We report on our recent electron-tunneling studies of bulk manganite samples that provide important information about the structure of the near-surface layers of the material and the nature of the charge transport across them. It is shown that the even part of the differential conductance of contacts formed by a metallic injector with the surface of a manganite is a power function of the voltage bias. High voltages applied to the sample are found to locally modify the conductance of the degraded native surface layer. Experiments aimed to monitor the force applied to a metal tip pressed into the surface of a manganite prove the presence of sub-surface layers with properties significantly different from those near the surface. Experimental data are analyzed and interpreted within the Glazman–Matveev theory taking into account inelastic tunneling through two metallic "drops" inside the insulating barrier.
ABSTRACT:Within the framework of the surface Green's function matching method, we carried out calculations of the differential conductance of symmetrically and asymmetrically connected armchair (n, n)-zigzag (2n, 0)-armchair (n, n) carbon nanotubes with different lengths of the middle section and infinitely long ends. It is shown that the (n, n)/(2n, 0)/(n, n) segment, when n is not a multiple of 3, behaves as a quantum dot and has a conduction gap even for short middle segments. The position of the conductance peeks closest to the Fermi energy is determined by the interface states of the (n, n)/(2n, 0) junction. In addition to conductance peaks originating from the interface states of the (n, n)/(2n, 0) heterojunction, for sufficiently long middle zigzag nanotube, there are more conductance peaks in the vicinity of the Fermi energy, and those stem from the electronic structure of an individual finite (2n, 0) zigzag nanotube. However, positions of the peaks farther away from the Fermi energy cannot be found in such a simple way. Thus, such (n, n)/(2n, 0)/(n, n) quantum dot has singularities in its electronic properties, which may yield the Coulomb blockade effect should those strongly localized discrete levels become occupied.
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