Extreme Electron-Phonon Coupling in Boron-based Layered Superconductors

The phonon-mode decomposition of the electron-phonon coupling in the MgB2-like system Li_{1-x}BC is explored using first principles calculations. It is found that the high temperature superconductivity of such systems results from extremely strong coupling to only ~2% of the phonon modes. Novel characteristics of E_2g branches include (1) ``mode lambda'' values of 25 and greater compared to a mean of $\sim 0.4$ for other modes, (2) a precipitous Kohn anomaly, and (3) E_2g phonon linewidths within a factor of ~2 of the frequency itself, indicating impending breakdown of linear electron-phonon theory. This behavior in borne out by recent inelastic x-ray scattering studies of MgB2 by Shukla et al.Comment: 4 two-column pages, 4 figures. Equations simplified. Figure 4 changed. Comparison with new data include

Ab Initio Evidence for the Formation of Impurity d(3z^2-r^2) Holes in Doped La_{2-x}Sr_xCuO_4

Using the spin unrestricted Becke-3-Lee-Yang-Parr density functional, we computed the electronic structure of explicitly doped La_{2-x}Sr_xCuO_4 (x = 0.125, 0.25, and 0.5). At each doping level, an impurity hole band is formed within the undoped insulating gap. This band is well-localized to CuO_6 octahedra adjacent to the Sr impurities. The nature of the impurity hole is A_{1g} in symmetry, formed primarily from the z^2 orbital on the Cu and p_z orbitals on the apical O's. There is a strong triplet coupling of this hole with the intrinsic B_{1g} Cu x^2-y^2/O1 p_{sigma} hole on the same site. Optimization of the c coordinate of the apical O's in the doped CuO_6 octahedron lead to an asymmetric anti-Jahn-Teller distortion of the O2 atoms toward the central Cu. In particular, the O2 atom between the Cu and Sr is displaced 0.26 A while the O2 atom between the Cu and La is displaced 0.10 A. Contrary to expectations, investigation of a 0.1 A enhanced Jahn-Teller distortion of this octahedron does not force formation of an x^2-y^2 hole, but instead leads to migration of the z^2 hole to the four other CuO_6 octahedra surrounding the Sr impurity. This latter observation offers a simple explanation for the bifurcation of the Sr-O2 distance revealed in x-ray absorption fine structure data.Comment: Submitted to Phys. Rev. B. See http://www.firstprinciples.com for more informatio

Probing Bogoliubov Quasiparticles in Superfluid 3He with a ‘Vibrating-Wire Like’ MEMS Device

International audienceWe have measured the interaction between superfluid 3 He-B and a micro-machined goalpost-shaped device at temperatures below 0.2 T c. The measured damping follows well the theory developed for vibrating wires, in which the An-dreev reflection of quasiparticles in the flow field around the moving structure leads to a nonlinear frictional force. At low velocities the damping force is proportional to velocity while it tends to saturate for larger excitations. Above a velocity of 2.6 mms −1 the damping abruptly increases, which is interpreted in terms of Cooper-pair breaking. Interestingly, this critical velocity is significantly lower than reported with other mechanical probes immersed in superfluid 3 He. Furthermore , we report on a nonlinear resonance shape for large motion amplitudes that we interpret as an inertial effect due to quasiparticle friction, but other mechanisms could possibly be invoked as well. PACS numbers: 85.85.+j, 67.30.H-, 67.30.e

Fermi Surfaces of Diborides: MgB2 and ZrB2

We provide a comparison of accurate full potential band calculations of the Fermi surfaces areas and masses of MgB2 and ZrB2 with the de Haas-van Alphen date of Yelland et al. and Tanaka et al., respectively. The discrepancies in areas in MgB2 can be removed by a shift of sigma-bands downward with respect to pi-bands by 0.24 eV. Comparison of effective masses lead to orbit averaged electron-phonon coupling constants lambda(sigma)=1.3 (both orbits), lambda(pi)=0.5. The required band shifts, which we interpret as an exchange attraction for sigma states beyond local density band theory, reduces the number of holes from 0.15 to 0.11 holes per cell. This makes the occurrence of superconductivity in MgB2 a somewhat closer call than previously recognized, and increases the likelihood that additional holes can lead to an increased Tc.Comment: 7 pages including 4 figure

Academic advocacy in public health: Disciplinary ‘duty’ or political ‘propaganda’?

The role of ‘advocacy’ within public health attracts considerable debate but is rarely the subject of empirical research. This paper reviews the available literature and presents data from qualitative research (interviews and focus groups conducted in the UK in 2011–2013) involving 147 professionals (working in academia, the public sector, the third sector and policy settings) concerned with public health in the UK. It seeks to address the following questions: (i) What is public health advocacy and how does it relate to research?; (ii) What role (if any) do professionals concerned with public health feel researchers ought to play in advocacy?; and (iii) For those researchers who do engage in advocacy, what are the risks and challenges and to what extent can these be managed/mitigated? In answering these questions, we argue that two deeply contrasting conceptualisations of ‘advocacy’ exist within public health, the most dominant of which (‘representational’) centres on strategies for ‘selling’ public health goals to decision-makers and the wider public. This contrasts with an alternative (less widely employed) conceptualisation of advocacy as ‘facilitational’. This approach focuses on working with communities whose voices are often unheard/ignored in policy to enable their views to contribute to debates. We argue that these divergent ways of thinking about advocacy speak to a more fundamental challenge regarding the role of the public in research, policy and practice and the activities that connect these various strands of public health research