H. Takagi

The muon-spin-relaxation rate $\ensuremath{\sigma}$ has been measured in sixteen specimens of high-${T}_{c}$ cuprate superconductors (the 2:1:4, 1:2:3, 2:2:1:2, and 2:2:2:3 series). This has allowed us to study the magnetic field penetration depth $\ensuremath{\lambda}$ and thus the superconducting carrier density ${n}_{s}$ divided by the effective mass ${m}^{*}(\ensuremath{\sigma}\ensuremath{\propto}\frac{1}{{\ensuremath{\lambda}}^{2}}\ensuremath{\propto}\frac{{n}_{s}}{{m}^{*}})$. A universal linear relation between ${T}_{c}$ and $\ensuremath{\sigma}(T\ensuremath{\rightarrow}0)\ensuremath{\propto}\frac{{n}_{s}}{{m}^{*}}$ has been found with increasing carrier doping. In heavily doped samples, however, ${T}_{c}$ shows saturation and suppression with increasing $\frac{{n}_{s}}{{m}^{*}}$. This saturation starts at different values of $\frac{{n}_{s}}{{m}^{*}}$ for materials with different multiplicities of CuO planes.

Optical reflectivity spectra are studied for single crystals of the prototypical high-${\mathit{T}}_{\mathit{c}}$ system ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ over a wide compositional range 0\ensuremath{\le}x\ensuremath{\le}0.34, which covers insulating, superconducting, and normal metallic phases. The measurements are made at room temperature over an energy range from 0.004 to 35 eV for the polarization parallel to the ${\mathrm{CuO}}_{2}$ planes. They are also extended to the perpendicular polarization to study anisotropy and to discriminate the contribution from the ${\mathrm{CuO}}_{2}$ plane. The present study focuses on the x dependence of the optical spectrum, which makes it possible to sort out the features of the excitations in the ${\mathrm{CuO}}_{2}$ plane and thus to characterize the electronic structure of the ${\mathrm{CuO}}_{2}$ plane in the respective phase. Upon doping into the parent insulator ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ with a charge-transfer energy gap of about 2 eV the spectral weight is rapidly transferred from the charge-transfer excitation to low-energy excitations below 1.5 eV. The low-energy spectrum is apparently composed of two contributions; a Drude-type one peaked at \ensuremath{\omega}=0 and a broad continuum centered in the midinfrared range. The high-${\mathit{T}}_{\mathit{c}}$ superconductivity is realized as doping proceeds and when the transfer of the spectrum weight is saturated. The resulting spectrum in the high-${\mathit{T}}_{\mathit{c}}$ regime is suggestive of a strongly itinerant character of the state in the moderately doped ${\mathrm{CuO}}_{2}$ plane while appreciable weight remains in the charge-transfer energy region. The spectrum exhibits a second drastic change for heavy doping (x\ensuremath{\sim}0.25) corresponding to the superconductor-to-normal-metal transition and becomes close to that of a Fermi liquid. The results are universal for all the known cuprate superconductors including the electron-doped compounds, and they reconcile the dc transport properties with the high-energy spectroscopic results.

We have discovered that the ${\mathrm{Ce}}^{4+}$ doping and subsequent annealing in reducing atmosphere give rise to 24-K superconductivity in the ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$-type structure with sheets of Cu-O squares. In contrast to the previously reported high-${\mathrm{T}}_{\mathrm{c}}$ cuprates, the charge carriers in the new superconductors are doped electrons, not holes; this was confirmed by the measurements of Hall and Seebeck coefficients as well as by chemical analysis of the effective copper valence. An anomalous dependence of ${\mathrm{T}}_{\mathrm{c}}$ on the concentration of doped electrons is shown for these electron-doped superconducting suprates.

Transport and magnetic properties of (${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${)}_{2}$${\mathrm{CuO}}_{4}$ are systematically investigated over a wide composition range up to x=0.175 including a nonsuperconducting metal phase in the heavily doped region. Remarkable changes associated with the superconductor-to-nonsuperconductor transition are observed both in the Hall coefficient and the magnetic susceptibility, suggesting the modification of both charge and spin states.