Direct visualization of defect density waves in 2D.
A scanning tunneling microscopy investigation of the 1/3 of a monolayer alpha phase of Sn on Si(111) reveals a new low temperature phase, which is electronic and not structural. This phase consists of a one-dimensional incommensurate electronic wave that coincides with a periodic modulation of the population of the substitutional Si defects, i.e., a defect density wave.