Observation of particle hole asymmetry and phonon ex- citations in non-Fermi liquid systems: A high-resolution photoemission study of ruthenates

Abstract

– We investigate the temperature evolution of the electronic states in the vicinity of the Fermi level of a non-Fermi liquid (NFL) system, CaRuO3 using ultra high-resolution photoemission spectroscopy; isostructural SrRuO3 exhibiting Fermi liquid behavior despite similar electron interaction parameters as that of CaRuO3, is used as a reference. High-energy resolution in this study helps to reveal particle-hole asymmetry in the excitation spectra of CaRuO3 in contrast to that in SrRuO3. In addition, we observe signature of phonon excitations in the photoemission spectra of CaRuO3 at finite temperatures while these are weak in SrRuO3. Recent observations of various bulk properties in normal phase of high temperature superconductors [1], d and f electrons at quantum critical points [2], low dimensional systems [3] etc., exhibit deviations from Fermi-liquid (FL) behavior, the most fundamental paradigm in solid-state physics [4]. Such non-Fermi liquid (NFL) behavior has often been attributed to strong electron correlations and/or charge fractionalizations [5]. Interestingly, CaRuO3, a 3dimensional orthorhombically distorted perovskite exhibits NFL behavior at low temperatures, while isostructural SrRuO3 exhibits Fermi liquid (FL) behavior [6]. Magnetic measurements reveal ferromagnetic ground state in SrRuO3 (Curie temperature, TC ∼ 165 K). However, CaRuO3 does not exhibit long range order. The average Ru-O-Ru bond angle is slightly different in these two compounds (150 in CaRuO3 and 165 ◦ in SrRuO3). It was believed that the decrease in the electron hopping interaction strength, t, due to smaller Ru-O-Ru angle in CaRuO3 leads to an increase in effective electron correlation strength, U/W (U = electron-electron Coulomb repulsion strength, t ∝ W ∼ bandwidth), and hence the NFL behavior appears in CaRuO3 [6]. However, a recent study based on band structure calculation and photoemission spectroscopy [7] shows that the width of various d-bands remains almost the same across the series and U/W is essentially (∗) Corresponding author: kbmaiti@tifr.res.in c © EDP Sciences 2 EUROPHYSICS LETTERS

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@inproceedings{Maiti2007ObservationOP, title={Observation of particle hole asymmetry and phonon ex- citations in non-Fermi liquid systems: A high-resolution photoemission study of ruthenates}, author={Kalobaran Maiti and Ravi S. Singh}, year={2007} }