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Register a new userEvolution of the Kondo state of YbRh2Si2 probed by high field ESR
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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 T_K ~ 25 K. A remarkable 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 which are formed below T_K 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 < T_K, 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.
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We present quantum oscillation measurements of YbRh2Si2 at magnetic fields above the Kondo-suppression scale H0 ~ 10 T. Comparison with electronic structure calculations is complicated because the small Fermi surface, where the Yb 4f-quasi-hole is not contributing to the Fermi volume, and large Fermi surface, where the Yb 4f-quasi-hole is contributing to the Fermi volume, are related by a rigid Fermi energy shift. This means that spin-split branches of the large Fermi surface can look like unsplit branches of the small surface, and vice-versa. Thus, although the high-field angle dependence of the experimentally-measured oscillation frequencies most resembles the electronic structure prediction for the small Fermi surface, this may instead be a branch of the spin-split large Fermi surface.
Below the Kondo temperature the heavy Fermion compound YbRh$_{2}$Si$_{2}$ shows a well defined Electron Spin Resonance (ESR) with local Yb$^{3+}$ 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 Yb$^{3+}$ in YPd$_3$:Yb. The ESR lineshape is investigated regarding contributions from itinerant centers. From the ESR of monoisotopic $^{174}$YbRh$_{2}$Si$_{2}$ we could exclude unresolved hyperfine contributions to the lineshape.
Below a characteristic temperature, due to hybridisation effects Kondo insulators exhibit a gap in the electronic density of states and behave like semiconductors. By using Gd3+ electron spin resonance (ESR), the compound CeNiSn was investigated as a representative of this class. In addition, the metal-to-insulator transition was studied as a function of doping for CeNi(1-x)Co(x)Sn and CeNi(1-y)Pt(y)Sn. The linewidth of the Gd resonance yields direct information about the density of states at the Fermi energy. So the size of the gap can clearly be estimated for the pure compound, and the closing of the gap by substitution of Ni by Co or Pt can be followed in detail. These results are compared to measurements of NMR, specific heat and susceptibility.
We present electron spin resonance data of Ti$^{3+}$ (3$d^1$) ions in single crystals of the novel layered quantum spin magnet TiOCl. The analysis of the g tensor yields direct evidence that the d_{xy} orbital from the t_{2g} set is predominantly occupied and owing to the occurrence of orbital order a linear spin chain forms along the crystallographic b axis. This result corroborates recent theoretical LDA+U calculations of the band structure. The temperature dependence of the parameters of the resonance signal suggests a strong coupling between spin and lattice degrees of freedom and gives evidence for a transition to a nonmagnetic ground state at 67 K.
Magnetic-field-induced changes of the Fermi surface play a central role in theories of the exotic quantum criticality of YbRh2Si2. We have carried out de Haas-van Alphen measurements in the magnetic-field range 8 T <= H <= 16 T, and directly observe field dependence of the extremal Fermi surface areas. Our data support the theory that a low-field large Fermi surface, including the Yb 4f quasihole, is increasingly spin split until a majority-spin branch undergoes a Lifshitz transition and disappears at H0 ~ 10 T, without requiring 4f localization at H0.
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