  We analyze the interference between tunneling paths that occurs for a spin
system with both fourth-order and second-order transverse anisotropy. Using an
instanton approach, we find that as the strength of the second-order transverse
anisotropy is increased, the tunnel splitting is modulated, with zeros
occurring periodically. This effect results from the interference of four
tunneling paths connecting easy-axis spin orientations and occurs in the
absence of any magnetic field.
