Quasiparticles in transition metals with strong local correlations: band formation and collective effects

Abstract

We address the question, whether low lying one-particle excitations in the Fermi-liquid phase of highly correlated electron systems can form well defined bands of nearly stable quasiparticles, and comment on features of a universal band picture for these systems. We outline how to derive a description of instabilities to magnetic, charged ordered or superconducting phases, which bears a close analogy to Stoner theory and lends itself to an interpretation in terms of quasiparticle bands and residual interactions at low temperatures. Concepts and problems are illustrated via calculations for some standard models of solid-state theory using modern many-body techniques like NRG, NCA and DMFT. Differences to conventional band structure theory are pointed out. We shortly comment on the relevance of these questions to the physics of inhomogeneous systems and small particles. The quasiparticle concept and bandsLandau's concept of a Fermi liquid [1] has been of great importance for our understanding of various properties of metals, since it reconciles the picture of independent electrons with our knowledge about their interactions and the correlations thereby induced. In simple metals the conduction bands are broad, the kinetic contribution to the energy is important and electrons acquire a moderate dressing only. They bear an apparent correspondence to the quasiparticles, which constitute the Fermi liquid at low temperatures. This concept also applies to systems of a quite different nature, in particular to the He-3 fluid, in which the fundamental Fermions are atoms with stable electron shells interacting dominantly via a strong shortranged force. Here, the quasiparticles differ considerably from these atoms, incorporating strong backflow effects [2]. It came as a surprise that transition metal compounds can host even more extreme types of Fermi liquids [3], with quasiparticles heavier by a factor of thousand than bare electrons [4]. Although the study of exotic groundstates with superconducting or magnetic order originally lay in the main focus of scientific activity [5,6], it seems worthwhile to consider the "normal" properties of these exotic Fermi liquids, which show strong anomalous temperature dependencies in various quantities like the specific heat or the BIS spectra [7]. Moreover, a number of these systems exhibit a "coherence regime" at low temperatures [7], in which the quasiparticles are supposed to form rather well defined bands. In this connection, the interesting question arises, whether a universal band structure concept for such systems exists, which combines common

Spectral properties of the two-impurity Anderson model with varying distance and various interactions

“…The separation of the peak maxima is roughly given by 2t, which is in accord with the simple reasoning from the isolated two-impurity model [see Eq. (32)]. This reflects Fig.…”

“…This reflects Fig. 8. a tendency, which has been outlined before, 32 namely, that features of the unperturbed structures of the one-particle states leave their traces in the quasiparticle band structure. 045133-10 In the low-energy region, hopping produces a gap similar to the case of a direct exchange coupling [see Figs.…”

Non-Fermi-liquid signatures in the Hubbard model due to van Hove singularities

“…For the finite temperature calculations we use the enhanced non-crossing approximation 15,16,[46][47][48] (ENCA) as the impurity solver. There are no adjustable parameters and the method works directly on the real frequency axis.…”

“…14 The Fermi liquid reveals itself via a low-energy many-body bandstructure forming around the Fermi level at low temperatures. 15,16 Due to the neglect of momentum dependent correlations in DMFT the heavy quasiparticles possess the non-interacting Fermi surface. 9 A characteristic low-energy scale T 0 is associated with this lattice version of the Kondo effect and marks the temperature scale for local-moment screening and the emergence of coherent quasiparticles.…”

Multi-orbital Anderson models and the Kondo effect: a NCA study enhanced by vertex corrections

“…Capturing electron correlations due to strong local interactions in realistic transition metal compounds, in the framework of band structure theory or in form of quasi-particle band structures derived from Greens functions, is an active field of research [21][22][23]. Difficulties arise from the effective local building blocks needed in the theories, which in most cases are more complicated than a simple Anderson impurity.…”