62 research outputs found
Green's function approach to the magnetic properties of the kagome antiferromagnet
The Heisenberg antiferromagnet is studied on the kagom\'e lattice by
using a Green's function method based on an appropriate decoupling of the
equations of motion. Thermodynamic properties as well as spin-spin correlation
functions are obtained and characterize this system as a two-dimensional
quantum spin liquid. Spin-spin correlation functions decay exponentially with
distance down to low temperature and the calculated missing entropy at T=0 is
found to be . Within the present scheme, the specific heat exhibits
a single peak structure and a dependence at low temperature.Comment: 6 (two-column revtex4) pages, 5 ps figures. Submitted to Phys. Rev.
Orbital liquid in three dimensional Mott insulator:
We present a theory of spin and orbital states in Mott insulator .
The spin-orbital superexchange interaction between ions in cubic
crystal suffers from a pathological degeneracy of orbital states at classical
level. Quantum effects remove this degeneracy and result in the formation of
the coherent ground state, in which the orbital moment of level is
fully quenched. We find a finite gap for orbital excitations. Such a disordered
state of local degrees of freedom on unfrustrated, simple cubic lattice is
highly unusual. Orbital liquid state naturally explains observed anomalies of
.Comment: 5 pages, 3 figure
Accounting for nutrition-related health impacts in food life cycle assessment: insights from an expert workshop
\ua9 The Author(s) 2024.Sub-optimal dietary patterns make major contributions to the Global Burden of Disease and are among the most pressing issues affecting human health. Consequently, they are key to consider when assessing the human health and other environmental impacts of foods and diets within life cycle assessments. The UN Environment Life Cycle Initiative convened a task force on nutrition-related human health impacts as part of the Global Life Cycle Impact Assessment Method (GLAM) project. The health impacts of dietary patterns can be expressed in disability-adjusted life years (DALYs), in line with reporting human health impacts of other impact categories within the life cycle impact assessment (LCIA) framework. The task force held a workshop with nutrition experts to receive guidance in its process to develop a consensus-based impact assessment framework for addressing nutrition-related health impacts in LCIA. The workshop aimed to (1) evaluate the general assessment framework, (2) discuss scientific questions for quantifying human health impacts from nutrition for food items and diets, and (3) provide initial guidance for further development. The proposed framework based on the Global Burden of Disease (GBD) risk ratios was regarded as a good starting point to assess the relative health risks of the general population, provided that the dietary context is considered and several limitations, such as incomplete disease coverage, are acknowledged. The experts advised against a potentially misleading use of adult-derived dietary risk factors for children. To improve global coverage of the GLAM framework, it is important to consider a wider range of dietary patterns. The experts also recommended using a metric complementary to DALYs, such as nutrient adequacy, also considering, e.g., vitamin A and iron, to complement the assessment
Magneto-thermodynamics of the spin-1/2 Kagome antiferromagnet
In this paper, we use a new hybrid method to compute the thermodynamic
behavior of the spin-1/2 Kagome antiferromagnet under the influence of a large
external magnetic field. We find a T^2 low-temperature behavior and a very low
sensitivity of the specific heat to a strong external magnetic field. We
display clear evidence that this low temperature magneto-thermal effect is
associated to the existence of low-lying fluctuating singlets, but also that
the whole picture (T^2 behavior of Cv and thermally activated spin
susceptibility) implies contribution of both non magnetic and magnetic
excitations. Comparison with experiments is made.Comment: 4 pages, LaTeX 2.09 and RevTeX with 3 figures embedded in the text.
Version to appear in Phys. Rev. Let
Magnetization of coupled spin clusters in Ladder Geometry
In this paper, we construct a class of spin-1/2 antiferromagnetic (AFM)
two-chain ladder models consisting of blocks of n-spin tetrahedral clusters
alternating with two-spin rungs. For n=4 and 6 and in extended parameter
regimes, the exact ground state of the ladder is shown to be a product of the
ground states of the rungs and the n-spin blocks, in both zero and finite
magnetic fields. In the latter case, magnetization/site (m) versus magenetic
field (h) plot exhibits well-defined magnetization plateaus.Comment: 9 pages, latex, 6 figures, To be published in Phys. Rev.
