456 research outputs found
Spectral properties of finite laser-driven lattices of ultracold Rydberg atoms
We investigate the spectral properties of a finite laser-driven lattice of
ultracold Rydberg atoms exploiting the dipole blockade effect in the frozen
Rydberg gas regime. Uniform one-dimensional lattices as well as lattices with
variable spacings are considered. In the case of a weak laser coupling, we find
a multitude of many-body Rydberg states with well-defined excitation properties
which are adiabatically accessible starting from the ground state. A
comprehensive analysis of the degeneracies of the spectrum as well as of the
single and pair excitations numbers of the eigenstates is performed. In the
strong laser regime, analytical solutions for the pseudo-fermionic eigenmodes
are derived. Perturbative energy corrections for this approximative approach
are provided.Comment: 17 pages, 12 figure
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Impact of a drier EarlyâMid-Holocene climate upon Amazonian forests
This paper uses a palaeoecological approach to examine the impact of drier climatic conditions of the EarlyâMid-Holocene (ca 8000â4000 years ago) upon Amazonia's forests and their fire regimes. Palaeovegetation (pollen data) and palaeofire (charcoal) records are synthesized from 20 sites within the present tropical forest biome, and the underlying causes of any emergent patterns or changes are explored by reference to independent palaeoclimate data and present-day patterns of precipitation, forest cover and fire activity across Amazonia. During the EarlyâMid-Holocene, Andean cloud forest taxa were replaced by lowland tree taxa as the cloud base rose while lowland ecotonal areas, which are presently covered by evergreen rainforest, were instead dominated by savannahs and/or semi-deciduous dry forests. Elsewhere in the Amazon Basin there is considerable spatial and temporal variation in patterns of vegetation disturbance and fire, which probably reflects the complex heterogeneous patterns in precipitation and seasonality across the basin, and the interactions between climate change, drought- and fire susceptibility of the forests, and Palaeo-Indian land use. Our analysis shows that the forest biome in most parts of Amazonia appears to have been remarkably resilient to climatic conditions significantly drier than those of today, despite widespread evidence of forest burning. Only in ecotonal areas is there evidence of biome replacement in the Holocene. From this palaeoecological perspective, we argue against the Amazon forest âdiebackâ scenario simulated for the future
Impact of Electric Fields on Highly Excited Rovibrational States of Polar Dimers
We study the effect of a strong static homogeneous electric field on the
highly excited rovibrational levels of the LiCs dimer in its electronic ground
state. Our full rovibrational investigation of the system includes the
interaction with the field due to the permanent electric dipole moment and the
polarizability of the molecule. We explore the evolution of the states next to
the dissociation threshold as the field strength is increased. The rotational
and vibrational dynamics are influenced by the field; effects such as
orientation, angular motion hybridization and squeezing of the vibrational
motion are demonstrated and analyzed. The field also induces avoided crossings
causing a strong mixing of the electrically dressed rovibrational states.
