C. Sulkowski

Electronic structure parameters of Sm1+xBa2−xCu3Oy solid solution of orthorhombic and tetragonal structure

Abstract Two series of Sm 1+ x Ba 2− x Cu 3 O y samples were prepared, both exhibiting a change from orthorhombic to tetragonal structure. The first is a series of solid solution of increasing Sm/Ba ratio, the second is a series of exact “123” cation stoichiometry with lowering oxygen content. Several quantities, such as electrical resistivity in magnetic field up to 8T (ϱ), Hall carrier concentration ( n H ) and thermopower ( S ), were measured. Despite the different chemical nature of the deviation from the initial SmBa 2 Cu 3 O 6.85 composition in the two series, the transport properties of the normal state change in an analogous manner. No significant differences in the Hall mobility at room temperature were revealed between 90 K superconducting and non-metallic samples; however, a wide maximum was observed for both series for samples with intermediate T c values. The critical temperature ( T c ) is depressed with increasing deviation from the initial composition in both series; however, for the solid solution there is no clear 60 K plateau as for the stoichiometric series. The electronic density of states ( g ( E F )) was assessed from in-field T c measurements on the basis of the GLAG theory and, independently, from analysis of S and n H values at room temperature. Both methods revealed no significant differences in the behavior of the g ( E F ) between the solid solution and stoichiometric series. However, the g ( E F ) calculated by the first method decreases fast with deviation from the initial composition, whereas the results of the second method remain nearly constant over a wide range of composition factors. The value of the g ( E F ) obtained for the initial SmBa 2 Cu 3 O 6.85 sample from the jump of the specific heat at T c based on BCS theory also agrees well with the values calculated from transport measurements.

Thermomagnetic properties of the overdoped La1.7Sr0.3CuO4 superconductor

Abstract We have studied the normal-state transport phenomena of the La 1.7 Sr 0.3 CuO 4 overdoped high- T c superconductor. The Ettingshausen ( P E ), Hall ( R H ) and Seebeck ( S ) coefficients as well as the thermal ( κ ) and electrical ( σ ) conductivities have been measured. The temperature dependence of the isothermal Nernst ( Q i ) coefficient has been estimated on the basis of the Bridgman relation. The results have been interpreted in terms of a rigid, single-band model, with an assumption that both doping and temperature change the band filling resulting in the variation of the Fermi surface curvature. The latter quantity appeared to be a key feature driving the temperature dependences of P E and R H . In opposition to the usual interpretation, the temperature dependence of the R H for La 1.7 Sr 0.3 CuO 4 is dominated by variations of the Fermi surface shape, and not by the changes in the charge carrier concentration. The consistency of this physical picture was controlled by an estimation of P E using measured values of σ , R H , S and κ and comparison with measured value of the Ettingshausen coefficient.

Magnetic features of La1−2xNaxKxMnO3+δ solid solution

Abstract Preliminary information on magnetic properties of the La 1−2 x Na x K x MnO 3+ δ type solid solution is reported. The DC-magnetization measurements have allowed determination of the temperature dependence of ferromagnetic transition temperature vs. composition. The results are compared with AC-magnetic susceptibility data obtained for samples of the La 1− x Na x MnO 3+ δ type. Thermo-power data have also been collected vs. sample composition at room temperature ( S 300 ) and vs. temperature ( S T ) to show the complex nature of the magnetic ordering appearing in these materials.

