G. Triscone

Variation of the superconducting properties of Bi2Sr2CaCu2O8+x with oxygen content

Abstract We have mapped the critical temperature of Bi 2 Sr 2 CaCU 2 O 8+δ (2212) in the ( p, T ) plane for 10 −5 ≤ p ≤ 80 bar (O 2 ) and 300 ≤ T ≤ 840°C. Equilibrium conditions are frozen by quenching the samples into liquid gallium. The DC diamagnetic transitions remain sharp, and span an unusual temperature range for the 2212 phase (51.5 K to 94.4 K). This variation is attributed to a measured 2.5% change of the oxygen content ( Δδ ⋍ 0.2 ) in the structure. At the same time, we do not observe any significant variation of the lattice parameters. The Meissner effect, measured in an external magnetic field of 20 Oe, increases with T c . The slope of the normal-state susceptibility ∂ χ / ∂T becomes markedly less negative with decreasing oxygen concentration. It vanishes for the optimum doping that leads to T c ⋍ 94 K .

Variation of the superconducting and crystallographic properties and their relation to oxygen stoichiometry of highly homogeneous YBa2Cu3Ox

Abstract A great number of YBa2Cu3Ox (if6.2 ⩽ x ⩽ 7.0) samples were investigated by a.c. susceptibility and field cooling magnetization measurements, X-ray and microprobe analyses, microstructural examinations in a light microscope and a scanning electron microscope. The oxygen content x was determined by the reduction of the samples in a hydrogen flow. To reach the (x-T-p) equilibrium, well-oxidized YBCO samples were annealed in pure oxygen flow (10−1 atm ⩽ P(O2) ⩽ 1 atm) at temperatures between 200 and 1000 °C and subsequently quenched into liquid gallium. Other samples were prepared under 80 atm O2 (200 °C ⩽ T⩽ 1000 °C). We were able to produce macroscopically very homogeneous oxygen deficient material with sharp χa.c. transitions in the entire range of critical temperatures. The evolution of the Meissner fraction is non-monotonic with a relative maximum near x = 6.6, a concentration where the orthorhombicity 2 (b − a) (a + b) also deviates from monotonic behaviour.

Variation of the superconducting and structural properties of Y2Ba4Cu7Oz with oxygen content (14.1 < z < 15.3, 30 K ⩽ Tc ⩾ 95 K)

A great number of pure and homogeneous Y2Ba4Cu7Oz samples were investigated by AC susceptibility, field cooling magnetization measurements, X-ray diffraction, microprobe analysis and microstructural examinations in a light microscope and a scanning electron microscope. The oxygen content z between 14.1 and 15.3 was determined by the reduction of the samples in a hydrogen flow. To reach the (z-T-p) equilibrium, well-crystallized samples were annealed in pure oxygen (10−6 ⩽p(O2)⩽102 bar) between 300 and 900°C and subsequently quenched. The specimens produced in this way show sharp χAC transitions in the entire range of critical temperatures (30 K ⩽ Tc ⩽ 95 K). The correlation Tc versus the orthorhombicity 2(b−a)/(b+a) is almost linear, similar to that observed in YBa2Cu3Ox. This is contrast to the distinct behaviour of the functions Tc versus z and Tc versus x in these two compounds. We present a stability-decomposition map 0for Y2Ba4Cu7Oz over a wide range of (T, p) coordinates.

Specific heat up to 14 tesla and magnetization of a Bi2Sr2CaCu2O8 single crystal thermodynamics of a 2D superconductor

Abstract A 0.25 g single crystal of Bi 2 Sr 2 CaCu 2 O 8 with a sharp superconducting transition at 85 K is characterized by AC susceptibility, X-ray diffraction, Meissner effect, normal-state susceptibility, magnetization in the mixed state, and specific heat from 1 to 300 K. The residual linear term γ ∗ T in the specific heat at low T is small and field independent. High-resolution ( t c show that: (a) there is no mean-field specific heat jump at T c ; (b) a symmetrical logarithmic ‘divergence’ occurs at the transition; (c) its amplitude decreases with B -2 3 ; (d) its width scales with B 1 2 ; (e) T c does not depend on the field, and coincides with the crossing point of the magnetization curves M ∗ ( H, T ∗ ); (f) a crossing point C x (H, T x ) is found in the specific heat 10 K below T c ; (g) the effect of a field along the CuO planes is at least 50 times smaller than that of a field along the c axis; (h) no transition is observed at the irreversibility line. An empirical formula for ϑ( C/T )/ϑ H that accounts for C ( H, T ) and M ( H, T ) data over a wide temperature and field range is proposed.

