NLTE analysis of Sr lines in spectra of late-type stars with new R-matrix atomic data

We investigate statistical equilibrium of neutral and singly-ionized strontium in late-type stellar atmospheres. Particular attention is given to the completeness of the model atom, which includes new energy levels, transition probabilities, photoionization and electron-impact excitation cross-sections computed with the R-matrix method. The NLTE model is applied to the analysis of Sr I and Sr II lines in the spectra of the Sun, Procyon, Arcturus, and HD 122563, showing a significant improvement in the ionization balance compared to LTE line formation calculations, which predict abundance discrepancies of up to 0.5 dex. The solar Sr abundance is log A = 2.93 \pm 0.04 dex, in agreement with the meteorites. A grid of NLTE abundance corrections for Sr I and Sr II lines covering a large range of stellar parameters is presented.Comment: 11 pages, 7 figures, accepted for publication in Astronomy & Astrophysic

Detailed Topography of the Fermi Surface of Sr2RuO4

We apply a novel analysis of the field and angle dependence of the quantum-oscillatory amplitudes in the unconventional superconductor Sr2RuO4 to map its Fermi surface in unprecedented detail, and to obtain previously inaccessible information on the band dispersion. The three quasi-2D Fermi surface sheets not only exhibit very diverse magnitudes of warping, but also entirely different dominant warping symmetries. We use the data to reassess recent results on c-axis transport phenomena.Comment: REVTeX, 4 page

Rigid-Band Shift of the Fermi Level in a Strongly Correlated Metal: Sr(2-y)La(y)RuO(4)

We report a systematic study of electron doping of Sr2RuO4 by non-isovalent substitution of La^(3+) for Sr^(2+). Using a combination of de Haas-van Alphen oscillations, specific heat, and resistivity measurements, we show that electron doping leads to a rigid-band shift of the Fermi level corresponding to one doped electron per La ion, with constant many-body quasiparticle mass enhancement over the band mass. The susceptibility spectrum is substantially altered and enhanced by the doping but this has surprisingly little effect on the strength of the unconventional superconducting pairing.Comment: 4 pages, 3 figure

Fermi Surface Reconstruction in CeRh1−x_{1-x}Cox_{x}In5_{5}

The evolution of the Fermi surface of CeRh$_{1-x}$Co$_x$In$_5$ was studied as a function of Co concentration $x$ via measurements of the de Haas-van Alphen effect. By measuring the angular dependence of quantum oscillation frequencies, we identify a Fermi surface sheet with $f$-electron character which undergoes an abrupt change in topology as $x$ is varied. Surprisingly, this reconstruction does not occur at the quantum critical concentration $x_c$, where antiferromagnetism is suppressed to T=0. Instead we establish that this sudden change occurs well below $x_c$, at the concentration x ~ 0.4 where long range magnetic order alters its character and superconductivity appears. Across all concentrations, the cyclotron effective mass of this sheet does not diverge, suggesting that critical behavior is not exhibited equally on all parts of the Fermi surface.Comment: 4 pages, 4 figure

Properties of the Nearly Free Electron Superconductor Ag5Pb2O6 Inferred from Fermi Surface Measurements

We measured the Fermi surface of the recently discovered superconductor Ag5Pb2O6 via a de Haas-van Alphen rotation study. Two frequency branches were observed and identified with the neck and belly orbits of a very simple, nearly free electron Fermi surface. We use the observed Fermi surface geometry to quantitatively deduce superconducting properties such as the in-plane and out-of-plane penetration depths, the coherence length in the clean limit, and the critical field; as well as normal state properties such as the specific heat and the resistivity anisotropy.Comment: 2 pages, 1 figure, submitted to Physica C (M2S Proceedings

Interpretation of the angular dependence of the de Haas-van Alphen effect in MgB_2

We present detailed results for the amplitude and field dependence of the de Haas-van Alphen (dHvA) signal arising from the electron-like $\pi$ sheet of Fermi surface in MgB_2. Our data and analysis show that the dip in dHvA amplitude when the field is close to the basal plane is caused by a beat between two very similar dHvA frequencies and not a spin-zero effect as previously assumed. Our results imply that the Stoner enhancement factors in MgB_2 are small on both the Sigma and Pi sheets.Comment: 4 pages with figures. Submitted to PR

Determining the superconducting gap structure in Sr2RuO4 from sound attenuation studies below Tc

This work presents a quantitative theoretical study of the sound attenuation in the unconventional multiband superconductor Sr2RuO4 below the superconducting transition temperature Tc. Sound attenuation in this material is shown to have the remarkable property of being able to identify different nodal structures on different bands. The nodal structures on the \gamma band on the one hand, and on the \alpha and \beta bands on the other, are both found to be characterized by the existence of point nodes, but are significantly different in their quantitative aspects.Comment: 7 pages, REVTe

ARES+MOOG - a practical overview of an EW method to derive stellar parameters

The goal of this document is to describe the important practical aspects in the use of an Equivalent Width (EW) method for the derivation of spectroscopic stellar parameters. A general description of the fundamental steps composing any EW method is given, together with possible differences that may be found in different methods used in the literature. Then ARES+MOOG is then used as an example where each step of the method is described in detail. A special focus is given for the specific steps of this method, namely the use of a differential analysis to define the atomic data for the adopted line list, the automatic EW determinations, and the way to find the best parameters at the end of the procedure. Finally, a practical tutorial is given, where we focus on simple exercises useful to illustrate and explain the dependence of the abundances with the assumed stellar parameters. The interdependences are described and a clear procedure is given to find the "final" stellar parameters.Comment: 15 pages, 4 figures, accepted for publication as a chapter in "Determination of Atmospheric Parameters of B, A, F and G Type Stars", Springer (2014), eds. E. Niemczura, B. Smalley, W. Pyc