${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ is the only homomorphic member of the entire rare-earth 1:2:3 family which is insulating and not metallic or superconducting. With this unusual behavior in mind, and with an eye to resolving certain questions regarding the widely held notion of otherwise mobile holes held captive in a supratrivalent Pr ionic state as its cause, we examined the effect of varying oxygen concentration on the structural, electrical, and magnetic properties of this compound. Powder x-ray diffraction studies revealed an orthorhombic-to-tetragonal order-disorder transition at oxygen levels close to those found for the superconducting lanthanide compounds. From Rietveld-refined powder-neutron-diffraction data taken on a fully oxygenated sample, key insights into the ambivalent character of Pr valency were obtained. Other aspects of Pr valency and carrier dynamics were uncovered by electrical and magnetic measurements as a function of y. We found that the room-temperature resistivity increases by about three orders of magnitude from y\ensuremath{\simeq}0 to y\ensuremath{\simeq}0.5, while the temperature dependence of the paramagnetic susceptibility remains quantitatively the same. We conclude from these results that removal of oxygen from ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ does not substantially alter the valence state of Pr, which we assert to be nominally 3+, but with strong overlap of its outer-lying 4f orbital with neighboring oxygen 2p levels results in a characteristically mixed or fluctuating valence situation reminiscent of heavy-fermion systems, and the principal effect on transport properties is on carrier concentration in the chains and steric hindrance of their motion by the resulting oxygen vacancies.

It is shown that samples of La2−xSrxCuO4 prepared under 100 bar oxygen pressure exhibit a metallic conductivity which increases as x increases up to x=0.34. In spite of this behavior, superconductivity disappears with Tc decreasing from x=0.15 to x=0.25. This decrease in Tc is shown to be continuous (and not dominated by phase separation). Such decreases in Tc are shown to be a common feature of high temperature superconductors, being observed in YBa2Cu3Oy-like systems as well as in doped Bi 2212 and Tl 1201 compounds.

Structural and magnetic data are presented and discussed for epitaxial films of rare earth metals ( Dy, Ho, Er) on LaF3 films on the GaAs(TTT) surface and Fe on Ag films on the GaAs(001) surface. Both systems exhibit unusual structural characteristics which influence the magnetic properties of the metal films. In the case of rare earth epitaxy on LaF3 we present evidence for epitaxy across an incommensurate or discommensurate interface. Coherency strain is not transmitted into the metal which behaves much like bulk crystals of the rare earths. In the case of Fe films , tilted epitaxy and long-range coherency strain are confirmed by X-ray diffractometry. Methods of controlling some of these structural effects by modifying the epitaxial structures are presented.

PrBa2Cu3O7−y is probably the most enigmatic member of the 1-2-3 family of compounds. It is the only non-superconducting example in this lanthanide series of isomorphic structures. We have studied its structural details, and dependence of its physical properties on oxygen concentration and solid solutions with Y, Ca and Zn, with the objective of understanding its peculiar behavior and the role Pr plays in suppressing superconductivity.

In the system ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$, as x and p (the [${\mathrm{CuO}}_{2}$${]}^{\mathrm{p}\mathrm{\ensuremath{-}}2}$ charge) are increased, the superconducting transition temperature first increases, then peaks becoming nonsuperconducting for x\ensuremath{\gtrsim}0.26. We report here a search for changes in physical properties at the values of x where ${T}_{c}$ is observed to change its behavior. The in-plane lattice constants and the normal resistivity both show a continued monotonic decrease over this entire region, suggesting that no major electronic changes occur. The tetragonal-to-orthorhombic transition temperature ${T}_{s}$ also decreases with increasing x and becomes unobservable for x\ensuremath{\gtrsim}0.19, suggesting that this structural transition itself is unrelated to the disappearance of superconductivity that occurs at higher doping levels. The magnetic spin susceptibility ${\ensuremath{\chi}}_{\mathrm{spin}(\mathrm{T}}$) generally rises gradually with increasing doping (reflecting decreasing spin-spin interactions), reaches a maximum near x\ensuremath{\sim}0.25, and then decreases. There is a weak peak in ${\ensuremath{\chi}}_{\mathrm{spin}(\mathrm{T}}$) as a function of temperature at T=${T}_{\mathrm{max}}$. As a function of increasing x,${T}_{\mathrm{max}}$ falls to zero near x\ensuremath{\sim}0.25. These two observations might be related to the disappearance of superconductivity, since all three occur near the same value of Sr content x.

