1,619 research outputs found
Theory of magnetization plateaux in the Shastry-Sutherland model
Using perturbative continuous unitary transformations, we determine the
long-range interactions between triplets in the Shastry-Sutherland model, and
we show that an unexpected structure develops at low magnetization with
plateaux progressively appearing at 2/9, 1/6, 1/9 and 2/15 upon increasing the
inter-dimer coupling. A critical comparison with previous approaches is
included. Implications for the compound SrCu(BO) are also
discussed: we reproduce the magnetization profile around localized triplets
revealed by NMR, we predict the presence of a 1/6 plateau, and we suggest that
residual interactions beyond the Shastry-Sutherland model are responsible for
the other plateaux below 1/3.Comment: 5 pages, 6 figure
Phase diagram of the fully frustrated transverse-field Ising model on the honeycomb lattice
Motivated by the current interest in the quantum dimer model on the
triangular lattice, we investigate the phase diagram of the closely related
fully-frustrated transverse field Ising model on the honeycomb lattice using
classical and semi-classical approximations. We show that, in addition to the
fully polarized phase at large field, the classical model possesses a multitude
of phases that break the translational symmetry which in the dimer language,
correspond to a plaquette phase and a columnar phase separated by an infinite
cascade of mixed phases. The modification of the phase diagram by quantum
fluctuations has been investigated in the context of linear spin-wave theory.
The extrapolation of the semiclassical energies suggests that the plaquette
phase extends down to zero field for spin 1/2, in agreement with the
phase of the quantum dimer model on the triangular
lattice with only kinetic energy.Comment: 15 Pages, 11 Figures, Accepted for PR
Field-induced gap in ordered Heisenberg antiferromagnets
Heisenberg antiferromagnets in a strong uniform magnetic field are
expected to exhibit a gapless phase with a global O(2) symmetry. In many real
magnets, a small energy gap is induced by additional interactions that can be
viewed as a staggered transverse magnetic field , where is a small
proportionality constant. We study the effects of such a perturbation,
particularly for magnets with long-range order, by using several complimentary
approaches: numerical diagonalizations of a model with long-range interactions,
classical equations of motion, and scaling arguments. In an ordered state at
zero temperature, the energy gap at first grows as and then may
dip to a smaller value, of order , at the quantum critical point
separating the ``gapless'' phase from the gapped state with saturated
magnetization. In one spatial dimension, the latter exponent changes to 4/5.Comment: 6 pages, 5 figure
Supersolid phase induced by correlated hopping in spin-1/2 frustrated quantum magnets
We show that correlated hopping of triplets, which is often the dominant
source of kinetic energy in dimer-based frustrated quantum magnets, produces a
remarkably strong tendency to form supersolid phases in a magnetic field. These
phases are characterized by simultaneous modulation and ordering of the
longitudinal and transverse magnetization respectively. Using Quantum Monte
Carlo and a semiclassical approach for an effective hard-core boson model with
nearest-neighbor repulsion on a square lattice, we prove in particular that a
supersolid phase can exist even if the repulsion is not strong enough to
stabilize an insulating phase at half-filling. Experimental implications for
frustrated quantum antiferromagnets in a magnetic field at zero and finite
temperature are discussed.Comment: 4 pages; 4 figures; published versio
Frustrated three-leg spin tubes: from spin 1/2 with chirality to spin 3/2
Motivated by the recent discovery of the spin tube
[(CuCltachH)Cl]Cl, we investigate the properties of a frustrated
three-leg spin tube with antiferromagnetic intra-ring and inter-ring couplings.
