In the spinel system Li,-,Ti204 the Li content was reduced by oxidative extraction by means of a solution of I, as well as Br, in CH3CN or by oxygen. The extraction is associated with a transition of Ti3+ to Ti4+: the system is L i , -Y T i~! Y T i~~Y 0 4 . Consequently, with increasing y the charge carrier concentration decreases from the starting value of 0.5 electrons/Ti ion for LiTi204. The extraction proceeds in two steps. At the beginning, the Ti20, framework remains intact and the transition to superconductivity increases with decreasing charge carrier concentration to a maximal value of 13.2K. Separated by a two-phase region for medium extraction levels for higher y values, a Li depleted material of approximate composition Lio,,Ti,04 is obtained. The crystal structure of Lio.3Ti,0, can be deduced from the original spinel lattice by a partial Ti migration from 16d to 16c and 8a sites (space group Fd3m). Consequently, the Ti,04 framework of Lio,3Ti,0, is disconnected and the material is no longer superconducting.
We have studied the spatial distribution of the critical current density in YBa2Cu30,-6 ramp edge Josephson junctions using low-temperature scanning electron microscopy. Applying this technique allows the imaging of the critical current density distribution with a spatial resolution of about 1 pm. Our measurements show that the geometry of the ramp-edge junction eases the trapping of magnetic Aux quanta in the YBa,Cu,O,-s layer covering the ramp edge. These trapped flux quanta result in a spatially inhomogeneous magnetic field parallel to the barrier layer, which in turn results in a spatially modulated supercurrent density and an unusual magnetic field dependence of the critical current.
With low-temperature scanning electron microscopy, the magnetic flux states in high critical temperature Josephson junctions have been imaged. The experiments were performed with YBa(2)Cu(3)O(7-delta) thin-film grain boundary Josephson junctions fabricated on [001] tilt SrTiO(3) bicrystals. For applied magnetic fields parallel to the grain boundary plane, which correspond to local maxima of the magnetic field dependence of the critical current, the images clearly show the corresponding magnetic flux states in the grain boundary junction. The spatial modulation of the Josephson current density by the external magnetic field is imaged directly with a spatial resolution of about 1 micrometer.
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