1,442 research outputs found
Topological quantum criticality from multiplicative topological phases
Symmetry-protected topological phases (SPTs) characterized by short-range
entanglement include many states essential to understanding of topological
condensed matter physics, and the extension to gapless SPTs provides essential
understanding of their consequences. In this work, we identify a fundamental
connection between gapless SPTs and recently-introduced multiplicative
topological phases, demonstrating that multiplicative topological phases are an
intuitive and general approach to realizing concrete models for gapless SPTs.
In particular, they are naturally well-suited to realizing higher-dimensional,
stable, and intrinsic gapless SPTs through combination of canonical topological
insulator and semimetal models with critical gapless models in
symmetry-protected tensor product constructions, opening avenues to far broader
and deeper investigation of topology via short-range entanglement
Lifshitz transition in the phase diagram of two-leg - ladder systems at low filling
We use a combination of numerical matrix product states (MPS) and analytical
approaches to investigate the phase diagram of the two-leg - ladder in
the region of low to intermediate fillings. We choose the same coupling
strength along the leg- and rung-directions, but study the effect of adding a
nearest-neighbor repulsion . We observe a rich phase diagram and
analytically identify a Lifshitz-like band filling transition, which can be
associated to a numerically observed crossover from s-wave to d-wave like
superconducting quasi-long range order (QLRO). Due to the strong interactions,
the Lifshitz transition is smeared into a crossover region which separates two
distinct Luttinger theories with unequal physical meaning of the Luttinger
parameter. Our numerically exact MPS results spotlight deviations from standard
Luttinger theory in this crossover region and is consistent with Luttinger
theory sufficiently far away from the Lifshitz transition. At very low
fillings, studying the Friedel-like oscillations of the local density
identifies a precursor region to a Wigner crystal at small values of the
magnetic exchange interaction . We discuss analytically how tuning
parameters at these fillings modifies the phase diagram, and find good
agreement with MPS results.Comment: 14 pages, 4 appendices, 17 figure
Índices para la evaluación de un seguidor solar
The usage of solar trackers in solar energy technologies represents an alternative to increase the capture of solar radiation. However, the costs involved convert it into a prohibitive technology, having indexes and procedures that allow for its evaluation and determination of the viability of its use is of vital importance at the time of deciding its implementation and so to form the objective of this study. In the investigation carried out, the indexes of average gain, comparative efficiency and profitability are introduced and a procedure for the evaluation is developed, as a contribution to the decision-making process as to whether or not to use a solar tracker. It is confirmed that the use of trackers is completely viable when used with higher power technologies. Furthermore, it is shown that theoretically, the average gain index is higher than the values currently registered in scientific literature. Such factors support the possibility of designing and developing solar trackers with a higher gain, lower production costs and lower consumption indexes, as a way for their mass use in low power photovoltaic generators.El uso de los seguidores solares como tecnología de aprovechamiento de energía solar representa una alternativa para incrementar la captación de radiación solar. Sin embargo, sus costos lo convierten en una tecnología prohibitiva, por lo que disponer de índices y procedimientos que permitan su evaluación para determinar la viabilidad de uso es una herramienta de extrema importancia a la hora de decidir sobre su utilización y constituye el objetivo de este trabajo. Con la investigación realizada se introducen los índices de ganancia media, eficiencia comparativa y de utilidad y se desarrolla un procedimiento de evaluación, como contribución a la toma de decisiones sobre el uso o no de un seguidor solar. Se corrobora que su uso es totalmente viable mientras mayor es la potencia de las tecnologías en las cuales se utiliza. Además, se muestra que teóricamente el índice medio de ganancia es mayor que los valores registrados actualmente en la literatura científica. Tales elementos apoyan la posibilidad de diseñar y desarrollar seguidores solares con mayor ganancia, menores costos de fabricación y menores índices de consumo, como vía para su utilización masiva en generadores fotovoltaicos de baja potencia
Estimating ecological metrics for holistic conservation management in a biodiverse but information‐poor tropical region
Conservation ecologists face the dual challenge of working with difficult‐to‐study species and providing ecological metrics that support conservation management at global, regional, and local levels. We present metrics identifying distributions, site‐level and global abundance, site‐contextualized habitat requirements, and threats for seven dry forest endemic birds (two threatened, one Near Threatened) in the globally important Tumbes region of Peru. Extents of occurrence ranged from 36,000 to 152,000 km2, and while broad distributions were generally congruent, nearly half of species overlapped 150,000. Site‐level population estimates varied hugely, reflecting size of site and extreme variation in local abundances. Large tree girths and dense low cover generally promoted bird abundance, but stem density acted in opposite directions for different species, implying the need for site‐ and species‐specific habitat management. Habitat quality varied across sites, further complicating management options at the local level (e.g., reduced grazing). We highlight the suitability of our methods in providing useful conservation metrics for data‐poor regions, and demonstrate their application. Importantly, we propose key sites and priority actions for the region, including extensions of existing protected areas
Paramagnetic defects in polycrystalline zirconia: An EPR and DFT study
The paramagnetic defects present in pristine zirconium dioxide (ZrO2) and those formed upon reductive treatments (either annealing or UV irradiation in H-2) are described and rationalized by the joint use of electron paramagnetic resonance (EPR) and DFT supercell calculations. Three types of Zr3+ reduced sites have been examined both in the bulk of the solid (one center) and at the surface (two centers). Trapping electron centers different from reduced Zr ions are also present, whose concentration increases upon annealing. A fraction of these sites are paramagnetic showing a symmetric signal at g = 2.0023, but the majority of them are EPR silent and are revealed by analysis of electron transfer from the reduced solid to oxygen. The presence of classic F-type centers (electrons in bulk oxygen vacancies) is disregarded on the basis of the g-tensor symmetry. This is expected, on the basis of theoretical calculations, to be anisotropic and thus incompatible with the observed signal. In general terms, ZrO2 has Some properties similar to typical reducible oxides, such as TiO2 and CeO2 (excess electrons stabilized at cationic sites), but it is much more resistant to reduction than this class of materials. While point defects in doped (Y3+, Ca2+) ZrO2 materials have been widely investigated for their role as ionic conductors, the defectivity of pristine ZrO2 is much less known; this paper presents a thorough analysis of this phenomenon
Beating noise with abstention in state estimation
We address the problem of estimating pure qubit states with non-ideal (noisy)
measurements in the multiple-copy scenario, where the data consists of a number
N of identically prepared qubits. We show that the average fidelity of the
estimates can increase significantly if the estimation protocol allows for
inconclusive answers, or abstentions. We present the optimal such protocol and
compute its fidelity for a given probability of abstention. The improvement
over standard estimation, without abstention, can be viewed as an effective
noise reduction. These and other results are exemplified for small values of N.
For asymptotically large N, we derive analytical expressions of the fidelity
and the probability of abstention, and show that for a fixed fidelity gain the
latter decreases with N at an exponential rate given by a Kulback-Leibler
(relative) entropy. As a byproduct, we obtain an asymptotic expression in terms
of this very entropy of the probability that a system of N qubits, all prepared
in the same state, has a given total angular momentum. We also discuss an
extreme situation where noise increases with N and where estimation with
abstention provides a most significant improvement as compared to the standard
approach
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