Oxygen Isotope Effects in Manganites: Evidence for (Bi)Polaronic Charge Carriers

Abstract

The isotope effects observed in conventional superconductors show that lattice vibrations play an essential role in bringing about superconductivity. In particular, one finds that the superconducting transition temperature Tc varies as Tc ∝ 1/M (α ≃ 0.5) when the isotopic mass M of the material is varied. If lattice vibrations were not important in this phenomenon there is no reason why Tc should change with the mass of the ions. Thus, the isotope effect provides crucial insight into the microscopic theory of superconductivity in conventional superconductors. The isotope effects observed in high-temperature superconductors (HTSC) should also place strong constraints on the microscopic pairing mechanism of high-temperature superconductivity. It has been shown that the isotope effects in HTSC are very unusual (see Ref. [1] [11]). The oxygen isotope shift of the superconducting transition temperature Tc is small in the optimally doped cuprate superconductors, but the shift is very large, and even much larger than the BCS prediction as the doping is reduced towards the deeply underdoped regime. The large isotope effects in the underdoped cuprate superconductors may suggest that lattice vibrations also play an important role in bringing about high-temperature superconductivity. However the isotope effects cannot be simply explained by the conventional theory of superconductivity. Zhao and coworkers have initiated studies of the oxygen isotope effects on both the penetration depth and the carrier concentration in some cuprate superconductors [6, 5, 4, 8]. The results indicate that the effective supercarrier mass depends strongly on the oxygen isotope mass in the underdoped region, while the carrier concentration is independent of the oxygen isotope mass [6, 4, 8]. These isotope-effect results might suggest that polaronic carriers exist and condense into supercarriers in HTSC, which may give experimental support to the theory of (bi)polaronic superconductivity in HTSC [12]. On the other hand, little attempt had been made to investigate the isotope effects