R. Rössler

Bi2Sr2Can-1CunO2(n+2)+δ thin films on c-axis oriented and vicinal substrates

Abstract Bi 2 Sr 2 Ca n −1 Cu n O 2( n +2)+ δ ( n =1,2,3) thin films are prepared on c -axis oriented and vicinal substrates by pulsed-laser deposition. Optimization of substrate temperature, laser fluence and post-annealing conditions produces single-phase, well-oriented and smooth films. The surface morphology, texture, crystallinity and stoichiometry of Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) thin films strongly depend on the fabrication parameters. On ( 0 0 1 ) SrTiO 3 substrates, c -axis oriented Bi-2212 films with J c (60 K )=2×10 6 A/cm 2 and T c0 =82 K are obtained. Vicinal Bi-2212 films grown on miscut substrates reveal anisotropic resistivities independent of film thickness (20–300 nm), high J c anisotropy and strong in-plane and out-of-plane texture.

Electrical properties, texture, and microstructure of vicinal YBa2Cu3O7−δ thin films

Vicinal YBa2Cu3O7?? (YBCO) thin films of thickness h = 20–480?nm are grown by pulsed-laser deposition on 10° miscut (001) SrTiO3 substrates. The anisotropic resistivities, c-axis texture, and critical temperature drastically depend on the thickness of vicinal films. High-resolution electron microscopy reveals a defect microstructure with strong bending of the YBCO lattice near the SrTiO3 interface and improved film microstructure at larger distances to the substrate. The required layer thickness for microstructure relaxation and increase of electrical conductivity are significantly larger than the critical thickness of c-axis oriented YBCO films.

Anomalous Hall effect and vortex pinning in high-Tc superconductors

Abstract Pulsed high-current-density measurements of the Hall effect in the mixed state in several high- T c superconductors allow for a systematical tuning of the balance between Lorentz and pinning forces on the vortices. The characteristic temperatures for the single sign reversal in YBa 2 Cu 3 O 7− δ and those for the double sign change of the Hall effect in optimally-doped Bi 2 Sr 2 CaCu 2 O x are independent of current density. This confirms that the observed sign changes of the vortex Hall effect are intrinsic properties and are not evoked by vortex pinning. In contrast, underdoped Bi 2 Sr 2 CaCu 2 O x does not exhibit a sign reversal of the Hall effect but a negative contribution to the Hall conductivity can be detected when pinning is reduced in high-current densities.

Anisotropic resistivity in off-c-axis grown Bi2Sr2CaCu2O8 films

Abstract Bi 2 Sr 2 CaCu 2 O 8 thin films were prepared on off- c -axis (001) SrTiO 3 substrates by pulsed-laser deposition. Oriented film growth, with a c -axis inclined with respect to the surface normal by a tilt angle α is found up to α ≃15° from SEM, XRD and resistivity measurements. From a tensorial description of the films resistivity, the in-plane and out-of-plane resistivities ρ ab and ρ c can be deduced. These measurements represent a new method to investigate anisotropic transport properties of materials, taking advantage of the availability of perfectly grown crystalline thin films instead of using bulk crystals.

Effect of varying material anisotropy on critical current anisotropy in vicinal YBa2Cu3O7−δ thin films

The high Tc cuprate superconductors are noted for their anisotropic layered structure, certain of these materials indeed tend toward the limit of a Lawrence–Doniach superconductor. However, YBa2Cu3O7−δ has a smaller anisotropy than would be expected from its interlayer spacing. This is due to the cuprate chains in the structure. To investigate the influence of the chain oxygen on transport properties critical current versus applied field angle measurements were performed on fully oxygenated and deoxygenated YBa2Cu3O7−δ thin films and optimally oxygenated Y0.75Ca0.25Ba2Cu3O7−δ thin films. The films were grown on 10° miscut SrTiO3 substrates to enable the intrinsic vortex channeling effect to be observed. The form of the vortex channeling minimum observed in field angle dependent critical current studies on the films was seen to depend on film oxygenation. The vortex channeling effect is dependent on a angular dependent crossover to a string-pancake flux line lattice. The results obtained appear to be consi...

