A local metallic state in globally insulating La1.24Sr1.76Mn2O7 well above the metal–insulator transition

Abstract

The distinction between metals, semiconductors and insulators depends on the behaviour of the electrons nearest the Fermi level E F , which separates the occupied from the unoccupied electron energy levels. For a metal, E F lies in the middle of a band of electronic states, whereas E F in insulators and semiconductors lies in the gap between states. The temperatureinduced transition from a metallic to an insulating state in a solid is generally connected to a vanishing of the low-energy electronic excitations 1 . Here we show the first direct evidence of a counter-example, in which a significant electronic density of states at the Fermi energy exists in the insulating regime. In particular, angle-resolved photoemission data from the colossal magnetoresistive oxide La 1.24 Sr 1.76 Mn 2 O 7 show clear Fermi-edge steps, both below the metal-insulator transition temperature T C , when the sample is globally metallic, and above T C , when it is globally insulating. Further, small amounts of metallic spectral weight survive up to temperatures more than twice T C . Such behaviour may also have close ties to a variety of exotic phenomena in correlated electron systems, including the pseudogap temperature in underdoped cuprates 2 .As shown in Fig. 1a, the colossal magnetoresistive (CMR) oxide La 2−2x Sr 1+2x Mn 2 O 7 (x = 0.36,0.38) shows a metal-insulator transition at a T C just below 130 K, at which point the system also switches from being a ferromagnet (low temperature T) to a paramagnet (high T) 3 . We carried out angle-resolved photoemission spectroscopy (ARPES) experiments on cleaved single crystals of these materials, with an experimental arrangement as described elsewhere 4 . ARPES is an ideal experimental probe of the electronic structure because it gives the momentum-resolved single-particle excitation spectrum. As discussed in ref. 4, the x = 0.36, 0.38 compounds studied here do not contain the lowenergy pseudogap of the x = 0.4 samples 5-8 (see the Methods section for more details on this, the possible issue of surface sensitivity of ARPES and of possible intergrowths at the surface). The much larger metallic spectral weight of these nonpseudogapped compounds also allows us to study the electronic behaviour in greater detail. Although we find only minimal differences between the x = 0.36 and x = 0.38 compounds, all ARPES spectra shown here are from x = 0.38 samples. Figure 1b shows a large-energy-scale experimental picture of a low-temperature d x 2 −y 2 symmetry band taken along the blue cut near the zone boundary, as shown in the inset. We are able to get clean data by isolating the various bilayer-split bands using different photon energies, as described in ref. 4. In particular, in this paper we show only data from the antibonding bilayer-split band, which has Fermi crossings at k x = ±0.17π/a, k y = 0.9π/a, corresponding to the solid Fermi surface of the inset of Fig. 1b. The energy-distribution curves (EDCs) at the Fermi wavevector k F (indicated by the red line in Fig. 1b) taken at a series of temper...

Observation of anomalous temperature dependence of spectrum on small Fermi surfaces in aBiS2-based superconductor

“…Loss of spectral weight in a large energy scale has been observed in a polaronic state of the colossal magnetoresistant manganites La 2−2x Sr 1+2x Mn 2 O 7 [32][33][34][35] . In the polaronic state induced by strong electron-phonon coupling, the spectral function consists of a low-energy "zero-phonon" quasiparticle peak and a hump-like high-energy incoherent resonance.…”

“…In the manganites, the incoherent branch loses its partial spectral weight over an energy range of up to 0.8 eV, which is accompanied by a disappearance of the quasiparticle peaks around the metal-insulator transition temperature. This could be associated with either loss of polaron coherence 34 or a decreased fraction of metallic regions in the scenario of phase separation 33 . Assuming that the polaronic picture could be applied to the BiS 2 system, the spectral weight of the incoherent branch, i.e.…”

Correlation between phase competition and the nucleation of a Griffiths-like phase in (La1-yPry)0.7Ca0.3Mn16/18O3

“…The immediate implication is that y c = 0.85 represents the compositional critical point below which a GP is nucleated in this system (comparable results occur with 18 O substitution, Figs 1 a2-e2, a point returned to below). The idea of a critical ratio in a phase-separated view of the manganites has been discussed recently [17], focusing principally on the thermal, not compositional, variation of this ratio. By contrast, and of immediate relevance to the present conclusion, is that analysis of previous neutron diffraction experiments [14] concluded that at this same composition the role of quenched disorder was pivotal in establishing a long-ranged phase separated state.…”

Systematic Analysis of ARPES Spectra of Transition-Metal Oxides: Nature of EffectivedBand

“…In order to address the issue of a proper description of the periodic systems with strong correlation, the experimental determination of band dispersions by angleresolved photoemission spectroscopy (ARPES) has been highly required, but such data have been largely limited to low dimensional compounds [9,10,11] of limited chemical variety. For three-dimensional perovskite oxides, which have a wide variety of chemical compositions, even when bulk single crystals are available, they do not have cleavage planes required for ARPES measurements.…”