C. Rossel

Magnetic relaxation effects in PbMo6S8 superconducting single crystals: A comparison with high-Tc superconductors

Abstract Magnetic measurements performed on PbMo6S8 single crystals show that this classic isotropic superconductor (Tc = 14.5 K) presents surprisingly similar irreversibility and relaxation properties as those generally attributed to layered high-Tc superconductors. An irreversibility line H ∗ = H ∗ 0 (1-t) m with m∼ 3 2 is observed for H ⪅ 1 kOe. From the magnetic relaxation rates measured below this irreversibility line, activation energies U0 around 15 meV are derived within the flux creep picture. The similarity with the high-Tc oxides is explained by the very short coherence length ξ ≈ 25 A of PbMo6S8 which also makes this compound sensitive to disorder and local defects.

Non-logarithmic magnetic relaxation in Bi2.2Sr1.7CaCu2O8 single crystals; evidence for collective flux pinning

High precision magnetic relaxation measurements have been performed on BiSrCaCuO single crystals with the field H applied parallel to the c-axis. At temperatures T/Tc > 0.5, the data exhibit two different field regimes. One for H H∗ where the relaxation follows a [ln(t/t0] -α dependence. This relaxation law is predicted in the collective flux creep theory for J ⪡ Jc as well as in the vortex-glass model. In the temperature range 0.25 < T/Tc < 0.4, there is a change in character of the measured decay of the magnetization. This change in character is interpreted to arise from thermal fluctuations of the flux-line lattice giving rise to a considerable reduction in the pinning strength above the depinning temperature.

Effect of structural changes on the transition temperatures in Y2Ba4Cu7O14+x single crystals

Abstract A structural investigation of 14 Y 2 Ba 4 Cu 7 O 14+ x single crystals with transition temperatures 0 T c 3 or barium metal as precursor) and by a high pressure flux-growth method, has been performed. X-ray structure refinements yielded changes in the single chain 123-unit only. With increasing T c a shortening of the Cu(2)ue5f8O(1) apical bondlength is observed, together with a reduction of buckling in the Ba(1)ue5f8O(1) plane. High- T c >90 K crystals have the ideal 247 structure, with full occupancies of all sites and well aligned single chains. Lower T c 247 crystals show a smaller electron density not only at the O(8), O(9) sites (oxygen nonstoichiometry), but also at the O(1) and Cu(1) site (Cu nonstoichiometry). These deficiencies are explained by the incorporation of aluminum, carbonate or vacancies at the single chain Cu(1) position. The resulting defects cause a misorientation of the single chains in the 123-unit, whereas the 124-unit is not affected by any distortions within the whole T c range. The results clearly support the explanation that Cu(1)-defects (and the resulting misorientation of single chains) are responsible for the decrease of T c . Electron density maps of some 247 crystals show a splitting of the apical oxygen O(1) or the plane copper Cu(2) site, indicating the possible coexistence of different apical bondlengths, requested by recent polaron models.

Microstructure of (001), (110) and (103) oriented thin films of YBa2Cu3O7−x investigated with STM, SEM and HRTEM

Abstract Epitaxial films of (110) or (103) YBa 2 Cu 3 O 7− x grown on (110) SrTiO 3 substrates by laser ablation are compared to (001) films. Their microstructure is investigated with scanning tunneling microscopy, scanning electron microscopy and high-resolution transmission electron microscopy. The growth of steps, terraces and spirals originating from screw dislocations commonly seen in c -axis films is inhibited in films grown on (110) SrTiO 3 . In the case of (103) oriented films, the growth of 90° twisted domains produces a tooth-like structure with preferred orientation. The influence of vicinal substrates (a few degrees of misorientation) on the film growth, on the nature of defects and on the resistive behavior is also investigated.

Measurement of the London penetration depths in YBa2Cu3Ox by means of muon spin rotation (μSR) experiments

Abstract To get accurate values of the London penetration depths in YBa2Cu3O x , μSR measurements were performed on a high quality, sintered sample and a c-axis-oriented polycrystal. For the sintered sample the temperature dependence of the effective penetration depth λeff is well described by the two-fluid model, with λeff(0) = 155(10) nm. This behavior of λeff(T) is consistent with conventional s-wave pairing. The anisotropy ratio λ c λ ab ⋍ 5(1) was determined from measurements on the polycrystal. These results were used to calculate λab(0) = 130(10) nm and λ c (0) ⋍ 500 – 800 nm .

Magnetic relaxation effects in YBa2Cu3O7−δ thin films

Abstract The time relaxation of the magnetization of YBa 2 Cu 3 O 7−δ thin films has been investigated. The slow nonexponential decay rates measured in the field-cooled (FC) and zero-field-cooled (ZFC) conditions show the same temperature dependence and vanish at T c . Our data suggest that flux creep is important below 60 K and might be dominated by glass effects close to T c .

