Quantum critical behavior of heavy fermions: Quasiparticles in the Gaussian fluctuation regime

Abstract

Key words Heavy fermions, quantum critical point, spin fluctuations, YbRh2Si2. This article is dedicated to Dieter Vollhardt on the occasion of his 60th birthday.Landau quasiparticle (qp) theory may be extended into the non-Fermi liquid regime of metallic compounds near a quantum critical point (QCP), provided the width of the quasiparticle states is less than their energy. Sufficiently far from the QCP, when the critical fluctuations are non-interacting, their influence on the qp properties may be calculated in a controlled way. A prominent example is the 'marginal Fermi liquid,' characterized by a logarithmic variation of the specific heat coefficient and a linear variation of the resistivity with temperature. The latter is induced by the interaction of qps with three-dimensional ferromagnetic (FM) or, in a disordered system, with quasi-two-dimensional antiferromagnetic (AFM) spin fluctuations. It is argued that both types of fluctuations are present in YbRh2Si2. It is suggested that the experimentally observed crossover line at T * marks the changeover from AFM to FM fluctuations with increasing magnetic field. The non-Fermi liquid behavior found in the electron spin resonance data on YbRh2Si2 may be completely interpreted using critical quasiparticle theory.

Electron spin resonance in Y bRh2Si2: local-moment, unlike-spin and quasiparticle descriptions

“…The theory outlined in this paper makes use of a general theory of ESR in systems with unlike spins [19], which in the current case are the spins of the conduction electrons and the pseudospins associated with the ground doublet of the Yb ions. We develop an expression for the g-factor of the coupled system in the weak field limit and compare it with the expression for the local-moment g-factor obtained in [11] as well as with the quasiparticle g-factors derived in [14,15]. We also comment on the quasiparticle theory outlined in [16] and its connection with the theories outlined in [11,[13][14][15].…”

“…We develop an expression for the g-factor of the coupled system in the weak field limit and compare it with the expression for the local-moment g-factor obtained in [11] as well as with the quasiparticle g-factors derived in [14,15]. We also comment on the quasiparticle theory outlined in [16] and its connection with the theories outlined in [11,[13][14][15].…”

“…The comparison with the quasiparticle results is more subtle. In the notation of this report, the expression for the g-factor presented in [14,15] takes the form…”

“…Currently, theories of the ESR in the non-Fermi liquid regime in YbRh 2 Si 2 , and Kondo lattice systems in general, fall into two categories: those based on the local-moment picture [10][11][12] and a quasiparticle approach which explicitly includes the exchange interaction between the conduction electrons and the 4f quasiparticles [13][14][15][16]. The theory for the g-factor developed in [11] is based on an earlier theory of ESR in anisotropic magnets [17] that, in turn, is an application of a general approach to collective motion in many-particle systems developed by Mori [18].…”

Ferromagnetic correlations in heavy fermions from an NMR point of view: YbNi4P2 vs. YbRh2Si2

“…With increasing field the upturn in K(α) and the related enhancement in χ''at finite q is suppressed. This quenching of afm fluctuations might be associated to the crossover from the NFL to the FL regime [37]. [28].…”