Showing papers by "Tsinghua University published in 2006"

Some geometric aggregation operators based on intuitionistic fuzzy sets

Abstract: The weighted geometric (WG) operator and the ordered weighted geometric (OWG) operator are two common aggregation operators in the field of information fusion. But these two aggregation operators are usually used in situations where the given arguments are expressed as crisp numbers or linguistic values. In this paper, we develop some new geometric aggregation operators, such as the intuitionistic fuzzy weighted geometric (IFWG) operator, the intuitionistic fuzzy ordered weighted geometric (IFOWG) operator, and the intuitionistic fuzzy hybrid geometric (IFHG) operator, which extend the WG and OWG operators to accommodate the environment in which the given arguments are intuitionistic fuzzy sets which are characterized by a membership function and a non-membership function. Some numerical examples are given to illustrate the developed operators. Finally, we give an application of the IFHG operator to multiple attribute decision making based on intuitionistic fuzzy sets.

Nearly Monodisperse Cu2O and CuO Nanospheres: Preparation and Applications for Sensitive Gas Sensors

Abstract: Nearly monodisperse single-crystalline Cu2O nanospheres were prepared by an effective solution method with NaBH4 as the reducing agent. Sensors based on them have good sensitivity to some flammable gases because thin films formed by nanospheres have capacious interspaces and enough spherical surfaces to interact with gases (ppm level).

Size dependence of Young's modulus in ZnO nanowires.

Abstract: We report a size dependence of Young's modulus in [0001] oriented ZnO nanowires (NWs) with diameters ranging from 17 to 550 nm for the first time. The measured modulus for NWs with diameters smaller than about 120 nm is increasing dramatically with the decreasing diameters, and is significantly higher than that of the larger ones whose modulus tends to that of bulk ZnO. A core-shell composite NW model in terms of the surface stiffening effect correlated with significant bond length contractions occurred near the {1010} free surfaces (which extend several layers deep into the bulk and fade off slowly) is proposed to explore the origin of the size dependence, and present experimental result is well explained. Furthermore, it is possible to estimate the size-related elastic properties of GaN nanotubes and relative nanostructures by using this model.

Movie review mining and summarization

Abstract: With the flourish of the Web, online review is becoming a more and more useful and important information resource for people. As a result, automatic review mining and summarization has become a hot research topic recently. Different from traditional text summarization, review mining and summarization aims at extracting the features on which the reviewers express their opinions and determining whether the opinions are positive or negative. In this paper, we focus on a specific domain - movie review. A multi-knowledge based approach is proposed, which integrates WordNet, statistical analysis and movie knowledge. The experimental results show the effectiveness of the proposed approach in movie review mining and summarization.

Frequency Recognition Based on Canonical Correlation Analysis for SSVEP-Based BCIs

Abstract: Canonical correlation analysis (CCA) is applied to analyze the frequency components of steady-state visual evoked potentials (SSVEP) in electroencephalogram (EEG). The essence of this method is to extract a narrowband frequency component of SSVEP in EEG. A recognition approach is proposed based on the extracted frequency features for an SSVEP-based brain computer interface (BCI). Recognition Results of the approach were higher than those using a widely used fast Fourier transform (FFT)-based spectrum estimation method

Understanding user behavior in large-scale video-on-demand systems

Abstract: Video-on-demand over IP (VOD) is one of the best-known examples of "next-generation" Internet applications cited as a goal by networking and multimedia researchers. Without empirical data, researchers have generally relied on simulated models to drive their design and developmental efforts. In this paper, we present one of the first measurement studies of a large VOD system, using data covering 219 days and more than 150,000 users in a VOD system deployed by China Telecom. Our study focuses on user behavior, content access patterns, and their implications on the design of multimedia streaming systems. Our results also show that when used to model the user-arrival rate, the traditional Poisson model is conservative and overestimates the probability of large arrival groups. We introduce a modified Poisson distribution that more accurately models our observations. We also observe a surprising result, that video session lengths has a weak inverse correlation with the video's popularity. Finally, we gain better understanding of the sources of video popularity through analysis of a number of internal and external factors.

