A simple interpretation of quantum mirages

In an interesting new experiment the electronic structure of a magnetic atom adsorbed on the surface of Cu(111), observed by STM, was projected into a remote location on the same surface. The purpose of the present paper is to interpret this experiment with a model Hamiltonian, using ellipses of the size of the experimental ones, containing about 2300 atoms. The charge distribution for the different wavefunctions is analyzed, in particular, for those with energy close to the Fermi energy of copper Ef. Some of them show two symmetric maxima located on the principal axis of the ellipse but not necessarily at the foci. If a Co atom is adsorbed at the site where the wavefunction with energy $E_F$ has a maximum and the interaction is small, the main effect of the adsorbed atom will be to split this particular wavefunction in two. The total charge density will remain the same but the local density of states will present a dip at Ef at any site where the charge density is large enough. We relate the presence of this dip to the observation of quantum mirages. Our interpretation suggests that other sites, apart from the foci of the ellipses, can be used for projecting atomic images and also indicates the conditions for other non magnetic adsorbates to produce mirages.Comment: 3 pages, 3 Fig

Supersymmetric Transformations in Optical Fibers

[EN] Supersymmetry (SUSY) has recently emerged as a tool to design unique optical structures with degenerate spectra. Here, we study several fundamental aspects and variants of one-dimensional SUSY in axially symmetric optical media, including their basic spectral features and the conditions for degeneracy breaking. Surprisingly, we find that the SUSY degeneracy theorem is partially (totally) violated in optical systems connected by isospectral (broken) SUSY transformations due to a degradation of the paraxial approximation. In addition, we show that isospectral constructions provide a dimension-independent design control over the group delay in SUSY fibers. Moreover, we find that the studied unbroken and isospectral SUSY transformations allow us to generate refractive-index superpartners with an extremely large phase-matching bandwidth spanning the S þ C þ L optical bands. These singular features define a class of optical fibers with a number of potential applications. To illustrate this, we numerically demonstrate the possibility of building photonic lanterns supporting broadband heterogeneous supermodes with large effective area, a broadband all-fiber true-mode (de)multiplexer requiring no mode conversion, and different mode-filtering, mode-conversion, and pulse-shaping devices. Finally, we discuss the possibility of extrapolating our results to acoustics and quantum mechanics.We thank Sergio Lechago for his valuable help with the numerical simulations. This work is supported by Spanish National Plan projects [No. MINECO/FEDER UE XCORE TEC2015-70858-C2-1-R, No. PHUTURE TEC2015-73581-JIN (AEI/FEDER, UE), and No. HIDRASENSE RTC-2014-2232-3]. A.M.'s work is supported by F.P.I. Grant No. BES-2013-062952.Macho-Ortiz, A.; Llorente, R.; García Meca, C. (2018). Supersymmetric Transformations in Optical Fibers. Physical Review Applied. 9(1):014024-1-014024-15. https://doi.org/10.1103/PhysRevApplied.9.014024S014024-1014024-159

Water and Land-surface Feedbacks in a Polygonal Tundra Environment

The Arctic, including Alaska, is currently experiencing an unprecedented degree of environmental change with increases in both the mean annual surface temperature and precipitation. These observed changes in the climate regime has resulted in a permafrost condition that is particularly sensitive to changes in both Changes in the surface energy balance and water balances and is susceptible to degradation. Thermokarst topography forms whenever ice-rich permafrost thaws and the ground subsides into the resulting voids. Extensive areas of thermokarst activity are currently being observed throughout the arctic and sub-arctic environments. The important processes involved with thermokarsting include surface ponding, surface subsidence, changes in drainage patterns, and related erosion. In this research, we are applying the land-surface evolution model, ERODE (http://csdms.colorado.edu/wiki/Model:Erode), to an area dominated by low- center, ice-wedge polygons. We are modifying the ERODE model to include land surface subsistence in areas where the maximum active layer depth exceeds the protective layer – the layer of soil above ice-rich soils that acts as a buffer to surface energy processes. The goal of this modeling study is to better understand and quantify the development of thermokarst features in the polygonal tundra environment, emphasizing the resulting feedbacks and connections between hydrologic processes and a dynamic surface topography. Further, we are working on understanding the balance between thermal and mechanical processes with regard to thermokarst processes. This unique application of a landscape evolution model may provide valuable insight related to the rates and spatial extent of thermokarst development and the subsequent hydrologic responses to degrading permafrost in a changing climate.Office of Biological and Environmental Research, Department of Energy Office of Science, Alaska Climate Science Cente

quasiharmonic equations of state for dynamically-stabilized soft-mode materials

We introduce a method for treating soft modes within the analytical framework of the quasiharmonic equation of state. The corresponding double-well energy-displacement relation is fitted to a functional form that is harmonic in both the low- and high-energy limits. Using density-functional calculations and statistical physics, we apply the quasiharmonic methodology to solid periclase. We predict the existence of a B1--B2 phase transition at high pressures and temperatures

Geometry of River Networks II: Distributions of Component Size and Number

The structure of a river network may be seen as a discrete set of nested sub-networks built out of individual stream segments. These network components are assigned an integral stream order via a hierarchical and discrete ordering method. Exponential relationships, known as Horton's laws, between stream order and ensemble-averaged quantities pertaining to network components are observed. We extend these observations to incorporate fluctuations and all higher moments by developing functional relationships between distributions. The relationships determined are drawn from a combination of theoretical analysis, analysis of real river networks including the Mississippi, Amazon and Nile, and numerical simulations on a model of directed, random networks. Underlying distributions of stream segment lengths are identified as exponential. Combinations of these distributions form single-humped distributions with exponential tails, the sums of which are in turn shown to give power law distributions of stream lengths. Distributions of basin area and stream segment frequency are also addressed. The calculations identify a single length-scale as a measure of size fluctuations in network components. This article is the second in a series of three addressing the geometry of river networks.Comment: 16 pages, 13 figures, 4 tables, Revtex4, submitted to PR