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Abstract. We develop a theoretical basis for understanding the spin relaxation processes in Kondo lattice systems with heavy fermions as experimentally observed by electron spin resonance (ESR). The Kondo effect leads to a common energy scale that regulates a logarithmic divergence of different spin kinetic coefficients and supports a collective spin motion of the Kondo ions with conduction electrons. We find that the relaxation rate of a collective spin mode is greatly reduced due to a mutual cancellation of all the divergent contributions even in the case of the strongly anisotropic Kondo interaction. The contribution to the ESR linewidth caused by the local magnetic field distribution is subject to motional narrowing supported by ferromagnetic correlations. The developed theoretical model successfully explains the ESR data of YbRh2Si2 in terms of their dependence on temperature and magnetic field.

We consider the local properties of the Yb 3+ ion in the crystal electric field in the Kondo lattice compounds YbRh 2 Si 2 and YbIr 2 Si 2 . On this basis we have calculated the magnetic susceptibility taking into account the Kondo interaction in the simplest molecular field approximation. The resulting Curie-Weiss law and Van Vleck susceptibilities could be excellently fitted to experimental results in a wide temperature interval where thermodynamic and transport properties show non-Fermiliquid behaviour for these materials.

We present the field and temperature behavior of the narrow Electron Spin Resonance (ESR) response in YbRh 2 Si 2 well below the single ion Kondo temperature. The ESR g-factor reflects a Kondo-like field and temperature evolution of the Yb 3+ magnetism. Measurements towards low temperatures (> 0.6K) have shown distinct crossover anomalies of the ESR parameters upon approaching the regime of a well defined heavy Fermi liquid. Comparison with the field dependence of specific heat and electrical resistivity reveal that the ESR parameters can be related to quasiparticle mass and cross section and, hence, contain inherent heavy electron properties.Copyright line will be provided by the publisher 1 Introduction The heavy fermion metal YbRh 2 Si 2 has proven to display a variety of unusual low temperature electronic properties which are related to the interplay between the Kondo interaction of Yb 3+ 4f spin / conduction electron spin and the indirect magnetic RKKY interaction of the 4f spins. It is located very close to a magnetic instability where a weak antiferromagnetic long range order below 70 mK is suppressed by a magnetic field of B c = 60 mT. In the vicinity of such a magnetic field induced quantum critical point and at low temperature (T ) pronounced non-Fermi liquid behavior is observed as evidenced by a divergence of the electronic specic heat and electrical resistivity ρ ∝ T . At higher fields electronic specific heat C, magnetic susceptibility, and ρ show Landau-Fermi liquid (LFL) behavior with a renormalized electronic mass and quasiparticle scattering [1]. YbRh 2 Si 2 is one of a few Kondo lattice compounds where a welldefined Electron Spin Resonance (ESR) signal allows to directly characterize the spin dynamics of the Kondo ion.

A model of spin relaxation in Kondo lattices is proposed to explain the presence of an electron spin resonance (ESR) signal in the heavy fermion compounds YbRh 2 Si 2 and YbIr 2 Si 2 . Coupled equations for dynamical susceptibilities of Kondo ions and conduction electrons are derived by means of the functional derivative method. The perturbational scaling approach reveals the collective spin motion of Yb-ions with conduction electrons in the bottleneck regime. A common energy scale due to the Kondo effect regulates the temperature dependence of the different kinetic coefficients and results in a mutual cancelation of all divergent parts in a collective spin mode. The angular dependence of the ESR linewidth is shown to be in a qualitative agreement with experimental data on YbRh 2 Si 2 and YbIr 2 Si 2 . Linewidth contributions other than the Kondo interaction are also discussed. Theoretical modelOur basic theoretical model includes the kinetic energy of conduction electrons, the Zeeman energy, the Kondo interaction of Yb-ions with conduction electrons, and the coupling between the Yb-ions via conduction electrons (RKKY interaction).A free Yb 3+ ion has a 4f 13 configuration with one term 2 F. The spin-orbit interaction splits the 2 F term into two multiplets: 2 F 7/2 with J = 7/2 and 2 F 5/2 with J = 5/2, where J is the value of the total momentum J = L + S with L and S as the orbital and spin momentum of the ion. The excited multiplet 2 F 5/2 is separated from the ground state 2 F 7/2 by about 1 eV. Since this value is much larger than the energy of the crystal electric field (CEF), we consider in the following the ground multiplet only. It is reasonable to express the Zeeman energy of the i-th Yb-ion ZJ H for the lowest multiplet J = 7/2 via the total electronic momentum of the ion. Using the Lande g-factor g J , we have ( 2 ).As a matter of fact all these results are simple consequences of the well known the Wigner-Eckart theorem.The same arguments can be used to reveal the anisotropy of the RKKY interaction between the Kondo ions. Although this interaction appears in the second order of the Kondo interaction (3) it is convenient to consider it independently. The result is given, in particular, in [8]. Starting with the isotropic exchange Hamiltonian for two Kondo ions 2 ( 1) ij RKKY RKKY ex ij i j ij J i j RKKY J ij ij J J Jg J Jg g g J Jg g g J J g J J J J J θ θ