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Nano Lett ; 18(4): 2623-2629, 2018 04 11.
Artículo en Inglés | MEDLINE | ID: mdl-29529377


It is well-known that superconductivity in thin films is generally suppressed with decreasing thickness. This suppression is normally governed by either disorder-induced localization of Cooper pairs, weakening of Coulomb screening, or generation and unbinding of vortex-antivortex pairs as described by the Berezinskii-Kosterlitz-Thouless (BKT) theory. Defying general expectations, few-layer NbSe2, an archetypal example of ultrathin superconductors, has been found to remain superconducting down to monolayer thickness. Here, we report measurements of both the superconducting energy gap Δ and critical temperature TC in high-quality monocrystals of few-layer NbSe2, using planar-junction tunneling spectroscopy and lateral transport. We observe a fully developed gap that rapidly reduces for devices with the number of layers N ≤ 5, as does their TC. We show that the observed reduction cannot be explained by disorder, and the BKT mechanism is also excluded by measuring its transition temperature that for all N remains very close to TC. We attribute the observed behavior to changes in the electronic band structure predicted for mono- and bi- layer NbSe2 combined with inevitable suppression of the Cooper pair density at the superconductor-vacuum interface. Our experimental results for N > 2 are in good agreement with the dependences of Δ and TC expected in the latter case while the effect of band-structure reconstruction is evidenced by a stronger suppression of Δ and the disappearance of its anisotropy for N = 2. The spatial scale involved in the surface suppression of the density of states is only a few angstroms but cannot be ignored for atomically thin superconductors.

Science ; 353(6299): 575-9, 2016 Aug 05.
Artículo en Inglés | MEDLINE | ID: mdl-27493182


Chirality is a fundamental property of electrons with the relativistic spectrum found in graphene and topological insulators. It plays a crucial role in relativistic phenomena, such as Klein tunneling, but it is difficult to visualize directly. Here, we report the direct observation and manipulation of chirality and pseudospin polarization in the tunneling of electrons between two almost perfectly aligned graphene crystals. We use a strong in-plane magnetic field as a tool to resolve the contributions of the chiral electronic states that have a phase difference between the two components of their vector wave function. Our experiments not only shed light on chirality, but also demonstrate a technique for preparing graphene's Dirac electrons in a particular quantum chiral state in a selected valley.

Phys Rev Lett ; 116(18): 186603, 2016 May 06.
Artículo en Inglés | MEDLINE | ID: mdl-27203338


We observe a series of sharp resonant features in the differential conductance of graphene-hexagonal boron nitride-graphene tunnel transistors over a wide range of bias voltages between 10 and 200 mV. We attribute them to electron tunneling assisted by the emission of phonons of well-defined energy. The bias voltages at which they occur are insensitive to the applied gate voltage and hence independent of the carrier densities in the graphene electrodes, so plasmonic effects can be ruled out. The phonon energies corresponding to the resonances are compared with the lattice dispersion curves of graphene-boron nitride heterostructures and are close to peaks in the single phonon density of states.

Rev. biol. trop ; 53(supl.1): 145-153, maio 2005. tab
Artículo en Inglés | LILACS | ID: lil-456505


This fishery was examined utilizing public records, stakeholder interviews, and operational site visits to describe the fishery for the Puerto Rico Coral Reef Advisory Committee as a first step toward development of policies for the effective management of these natural resources. The fishery is not large, including fewer than 20 licensed fishers operating primarily on the west end of the island. Only three operators export product, with the remaining fishers providing specimens to the exporters based upon customer orders. Most collection of coral reef species occurs over hard rubble zones mixed with relic reef structures and rock, or on the sides and frontal areas of active reefs. Other species are collected from among mangrove prop root zones, tidal flats, and seagrass beds. Collections are made using simple barrier and dip nets for fish and motile invertebrates such as shrimp. Invertebrates such as crabs, starfish, and sea cucumbers are commonly collected by overturning small rocks, gathering the specimens, and then replacing the rocks in their original positions. Specimens are carried to the boat and transferred to individual cup holders to maximize survival. Although statements concerning former use of chemicals to assist capture were noted, no evidence of current chemical use was observed. Specimens are held in re-circulating seawater systems onshore until collections are aggregated and shipped. The fishery strives to operate with mortality of<1%, as mortalities of>3% are described as unacceptable to customers. More than 100 fish species are collected in this fishery, but the top ten species account for >70% of the total numbers and >60% of the total value of the fishery, with a single species, Gramma loreto (Royal Gramma), comprising >40% of the numbers. More than 100 species of invertebrates are collected, but this fishery is also dominated by a handful of species, including anemones, hermit crabs, turbo snails, serpent starfish, and feather duster polychaetes

Animales , Comercio/organización & administración , Conservación de los Recursos Naturales/legislación & jurisprudencia , Peces , Explotaciones Pesqueras/economía , Cooperación Internacional , Invertebrados , Comercio/economía , Ecosistema , Dinámica Poblacional , Puerto Rico , Factores de Tiempo