When electrons experience Coulomb repulsion, their kinetic energy becomes significantly reduced. This effect has now been measured in the pnictide superconductor LaFePO, and shows that correlations between electrons in these materials are just as strong as in some copper oxide and ruthenate superconductors. In correlated metals derived from Mott insulators, the motion of an electron is impeded by Coulomb repulsion due to other electrons. This phenomenon causes a substantial reduction in the electron’s kinetic energy, leading to remarkable experimental manifestations in optical spectroscopy1. The high-transition-temperature (Tc) superconducting cuprates are perhaps the most studied examples of such correlated metals. The occurrence of high-Tc superconductivity in the iron pnictides2,3,4 puts a spotlight on the relevance of correlation effects in these materials5. Here, we present an infrared and optical study on single crystals of the iron pnictide superconductor LaFePO. We find clear evidence of electronic correlations in metallic LaFePO with the kinetic energy of the electrons reduced to half of that predicted by band theory of nearly free electrons. We deduce that electronic many-body effects are important in the iron pnictides despite the absence of a Mott transition.

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Electronic correlations in the iron pnictides

We investigated the transport and optical properties of $\mathrm{Sr}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}{\mathrm{O}}_{3}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}1)$ which shows a metal-insulator transition at $x$ about 0.5 at room temperature. While $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_{3}$ is a ferromagnetic metal with 4 ${t}_{2g}$ electrons, $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ is a band gap insulator with an optical gap of about $3.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Because the ${t}_{2g}$ states of Ru and Ti are well decoupled, the electronic structure of $\mathrm{Sr}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}{\mathrm{O}}_{3}$ near the Fermi level is dominated by Ru ${t}_{2g}$ states. The temperature dependent transport data of samples near the transition showed characteristics of various localization phenomena. The optical conductivity spectra showed that as $\mathrm{Sr}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}{\mathrm{O}}_{3}$ changes from a metal to an insulator, the Drude-like peak decreases and evolves into an incoherent peak, which shifts to higher energy gradually and disappears. This evolution of the optical conductivity could not be explained by either disorder or correlation mechanisms. From our transport and optical data, we could categorize six kinds of electronic states in this system depending on $x$: a correlated metal $(x\ensuremath{\sim}1.0)$, a disordered metal $(x\ensuremath{\sim}0.7)$, an Anderson insulator $(x\ensuremath{\sim}0.5)$, a soft Coulomb gap insulator $(x\ensuremath{\sim}0.4)$, a disordered correlation insulator $(x\ensuremath{\sim}0.2)$, and a band insulator $(x\ensuremath{\sim}0.0)$. To understand these electronic structure evolutions, the disorder and the electron correlation effects should be considered together. We believe that $\mathrm{Sr}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Ru}}_{x}{\mathrm{O}}_{3}$ is a prototype system experiencing a Mott-Hubbard like transition in the Ru ${t}_{2g}$ alloy band, which is derived from the combined effects of the disorder and the electron correlation.

Metal-insulator transition in a disordered and correlatedSrTi1−xRuxO3system: Changes in transport properties, optical spectra, and electronic structure

Local density approximation plus on-site Coulomb interaction $U$ band structure calculations reveal that ${\mathrm{SrRuO}}_{3}$ exhibits a half-metallic ground state with an integer spin moment of $2.0{\ensuremath{\mu}}_{B}/{\mathrm{SrRuO}}_{3}$. An associated tilting $4d{t}_{2g}$ orbital ordering on a Ru sublattice is observed under the on-site Coulomb interaction $U$ in the presence of lattice distortion. This finding unravels the on-site Coulomb correlation as the driving force of the $4d$ orbital ordering and Jahn-Teller distortion as well as of the half-metallic ground state.

Orbital Ordering and Jahn-Teller Distortion in Perovskite Ruthenate SrRuO~3

We investigate the electronic structure of $4d$ transition-metal oxides ${\mathrm{CaRuO}}_{3}$ and ${\mathrm{SrRuO}}_{3}$. The analysis of the photoemission spectra reveals significantly weak electron correlation strength $(U∕W\ensuremath{\sim}0.2)$ as expected in $4d$ systems and resolves the long-standing issue that arose due to the prediction of large $U∕W$ similar to $3d$ systems. It is shown that the bulk spectra, thermodynamic parameters, and optical properties in these systems can consistently be described using first-principles calculations. The observation of different surface and bulk electronic structures in these weakly correlated $4d$ systems is unusual.

Evidence against strong correlation in 4 d transition-metal oxides CaRuO 3 and SrRuO 3

We have measured soft x-ray photoemission and O $1s$ x-ray absorption spectra of ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ru}{\mathrm{O}}_{3}$ thin films prepared in situ. The coherent and incoherent parts have been identified in the bulk component of the photoemission spectra, and spectral weight transfer from the coherent to the incoherent part has been observed with decreasing $x$, namely, with increasing orthorhombic distortion. We propose that, while the Ru $4d$ one-electron bandwidth does not change with $x$, the distortion, and hence the splitting of the ${t}_{2g}$ band, effectively increases electron correlation strength. Although strong mass enhancement is found in the electronic specific heat data, the coherent part remains wide, suggesting enhanced band narrowing only in the vicinity of ${E}_{F}$.

https://arxiv.org/pdf/cond-mat/0412718.pdf

Manifestation of correlation effects in the photoemission spectra of Ca1-xSrxRuO3

We investigated optical properties of (Ca,Sr)RuO {sub 3} films on the borderline of a metal-insulator transition. Our results show all of the predicted characteristics for a metallic Mott-Hubbard system, including (i) a mass enhancement in dc limit, (ii) a U/2 excitation, and (iii) a U excitation. Also, self-consistency is exploited within the Gutzwiller-Brinkman-Rice picture for the Mott transition. Our finding displays that electron correlation should be important even in 4d materials. However, low frequency behaviors of electrodynamic quantities suggest extra scattering mechanisms in addition to the Mott-Hubbard correlation. {copyright} {ital 1999} {ital The American Physical Society }

https://arxiv.org/pdf/cond-mat/9807209.pdf

J. Bak

Papers

Spectral Evolution in ( C a , S r ) RuO 3 near the Mott-Hubbard Transition