We have measured the low-temperature specific heat (1.3≤T≤20 K) and the dc magnetic susceptibility (100≤T≤250 K) of eight samples of the high-T c superconductor Y x Ba3−xCu3O7−δ (x=0.9, 1.0, 1.1) and of two samples of nonsuperconducting YBa2Cu3O6+δ. We have also performed specific heat measurements on the possible impurity phases: YBa3Cu2O7, Y2BaCuO5, CuO, and BaCuO2+x. The superconducting samples all have a nonzero, sample-dependent linear term γ* and an upturn inC/T at very low temperature. We show that this anomalous behavior is at least partly due to the presence of a small amount (≈1%) of BaCuO2+x impurity phase in the measured samples. This is evidenced by the correlation between γ* and the Curie component of the susceptibility, which is proportional to the amount of paramagnetic impurities.

The magnetic susceptibility of several YBa 2 Cu 3 O 7−δ samples has been measured above the superconducting critical temperature in a SQUIS magnetometer. The normal state susceptibility extrapolated to T=0 varies between χ (0) = 174 and 294.10 −6 cm 3 /mol, depending on the sample preparation. The specific heat jump measured on the same samples varies linearly with χ (0) between ΔC/T c = 36 and 57 mJ/(K 2 mol). These measuremets permit to separate out the spin paramagnetic contribution to the susceptibility. It appears that the DOS is very sensitive to minute variations in the oxygen content, as predicted by band structure calculations.

Les proprietes physiques obtenues sur ces echantillons sont meilleures que celles des echantillons prepares par des methodes d'etat solide (amelioration de l'effet Meissner, saut bien defini a T c de la chaleur massique etc …)

YBa2(Cu1-xFex)3O7-δ with x=0, 1%, 2% and 4% was prepared by the citrate pyrolysis method. The samples were characterized by X-ray diffraction, micrographs, electron microprobe, a.c. susceptibility near the critical temperature Tc, field cooling Meissner effect, d.c. susceptibility from 100 to 250 K and specific heat from 1 to 300 K.

A number of high-quality polycrystalline YBa2Cu3O7 samples were investigated by specific-heat measurements from 30 to 300 K, with emphasis on the second-order transition at Tc. The same samples were examined for their Meissner effect, the normal state magnetic susceptibility and crystal structure parameters. A clear correlation was established between the normal state susceptibility and the specific-heat discontinuity ΔC at Tc, a fact allowing us to separate the exchange enhanced Pauli contribution. The results impose a constraint on the ratio of the Stoner factor S and the mass renormalization 1 + λ through S/(1 + λ) = 0.42A(λ), where A(λ) = ΔC/γTc. The transition temperature is insensitive even to pronounced variations of the density of states.

The resistivity ϱ and the susceptibility χ of very small single crystals of CeAl3 have been measured. The anisotropy of χ reflects the magnetic property of the crystal field doublet ground state |Jz = ± 3/2〉. At low temperature, both the resistivity and the magnetoresistance provide evidence that some kind of magnetic order starts to develop at 1.6 K.

We have performed low temperature (≤20 K) specific heat measurements on eight samples of super-conducting YxBa3−xCu3O7−δ (with x = 0.9, 1.0 and 1.1 respectively), and on two samples of non-superconducting YBa2Cu3O6. We have also measured their d.c. magnetic susceptibility between 100 and 250 K. The superconducting samples all have a non-zero, sample-dependent linear term γ∗, and an upturn in C/T at very low temperature. The non-superconducting samples exhibit a similar behaviour. We find that γ∗ is correlated with the Curie component of the suceptibility. As the latter is proportional to the amount of paramagnetic impurities, this means that γ∗ is largely due to the presence of impurities in the samples.

The R2Fe14C compounds were prepared from the elements by arc melting and long-time annealing (more than 480 hrs) at temperatures below 1200 K. They crystallize with the tetragonal Nd2Fe14B structure type as shown by X-ray powder diffraction analysis. All samples contain at least 70 vol.% of the R2Fe14C phase. Both saturation magnetization values (42–133 Am2/kg) and Curie temperatures (493–613 K) are lower than those measured on the corresponding borides.

YBa2Cu3O7 superconductors with inductive transitions as narrow as 0.45 K above 90 K were synthetized. Samples were characterized by thermogravimetry, differential thermal analysis, X-ray and neutron diffraction. The structure is characterized by a two-dimensional Cu-O network with square-pyramidal and square-planar coordinated Cu atoms. Results show a clear metallic behaviour of the resistivity. An orbital critical field as high as 300 T is extrapolated. Meissner flux expulsion up to 40% is observed. Small amounts of magnetic Cu2+ ions are correlated with the presence of the impurity phase BaCuO2. The Pauli susceptibility and the specific-heat jump at Tc are consistent with γ ≃ 2 mJ/(K2 gat) (9 mJ/(K2 mole-Cu)), neglecting all renormalizations.