STATE SCARRING BY GHOSTS OF PERIODIC-ORBITS

In this paper we discover a generic structure in the eigenfunctions of quantum billiards, namely, scarring by families of stable periodic or bits in a nonchaotic system. Our study is conducted on two different t riangular billiards, one an ergodic system and the other a ''pseudoint egrable'' billiard. Surprisingly, we detect scars in regions which con tain no periodic orbits. The periodic orbits responsible for scarring reside in a ''neighboring'' triangle. Such orbits show a more complex phase space structure than the ''bouncing ball'' trajectories of the s tadium billiard. While diffuse nodal structure is usually the antithes is of scarring, we show that in some eigenstates it is supported by ex tensive families of stable periodic orbits.