Critical current density and upper critical field of YBa2Cu3O7 thin films

Epitaxial YBa2Cu4O7-thin films with thec-axis oriented perpendicular to the film plane were prepared bydc-sputtering from a single stoichiometric target on (100) SrTiO3-substrates. Typical values of the inductively measured superconducting transitions were about 90 K with a width less than 0.5 K. Critical current densities were measured on 5 to 10 m wide strips as function of magnetic field and temperature. The temperature dependences ofj c follow a universal functionj c(B,T)=j c * (T=0,B)·(1?T/T c (B)) with =1.5±0.1. ForB=0 andT=77 K we obtainedj c =4·106 A/cm2. The field dependence of the resistive transitions was measured with the magnetic field parallel to thec-axis. The slope of the upper critical fieldB c2 (T) was determined for different criteria. The carrier concentration evaluated from Hall-effect measurements was found to decrease linearly from one per unit cell at 240 K with decreasing temperature extrapolating nearly through zero forT=0. Highly resolved angular dependent measurements of the critical current density withB perpendicular to the current but tilted from thec-axis show a very strong and sharp enhancement ofj c for the magnetic field parallel to the (CuO2)-layers (Bc). Additionally to this phenomenon, which is caused by an intrinsic pinning mechanism due to the layered structure of high-T c -superconductors the influence of the anisotropy of the upper critical field onj c (B, T, ) is evident nearT c