OBSERVATION OF A NEW SURFACE-STATE ON HE-4 CRYSTAL INTERFACES

The equilibrium shape of hcp He-4 crystals has been studied at tempera tures 0.05 less than or equal to T less than or equal to 0.7 K by mean s of a high-precision optical interferometer. We find that the profile of the interfacial boundary, close to an almost horizontal c-facet, h as a well-defined slope discontinuity separating two angular regions w ith different behavior of the surface stiffness <(alpha)over tilde>. F or surfaces tilted by an angle phi less than or similar to 100 mu rad with respect to the c-facet we obtain <(alpha)over tilde> = kappa/phi, contrary to the linear angular dependence predicted by current theori es. Our results on four large crystals yield kappa = (11 +/- 3) x 10(- 4)T erg/cm(2) K.