In order to probe the order parameter symmetry of the heavy-fermion superconductor (HFS) CeCoIn5, we employ point-contact spectroscopy, where dynamic conductance spectra are taken from a nano-scale junction between a normal-metal (N) Au tip and a single crystal of CeCoIn5. The point-contact junction (PCJ) is formed on a single crystal surface with two crystallographic orientations, (001) and (110). Our conductance spectra, reproducibly obtained over wide ranges of temperature, constitute the cleanest data sets ever reported for HFSs. The point contacts are shown to be in the Sharvin limit, ensuring spectroscopic nature of the measured data. A signature for the emerging heavy-fermion liquid is evidenced by the development of the asymmetry in the background conductance, starting at T* (~ 45 K) and increasing with decreasing temperature down to Tc (2.3 K). Below Tc, an enhancement of the sub-gap conductance arising from Andreev reflection is observed, with the magnitude of ~ 13.3% and ~ 11.8% for the (001) and the (110) PCJ, respectively. These values are an order of magnitude smaller than those observed in conventional superconductors, but consistent with those in other HFSs. Our zero-bias conductance data for the (001) PCJ are best fit with the extended Blonder-Tinkham-Klapwijk model using the d-wave order parameter. The fit to the full conductance curve of the (001) PCJ at 400 mK indicates the strong coupling nature (2Δ/kBTc = 4.64). However, our observed suppression of both the Andreev reflection signal and the energy gap indicates the failure of existing models. We provide possible directions for theoretical formulations of the electronic transport across an N/HFS interface in general, and the Au/CeCoIn5 interface in particular. Several qualitative features observed in the (110) PCJ provide the first clear spectroscopic evidence for the dx2-y2 symmetry of the superconducting order parameter in CeCoIn5.
Structural, magnetic and electronic properties of compounds in the series R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$ (R=Nd,Sm,..., and Lu) were studied. Resistivity, Meissner-effect, and shielding measurements have revealed superconductivity among all the rare-earth compounds, except La, Pr, and Tb, with critical temperatures ${T}_{c}$ measured at the midpoint of the resistive transition ranging from 87 to 95 K. No depression of ${T}_{c}$ was observed upon introduction of most of the rare-earth magnetic ions. Susceptibility measurements down to 1.6 K have shown that an antiferromagnetic ordering (most likely due to dipole-dipole interactions) occurs only for the Gd compound. Changes in oxygen content in these materials drastically affect their physical properties. The importance of the cooling rate during the synthesis of the sample has been correlated to oxygen content. ${T}_{c}$'s are optimized by slow cooling. Annealing at 700\ifmmode^\circ\else\textdegree\fi{}C in oxygen pressure of 40 atmospheres slightly increase ${T}_{c}$, while annealing under vacuum at 420\ifmmode^\circ\else\textdegree\fi{}C destroys ${T}_{c}$ and induces a semiconducting behavior. These changes in oxygen content and ${T}_{c}$ are perfectly reversible.
Planar tunneling spectroscopy is performed into the a-b plane of the high-temperature superconductor Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}. The tunneling spectra exhibit a zero-bias conductance peak (ZBCP). Preliminary studies as a function of temperature, crystallographic orientation, magnetic field magnitude and direction confirm the ZBCP is an Andreev bound state (ABS) at zero energy. Below 5K, a depletion in the density of states at zero energy is observed.
High-quality thin Nb and NbN films (60-100 \AA{}) are grown on (100) ${n}^{+}\ensuremath{-}\mathrm{InAs}$ ${(n=10}^{19}{\mathrm{cm}}^{\ensuremath{-}3})$ substrates by dc-magnetron sputter deposition. Studies of the electronic properties of interfaces between the superconductor and the semiconductor are done by Raman scattering measurements. The superconducting proximity effect at superconductor-semiconductor interfaces is observed through its impact on inelastic light scattering intensities originating from the near-interface region of InAs. The InAs longitudinal optical phonon LO mode $(237{\mathrm{cm}}^{\ensuremath{-}1})$ and the plasmon-phonon coupled modes ${L}_{\ensuremath{-}}$ $(221{\mathrm{cm}}^{\ensuremath{-}1})$ and ${L}_{+}$ (1100 to $1350{\mathrm{cm}}^{\ensuremath{-}1}),$ for ${n}^{+}=1\ifmmode\times\else\texttimes\fi{}{10}^{19}\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}{10}^{19}{\mathrm{cm}}^{\ensuremath{-}3}$ are measured. The intensity ratio of the LO mode (associated with the near-surface charge accumulation region, in InAs) to that of the ${L}_{\ensuremath{-}}$ mode (associated with bulk InAs), is observed to increase by up to 40% below the superconducting transition temperature. This temperature-dependent change in light scattering properties is only observed with high quality superconducting films and when the superconductor and the semiconductor are in good electrical contact. A few possible mechanisms of the observed effect are proposed.
Superconducting critical transitions with an onset at 112 K and zero resistance at 107 K are obtained within the Bi-Sr-Ca-Cu-O system. The synthesis and formation of the 110-K superconducting phase using the 85-K material as a precursor is explained. The 110-K phase grows from the 85-K phase such that the resulting faceted crystal (a pseudomorph) can contain some of the 85-K phase in the core. With such a microstructure our magnetic data can be simply explained. A major structural difference between the 85- and 110-K materials is that the 85-K material can grow (relatively) large single crystals having long-range order whereas the 110-K material has only intermediate-range order (cryptocrystalline) of about 100-200 \AA{}.
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