Slightly doped systems

The undoped or slightly doped compound La2Cu04+ t. 0.03, was studied by ARPES by Zakharov et al. (1997). They observed that this compound has no natural cleavage planes, so cleavage probably results in non-uniform surfaces, and at the very least it results in broken covalent or ionic bonds that may redistribute electronic charge compared to the underlying crystalline compound. Until confirmation from more extensive mapping of the Brillouin zone, results on this system should be considered tentative. 

Phosphorescence. Phosphorescence of Fe3+-doped Gels. In H20. In all systems studied the phosphorescence of the initial Fe3+-doped gel showed a wreak emission at 690 nm with a shoulder at ca. 720 nm. Emission and excitation spectra were similar to those shown in Figure 1. The phosphorescence comes from the Ai - 47 transition while the various other transitions in the Fe3+ ion in tetrahedral coordination are apparent in the excitation spectrum (8, 10). Throughout the induction period emission intensity increased only slightly, the peak at 690 nm increasing somewhat more than the shoulder at 720 nm. After induction the signal rose rapidly until a constant intensity was attained. The rapid increase in intensity of the phosphorescence signal after induction ended paralleled the intensity increase of the x-ray diffraction lines. 

To date CuF " fluorescence has only been observed for Cu(II)/KZnF3 and Cu(II)/K2ZnF4 [16, 17]. These systems are favorable as they are doped rather than pure crystals, and are in cubic/slightly compressed sites, all of which will tend to minimize radiationless deactivation. The fluorescence spectrum confirms the general picture given by Fig. 7 [16]. 

Current-voltage (I-V) characteristics were also measured at different temperatures in magnetic fields applied perpendicularly to the surface of the substrate, i.e. parallel to the c-axis of the film. The critical current densities at T=2.1 K and in zero field are above 10 A/m. They have been defined by the electric field criterion E=Ec=10 V/cm. As an example, I-V characteristics for different values of the external magnetic field at T = 4.2 K are reported in Fig. 2. The values of the measured critical current density are slightly lower with respect to some others reported in the literature which, however, refer to epitaxial thin films in which Ce doping level was equal to its optimum value x = 0.15 [9]. We believe this is related to the reduced value of the critical temperature T in this system with respect to the optimally Ce-doped samples. 

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