2,923 research outputs found
Modified conjugated gradient method for diagonalising large matrices
We present an iterative method to diagonalise large matrices. The basic idea
is the same as the conjugated gradient (CG) method, i.e, minimizing the
Rayleigh quotient via its gradient and avoiding reintroduce errors to the
directions of previous gradients. Each iteration step is to find lowest
eigenvector of the matrix in a subspace spanned by the current trial vector and
the corresponding gradient of the Rayleigh quotient, as well as some previous
trial vectors. The gradient, together with the previous trail vectors, play a
similar role of the conjugated gradient of the original CG algorithm. Our
numeric tests indicate that this method converges significantly faster than the
original CG method. And the computational cost of one iteration step is about
the same as the original CG method. It is suitably for first principle
calculations.Comment: 6 Pages, 2EPS figures. (To appear in Phys. Rev. E
How do human resource management practices affect employee well-being? A moderated mediation model
Purpose – The study examines how human resource management practices (HRMPs) – including ability practice, motivation practice, and opportunity practice – affect employee well-being (EWB) – including life well-being, job well-being, and psychological well-being – in the Chinese cultural context. Design/methodology/approach – A sample of 529 employees from various industries in China participated in the survey for this study. Data were analyzed using structural equation modeling. Findings – The findings indicate that HRMPs have a significant positive effect on EWB. Specifically, practices based on ability, motivation, and opportunity have a significant positive effect on job well-being, life well-being, and psychological well-being respectively. Integrity leadership moderates the impact of HRMPs on EWB. Organizational justice has a partial mediating effect on the relationship between HRMPs and EWB. Integrity leadership moderates the mediation effect of organizational justice in the relationship between HRMPs and EWB. Practical implications –Human resource policies and practices need to create a fair organizational atmosphere, and managers implementing them must have integrity leadership. When selecting and promoting managers, organizations should not only pay attention to a candidate’s ability, but also his or her integrity. Originality/value – This study uncovers how the important roles of organizational justice and integrity leadership act on the relationship between HRMPs and EWB, thus advancing our understanding of how HRMPs can effectively increase EWB
Two-dimensional negative donors in magnetic fields
A finite-difference solution of the Schroedinger equation for negative donor
centers D^- in two dimensions is presented. Our approach is of exact nature and
allows us to resolve a discrepancy in the literature on the ground state of a
negative donor. Detailed calculations of the energies for a number of states
show that for field strengths less than \gamma=0.117 a.u. the donor possesses
one bound state, for 0.117<\gamma<1.68 a.u. there exist two bound states and
for field strengths \gamma>1.68 a.u. the system possesses three bound states.
Further relevant characteristics of negative donors in magnetic fields are
provided.Comment: 7 pages, 1 figur
Comparison between unipolar and bipolar single phase grid-connected inverters for PV applications
An inverter is essential for the interfacing of photovoltaic panels with the AC network. There are many possible inverter topologies and inverter switching schemes and each one will have its own relative advantages and disadvantages. Efficiency and output current distortion are two important factors governing the choice of inverter system. In this paper, it is argued that current controlled inverters offer significant advantages from the point of view of minimisation of current distortion. Two inverter switching strategies are explored in detail. These are the unipolar current controlled inverter and the bipolar current controlled inverter. With respect to low frequency distortion, previously published works provide theoretical arguments in favour of bipolar switching. On the other hand it has also been argued that the unipolar switched inverter offers reduced switching losses and generates less EMI. On efficiency grounds, it appears that the unipolar switched inverter has an advantage. However, experimental results presented in this paper show that the level of low frequency current distortion in the unipolar switched inverter is such that it can only comply with Australian Standard 4777.2 above a minimum output current. On the other hand it is shown that at the same current levels bipolar switching results in reduced low frequency harmonics
Semantic patterns for sentiment analysis of Twitter
Most existing approaches to Twitter sentiment analysis assume that sentiment is explicitly expressed through affective words. Nevertheless, sentiment is often implicitly expressed via latent semantic relations, patterns and dependencies among words in tweets. In this paper, we propose a novel approach that automatically captures patterns of words of similar contextual semantics and sentiment in tweets. Unlike previous work on sentiment pattern extraction, our proposed approach does not rely on external and fixed sets of syntactical templates/patterns, nor requires deep analyses of the syntactic structure of sentences in tweets. We evaluate our approach with tweet- and entity-level sentiment analysis tasks by using the extracted semantic patterns as classification features in both tasks. We use 9 Twitter datasets in our evaluation and compare the performance of our patterns against 6 state-of-the-art baselines. Results show that our patterns consistently outperform all other baselines on all datasets by 2.19% at the tweet-level and 7.5% at the entity-level in average F-measure
