Dynamics of doublon-holon pairs in Hubbard two-leg ladders

The dynamics of holon-doublon pairs is studied in Hubbard two-leg ladders using the time-dependent Density Matrix Renormalization Group method. We find that the geometry of the two-leg ladder, that is qualitatively different from a one-dimensional chain due to the presence of a spin-gap, strongly affects the propagation of a doublon-holon pair. Two distinct regimes are identified. For weak inter-leg coupling, the results are qualitatively similar to the case of the propagation previously reported in Hubbard chains, with only a renormalization of parameters. More interesting is the case of strong inter-leg coupling where substantial differences arise, particularly regarding the double occupancy and properties of the excitations such as the doublon speed. Our results suggest a connection between the presence of a spin gap and qualitative changes in the doublon speed, indicating a weak coupling between the doublon to magnetic excitations.Comment: 8 pages, 9 figures. Published versio

Hornitos Hotel

https://openscholarship.wustl.edu/bcs/1268/thumbnail.jp

La situación económica del Magreb

Tras los éxitos iniciales en materia de estabilización macroeconómica, el Magreb lleva desde la década de 1980 intentando implantar un proceso de reformas económicas y administrativas con el objetivo de liberalizar su sistema económico. Pero este proceso es lento y, en ocasiones, los obstáculos políticos y sociales lo paralizan. El objeto de este trabajo es ofrecer un análisis económico de la región magrebí, de su situación económica y del estado de las reformas, así como de las perspectivas económicas a medio plazo de la región. Nos referiremos en la medida de lo posible al Magreb en general, salvo cuando se lleva a cabo un análisis a nivel de país debido a las particularidades del mismo. El trabajo se estructura como sigue: en primer término, se analiza el elemento geográfico como factor unificador; en segundo término, se lleva a cabo un análisis de los principales aspectos económicos, como el crecimiento y los indicadores sociales, las estrategias de desarrollo, las políticas macroeconómicas o el sector exterior; sigue, en tercer término, un análisis de las reformas económicas, básicamente la liberalización exterior e interior, la privatización, las reformas fiscales y de la administración o la reforma del sector financiero. Finalmente, en el último apartado se esbozan una serie de perspectivas a medio plazo y se destaca que la ventana de oportunidad para llevar a cabo las reformas económicas es estrecha y que resulta urgente acelerarlas si no se quiere poner en entredicho la credibilidad del proceso y la sostenibilidad de la estabilización macroeconómica y de los Acuerdos de Asociación con la UE

On the Definition of Effective Permittivity and Permeability For Thin Composite Layers

The problem of definition of effective material parameters (permittivity and permeability) for composite layers containing only one-two parallel arrays of complex-shaped inclusions is discussed. Such structures are of high importance for the design of novel metamaterials, where the realizable layers quite often have only one or two layers of particles across the sample thickness. Effective parameters which describe the averaged induced polarizations are introduced. As an explicit example, we develop an analytical model suitable for calculation of the effective material parameters $\epsilon_{\rm{eff}}$ and $\mu_{\rm{eff}}$ for double arrays of electrically small electrically polarizable scatterers. Electric and magnetic dipole moments induced in the structure and the corresponding reflection and transmission coefficients are calculated using the local field approach for the normal plane-wave incidence, and effective parameters are introduced through the averaged fields and polarizations. In the absence of losses both material parameters are purely real and satisfy the Kramers-Kronig relations and the second law of thermodynamics. We compare the analytical results to the simulated and experimental results available in the literature. The physical meaning of the introduced parameters is discussed in detail.Comment: 6 pages, 5 figure

Decoherence under many-body system-environment interactions: a stroboscopic representation based on a fictitiously homogenized interaction rate

An environment interacting with portions of a system leads to multiexponential interaction rates. Within the Keldysh formalism, we fictitiously homogenize the system-environment interaction yielding a uniform decay rate facilitating the evaluation of the propagators. Through an injection procedure we neutralize the fictitious interactions. This technique justifies a stroboscopic representation of the system-environment interaction which is useful for numerical implementation and converges to the natural continuous process. We apply this procedure to a fermionic two-level system and use the Jordan-Wigner transformation to solve a two-spin swapping gate in the presence of a spin environment.Comment: 11 pages, 3 figures, title changed, some typos change

Vacuum polarization and induced Maxwell and Kalb-Ramond effective action in very special relativity

This work investigates the implications of very special relativity (VSR) on the calculation of vacuum polarization for fermions in the presence of Maxwell and Kalb-Ramond gauge fields in four-dimensional spacetime. We derive the $SIM(2)$-covariant gauge theory associated with an Abelian antisymmetric 2-tensor and its corresponding field strength. We demonstrate that the free VSR-Kalb-Ramond electrodynamics is equivalent to a massive scalar field with a single polarization. Furthermore, we determine an explicit expression for the effective action involving Maxwell and Kalb-Ramond fields due to fermionic vacuum polarization at one-loop order. The quantum corrections generate divergences free of nonlocal terms only in the VSR-Maxwell sector. At the same time, we observe UV/IR mixing divergences due to the entanglement of VSR-nonlocal effects with quantum higher-derivative terms for the Kalb-Ramond field. However, in the lower energy limit, the effective action can be renormalized like in the Lorentz invariant case.Comment: 26 pages, 3 figure