M.G. Karkut

X‐ray photoelectron spectroscopy study of thin Ag overlayers on YBa2Cu3O7−x: Effects of oxygen annealing

X‐ray photoelectron spectroscopy was used to examine vapor deposited Ag overlayers on YBa2Cu3O7−x thin films and cleaved single‐crystal surfaces (x=1), including the effects of subsequent heat treatment in oxygen. The core level spectra of the present tetragonal crystals were found to closely resemble those previously reported for nominally orthorhombic superconducting samples. Minor differences, notably an increased fraction of Cu in the +1 oxidation state and a slightly higher binding energy of the Ba 3d levels, may be attributed to the lower oxygen content of the tetragonal semiconducting phase. Deposition of Ag on clean monocrystal surfaces was found to cause some disruption of CuO bonds at the Ag/YBa2Cu3O7−x interface. We also find weak indications of surface band bending (i.e., Schottky barrier formation) from a uniform shift in the measured binding energies of the substrate core levels upon Ag deposition. Heat treatment in pure oxygen (1 h at 500u2009°C) of Ag‐coated superconducting thin films was foun...

Anisotropic magnetotransport in high temperature superconductor multilayers

The high temperature superconducting cuprates are layered materials in which the CuO2 layers are found to play an important role for the superconducting properties. When produced in thin film form, these materials can be grown in a layer by layer sequence and this has allowed us to make superlattices of various combinations of these compounds. In particular, in the YBa2Cu3O7/PrBa2Cu3O7 system it is possible to produce superlattices where the individual layers are as thin as one c-axis unit cell. This allows us to modify the anisotropy of these materials in a controlled manner. In this contribution we present a study of the resistive transition in a magnetic field perpendicular or parallel to the layers, and how the results can be used to gain insight into the role of the anisotropy for the dissipative behaviour of these high temperature superconductors.

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.

Verification of the Takahashi-Tachiki effect using multilayers consisting of Nb and Nb0.6Ti0.4

Abstract We observe, in Nb/Nb0.6Ti0.4 multilayers, the predicted anomalous upturn in the parallel upper critical field which reflects the shift of the nucleation field from the clean Nb layers to the dirty NbTi layers. The temperature at which the upturn occurs increases with modulation wavelength in accord with theory. We also present critical fields as a function of angle.

Segregation, interface reactions and surface topography of YBa2Cu3O7−δ thin films, investigated by Auger depth profiling, microprobe analysis, STM and SEM

Abstract Thin films of the high T c superconductor YBaCuO, prepared by co-evaporation on substrates of ZrO 2 and SrTiO 3 , have been examined with Auger electron depth profile analysis, microprobe analysis, scanning electron microscopy and scanning tunneling microscopy. The Auger depth profiles unveil pronounced diffusion of Ba into the substrate for films prepared on ZrO 2 . Introduction of a Pt buffer layer at the interface promotes strong segregation of the metallic elements in the film. The STM images provide quantitative data on the surface topography and are shown to correlate excellently with SEM micrographs.

A time efficient SQUID magnetometer. Magnetic relaxation in a YBa2Cu3O7−x thin film

Abstract We have constructed and tested a DC SQUID magnetometer for magnetic relaxation measurements in high-T c superconductors in a time window starting as early as 0.1 s after field switching. Quick energizing or deenergizing of a copper coil DC magnet inside a superconducting 5 T magnet gives a time window of > 5 decades within 3 hours, providing reliable determination of the magnetization decay rate. The available field range is 0.05 mT to 5 T. The sample temperature can be varied between 4.2 and 200 K with a temperature stability of ± mK. We present measurements of the magnetization versus temperature and time in a thin film of YBa 2 Cu 3 O 7−x .

Investigation of thermally activated flux flow of YBa2Cu3O7/ PrBa2Cu3O7 superlattices in magnetic fields parallel to the a,b plane

Abstract YBa 2 Cu 3 O 7 is a strongly anisotropic 3D material. Our observations of thermally activated flux flow in magnetic fields perpendicular to the a,b plane show that the vortices are 3D-like in this material. Making multilayers of the type YBa 2 Cu 3 O 7 /PrBa 2 Cu 3 O 7 (YBCO/PrBCO) allows to increase the anisotropy and turn the material into a 2D system. We explore this 3D to 2D transition by investigating the thermally activated flux flow behavior in parallel applied fields. We find the parallel field dependence to be markedly different from the perpendicular dependence. At low fields and in multilayers with relatively thick individual layers we find that the activation is largely field independent. At higher fields we observe a crossover to a power law behaviour similar to what is observed in thick YBCO films. In multilayers with 24A YBCO layers decoupled by thick PrBCO layers (>> 24A) we find no broadening of the resistive transition up to 20 Tesla.

Anomalous ac magnetic response in an YBa2Cu3O7 crystal with H⊥c

Abstract We have observed anomalous loss peaks for a single crystal YBa 2 Cu 3 O 7 cube with H ⊥ c in the field range 0.1–2 T and investigate their response as a function of orientation and ac field amplitude and frequency. We consider various mechanisms for the observed behaviour.

Angle dependence of the magnetoresistance in the mixed state of YBa2Cu3O7 and Y0.8Pr0.2Ba2Cu3O7 thin films

Abstract The effect of adding praseodymium (Pr) to YBa 2 Cu 3 O 7 is to reduce T c and increase resistivity. We have measured the magnetoresistance in epitaxial films of YBa 2 Cu 3 O 7 and Y 0.8 Pr 0.2 Ba 2 Cu 3 O 7 in magnetic fields up to 5 T and temperatures between 50K and 90K. Large variations in the magnetoresistance are observed when the magnetic field direction is rotated with respect to the crystalline c-axis of the films. In the YBa 2 Cu 3 O 7 films the effect of decreasing the temperature is to move from a 2D picture close to T c where the effect of the magnetic field scales with the component perpendicular to the planes, to a less anisotropic behaviour at lower temperatures. For Y 0.8 Pr 0.2 Ba 2 Cu 3 O 7 the behaviour of the magnetoresistance is opposite. Here, increasing the temperature towards T c decreases the anisotropy.

Direct measurements of the local electron transport properties in YBaCuO superconducting thin films

Abstract We have made measurements of the local potential distribution and topography of current carrying Y 1 Ba 2 Cu 3 O 6.9 thin films using a scanning tunneling microscope. We have compared the electrostatic potential distribution in the normal state on post-annealed thin films and in-situ prepared films. On post-annealed films we find steps in the potential image indicative of insulating boundaries in the material which occur at boundaries between clusters of grains whereas the in-situ films show a nearly uniform potential drop. These results provide a clear indication of the nature of disorder in these thin films and what length scale ultimately determines the microscopic resistivity.