Seminars Archive

Colossal Magnetoresistive Manganites and High Temperature Superconductors: so different, …. yet so similar.

Abstract
In this talk, I will discuss the results of some recent angle-resolved photoemission spectroscopy (ARPES) investigations in the prototypical colossal magnetoresistive (CMR) bilayer compound La1.2Sr1.8Mn2O7 (LSMO, x = 0.4) [1,2]. Our results allowed elucidating the controversial nature of the ferromagnetic metallic (FM) groundstate in LSMO by showing that the FM phase is a polaronic metal, albeit with a strong anisotropic band structure. Its electronic structure has been found to be strikingly similar to that of the pseudogap phase in heavily underdoped cuprates high temperature superconductors [3]. In particular, the distribution of spectral weight in momentum space exhibits a nodal–antinodal dichotomous character. The spectra along the parallel sections of the Fermi surface (FS) (antinodal) exhibit a pseudogap. On the other hand, quasiparticle excitations (QP) have been detected for the first time along the nodal direction (i.e. diagonal), and they are found to determine the metallic transport properties in the FM phase. This dichotomy between the electronic excitations along the nodal and antinodal directions in momentum space was so far considered a characteristic unique feature of the copper oxide high-temperature superconductors (HTSC). These findings therefore cast doubt on the assumption that the pseudogap state and the nodal-antinodal dichotomy in the copper oxides HTSC are hallmarks of the superconductivity state. Furthermore, we found that the temperature dependent evolution of the nodal QP in LSMO tracks remarkably well the DC conductivity, thus accounting for the macroscopic transport properties and the metal to insulator transition [2]. Our results indicate that the microscopic mechanism leading to the MIT and the CMR effect in manganites is intrinsically a quantum effect linked to a crossover via the nodal QP collapse from a coherent polaronic conductor in the FM state below TC to a hopping regime with thermally activated single polarons in the paramagnetic state above TC. The role of the exchange interaction is crucial in controlling the competition between coherence and localization effects.[1] N. Mannella et al., Nature 438, 474 (2005) [2] N. Mannella et al., Phys. Rev. B 76, 233102 (2007) [3] K. M Shen et al., Science 307, 901 (2005).