Analog of the Anderson Theorem for the Polar Phase of Liquid ³He in a Nematic Aerogel

The effect of an aerogel with parallel fibers on the temperatures of ³He transition to superfluid phases differing in the l_z projections of the orbital angular momentum on the directions of the fibers is considered. It is shown that at the specular reflection of Fermi excitations of liquid ³He from the fibers, the temperature of transition to the polar phase corresponding to l_Z = 0 remains the same as the temperature of transition to any phase with orbital angular momentum l = 1 in the absence of the aerogel. The temperature of transition into phases with l_z = ±1 turns out to be lower, and there appears to be a finite temperature interval in which only the polar phase is stable. This interval has been determined. The effect of the magnetic (exchange) scattering of Fermi excitations at adsorbed ³He atoms on the temperature of transition of ³He to the superfluid state has been estimated.