.g. ABAB, ABCABC), leading to different physical properties. Here, we show that regardless of the space group of the 2D material, the Raman frequencies of the interlayer shear modes observed under the typical configuration blue shift for AB stacked materials, and red shift for ABC stacked materials, as the number of layers increases. Our predictions are made using an intuitive bond polarizability model which shows that stacking sequence plays a key role in determining which interlayer shear modes lead to the largest change in polarizability (Raman intensity); the modes with the largest Raman intensity determining the frequency trends. We present direct evidence for these conclusions by studying the Raman modes in few layer graphene, MoS2, MoSe2, WSe2 and Bi2Se3, using both first principles calculations and Raman spectroscopy. This study sheds light on the influence of stacking sequence on the Raman intensities of intrinsic interlayer modes in 2D layered materials in general, and leads to a practical way of identifying the stacking sequence in these materials.

cond-mat.mtrl-sci↗
arXiv2015

On the Hyperbolizing metric spaces

In this paper, we prove that the metric space $(Z\setminus M,u_Z)$ defined by Z.Ibragimov is asymptotically $PT_{-1}$ if the metric space $(Z,d)$ is $PT_{0}$, where $M$ is a nonempty closed proper subset of $Z$. Secondly, based on the metric $u_Z$, we define a new kind of metric $k_{z}$ on the set $Z\setminus M$ and show that the new metric space $(Z\setminus M,k_{Z})$ is also asymptotically $PT_{-1}$ without the assumption of $PT_{0}$ on the metric space $(Z,d)$.

math.MG↗
arXiv2014

Low Resistance Metal Contacts to MoS2 Devices with Nickel-Etched-Graphene Electrodes

We report an approach to achieve low-resistance contacts to MoS2 transistors with the intrinsic performance of the MoS2 channel preserved. Through a dry transfer technique and a metal-catalyzed graphene treatment process, nickel-etched-graphene electrodes were fabricated on MoS2 that yield contact resistance as low as 200 ohm-um. The substantial contact enhancement (~2 orders of magnitude) as compared to pure nickel electrodes, is attributed to the much smaller work function of nickel-graphene electrodes, together with the fact that presence of zigzag edges in the treated graphene surface enhances tunneling between nickel and graphene. To this end, the successful fabrication of a clean graphene-MoS2 interface and a low resistance nickel-graphene interface is critical for the experimentally measured low contact resistance. The potential of using graphene as an electrode interlayer demonstrated in this work paves the way towards achieving high performance next-generation transistors.

cond-mat.mes-hall↗
arXiv2014

Effects of Lower Symmetry and Dimensionality on Raman Spectra in 2D WSe2

We report the observation and interpretation of new Raman peaks in few-layer tungsten diselenide (WSe2), induced by the reduction of symmetry going from 3D to 2D. In general, Raman frequencies in 2D materials follow quite closely the frequencies of corresponding eigenmodes in the bulk. However, while the modes that are Raman active in the bulk are also Raman active in the thin films, the reverse is not always true due to the reduced symmetry in thin films. Here, we predict from group theory and density functional calculations that two intra-layer vibrational modes which are Raman inactive in bulk WSe2 in our experimental configuration become Raman active in thin film WSe2, due to reduced symmetry in thin films. This phenomenon explains the Raman peaks we observe experimentally at ~310 cm-1 and 176 cm-1 in thin film WSe2. Interestingly, the bulk mode at ~310 cm-1 that is Raman inactive can in fact be detected in Raman measurements under specific wavelengths of irradiation, suggesting that in this case, crystal symmetry selection rules may be broken due to resonant scattering. Both theory and experiment indicate that the and modes blue-shift with decreasing thickness, which we attribute to surface effects. Our results shed light on a general understanding of the Raman/IR activities of the phonon modes in layered transition metal dichalcogenide materials and their evolution behavior from 3D to 2D.

cond-mat.mtrl-sci↗
arXiv2013

Anomalous Frequency Trends in MoS2 Thin Films Attributed to Surface Effects

The layered dichalcogenide MoS2 has many unique physical properties in low dimensions. Recent experimental Raman spectroscopies report an anomalous blue shift of the in-plane E2g1 mode with decreasing thickness, a trend that is not understood. Here, we combine experimental Raman scattering and theoretical studies to clarify and explain this trend. Special attention is given to understanding the surface effect on Raman frequencies by using a force constants model based on first-principles calculations. Surface effects refer to the larger Mo-S force constants at the surface of thin film MoS2, which results from a loss of neighbours in adjacent MoS2 layers. Without surface effects, the frequencies of both out-of-plane A1g and in-plane E2g1 modes decrease with decreasing thickness. However, the E2g1 mode blue shifts while the A1g mode red shifts once the surface effect is included, in agreement with the experiment. Our results show that competition between the thickness effect and the surface effect determines the mechanical properties of two-dimensional MoS2, which we believe applies to other layered materials.

cond-mat.mtrl-sci↗
arXiv2013

First Principles Investigations of the Atomic, Electronic, and Thermoelectric Properties of Equilibrium and Strained Bi2Se3 & Bi2Te3, with van der Waals Interactions

Bi2Se3 and Bi2Te3 are layered compounds of technological importance, being excellent thermoelectric materials as well as topological insulators. We report density functional theory calculations of the atomic, electronic and thermoelectric properties of strained bulk and thin film Bi2Se3 and Bi2Te3, focusing on an appropriate description of van der Waals interactions. The calculations show that the van der Waals Density functional with Cooper's exchange can reproduce closely the experimental interlayer distances in unstrained Bi2Se3 and Bi2Te3. Interestingly, we predict atomic structures that are in much better agreement with the experimentally determined structure from Nakajima than that obtained from Wyckoff, especially for Bi2Se3 where the difference in atomic structures qualitatively changes the electronic band structure. The band structure obtained using the Nakajima structure, and the theoretically optimized structure, are in much better agreement with previous reports of photoemission measurements, than that obtained using the Wyckoff structure. Fully optimizing atomic structures of bulk and thin film Bi2Se3 and Bi2Te3 under different in-plane and uniaxial strains, we predict that the electronic band gap of both the bulk materials and thin films decreases with tensile in-plane strain and increases with compressive in-plane strain. We also predict, using the semiclassical Boltzmann approach, that the magnitude of the n-type Seebeck coefficient of Bi2Te3 can be increased by the compressive in-plane strain, while that of Bi2Se3 can be increased with tensile in-plane strain. Further, the in-plane power factor of n-doped Bi2Se3 can be increased with compressive uniaxial strain, while that of n-doped Bi2Te3 can be increased by compressive in-plane strain. ...

cond-mat.mtrl-sci↗
arXiv2013

Interlayer breathing and shear modes in few-trilayer MoS2 and WSe2

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have recently attracted tremendous interest as potential valleytronic and nano-electronic materials, in addition to being well-known as excellent lubricants in the bulk. The interlayer van der Waals (vdW) coupling and low frequency phonon modes, and how they evolve with the number of layers, are important for both the mechanical and electrical properties of 2D TMDs. Here we uncover the ultra-low frequency interlayer breathing and shear modes in few-layer MoS2 and WSe2, prototypical layered TMDs, using both Raman spectroscopy and first principles calculations. Remarkably, the frequencies of these modes can be perfectly described using a simple linear chain model with only nearest-neighbour interactions. We show that the derived in-plane (shear) and out-of-plane (breathing) force constants from experiment remain the same from two-layer 2D crystals to the bulk materials, suggesting that the nanoscale interlayer frictional characteristics of these excellent lubricants should be independent of the number of layers.

cond-mat.mtrl-sci↗