MiRNA-Directed Regulation of VEGF and Other Angiogenic Factors under Hypoxia

Abstract: MicroRNAs (miRNAs) are a class of 20–24 nt non-coding RNAs that regulate gene expression primarily through post-transcriptional repression or mRNA degradation in a sequence-specific manner. The roles of miRNAs are just beginning to be understood, but the study of miRNA function has been limited by poor understanding of the general principles of gene regulation by miRNAs. Here we used CNE cells from a human nasopharyngeal carcinoma cell line as a cellular system to investigate miRNA-directed regulation of VEGF and other angiogenic factors under hypoxia, and to explore the principles of gene regulation by miRNAs. Through computational analysis, 96 miRNAs were predicted as putative regulators of VEGF. But when we analyzed the miRNA expression profile of CNE and four other VEGF-expressing cell lines, we found that only some of these miRNAs could be involved in VEGF regulation, and that VEGF may be regulated by different miRNAs that were differentially chosen from 96 putative regulatory miRNAs of VEGF in different cells. Some of these miRNAs also co-regulate other angiogenic factors (differential regulation and co-regulation principle). We also found that VEGF was regulated by multiple miRNAs using different combinations, including both coordinate and competitive interactions. The coordinate principle states that miRNAs with independent binding sites in a gene can produce coordinate action to increase the repressive effect of miRNAs on this gene. By contrast, the competitive principle states when multiple miRNAs compete with each other for a common binding site, or when a functional miRNA competes with a false positive miRNA for the same binding site, the repressive effects of miRNAs may be decreased. Through the competitive principle, false positive miRNAs, which cannot directly repress gene expression, can sometimes play a role in miRNA-mediated gene regulation. The competitive principle, differential regulation, multi-miRNA binding sites, and false positive miRNAs might be useful strategies in the avoidance of unwanted cross-action among genes targeted by miRNAs with multiple targets.

A bayesian network approach to traffic flow forecasting

Abstract: A new approach based on Bayesian networks for traffic flow forecasting is proposed. In this paper, traffic flows among adjacent road links in a transportation network are modeled as a Bayesian network. The joint probability distribution between the cause nodes (data utilized for forecasting) and the effect node (data to be forecasted) in a constructed Bayesian network is described as a Gaussian mixture model (GMM) whose parameters are estimated via the competitive expectation maximization (CEM) algorithm. Finally, traffic flow forecasting is performed under the criterion of minimum mean square error (mmse). The approach departs from many existing traffic flow forecasting models in that it explicitly includes information from adjacent road links to analyze the trends of the current link statistically. Furthermore, it also encompasses the issue of traffic flow forecasting when incomplete data exist. Comprehensive experiments on urban vehicular traffic flow data of Beijing and comparisons with several other methods show that the Bayesian network is a very promising and effective approach for traffic flow modeling and forecasting, both for complete data and incomplete data

Fabrication of Single-Crystalline Silicon Nanowires by Scratching a Silicon Surface with Catalytic Metal Particles†

Abstract: A novel strategy for preparing large-area, oriented silicon nanowire (SiNW) arrays on silicon substrates at near room temperature by localized chemical etching is presented. The strategy is based on metal-induced (either by Ag or Au) excessive local oxidation and dissolution of a silicon substrate in an aqueous fluoride solution. The density and size of the as-prepared SiNWs depend on the distribution of the patterned metal particles on the silicon surface. High-density metal particles facilitate the formation of silicon nanowires. Well-separated, straight nanoholes are dug along the Si block when metal particles are well dispersed with a large space between them. The etching technique is weakly dependent on the orientation and doping type of the silicon wafer. Therefore, SiNWs with desired axial crystallographic orientations and doping characteristics are readily obtained. Detailed scanning electron microscopy observations reveal the formation process of the silicon nanowires, and a reasonable mechanism is proposed on the basis of the electrochemistry of silicon and the experimental results.

Settling the Complexity of Two-Player Nash Equilibrium

Abstract: Even though many people thought the problem of finding Nash equilibria is hard in general, and it has been proven so for games among three or more players recently, it's not clear whether the two-player case can be shown in the same class of PPAD-complete problems. We prove that the problem of finding a Nash equilibrium in a two-player game is PPAD-complete

Thickness of graphene and single-wall carbon nanotubes

Abstract: Young's modulus and the thickness of single wall carbon nanotubes (CNTs) obtained from prior atomistic studies are largely scattered. In this paper we establish an analytic approach to bypass atomistic simulations and determine the tension and bending rigidities of graphene and CNTs directly from the interatomic potential. The thickness and elastic properties of graphene and CNTs can also be obtained from the interatomic potential. But the thickness, and therefore elastic moduli, also depend on type of loading (e.g., uniaxial tension, uniaxial stretching, equibiaxial stretching), as well as the nanotube radius $R$ and chirality when $Rl1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. This explains why the thickness obtained from prior atomistic simulations is scattered. This analytic approach is particularly useful in the study of multiwall CNTs since their stress state may be complex even under simple loading (e.g., uniaxial tension) due to the van der Waals interactions between nanotube walls. The present analysis also provides an explanation of Yakobson's paradox that the very high Young's modulus reported from the atomistic simulations together with the shell model may be due to the not-well-defined CNT thickness.

