Mechanism of cerium doping-induced formation and modification of the energy spectrum in the Nd$_{2-x}$Ce$_x$CuO$_y$ system

This paper reports on a study of the electrical resistivity and thermopower coefficient of samples of the Nd$_{2-x}$Ce$_x$CuO$_y$ system ($x$ = 0.1–0.2), as well as their analysis in conjunction with the literature data on electron transport in similar samples occurring under variation of cerium content within a broad range ($x$ = 0.025–0.220). The study revealed features in the temperature and concentration dependences of the thermopower coefficient characteristic of neodymium HTSC. The asymmetric narrow band model was shown to fit satisfactorily all experimentally obtained temperature dependences of the thermopower coefficient of neodymium-based HTSC for $x >$ 0.06, which supports the possibility of applying this model as a universal method in description and analysis of the specific features revealed in electron transport in HTSC of various systems. Parameters of the energy spectrum and the carrier system in the normal phase were determined, and it was found that their values measured in the Nd$_{2-x}$Ce$_x$CuO$_y$ system are, on the whole, close to those characteristic of other HTSC systems. The trends observed in the variation of these parameters with increasing cerium content were analyzed and shown to differ substantially in the cases of under- and over-doped regimes. An assumption was made on the cerium doping-induced formation in Nd$_2$CuO$_y$ of a new narrow band, which originates in the Mott–Hubbard gap as a result of state transport from the lower Hubbard subband. The “midgap” model was demonstrated to account for all the results obtained.