Importantly, we show how some of these highly excited levels can be shifted to
the continuum as the field strength is increased, and reversely how two atoms
in the continuum can be brought into a bound state by lowering the electric
field strength.Comment: 10 pages, 4 figure
Binary Induced Neutron-Star Compression, Heating, and Collapse
We analyze several aspects of the recently noted neutron star collapse
instability in close binary systems. We utilize (3+1) dimensional and spherical
numerical general relativistic hydrodynamics to study the origin, evolution,
and parametric sensitivity of this instability. We derive the modified
conditions of hydrostatic equilibrium for the stars in the curved space of
quasi-static orbits. We examine the sensitivity of the instability to the
neutron star mass and equation of state. We also estimate limits to the
possible interior heating and associated neutrino luminosity which could be
generated as the stars gradually compress prior to collapse. We show that the
radiative loss in neutrinos from this heating could exceed the power radiated
in gravity waves for several hours prior to collapse. The possibility that the
radiation neutrinos could produce gamma-ray (or other electromagnetic) burst
phenomena is also discussed.Comment: 17 pages, 7 figure
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Long-term impacts of mid-Holocene drier climatic conditions on Bolivian tropical dry forests
The Bolivian Chiquitano dry forest is the largest block of intact seasonally dry tropical forest in South America and is a priority ecoregion for conservation due to its high threat status. However, the long-term impacts of drier climatic conditions on tropical dry forests are not well understood, despite climate models predicting increased droughts over Bolivia in the coming century. In this paper, we assess the impacts of drier climatic conditions during the mid-Holocene on the Bolivian Chiquitano tropical dry forest using fossilised pollen, phytoliths, macro-charcoal, and geochemical proxies from a sediment core from a large lake (Laguna MandiorĂ©) on the BoliviaâBrazil border. Our results show that drier climatic conditions during the mid-
Holocene caused a local-scale, ecotonal expansion of upland savannah at the expense of dry forest. Interaction between drier climatic conditions and fire regime likely exerted a stronger control over the position of the dry forestâsavannah ecotone than edaphic factors. However, the majority of the dry forest within the lake catchment maintained a closed canopy throughout the drier conditions of the mid-Holocene, despite floristic turnover towards more drought-tolerant taxa. These findings imply overall resilience of the Chiquitano dry forest biome to future drought, albeit with floristic changes and upland savannah encroachment at ecotones
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Effects of past climate variability on fire and vegetation in the cerrĂŁdo savanna of the Huanchaca Mesetta, NE Bolivia
CerrĂŁdo savannas have the greatest fire activity
of all major global land-cover types and play a significant
role in the global carbon cycle. During the 21st century,
temperatures are projected to increase by ⌠3
âŠC coupled
with a precipitation decrease of ⌠20 %. Although these conditions
could potentially intensify drought stress, it is unknown
how that might alter vegetation composition and fire
regimes. To assess how Neotropical savannas responded to
past climate changes, a 14 500-year, high-resolution, sedimentary
record from Huanchaca Mesetta, a palm swamp located
in the cerrĂŁdo savanna in northeastern Bolivia, was analyzed
with phytoliths, stable isotopes, and charcoal. A nonanalogue,
cold-adapted vegetation community dominated the
Lateglacialâearly Holocene period (14 500â9000 cal yr BP,
which included trees and C3 Pooideae and C4 Panicoideae
grasses. The Lateglacial vegetation was fire-sensitive and fire
activity during this period was low, likely responding to fuel
availability and limitation. Although similar vegetation characterized
the early Holocene, the warming conditions associated
with the onset of the Holocene led to an initial increase
in fire activity. Huanchaca Mesetta became increasingly firedependent
during the middle Holocene with the expansion
of C4 fire-adapted grasses. However, as warm, dry conditions,
characterized by increased length and severity of the
dry season, continued, fuel availability decreased. The establishment
of the modern palm swamp vegetation occurred at
5000 cal yr BP. Edaphic factors are the first-order control on
vegetation on the rocky quartzite mesetta. Where soils are
sufficiently thick, climate is the second-order control of vegetation
on the mesetta. The presence of the modern palm
swamp is attributed to two factors: (1) increased precipitation
that increased water table levels and (2) decreased frequency
and duration of surazos (cold wind incursions from
Patagonia) leading to increased temperature minima. Natural
(soil, climate, fire) drivers rather than anthropogenic
drivers control the vegetation and fire activity at Huanchaca
