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تسجيل مستخدم جديدTemperature dependence of the electronic structure of A-site ordered perovskite CaCu$_3$Ti$_4$O$_{12}$: Angle-integrated and -resolved photoemission studies
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We have investigated the electronic structure of A-site ordered CaCu$_3$Ti$_4$O$_{12}$ as a function of temperature by using angle-integrated and -resolved photoemission spectroscopies. Intrinsic changes of the electronic structure have been successfully observed in the valence band region by the careful consideration of charging effects. The obtained photoemission results have revealed that the intensity of the nearly non-dispersive Cu 3$d$-O 2$p$ hybridized bands at the binding energy of $sim$2 eV increases with decreasing temperature from 300 to 120 K. This suggests that the density of the localized states, caused by the strong correlation effects, enhances as temperature decreases.
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CaCu$_3$Fe$_4$O$_{12}$ exhibits a temperature-induced transition from a ferrimagnetic-insulating phase, in which Fe appears charge disproportionated, as Fe$^{3+}$ and Fe$^{5+}$, to a paramagnetic-metallic phase at temperatures above 210 K, with Fe$^{
4+}$ present. To describe it, we propose a microscopic effective model with two interpenetrating sublattices of Fe$^{(4-delta)+}$ and Fe$^{(4+delta)+}$, respectively, being $delta$ the Fe-charge disproportionation. We include all $3d$-Fe orbitals: $t_{2g}$ localized orbitals, with spin 3/2 and magnetically coupled, plus two degenerate itinerant $e_g$ orbitals with local and nearest-neighbor (NN) electron correlations, and hopping between NN $e_g$ orbitals of the same symmetry. Allub and Alascio previously proposed a model to describe the phase transition in LaCu$_3$Fe$_4$O$_{12}$ from a paramagnetic-metal to an antiferromagnetic-insulator, induced by temperature or pressure, involving charge transfer between Fe and Cu ions, in contrast to Fe-charge disproportionation. With the model proposed for CaCu$_3$Fe$_4$O$_{12}$, modified to account for this difference between the two compounds, the density of states of the itinerant Fe orbitals was obtained, using Greens functions methods. The phase diagram for CaCu$_3$Fe$_4$O$_{12}$ was calculated, including phases exhibiting Fe-charge disproportionation, where the two eg orbitals in each site are symmetrically occupied, as well as novel phases exhibiting local orbital selectivity/asymmetric occupation of $e_g$ orbitals. Both kinds of phases may exhibit paramagnetism and ferromagnetism. We determined the model parameters which best describe the phase transition observed in CaCu$_3$Fe$_4$O$_{12}$, and found other phases at different parameter ranges, which might be relevant for other compounds of the ACu$_3$Fe$_4$O$_{12}$ family, which present both types of transitions.
We report angle-resolved photoemission spectroscopy (ARPES) results of A-site ordered perovskite CaCu$_3$Ti$_4$O$_{12}$. We have observed the clear band dispersions, which are shifted to the higher energy by 1.7 eV and show the band narrowing around
2 eV in comparison with the local density approximation calculations. In addition, the high energy multiplet structures of Cu 3$d^8$ final-states have been found around 8 - 13 eV. These results reveal that CaCu$_3$Ti$_4$O$_{12}$ is a Mott-type insulator caused by the strong correlation effects of the Cu 3$d$ electrons well hybridized with O 2$p$ states. Unexpectedly, there exist a very small spectral weight at the Fermi level in the insulator phase, indicating the existence of isolated metallic states.
We studied the electronic structure of the heavy fermion compound Yb(Ru$_{1-x}$Rh$_{x}$)$_2$Ge$_2$ with $x=0$ and nominally $x=0.125$ using ARPES and LDA calculations. We find a valence band structure of Yb corresponding to a non-integer valence clos
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