Negative charge carriers in the 90 K plateau of REBa2Cu3O7−δ

Abstract Specific heat, thermopower and electrical resistivity in magnetic field were measured in the series of polycrystalline REBa 2 Cu 3 O 7−δ samples from the so-called 90 K plateau of T c versus δ dependence. Clear correlations between such electronic quantities as specific heat jump, Δ C p , thermopower at 300 K, S 300 , and slope of the upper critical field, d H c2 /d T , were observed for all the investigated samples. We proposed some possible explanation of the observed effects in terms of the simple theory of metals and Ginzburg-Landau-Abrikosov-Gorkov theory of type-II superconductors. This analysis suggests that in the REBa 2 Cu 3 O 7−δ superconductors with T c from the region of the 90 K plateau there exist charge carriers of negative sign, in addition to holes dominating the Hall effect. The electronic density of states of the negative charge carriers is the highest for δ ≈ 0 and substantially lower near the edge of the plateau. Its value is responsible for the changes of Δ C p , d H c2 /d T and the negative component of S 300 in the region of the 90 K plateau.

Effect of the RE3+ ionic size on the REBa2Cu3O7−δ ceramics oxygenated at 250 bar

Abstract REBa 2 Cu 3 O 7− δ (RE=Ho, Y, Dy, Gd, Sm, Nd) ceramics have been oxygenated at 1 bar pressure (LP) and, subsequently, at 250 bar (HP). Despite the noticed slight uptake of oxygen (up to the value of 0.07), after the HP processing, the electrical resistivity ( ρ ) of all samples increased, what was attributed to the deterioration of the grain boundaries. The increase of ρ was much more pronounced and also accompanied by a change of ρ ( T ) characteristics into a semiconducting-like one in the case of 123 compounds based on REs which ionic size is large enough to form solid solutions of RE 1+ x Ba 2− x Cu 3 O y type (i.e., RE=Gd, Sm, Nd). As shown in the literature, such 123s usually contain more structural defects. Thus, the observed effect may be attributed to the migration of the defects induced by the elevated pressure oxygenation. The defects could be trapped near the grain boundaries resulting in the deterioration of their electrical properties. The possible role of the oxygen-pressure-induced modifications of the impurity phases has been also discussed. The materials obtained in the HP process may be regarded as 3D arrays of superconducting grains coupled by the Josephson junction or weak links only, as was shown in a tunneling experiment.

Superconducting and transport properties of TmBa2Cu3O7−δ and comparison to YBa2Cu3O7−δ

Abstract TmBa 2 Cu 3 O 7− δ ceramic samples with oxygen contents between ∼6.7 and 7 have been characterized in terms of their superconducting transition temperatures ( T c ), temperature dependencies of resistivity ( ρ ( T )), thermopowers ( S ( T )) and Hall coefficients ( R H ( T )) and specific heat jumps at T c (Δ C p ). The behaviors of these parameters are compared with those exhibited by YBa 2 Cu 3 O 7− δ samples. In general, the temperature dependencies of ρ , S and R H found for TmBCO are similar to those for YBCO. However, some clear quantitative differences between the compounds have been observed for the oxygen content dependencies of T c , S and Δ C p / T c , especially at low oxygen contents. TmBCO samples with oxygen contents 7− δ =6.7–6.8 show significantly lower T c and Δ C p / T c , and higher S compared toYBCO, although the carrier concentration n H at 7− δ ≈6.7 is similar for both compounds (4–5×10 21 cm −3 ). The observed difference in the values of T c , Δ C p / T c and S seem to be caused by the large mass and magnetic moment of Tm. Thermopower, S , and carrier concentration, n H , characterize the electronic structure of the compounds. The dependence of S on 1/ n H for TmBCO and YBCO is similar and nearly linear, in agreement with the Mott relation.

Superconductivity and Magnetism in Pure and Substituted RuSr2GdCu2O8

We describe results of several macroscopic and local measurements of magnetic and superconducting properties for pure and Ce and Cu substituted RuSr2GdCu2O8. From various experiments the physical properties phase diagram is derived as a function of the hole doping. We show that the large contribution of polarized Gd3+ magnetic moments to magnetization causes apparent absence of the Meissner state and augments the weak Ru ferromagnetism. This weak ferromagnetism in the superconducting state can be significantly enhanced by the external magnetic field. These results indicate that the low-temperature behavior can be qualitatively explained assuming a quasi-two-dimensional character of the superconducting regions.

