General Considerations

The electronic properties of high-T_c 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 $\mbox{YBa$_{2}$ Cu$_{3}$ O$_{7- \delta}$ }$, the crystal structure is orthorhombic (see Fig. 2.9). The layered crystal structure of $\mbox{YBa$_{2}$ Cu$_{3}$ O$_{7- \delta}$ }$ results in a strong $\vec{a}$-$\vec{c}$anisotropy, while the orthorhombic distortions due to the Cu-O chains introduce another $\vec{a}$-$\vec{b}$ anisotropy. The anisotropic nature of various physical quantities measured in single crystals of $\mbox{YBa$_{2}$ Cu$_{3}$ O$_{7- \delta}$ }$have been well documented. For instance, the electrical resistivity along the $\vec{a}$ and $\vec{c}$-directions of the unit cell varies linearly with temperature, while along the $\vec{b}$-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 $\mbox{YBa$_{2}$ Cu$_{3}$ O$_{7- \delta}$ }$ also exhibit strong anisotropy. The anisotropy of the magnetic penetration depth $\lambda$ in the uniaxial high-T_cmaterials is a direct consequence of the superconducting currents not being isotropic [66]. In addition to the penetration depth exhibiting a large $\vec{a}$-$\vec{c}$ anisotropy, recent measurements on untwinned $\mbox{YBa$_{2}$ Cu$_{3}$ O$_{6.95}$ }$ single crystals show a small but significant $\vec{a}$-$\vec{b}$ anisotropy [10].