Confidence Intervals for Diffusion Index Forecasts and Inference for Factor-Augmented Regressions

Abstract: We consider the situation when there is a large number of series, N ,e ach withT observations, and each series has some predictive ability for some variable of interest. A methodology of growing interest is first to estimate common factors from the panel of data by the method of principal components and then to augment an otherwise standard regression with the estimated factors. In this paper, we show that the least squares estimates obtained from these factor-augmented regressions are √ T consistent and asymptotically normal if √ T/ N→ 0. The conditional mean predicted by the estimated factors is min[ √ T� √ N ] consistent and asymptotically normal. Except when T/N goes to zero, inference should take into account the effect of “estimated regressors” on the estimated conditional mean. We present analytical formulas for prediction intervals that are valid regardless of the magnitude of N/T and that can also be used when the factors are nonstationary.

Spinning and processing continuous yarns from 4-inch wafer scale super-aligned carbon nanotube arrays

Abstract: Carbon nanotubes (CNTs) are an extremely interesting type of material due to their unique 1D structure, and their excellent mechanical, thermal, and electrical properties. Many promising applications have been demonstrated. To exploit their excellent physical properties at a macroscopic level, it is desirable to create CNTs with macroscopic length. However, it has been very challenging to grow arbitrarily long CNTs. An alternative approach is to create long nanotube structures with many of them aligned into continuous yarns or ropes. In 2002, a breakthrough was made by our group to fabricate pure CNT yarns by directly drawing CNTs from super-aligned CNT arrays. Following that, Zhang et al. developed a draw– twisting spin method and demonstrated more interesting applications of the raw yarns. Since CNTs in the yarn are nearly parallel aligned, the CNT yarn is intrinsically an anisotropic material and has a special axis along the drawing direction, which demonstrates many fascinating properties and applications such as filaments for light bulbs, polarizers working in the UV region, thermal-field emitters, polarized-light emitters, transparent conducting membranes, etc. As these yarns are macroscopic objects, there is no doubt that they will be sought after for more and more applications as time goes on. However, to achieve real applications in the industry, some key issues have to be solved in advance. Currently many groups have achieved the growth of CNT arrays, but of these only two groups have reported spinning yarns from their arrays. The first question is why our CNT arrays can give rise to CNT yarn while others cannot? What is the critical factor that determines the ability for yarn formation? In our first paper and the recent paper of Zhang et al., the reported syntheses were carried out at atmospheric pressure (AP) in a tube furnace with diameters of 1–2 in. Can this synthesis be expanded to a larger scale at low pressure (LP) for commercial chemical vapor deposition (CVD) systems? The third problem is that, even though many applications have been demonstrated, the directly drawn-out yarns are very sticky due to their clean surfaces and extremely high surfaceto-volume ratio. It can easily to stick to the surfaces of other objects and can never be separated again, which greatly inhibits the real application of CNT yarns. Here, we show how these crucial problems were tackled by our group in the past two years. CNT arrays (or termed “CNT forests”) represent high-quality and highly ordered CNT structures, in which CNTs are nearly parallel aligned and perpendicular to the substrate. To date, a lot of groups have successfully synthesized CNT arrays since the initial papers were published. However, only two groups have reported that CNT yarns can be drawn from their arrays, which were defined as super-aligned arrays by us. We have tried to pull yarns from CNT arrays derived by a previously published method, which is denoted as a normal array, no yarns could be drawn out even though the height of the CNT array was above 1 mm. Now the question is what the difference is between normal and super-aligned arrays? Zhang et al. claimed that the formation of yarn was due to the disordered region at the top and bottom of the CNT array, which entangled together forming a loop. However we cannot agree with them based on the following points. First, in the normal array, the top and bottom part are more disordered and entangled than in the super-aligned array. Second, in our super-aligned array, the bottom part is highly ordered (see supporting information, Fig. S1) and without entanglement. Therefore, the disordered entanglement at the top may help the formation of yarn, but it cannot be the key factor for yarn formation. To elucidate the difference between normal and superaligned arrays, we performed comparative studies. Figure 1A is the transmission electron microscopy (TEM) image of C O M M U N IC A IO N S

Topological quantization of the spin Hall effect in two-dimensional paramagnetic semiconductors

Abstract: We propose models of two-dimensional paramagnetic semiconductors where the intrinsic spin Hall effect is exactly quantized in integer units of a topological charge. The model describes a topological insulator in the bulk and a ``holographic metal'' at the edge, where the number of extended edge states crossing the Fermi level is dictated by (exactly equal to) the bulk topological charge. We also demonstrate the spin Hall effect explicitly in terms of the spin accumulation caused by the adiabatic flux insertion.

