Ab initio simulations of excited carrier dynamics in carbon nanotubes

Combining time-dependent density functional calculations for electrons with molecular dynamics simulations for ions, we investigate the dynamics of excited carriers in a (3,3) carbon nanotube at different temperatures. Following an hv=6.8 eV photoexcitation, the carrier decay is initially dominated by efficient electron-electron scattering. At room temperature, the excitation gap is reduced to nearly half its initial value after ~230 fs, where coupling to phonons starts dominating the decay. We show that the onset point and damping rate in the phonon regime change with initial ion velocities, a manifestation of temperature dependent electron-phonon coupling.Comment: 8 pages, 3 figures, 1 EPAPS supplementary fil

Decentralized Cooperative Stochastic Bandits

We study a decentralized cooperative stochastic multi-armed bandit problem with $K$ arms on a network of $N$ agents. In our model, the reward distribution of each arm is the same for each agent and rewards are drawn independently across agents and time steps. In each round, each agent chooses an arm to play and subsequently sends a message to her neighbors. The goal is to minimize the overall regret of the entire network. We design a fully decentralized algorithm that uses an accelerated consensus procedure to compute (delayed) estimates of the average of rewards obtained by all the agents for each arm, and then uses an upper confidence bound (UCB) algorithm that accounts for the delay and error of the estimates. We analyze the regret of our algorithm and also provide a lower bound. The regret is bounded by the optimal centralized regret plus a natural and simple term depending on the spectral gap of the communication matrix. Our algorithm is simpler to analyze than those proposed in prior work and it achieves better regret bounds, while requiring less information about the underlying network. It also performs better empirically

Deregulation, Consolidation, and Efficiency: Evidence From the Spanish Insurance Industry

This paper provides new information on the effects of deregulation and consolidation in financial services markets by analyzing the Spanish insurance industry. The sample period 1989-1998 spans the introduction of the European Union’s Third Generation Insurance Directives, which deregulated the EU insurance market. Deregulation has led to dramatic changes in the Spanish insurance market; the number of firms declined by 35 percent and average firm size increased by 275 percent. We analyze the causes and effects of consolidation using modern frontier efficiency analysis to estimate cost, technical, and allocative efficiency, as well as using Malmquist analysis to measure total factor productivity change. The results show that many small, inefficient, and financially under-performing firms were eliminated from the market due to insolvency or liquidation and that acquirers in the mergers and acquisitions market prefer relatively efficient target firms. As a result, the market experienced significant growth in total factor productivity over the sample period. Consolidation reduced the number of firms operating with increasing returns to scale but also increased the number operating with decreasing returns to scale. Hence, many large firms should focus on improving efficiency rather than on further growth.

Symmetry reduction induced by anyon condensation: a tensor network approach

Topological ordered phases are related to changes in the properties of their quasi-particle excitations (anyons). We study these relations in the framework of projected entanglement pair states (\textsf{PEPS}) and show how condensing and confining anyons reduces a local gauge symmetry to a global on-site symmetry. We also study the action of this global symmetry over the quasiparticle excitations. As a byproduct, we observe that this symmetry reduction effect can be applied to one-dimensional systems as well, and brings about appealing physical interpretations on the classification of phases with symmetries using matrix product states (\textsf{MPS}). The case of $\mathbb{Z}_2$ on-site symmetry is studied in detail.Comment: 21+5 pages, 15+3 figures. Introduction and conclusions enlarged, references and figure added, minor typos corrected, appendix about dyons adde

Mathematical open problems in Projected Entangled Pair States

Projected Entangled Pair States (PEPS) are used in practice as an efficient parametrization of the set of ground states of quantum many body systems. The aim of this paper is to present, for a broad mathematical audience, some mathematical questions about PEPS.Comment: Notes associated to the Santal\'o Lecture 2017, Universidad Complutense de Madrid (UCM), minor typos correcte

Sánchez de Toca, the failure of a minister Marina

Joaquín Sánchez de Toca, político conservador, desde el ministerio de Marina planteó una reforma integral de la Armada. Tendría un presupuesto real, una nueva organización administrativa, unas infraestructuras modernas en sus astilleros y arsenales y, por último, una potente escuadra capaz de defender el litoral español. Fracasó. ¿Por qué? Por la oposición de los oficiales de la Armada, temerosos de perder sus privilegios; por la oposición de Fernández Villaverde, temeroso de ver anuladas sus aspiraciones a la sucesión de Silvela y de Alfonso XIII, temeroso del avance republicano en las elecciones, achacable a la inacción de Maura, ministro de GobernaciónFrom the Ministre of the Marine, the conservative politician Joaquín Sánchez de Toca contemplated a comprehensive reform of the Navy. It would benefit from a real budget, a new administrative organization, a modern infrastructure for its arsenals and shypyards and, finally, a powerful fleet ready to defend spanish coast. However, he failed. Why? Because of the opposition he faced In different fronts: the Army officers, afraid of losing their privileges, Fernández Villaverde, fearful of destroying his aspiration to succeed Silvela, and Alfonso XIII, frightened of the Republican advance in the election due to the passivity on the part of Maura, the Minister of the Interio