Origin of the heavy fermion behavior in Ca2−xSrxRuO4

Abstract

We study the electronic states for Ca2−xSrxRuO4 in 0.5 ≤ x ≤ 2 within the Gutzwiller approximation (GA) on the basis of the three-orbital Hubbard model for the Ru t2g orbitals. The main effects of the Ca-substitution are taken into account as the changes of the dp hybridizations between the Ru 4d and O 2p orbitals. Using the numerical minimization of the energy obtained in the GA, we obtain the renormalization factor (RF) of the kinetic energy and total RF, which estimates the inverse of the mass enhancement, for three cases with the effective models of x = 2 and 0.5 and a special model. We find that the inverse of the total RF becomes the largest for the case of x = 0.5, and that the van Hove singularity, which is located on (below) the Fermi level for the special model (the effective model of x = 0.5), plays a secondary role in enhancing the effective mass. Our calculation suggests that the heavy fermion behavior around x = 0.5 comes from the cooperative effects between moderately strong Coulomb interaction compared to the total bandwidth and the modification of the electronic structures due to the rotation of RuO6 octahedra (i.e., the variation of the dpπ hybridizations and the downward shift for the dxy orbital). We propose that moderately strong electron correlation and the orbital-dependent modifications of the electronic structures due to the lattice distortions play important roles in the electronic states for Ca2−xSrxRuO4.

Mott transition, spin-orbit effects, and magnetism in Ca2RuO4

“…Ca 2 RuO 4 itself exhibits a peculiar paramagnetic metal-insulator transition [6] (MIT) at T MIT = 360 K, basically concurrent with the change from L-Pbca (long c axis) to S-Pbca (short c axis) structure [8,9] at T S = 356 K. Similar transitions have been reported when Ca is partially replaced by Sr (x 0.2) or under pressure [10,11]. The origin of the MIT has been intensively investigated, both experimentally [5][6][7][8][9][10][11][12][13][14][15] and theoretically [16][17][18][19][20][21][22][23][24][25][26]. Electronically, Ca 2 RuO 4 is characterized by 2/3-filled t 2g bands (t 4 2g e 0 g electronic configuration).…”

Competition between spin fluctuations in Ca2−xSrxRuO4Arakawa1, Ogata2 2013Phys. Rev. BSelf Cite6040View full textAdd to dashboardCite

“…8. In addition to the change of the hopping integrals due to the rotation of RuO 6 octahedra, we have taken account of the effect of the rotation-induced hybridization of the d xy orbital to the d x 2 −y 2 orbital as the difference of the crystalline-electric-field (CEF) energies between the d xz/yz and d xy orbitals, t 2g .…”

“…We proposed that this HF behavior can be qualitatively understood as the cooperative effect between moderately strong electron correlation and the orbital-dependent modification of the electronic structures due to the rotation of RuO 6 octahedra. 8 In that study, however, we considered only the local correlations. It is thus needed to study the effects of the nonlocal correlation on the electronic structures since both local and nonlocal correlations play important roles in discussing the electronic structure for a strongly correlated electron system in general.…”

“…8 In particular, we analyze these static susceptibilities for the models of x = 2 and 0.5, and a special model to clarify effects of the rotation of RuO 6 octahedra, the van Hove singularity (vHs) for the d xy orbital, and the Hund's rule coupling. In this special model, the vHs for the d xy orbital is located on the Fermi level.…”

Microscopic theory on charge transports of a correlated multiorbital system

“…Thus, those experimental results indicate the importance of many-body effects beyond Landau's FL theory near a magnetic QCP. Note 22 , which drastically affect the electronic structure 23,24 .…”