A Hybrid Supercapacitor Fabricated with a Carbon Nanotube Cathode and a TiO2–B Nanowire Anode

Abstract: Recently, a new hybrid supercapacitor, integrating both the advantages of supercapacitors and lithium-ion batteries, was proposed and rapidly turned into state-of-the-art energy-storage devices with a high energy density, fast power capability, and a long cycle life. In this paper, a new hybrid supercapacitor is fabricated by making use of the benefits of 1D nanomaterials consisting of a carbon nanotube (CNT) cathode and a TiO2–B nanowire (TNW) anode, and the preliminary results for such an energy-storage device operating over a wide voltage range (0–2.8 V) are presented. The CNT–TNW supercapacitor is compared to a CNT–CNT supercapacitor, and discussed with regards to available energy densities, power capabilities, voltage profiles, and cycle life. On the basis of the total weight of both active materials, the CNT–TNW supercapacitor delivers an energy density of 12.5 W h kg–1 at a rate of 10 C, double the value of the CNT–CNT supercapacitor, while maintaining desirable cycling stability. The combination of a CNT cathode and a TNW anode in a non-aqueous electrolyte is proven to be suitable for high-performance hybrid supercapacitor applications; this can reasonably be assigned to the interesting synergistic effects of the two nanomaterials. It is hoped that the results presented in this study might renew interest in the design of nanomaterials that are applicable not only to hybrid supercapacitors, but also to energy conversion and storage applications of the future.

Utilization and fairness in spectrum assignment for opportunistic spectrum access

Abstract: The Open Spectrum approach to spectrum access can achieve near-optimal utilization by allowing devices to sense and utilize available spectrum opportunistically. However, a naive distributed spectrum assignment can lead to significant interference between devices. In this paper, we define a general framework that defines the spectrum access problem for several definitions of overall system utility. By reducing the allocation problem to a variant of the graph coloring problem, we show that the global optimization problem is NP-hard, and provide a general approximation methodology through vertex labeling. We examine both a centralized strategy, where a central server calculates an allocation assignment based on global knowledge, and a distributed approach, where devices collaborate to negotiate local channel assignments towards global optimization. Our experimental results show that our allocation algorithms can dramatically reduce interference and improve throughput (as much as 12-fold). Further simulations show that our distributed algorithms generate allocation assignments similar in quality to our centralized algorithms using global knowledge, while incurring substantially less computational complexity in the process.

Surface effects on elastic properties of silver nanowires: Contact atomic-force microscopy

Abstract: Silver nanowires with different diameters were synthesized by a hydrothermal chemical method. The elastic properties of the nanowires with outer diameters ranging from 20 to $140\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ were measured using contact atomic force microscopy. The apparent Young modulus of the nanowires is found to decrease with the increase of the diameter. When the diameter of the silver nanowires is larger than $100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the Young modulus approaches a constant value. The size dependence of the apparent Young modulus of the silver nanowires is attributed to the surface effect, which includes the effects of the surface stress, the oxidation layer, and the surface roughness. Thus, a theoretical analysis is presented to explain the size dependence. This analysis is different from the previous models in that both the surface stress and the surface moduli are included in it. We also show that the apparent surface modulus and the surface stress of the silver nanowires can be experimentally determined.

Adhesion and friction in gecko toe attachment and detachment.

Abstract: Geckos can run rapidly on walls and ceilings, requiring high friction forces (on walls) and adhesion forces (on ceilings), with typical step intervals of ≈20 ms. The rapid switching between gecko foot attachment and detachment is analyzed theoretically based on a tape model that incorporates the adhesion and friction forces originating from the van der Waals forces between the submicron-sized spatulae and the substrate, which are controlled by the (macroscopic) actions of the gecko toes. The pulling force of a spatula along its shaft with an angle θ between 0 and 90° to the substrate, has a “normal adhesion force” contribution, produced at the spatula-substrate bifurcation zone, and a “lateral friction force” contribution from the part of spatula still in contact with the substrate. High net friction and adhesion forces on the whole gecko are obtained by rolling down and gripping the toes inward to realize small pulling angles θ between the large number of spatulae in contact with the substrate. To detach, the high adhesion/friction is rapidly reduced to a very low value by rolling the toes upward and backward, which, mediated by the lever function of the setal shaft, peels the spatulae off perpendicularly from the substrates. By these mechanisms, both the adhesion and friction forces of geckos can be changed over three orders of magnitude, allowing for the swift attachment and detachment during gecko motion. The results have obvious implications for the fabrication of dry adhesives and robotic systems inspired by the gecko's locomotion mechanism.

