D. Cattani

Structure, Resistivity, Critical Field, Specific-Heat Jump at Tc, Meissner Effect, a.c. and d.c. Susceptibility of the High-Temperature Superconductor La2-xSrxCuO4

The temperature dependence of the resistivity, the magnetic properties and the specific heat were investigated on sintered samples of La1.85Sr0.15CuO4 having zero resistance below 35 K. The crystal structure at 300 K (tetragonal K2NiF4-type) was refined from X-ray powder diffraction data. The d.c. susceptibility shows no indication for the existence of localized Cu2+ moments. The observation of a 60% Meissner effect and a smeared jump at Tc in the specific-heat curve prove the intrinsic character of this superconducting state. The amplitude of this jump is compatible with the DOS estimated from the Pauli susceptibility. With a critical magnetic field slope dHc2/dT|Tc = -2.5 T/K, the orbital critical field is expected to be of the order of 64 T.

Superconducting and magnetic properties of Y1Ba2Cu4O8±δ

Abstract We have carried out extensive characterization and measurements on a high purity Y 1 Ba 2 Cu 4 O 8±δ (124) sample. Metallographic investigations, X-ray diffraction and microprobe analysis have been performed and reveal a single phase with traces of CuO. Resistivity measurements give a resistivity ratio R (300 K)/ R (100 K) of ≈ 6.1, and a critical temperature T co of 82.5 K. The AC susceptibility shows a sharp transition at about same temperature. The Meissner effect, with a 55% fraction (at 4.2 K with H =20.3 Oe) confirms the bulk nature superconductivity. The specific heat difference C (0, T )− C ( H , T ) has been estimated from magnetization measurements in the vicinity of T c for μ 0 H =8 T and compared to the direct calorimetric measurement. Both methods indicate that the specific heat jump amounts to approximately 15 mJ/K 2 mol. Fluctuation contributions are found up to several degrees above T c . Inter- and intra-grain critical currents were determined at 4.2 K by AC inductive measurements and by measuring the irreversibility of the magnetization curve at a constant temperature (Beans method). The results shows that there is no significant difference between the critical currents of the 124 phase and those of the 123 phase (Y 1 Ba 2 Cu 3 O 7 ).

Magnetic Screening and Flux Distribution in the High-Tc Superconductor La1.85Sr0.15CuO4

The magnetic screening and flux distribution within a polycrystalline sample of La1.85Sr0.15CuO4 has been measured by means of muon spin rotation spectrocopy (μSR) between 7.6 K and Tc = 34 K in applied fields of 10 Oe to 2 kOe. Above 500 Oe the transverse field relaxation rate of the μ+ spin polarization is quasi-field-independent, as expected for a superconducting material in the mixed state. At low field, a clear reduction of the relaxation rate is observed, interpreted as the disappearance of the mixed state. However, the average internal field at the muon sites, as deduced from the μ+ Larmor frequency does not reveal a sizeable Meissner effect in clear contradiction with the experimental results of the d.c. magnetization carried out on the same sample.

Magnetic penetration depth in the Chevrel phase superconductors SnMo6SySe8−y and PbMo6SzSe8−z

We report on transverse field (300 mT)μ+ SR measurements in the Chevrel phase superconductors SnMo6SySe8−y (y=1, 4, 7, 8) and PbMo6SzSe8−z (z=4.8) using the field cooling procedure (from room temperature down to 2.2 K). The London penetration depth λL was determined by the analysis of the fits with a single Gaussian relaxation function and by comparing the measured field distributions with calculated ones, where a broadening due to small random displacements of the vortices was taken into account.

Granular behavior of the PbMo6S8 Chevrel phase

PbMo6S8 is a potential superconducting mial for high magnetic field applications, but, up to now, the critical current density Jc, is still slightly to low to make it competitive for technical development. Analyses based on specific heat and ac susceptibility experiments m zero magnetic field show that the poor connections between the grains of this material limit already Jc near Tc.

The critical field of PbMo6S8 measured by specific heat up to 14 T

Calorimetric determination of the initial slope dH c2 /dT was carried out on a set of homogeneous high quality PbMo 6 S 8 samples. The specific heat jump at T c was measured in magnetic fields up to 14 T. The measurements unambigously confirm this compound as a high-field bulk superconductor (H c2 ∼50–60 tesla) and bring some insight into the nature of its upper critical field.

Calorimetric determination of Hc2 of PbMo6S8

Abstract Upper critical field H c2 of PbMo 6 S 8 was measured by means of a calorimetric experiment. This allows us to demonstrate the bulk nature of the high H c2 (54 Tesla at T=0 K).

Intrinsic critical current densities of PbMo6S8

The high upper critical field B c2 is now demonstrated to be an intrinsic property of the PbMo 6 S 8 (PMS) Chevrel phase compound. However lower values of B c2 determined using measurements based on macroscopic screening currents (like ac susceptibility and critical current densities), suggest that the granular nature of this superconductor may limit transport current by possible weak links between grains. We report inductive measurements of critical current densities on single-phased PMS powder and sintered bulk samples, those results confirm the existence of intrinsic high critical currents on PMS.

μ+SR studies in the chevrel phase superconductors

The magnetic properties of the superconducting Chevrel compound Eu0.75Sn0.25Mo6S7.6Se0.4 have been studied by μ+SR. The measurements covered the field range 0–30 kOe at various temperatures between 0.05 K and 1.5 K as well as studied in fields between 0 and 3 kOe from 2 K up to room temperature. Strong effects due to the Eu 4f-moments are observed over the whole temperature range. The relaxation rate seems to have its origin mainly in the dynamics of the Eu spins. For comparison the nonmagnetic Chevrel phase superconductor SnMo6S4Se4 was measured too.

Homogeneity and critical current density of Sn-doped PbMo6S8 superconductors

With the overall critical current density,Jc, remaining low for PbMo6S8, the addition of 0.3 at. % Sn noticcably improved the performance. The enhancement occurs despite the presence of excess Pb and a slightly suppressedTc. The increase inJc can be attributed to a better homogeneity with respect to the chemical composition of the samples, which may be as a consequence of Sn substitution in the lattice. Detailed calorimetric investigation of undoped PbMo6S8 showed aTc distribution down to 9 K evidenced by a smearing ofTc. This may be due to a progressive degradation of the sample quality leading to a plausible off-stoichiometry. High-resolution Auger electron spectroscopic experiments suggest a variation in composition indicating excellent correspondence with the specific heat data. However, no such deviation was detected for the Sn-doped samples and therefore the observed raise inJc. The results are discussed in conjunction with oxygen, lead and tin incorporated PbMo6S8.