Hubbard band versus oxygen vacancy states in the correlated electron metal SrVO3

Abstract

We study the effect of oxygen vacancies on the electronic structure of the model strongly correlated metal SrVO3. By means of angle-resolved photoemission (ARPES) synchrotron experiments, we investigate the systematic effect of the UV dose on the measured spectra. We observe the onset of a spurious dose-dependent prominent peak at an energy range were the lower Hubbard band has been previously reported in this compound, raising questions on its previous interpretation. By a careful analysis of the dose dependent effects we succeed in disentangling the contributions coming from the oxygen vacancy states and from the lower Hubbard band. We obtain the intrinsic ARPES spectrum for the zero-vacancy limit, where a clear signal of a lower Hubbard band remains. We support our study by means of state-of-the-art ab initio calculations that include correlation effects and the presence of oxygen vacancies. Our results underscore the relevance of potential spurious states affecting ARPES experiments in correlated metals, which are associated to the ubiquitous oxygen vacancies as extensively reported in the context of a two-dimensional electron gas (2DEG) at the surface of insulating d 0 transition metal oxides. A major challenge of modern physics is to understand the fascinating phenomena in strongly-correlated transition metal oxides (TMOs), which emerge in the neighborhood of the Mott insulator state. Some preeminent examples that have gathered the interest for almost 30 years are high temperature superconductivity, colossal magnetoresistance, heavy fermion physics and, of course, the Mott metal-insulator transition itself [1]. Significant theoretical progress was made with the introduction of Dynamical Mean Field Theory (DMFT) and its combination with ab initio Density Functional methods (LDA+DMFT), which allows treatment of the interactions promoting itinerancy and localization of electrons on equal footing [2][3][4]. Among the most emblematic achievements of DMFT is the prediction of a Hubbard satellite, which splits off of the conduction band of a metal. This satellite results from the partial localization of conduction electrons due to their mutual Coulomb repulsion. Early DMFT studies also showed that it is the precursor of the localized electronic states of a Mott insulator [5]. Since then, these predictions promoted a large number of studies using photoemission spectroscopy, which is a technique to directly probe the presence of Hubbard bands. In this context, the TMO system SrVO 3 has emerged as the drosophila model system to test the predictions of strongly correlated electron theories. In fact, SrVO 3 is arguably the simplest correlated metal. It is a simple cubic perovskite, with nominally one electron per V site, which occupies a 3 fold degenerate t 2g conduction band. While the presence of a satellite in the photoemission spectra of Ni metal was already well known, in the context of correlated TMOs, the Hubbard band was originally reported in a systematic investigation of Ca 1−x Sr x VO 3 [6], which was follo...

Recent Progress in First-Principles Methods for Computing the Electronic Structure of Correlated Materials

“…Within the LDA, the t 2g states form a narrow isolated conduction band occupied with one electron/unit cell, whereas the e g states are unoccupied (right panel of Figure 5). Experimentally, the material exhibits a renormalized quasiparticle peak in between a weak, broad satellite feature centered around −1.5 eV [79][80][81][82][83] and more pronounced upper satellite feature at around 3 eV [79]. The bottom of the t 2g -band is located around −0.7 eV [80,83] and the effective mass enhancement compared to the LDA is approximately 2 [84], which also agrees with estimations from the specific heat coefficient [85].…”

“…Experimentally, the spectral function of SrVO 3 is characterized by the presence of a quasiparticle peak around the Fermi level sandwiched by two satellite features, which have been interpreted as lower and upper Hubbard bands. LDA+DMFT calculations [83,[90][91][92] did indeed reproduce such features and so did the first few GW+EDMFT calculations [26,27,86]. The latter calculations, however, are rather similar to the former in that, instead of starting with the LDA band structure, one starts with a one-shot GW band structure, although they have the advantage of having a more accurate GW band structure and a more rigorous removal of the double-counting term.…”

“…Experimentally, the material exhibits a renormalized quasiparticle peak in between a weak, broad satellite feature centered around −1.5 eV [79][80][81][82][83] and more pronounced upper satellite feature at around 3 eV [79]. The bottom of the t 2g -band is located around −0.7 eV [80,83] and the effective mass enhancement compared to the LDA is approximately 2 [84], which also agrees with estimations from the specific heat coefficient [85]. Within the GWA, the bandwidth of the t 2g -bands is 2.1 eV, which corresponds to an effective mass enhancement of approximately 1.2 [38,86,87].…”

“…Although GW-based methods seem to be able to describe the satellites as plasmons traditionally the satellite features are explained as Hubbard bands since they appear in LDA+DMFT calculations with a static U (See e.g., [83,[90][91][92]), which do not allow for long-range charge fluctuations responsible for plasmon excitations. While LDA+DMFT provides a better description of the lower satellite, the position of the upper satellite is too close to the Fermi energy, even when the effective interaction for the impurity problem is adjusted to give the correct band renormalization.…”

Influence of oxygen vacancies on two-dimensional electron systems at SrTiO3-based interfaces and surfaces

“…6a and c). While in the case of defect free SrVO 3 , a t 2g quasiparticle peak at the Fermi level and a lower Hubbard band (LHB) at negative energies of the same t 2g nature is found in agreement with the ARPES result [34], the oxygen vacancies produce a dispersionless in-gap state at ∼ −1 eV which is of e g character as in SrTiO 3 . This is in very good agreement with the oxygen vacancy states observed in ARPES which are created by UV or X-ray irradiation.…”