| dc.description.abstract | In this chapter we discuss the semiconductor-metal transition in La???Sr?CoO?.
We study the electrical resistivity, X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), Bremsstrahlung isochromat spectroscopy (BIS) and X-ray absorption spectroscopy (XAS) of well-characterized samples of La???Sr?CoO?, x = 0.0, 0.1, 0.2, 0.3 and 0.4.
The electrical resistivity was measured over a wide temperature range (2 K–300 K) to confirm the semiconductor-metal transition. At low temperatures (T < 30 K), we see a rise in the resistivity for the metallic compositions. This rise in resistivity has a power-law dependence characteristic of disorder effects.
The XPS studies show systematic changes in the valence band and Co 2p core-level spectra consistent with the electrical and magnetic properties of the compounds. The UPS spectra show a finite intensity at the Fermi level for the metallic compositions. The BIS data show hole states developing within the band gap of LaCoO? for increasing Sr content. These states overlap the valence band for x > 0.2 leading to a semiconductor-metal transition. The results of XAS at the O K-edge also show the development of hole states induced by the Sr substitution. These states overlap the Fermi level for x > 0.2.
Comparison of BIS and XAS results indicate a strongly mixed character for the ground state of LaCoO?. Analysis of the core-valence-valence (CVV) Auger spectra indicates Udd = 3.4 eV and Upp = 6.7 eV for the on-site Coulomb interaction energies in the Co 3d and O 2p manifolds, respectively. The Co 2p core-level spectrum of LaCoO?, simulated in terms of model many-body calculations, provides estimates for various parameter values, such as the charge-transfer energy ? = 4.0 eV and the hybridization between Co 3d and O 2p states, t? = 3.8 eV, and is consistent only with a low-spin configuration of the ground state. The large hybridization strength, t?, is comparable to the bare charge-transfer energy ? and the Coulomb interaction strength, and is in agreement with the mixed character of the parent insulator, as deduced from the analysis of the valence band spectra. Moreover, these parameter values indicate that the doped hole states will locally convert the formally low-spin Co³? state to high-spin Co?? state, in agreement with earlier magnetic measurements.
In this chapter we study the electronic structure and semiconductor-metal transition in La???Sr?MnO? (x = 0.0–0.4) using various techniques. The electrical resistivity (?) measured over a wide temperature range (2 K–450 K) shows an unusual semiconductor-metal transition for x > 0.2. The metallic compositions exhibit a broad maximum in ? versus T, in the temperature range 150 K–250 K depending on the composition. Magnetic measurements show a Curie temperature for the doped compounds, which cannot be directly related to the resistivity anomaly.
In the metallic phase, the resistivity over the entire temperature range remains several orders of magnitude higher compared to other metallic oxides. The high-temperature resistivity indicates an activated behaviour for all the compositions with very similar band gaps. At low temperatures (T < 30 K) we see an upturn in the resistivity which fits a T??·? dependence, indicating the importance of electron-electron interactions in the presence of disorder in the system. The conductivity above 100 K, for x > 0.2, fits an equation of the type:
? = ??e??°/²?? + A/(1 + BT)
implying the total conductivity to be a sum of semiconducting and metallic components.
Electron spectroscopy studies employing photoemission and inverse photoemission show negligible intensity at E? even for the metallic compositions, indicating an unusual semiconductor-metal transition, consistent with the resistivity data. BIS spectra show doped hole states developing as a function of Sr content, about 1.4 eV above E?. Soft X-ray absorption studies at the oxygen K-edge also show the formation of doped hole states above E? for increasing x, implying substantial oxygen 2p character of the doped states.
The Mn 2p core-level spectrum of LaMnO?, analysed in terms of a model many-body configuration interaction calculation, gives parameter values for ? (= 5.0 eV), t? (= 3.8 eV) and Udd (= 4.0 eV). AES gives an estimate of Upp, the on-site Coulomb interaction strength in the oxygen 2p states, to be 6.8 eV. The values of ?, t? and Udd classify LaMnO? as a mixed character oxide with large electron-electron correlations. These parameter values also indicate that the doped hole states in this series should have considerably mixed Mn 3d–O 2p character, in conformity with BIS and XAS results.
The absence of any perceivable density of states at E? even for the metallic compositions (x > 0.2) in conjunction with the anomalous resistivity behaviour suggests the interpretation that doping of Sr in place of La produces a local density of states with a gap, in conformity with bulk SrMnO? being an insulator; however, metallic percolating paths, presumably arising from the interface or the boundary of Sr-doped and undoped regions, are established at high enough Sr concentrations, resulting in an apparent semiconductor-metal transition as a function of temperature.
In this chapter, we study the effect of Sr substitution in LaFeO? using various electron spectroscopy techniques. From resistivity measurements, it is found that the samples remain semiconducting up to 40% substitution of Sr for La. We determine the electronic structure of well-characterized samples of La???Sr?FeO? (x = 0.0–0.4) by X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), Bremsstrahlung isochromat spectroscopy (BIS) and X-ray absorption spectroscopy (XAS).
We find systematic changes in the occupied and unoccupied density of states (DOS). The spectral features, particularly of the unoccupied states obtained from BIS spectra, indicate the probable reason for the absence of an insulator-metal transition in this series. AES provides us with estimates of Udd and Upp, the on-site Coulomb interaction strengths in Fe 3d and O 2p states, respectively. Fe 2p core-level XPS in conjunction with a model many-body calculation gives us the parameter values for the bare charge-transfer energy ? and the Fe 3d–O 2p hybridization strength, t? for LaFeO?. The interaction parameters thus obtained suggest a substantially different nature of the ground state in LaFeO?, as compared to an earlier analysis; we also discuss the character of the doped hole states in La???Sr?FeO?, based on these parameter values. | |
