  Electrons at interfaces, driven to minimize their free energy, are
distributed differently than in bulk. This can be dramatic at interfaces
involving heterovalent compounds. Here we profile an abrupt interface between V
3d2 LaVO3 and V 3d0 LaVO4 using electron energy loss spectroscopy. Although no
bulk phase of LaVOx with a V 3d1 configuration exists, we find a nanometer-wide
region of V 3d1 at the LaVO3/LaVO4 interface, rather than a mixture of V 3d0
and V 3d2. The two-dimensional sheet of 3d1 electrons is a prototypical
electronic reconstruction at an interface between competing ground states.
