Spin-dependent orbital selectivity and partial Kondo-screening in magnetically ordered Hund's metal

TL;DR

Sr2CoO4 is a 3d Hund's metal with anomalous ferromagnetism and transport traced to spin–orbital differentiation. The study employs fully charge self-consistent spin-polarized DFT+DMFT with a rotationally invariant Coulomb interaction and CTQMC impurity solver to compute spin-orbital resolved DOS, self-energies, and hybridization. Key findings include a spin-dependent orbital selectivity and a spin-orbital selective incoherence that produce a sharp SOS Kondo resonance in the spin-up t2g states around TK ~ 70 K, leading to partial screening of local moments and explaining the reduced saturation magnetization and resistivity upturn; an intermediate 110–200 K window shows t2g↑ gap formation and strong correlation effects while e_g bands remain metallic. The work suggests a Hund’s coupling–driven spin-selectivity mechanism beyond conventional orbital selectivity, advocating a Kondo-lattice-like framework to understand long-range order in Hund's metals.

Abstract

Hund's metallicity in 3$d$ transition metal oxides constitutes a rare class of compounds, since they have been long understood considering the dominance of Hubbard $U$. $\mathrm{LiV_2O_4}$\& $\mathrm{Sr_2CoO_4}$ belong to this rare class of metals; among them, $\mathrm{LiV_2O_4}$ has been the subject of extensive investigations for its unconventional heavy-fermion behavior, while studies on $\mathrm{Sr_2CoO_4}$ remain limited despite its anomalous ferromagnetic ground state. In this study, we report an unusual spin-orbital selective localization in $\mathrm{Sr_2CoO_4}$ leading to a sharp Kondo resonance at $\sim$70 K in the spin-$up$ channel of orbitals of $t_{2g}$ symmetry using a combination of Density functional theory and Dynamical mean field theory (DFT+DMFT) calculations. Correspondingly, an appreciable reduction in the magnetization below $T$=100 K further suggests partial Kondo screening of local moments active at low temperatures, explaining its effective spin magnetization state and upturn in its resistivity observed in experimental reports. We note a significant effect of Hund's induced spin-orbital selective incoherence in dictating the temperature evolution of its macroscopic observables e.g. spin-spin correlation function and effective local moment. Our results reveal a potentially distinct/new form of spin-dependent selectivity induced via Hund's coupling in addition to the conventional orbital-selectivity in the Hund's metals, as a plausible key mechanism in stabilizing their long-range magnetic order.

Figures (6)

• Figure 1: (a) The plot of $T \chi_{\text{loc}}$ and (b) inverse local spin-susceptibility ($\chi_{\text{loc}}^{-1}$) in the temperature range 50 -- 3000 K..
• Figure 2: Temperature dependent variation of spin-orbital resolved spectral density plot for Co 3$d$ orbitals.
• Figure 3: Temperature dependent variation of spin-orbital resolved hybridization function ($\mathrm{Im} \Delta(\omega)$) of Co 3$d$ orbitals.
• Figure 4: Temperature dependent variation of inverse quasiparticle lifetime (imaginary part of self-energy, $\mathrm{Im} \Sigma(i \omega)$) of Co 3$d$ orbitals for spin-up (Left ) and spin-down (right) channel.
• Figure 5: Temperature-dependent magnetization curve using rotationally-invariant form of Coulomb interaction parametrization (full-type) obtained from DFT+DMFT calculations.