We report an electron spin resonance (ESR) study in single crystals of the new heavy-fermion metal YbIr2Si2 whose spin dynamics is characterized by a Kondo energy scale  K and which acquires a Landau Fermi-liquid state below 0.2 K. The ESR spectra are observed well below TK and show typical properties of a Yb3+ resonance. This enables the direct observation of the spin dynamics and static magnetic properties of YbIr2Si2 below the Kondo temperature.

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.

Resistivity measurements have been performed on a low (LR)-and high (HR)-resistance variant of the κ-(BEDT-TTF)2Cu[N(CN)2]Br superconductor. While the HR sample was synthesized following the standard procedure, the LR crystal is a result of a somewhat modified synthesis route. According to their residual resistivities and residual resistivity ratios, the LR crystal is of distinctly superior quality. He-gas pressure was used to study the effect of hydrostatic pressure on the different transport regimes for both variants. The main results of these comparative investigations are (i) a significant part of the inelastic-scattering contribution, which causes the anomalous ρ(T ) maximum in standard HR crystals around 90 K, is sample dependent, i.e. extrinsic in nature, (ii) the abrupt change in ρ(T ) at T * ≈ 40 K from a strongly temperature-dependent behavior at T > T * to an only weakly T-dependent ρ(T ) at T < T * is unaffected by this scattering contribution and thus marks an independent property, most likely a second-order phase transition, (iii) both variants reveal a ρ(T) ∝ AT 2 dependence at low temperatures, i.e. for Tc ≤ T ≤ T0, although with strongly sample-dependent coefficients A and upper bounds for the T 2 behavior measured by T0. The latter result is inconsistent with the T 2 dependence originating from coherent Fermi-liquid excitations.

An electron spin resonance (ESR) study of the heavy fermion compound YbRh2Si2 for fields up to ∼ 8 T reveals a strongly anisotropic signal below the single ion Kondo temperature TK ∼ 25 K. A remarkable similarity between the T -dependence of the ESR parameters and that of the specific heat and the 29 Si nuclear magnetic resonance data gives evidence that the ESR response is given by heavy fermions which are formed below TK and that ESR properties are determined by their field dependent mass and lifetime. The signal anisotropy, otherwise typical for Yb 3+ ions, suggests that, owing to a strong hybridization with conduction electrons at T < TK, the magnetic anisotropy of the 4f states is absorbed in the ESR of heavy quasiparticles. Tuning the Kondo effect on the 4f states with magnetic fields ∼ 2 − 8 T and temperature 2 − 25 K yields a gradual change of the ESR g-factor and linewidth which reflects the evolution of the Kondo state in this Kondo lattice system. PACS numbers: 71.27.+a, 75.20.Hr, Strong electron-electron (EE) interactions in metals yield a fascinating variety of novel and often interrelated quantum phenomena, such as quantum phase transitions, breakdown of the Landau Fermi-liquid (LFL) state, unconventional superconductivity, etc. (for an overview see, e.g., [1]). In intermetallic compounds where 4f (5f ) magnetic ions (e.g. Yb, Ce, U etc.) build up a regular Kondo lattice, strong EE correlations are established by the coupling of local f -magnetic moments with the conduction electrons (CE). As a consequence, a large effective mass enhancement of the quasiparticles (QP) hallmarks the properties of paramagnetic heavy fermion metals. A competing interaction, the so-called RKKY-interaction between the local f -states via the sea of CE, favors a magnetically ordered ground state.An important realization of a system where the delicate balance between Kondo and RKKY interactions can be investigated is the intermetallic compound YbRh 2 Si 2 where antiferromagnetic order, quantum criticality, heavy fermion-and non-LFL (NFL) behavior can be tuned by a magnetic field B and temperature T [2,3,4,5,6] (Fig. 1). In the parameter domain where these remarkable electronic crossovers take place a strong hybridization of 4f electrons with CE significantly broadens the otherwise atomically sharp f -states. That is why the observation of a narrow electron spin resonance (ESR) signal in the Kondo state of YbRh 2 Si 2 was very surprising [7]. While the reported pronounced anisotropy of the signal is indeed in accordance with an ESR of localized Yb 3+ 4f moments, a non-local picture is suggested by the observation of this signal down to the lowest accessible temperatures of 0.69 K [8] where the single ion Kondo effect is expected to screen the magnetic moments. On the other hand the conduction electron ESR seems also unlikely because in this compound comprising heavy metal elements the spin-orbit (SO) coupling drastically shortens the electron spin lifetime [9].To unravel a controversial nature of this resonance response w...

Below the Kondo temperature TK electron spin resonance (ESR) usually is not observable from the Kondo-ion itself because the characteristic spin fluctuation energy results in a huge width of the ESR line. The heavy fermion metal YbRh2Si2 seems to be an exceptional case where definite ESR spectra show characteristic properties of the Kondo-ion Yb 3+ well below TK. We found that the spin dynamics of YbRh2Si2, as determined by its ESR relaxation, is spatially characterized by an anisotropy of the zero temperature residual relaxation only.