We present new upper critical field Hc2(T ) data in a broad temperature region 0.3K ≤ T ≤ Tc for LuNi2B2C and YNi2B2C single crystals with well characterized low impurity scattering rates.The absolute values for all T , in particular Hc2(0), and the sizeable positive curvature (PC) of Hc2(T ) at high and intermediate T are explained quantitatively within an effective two-band model. The failure of the isotropic single band approach is discussed in detail. Supported by de Haas van Alphen data, the superconductivity reveals direct insight into details of the electronic structure. The observed maximal PC near Tc gives strong evidence for clean limit type II superconductors. 74.60.Ec, 74.70.Ad, 74.72Ny The discovery [1,2] of superconductivity in transition metal borocarbides has generated large interest due to their relatively high transition temperatures T c ∼ 15 to 23 K and due to the relation between the mechanisms of superconductivity in these compounds, in cuprates, and in ordinary transition metals. Another highlight is the coexistence of magnetism and superconductivity in some of these compounds containing rare earth elements [3][4][5]. A study of the non-magnetic compounds such as LNi 2 B 2 C, with L=Lu,Y,Th,Sc [6], is a prerequisite for the understanding of their magnetic counterparts. Experimental data for LuNi 2 B 2 C [7] demonstrate beside a maximal positive curvature (PC) of H c2 (T ) near T c , observed also for YNi 2 B 2 C [8,4,9], a weak T -dependent anisotropy within the tetragonal basal plane and a Tindependent out-of-plane anisotropy of the upper critical field H c2 . Both anisotropies have been described [7] in terms of nonlocal corrections to the Ginzburg-Landau (GL) equations. In this picture the PC of H c c2 ( H to the tetragonal c-axis) is caused, almost purely, by the basal plane anisotropy. However, it should be noted that the reported anisotropy of H c2 for YNi 2 B 2 C is significantly smaller than for LuNi 2 B 2 C [7,9,10] whereas its PC is comparable or even larger. Further explanations of the unusual PC of H c2 (T ), such as quasi-2D fluctuations [11], are excluded by the underestimation of H c2 (T ) at low-T [9] and the observed weak anisotropy. The quantum critical point scenario [12] as well as the bipolaronic one [13] can be disregarded because the slope of H c2 (T ) decreases for T →0 (see Fig. 1). Local density approximation (LDA) band structure calculations [14,15] predict a nearly isotropic electronic structure with rather complicated bands near the Fermi level E F . However, in analyzing the superconductivity in terms of an isotropic single-band (ISB) Eliashberg model, the multi-band character and the anisotropic Fermi surface have been widely ignored so far.Here we present and analyze theoretically new data of H c2 (T ) in a broad interval 0.3K≤ T ≤ T c for high purity LuNi 2 B 2 C and YNi 2 B 2 C single crystals. We show that typical features of both compounds, such as H c2 (0) ∼ 8 to 10 T and the unusual PC of H c2 (T ) for T > ∼ 0.5T c , cannot in any way be explained ...

The interface of Cu͑001͒ electrode surfaces in 10 mM HCl solution was studied by in situ surface x-ray diffraction and density functional theory, focusing on the precise structure of the c͑2 ϫ 2͒ Cl adlayer formed at positive potentials. Crystal truncation rod measurements in this adsorbate phase at a potential of −0.20 V Ag/AgCl reveal distinct differences to corresponding data by Tolentino et al. ͓Surf. Sci. 601, 2962 ͑2007͔͒ for the c͑2 ϫ 2͒ Cl structure formed at the Cu͑001͒-vacuum interface. Although in both environments, the atoms in the second Cu layer exhibit a small vertical corrugation, the sign of this corrugation is reversed. Furthermore, also the Cu-Cl bond distance and the average Cu interlayer spacings at the surface differ. Ab initio calculations performed for this adsorbate system reproduce these effects-specifically the reversal of the subsurface second-layer buckling caused in the presence of coadsorbed water molecules and cations in the outer part of the electrochemical double layer. In addition, studies at more negative potentials reveal a continuous surface phase transition to a disordered Cl adlayer at −0.62 V Ag/AgCl , but indicate a substantial Cl coverage even at the onset of hydrogen evolution.

Direct in situ x-ray surface scattering studies of growth at a solid-liquid interface are demonstrated using the homoepitaxial electrodeposition on Au(100) as an example. With decreasing potential transitions from step-flow to layer-by-layer growth, manifested by layering oscillations in the x-ray intensity, then to multilayer growth, and finally back to layer-by-layer growth were observed. This complex growth behavior can be explained by the effect of anion coadsorbates and the potential-dependent Au surface reconstruction on the Au surface mobility.

The adsorption and desorption of bis-(3-sulfopropyl) disulfide (SPS) on Cu and Au electrodes and its electrochemical effect on Cu deposition and dissolution were examined using cyclic voltammetry stripping (CVS), field-emission scanning electron microscopy (FESEM), and X-ray photoelectron spectroscopy (XPS). SPS dissociates into 3-mercapto-1-propanesulfonate when it is contacted with Au and Cu electrodes, producing Cu(I)- and Au(I)-thiolate species. These thiolates couple with chloride ions and promote not only the reduction of Cu(2+) in Cu deposition but also the oxidation of Cu(0) to Cu(+) in Cu stripping. During Cu electrodeposition on the SPS-modified Au electrode, thiolates transfer from Au onto the Cu underpotential deposition (UPD) layer. The Cu UPD layer stabilizes a large part of the transferred thiolates which subsequently is buried by the Cu overpotential deposition (OPD) layer. The buried thiolates reappear on the Au electrode after the copper deposit is electrochemically stripped off. A much smaller part of thiolates transfers to the top of the Cu OPD layer. In contrast, when SPS preadsorbs on a Cu-coated Au electrode, almost all of the adsorbed SPS leaves the Cu surface during Cu electrochemical stripping and does not return to the uncovered Au surface. A reaction mechanism is proposed to explain these results.

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