A practical VEP-based brain-computer interface

Abstract: This paper introduces the development of a practical brain-computer interface at Tsinghua University. The system uses frequency-coded steady-state visual evoked potentials to determine the gaze direction of the user. To ensure more universal applicability of the system, approaches for reducing user variation on system performance have been proposed. The information transfer rate (ITR) has been evaluated both in the laboratory and at the Rehabilitation Center of China, respectively. The system has been proved to be applicable to >90% of people with a high ITR in living environments.

A Cross-national study of subjective sexual well-being among older women and men: findings from the global study of sexual attitudes and behaviors

Abstract: Subjective sexual well-being refers to the cognitive and emotional evaluation of an individual's sexuality. This study examined subjective sexual well-being, explored its various aspects, examined predictors across different cultures, and investigated its possible associations with overall happiness and selected correlates, including sexual dysfunction. Data were drawn from the Global Study of Sexual Attitudes and Behaviors, a survey of 27,500 men and women aged 40-80 years in 29 countries. The cross-national variation of four aspects of sexual well-being (the emotional and physical satisfaction of sexual relationships, satisfaction with sexual health or function, and the importance of sex in one's life) was explored using cluster analysis, and relationships among sexual well-being, general happiness, and various correlates were examined using ordinary least squares regression and ordered logistic regression. Results from the cluster analysis identified three clusters: a gender-equal regime and two male-centered regimes. Despite this cultural variation, the predictors of subjective sexual well-being were found to be largely consistent across world regions.

Search for neutral MSSM Higgs bosons at LEP

Abstract: The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for the neutral Higgs bosons which are predicted by the Minimal Supersymmetric standard model (MSSM). The data of the four collaborations are statistically combined and examined for their consistency with the background hypothesis and with a possible Higgs boson signal. The combined LEP data show no significant excess of events which would indicate the production of Higgs bosons. The search results are used to set upper bounds on the cross-sections of various Higgs-like event topologies. The results are interpreted within the MSSM in a number of “benchmark” models, including CP-conserving and CP-violating scenarios. These interpretations lead in all cases to large exclusions in the MSSM parameter space. Absolute limits are set on the parameter cosβ and, in some scenarios, on the masses of neutral Higgs bosons.

Quantum spin-Hall effect and topologically invariant Chern numbers.

Abstract: We present a topological description of the quantum spin-Hall effect (QSHE) in a two-dimensional electron system on a honeycomb lattice with both intrinsic and Rashba spin-orbit couplings. We show that the topology of the band insulator can be characterized by a 2 x 2 matrix of first Chern integers. The nontrivial QSHE phase is identified by the nonzero diagonal matrix elements of the Chern number matrix (CNM). A spin Chern number is derived from the CNM, which is conserved in the presence of finite disorder scattering and spin nonconserving Rashba coupling. By using the Laughlin gedanken experiment, we numerically calculate the spin polarization and spin transfer rate of the conducting edge states and determine a phase diagram for the QSHE.

Performance of sub-pixel registration algorithms in digital image correlation

Abstract: Developments in digital image correlation in the last two decades have made it a popular and effective tool for full-field displacement and strain measurements in experimental mechanics In digital image correlation, the use of the sub-pixel registration algorithm is regarded as the key technique to improve accuracy Different types of sub-pixel registration algorithms have been developed However, little quantitative research has been carried out to compare their performances This paper investigates three types of the most commonly used sub-pixel displacement registration algorithms in terms of the registration accuracy and the computational efficiency using computer-simulated speckle images A detailed examination of the performances of each algorithm reveals that the iterative spatial domain cross-correlation algorithm (Newton–Raphson method) is more accurate, but much slower than other algorithms, and is recommended for use in these applications

One-pot synthesis and bioapplication of amine-functionalized magnetite nanoparticles and hollow nanospheres

Abstract: To demonstrate their applications in biological and medical fields such as in immunoassays, magnetic separation of cells or proteins, drug or gene delivery, and magnetic resonance imaging, the template-free syntheses of water-soluble and surface functionalized magnetic nanomaterials have become essential and are challenging. Herein, we developed a facile one-pot template-free method for the preparation of amine-functionalized magnetite nanoparticles and hollow nanospheres by using FeCl(3)6 H(2)O as single iron source. These magnetic nanomaterials were characterized by TEM, SEM, XRD, and FTIR technologies. Their magnetic properties were also studied by using a superconducting quantum interference device (SQUID) magnetometer at room temperature. Then the amine-functionalized magnetite nanoparticles were applied to immunoassays and magnetic resonance imaging in live mice.