On the magnetic and superconducting properties of Ru1−xSr2RECu2+xO8−d, RE=Gd, Eu, compounds

The temperature of the superconducting transition of Ru1−xSr2RECu2+xO8−d (RE=Gd, Eu) compounds was raised up to 72 K (for x=0.4; compare to TC=45 K observed for RuSr2GdCu2O8) by changing the ratio between Ru and Cu. Magnetization experiments performed under external pressure for RE=Gd: x=0.1, 0.2, 0.3, and 0.4, and RE=Eu: x=0.4 samples reveal that TC increases at a rate of 5–6 K/GPa. The hole doping achieved along the series has been probed in Hall effect, thermopower, and XANES experiments and is found to increase with x. Muon spin rotation spectroscopy experiments reveal the presence of a low temperature magnetic transition for Ru0.6Sr2EuCu2.4O8−d (Tm≈5 K), similar to that recently observed in the Ru1−xSr2GdCu2+xO8−d system [P. W. Klamut, et al., Physica C 364, 313 (2001)]. Low temperature magnetization data suggest the constrained dimensionality of the superconducting phase in both RE=Gd and Eu based compounds.

A direct correlation between Tc and EDOS for 1-2-3 type (Ca0.1La0.9)(Ba1.65La0.35)Cu3Oy superconductor

Abstract We have prepared a series of (Ca 0.1 La 0.9 )(Ba 1.65 La 0.35 )Cu 3 O y ceramic samples where y varies from 6.75 to 7.23 by annealing in oxygen at various pressures (up to 1330 bar). This compound (called CLBLCO) is one of few high- T c superconductors for which the oxygen content can be varied throughout the entire doping range. In our series the critical temperature increases from T c =16.9 K for y =7.00 (underdoped region) (the sample with y =6.75 is non-superconducting) to T c =30.1 K for y =7.10 (optimal doping). With further increase of the oxygen content T c decreases to 8.8 K for y =7.23 (overdoped region). Here, we present measurements of the resistivity ( ρ ), thermoelectric power ( S ), the Hall coefficient ( R H ), and their interpretation in terms of a simple metal theory. In this approach we indicate a correlation between the transition temperature ( T c ) and the density of the electronic states (EDOS) at the Fermi surface ( g ( E F )).

Magnetism and Superconductivity in Ru 1− x Sr 2 RECu 2+ x O 8− d (RE=Gd, Eu) and RuSr 2 Gd 1− y Ce y Cu 2 O 8 Compounds

We discuss the properties of new superconducting compositions of ruthenocuprates Ru1−x Sr2RECu2+x O8−d (RE=Gd, Eu) that were synthesized at 600 atm. of oxygen at 1080°C. By changing ratio between the Ru and Cu, the temperature of superconducting transition (TC) raises up to T C max = 72 K for x=0.3, 0.4. The hole doping achieved along the series increases with Cu→Ru substitution. For x ≠ 0, TC can be subsequently tuned between T C max and 0 K by changing oxygen content in the compounds. The magnetic characteristics of the RE=Gd and Eu based compounds are interpreted as indicative of constrained dimensionality of the superconducting phase. Muon spin rotation experiments reveal the presence of the magnetic transitions at low temperatures (T m=14-2 K for x=0.1–0.4) that can originate in the response of Ru/Cu sublattice. RuSr2Gd1−y Ce1−y Cu2O8 (0 ≤ y ≤ 0.1) compounds show the simultaneous increase of TN and decrease of TC with y. The effect should be explained by the electron doping that occurs with Ce→Gd substitution. Properties of these two series allow us to propose phase diagram for 1212-type ruthenocuprates that links their properties to the hole doping achieved in the systems. Non-superconducting single-phase RuSr2GdCu2O8 and RuSr2EuCu2O8 are reported and discussed in the context of the properties of substituted compounds.