621 research outputs found
Scope and precision of sustainability assessment approaches to food systems
With sustainability within food systems becoming an increasingly important issue, several approaches that claim to assess the sustainability of farms, farming systems, and supply chains have been developed. Looking more closely at these sustainability impact assessment approaches, we discerned considerable differences between them in terms of scope, the level of assessment, and the precision of indicators used for impact assessment. Our aim was to classify and analyze a range of available sustainability impact assessment approaches with respect to scope and precision. From a total of 35 sustainability assessment approaches, we selected 6 for a detailed comparison. From our analysis, we concluded that there are 3 different types of trade-offs in these approaches: between different kinds of scope, between different indicators for precision and trade-offs, and between the scope and precision. Thus, one-size-fits-all solutions, with respect to tool selection, are rarely feasible. Furthermore, as indicator selection determines the assessment results, different and inconsistent indicators can lead to contradictory assessment results that may not be comparable. To overcome these shortcomings, sustainability impact assessments should include a precise definition of the notion of “sustainability” along with a description of the methodological approach and the indicator sets and should aim for harmonization of indicators and assumptions. Global initiatives such as the Sustainability Assessment in Food and Agriculture Systems (SAFA) Guidelines are a helpful step toward shedding light on the differences of these approaches and making the assessment results more comparable
COMPLETE SOLUTION OF THE XXZ-MODEL ON FINITE RINGS. DYNAMICAL STRUCTURE FACTORS AT ZERO TEMPERATURE.
The finite size effects of the dynamical structure factors in the XXZ-model
are studied in the euclidean time -representation. Away from the
critical momentum finite size effects turn out to be small except for
the large limit. The large finite size effects at the critical momentum
signal the emergence of infrared singularities in the spectral
-representation of the dynamical structure factors.Comment: PostScript file with 12 pages + 11 figures uuencoded compresse
Quantum algorithm for simulating the dynamics of an open quantum system
In the study of open quantum systems, one typically obtains the decoherence
dynamics by solving a master equation. The master equation is derived using
knowledge of some basic properties of the system, the environment and their
interaction: one basically needs to know the operators through which the system
couples to the environment and the spectral density of the environment. For a
large system, it could become prohibitively difficult to even write down the
appropriate master equation, let alone solve it on a classical computer. In
this paper, we present a quantum algorithm for simulating the dynamics of an
open quantum system. On a quantum computer, the environment can be simulated
using ancilla qubits with properly chosen single-qubit frequencies and with
properly designed coupling to the system qubits. The parameters used in the
simulation are easily derived from the parameters of the system+environment
Hamiltonian. The algorithm is designed to simulate Markovian dynamics, but it
can also be used to simulate non-Markovian dynamics provided that this dynamics
can be obtained by embedding the system of interest into a larger system that
obeys Markovian dynamics. We estimate the resource requirements for the
algorithm. In particular, we show that for sufficiently slow decoherence a
single ancilla qubit could be sufficient to represent the entire environment,
in principle.Comment: 5 figures, two table
Decoherence in a scalable adiabatic quantum computer
We consider the effects of decoherence on Landau-Zener crossings encountered
in a large-scale adiabatic-quantum-computing setup. We analyze the dependence
of the success probability, i.e. the probability for the system to end up in
its new ground state, on the noise amplitude and correlation time. We determine
the optimal sweep rate that is required to maximize the success probability. We
then discuss the scaling of decoherence effects with increasing system size. We
find that those effects can be important for large systems, even if they are
small for each of the small building blocks.Comment: 6 pages (two-column), 1 figur
Charge and spin dynamics in the one-dimensional and models
The impact of the spin-flip terms on the (static and dynamic) charge and spin
correlations in the Luttinger-liquid ground state of the 1D model is
assessed by comparison with the same quantities in the 1D model, where
spin-flip terms are absent. We employ the recursion method combined with a
weak-coupling or a strong-coupling continued-fraction analysis. At
we use the Pfaffian representation of dynamic spin correlations. The changing
