Conductivity of the Wigner Solid on the Free Surface of Superfluid ³He-B

We have measured the conductivity of the 2D electron crystal (the Wigner solid) trapped on the free surface of superfluid ³He-B down to 230 μK. The resistance R(T) greatly decreases as temperature T decreases, obeying Arrhenius' law, R(T) ∝ exp(-Δ(T)/k_BT), with Δ(T) which we assign to the superfluid energy gap. This reveals that the scattering of ³He quasiparticles by the WS determines the transport. As the input voltage increases, an anomalous behavior dominates the resistance just below the superfluid transition. The WS provides a new experimental method for probing both the bulk and free surface of superfluid ³He.