We pay special attention to the evolution of the properties from weak to strong
inter-ring coupling and show on the basis of extensive density matrix
renormalization group and exact diagonalization calculations that the system
undergoes a first-order phase transition between a dimerized gapped phase at
weak coupling that can be described by the usual spin-chirality model and a
gapless critical phase at strong coupling that can be described by an effective
spin-3/2 model. We also show that there is a magnetization plateau at 1/3 in
the gapped phase and slightly beyond. The implications for
[(CuCltachH)Cl]Cl are discussed, with the conclusion that this
system behaves essentially as a spin-3/2 chain.Comment: 8 pages, 9 figures, revised versio
Global land use implications of biofuels: State of the art conference and workshop on modelling global land use implications in the environmental assessment of biofuels
Background, Aims and Scope On 4¿5 June 2007, an international conference was held in Copenhagen. It provided an interdisciplinary forum where economists and geographers met with LCA experts to discuss the challenges of modelling the ultimate land use changes caused by an increased demand for biofuels. Main Features The main feature of the conference was the cross-breeding of experience from the different approaches to land use modelling: The field of LCA could especially benefit from economic modelling in the identification of marginal crop production and the resulting expansion of the global agricultural area. Furthermore, the field of geography offers insights in the complexity behind new land cultivation and practical examples of where this is seen to occur on a regional scale. Results Results presented at the conference showed that the magnitude and location of land use changes caused by biofuels demand depend on where the demand arises. For instance, mandatory blending in the EU will increase land use both within and outside of Europe, especially in South America. A key learning for the LCA society was that the response to a change in demand for a given crop is not presented by a single crop supplier or a single country, but rather by responses from a variety of suppliers of several different crops in several countries. Discussion The intensification potential of current and future crop and biomass production was widely discussed. It was generally agreed that some parts of the third world hold large potentials for intensification, which are not realised due to a number of barriers resulting in so-called yield gaps. Conclusions Modelling the global land use implications of biofuels requires an interdisciplinary approach optimally integrating economic, geographical, biophysical, social and possibly other aspects in the modelling. This interdisciplinary approach is necessary but also difficult due to different perspectives and mindsets in the different disciplines. Recommendations and Perspectives The concept of a location dependent marginal land use composite should be introduced in LCA of biofuels and it should be acknowledged that the typical LCA assumption of linear substitution is not necessarily valid. Moreover, fertiliser restrictions/accessibility should be included in land use modelling and the relation between crop demand and intensification should be further explored. In addition, environmental impacts of land use intensification should be included in LCA, the powerful concept of land use curves should be further improved, and so should the modelling of diminishing returns in crop production
A national facilitation project to improve primary palliative care : impact of the Gold Standards Framework on process and self-ratings of quality
Background: Improving quality of end-of-life care is a key driver of UK policy. The Gold Standards Framework (GSF) for Palliative Care aims to strengthen primary palliative care through facilitating implementation of systematic clinical and organisational processes.
Objectives: To describe the general practices that participated in the GSF programme in 2003–5 and the changes in process and perception of quality that occurred in the year following entry into the programme, and to identify factors associated with the extent of change.
Methods: Participating practices completed a questionnaire at baseline and another approximately 12 months later. Data were derived from categorical questions about the implementation of 35 organisational and clinical processes, and self-rated assessments of quality, associated with palliative care provision.
Participants: 1305 practices (total registered population almost 10 million). Follow-up questionnaire completed by 955 (73.2%) practices (after mean (SD) 12.8 (2.8) months; median 13 months).
Findings: Mean increase in total number of processes implemented (maximum = 35) was 9.6 (95% CI 9.0 to 10.2; p<0.001; baseline: 15.7 (SD 6.4), follow-up: 25.2 (SD 5.2)). Extent of change was largest for practices with low baseline scores. Aspects of process related to coordination and communication showed the greatest change. All dimensions of quality improved following GSF implementation; change was highest for the "quality of palliative care for cancer patients" and "confidence in assessing, recording and addressing the physical and psychosocial areas of patient care".
Conclusion: Implementation of the GSF seems to have resulted in substantial improvements in process and quality of palliative care. Further research is required of the extent to which this has enhanced care (physical, practical and psychological outcomes) for patients and carers
Single-particle versus pair condensation of hard-core bosons with correlated hopping
We investigate the consequences of correlated hopping on the ground state
properties of hard-core bosons on a square lattice as revealed by extensive
exact diagonalizations and quantum Monte Carlo simulations. While for non
interacting hard-core bosons the effective attraction induced by the correlated
hopping leads to phase separation at low density, we show that a modest
nearest-neighbor repulsion suppresses phase separation, leading to a remarkable
low-density pairing phase with no single particle Bose-Einstein condensation
but long-range two-particle correlations, signaling a condensation of pairs. We
also explain why the unusual properties of the pairing phase are a real
challenge for standard one-worm quantum Monte Carlo simulations.Comment: 8 pages, 7 figure
One particle interchain hopping in coupled Hubbard chains
Interchain hopping in systems of coupled chains of correlated electrons is
investigated by exact diagonalizations and Quantum-Monte-Carlo methods. For two
weakly coupled Hubbard chains at commensurate densities (e.g. n=1/3) the
splitting at the Fermi level between bonding and antibonding bands is strongly
reduced (but not suppressed) by repulsive interactions extending to a few
lattice spacings. The magnitude of this reduction is directly connected to the
exponent of the 1D Luttinger liquid. However, we show that the
incoherent part of the single particle spectral function is much less affected
by the interchain coupling. This suggests that incoherent interchain hopping
could occur for intermediate values.Comment: 4 pages, LaTeX 3.0, 7 PostScript figures in uuencoded for
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