135,838 research outputs found
Effective Charge and Spin Hamiltonian for the Quarter-Filled Ladder Compound -NaVO
An effective intra- and inter-ladder charge-spin hamiltonian for the
quarter-filled ladder compound -NaVO has been derived by using
the standard canonical transformation method. In the derivation, it is clear
that a finite inter-site Coulomb repulsion is needed to get a meaningful result
otherwise the perturbation becomes ill-defined. Various limiting cases
depending on the values of the model parameters have been analyzed in detail
and the effective exchange couplings are estimated. We find that the effective
intra-ladder exchange may become ferromagnetic for the case of zig-zag charge
ordering in a purely electronic model.
We estimate the magnitude of the effective inter-rung Coulomb repulsion in a
ladder and find it to be about one-order of magnitude too small in order to
stabilize charge-ordering.Comment: Eur. Phys. J. B (submitted
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Site-specific mutations in a minimal voltage-dependent K+ channel alter ion selectivity and open-channel block.
MinK is a small membrane protein of 130 amino acids with a single potential membrane-spanning alpha-helical domain. Its expression in Xenopus oocytes induces voltage-dependent, K(+)-selective channels. Using site-directed mutagenesis of a synthetic gene, we have identified residues in the hydrophobic region of minK that influence both ion selectivity and open-channel block. Single amino acid changes increase the channel's relative permeability for NH4+ and Cs+ without affecting its ability to exclude Na+ and Li+. Blockade by two common K+ channel pore blockers, tetraethylammonium and Cs+, was also modified. These results suggest that an ion selectivity region and binding sites for the pore blockers within the conduction pathway have been modified. We conclude that the gene encoding minK is a structural gene for a K+ channel protein
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Amy's story: a research agenda for smoking cessation in pregnancy
The purpose of this paper is to report on a case from Nottinghamshire County Primary Care Trust (PCT) as an exploratory study examining the role of social marketing’s contribution to smoking cessation during pregnancy. Insights from the case will be used to inform and stimulate debate around future inquiry regarding the effectiveness of such campaigns, specifically with respect to smoking cessation in pregnancy, the role of low-budget highly-localised programmes and, in response to a recent Chartered Institute of Marketing (CIM) paper (Thorp, 2009), the extent to which social marketers lead the way in producing behavioural change in populations. Whilst well discussed in the health literature, smoking cessation during pregnancy remains under-researched in the marketing literature offering opportunities for research and practice. Also, this study is of contemporary interest in light of the proposed public sector cuts which will restrict social marketing budgets, the move of Public Health to local authority control placing it as central to the Government’s public health plans, and the Government’s reported interest in behavioural change techniques such as nudge theory (Stamp, 2010). The paper is structured by presenting the case using The National Social Marketing Centre’s (NSMC, 2010) benchmark criteria for effective social marketing, whilst identifying findings and themes from the literature
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The charybdotoxin receptor of a Shaker K+ channel: peptide and channel residues mediating molecular recognition.
Charybdotoxin (CTX) is a peptide of known structure that inhibits Shaker K+ channels by a pore-blocking mechanism. Point mutagenesis of all 30 solvent-exposed residues identified the part of the CTX molecular surface making contact with the receptor in the K+ channel. All close-contact residues are clustered in a well-defined interaction surface; the shape of this surface implies that the outer opening of the Shaker channel conduction pore abruptly widens to a 25 x 35 A plateau. A mutagenic scan of the S5-S6 linker sequence of the Shaker K+ channel identified those channel residues influencing CTX binding affinity. The Shaker residues making the strongest contribution to toxin binding are located close to the pore-lining sequence, and more distant residues on both sides of this region influence CTX binding weakly, probably by an electrostatic mechanism. Complementary mutagenesis of both CTX and Shaker suggests that Shaker-F425 contacts a specific area near T8 and T9 on the CTX molecular surface. This contact point constrains Shaker-F425 to be located at a 20 A radial distance from the pore axis and 10-15 A above the "floor" of the CTX receptor
A Generalized Ginzburg-Landau Approach to Second Harmonic Generation
We develop a generalized Ginzburg-Landau theory for second harmonic
generation (SHG) in magnets by expanding the free energy in terms of the order
parameter in the magnetic phase and the susceptibility tensor in the
corresponding high-temperature phase. The non-zero components of the SHG
susceptibility in the ordered phase are derived from the symmetries of the
susceptibility tensor in the high-temperature phase and the symmetry of the
order parameter. In this derivation, the dependence of the SHG susceptibility
on the order parameter follows naturally, and therefore its nonreciprocal
optical properties.
We examine this phenomenology for the magnetoelectric compound CrO as
well as for the ferroelectromagnet YMnO.Comment: European Journal of Physics B (accepted
Series active variable geometry suspension application to comfort enhancement
This paper explores the potential of the Series Active Variable Geometry Suspension (SAVGS) for comfort and road holding enhancement. The SAVGS concept introduces significant nonlinearities associated with the rotation of the mechanical link that connects the chassis to the spring-damper unit. Although conventional linearization procedures implemented in multi-body software packages can deal with this configuration, they produce linear models of reduced applicability. To overcome this limitation, an alternative linearization approach based on energy conservation principles is proposed and successfully applied to one corner of the car, thus enabling the use of linear robust control techniques. An H∞ controller is synthesized for this simplified quarter-car linear model and tuned based on the singular value decomposition of the system's transfer matrix. The proposed control is thoroughly tested with one-corner and full-vehicle nonlinear multi-body models. In the SAVGS setup, the actuator appears in series with the passive spring-damper and therefore it would typically be categorized as a low bandwidth or slow active suspension. However, results presented in this paper for an SAVGS-retrofitted Grand Tourer show that this technology has the potential to also improve the high frequency suspension functions such as comfort and road holding
Superfluid and insulating phases of fermion mixtures in optical lattices
The ground state phase diagram of fermion mixtures in optical lattices is
analyzed as a function of interaction strength, fermion filling factor and
tunneling parameters. In addition to standard superfluid, phase-separated or
coexisting superfluid/excess-fermion phases found in homogeneous or
harmonically trapped systems, fermions in optical lattices have several
insulating phases, including a molecular Bose-Mott insulator (BMI), a
Fermi-Pauli (band) insulator (FPI), a phase-separated BMI/FPI mixture or a
Bose-Fermi checkerboard (BFC). The molecular BMI phase is the fermion mixture
counterpart of the atomic BMI found in atomic Bose systems, the BFC or BMI/FPI
phases exist in Bose-Fermi mixtures, and lastly the FPI phase is particular to
the Fermi nature of the constituent atoms of the mixture.Comment: 4 pages with 3 figures (Published version
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