9,152 research outputs found
Antiferromagnetism and Superconductivity in layered organic conductors: Variational cluster approach
The -(ET)X layered conductors (where ET stands for BEDT-TTF) are
studied within the dimer model as a function of the diagonal hopping
and Hubbard repulsion . Antiferromagnetism and d-wave superconductivity are
investigated at zero temperature using variational cluster perturbation theory
(V-CPT). For large , N\'eel antiferromagnetism exists for ,
with . For fixed , as is decreased (or pressure
increased), a superconducting phase appears. When is
decreased further, the a order takes over. There is a critical value
of of beyond which the AF and dSC phases are separated
by Mott disordered phase.Comment: 4 pages, 4 figures. Investigation of the d_xy phase added +
discussion of gap symmetr
Directly Indecomposables in Semidegenerate Varieties of Connected po-Groupoids
We study varieties with a term-definable poset structure, "po-groupoids". It
is known that connected posets have the "strict refinement property" (SRP). In
[arXiv:0808.1860v1 [math.LO]] it is proved that semidegenerate varieties with
the SRP have definable factor congruences and if the similarity type is finite,
directly indecomposables are axiomatizable by a set of first-order sentences.
We obtain such a set for semidegenerate varieties of connected po-groupoids and
show its quantifier complexity is bounded in general
Spin Bose-Metal phase in a spin-1/2 model with ring exchange on a two-leg triangular strip
Recent experiments on triangular lattice organic Mott insulators have found
evidence for a 2D spin liquid in proximity to the metal-insulator transition. A
Gutzwiller wavefunction study of the triangular lattice Heisenberg model with
appropriate four-spin ring exchanges has found that the projected spinon Fermi
sea state has a low variational energy. This wavefunction, together with a
slave particle gauge theory, suggests that such spin liquid possesses spin
correlations that are singular along surfaces in momentum space ("Bose
surfaces"). Signatures of this state, which we refer to as a "Spin Bose-Metal"
(SBM), are expected to be manifest in quasi-1D ladder systems: The discrete
transverse momenta cut through the 2D Bose surface leading to a distinct
pattern of 1D gapless modes. Here we search for a quasi-1D descendant of the
triangular lattice SBM state by exploring the Heisenberg plus ring model on a
two-leg strip (zigzag chain). Using DMRG, variational wavefunctions, and a
Bosonization analysis, we map out the full phase diagram. Without ring exchange
the model is equivalent to the J_1 - J_2 Heisenberg chain, and we find the
expected Bethe-chain and dimerized phases. Remarkably, moderate ring exchange
reveals a new gapless phase over a large swath of the phase diagram. Spin and
dimer correlations possess particular singular wavevectors and allow us to
identify this phase as the hoped for quasi-1D descendant SBM state. We derive a
low energy theory and find three gapless modes and one Luttinger parameter
controlling all power laws. Potential instabilities out of the zigzag SBM give
rise to other interesting phases such as a period-3 VBS or a period-4 Chirality
order, which we discover in the DMRG; we also find an interesting SBM state
with partial ferromagnetism.Comment: 30 pages, 23 figure
Automated extraction of knowledge for model-based diagnostics
The concept of accessing computer aided design (CAD) design databases and extracting a process model automatically is investigated as a possible source for the generation of knowledge bases for model-based reasoning systems. The resulting system, referred to as automated knowledge generation (AKG), uses an object-oriented programming structure and constraint techniques as well as internal database of component descriptions to generate a frame-based structure that describes the model. The procedure has been designed to be general enough to be easily coupled to CAD systems that feature a database capable of providing label and connectivity data from the drawn system. The AKG system is capable of defining knowledge bases in formats required by various model-based reasoning tools
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Developing theory-informed interventions to implement evidence into practice: a systematic approach using the Theoretical Domains Framework
Background: There is little systematic operational guidance about how best to develop complex interventions to reduce the gap between practice and evidence. This article is one in a series of articles documenting the development and use of the Theoretical Domains Framework (TDF) to advance the science of implementation research.
Methods: The intervention was developed considering three main components: theory, evidence, and practical issues. We used a four-step approach, consisting of guiding questions, to direct the choice of the most appropriate components of an implementation intervention: Who needs to do what, differently? Using a theoretical framework, which barriers and enablers need to be addressed? Which intervention components (behaviour change techniques and mode(s) of delivery) could overcome the modifiable barriers and enhance the enablers? And how can behaviour change be measured and understood?
Results: A complex implementation intervention was designed that aimed to improve acute low back pain management in primary care. We used the TDF to identify the barriers and enablers to the uptake of evidence into practice and to guide the choice of intervention components. These components were then combined into a cohesive intervention. The intervention was delivered via two facilitated interactive small group workshops. We also produced a DVD to distribute to all participants in the intervention group. We chose outcome measures in order to assess the mediating mechanisms of behaviour change.
Conclusions: We have illustrated a four-step systematic method for developing an intervention designed to change clinical practice based on a theoretical framework. The method of development provides a systematic framework that could be used by others developing complex implementation interventions. While this framework should be iteratively adjusted and refined to suit other contexts and settings, we believe that the four-step process should be maintained as the primary framework to guide researchers through a comprehensive intervention development process
Quantum entanglement between a nonlinear nanomechanical resonator and a microwave field
We consider a theoretical model for a nonlinear nanomechanical resonator
coupled to a superconducting microwave resonator. The nanomechanical resonator
is driven parametrically at twice its resonance frequency, while the
superconducting microwave resonator is driven with two tones that differ in
frequency by an amount equal to the parametric driving frequency. We show that
the semi-classical approximation of this system has an interesting fixed point
bifurcation structure. In the semi-classical dynamics a transition from stable
fixed points to limit cycles is observed as one moves from positive to negative
detuning. We show that signatures of this bifurcation structure are also
present in the full dissipative quantum system and further show that it leads
to mixed state entanglement between the nanomechanical resonator and the
microwave cavity in the dissipative quantum system that is a maximum close to
the semi-classical bifurcation. Quantum signatures of the semi-classical
limit-cycles are presented.Comment: 36 pages, 18 figure
Spin Bose-Metal and Valence Bond Solid phases in a spin-1/2 model with ring exchanges on a four-leg triangular ladder
We study a spin-1/2 system with Heisenberg plus ring exchanges on a four-leg
triangular ladder using the density matrix renormalization group and Gutzwiller
variational wave functions. Near an isotropic lattice regime, for moderate to
large ring exchanges we find a spin Bose-metal phase with a spinon Fermi sea
consisting of three partially filled bands. Going away from the triangular
towards the square lattice regime, we find a staggered dimer phase with dimers
in the transverse direction, while for small ring exchanges the system is in a
featureless rung phase. We also discuss parent states and a possible phase
diagram in two dimensions.Comment: 4 pages, 5 figures, v3 is the print versio
The effect of GSM-like ELF radiation on the alpha band of the human resting EEG
Mobile phone handsets such as those operating in the GSM network emit extremely low frequency electromagnetic fields ranging from DC to at least 40 kHz. As a subpart of an extended protocol, the influence of these fields on the human resting EEG has been investigated in a fully counter balanced, double blind, cross-over design study that recruited 72 healthy volunteers. A decrease in the alpha frequency band was observed during the 20 minutes of ELF exposure in the exposed hemisphere only. This result suggests that ELF fields as emitted from GSM handsets during the DTX mode may have an effect on the resting alpha band of the human EEG
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