464 research outputs found
Neel order, quantum spin liquids and quantum criticality in two dimensions
This paper is concerned with the possibility of a direct second order
transition out of a collinear Neel phase to a paramagnetic spin liquid in two
dimensional quantum antiferromagnets. Contrary to conventional wisdom, we show
that such second order quantum transitions can potentially occur to certain
spin liquid states popular in theories of the cuprates. We provide a theory of
this transition and study its universal properties in an expansion.
The existence of such a transition has a number of interesting implications for
spin liquid based approaches to the underdoped cuprates. In particular it
considerably clarifies existing ideas for incorporating antiferromagnetic long
range order into such a spin liquid based approach.Comment: 18 pages, 17 figure
Scientistsʼ warning on climate change and medicinal plants
The recent publication of a World Scientistsʼ Warning to Humanity highlighted the fact that climate change, absent strenuous mitigation or adaptation efforts, will have profound negative effects for humanity and other species, affecting numerous aspects of life. In this paper, we call attention to one of these aspects, the effects of climate change on medicinal plants. These plants provide many benefits for human health, particularly in communities where Western medicine is unavailable. As for other species, their populations may be threatened by changing temperature and precipitation regimes, disruption of commensal relationships, and increases in pests and pathogens, combined with anthropogenic habitat fragmentation that impedes migration. Additionally, medicinal species are often harvested unsustainably, and this combination of pressures may push many populations to extinction. A second issue is that some species may respond to increased environmental stresses not only with declines in biomass production but with changes in chemical content, potentially affecting quality or even safety of medicinal products. We therefore recommend actions including conservation and local cultivation of valued plants, sustainability training for harvesters and certification of commercial material, preservation of traditional knowledge, and programs to monitor raw material quality in addition to, of course, efforts to mitigate climate change
Scientists’ Warning on Climate Change and Medicinal Plants
The recent publication of a World Scientists’ Warning to Humanity highlighted the fact that climate change, absent strenuous mitigation or adaptation efforts, will have profound negative effects for humanity and other species, affecting numerous aspects of life. In this paper, we call attention to one of these aspects, the effects of climate change on medicinal plants. These plants provide many benefits for human health, particularly in communities where Western medicine is unavailable. As for other species, their populations may be threatened by changing temperature and precipitation regimes, disruption of commensal relationships, and increases in pests and pathogens, combined with anthropogenic habitat fragmentation that impedes migration. Additionally, medicinal species are often harvested unsustainably, and this combination of pressures may push many populations to extinction. A second issue is that some species may respond to increased environmental stresses not only with declines in biomass production but with changes in chemical content, potentially affecting quality or even safety of medicinal products. We therefore recommend actions including conservation and local cultivation of valued plants, sustainability training for harvesters and certification of commercial material, preservation of traditional knowledge, and programs to monitor raw material quality, in addition to, of course, efforts to mitigate climate change
Exploring Entrepreneurial Skills and Competencies in Farm Tourism
Diversification to farm tourism is increasingly seen as a viable development strategy to promote a more diverse and sustainable rural economy and to counter declining farm incomes. However, our understanding of the dynamics of the modern farm tourism business and the entrepreneurial and competitive skills farmers require in making the transition from agriculture to a diversified - and service based - enterprise remains limited. Hence, the aim of this paper is to explore the range of skills and competencies that farmers in the North West of England identify as important when adopting a diversification strategy to farm tourism. With the findings indicating that that whilst a range of managerial skills are valued by farmers, they lack many of the additional business and entrepreneurial competencies required for success. Moreover, this paper acknowledges the need to generate consensus on the requisite skill-set that farm tourism operators require, along with a need for a currently fragmented rural tourism literature to acknowledge the significance of rural entrepreneurship and the characteristics of successful farmers and farm tourism ventures
Interlayer pair tunneling and gap anisotropy in YBaCuO
Recent ARPES measurement observed a large -axis gap anisotropy,
, in clean YBaCuO. This
indicates that some sub-dominant component may exist in the -wave
dominant gap. We propose that the interlayer pairing tunneling contribution can
be determined through the investigation of the order parameter anisotropy.
Their potentially observable features in transport and spin dynamics are also
studied.Comment: 4 pages, 3 figure
A minimum single-band model for low-energy excitations in superconducting KFeSe
We propose a minimum single-band model for the newly discovered iron-based
superconducting KFeSe. Our model is found to be numerically
consistent with the five-orbital model at low energies. Based on our model and
the random phase approximation, we study the spin fluctuation and the pairing
symmetry of superconducting gap function. The spin excitation
and the pairing symmetry are revealed. All of the results can
well be understood in terms of the interplay between the Fermi surface topology
and the local spin interaction, providing a sound picture to explain why the
superconducting transition temperature is as high as to be comparable to those
in pnictides and some cuprates. A common origin of superconductivity is
elucidated for this compound and other high-T materials.Comment: 5 pages, 4 figure
A link between the spin fluctuation and Fermi surface in high T_C cuprates --- A consistent description within the single-band Hubbard model
A link between the spin fluctuation and the "fermiology" is explored for the
single-band Hubbard model within the fluctuation exchange (FLEX) approximation.
We show that the experimentally observed peak position of the spin structure in
the high T_C cuprates can be understood from the model that reproduces the
experimentally observed Fermi surface. In particular, both the variation of the
incommensurability of the peak in the spin structure and the evolution of the
Fermi surface with hole doping in La_{2-x}Sr_xCuO_4 may be understood with a
second nearest neighbor hopping decreasing with hole doping.Comment: 5 pages, RevTeX, uses epsf.sty and multicol.st
Predicting a Gapless Spin-1 Neutral Collective Mode branch for Graphite
Using the standard tight binding model of 2d graphite with short range
electron repulsion, we find a gapless spin-1, neutral collective mode branch
{\em below the particle-hole continuum} with energy vanishing linearly with
momenta at the and points in the BZ. This spin-1 mode has a wide
energy dispersion, 0 to and is not Landau damped. The `Dirac cone
spectrum' of electrons at the chemical potential of graphite generates our
collective mode; so we call this `spin-1 zero sound' of the `Dirac sea'.
Epithermal neutron scattering experiments, where graphite single crystals are
often used as analyzers (an opportunity for `self-analysis'!), and spin
polarized electron energy loss spectroscopy (SPEELS) can be used to confirm and
study our collective mode.Comment: 4 pages of LaTex file, 3 eps figure file
N\'eel transition, spin fluctuations, and pseudogap in underdoped cuprates by a Lorentz invariant four-fermion model in 2+1 dimensions
We show that the N\'eel transition and spin fluctuations near the N\'eel
transition in planar cuprates can be described by an SU(2) invariant
relativistic four-fermion model in 2+1 dimensions. Features of the pseudogap
phenomenon are naturally described by the appearance of an anomalous dimension
for the spinon propagator.Comment: 5 pages, 2 figures (revtex4). Final revised and corrected versio
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