Bilayer manganites reveal polarons in the midst of a metallic breakdown

Abstract

The exact nature of the low temperature electronic phase of the manganite materials family, and hence the origin of their colossal magnetoresistant (CMR) effect, is still under heavy debate. By combining new photoemission and tunneling data, we show that in La 2-2x Sr 1+2x Mn 2 O 7 the polaronic degrees of freedom win out across the CMR region of the phase diagram. This means that the generic ground state is that of a system in which strong electron-lattice interactions result in vanishing coherent quasi-particle spectral weight at the Fermi level for all locations in k-space. The incoherence of the charge carriers offers a unifying explanation for the anomalous charge-carrier dynamics seen in transport, optics and electron spectroscopic data. The stacking number N is the key factor for true metallic behavior, as an intergrowth-driven breakdown of the polaronic domination to give a metal possessing a traditional Fermi surface is seen in the bilayer system.Competition between local lattice distortions leading to anti-ferromagnetic, charge and orbital (CO) ordering on the one hand, and mixed valence character promoting metallic ferromagnetic double exchange on the other, determines the transport transport properties LSMO [5], whereas the bilayer analogue is metallic only in a narrow Sr-doping and temperature regime [6], giving rise to the largest CMR effect [7]. The more strongly 2D, single layer compound shows neither metallic nor CMR behavior [8].Focusing on bilayer LSMO within the CMR-region of * These authors contributed equally to this work the phase diagram, the prevailing picture from structural studies is one of polarons existing above T C . On cooling towards T C , these short range versions of the CO order typical of the insulating compositions, become increasingly correlated [9, 10]. Eventually double exchange, leading to an itinerant, metallic state, takes over.This metallic state for x = 0.4 has been shown to support small quasi-particles (QPs) in the spectral function measured by angle-resolved photoemission (ARPES) [11]. These signal coherent electronic excitations, albeit strongly dressed with lattice distortions, and are seen as evidence for a novel and elusive state of matter known as a polaronic metal [11, 12].Other ARPES studies paint a different picture, with stronger QP features observed at low T that persist up to temperatures of order 1.5T C [13][14][15], despite the system being nominally insulating. In contrast, scanning tunneling microscopy / spectroscopy (STM/S) studies reported gaps in the local density of states near-E F for x = 0.30 [16] and 0.325 [17], both in the metallic and insulating temperature regimes. Finally, new neutron diffraction data for x = 0.4 has shown that even far below T C -at 10 K in the metallic state -polarons remain as fluctuations that strongly broaden and soften phonons near the wave vectors where the charge order peaks would appear in the insulating phase [18].Here, a combination of ARPES and STM/S reveals that the bilayer manganites still have a number of...

Polaron framework to account for transport properties in metallic epitaxial manganite films

“…Nevertheless, this basic picture appears to be conceptually insufficient to describe the physics of manganites, in general, and LSMO, in particular [5]: the strong role of polaron effects in manganites has been pointed out [6], and good experimental evidence has been provided by a variety of methods [6][7][8][9][10][11][12][13]. Moreover, manganites were proposed as an example of a polaron Fermi liquid [7].Although the presence of polarons in the metallic phase of LSMO (down to 6 K) has been demonstrated by optical * patrizio.graziosi@gmail.com conductivity and reflectivity [8,9] and a polaron metal phase has been indicated in metallic La 2−2x Sr 1+2x Mn 2 O 7 by angleresolved photoemission spectroscopy [14] and neutron scattering [15], explicit evidence of the polaron effects from transport characterizations was, up to now, still missing. Indeed, despite these convincing proves, the dominating trend in literature is to describe the resistivity of manganites via a T 2 + T 5 behavior, ignoring, thus, the polaron effects (see, for instance, a very recent communication in Ref.…”

“…Although the presence of polarons in the metallic phase of LSMO (down to 6 K) has been demonstrated by optical * patrizio.graziosi@gmail.com conductivity and reflectivity [8,9] and a polaron metal phase has been indicated in metallic La 2−2x Sr 1+2x Mn 2 O 7 by angleresolved photoemission spectroscopy [14] and neutron scattering [15], explicit evidence of the polaron effects from transport characterizations was, up to now, still missing. Indeed, despite these convincing proves, the dominating trend in literature is to describe the resistivity of manganites via a T 2 + T 5 behavior, ignoring, thus, the polaron effects (see, for instance, a very recent communication in Ref.…”

Speed limit of the insulator–metal transition in magnetite

“…3 In many oxide materials [13,14] the insulator-metal transition has been discussed in terms of freezing in fluctuating lattice and electronic (charge, spin and orbital) order, sometimes in combination with the occurrence of phase separation [15,16]. However, due to the many competing degrees of freedom, the close energetic proximity of the different phases often obscures the exact nature of these phase transitions as they are probed in thermal equilibrium [17]. In magnetite, the Verwey transition does not involve changes in the spin ordering, allowing a focus purely on the role of the lattice and electronic ordering.…”

On the Origin of CE-Type Orbital Fluctuations in the Ferromagnetic Metallic Phase of La2−2xSr1+2xMn2O7 near x = 0.4

“…16,17 On the other hand, quasi-particle peaks observed in several experiments subsequently [19][20][21] has been attributed to the intergrowth present in the experimental samples by a recent scanning tunneling microscopy (STM) combined with ARPES, thus reaffirming the pseudogap structure associated with the Fermi surface. 22 The persistent CE-type dynamical short-range charge-orbital correlations in the ferromagnetic metallic phase at x = 0.4, apart from being responsible for the pseudo-gap structure in the Fermi surface, yields a strong renormalization of phonons near the wavevector (0.5π, 0.5π, 0) observed in the inelastic neutron scattering, 4 which is believed to be resulting from the strong Fermi surface nesting in the ferromangetic metallic state. As the hole pocket corresponding to the bonding portion of the Fermi surface has nearly straight segments in comparison to the antibonding portions, bonding-bonding nesting has been suggested 16,23 to be responsible for the nano-scale charge-orbital correlations in the ferromagnetic metallic phase.…”