290 research outputs found
Strongly Correlated States of Ultracold Rotating Dipolar Fermi Gases
We study strongly correlated ground and excited states of rotating quasi-2D
Fermi gases constituted of a small number of dipole-dipole interacting
particles with dipole moments polarized perpendicular to the plane of motion.
As the number of atoms grows, the system enters {\it an intermediate regime},
where ground states are subject to a competition between distinct bulk-edge
configurations. This effect obscures their description in terms of composite
fermions and leads to the appearance of novel composite fermion quasi-hole
states. In the presence of dipolar interactions, the principal Laughlin state
at filling exhibits a substantial energy gap for neutral (total
angular momentum conserving) excitations, and is well-described as an
incompressible Fermi liquid. Instead, at lower fillings, the ground state
structure favors crystalline order.Comment: 5 pages, 5 figures, paper presented at DPG Meeting 2006, as well as
Fritz Haber Institute Colloquiu
Using network analysis to explore co-ocurrence patterns in soil microbial communities.
9 páginas, 1 tabla, 4 figuras.Exploring large environmental datasets generated by high-throughput DNA sequencing technologies requires new analytical approaches to move beyond the basic inventory descriptions of the composition and diversity of natural microbial communities. In order to investigate potential interactions between microbial taxa, network analysis of significant taxon co-occurrence patterns may help to decipher the structure of complex microbial communities across spatial or temporal gradients. Here, we calculated associations between microbial taxa and applied network analysis approaches to a 16S rRNA gene barcoded pyrosequencing dataset containing >160 000 bacterial and archaeal sequences from 151 soil samples from a broad range of ecosystem types. We described the topology of the resulting network and defined operational taxonomic unit categories based on abundance and occupancy (that is, habitat generalists and habitat specialists). Co-occurrence patterns were readily revealed, including general non-random association, common life history strategies at broad taxonomic levels and unexpected relationships between community members. Overall, we demonstrated the potential of exploring inter-taxa correlations to gain a more integrated understanding of microbial community structure and the ecological rules guiding community assembly.AB is supported by the Spanish FPU
predoctoral scholarship program and EOC by grants
PIRENA CGL2009-13318 and CONSOLIDER-INGENIO
2010 GRACCIE CSD2007-00067 from the Spanish
Ministerio de Ciencia e Innovacio´n (MICINN).Peer reviewe
Non-Abelian spin singlet states of two-component Bose gases in artificial gauge fields
We study strongly correlated phases of a pseudo-spin-1/2 Bose gas in an
artificial gauge field using the exact diagonalization method. The atoms are
confined in two dimensions and interact via a two-body contact potential. In
Abelian gauge fields, pseudo-spin singlets are favored by pseudo-spin
independent interactions. We find a series of incompressible phases at fillings
\nu=2k/3. By comparison with the non-Abelian spin singlet (NASS) states,
constructed as zero-energy eigenstates of a (k+1)-body contact interaction, we
classify the non-trivial topology of the states. An additional spin-orbit
coupling is shown to switch between NASS-like states and spin-polarized phases
from the Read-Rezayi series.Comment: 4 pages, 3 figure
Scissors mode of trapped dipolar gases
We study the scissors modes of dipolar boson and fermion gases trapped in a
spherically symmetric potential. We use the harmonic oscillator states to solve
the time-dependent Gross-Pitaevskii equation for bosons and the time-dependent
Hartree-Fock equation for fermions. It is pointed out that the scissors modes
of bosons and fermions can be of quite different nature
Far-infrared edge modes in quantum dots
We have investigated edge modes of different multipolarity sustained by
quantum dots submitted to external magnetic fields. We present a microscopic
description based on a variational solution of the equation of motion for any
axially symmetric confining potential and multipole mode. Numerical results for
dots with different number of electrons whose ground-state is described within
a local Current Density Functional Theory are discussed. Two sum rules, which
are exact within this theory, are derived. In the limit of a large neutral dot
at B=0, we have shown that the classical hydrodynamic dispersion law for edge
waves \omega(q) \sim \sqrt{q \ln (q_0/q)} holds when quantum and finite size
effects are taken into account.Comment: We have changed some figures as well as a part of the tex
Fractional quantum Hall states of few bosonic atoms in geometric gauge fields
We employ the exact diagonalization method to analyze the possibility of
generating strongly correlated states in two-dimensional clouds of ultracold
bosonic atoms which are subjected to a geometric gauge field created by
coupling two internal atomic states to a laser beam. Tuning the gauge field
strength, the system undergoes stepwise transitions between different ground
states, which we describe by analytical trial wave functions, amongst them the
Pfaffian, the Laughlin, and a Laughlin quasiparticle many-body state. The
adiabatic following of the center of mass movement by the lowest energy dressed
internal state, is lost by the mixing of the second internal state. This
mixture can be controlled by the intensity of the laser field. The
non-adiabaticity is inherent to the considered setup, and is shown to play the
role of circular asymmetry. We study its influence on the properties of the
ground state of the system. Its main effect is to reduce the overlap of the
