  The propagation of Dyakonov-Tamm (DT) surface waves guided by the planar
interface of two nondissipative materials $A$ and $B$ was investigated
theoretically and numerically, via the corresponding canonical boundary-value
problem. Material $A$ is a homogeneous uniaxial dielectric material whose optic
axis lies at an angle $\chi$ relative to the interface plane. Material $B$ is
an isotropic dielectric material that is periodically nonhomogeneous in the
direction normal to the interface. The special case was considered in which the
propagation matrix for material $A$ is non-diagonalizable because the
corresponding surface wave-named the Dyakonov-Tamm-Voigt (DTV) surface wave-has
unusual localization characteristics. The decay of the DTV surface wave is
given by the product of a linear function and an exponential function of
distance from the interface in material $A$; in contrast, the fields of
conventional DT surface waves decay only exponentially with distance from the
interface. Numerical studies revealed that multiple DT surface waves can exist
for a fixed propagation direction in the interface plane, depending upon the
constitutive parameters of materials $A$ and $B$. When regarded as functions of
the angle of propagation in the interface plane, the multiple DT surface-wave
solutions can be organized as continuous branches. A larger number of DT
solution branches exist when the degree of anisotropy of material $A$ is
greater. If $\chi = 0^\circ$ then a solitary DTV solution exists for a unique
propagation direction on each DT branch solution. If $\chi > 0^\circ$, then no
DTV solutions exist. As the degree of nonhomogeneity of material $B$ decreases,
the number of DT solution branches decreases.