Transition to Long Range Magnetic Order in the Highly Frustrated Insulating Pyrochlore Antiferromagnet Gd_2Ti_2O_7
Experimental evidence from measurements of the a.c. and d.c. susceptibility,
and heat capacity data show that the pyrochlore structure oxide, Gd_2Ti_2O_7,
exhibits short range order that starts developing at 30K, as well as long range
magnetic order at K. The Curie-Weiss temperature, =
-9.6K, is largely due to exchange interactions. Deviations from the Curie-Weiss
law occur below 10K while magnetic heat capacity contributions are found
at temperatures above 20K. A sharp maximum in the heat capacity at K
signals a transition to a long range ordered state, with the magnetic specific
accounting for only 50% of the magnetic entropy. The heat capacity above
the phase transition can be modeled by assuming that a distribution of random
fields acts on the ground state for Gd. There is no
frequency dependence to the a.c. susceptibility in either the short range or
long range ordered regimes, hence suggesting the absence of any spin-glassy
behavior. Mean field theoretical calculations show that no long range ordered
ground state exists for the conditions of nearest-neighbor antiferromagnetic
exchange and long range dipolar couplings. At the mean-field level, long range
order at various commensurate or incommensurate wave vectors is found only upon
inclusion of exchange interactions beyond nearest-neighbor exchange and dipolar
coupling. The properties of Gd$_2Ti_2O_7 are compared with other geometrically
frustrated antiferromagnets such as the Gd_3Ga_5O_{12} gadolinium gallium
garnet, RE_2Ti_2O_7 pyrochlores where RE = Tb, Ho and Tm, and Heisenberg-type
pyrochlore such as Y_2Mo_2O_7, Tb_2Mo_2O_7, and spinels such as ZnFe_2O_4Comment: Letter, 6 POSTSCRIPT figures included. (NOTE: Figure 5 is not
included --) To appear in Physical Review B. Contact:
[email protected]
Susceptibility and dilution effects of the kagome bi-layer geometrically frustrated network. A Ga-NMR study of SrCr_(9p)Ga_(12-9p)O_(19)
We present an extensive gallium NMR study of the geometrically frustrated
kagome bi-layer compound SrCr_(9p)Ga_(12-9p)O_(19) (Cr^3+, S=3/2) over a broad
Cr-concentration range (.72<p<.95). This allows us to probe locally the kagome
bi-layer susceptibility and separate the intrinsic properties due to the
geometric frustration from those related to the site dilution. Our major
findings are: 1) The intrinsic kagome bi-layer susceptibility exhibits a
maximum in temperature at 40-50 K and is robust to a dilution as high as ~20%.
The maximum reveals the development of short range antiferromagnetic
correlations; 2) At low-T, a highly dynamical state induces a strong wipe-out
of the NMR intensity, regardless of dilution; 3) The low-T upturn observed in
the macroscopic susceptibility is associated to paramagnetic defects which stem
from the dilution of the kagome bi-layer. The low-T analysis of the NMR
lineshape suggests that the defect can be associated with a staggered
spin-response to the vacancies on the kagome bi-layer. This, altogether with
the maximum in the kagome bi-layer susceptibility, is very similar to what is
observed in most low-dimensional antiferromagnetic correlated systems; 4) The
spin glass-like freezing observed at T_g=2-4 K is not driven by the
dilution-induced defects.Comment: 19 pages, 19 figures, revised version resubmitted to PRB Minor
modifications: Fig.11 and discussion in Sec.V on the NMR shif
Area of Concern: A new paradigm in life cycle assessment for the development of footprint indicators
Purpose As a class of environmental metrics, footprints have been poorly defined, have shared an unclear relationship to Life Cycle Assessment (LCA), and the variety of approaches to quantification have sometimes resulted in confusing and contradictory messages in the marketplace. In response, a task force operating under the auspices of the UNEP/SETAC Life Cycle Initiative project on environmental Life Cycle Impact Assessment (LCIA) has been working to develop generic guidance for developers of footprint metrics. The purpose of this paper is to introduce a universal footprint definition and related terminology as well as to discuss modelling implications.
Methods The task force has worked from the perspective that footprints should be underpinned by the same data systems and models as used in LCA. However, there are important differences in purpose and orientation relative to LCA impact category indicators. Footprints have a primary orientation toward society and nontechnical stakeholders. They are also typically of narrow scope, having the purpose of reporting only in relation to specific topics. In comparison, LCA has a primary orientation toward stakeholders interested in comprehensive evaluation of overall environmental performance and trade-offs among impact categories. These differences create tension between footprints, the existing LCIA framework based on the Area of Protection paradigm, and the core LCA standards ISO14040/44.
Results In parallel to Area of Protection, we introduce Area of Concern as the basis for a universal footprint definition. In the same way that LCA uses impact category indicators to assess impacts that follow a common cause-effect pathway toward Areas of Protection, footprint metrics address Areas of Concern. The critical difference is that Areas of Concern are defined by the interests of stakeholders in society rather than the LCA community. In addition, Areas of Concern are stand-alone and not necessarily part of a framework intended for comprehensive environmental performance assessment. The Area of Concern paradigm is needed to support the development of footprints in a way that fulfils their distinctly different purpose. It is also needed as a
mechanism to extricate footprints from some of the provisions of ISO 14040/44 which are not considered relevant. Specific issues are identified in relation to double counting, aggregation, and the selection of relevant indicators.
Conclusions The universal footprint definition and related terminology introduced in this paper create a foundation that will support the development of footprint metrics in parallel with LCA
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