Interaction effects in mixed-valent Kondo insulators

We study theoretically the class of mixed-valent Kondo insulators, employing a recently developed local moment approach to heavy Fermion systems using the asymmetric periodic Anderson model (PAM). Novel features in spectra and transport, observable experimentally but lying outside the scope of the symmetric PAM or the Kondo lattice model, emerge naturally within the present theory. We argue in particular that a shoulder-like feature in the optical conductivity, that is distinct from the usual mid-infrared or direct gap peak and has been observed experimentally in mixed-valent compounds such as CeOs 4Sb12 and YbAl3, is of intrinsic origin. Detailed comparison is made between the resultant theory and transport/optical experiments on the filled-skutterudite compound CeOs4Sb12, and good agreement is obtained. © 2007 IOP Publishing Ltd

Exhaustion physics in the periodic Anderson model from iterated perturbation theory

We discuss the "exhaustion" problem in the context of the periodic Anderson model using iterated perturbation theory (IPT) within the dynamical mean-field theory. We find that, despite its limitations, IPT captures the exhaustion physics, which manifests itself as a dramatic, strongly energy-dependent, suppression of the effective hybridization of the self-consistent Anderson impurity problem. As a consequence, low-energy scales in the lattice case are strongly suppressed compared to the "Kondo scale" in the single impurity picture. The IPT results are in qualitative agreement with recent Quantum Monte Carlo results for the same problem

First-principles investigation of cubic BaRuO3: A Hund's metal

A first-principles investigation of cubic BaRuO3, by combining density functional theory with dynamical mean-field theory and a hybridization expansion continuous time quantum Monte Carlo solver, has been carried out. Nonmagnetic calculations with appropriately chosen on-site Coulomb repulsion U and Hund's exchange J for single-particle dynamics and static susceptibility show that cubic BaRuO3 is in a spin-frozen state at temperatures above the ferromagnetic transition point. A strong redshift with increasing J of the peak in the real frequency dynamical susceptibility indicates a dramatic suppression of the Fermi liquid coherence scale as compared to the bare parameters in cubic BaRuO3. The self-energy also shows clear deviation from Fermi liquid behavior that manifests in the single-particle spectrum. Such a clean separation of energy scales in this system provides scope for an incoherent spin-frozen (SF) phase that extends over a wide temperature range, to manifest in non-Fermi liquid behavior and to be the precursor for the magnetically ordered ground state