The Iso-electronic Series $Ca_{2-x}Sr_{x}RuO_{4}$: Structural Distortion, Effective Dimensionality, Spin Fluctuations and Quantum Criticality

Abstract

The iso-electronic d 4 compounds of the 4d series show rich phase diagrams due to competing spin, charge and orbital degrees of freedom in presence of strong correlations and structural distortions. One such iso-electronic series, Ca2−xSrxRuO4, is studied within the GGA (and spin-orbit coupled GGA) plus DMFT formalism using the hybridization expansion of exact low temperature continuous time Quantum Monte Carlo impurity solver. While the local dynamical correlations make Sr2RuO4 a Hund's metal, they drive Ca2RuO4, the compound at the other end of the series, to a Mott insulating ground state. We study the dynamic and static single-particle and local irreducible vertex-corrected two-particle responses across the series at three different points (x = 2.0, 0.5, 0.0) to understand the anomalous cross-over from Hund's metal (x = 2.0) to a Mott insulator (x = 0) and find that a structural distortion is likely to be responsible for the cross-over. Further, dynamical correlations reveal that the band-width (W ) of the Hund's metal is larger than its effective local Hubbard U , and a finite Hund's coupling JH helps it remain in a bad metallic and nearly spinfrozen state over a large temperature range. Ca2RuO4, on the other hand, is intrinsically driven to the proximity of a Mott transition due to narrowing of band width (U/W > 1.5), though its finite temperature excitations indicate bad metallicity. We show that there is a critical end point of second-order structural transition at x = 0.5, where spin fluctuations become critically singular and follow the exact scaling of conformally invariant boundary field theory. We also find that this critical end point of quasi-3D nature is associated with an effective dimensional cross-over between the x = 2.0 and x = 0.0 quasi-2D structures. Finally we draw a modified magnetic phase diagram of the material, showing a fan-like region starting from the quantum critical end point at x = 0.5.

Hallmarks of Hunds coupling in the Mott insulator Ca2RuO4

“…As a first step, comparison to the band structure calculations is made. Although details can varies depending on exact methodology, all band structure calculations of Ca 2 RuO 4 find a single fast dispersing branch [22][23][24][25] . Our DFT calculation reveals that the fast dispersing band has predominantely d xy character (Fig.…”