X-ray single-crystal structure refinement of the 129 K superconductor HgxPb1−xBa2Ca3Cu4O10+δ

Abstract Crystals of Hg 0.8 Pb 0.2 Ba 2 Ca 3 Cu 4 O 10.14 were measured on an X-ray four-circle single-crystal diffractometer and the crystal structure was refined to R = 0.023. The space group is P4/mmm and the lattice parameters are a = 3.8530(5) A , and c = 18.968(3) A . Lead is incorporated at the Hg site, but it is shifted slightly from the origin in the z = 0 plane. A model of Pb 4+ clusters in the basal plane is proposed, which is in agreement with recent TEM results. A reduction of electron density at the Hg site is attributed to the substitution by Cu or CO 3 groups. Stacking faults of 1223 or 1245 material are manifested in the Fourier maps, and it was possible to refine the amount to 3.8%. However, these stacking faults induce additional electron density at the O(4) site ( 1 2 , 1 2 0 ), and cause an overestimation of the excess oxygen content, if it is not corrected carefully. The bondlengths are distances are in agreement with those of other members of the Hg-12( n −1) n series, and show systematic changes with increasing number n of CuO 2 layers. The effect of stacking faults on the diffraction of all members of the Hg-12( n −1) n family is discussed, and a method for their determination is given.

Evidence for Al doping in the CuO2 planes of YBa2Cu4O8 single crystals

Abstract Transition temperatures of YBa 2 Cu 4 O 8 (124) single crystals vary between 70 and 81 K, dependent on the crucible material used for crystal growth. Crystals grown in Al 2 O 3 crucibles (Al-124) have the lowest T c and incorporate 0.1-0.5% Al in the structure, but have an increased pinning as compared to undoped crystals. X-ray structure refinements of 23 YBa 2 Cu 4 O 8 single crystals showed a significantly reduced electron density at the Cu(2) site of Al-124, corresponding to an incorporation of 1–2% Al in the superconducting CuO 2 planes. The electron density at the Cu(1) chain site showed no difference between crystals grown in Al 2 O 3 , ZrO 2 and Y 2 O 3 crucibles, but it was generally lower than at the Cu(2) plane site. A significant increase of the anisotropic thermal factor U 33 of the apical oxygen atom is observed in Al-124, due to a shorter Alue5f8O bondlength. 63 Cu(1) and 63 Cu(2) NQR in undoped and Al-doped 124 single crystals showed a much stronger additional broadening of the plane than the chain Cu NQR lines in Al-124, indicating an Al incorporation in the CuO 2 planes.

Magnetic and electronic properties of YBa2Cu3Ox (6.5 < x < 7.0) studied by muon spin rotation

Abstract A systematic study of the local magnetic field distribution p(B) in the perovskite-type superconductor YBa2Cu3Ox is presented. Special attention is given to the second moment 〈 ΔB2 〉 of p(B) which is closely related to the superconducting carrier density ns and the effective mass m ∗ of the superconducting carriers. The influence of temperature, anisotropy and hole doping are investigated in detail. It is found that the temperature dependence of the magnetic penetration depth in YBa2Cu3Ox is in agreement with conventional s-wave pairing and suggests strong coupling. The anisotropic behavior of p(B) is well described by a theory for uniaxial type II superconductors, with an anisotropy ratio γ ∼- 5(1). Moreover, the zero-temperature values λab(0) = 130(10) nm and λc(0) ∼- 500–800 nm (penetration depth for screening currents flowing in the ab-planes and along the c-axis, respectively) were extracted from our data. μSR experiments performed on oxygen deficient samples indicate that the structural and electronic properties of YBa2Cu3Ox are strongly correlated via the oxygen content x.

μSR in oxygen deficient YBa2Cu3O x (6.5⩽x⩽7.0)

A systematic μSR study of the local magnetic field distribution in a series of oxygen deficient YBa2Cu3Ox samples with 6.5⩽x⩽7.0 is reported. Special attention was given to perform the experiments under the same conditions, so that the oxygen content of the measured samples was the only parameter varied. The behavior of the depolarization rate σ as a function of the oxygen contentx was found to have strong similarities with the behavior of the critical temperatureTc as a function ofx. In particular, two step-like increases of σ were observed abovex=6.7 and 6.9. The temperature dependence of the normalized depolarization rate σ(T)/σ(0) is well described by the two-fluid model forx⩾6.781(1) and clearly deviates from this behavior forx⩽6.704(1). Our results are compared to those obtained by other groups.