Anomalous Surface Behavor in Temperature-Dependent LEED Study of Sr₃Ir₂O₇

The temperature dependent low-energy electron diffraction (LEED) studies on Sr₃Ir₂O₇(001) show a behavior that contradicts the expected Debye-Waller scattering. According to Debye-Waller scattering, a finite Debye temperature would imply a decrease in LEED scattering intensities with increasing temperature, but this is not observed between room temperature and roughly 400 K in Sr₃Ir₂O₇ at low electron kinetic energies. Instead, there is a significant enhancement in the intensities of the LEED diffraction spots before an attenuation in intensity sets in, with increasing temperature. Hence, there is an anomaly in the surface behavior of Sr₃Ir₂O₇(001) which contradicts the Debye-Waller scattering theory. However, at higher electron kinetic energies, the LEED intensity behavior becomes more characteristic of a finite Debye temperature. A Debye temperature of 270±22 K was obtained at an electron kinetic energy of 51.7 eV which is very surface sensitive because of the limited electron mean free path. At 162.0 eV electron kinetic energy, a Debye temperature of 457±19 K was derived from the temperature-dependent LEED. This is much more representative of the bulk. In fact, this bulk Debye temperature is in-line with known Debye temperatures of other similar oxides.
This anomaly in the surface behavior of Sr₃Ir₂O₇(001) might be attributed to temperature-induced surface segregation, strong correlation, a structural transition, or an insulator-metal phase transition. By carrying out angle resolved X-ray photoemission spectroscopy, surface segregation has been excluded as a cause of the observed anomaly. However, electron transport measurements indicate a carrier activation energy that is in close agreement with the thermal energy at which the surface goes from anomalous behavior to Debye-Waller behavior in LEED, suggesting a phase transition associated with changing carrier concentration. This suggests that the LEED intensity at the surface of Sr₃Ir₂O₇(001) does not follow the expected Debye-Waller behavior, up to roughly 400 K due to increasing charge carrier concentration which dampens vibrational amplitudes.