Non-Fermi-Liquid Behavior ofSrRuO3: Evidence from Infrared Conductivity

Abstract

The reflectivity of the itinerant ferromagnet SrRuO3 has been measured between 50 and 25,000 cm −1 at temperatures ranging from 40 to 300 K, and used to obtain conductivity, scattering rate, and effective mass as a function of frequency and temperature. We find that at low temperatures the conductivity falls unusually slowly as a function of frequency (proportional to 1/ω 1/2 ), and at high temperatures it even appears to increase as a function of frequency in the far-infrared limit. The data suggest that the charge dynamics of SrRuO3 are substantially different from those of Fermi-liquid metals.The occurrence of novel phenomena and new physics in correlated-electron systems is a recurring theme in condensed matter physics. d-electron based systems, in particular, present the combined intrigue of a range of dramatic phenomena, including superconductivity and itinerant ferromagnetism, and a tendency towards inscrutability, associated with the fact that key electronic states are often intermediate between the ideals of localization and itinerancy, which provide the starting points for most theory.Ruthenates constitute a class of transition-metal oxides occuring in both layered 1,2 and nearly cubic structures. Both electronic conduction and magnetic properties are associated with bands involving Ru-4d orbitals hybridized with O-2p levels, which manifest a range of phenomena including superconductivity and magnetism. For SrRuO 3 , which exhibits a transition from a paramagnetic to a ferromagnetic state at T c ≈ 150 K, bandstructure calculations are able to reproduce basic aspects of the magnetic behavior 3-6 . Specific heat and photoemission studies 4,7 , however, produce results for density of states and bandwidth that differ from band theory in manner that indicates the importance of electron correlation effects. In addition, transport studies 4,[8][9][10]

Three-dimensional flux states as a model for the pseudogap phase of transition metal oxides

“…Within our model, the location of the peak is proportional to the exchange energy J. Assuming this takes the value 0.1 eV, we find that the peak occurs at approximately 1000 cm −1 , which gives order of magnitude agreement with the observed value of 250 cm −1 in SrRuO 3 [1]. Due to the simplicity of the model we are solving, one would not expect more accurate agreement.…”

“…In this pseudogapped regime, ρ (T ) increases linearly with temperature, passing through the Ioffe-Regel limit without saturation [3], behavior indicative of a "bad metal" [4]. The optical conductivity in this state is proportional to the non-Fermi liquid behavior of ω −1/2 at high frequency and has a peak at low frequencies [1] at approximately 250 cm −1 , the precise location of the peak being temperature dependent.We propose that this pseudogap state can be understood by considering a ground state with spontaneously generated electronic currents circulating around the plaquettes. The currents arise from electron-electron correlations, due to the bi-quadratic terms in the Hamiltonian.…”

“…The motivation for this work is the observation of a pseudogap that opens up in optical conductivity measurements of the three-dimensional transition metal oxide SrRuO 3 [1] above its ferromagnetic transition temperature of T C ≈ 150 K. A pseudogap has also recently been seen in BaRuO 3 [2]. In this pseudogapped regime, ρ (T ) increases linearly with temperature, passing through the Ioffe-Regel limit without saturation [3], behavior indicative of a "bad metal" [4].…”

Electronic properties of bulk and thin filmSrRuO3: Search for the metal-insulator transition

“…depending on the details of the exchange and correlation functional and the treatment of the core and valence electrons. [22][23][24][25][26][27][28][29][30][31][32][33][34] The reduced calculated magnetic moment in the solid compared to that in the free ion limit is due in part to the large spatial extent of the Ru 4d orbitals, which results in a significant overlap ͑hy-bridization͒ with the oxygen 2p. Furthermore, due to the metallic character of SRO, an overlap of the majority and minority Ru 4d bands occurs at the Fermi level; as a result partial occupation of the minority band also leads to a reduced magnetic moment.…”

“…Due to the large spatial extent of the 4d orbitals in the ruthenates, correlation effects are anticipated to be less important as stronger hybridization provides more effective screening and a reduced Hubbard U ͑Cou-lomb repulsion energy͒. Many experimental studies have already addressed the degree of electron-electron correlation in SrRuO 3 including x-ray and ultraviolet photoemission spectroscopy, [18][19][20][21] specific-heat measurements, 22 infrared and optical conductivity measurements, 23 and transport experiments. 24 For example, Kim and co-workers 19 use x-ray photoemission spectroscopy ͑XPS͒ to identify how such correlations change within the ruthenate family, and Toyota et al 18 use photoemission spectroscopy ͑PES͒ to detail the metal-insulator transition in SrRuO 3 as a function of film thickness concomitant with the onset of magnetism.…”