J. Wykhoff

Resistivity studies under hydrostatic pressure on a low-resistance variant of the quasi-two-dimensional organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br: Search for intrinsic scattering contributions

Resistivity measurements have been performed on a low (LR)- and high (HR)-resistance variant of the kappa-(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 rho(T) maximum in standard HR crystals around 90 K, is sample dependent, i.e. extrinsic in nature, (ii) the abrupt change in rho(T) at T* approx. 40 K from a strongly temperature-dependent behavior at T > T* to an only weakly T-dependent rho(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 rho(T) proportional to AT^2 dependence at low temperatures, i.e. for T_c < T < T_0, although with strongly sample-dependent coefficients A and upper bounds for the T^2 behavior measured by T_0. The latter result is inconsistent with the T^2 dependence originating from coherent Fermi-liquid excitations.

Evolution of the Kondo state of YbRh2Si2 probed by high-field ESR.

An electron spin resonance (ESR) study of the heavy fermion compound YbRh2Si2 for fields up to approximately 8 T reveals a strongly anisotropic signal in the Kondo state below approximately 25 K. A similarity between the T dependence of the ESR parameters and that of the specific heat and the 29Si nuclear magnetic resonance data gives evidence that the ESR response is given by heavy fermions. Tuning the Kondo effect on the 4f states with magnetic fields approximately 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.

Electron spin resonance of YbIr2Si2 below the Kondo temperature

We report an electron spin resonance (ESR) study in single crystals of the new heavy-fermion metal YbIr 2 Si 2 whose spin dynamics is characterized by a Kondo energy scale T K ≃ 40 K and which acquires a Landau Fermi-liquid state below 0.2 K. The ESR spectra are observed well below T K and show typical properties of a Yb 3+ resonance. This enables the direct observation of the spin dynamics and static magnetic properties of YbIr 2 Si 2 below the Kondo temperature.

Why could electron spin resonance be observed in a heavy fermion Kondo lattice

AbstractWe 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.

Anisotropic electron spin resonance of YbIr2Si2

A series of electron spin resonance (ESR) experiments were performed on a single crystal of the heavy fermion metal Y bIr₂Si₂ to map out the anisotropy of the ESR-intensity I(ESR) which is governed by the microwave field component of the g-factor. The temperature dependencies of I(ESR)(T) and g(T) were measured for different orientations and compared within the range 2.6 K ≤ T ≤ 16 K. The analysis of the intensity dependence on the crystal orientation with respect to both the direction of the microwave field and the static magnetic field revealed remarkable features: the intensity variation with respect to the direction of the microwave field was found to be one order of magnitude smaller than expected from the g-factor anisotropy. Furthermore, we observed a weak basal plane anisotropy of the ESR parameters which we interpret to be an intrinsic sample property.

Magnetic susceptibility of YbRh2Si2 and YbIr2Si2 on the basis of a localized 4f electron approach

We consider the local properties of the Yb 3+ ion in the crystal electric field in the Kondo lattice compounds YbRh2Si2 and YbIr2Si2. 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 over a wide temperature interval where thermodynamic and transport properties show non-Fermi-liquid behavior for these materials. (Some figures in this article are in colour only in the electronic version)

Spin dynamics of YbRh2Si2observed by electron spin resonance

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 in which definite ESR spectra show characteristic properties of the Kondo ion Yb3+ 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.

Low temperature properties of the electron spin resonance in YbRh2Si2

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 toward low temperatures (> 0.5 K) 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.

ESR of YbRh2Si2 and 174YbRh2Si2 : local and itinerant properties

Abstract Below the Kondo temperature the heavy Fermion compound YbRh2Si2 shows a well defined electron spin resonance (ESR) with local Yb3+ properties. We report a detailed analysis of the ESR intensity which gives information on the number of ESR active centers relative to the ESR of well localized Yb3+ in YPd3:Yb. The ESR lineshape is investigated regarding contributions from itinerant centers. From the ESR of monoisotopic 174YbRh2Si2 we could exclude unresolved hyperfine contributions to the lineshape.Below the Kondo temperature the heavy Fermion compound YbRh

Spin dynamics of YbRh2Si2 observed by electron spin resonance

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