nature of the dynamically relevant charge and spin excitations on approach of
the transition to phase separation is investigated in detail. The
charge excitations (but not the spin excitations) at the transition have a
single-mode nature, whereas charge and spin excitations have a complicated
structure in the model. In the model, phase separation is
accompanied by N\'eel long-range order, caused by the condensation of electron
clusters with an already existing alternating up-down spin configuration
(topological long-range order). In the model, by contrast, the spin-flip
processes in the exchange coupling are responsible for continued strong spin
fluctuations (dominated by 2-spinon excitations) in the phase-separated state.Comment: 11 pages (RevTex). 14 Figures available from author
The Mott-Hubbard Transition on the D=infinity Bethe Lattice
In view of a recent controversy we investigated the Mott-Hubbard transition
in D=infinity with a novel cluster approach. i) We show that any truncated
Bethe lattice of order n can be mapped exactly to a finite Hubbard-like
cluster. ii) We evaluate the self-energy numerically for n=0,1,2 and compare
with a series of self-consistent equation-of-motion solutions. iii) We find the
gap to open continously at the critical U_c~2.5t* (t = t* / sqrt{4d}). iv) A
low-energy theory for the Mott-Hubbard transition is developed and relations
between critical exponents are presented.Comment: Replaced with the published versio
Distributions of gaps and end-to-end correlations in random transverse-field Ising spin chains
A previously introduced real space renormalization-group treatment of the
random transverse-field Ising spin chain is extended to provide detailed
information on the distribution of the energy gap and the end-to-end
correlation function for long chains with free boundary conditions. Numerical
data, using the mapping of the problem to free fermions, are found to be in
good agreement with the analytic finite size scaling predictions.Comment: 12 pages revtex, 10 figures, submitted to Phys. Rev.
Dynamic properties of the spin-1/2 XY chain with three-site interactions
We consider a spin-1/2 XY chain in a transverse (z) field with multi-site
interactions. The additional terms introduced into the Hamiltonian involve
products of spin components related to three adjacent sites. A Jordan-Wigner
transformation leads to a simple bilinear Fermi form for the resulting
Hamiltonian and hence the spin model admits a rigorous analysis. We point out
the close relationships between several variants of the model which were
discussed separately in previous studies. The ground-state phases (ferromagnet
and two kinds of spin liquid) of the model are reflected in the dynamic
structure factors of the spin chains, which are the main focus in this study.
First we consider the zz dynamic structure factor reporting for this quantity a
closed-form expression and analyzing the properties of the two-fermion
(particle-hole) excitation continuum which governs the dynamics of transverse
spin component fluctuations and of some other local operator fluctuations. Then
we examine the xx dynamic structure factor which is governed by many-fermion
excitations, reporting both analytical and numerical results. We discuss some
easily recognized features of the dynamic structure factors which are
signatures for the presence of the three-site interactions.Comment: 28 pages, 10 fugure
Deeply sequenced metagenome and metatranscriptome of a biogas-producing microbial community from an agricultural production-scale biogas plant
Bremges A, Maus I, Belmann P, et al. Deeply sequenced metagenome and metatranscriptome of a biogas-producing microbial community from an agricultural production-scale biogas plant. GigaScience. 2015;4(1): 33.Background
The production of biogas takes place under anaerobic conditions and involves microbial decomposition of organic matter. Most of the participating microbes are still unknown and non-cultivable. Accordingly, shotgun metagenome sequencing currently is the method of choice to obtain insights into community composition and the genetic repertoire.
Findings
Here, we report on the deeply sequenced metagenome and metatranscriptome of a complex biogas-producing microbial community from an agricultural production-scale biogas plant. We assembled the metagenome and, as an example application, show that we reconstructed most genes involved in the methane metabolism, a key pathway involving methanogenesis performed by methanogenic Archaea. This result indicates that there is sufficient sequencing coverage for most downstream analyses.
Conclusions
Sequenced at least one order of magnitude deeper than previous studies, our metagenome data will enable new insights into community composition and the genetic potential of important community members. Moreover, mapping of transcripts to reconstructed genome sequences will enable the identification of active metabolic pathways in target organisms
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