2,951 research outputs found
Topological phase transitions driven by next-nearest-neighbor hopping in two-dimensional lattices
For two-dimensional lattices in a tight-binding description, the intrinsic
spin-orbit coupling, acting as a complex next-nearest-neighbor hopping, opens
gaps that exhibit the quantum spin Hall effect. In this paper, we study the
effect of a real next-nearest-neighbor hopping term on the band structure of
several Dirac systems. In our model, the spin is conserved, which allows us to
analyze the spin Chern numbers. We show that in the Lieb, kagome, and T_3
lattices, variation of the amplitude of the real next-nearest-neighbor hopping
term drives interesting topological phase transitions. These transitions may be
experimentally realized in optical lattices under shaking, when the ratio
between the nearest- and next-nearest-neighbor hopping parameters can be tuned
to any possible value. Finally, we show that in the honeycomb lattice,
next-nearest-neighbor hopping only drives topological phase transitions in the
presence of a magnetic field, leading to the conjecture that these transitions
can only occur in multigap systems.Comment: 10 pages, 9 figures [erratum: corrected colors in Fig. 7(a)
The Weibull-Geometric distribution
In this paper we introduce, for the first time, the Weibull-Geometric
distribution which generalizes the exponential-geometric distribution proposed
by Adamidis and Loukas (1998). The hazard function of the last distribution is
monotone decreasing but the hazard function of the new distribution can take
more general forms. Unlike the Weibull distribution, the proposed distribution
is useful for modeling unimodal failure rates. We derive the cumulative
distribution and hazard functions, the density of the order statistics and
calculate expressions for its moments and for the moments of the order
statistics. We give expressions for the R\'enyi and Shannon entropies. The
maximum likelihood estimation procedure is discussed and an algorithm EM
(Dempster et al., 1977; McLachlan and Krishnan, 1997) is provided for
estimating the parameters. We obtain the information matrix and discuss
inference. Applications to real data sets are given to show the flexibility and
potentiality of the proposed distribution
Robust zero-energy modes in an electronic higher-order topological insulator: the dimerized Kagome lattice
Quantum simulators are an essential tool for understanding complex quantum
materials. Platforms based on ultracold atoms in optical lattices and photonic
devices led the field so far, but electronic quantum simulators are proving to
be equally relevant. Simulating topological states of matter is one of the holy
grails in the field. Here, we experimentally realize a higher-order electronic
topological insulator (HOTI). Specifically, we create a dimerized Kagome
lattice by manipulating carbon-monoxide (CO) molecules on a Cu(111) surface
using a scanning tunneling microscope (STM). We engineer alternating weak and
strong bonds to show that a topological state emerges at the corner of the
non-trivial configuration, while it is absent in the trivial one. Contrarily to
conventional topological insulators (TIs), the topological state has two
dimensions less than the bulk, denoting a HOTI. The corner mode is protected by
a generalized chiral symmetry, which leads to a particular robustness against
perturbations. Our versatile approach to quantum simulation with artificial
lattices holds promises of revealing unexpected quantum phases of matter
Topological phase transitions between chiral and helical spin textures in a lattice with spin-orbit coupling and a magnetic field
We consider the combined effects of large spin-orbit couplings and a
perpendicular magnetic field in a 2D honeycomb fermionic lattice. This system
provides an elegant setup to generate versatile spin textures propagating along
the edge of a sample. The spin-orbit coupling is shown to induce topological
phase transitions between a helical quantum spin Hall phase and a chiral
spin-imbalanced quantum Hall state. Besides, we find that the spin orientation
of a single topological edge state can be tuned by a Rashba spin-orbit
coupling, opening an interesting route towards quantum spin manipulation. We
discuss the possible realization of our results using cold atoms trapped in
optical lattices, where large synthetic magnetic fields and spin-orbit
couplings can be engineered and finely tuned. In particular, this system would
lead to the observation of a time-reversal-symmetry-broken quantum spin Hall
phase.Comment: 8 pages, 3 figures, Accepted in Europhys. Lett. (Dec 2011
Isolation by distance, not rivers, control the distribution of termite species in the Amazonian rain forest
The spatial distribution of species is affected by dispersal barriers, local environmental conditions and climate. However, the effect of species dispersal and their adaptation to the environment across geographic scales is poorly understood. To investigate the distribution of species from local to broad geographic scales, we sampled termites in 198 transects distributed in 13 sampling grids in the Brazilian Amazonian forest. The sampling grids encompassed an area of 271 500 km2 and included the five major biogeographic regions delimited by Amazonian rivers. Environmental data for each transect were obtained from local measurements and remote sensing. Similar to previous studies, termite species composition at the local scale was mostly associated with measures of soil texture and chemistry. In contrast, termite species composition at broad geographic scales was associated with soil nutrients, and the geographic position of the transects. Between 17 and 30% of the variance in termite species composition could be attributed exclusively to the geographic position of the transects, but could not be attributed to measured environmental variables or the presence of major rivers. Isolation by distance may have strong effects on termite species composition due to historic processes and the spatially structured environments along distinct geological formations of Amazonia. However, in contrast to many taxa in Amazonia, there is no evidence that major rivers are important barriers to termite dispersal
A new species of Calacarus Keifer (Acari: Eriophyidae) on papaya in Northeastern Brazil.
Calacarus flagelliseta n.sp. (Acari: Prostigmata: Eriophyidae) is described from Northeastern Brazil based on adults of both sexes collected from papaya leaves
The gas of elastic quantum strings in 2+1 dimensions: finite temperatures
The finite temperature physics of the gas of elastic quantum strings as
introduced in J. Zaanen, Phys. Rev. Lett. 84, 753 is investigated. This model
is inspired on the stripes in the high Tc superconductors. We analyze in detail
how the kinetic interactions of the zero temperature quantum problem crossover
into the entropic interactions of the high temperature limit.Comment: 14 pages, 2 figure
Soil termites in a rainforest, a secondary forest and mixed-culture plantation sites in central Amazonia.
Soil termites have been studied in detail in a rain forest, a secondary forest and two agroforestry plantation sites at the Embrapa Amazonia Ocidental, Manaus-AM (Brasil), using soil (0 cm-5 cm) and litter samples taken at random within the study sites, using a soilsampler of 21 cm diameter. As results is presented a list of termite genus diversity, then compare termite biomass and individuals numbers in litter and in soil at the different sites, and discuss possible factors that determine termite distribution in the field
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