K. Fossheim

Elastic and thermal behaviour of ceramic high-Tc superconductors studied by ultrasound, vibrating reed, and specific heat measurements

Abstract We report measurements of the elastic properties and specific heat performed on ceramic high-Tc superconductors. Characteristic features are observed in a wide range of temperatures. In YBCO we emphasize the dynamic behaviour observed between 220K and 270K, and suggest that a glass-like ordering phenomenon may lead to the observations. Hysteretic behaviour in the sound velocity, together with a broad lossy region, is found below 200K. Near Tc possible evidence of a structural instability is found. A specific heat anomaly is observed at about 220K. The elastic response of LABCO shows features which in many respects are similar to those observed in YBCO.

Restricted reversible region and strongly enhanced pinning in MPMG YBa2Cu3O7 with Y2BaCuO5 inclusions

Abstract Recent work has shown that the melt-powder-melt-grown (MPMG) high temperature superconductor YBa2Cu3O7 with Y2BaCuO5 inclusions sustains unusually high critical current in the bulk. In the present paper the magnetic irreversibility line of this material is studied by AC permeability measurements. The irreversibility line is much steeper and is moved to higher temperatures compared to single crystal YBa2Cu3O7 and the reversible region between the irreversibility line and the Hc2 line is drastically shrunk. From standard flux creep analysis the pinning energy is found to be increased by one order of magnitude as compared to YBa2Cu3O7. The measurements further show that the irreversibility temperature increases in a simple logarithmic fashion with increasing frequency. The magnetic AC field response is found to be highly nonlinear even when B ac B dc ∼10 -5 .

The AC magnetic response of YBa2Cu3O7 with Y2BaCuO5 inclusions and of single-crystal YBa2Cu3O7. A comparative study

Abstract We have measured the AC magnetic permeability response function μ ′ μ + iμ ″ in melt-process-melt-growth YBa 2 Cu 3 O 7 (Y123) with Y 2 BaCuO 5 (Y211) inclusions (MPMG) and in single-crystal YBa 2 Cu 3 O 7 (SC) in applied magnetic fields up to 8 T, oriented either parallel or perpendicular to the crystalline c -axis. The AC response of the two samples has been mapped out as a function of temperature, AC field amplitude and frequency, DC applied field, and field orientation. The peak in the lossy part of the susceptibility μ″ ( H, T ) is used to probe the flux dynamics in the vicinity of the “irreversibility” line (IL), defined here as the locus of μ″ maxima in the H-T plane. For H ∥ c , the IL ofthe MPMG sample exists at higher temperature and field than the SC sample, indicating that the Y211 insulating inclusions are responsible for additional strong pinning of the vortex structure. For H ⊥ c , the ILs, determined by the peak position of μ″ ( H, T ), are the same for both samples. However the amount of dissipation measured by the temperature width of μ″ ( H, T ), is greater for the MPMG sample for this orientation implying that the Y211 inclusions reduce the vortex pinning in this orientation. The precise natures of these effects due to the Y211 inclusions are still unknown. The frequency dependence and non-linear response differ for the two field orientations. In the range of available parameters and variables we find the system to be in the solid vortex phase for H ⊥ c since there is strong H AC dependence and only weak frequency dependence on the temperature position of the loss peak. For H ∥ c , however, the loss peak depends only weakly on H AC and strongly on the frequency.

High resolution specific heat measurements in the ceramic superconductor YBa2Cu3O7−x: anomalies near 90K and 220K

Specific heat measurements in six YBCO samplesare reported in the temperature range 60K to 300K. The specific heat jumps at the 90K superconducting transition are measured. Variations are explained by varying volume fraction of superconducting material in different samples. An additional anomaly in the specific heat is found at 220K. Glass-like ordering in the oxygen system is suggested to explain the observation.

Lattice instabilities in single-phase polycrystalline Y1-xCaxBa2Cu4O8 (x=0, 0.005, 0.1)

Ultrasonic and specific heat measurements have been performed on single-phase polycrystalline Y1-xCaxBa2Cu4O8 (x=0, 0.05, 0.1 denoted as Y124, Y124Ca0.05, and Y124Ca0.1 respectively) samples in the temperature region 65 to 320 K. It is found that partial replacement of Y by Ca may enhance the interatomic couplings of the lattice, leading to a higher Debye temperature in Ca-substituted samples. A Ca-induced phase transition at 150 K is revealed in Ca-doped samples. A pronounced attenuation peak near 260 K accompanied by a simultaneous change of slope in velocity is observed in pure and Ca-doped Y124 samples. For pure Y124 sample, the corresponding specific heat anomaly is weak and may be clearly seen only after substracting an appropriate background. While for Ca-doped samples, the corresponding anomaly in specific heat is difficult to isolate, due to the influence of the phase transition at 150 K. It is suggested that the anomalies near 260 K are caused by the formation of an antiferroelectric (AFE) or ferroelectric (FE) state associated with the ordering of the oxygen atoms in some off-centre positions of the structure.

