Evidence for a quantum phase transition in two dimensions

For a broad range of electron densities n and temperatures T, data for the in-plane magnetoconductance of the two-dimensional system of electrons in silicon MOSFET's is shown to collapse onto a universal curve using a single parameter Hσ(n,T); the scaling parameter Hσ obeys the empirical relation Hσ=A(n)[Δ(n)2+T2]1/2. The characteristic energy kBΔ associated with the magnetic field dependence of the conductivity decreases with decreasing density, and extrapolates to 0 at a critical density n0, signaling the approach to a zero-temperature quantum phase transition. For densities near n0Hσ=AT so that the magnetoconductance scales with H/T. We suggest that this behavior is associated with a ferromagnetic instability.