D. Włosewicz

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.

Order-disorder phase transitions in

Measurements of the electrical resistivity under hydrostatic pressure have been made and an x-ray analysis has been performed for the interstitial alloy . The results have been compared with specific heat and thermal expansion results for the same sample. An anomaly in the specific heat observed at 388-394 K was attributed to a phase transition due to the ordering of H atoms. The trapezoidal shape of the anomaly was explained in terms of the equilibrium phase separation occurring within the two-phase region, as in the case of the liquid-solution-vapour transition. At lower temperatures a series of specific heat anomalies at (small), (medium) and (large) was observed and assigned to the subsequent orderings of the H atoms. The smallest anomaly coincides with the change of the Nb-sublattice symmetry which is cubic below and orthorhombic above the temperature at which this anomaly is found and also, with a bend in the curve representing the temperature dependence of the resistivity. The temperature location of this bend has been found to be insensitive to the applied pressure (up to 19.5 kbar). The biggest low-temperature specific heat anomaly coincides with a jump in the relative elongation.

Specific heat of Sm1+xBa2−xCu3Oysolid solution of orthorhombic and tetragonal structure

Abstract The specific heats of Sm 1+ x Ba 2− x Cu 3 O y solid solution of orthorhombic and tetragonal structure were measured in the temperature range 80–300 K. The data were analyzed in the framework of the Debye model with dilatation correction. The tendency to lower the high-temperature limit of the Debye temperature, θ HT D , with the oxygen deficiency was noticed. In contrast, the increase of Sm substitution causes a rise of θ HT D . The temperature dependence θ D ( T ) was calculated for each compound from the series for the whole temperature region investigated.

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.

Physical Properties of (TEA)2MnCl4 Crystals

Results of calorimetric, dilatometric, dielectric and magnetic investigations of TEA 2 MnCl 4 crystals in a wide temperature range are presented. Two anomalies typical for the first-order phase transition at T 1 = 224 K and T 2 = 228 K were observed. The entropy jumps of 17 J/mol and 11 J/mol were determined at T 1 and T 2 , respectively. The coefficients dT i /dp) are positive and equal to dT1/dp ≅ 0.52 K/MPa and dT 2 /dp ≅ .30 K/MPa. The triple point existence under about p trip = 100 MPa at T trip = 255 K can be expected. The TEA 2 MnCl 4 crystals exhibit paramagnetic properties in the temperature range from 4.2 to 300 K. The Curie-Weiss law is fulfilled and any anomalies of magnetic properties were not observed near the phase transitions temperature.

The specific heat studies of (NH3CH3)5Bi2Br11: Evidence for the critical end-point in crystals

Abstract The specific heat, Cp , of (NH3CH3)5Bi2Br11 was measured by adiabatic calorimetry in the temperature range 30–350 K. The “peculiar” character of Cp (T) anomaly (from residual to giant effect) observed in the region 70–100 K in different measurement runs, was interpreted in terms of the critical end-point existence on the temperature-stress phase diagram. For the first time, a full set of possible Cp (T) curves predicted by theory was registered for near-critical behaviour and for the crystal with a given stoichiometry.

Thermal transport in Sm1+xBa2−xCu3Oy solid solution

Thermal conductivity (κ) and thermopower (S) of two series of high-temperature superconductors (HTSCs) with a general formula Sm1+xBa2−xCu3Oy have been measured. Both series originate from the same sample of Tc=90.4 K and extend to the nonsuperconducting, antiferromagnetic phases. The first series is of a 1:2:3 cation stoichiometry (x=0) and variable oxygen content (6.11<y<6.85), the second one is a series of solid solutions with a variable Sm/Ba ratio (0<x<0.6). For a solid solution series, regular changes in transport properties were observed with decreasing Tc: a gradual decrease of κ and increase of S at all temperatures, while the type of temperature dependences of respective quantities remained still characteristic for HTSCs. Therefore, the last, nonsuperconducting sample from this series may be used as a reference for estimation of the lattice thermal conductivity for superconducting samples. As it turned out, the stoichiometric series is different. The thermal transport properties of the most oxygen deficient sample (y=6.11) resembles a classical, poorly conducting material more than a HTSC. The κ(T) dependence for this sample reveals a broad maximum around 210 K. The S(T) curve for this sample follows a 1/T dependence like for a nondegenerate semiconductor with the activation energy of ∼0.3 eV. This value agrees well with that estimated from the ρ(T) dependence. The dc magnetization measurements carried out for the nonsuperconducting samples suggest that the origin of the difference in their thermal transport properties is not magnetic.