ON THE SPECIFIC HEAT ANOMALIES IN THE TEMPERATURE RANGE 200 K TO 240 K IN CuO AND YBa2Cu3O7−δ(YBCO)

Specific heat anomalies in YBa2Cu3O7−δ(YBCO) and CuO were investigated in the temperature range 200 K to 240 K. It was found that small amounts of CuO contained as a separate phase in YBCO produce easily observable anomalies caused by two consecutive antiferromagnetic (AF) transitions at 211.6 K and 228.3 K in CuO. The anomalies hence were observed in both pure CuO and YBCO samples containing some free CuO. The anomalies found here are of a different character than the broad peak we observed earlier around 220 K in pure ceramic YBCO. This lends support to the earlier interpretation of the 220 K anomaly as a structurally related one.

Short time magnetic relaxation in Bi2Sr2Ca2Ca3O10 tapes: a study of thermally activated flux motion and vortex glass transition

Abstract Short time (0.1–3600 s) magnetic relaxation M(t) after applying small field steps (0–10 mT) has been studied in field (0–0.2 T) cooled Bi2Sr2Ca2Cu3O10 tape samples. We plot the magnetization as a function of logarithmic time for a range of temperatures between 20 and 80 K. The absolute creep rates (dM/dln t) determined at fixed times display maxima at characteristic temperatures which we denote as T ∗ (B) . These maxima are related to the crossover between incomplete and complete flux penetration. The T ∗ (B) lines are shown to be clearly dependent on the time window, the sample size and the field step amplitude. They resemble in all essentials the irreversibility line usually measured by AC susceptibility experiments. In addition, the relaxation measurements done in flux exit and flux entry mode show the presence of surface/geometrical barriers. For full flux penetration, the relaxation curves can be transformed into V-I characteristics in good agreement with the ones measured by transport in the same sample. The vortex glass transition TG(B) line in both kinds of experiments is found to be the same. In the studied range of temperature and field, we also observe a unique change of curvature in M(ln t) at TG(B) the curvature is found to be negative and above TG(B) the curvature becomes positive. Our results highlight the close relation between the relaxation of the flux generated by a small magnetic field step, AC susceptibility and V-I characteristics.

Anisotropy of the irreversibility line in MPMG YBa2Cu3O7−δ with Y2BaCuO5 inclusions

Abstract A systematic study of the magnetic irreversibility behavior in the MPMG (melt-powder-melt-grown) material with Y 2 BaCuO 5 inclusions have been performed. The irreversibility line was measured in external fields up to 8 T using an ac susceptibility technique. Large anisotropy of the irreversibility line measured in fields parallel and perpendicular to the ab-plane was observed. The frequency dependence of the irreversibility line was found to be logarithmic. The excitation field dependence of the irreversibility line is also determined.

Temperature and field dependence of the weak-link behavior in superconducting YBa2Cu3O7-x

Abstract By measurement of the ac permeability of ceramic YBCO we determine a line in the B-T plabe separating regions of flux expulsion from those of large penetration. The line intersects the temperature axis below T c and does not scale with the sample radius, implying that simple critical state models are not appropriate to describe the nature of the magnetic hysteresis. A model where the flux penetration is caused by the onset of strong thermally activated creep across grain boundaries is proposed. The magnetic hysteresis are mainly governed by the time scale of the measurement and depends only weakly on the sample radius.

A.c. response of YBa2Cu3O7 with Y2BaCuO5 inclusions and of single crystal YBa2Cu3O7 : similarities, differences and anomalies

Abstract We have measured the a.c. magnetic permeability response function μ = μ′ + iμ″ in melt-processed melt-grown YBa2Cu3O7 with Y2BaCuO5 inclusions (MPMG) and in single crystal YBa2Cu3O7 (SC) in magnetic fields of up to 8 T oriented either parallel or perpendicular to the crystalline c-axis. The lossy part of the permeability μ″ (H,T) is used to probe the flux dynamics in the vicinity of the irreversibility line. For H | c, the MPMG has a higher irreversibility temperature Tirr(H) than the SC sample, while for H ⊥ c, Tirr(H) is similar for both samples. Because of the strong Hac dependence and the weak frequency dependence of μ″ (H,T) for H ⊥ c, we contend that the vortex system is in the glass phase. For H | c, the Hac dependence of μ″ is weak and its frequency dependence is strong, indicating that here the system is much closer to the glass/liquid transition. For the SC sample with H ⊥ c, μ″ (T) exhibits an anomalous structure which appears only in finite applied d.c. fields and disappears at fields greater than 1.5 T.