  The interfacial behavior of quantum materials leads to emergent phenomena
such as two dimensional electron gases, quantum phase transitions, and
metastable functional phases. Probes for in situ and real time surface
sensitive characterization are critical for active monitoring and control of
epitaxial synthesis, and hence the atomic-scale engineering of heterostructures
and superlattices. Termination switching, especially as an interfacial process
in ternary complex oxides, has been studied using a variety of probes, often ex
situ; however, direct observation of this phenomena is lacking. To address this
need, we establish in situ and real time reflection high energy electron
diffraction and Auger electron spectroscopy for pulsed laser deposition, which
provide structural and compositional information of the surface during film
deposition. Using this unique capability, we show, for the first time, the
direct observation and control of surface termination in complex oxide
heterostructures of SrTiO3 and SrRuO3. Density-functional-theory calculations
capture the energetics and stability of the observed structures and elucidate
their electronic behavior. This demonstration opens up a novel approach to
monitor and control the composition of materials at the atomic scale to enable
next-generation heterostructures for control over emergent phenomena, as well
as electronics, photonics, and energy applications.
