A study of the upper critical field and anisotropy in YBa2Cu3O7 single crystals

Il Nuovo Cimento D - We report a set of experimental results on second- and third-harmonic generation at microwave frequencies in YBa2Cu3O7 single crystals near T c. The investigation of the...     

Critical current and upper critical field of sintered and powdered superconducting YBa2Cu3O7

A sintered bulk sample of YBa₂Cu₃O₇ has been investigated resistively and inductively with respect to its resistivity (B, T), upper critical fieldB _c2 (T), and current carrying capabilityj _c (B, T). The inductive measurements were repeated after powdering the sample to a maximum particle size of 60 m. Results show that the intrinsic-intragrain properties as approximately revealed by the powder measurements, are severely masked in bulk measurements by the intergrain weak-link structure. As an example, the current densityj _c (0 T, 4.2 K) in the powdered YBa₂Cu₃O₇ has a value of 4·10⁶ A/cm², which is about three orders of magnitude higher than in the bulk material. The high values of the intragrainj _c are probably due to pinning at twin boundaries. — Besidesj _c the normal state resistivity just aboveT _c , _n , is influenced by the interconnecting structure, both properties being directly related: a smaller _n leads to a higherj _c in bulk samples. — The upper critical field shows a pronounced anisotropyB _c2 |/B _c2 of about 20 to 30, whereB _c2 |=330 T is the extrapolated upper critical field at zero temperature for the field orientation parallel to the (Cu–O)-planes. — For the time being, theB _c2 anisotropy and the weak-link structure of sintered bulk samples are the main obstacles for high field applications.     

Resistive transition and upper critical field in underdoped YBa2Cu3O6+x single crystals

A superconducting transition in the temperature dependence of the ab-plane resistivity of underdoped YBa₂Cu₃O_6+x crystals in the range T _c≲30 K has been investigated. Unlike the case of samples with the optimal level of doping, the transition width increased insignificantly with magnetic field, and in the range T≲13 K it decreased with increasing magnetic field. The transition point T _c(B) was determined by analyzing the fluctuation conductivity. The curves of B _c2(T) measured in the region T/T _c≳0.1 did not show a tendency to saturation and had a positive second derivative everywhere, including the immediate neighborhood of T _c. The only difference among the curves of B _c2(T) for different crystal states is the scales of Tand B, so they can be described in terms of a universal function, which fairly closely follows Alexandrov’s model of boson superconductivity. Zh. éksp. Teor. Fiz. 115, 268–284 (January 1999)