How Many Topics? Stability Analysis for Topic Models
Topic modeling refers to the task of discovering the underlying thematic
structure in a text corpus, where the output is commonly presented as a report
of the top terms appearing in each topic. Despite the diversity of topic
modeling algorithms that have been proposed, a common challenge in successfully
applying these techniques is the selection of an appropriate number of topics
for a given corpus. Choosing too few topics will produce results that are
overly broad, while choosing too many will result in the "over-clustering" of a
corpus into many small, highly-similar topics. In this paper, we propose a
term-centric stability analysis strategy to address this issue, the idea being
that a model with an appropriate number of topics will be more robust to
perturbations in the data. Using a topic modeling approach based on matrix
factorization, evaluations performed on a range of corpora show that this
strategy can successfully guide the model selection process.Comment: Improve readability of plots. Add minor clarification
Quantum Phase Transition of Spin-2 Cold Bosons in an Optical Lattice
The Bose-Hubbard Hamiltonian of spin-2 cold bosons with repulsive interaction
in an optical lattice is proposed. After neglecting the hopping term, the
site-independent Hamiltonian and its energy eigenvalues and eigenstates are
obtained. We consider the hopping term as a perturbation to do the calculations
in second order and draw the phase diagrams for different cases. The phase
diagrams show that there is a phase transition from Mott insulator with integer
number bosons to superfluid when the ratio ( is the
spin-independent on-site interaction and the hopping matrix element between
adjacent lattice sites) is decreased to a critical value and that there is
different phase boundary between superfluid and Mott insulator phase for
different Zeeman level component in some ground states. We find that the
position of phase boundary for different Zeeman level component is related to
its average population in the Mott ground state.Comment: 16 pages, 6 figure
Nafion-stabilized Pt Nanoparticles and the Effect of Pt Distribution on the Electrochemical Reduction of Oxygen
Nafion stabilized Pt sols was prepared by using methanol to reduce the precursor H2PtCl6. No other ingredient was used in the preparation and Pt nanoparticles of 2 -3 nm size were obtained. Platinum nanoparticles were found to form aggregates. The aggregate size and the stability of Pt sols could be changed by solvent treatment. This suggests that SO3¯ functional groups of Nafion stabilized Pt nanoparticles and that the aggregates formed due to the formation of ionic clusters of Nafion. The electrochemical reduction of oxygen was examined with carbon supported Nafion-Pt catalysts of different aggregate size. At low overpotential, the kinetic rate was not changed by the aggregate size. At high overpotential region where mass-transfer limitation phenomenon was observed, the reaction rate increased with decreasing aggregate size. It indicates that smaller aggregates, i.e., the more even Pt distribution, are less subjected to mass-transfer limitation and could consequently provide better electrode performance
Evaluation of the BCS Approximation for the Attractive Hubbard Model in One Dimension
The ground state energy and energy gap to the first excited state are
calculated for the attractive Hubbard model in one dimension using both the
Bethe Ansatz equations and the variational BCS wavefunction. Comparisons are
provided as a function of coupling strength and electron density. While the
ground state energies are always in very good agreement, the BCS energy gap is
sometimes incorrect by an order of magnitude, particularly at half-filling.
Finite size effects are also briefly discussed for cases where an exact
solution in the thermodynamic limit is not possible. In general, the BCS result
for the energy gap is poor compared to the exact result.Comment: 25 pages, 5 Postscript figure
Constructing 3D crystal templates for photonic band gap materials using holographic optical tweezers
A simple and robust method is presented for the construction of 3-dimensional crystals from silica and polystyrene microspheres. The crystals are suitable for use as templates in the production of three-dimensional photonic band gap (PBG) materials. Manipulation of the microspheres was achieved using a dynamic holographic assembler (DHA) consisting of computer controlled holographic optical tweezers. Attachment of the microspheres was achieved by adjusting their colloidal interactions during assembly. The method is demonstrated by constructing a variety of 3-dimensional crystals using spheres ranging in size from 3 µm down to 800 nm. A major advantage of the technique is that it may be used to build structures that cannot be made using self-assembly. This is illustrated through the construction of crystals in which line defects have been deliberately included, and by building simple cubic structures
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