Non-gapped Fermi surfaces, quasiparticles and the anomalous temperature dependence of the near-$E_F$ electronic states in the CMR oxide La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ with $x=0.36$

TL;DR

This study uses temperature-dependent angle-resolved photoemission to investigate the electronic states of La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ with x=0.36, revealing no pseudogap below T_C and a loss of quasiparticle coherence above T_C.

Contribution

It provides new temperature-resolved ARPES data showing the absence of a pseudogap and challenges phase separation models in CMR manganites.

Findings

No pseudogap observed below T_C.

Quasiparticle coherence disappears above T_C.

Spectral changes are not consistent with phase separation.

Abstract

After years of research into colossal magnetoresistant (CMR) manganites using bulk techniques, there has been a recent upsurge in experiments directly probing the electronic states at or near the surface of the bilayer CMR materials La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ using angle-resolved photoemission or scanning probe microscopy. Here we report new, temperature dependent, angle resolved photoemission data from single crystals with a doping level of $x=0.36$. The first important result is that there is no sign of a pseudogap in the charge channel of this material for temperatures below the Curie temperature $T_C$. The second important result concerns the temperature dependence of the electronic states. The temperature dependent changes in the Fermi surface spectra both at the zone face and zone diagonal regions in $k$-space indicate that the coherent quasiparticle weight disappears for temperatures significantly above $T_C$, and that the $k$-dependence of the T-induced changes in the spectra invalidate an interpretation of these data in terms of the superposition of a `universal' metallic spectrum and an insulating spectrum whose relative weight changes with temperature. In this sense, our data are not compatible with a phase separation scenario.