On the two-fluid model of the Kondo lattice

Abstract

The normal state of conduction electrons in metals at low temperatures has been described in terms of the standard theory of a Fermi liquid introduced soon after the advent of quantum mechanics and completed by Landau and others by the middle of the 20th century (1). Fermi liquid theory describes the nature of a quantum liquid of interacting itinerant fermions below a characteristic temperature T FL, which can be far below the bare Fermi temperature T F. The state above T FL, but below T F, is itself a correlated quantum liquid that can extend down to very low temperatures close to a quantum critical point (Fig. 1 A ) (2). Fig. 1. Two-fluid model of the Kondo lattice. ( A ) Schematic temperature-vs.-hybridization effectiveness phase diagram about a quantum critical point (midpoint of lower horizontal axis). f can be tuned in practice via hydrostatic pressure or chemical substitution. T N is the Neel temperature and T * is the hybridization crossover temperature. ( B and C ) the heavy carrier fraction f h( T ) and local moment fraction f l( T ), respectively, as functions of temperature and hybridization effectiveness in the two-fluid model. The quantum critical point corresponds to a T of 0 and an f of 1. The heavy carrier fraction saturates and the local moment fraction vanishes below T L for f > 1. The Kondo liquid tends to be unstable to superconductivity or other subtle forms of quantum order. The transitions or crossovers into such states as well as the relocalization temperature just above T N (and T N itself in B and C ) are not indicated. Yang and Pines (3) and Shirer et al. (4) present a phenomenological description of such a correlated quantum liquid known as the Kondo liquid (5). In the simplest case … 1E-mail: gl238{at}cam.ac.uk.