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arXiv:cond-mat/9609111 · Physical Review Letters

Phase Diagram of Electronic Models for Transition Metal Oxides in One Dimension

José A. Riera, K. Hallberg, Elbio R. Dagotto

Published Jul 28, 1997• 94 citations
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Abstract

The zero temperature phase diagram of the ferromagnetic Kondo model in one dimension is studied using numerical techniques, especially at large Hund coupling. A robust region of fully saturated ferromagnetism (FM) is identified at all densities. Phase separation between hole-rich and hole-poor regions and a paramagnetic regime with quasilocalized holes were also observed. It is argued that these phases will also appear in two and three dimensions. Our results apply both to manganites and one-dimensional compounds such as ${\mathrm{Y}}_{2\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{BaNiO}}_{5}$. As the transition metal ion spin grows, the hole mobility rapidly decreases, explaining the differences between Cu oxides and Mn oxides.

DOI
10.1103/physrevlett.79.713

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Using the density matrix renormalization group, we study metallic ferromagnetism in a one-dimensional copper-oxide model that contains one oxygen p orbital and one copper d orbital. The parameters for the $d\ensuremath{-}p$ model can be chosen so that it is similar to the one-dimensional periodic Anderson model. For these parameters, we compare the ground-state phase diagram with that of the Anderson model and find a ferromagnetic region analogous to one found in the Anderson model, but which is pushed to somewhat higher densities and interaction strengths. In both models, we find a region within the ferromagnetic phase in which phase separation between a localized ferromagnetic domain and a weakly antiferromagnetic regime occurs. We then choose a set of parameter values appropriate for copper-oxide materials and explore the ground-state phase diagram as a function of the oxygen-oxygen hopping strength and the electron density. We find three disconnected regions of metallic ferromagnetism and give physical pictures of the three different mechanisms for ferromagnetism in these phases.
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Many correlated metallic materials are described by Landau Fermi-liquid theory at low energies, but for Hund metals the Fermi-liquid coherence scale T_{FL} is found to be surprisingly small. In this Letter, we study the simplest impurity model relevant for Hund metals, the three-channel spin-orbital Kondo model, using the numerical renormalization group (NRG) method and compute its global phase diagram. In this framework, T_{FL} becomes arbitrarily small close to two new quantum critical points that we identify by tuning the spin or spin-orbital Kondo couplings into the ferromagnetic regimes. We find quantum phase transitions to a singular Fermi-liquid or a novel non-Fermi-liquid phase. The new non-Fermi-liquid phase shows frustrated behavior involving alternating overscreenings in spin and orbital sectors, with universal power laws in the spin (ω^{-1/5}), orbital (ω^{1/5}) and spin-orbital (ω^{1}) dynamical susceptibilities. These power laws, and the NRG eigenlevel spectra, can be fully understood using conformal field theory arguments, which also clarify the nature of the non-Fermi-liquid phase.