Strong-coupling scenario of a metamagnetic transition

Abstract

We investigate the periodic Anderson model in the presence of an external magnetic field, using dynamical mean-field theory in combination with the modified perturbation theory. A metamagnetic transition is observed which exhibits a massive change in the electronic properties. These are discussed in terms of the quasiparticle weight and densities of states. The results are compared with the experimental results of the metamagnetic transition in CeRu2Si2.By "metamagnetic transition", we describe an anomalous behavior of the magnetization as function of external field b ext , namely a sudden increase at a finite field b * ext . Many rare-earth materials exhibit this kind of behavior. One has to distinguish between those materials which already show long-range (e.g. antiferromagnetic) order for zero-field and those which are paramagnetic.In the first case, the materials already have finite local moments at zero-field. Here a metamagnetic transition occurs when the external field is stronger than the internal (antiferromagnetic) exchange between these moments. This situation is found for example in CeFe 2 -based alloys 1 .The other possibility, where a paramagnetic substance enters a high-magnetization state at a critical field b * ext is realized by some heavy-fermion metals as for example CeRu 2 Si 2 2,3,4,5 or UCoAl 5,6 . As will be discussed further below, experiments indicate a substantial change in the electronic structure at the transition, which is not yet fully understood.In this paper, we investigate a similar transition found in a relatively simple electronic model of heavy-fermion compounds. We examine the periodic Anderson model (PAM) with an external magnetic field. The periodic Anderson model describes the interplay between strongly correlated localized electrons with a band of uncorrelated conduction electrons. These two electronic sub-systems are coupled by a hybridization term. The Hamiltonian of the PAM reads:Here, s kσ (f iσ ) and s † kσ (f † iσ ) are the creation and annihilation operators for a conduction electron with Bloch vector k and spin σ (a localized electron on site i and spin σ) andThe dispersion of the conduction band is ǫ( k) and ǫ f is the position of the localized level. The hybridization strength V is taken to be k-independent, and finally U is the on-site Coulomb interaction strength between two felectrons. Throughout this paper, the conduction band will be described by a free (Bloch) density of states,, of semi-elliptic shape. Its width W = 1 sets the energy scale, and its center of gravity the energy-zero:The magnetic field b ext is given in energy units of the band width W ( z σ = +1(−1) for σ =↑ (↓)). The external field couples equally to the spin of the f and conduction band electrons. This is in our opinion most appropriate for the model Hamiltonian (1) where the f -states are taken to be non-degenerate (s-type). Without using the full orbital degeneracy, one could alternatively use the g-factors of the real materials. Other authors have even completely neglected the coupli...

Dynamical Mean-Field Study of Metamagnetism in Heavy Fermion Systems

“…As for * E-mail address: takemi@phys.sc.niigata-u.ac.jp the metallic case, the PAM away from the half-filling has been studied by using the DMFT in combination with the iterated perturbation theory (IPT). 15) In contrast to the experiment of CeRu 2 Si 2 , however, the metamagnetism occurs together with the ferromagnetic transition in ref. 15).…”

“…15) In contrast to the experiment of CeRu 2 Si 2 , however, the metamagnetism occurs together with the ferromagnetic transition in ref. 15). Hence, we need more realistic model for the study on the metamagnetism of CeRu 2 Si 2 .…”

“…9) Dynamical mean-field theory (DMFT) has been known as one of the most powerful method to describe the strongly correlated electron systems, since the DMFT includes the local quantum fluctuation effects which plays an essential role for the heavy fermion systems. [10][11][12][13][14][15] The early DMFT studies on the PAM for the heavy fermion systems have succeeded in describing the strongly renormalized quasiparticles and the magnetization process. [12][13][14] These studies were restricted to the case with the half-filling and with a k-independent c − f mixing, where the Kondo insulator state is realized.…”

Mean-field study of the heavy-fermion metamagnetic transition

“…A more common assumption is to set the two g-factors equal. 24,25,26,27 This choice, however, is a somewhat artificial limit 28 and, at the mean field level, leads to a nongeneric (g c = g f is a special tuning) magnetization plateau of width equal to the zero-field Kondo energy. 9,12 We do not attempt to model the anisotropy of g (it is in principle a tensor 29 ) by introducing explicit crystal field terms into the Hamiltonian.…”

Universal signatures of the metamagnetic quantum critical endpoint: Application toCeRu2Si2

“…However, it did not provide an explanation for the huge anomalies themselves like, for example, the large Grüneisen parameter. Since then, a number of microscopic theories based on the periodic Anderson or Hubbard model have been put forward for metamagnetism in heavy-fermion materials [18][19][20][21] that qualitatively explained many of the observed features in CeRu 2 Si 2 .…”