Polaron freezing and the quantum liquid-crystal phase in the ferromagnetic metallic La_0.67Ca_0.33MnO₃

The remarkable electronic properties of colossal magnetoresistive manganites are widely believed to be caused by the competition between a ferromagnetic metallic state and an antiferromagnetic insulating state with complex spin, charge, and orbital ordering. However, the physics underlying their magnetotransport properties is still not clear, especially the role of correlated Jahn-Teller polarons, which depending on temperature and doping, might form a liquid, glass or stripe polaron state. This question touches one of the most fundamental problems in the physics of doped Mott insulators, i.e. understanding the mechanism that chemical doping makes an insulator becoming superconductive as in the case of cuprates, or exhibiting the colossal magnetoresistance effect, as in the case of manganites. Here, by using ¹³⁹La NMR and high resolution transmission electron microscopy in the temperature range 3.2-1000 K, we have monitored the formation and evolution of CE-type polarons in optimally doped La_0.67Ca_0.33MnO₃. While NMR experiments show that correlated polarons dominate electron spin dynamics in the ferromagnetic phase, at very low temperatures they appear to form a quantum liquid-crystal like ferromagnetic phase, embedded into a ferromagnetic matrix with 3D polaron correlations. This is evidence that similarly to high T _c cuprates, quantum soft phases underlie the exotic physical properties of colossal magnetoresistive manganites.