This is a copy of the slides presented at the meeting but not formally written up for the volume.

We have studied the effects of epitaxial strains on structural phase transition behavior of fully-commensurate single crystal thin films of SrRuO3 using in situ temperature-dependent reflection high-energy electron diffraction (RHEED) and ex situ temperature-dependent x-ray diffraction (XRD) measurements. From RHEED measurements on compressively-strained (110) SrRuO3 grown on (001) SrTiO3 substrate, it was found that the surface of SrRuO3 showed no orthorhombic distortion even at room temperature (25°C) below the thickness of 10 monolayers (MLs). The orthorhombic transition temperature (TC) was determined to be 90°C at the thickness of 13 ML;s and then systematically increased up to 238°C at 35 ML. More interestingly, however, tensilely-strained SrRuO3 films grown on (110) DyScO3 and (110) GdScO3 substrates have simple cubic perovskite structure at room temperature irrespectively of SrRuO3 thicknesses. The shift of TC is very dramatic, considering that the orthorhombic transition temperature of SrRuO3 is known to be 547°C in bulk form. These unique transition behaviors were also confirmed by temperature-dependent four-circle x-ray diffraction measurements. TC of 35 ML thick SrRuO3 on SrTiO3 were determined to be 250°C and which was consistent with that determined from RHEED measurements.