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Register a new userElectron-polaron dichotomy of charge carriers in perovskite oxides
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Added by
Marius-Adrian Husanu PhD
Publication date
2020
fields
Physics
and research's language is
English
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Many transition metal oxides (TMOs) are Mott insulators due to strong Coulomb repulsion between electrons, and exhibit metal-insulator transitions (MITs) whose mechanisms are not always fully understood. Unlike most TMOs, minute doping in CaMnO3 induces a metallic state without any structural transformations. This material is thus an ideal platform to explore band formation through the MIT. Here, we use angle-resolved photoemission spectroscopy to visualize how electrons delocalize and couple to phonons in CaMnO3. We show the development of a Fermi surface where mobile electrons coexist with heavier carriers, strongly coupled polarons. The latter originate from a boost of the electron-phonon interaction (EPI). This finding brings to light the role that the EPI can play in MITs even caused by purely electronic mechanisms. Our discovery of the EPI-induced dichotomy of the charge carriers explains the transport response of Ce-doped CaMnO3 and suggests strategies to engineer quantum matter from TMOs.
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With their broad range of magnetic, electronic and structural properties, transition metal perovskite oxides ABO3 have long served as a platform for testing condensed matter theories. In particular, their insulating character - found in most compounds - is often ascribed to dynamical electronic correlations through the celebrated Mott-Hubbard mechanism where gaping arises from a uniform, symmetry-preserving electron repulsion mechanism. However, structural distortions are ubiquitous in perovskites and their relevance with respect to dynamical correlations in producing this rich array of properties remains an open question. Here, we address the origin of band gap opening in the whole family of 3d perovskite oxides. We show that a single-determinant mean-field approach such as density functional theory (DFT) successfully describes the structural, magnetic and electronic properties of the whole series, at low and high temperatures. We find that insulation occurs via energy-lowering crystal symmetry reduction (octahedral rotations, Jahn-Teller and bond disproportionation effects), as well as intrinsic electronic instabilities, all lifting orbital degeneracies. Our work therefore suggests that whereas ABO3 oxides may be complicated, they are not necessarily strongly correlated. It also opens the way towards systematic investigations of doping and defect physics in perovskites, essential for the full realization of oxide-based electronics.
Spin reorientation and magnetisation reversal are two important features of the rare-earth orthorhombic provskites ($RM$O$_{3}$s) that have attracted a lot of attention, though their exact microscopic origin has eluded researchers. Here, using density functional theory and classical atomistic spin dynamics we build a general Heisenberg magnetic model that allows to explore the whole phase diagram of the chromite and ferrite compounds and to scrutinize the microscopic mechanism responsible for spin reorientations and magnetisation reversals. We show that the occurrence of a magnetization reversal transition depends on the relative strength and sign of two interactions between rare-earth and transition-metal atoms: superexchange and Dzyaloshinsky-Moriya. We also conclude that the presence of a smooth spin reorientation transition between the so-called $Gamma_4$ and the $Gamma_2$ phases through a coexisting region, and the temperature range in which it occurs, depends on subtle balance of metal--metal (superexchange and Dzyaloshinsky-Moriya) and metal--rare-earth (Dzyaloshinsky-Moriya) couplings. In particular, we show that the intermediate coexistence region occurs because the spin sublattices rotate at different rates.
At the interface between complex insulating oxides, novel phases with interesting properties may occur, such as the metallic state reported in the LaAlO3/SrTiO3 system. While this state has been predicted and reported to be confined at the interface, some works indicate a much broader spatial extension, thereby questioning its origin. Here we provide for the first time a direct determination of the carrier density profile of this system through resistance profile mappings collected in cross-section LaAlO3/SrTiO3 samples with a conducting-tip atomic force microscope (CT-AFM). We find that, depending upon specific growth protocols, the spatial extension of the high-mobility electron gas can be varied from hundreds of microns into SrTiO3 to a few nanometers next to the LaAlO3/SrTiO3 interface. Our results emphasize the potential of CT-AFM as a novel tool to characterize complex oxide interfaces and provide us with a definitive and conclusive way to reconcile the body of experimental data in this system.
We apply ultrafast X-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO$_3$ after above-bandgap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO$_3$: the relevant excited charge carriers must remain localized to be consistent with the data.
Rare-earth nickelates R$^{3+}$Ni$^{3+}$O$_3$ (R=Lu-Pr, Y) show a striking metal-insulator transition in their bulk phase whose temperature can be tuned by the rare-earth radius. These compounds are also the parent phases of the newly identified infinite layer RNiO2 superconductors. Although intensive theoretical works have been devoted to understand the origin of the metal-insulator transition in the bulk, there have only been a few studies on the role of hole and electron doping by rare-earth substitutions in RNiO$_3$ materials. Using first-principles calculations based on density functional theory (DFT) we study the effect of hole and electron doping in a prototypical nickelate SmNiO3. We perform calculations without Hubbard-like U potential on Ni 3d levels but with a meta-GGA better amending self-interaction errors. We find that at low doping, polarons form with intermediate localized states in the band gap resulting in a semiconducting behavior. At larger doping, the intermediate states spread more and more in the band gap until they merge either with the valence (hole doping) or the conduction (electron doping) band, ultimately resulting in a metallic state at 25% of R cation substitution. These results are reminiscent of experimental data available in the literature and demonstrate that DFT simulations without any empirical parameter are qualified for studying doping effects in correlated oxides and to explore the mechanisms underlying the superconducting phase of rare-earth nickelates.
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