B. Holzapfel

Temperature variations of the energy spectrum of high temperature superconductors near the fermi level seen by optical absorption

Abstract The temperature dependence of optical absorption spectra in the visible range has been measured between 60 and 180 K on superconducting, single-crystalline films of YBa2Cu3O7−δ and Bi2Sr2CaCu2O8. The analysis of the experimental results was performed under the assumption of a high ratio between the total valence band width and the width of the narrow hole band. The linear increase of the absorption above Tc is attributed to a variation of the number density of holes, which is caused by a temperature dependent redistribution of electrons between the conducting CuO2 plane and a second electron subsystem that acts as a charge carrier reservoir. The occurrence of superconductivity at Tc reflected in a peculiar behaviour of the optical absorption at T ≤ Tc. It is shown that optical spectroscopy in the visible range provides valuable information about temperature dependent changes of the electronic structure of HTSC near the Fermi level.

Anomalies in the temperature behaviour of absorption in the optical 1.5–3.0 eV range for the high-Tc superconductor YBa2Cu3O7-δ

Abstract Spectral and temperature measurements of the absorption coefficient, K ( E, T ), have been performed within a dielectric transparency window (from 1.5 to 3 eV) which persists in the metal phase. An unusual temperature-dependent increase in absorption was observed above T c for the metal phase. In combination with the measured absorption spectra and their concentration dependences it indicates an important role for the metal-type intraband transitions which is enhanced with oxygen concentration. It is concluded that there occurs a noticeable temperature-dependent redistribution of electrons between the CuO 2 plane and the reservoir of charges. The K ( T ) curve has a peculiarity at T c and follows the superconducting gap width below T c . All the peculiar features of K ( T ) are conditioned by the narrowness of the conduction hole band and provide information on the electron structure near the Fermi level.

Effect of columnar defects on the critical current anisotropy of epitaxial YBa2Cu3O7-δ thin films and YBa2Cu3O7-δ/PrBa2Cu3O7-δ multilayers

Abstract Epitaxial YBa 2 Cu 3 O 7−δ (YBCO) thin films and YBa 2 Cu 3 O 7−δ /PrBa 2 Cu 3 O 7−δ (Y/Pr) multilayers were irradiated with high energy heavy ions (770 MeV 208 Pb and 340 MeV 129 Xe) under varius directions Φ relative to the c-axis. The irradiation resulted in columnar defects tilted by Φ from the c-axis. The angular dependence of their pinning activity was studied by measuring the anisotropy of the critical current density. The J c (B,T,θ) behaviour of the irradiated YBCO thin films showed an additional peak, which exceeds the intrinsic pinning peak, exactly at the irradiation direction. The Y/Pr multilayers, however, showed an isotropic J c -enhancement by a factor of 5, without any additional structure in the J c (B,T,θ)-curve.

Optical study of thin YBa2Cu3O7−δ films: Change of color at Tc and temperature redistribution of holes in underdoping and overdoping

Abstract Changes of optical absorption have been investigated vs temperature and doping. An unusual temperature dependence is of increasing or decreasing character in case of underdoping or overdoping, respectively.

Preparation and properties of thin film YBa2Cu3OX devices

Abstract Low-noise YBa 2 Cu 3 O x dc SQUIDs and flux transformers were fabricated by laser deposition. The Josephson junctions were of grain boundary type, formed by epitaxial thin filmes, covering Ar + ion milled step edge substrates. The 5 μm wide dc SQUIDs were patterned by a SiO inhibit technique. The SQUID hole was varied between 5μm ∗ 10μm and 50μm ∗ 50μm, representing SQUID inductances between 20 pH and 140 pH. The optimum transfer function δV/δΦ at the appropriate flux bias was greater than 100 μV/Φo at 77K for samples with 20 pH. The best value of the energy resolution for samples with 20 pH is ϵ n,w (f > 30 Hz ) = 6 ∗ 10 −31 J/Hz in the white noise region and dc bias modulation. The energy sensitivity was measured as a function of the SQUID inductance. The coloured noise could be reduced by one order of magnitude applying an ac-bias to the SQUIDs obtaining an energy resolution for samples with 20 pH of 3 ∗ 10 −30 J/Hz at 1 Hz. Noise contributions from conductance fluctuations or flux hopping could be seperated by measuring the equivalent flux noise as a function of the bias current. A preparation technique for thin-film input coils has been developed, in which the three epitaxial layers were patterned either by deposition through shadow masks or by Ar + ion milling. A 12 turn flux transformer with 25 μm wide lines carried a persistent current of about 1 mA for more than two hours. To fabricate a homogeneous multilayer sequence of YBa 2 Cu 3 O x -SrTiO 3 -YBa 2 Cu 3 O x on a SrTiO 3 -substrate, an off axis laser deposition geometry was established, providing smooth films without laser particulates.

Hall effect on epitaxial YBa2Cu3Oτ and Bi2Sr2CaCu2Oy films

Abstract We report Hall effect measurements on epitaxial YBa 2 Cu 3 O τ and Bi 2 Sr 2 CaCu 2 O y films in the normal state and in the phase transition. The Hall coefficient R H (B∥c-axis, j∥ab-plane) of both compounds in the normal state is positive and strongly temperature dependent. The unusual temperature dependence of R H will be compared to the prediction of our calculation based on a renormalized two-dimensional tight binding model, which accounts to some extent for strong correlation effects. Below T c the Hall coefficient, which is strongly dependent on both the temperature and the magnetic field, exhibits a peculiar sign reversal.

Off-Axis Laserdeposition of YBa2Cu3O7-δ thin films and SrTiO3 insulation layers

Abstract We report on YBa 2 Cu 3 O 7-δ thin film preparation by a new laser deposition geometry, the so-called off-axis laser deposition. The main modification of off-axis laser deposition is the change of the substrate orientation in relation to the laser-induced plume. This results in c-axis oriented, epitaxial YBa 2 Cu 3 O 7-δ thin films with standard J c -values (>10 6 A/cm 2 at 77K and zero field) and an exceptionally good surface quality. Due to this low surface roughness it is possible to prepare thin (30nm) SrTiO 3 insulating layers. The insulation resistance of such a SrTiO layer (between two YBCO layers) is >5MΩ. Using this off-axis geometry it is also possible to coat both sides of a substrate simultaneously, providing a one-step process in double-sided thin film deposition even on large and relatively thin substrates. Identical critical current densities (above 10 6 A/cm 2 ) were obtained for both films on the substrate.

Hall Effect of Epitaxial YBa2Cu3O7−x and Bi2Sr2CaCu2Oy Films

The Hall effect of epitaxial YBa2Cu3O7−x and Bi2Sr2CaCu2O y films has been investigated experimentally and theoretically. In both compounds the normal state Hall coefficient R H (B || c-axis) is positive and exhibits a strong temperature dependence, which is more pronounced in YBa2Cu3O7−x than in Bi2Sr2CaCu2O y . The Hall coefficient R xyz H in the normal phase has been calculated on the basis of a renormalized two-dimensional tight binding model using the relaxation time approximation. Strong correlation effects are accounted for via doping dependent hopping terms leading to band narrowing. The influence of the choice of the relaxation time on the Hall coefficient is explored. The calculations have been performed for a wide range of parameters and are compared to experimental data.