Observation of tunneling and magneto-tunneling out of a 2D Wigner crystal

Abstract

We measured the escape rates of surface state electrons from an electron layer confined at the liquid helium-vacuum interface in the temperature range of 30–450mK, and for densities 0.02–2.2×10⁸ cm ⁻². Below 200mK the escape rates were temperature independent and converged to the expected single particle tunneling rates in the zero density limit, where correlations are negligible. The single particle rates were enhanced exponentially as the density was increased up to a critical densityn _c. Atn _c the escape process becomes extremely nonlinear. As the barrier is raised so that the escape rates decrease below 5.0×10⁻⁴sec⁻¹, a new very weakly density and external field dependent mechanism seems to dominate the escape. Thermally activated escape was observed above 250mK and the activation energies were in good agreement with the expected values. Upon application of a magnetic field in the plane of the electron layer, the rates become strongly temperature and field dependent even at the lowest temperaturesT≤40mK. At these temperatures and low densities rates are suppressed four orders of magnitude in a few kGauss magnetic fields.