10,000 publications from this institution
Second harmonic generation (SHG) is a fundamental nonlinear optical phenomenon widely used both for experimental probes of materials and for application to optical devices. Even-order nonlinear optical responses including SHG generally require breaking of inversion symmetry, and thus have been utilized to study noncentrosymmetric materials. Here, we study theoretically the SHG in inversion-symmetric Dirac and Weyl semimetals under a DC current which breaks the inversion symmetry by creating a nonequilibrium steady state. Based on analytic and numerical calculations, we find that Dirac and Weyl semimetals exhibit strong SHG upon application of finite current. Our experimental estimation for a Dirac semimetal Cd$_3$As$_2$ and a magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ suggests that the induced susceptibility $χ^{(2)}$ for practical applied current densities can reach $10^5~\mathrm{pm}\cdot\mathrm{V}^{-1}$ with mid-IR or far-IR light. This value is 10$^2$-10$^4$ times larger than those of typical nonlinear optical materials. We also discuss experimental approaches to observe the current-induced SHG and comment on current-induced SHG in other topological semimetals in connection with recent experiments.
We describe an innovative and scalable recommendation system successfully deployed at eBay. To build recommenders for long-tail marketplaces requires projection of volatile items into a persistent space of latent products. We first present a generative clustering model for collections of unstructured, heterogeneous, and ephemeral item data, under the assumption that items are generated from latent products. An item is represented as a vector of independently and distinctly distributed variables, while a latent product is characterized as a vector of probability distributions, respectively. The probability distributions are chosen as natural stochastic models for different types of data. The learning objective is to maximize the total intra-cluster coherence measured by the sum of log likelihoods of items under such a generative process. In the space of latent products, robust recommendations can then be derived using naive Bayes for ranking, from historical transactional data. Item-based recommendations are achieved by inferring latent products from unseen items. In particular, we develop a probabilistic scoring function of recommended items, which takes into account item-product membership, product purchase probability, and the important auction-end-time factor. With the holistic probabilistic measure of a prospective item purchase, one can further maximize the expected revenue and the more subjective user satisfaction as well. We evaluated the latent product clustering and recommendation ranking models using real-world e-commerce data from eBay, in both forms of offline simulation and online A/B testing. In the recent production launch, our system yielded 3-5 folds improvement over the existing production system in click-through, purchase-through and gross merchandising value; thus now driving 100% related recommendation traffic with billions of items at eBay. We believe that this work provides a practical yet principled framework for recommendation in the domains with affluent user self-input data.
A model of finite-deformation elastoplasticity theory that accommodates finite elastic strain is discussed. This is based on a polyconvex extension of the classical Hookean relation between stress and elastic strain. A framework for the description of scale effects associated with strain hardening is also developed, based on the theory of materially uniform bodies with inhomogeneities.
The reforming reaction is one of the most important catalytic processes in the petroleum industry. The object of this process is to increase the octane number of the naphtha fraction of the crude oil distillate by converting paraffins into aromatic compounds. The presence of Re in the Pt-Re bimetallic system significantly enhanced the stability and reactivity of the catalyst. The Pt-Re bimetallic catalyst shows its unique property of naphtha reforming when it is sulfided to suppress the high hydrogenolysis activity of Re. All experiments were performed in a stainless steel ultrahigh vacuum system equipped with an Auger electron spectrometer, low-energy electron diffraction optics, a quadrupole mass spectrometer, and an internal isolation cell for catalytic reactions. Initial sulfiding changed the selectivity of the catalysts. The most significant change was a decrease in hydrogenolysis activity and an increase in cyclization activity.
