The room-temperature Raman spectrum of YBa2Cu3O7−x is sensitive both to stoichiometry and to the transition from the semiconducting to the superconducting state as a function of oxygen annealing. In particular, it is shown that a good quality film with a Tc of ∼90 K and a sharp resistivity transition has not been produced if the frequency of the O(1) stretch along the c axis shifts from 500 cm−1 to lower frequencies. In this paper, two types of processing failures are illustrated: (1) lack of sufficient oxygen and (2) incorrect stoichiometry. No attempt has been made to predict Tc from the Raman data for less than optimum films.
We have measured the upper critical field ${B}_{c2}$(T) of ${\mathrm{Y}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{y}}$ with x=0.4 near ${T}_{c}$ in dc fields up to 20 T and at 77 K with pulsed fields to 45 T. The data yield (${\mathrm{dB}}_{c2}$/dT${)}_{T={T}_{c}}$=1.3 T/K and ${T}_{c}$=89.5 K for the midpoint of the resistive transitions, and (${\mathrm{dB}}_{c2}$/dT${)}_{T={T}_{c}}$=5\ifmmode\pm\else\textpm\fi{}1 T/K and ${T}_{c}$=95.1 K for the onset. The estimates for ${B}_{c2}$(0) range from 80 to 320 T. Pulsed-field data suggest that a fraction of the material is still superconducting above 45 T at 77 K.
We discuss the effects of doping on the Cu chain sites in YBa{sub 2}Cu{sub 3-x}M{sub x}O{sub 6+y}. The relationship between bond lengths obtained from neutron scattering and charge transfer is evaluated in terms of bond valence. In particular, it is concluded that removing an oxygen from the chains transfers one electron to the planes. 24 refs., 3 figs.
We review recent progress in point contact spectroscopy (PCS) to extract spectroscopic information out of correlated electron materials, with the emphasis on non-superconducting states. PCS has been used to detect bosonic excitations in normal metals, where signatures (e.g. phonons) are usually less than 1$\%$ of the measured conductance. In the superconducting state, point contact Andreev reflection (PCAR) has been widely used to study properties of the superconducting gap in various superconductors. In the last decade, there have been more and more experimental results suggesting that the point contact conductance could reveal new features associated with the unusual single electron dynamics in non-superconducting states, shedding a new light on exploring the nature of the competing phases in correlated materials. We will summarize the theories for point contact spectroscopy developed from different approaches and highlight these conceptual differences distinguishing point contact spectroscopy from tunneling-based probes. Moreover, we will show how the Schwinger-Kadanoff-Baym-Keldysh (SKBK) formalism together with the appropriate modeling of the nano-scale point contacts randomly distributed across the junction leads to the conclusion that the point contact conductance is proportional to the {\it effective density of states}, a physical quantity that can be computed if the electron self energy is known. The experimental data on iron based superconductors and heavy fermion compounds will be analyzed in this framework. These recent developments have extended the applicability of point contact spectroscopy to correlated materials, which will help us achieve a deeper understanding of the single electron dynamics in strongly correlated systems.
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