numerical solutions with the analytical trial expressions by occupying states
with higher angular momentum. Thus, we propose generalized wave functions
arising from the Laughlin and Pfaffian wave function by including components,
where extra Jastrow factors appear, while preserving important features of
these states. We analyze quasihole excitations over the Laughlin and
generalized Laughlin states, and show that they possess effective fractional
charge and obey anyonic statistics. Finally, we study the energy gap over the
Laughlin state as the number of particles is increased keeping the chemical
potential fixed. The gap is found to decrease as the number of particles is
increased, indicating that the observability of the Laughlin state is
restricted to a small number of particles.Comment: 28 pages, 16 figure
Far-infrared edge modes in quantum dots
We have investigated edge modes of different multipolarity sustained by
quantum dots submitted to external magnetic fields. We present a microscopic
description based on a variational solution of the equation of motion for any
axially symmetric confining potential and multipole mode. Numerical results for
dots with different number of electrons whose ground-state is described within
a local Current Density Functional Theory are discussed. Two sum rules, which
are exact within this theory, are derived. In the limit of a large neutral dot
at B=0, we have shown that the classical hydrodynamic dispersion law for edge
waves \omega(q) \sim \sqrt{q \ln (q_0/q)} holds when quantum and finite size
effects are taken into account.Comment: We have changed some figures as well as a part of the tex
Consistent responses of soil microbial communities to elevated nutrient inputs in grasslands across the globe
Soil microorganisms are critical to ecosystem functioning and the maintenance of soil fertility. However, despite global increases in the inputs of nitrogen (N) and phosphorus (P) to ecosystems due to human activities, we lack a predictive understanding of how microbial communities respond to elevated nutrient inputs across environmental gradients. Here we used high-throughput sequencing of marker genes to elucidate the responses of soil fungal, archaeal, and bacterial communities using an N and P addition experiment replicated at 25 globally distributed grassland sites. We also sequenced metagenomes from a subset of the sites to determine how the functional attributes of bacterial communities change in response to elevated nutrients. Despite strong compositional differences across sites, microbial communities shifted in a consistent manner with N or P additions, and the magnitude of these shifts was related to the magnitude of plant community responses to nutrient inputs. Mycorrhizal fungi and methanogenic archaea decreased in relative abundance with nutrient additions, as did the relative abundances of oligotrophic bacterial taxa. The metagenomic data provided additional evidence for this shift in bacterial life history strategies because nutrient additions decreased the average genome sizes of the bacterial community members and elicited changes in the relative abundances of representative functional genes. Our results suggest that elevated N and P inputs lead to predictable shifts in the taxonomic and functional traits of soil microbial communities, including increases in the relative abundances of faster-growing, copiotrophic bacterial taxa, with these shifts likely to impact belowground ecosystems worldwide
Corrigendum: Stoichiometric Shifts in Soil C:N:P Promote Bacterial Taxa Dominance, Maintain Biodiversity, and Deconstruct Community Assemblages
Stoichiometric Shifts in Soil C:N:P Promote Bacterial Taxa Dominance, Maintain Biodiversity, and Deconstruct Community Assemblages
Imbalances in C:N:P supply ratios may cause bacterial resource limitations and constrain biogeochemical processes, but the importance of shifts in soil stoichiometry are complicated by the nearly limitless interactions between an immensely rich species pool and a multiple chemical resource forms. To more clearly identify the impact of soil C:N:P on bacteria, we evaluated the cumulative effects of single and coupled long-term nutrient additions (i.e., C as mannitol, N as equal concentrations NH4+ and NO3−, and P as Na3PO4) and water on communities in an Antarctic polar desert, Taylor Valley. Untreated soils possessed relatively low bacterial diversity, simplified organic C sources due to the absence of plants, limited inorganic N, and excess soil P potentially attenuating links between C:N:P. After 6 years of adding resources, an alleviation of C and N colimitation allowed one rare Micrococcaceae, an Arthrobacter species, to dominate, comprising 47% of the total community abundance and elevating soil respiration by 136% relative to untreated soils. The addition of N alone reduced C:N ratios, elevated bacterial richness and diversity, and allowed rare taxa relying on ammonium and nitrite for metabolism to become more abundant [e.g., nitrite oxidizing Nitrospira species (Nitrosomonadaceae), denitrifiers utilizing nitrite (Gemmatimonadaceae) and members of Rhodobacteraceae with a high affinity for ammonium]. Based on community co-occurrence networks, lower C:P ratios in soils following P and CP additions created more diffuse and less connected communities by disrupting 73% of species interactions and selecting for taxa potentially exploiting abundant P. Unlike amended nutrients, water additions alone elicited no lasting impact on communities. Our results suggest that as soils become nutrient rich a wide array of outcomes are possible from species dominance and the deconstruction of species interconnectedness to the maintenance of biodiversity
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