Specific heat of the superconductor YBa2Cu4O8 from 1.5 to 330 K

Abstract The specific heat of four samples of the 81 K-superconductor Yba 2 Cu 4 O 8 prepared under high oxygen pressure is investigated. The low-temperature specific heat is sample-dependent. For the most homogeneous sample, the coefficient of the linear term at T ⪡ T c is γ ∗ =4.9±0.2mJ/(K 2 mole) and the initial Debye temperature is θ (0)=350±10K. A small specific heat jump Δ C / T c =12 to 16 mJ/(K 2 mole) is observed at the superconducting transition; it is only weakly correlated with the field cooling Meissner fraction (14 to 55% in 20 Oe). A tentative separation of the total specific heat into electron and phonon terms is obtained by a least-squares fit of high-temperature data using a model of the phonon density of states. The electron contribution is found to be much smaller than for YBa 2 CU 3 O 7 . Thermochemical data are tabulated.

Specific heat (1–330 K), Meissner effect and magnetic susceptibility of Tl2Ba2Ca2Cu3O10, Tl2Ba2CaCu2O8 and Tl2Ba2CuO6 ceramic samples

Abstract The specific heat (T⩽330 K), magnetic susceptibility (T⩽250 K) and Meissner effect of three samples of the series Tl2Ba2Can−1CunO4+2n+x with n=1, n=2 and n=3 are reported. The critical temperatures measured by AC susceptibility are Tc(onset)≈10 K for n=1, Tc(midpoint)=99.5 K for n=2 and Tc(midpoint)=121.4 K for n=3, with transition widths (10%–90%) smaller than 10 K. The Meissner field expulsion at 4 K in 20 G amounts to 8%, 51% and 24%, respectively. The constant component of the susceptibility increases with the number of Cu layers and with Tc. The linear term in the low-temperature specific heat is lower than 7 mJK-2mol-1 and scales with the impurity Curie term of the magnetic susceptibility. Small anomalies are observed in the specific heat at Tc; their shape and their amplitude are consistent with those of the Meissner transitions. The origin of smearing is discussed in relation with the small coherence length. Thermochemical data are tabulated.

Specific heat of an YBa2Cu3O7 single crystal in fields up to 20 T

The specific heat of a single crystal of YBa2Cu3O7 was measured by adiabatic calorimetry between 40 and 150 K. The typical scatter of the data is well below 0.1%. Fields up to 20 T were applied either parallel or perpendicular to the c-axis. The shape of the anomaly at Tc changes with the magnetic field in the same manner for both directions, within a scale factor that determines the bulk anisotropy ratio, 5.5 ± 0.5, for (dHc2/dT)Tc. The superconducting transition is still observable in 20 T. A growing mixed-state Maki term is present. The characteristic non-classical behavior of YBa2Cu3O7 in a field, featuring a considerable smearing of the transition and an apparently field-independent onset, may be qualitatively understood as a phenomenological consequence of the small and anisotropic value of the coherence length ξ alone, using the relevant models of field-induced critical 1D fluctuations on the one hand, and a London-like regime for the mean-field part of the transition on the other hand.

Preparation, resistivity, magnetic properties and specific heat of the 95 K superconductor YBa2Cu3.5O7.5+x (“247”)

Abstract Under suitable preparation conditions, Y2Ba4Cu7O15 (“247”) has the highest critical temperature of the YBCO superconductor family. We present an extensive characterization of 247 samples with Tc onsets as high as 95K in bulk ceramic form. The experiments performed include metallographic examinations, X-ray diffraction, microprobe analysis, resistivity, AC susceptibility, Meissner and normal-state susceptibility, magnetic determinations of the slope of the critical field at Tc and of the critical current density, and specific heat from 1 to 330 K. Several properties (e.g., normal-state susceptibility, specific heat jump at Tc, slope of Hc2(T) at Tc, resistivity) may be considered as an approximate average between those of YBa2Cu4O8 (“124”) and those of YBa2Cu3O7 (“123”). The signature of fluctuations in magnetic properties is enhanced with respect to both 124 and 123. The behavior of the specific heat close to T = 0 is similar to that of the other members of the family, with the presence of a quasi linear T-term and the absence of any upturn in C/T. The maximum Tc appears to be related to optimal doping, consistent with the behavior observed in 123 as a function of the oxygen concentration.

Split superconducting transitions in the specific heat and magnetic susceptibility of YBa2Cu3Ox versus oxygen content

A homogeneous batch of YBa2Cu3Ox porous ceramics was studied in detail as a function of the annealing temperature Ta. X-ray diffraction patterns and optical micrographs show definitely one single phase. Well-separated double superconducting transitions are, however, observed; most clearly in the specific heat curves, except for Ta≈460°C. Initial values are Tc1=87 K and Tc2=92 K for Ta=300°C. Upon increasing the annealing temperature Ta to 500°C, Tc1 increases to 93.5 K while Tc2 remains almost constant. Beyond Ta=500°C, both Tc1 and Tc2 decrease. The essentially reversible splitting of the superconducting transition appears to be due to the complex kinetics of the oxygen uptake. The two critical temperatures Tc1 and Tc2 are attributed to the high oxygen content external layer of the grains and to the low oxygen content grain cores, respectively.

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 ).