TRAPPED 2ND SOUND-WAVES ON A NONEQUILIBRIUM SUPERFLUID-NORMAL INTERFACE

When a heat current Q is driven through a near-critical sample of He-4 a situation may be realized in which a nonequilibrium superfluid-norm al interface appears. We derive the effective two-dimensional dynamics of this interface, showing that a surface sound mode exists whose amp litude decays exponentially into the bulk on either side. These modes are singularly damped, with D(q) proportional to q(3/2) rather than q( 2). We find that thermal fluctuations do not roughen the interface, th us aiding the DYNAMX microgravity experiment which proposes to measure its detailed properties.