Fingerprints of spin-orbital polarons and of their disorder in the photoemission spectra of doped Mott insulators with orbital degeneracy

Abstract

We explore the effects of disordered charged defects on the electronic excitations observed in the photoemission spectra of doped transition metal oxides in the Mott insulating regime by the example of the R1−xCaxVO3 perovskites, where R = La, . . . , Lu. A fundamental characteristic of these vanadium d 2 compounds with partly filled t2g valence orbitals is the persistence of spin and orbital order up to high doping, in contrast to the loss of magnetic order in high-Tc cuprates at low defect concentration. We demonstrate that the disordered electronic structure of doped Mott-Hubbard insulators can be obtained with high precision within the unrestricted Hartree-Fock approximation. In particular: (i) the atomic multiplet excitations in the inverse photoemission spectra and the various defect-related states and satellites are well reproduced, (ii) a robust Mott gap survives up to large doping, and (iii) we show that the defect states inside the Mott gap develop a soft gap at the Fermi energy. The soft defect states gap, that separates the highest occupied from the lowest unoccupied states, can be characterized by a shape and a scale parameter extracted from a Weibull statistical sampling of the density of states near the chemical potential. These parameters provide a criterion and a comprehensive schematization for the insulator-metal transition in disordered systems. We demonstrate that charge defects trigger small spinorbital polarons, with their internal kinetic energy responsible for the opening of the soft defect states gap. This kinetic gap survives disorder fluctuations of defects and is amplified by the long-range e-e interactions, whereas in the atomic limit we observe a Coulomb singularity. The small size of spin-orbital polarons is inferred by an analysis of the inverse participation ratio which explains the origin of the robustness of spin and orbital order. Using realistic parameters for the perovskite vanadate system La1−xCaxVO3, we show that its soft gap is well reproduced as well as the marginal doping dependence of the position of the chemical potential relative to the center of the lower Hubbard band. The present theory uncovers also the reasons why the d 1 → d 0 satellite excitations, which directly probe the effect of the random defect fields on the polaron state, are not well resolved in the available experimental photoemission spectra for La1−xCaxVO3.

Defect-Induced Orbital Polarization and Collapse of Orbital Order in Doped Vanadium Perovskites

“…For D = 0, the order parameter m o a+b decreases almost linearly with x. This case has been studied in a polaron theory using a small t expansion [31] where m o a+b 1 − x(1 + 2δ c ). The 1 − x describes the dilution of electrons in a or b orbitals upon doping.…”

“…A convenient measure of the degree of localization of a UHF wave function ψ n,s (r) is the participation number (PN) P n,s , which is 1 for a state localized on a single site and N for a Bloch state. Usually, one considers the inverse participation number (IPN) which takes the form [31], P −1 n,s = i ( ασ | ψ n,s |iασ | 2 ) 2 ∈ [0, 1] for systems with spin-orbital degeneracy. P −1 n,s is plotted in Fig.…”

“…There are three different types of d 2 → d 3 transitions that fall into this energy window. Namely, either one of the two low spin (LS) transitions in the host or the high spin (HS) transition on a defect cube, where excitation energies are increased by V D , i.e., ω HS = U − 3J H + V D + ω LHB ≈ 3.0 eV, where the position of the LHB is given by ω LHB = E LHB − µ ≈ −V D [31]. It is the ω HS transitions that are sensitive to the melting of OO.…”

Mott transition and collective charge pinning in electron doped Sr2IrO4

“…Since both electron-phonon coupling and Mott physics appear to be important in the iridates, and moreover these materials are doped by chemical substitution of donor atoms, it is of interest to consider the effects of doping a Mott insulating state by chemical substitution. In a recent theoretical discussion of R 1−x Ca x VO 3 the effects of disordered charged defects on the spectral function in the Mott insulating regime [39] were shown to trigger small spin-orbital polarons, with their internal kinetic energy responsible for the opening of the soft defect states gap inside the Mott gap. Breaking of translation invariance is a natural consequence of the disorder potential due to the randomly substituted La 3+ donor atoms.…”

Spin-orbital order in LaMnO3 : d−p model study

“…III B as a method of choice to describe the states with broken spin-orbital symmetries. It was shown recently that this approach gives very reliable results for doped vanadium perovskites [36]. The magnetization direction is there one of the open questions and we discuss possible states in Sec.…”