Sharp Transition to a Spin-Ordered Phase in the 2D Electron System in Silicon MOSFET's

This paper has been withdrawn by the authors. Measurements at many different angles spanning a broad range of electron densities indicate that the observed doubling of the period of Shubnikov-de Haas oscillations, which we had attributed to a transition to a new spin-ordered phase, is also consistent with an alternative explanation. A full interpretation of our data will be posted at a later date.