Magnetic field and current density dependence of the mixed-state Hall effect in Bi2Sr2CaCu2Ox

Abstract We report on measurements of the mixed-state Hall effect in Bi2Sr2CaCu2Ox thin films in magnetic fields ranging from B=60 mT to B=1 T and current densities j up to 3 MA cm−2. To reduce the problem of sample heating, we employed a pulsed current technique and developed a new heating correction procedure. We observe a double sign reversal of the Hall resistivity ρyx in the mixed state, resulting in two peaks which both depend on B and j. The position of the sign changes only varies with B, but is found to be independent of j. The Hall conductivity σxy turns positive again in the superconducting regime and dramatically increases towards lower temperatures and current densities, as the pinning is reduced by intense currents.

Large photoinduced enhancement of the electrical anisotropy in semiconducting YBa2Cu3Ox

Abstract The impact of long-term light excitation on YBa 2 Cu 3 O x (YBCO) ( x ≈6.4) thin films grown on “wedged” SrTiO 3 substrates was studied at several temperatures between 50 and 275 K. From the anisotropic resistance of the films, the in-plane ( ρ ab ) and the out-of-plane ( ρ c ) resistivities were calculated. The electrical anisotropy ρ c / ρ ab increased substantially during the light excitation at all studied temperatures. At 50 K, an increase of more than a factor of two was observed, whereas at higher temperatures the increases were smaller. At 275 K, ρ c / ρ ab showed a slightly falling tendency after about 1 h of illumination indicating oxygen ordering. The results strongly support our previously proposed two-mechanism picture of persistent photoconductivity in YBCO.

In-plane and out-of-plane conductivity enhancement in oxygen-depleted YBa2Cu3Ox thin films during long-term photoexcitation

Abstract Long-term photoexcitation experiments were performed on oxygen depleted YBa 2 Cu 3 O x ( x ≈6.6) thin films, prepared by pulsed laser ablation on tilted SrTiO 3 substrates. From the anisotropic resistivity of these films, the in-plane ( σ ab ) and out-of-plane ( σ c ) conductivities were calculated. White-light photoexcitation at various temperatures from 70 to 305 K yielded strongly temperature dependent increases of both σ ab and σ c . The photoinduced changes of the anisotropy σ ab / σ c strongly depended on the temperature at which the experiment was performed. At 70 K, the anisotropy increased monotonically during 24 h of illumination, whereas, at higher temperatures, a short increase at the very beginning of the illumination was followed by a long-term decrease. Additionally, spectroscopic investigations at the above temperatures were performed to determine the influence of different excitation energies.

Spectroscopic Studies of the Persistent In-Plane and Out-of-Plane Photoconductivity in 60 K YBa2Cu3Ox

Investigations of the wavelength dependence of the so-called photodoping effect in 60 K YBa2Cu3Ox were performed using samples grown on “tilted” substrates, allowing to study the photo-induced changes of the in-plane and out-of-plane resistivities, ρab and ρc, respectively. The photo-excitation was carried out at photon energies from 1.16 to 2.05 eV and at several temperatures between 70 and 305 K. At 100 up to 305 K, the spectral efficiencies of the in-plane and out-of-plane photodoping showed a single maximum at about 1.8 eV. The anisotropy of the spectral efficiency at this maximum was significant at low temperatures, but decreased rapidly when the temperature approached room temperature, indicating an additional contribution to the out-of-plane spectral efficiency at high temperatures. At 70 K, we observed an additional maximum at 1.4 to 1.5 eV that is not predicted by the proposed models for the persistent photoconductivity in YBa2Cu3Ox, which assume electronic excitations across the charge-transfer gap of 1.7 eV.

Sign Change of the Out-of-plane Magnetoresistance in Oxygen-deficient YBCO

The out-of-plane magnetoresistance in optimally doped (Tc ≈ 90 K) and oxygen-deficient (Tc ≈ 58 K) thin films of YBa2Cu3O7−δgrown on vicinal SrTiO3substrates was investigated. The magnetoresistance of both compounds changes sign above the critical temperature. The results are compared to theoretical predictions for the magnetic-field suppression of superconducting order parameter fluctuations in anisotropic materials. It is suggested that the competition of Aslamazov-Larkin and density-of-states fluctuations, which contribute with different sign to the magnetoresistance, are responsible for the observed sign change.