Resonance photoemission of LaCoO3(111) and La0.9Sr0.1CoO3(111)

Resonant photoemission performed at the SRS Daresbury Laboratory is used to investigate the temperature- and doping-dependent spin states in single cr ystal LaCoO3(111) and La0.9Sr0.1CoO3(111). In an initial study, the surface reactivity of the LaCoO3(111) surface is investigated using H2O as a probe molecule. The results are used to interpret changes which may occur in the spectra as a function of time and temperature in UHV, due to surface react ions. This allows us to distinguish effects on the valence band spectra due to spin changes as a function of temperature. The temperature dependence o f low binding energy features primarily associated with Co in the low spin (LS) state in LaCoO3 is investigated in detail. In particular, the intensit y and onset energy of the Co 3p -> 3d resonance associated with these featu res is measured at small binding energy intervals. Delayed resonance onset associated with the LS state is consistently observed. This is used to loca te the separate contributions of the low spin and higher spin states to the low binding energy spectrum. We find the contribution due to the LS state lies around 1 eV to lower binding energy than that due to higher spin state s in LaCoO3. In contrast, we find the spectrum of La0.9Sr0.1CoO3(111) to be invariant with temperature, with no delayed resonance effect. The data are discussed in the light of recent models for the spin transitions, includin g the two stage spin-state model involving an intermediate spin state. Simu lation of the temperature variation of the valence band spectra of LaCoO3 u sing this model provides good agreement with experiment.