Mesetta. Thus the cerrĂŁdo savanna ecosystem of the Huanchaca
Plateau has exhibited ecosystem resilience to major
climatic changes in both temperature and precipitation since
the Lateglacial period
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Pre-Columbian ring ditch construction and land use on a âchocolate forest islandâ in the Bolivian Amazon
We present a palaeoecological investigation of pre-Columbian land use in the savannah âforest islandâ landscape of north-east Bolivian Amazonia. A 5700 year sediment core from La Luna Lake, located adjacent to the La Luna forest island site, was analysed for fossil pollen and charcoal. We aimed to determine the palaeoenvironmental context of pre-Columbian occupation on the site and assess the environmental impact of land use in the forest island region. Evidence for anthropogenic burning and Zea mays L. cultivation began ~2000 cal a BP, at a time when the island was covered by savannah, under drier-than-present climatic conditions. After ~1240 cal a BP burning declined and afforestation occurred. We show that construction of the ring ditch, which encircles the island, did not involve substantial deforestation. Previous estimates of pre-Columbian population size in this region, based upon labour required for forest clearance, should therefore be reconsidered. Despite the high density of economically useful plants, such as Theobroma cacao, in the modern forest, no direct pollen evidence for agroforestry was found. However, human occupation is shown to pre-date and span forest expansion on this site, suggesting that here, and in the wider forest island region, there is no truly pre-anthropogenic âpristineâ forest
Instabilities in neutrino-plasma density waves
One examines the interaction and possible resonances between supernova
neutrinos and electron plasma waves. The neutrino phase space distribution and
its boundary regions are analyzed in detail. It is shown that the boundary
regions are too wide to produce non-linear resonant effects. The growth or
damping rates induced by neutrinos are always proportional to the neutrino flux
and .Comment: 9 pages, a few words modified to match PRD publicatio
A Fresh Look at Axions and SN 1987A
We re-examine the very stringent limits on the axion mass based on the
strength and duration of the neutrino signal from SN 1987A, in the light of new
measurements of the axial-vector coupling strength of nucleons, possible
suppression of axion emission due to many-body effects, and additional emission
processes involving pions. The suppression of axion emission due to nucleon
spin fluctuations induced by many-body effects degrades previous limits by a
factor of about 2. Emission processes involving thermal pions can strengthen
the limits by a factor of 3-4 within a perturbative treatment that neglects
saturation of nucleon spin fluctuations. Inclusion of saturation effects,
however, tends to make the limits less dependent on pion abundances. The
resulting axion mass limit also depends on the precise couplings of the axion
and ranges from 0.5x10**(-3) eV to 6x10**(-3) eV.Comment: 32 latex pages, 13 postscript figures included, uses revtex.sty,
submitted to Physical Review
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Ecosystem turnover in palaeoecological records: the sensitivity of pollen and phytolith proxies to detecting vegetation change in southwestern Amazonia
Identification of ecosystem turnover in the palaeo-vegetation record is important for understanding the resilience of ecosystems to past environmental change. There is uncertainty over the ability of different types of palaeo-vegetation proxy to detect ecosystem turnover. The aim of this paper is to compare the sensitivity of two palaeo-vegetation proxies - pollen and phytoliths - to changes within and between three key tropical South American ecosystems: evergreen forest, dry forest and savannah. A quantitative approach is used to assess the sensitivity of these two proxies to vegetation changes, based on the variability of proxy assemblages from 1-hectare ecological plots in ecotonal south west Amazonia. This modern dataset of proxy variability within evergreen forest, dry forest and savannah plots is then used to define thresholds for proxy variability which differentiate floristic changes within an ecosystem from ecosystem turnover. These thresholds are applied to two palaeo-vegetation records from NE Bolivia. Our results show that pollen is more sensitive than phytoliths to changes within evergreen forest, but phytoliths are more sensitive than pollen to changes within dry forest. Both proxies were equally sensitive to changes within savannas. These are important considerations for palaeoecologists selecting proxies for the study of ecosystem turnover in the palaeo-record. Application of the thresholds to the palaeo-record demonstrated the utility of this quantitative approach for assessing the magnitude of vegetation change in the palaeo-record. This quantitative approach is therefore a useful tool to improve the identification of ecosystem turnover in the palaeo-record
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