Direct photoemission spectroscopy and electronic properties of in situ grown, strained high-Tc and related oxide films Original Research Article

We have optimized laser ablation thin film deposition and transfer procedures within synchrotron vault, specifically to perform angle-integrated and angle-resolved photoemission spectroscopy (ARPES) on high-Tc and related films without cleaving the samples. However, the chain-containing phases like YBCO-123 easily loose surface oxygen and do not exhibit stable Fermi edge, hence are not suitable for ARPES studies. Direct in situ ARPES studies on strained LSCO-214 films show striking strain effects on the electronic structure. The Fermi surface (FS) of LSCO evolves with doping, yet changes even more significantly with strain. The strain changes the FS topology from hole-like to electron-like, and causes band dispersion along kx and the Fermi level crossing before the Brillouin zone boundary, in sharp contrast to the ‘usual’ flat band remaining ≈30 meV below EF measured on unstrained samples. The associated reduction of the density of states does not diminish the superconductivity; Tc is enhanced in all our strained samples. Implications for the evolving high-Tc theory and studies of nano-engineered film heterostructures are briefly discussed.