The electronic properties of high-Tc
superconductors are extremely anisotropic. Consequently
theories which describe their behaviour must be able to distinguish between
the motion of the superconducting carriers in different directions.
For
,
the crystal structure is orthorhombic
(see Fig. 2.9).
The layered crystal structure of
results in a strong
-
anisotropy, while the orthorhombic distortions due to the Cu-O chains
introduce another
-
anisotropy.
The anisotropic nature of various
physical quantities measured in single crystals of
have been well documented. For instance, the electrical
resistivity along the
and
-directions of the unit cell
varies linearly with temperature, while along the
-direction there
is a distinct upward curvature for increasing temperature, which has
been attributed to conductivity along the Cu-O chains [64,65].
Critical-field, critical-current and magnetic penetration depth measurements in
also exhibit strong anisotropy. The anisotropy
of the magnetic penetration depth
in the uniaxial high-Tcmaterials is a direct consequence of the superconducting currents not
being isotropic [66]. In addition to the penetration depth
exhibiting a large
-
anisotropy, recent measurements on untwinned
single crystals show a small but significant
-
anisotropy [10].