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Research
Manganites
We have shown with angle-resolved photoemission spectroscopy (ARPES)
that the electronic structure in the ferromagnetic metallic (FM)
groundstate of the colossal magnetoresistive (CMR) bilayer manganite
La1.2Sr1.8Mn2O7 (LSMO)
shows striking similarities to that of the pseudogap phase in heavily
underdoped cuprates high temperature superconductors (HTSC). The
FM phase is a polaronic metal, albeit with a strong anisotropic distribution
of spectral weight in momentum space exhibiting a nodal–antinodal
dichotomy. 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. Since this nodal-antinodal dichotomy in momentum space was
so far considered a characteristically unique feature of the copper
oxide HTSC, these findings 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 in LSMO. Our results indicate
that the microscopic mechanism leading to the CMR effect in manganites
is intrinsically a quantum phase transition which is kinetic energy
driven and linked to a crossover from a small polaron hopping regime
in the paramagnetic state to a coherent polaronic conductor in the
FM state.
