We deduce a model relevant for the description of the ferromagnetic half-metal Chromium dioxide (\(CrO_{2}\)), widely used in magnetic recording technology. The model describes the effect of ...dynamical, local orbital correlations arising from local quantum chemistry of the material. A finite temperature solution of the model in \(d=\infty\) provides a natural explanation of the optical response, photoemission, resistivity and the large Woods-Saxon ratio observed in experiments. Our study confirms the important role of many body dynamical correlation effects for a proper understanding of the metallic phase of \(CrO_{2}\).
We investigate the role of orbital degeneracy in the double exchange (DE) model. In the \(J_{H}\to\infty\) limit, an effective generalized ``Hubbard'' model incorporating orbital pseudospin degrees ...of freedom is derived. The model possesses an exact solution in one- and in infinite dimensions. In 1D, the metallic phase off ``half-filling'' is a Luttinger liquid with pseudospin-charge separation. Using the \(d=\infty\) solution for our effective model, we show how many experimental observations for the well-doped (\(x\simeq 0.3\)) three-dimensional manganites \(La_{1-x}Sr_{x}MnO_{3}\) can be qualitatively explained by invoking the role of orbital degeneracy in the DE model.
We investigate the effects of static, diagonal disorder in the \(d=\infty\) Hubbard model by treating the dynamical effects of local Hubbard correlations and disorder on an equal footing. This is ...achieved by a proper combination of the iterated perturbation theory and the coherent potential approximation. Within the paramagnetic phase, we find that the renormalized Fermi liquid metal phase of the pure Hubbard model is stable against disorder for small disorder strengths. With increasing disorder, strong resonant scattering effects destroy low-energy Fermi liquid coherence, leading to an {\it incoherent} metallic non-FL state off half-filling. Finally, for large enough disorder, a {\it continuous} transition to the Mott-Anderson insulating phase occurs. The nature of the non-FL metallic phase, as well as the effects of the low energy coherence (incoherence) on optical conductivity and electronic Raman spectra, are considered in detail.
Realistic electronic-structure calculations for correlated Mott insulators are notoriously hard. Here we present an ab initio multiconfiguration scheme that adequately describes strong correlation ...effects involving Cu 3d and O 2p electrons in layered cuprates. In particular, the O 2p states giving rise to the Zhang-Rice band are explicitly considered. Renormalization effects due to nonlocal spin interactions are also treated consistently. We show that the dispersion of the lowest band observed in photoemission is reproduced with quantitative accuracy. Additionally, the evolution of the Fermi surface with doping follows directly from our ab initio data. Our results thus open a new avenue for the first-principles investigation of the electronic structure of correlated Mott insulators.
We present a family of two-dimensional frustrated quantum magnets solely based on pure nearest-neighbor Heisenberg interactions which can be solved quasi-exactly. All lattices are constructed in ...terms of frustrated quantum cages containing a chiral degree of freedom protected by frustration. The ground states of these models are dubbed ultimate quantum paramagnets and exhibit an extensive entropy at zero temperature. We discuss the unusual and extensively degenerate excitations in such phases. Implications for thermodynamic properties as well as for decoherence free quantum computation are discussed.
