Evolution and the concomitant disappearance of high-T_c superconductivity with carrier concentration in the La_2-xSr_xCuO_4-δ system (0.0≤x≤1.2): crossover from a mott insulator to a band metal

Abstract

Electrical resistivity, magnetic susceptibility, thermogravimetric analysis, and infrared absorption spectra of the compound La_2-xSr_xCuO_4-δ have been studied for a wide range of Sr concentrations (0.0≤x≤1.2). The samples annealed at an oxygen pressure of 1 bar were stoichiometric (δ=0.0) in the range 0.0<x<0.33. In this range the compounds are characterized by a decrease in the a parameter, an increase in the c parameter, and a maximum in the c/a ratio (at x=0.33) typical of the formation of low-spin Cu³⁺ ions. In the range 0.15<x<0.33, the compounds show a positive temperature coefficient of resistivity, decrease in the magnitude of the Pauli magnetic susceptibility, infrared oscillator strengths, thermopower S, as well as the Hall coefficient R_H. The superconducting transition temperature T_c as well as the percentage Meissner fraction also decrease with x in this range. In particular the stoichiometric x=0.33 composition having a hole concentration of ~0.33 holes/Cu shows a minimum in the Pauli magnetic susceptibility and disappearance of all infrared absorption bands. The superconductivity also disappears down to 4.2 K at this composition, even though it is more metallic. These results have been attributed to the occurrence of a transition from a highly correlated narrow-band "Mott conductor" to a broadband metal at high carrier concentrations. At still higher Sr concentrations (0.33<x<1.0), holes and oxygen vacancies coexist. The decrease in the c/a ratio, increase in resistivity, reappearance of the infrared bands, and the Curie-type magnetic susceptibility observed in the range 0.66≤x≤1.2 indicate the dominating role of oxygen vacancies which induce disorder and localization.