Dynamical conductivity spectra [σ(ω)] have been measured for many heavy-fermion (HF) Ce and Yb compounds. A characteristic excitation peak has been observed in the infrared region of σ(ω) for all the compounds, and has been analyzed in terms of a simple model based on conduction (c)-f electron hybridized band. A universal scaling is found between the observed peak energies and the estimated c-f hybridization strengths of these HF compounds. This scaling demonstrates that the model of c-f hybridized band can generally and quantitatively describe the low-energy charge excitations in a wide range of HF compounds.
In Ref. 1, Schubert et al. [Phys. Rev. Research 1, 032004 (2019)] reported measurements of the isothermal magnetoresistance of Fe-and Ni-substituted YbRh2Si2, based on which they raised questions about the Kondo destruction description for the magnetic field-induced quantum critical point (QCP) of pristine YbRh2Si2. Here we make three points. Firstly, as shown by studies on pristine YbRh2Si2 in Paschen et al. and Friedemann et al., isothermal crossed-field and single-field Hall effect measurements are necessary to ascertain the evolution of the Fermi surface across this QCP. Because Schubert et al. did not carry out such measurements, their results on Fe-and Ni-substituted YbRh2Si2 cannot be used to assess the validity of the Kondo destruction picture neither for substituted nor for pristine YbRh2Si2. Secondly, when referring to the data of Friedemann et al. on the isothermal crossover of YbRh2Si2, they did not recognize the implications of the crossover width, quantified by the full width at half maximum (FWHM), being linear in temperature, with zero offset, over about 1.5 decades in temperature, from 30 mK to 1 K. Finally, in claiming deviations of Hall crossover FWHM data of Friedemann et al. from the above linear-in-T dependence they neglected the error bars of these measurements and discarded some of the data points. The claims of Schubert et al. are thus not supported by data, neither previously published nor new (Ref. 1). As such they cannot invalidate the evidence that has been reported for Kondo destruction quantum criticality in YbRh2Si2.Quantum criticality is a topic of considerable interest for a variety of strongly correlated electron systems, with antiferromagnetic heavy fermion systems representing a prototype. From extensive experimental measurements across QCPs of several heavy fermion metals, a variety of properties are found 2-16 to be inconsistent with spin-density-wave quantum criticality [17][18][19] , which is based on Landau's framework of order-parameter fluctuations. Instead, they support Kondo destruction quantum criticality 20-22 , which goes beyond the Landau framework through a critical destruction of the static Kondo entanglement. In particular, across the magnetic field-induced QCP in YbRh 2 Si 2 , the linear-response Hall coefficient determined from a crossed-field Hall measurement 3,5 , along with single-field Hall effect 3,5 , magnetoresistance 3,5 , and thermodynamic properties 4 , provided evidence for an extra energy scale, T * , in the T -B plane. This energy scale goes to zero as the QCP is approached from the non-magnetic side. Isothermal magnetotransport and thermodynamic properties undergo a rapid crossover across the T * -line, which extrapolates to a jump in the T = 0 limit, across generations of YbRh 2 Si 2 samples. These properties are in contrast with the po-larization crossover scenario 1 .Recently, Schubert et al. 1 studied the magnetoresistance of Fe-and Ni-substituted YbRh 2 Si 2 . Primarily based on the isothermal behavior of the magnetoresistance in these dope...
International audienceThe cubic lattice parameter temperature dependence of SmB6 between 300 K and 4.2 K as well as the X-ray absorption at the LIII edge measured in the same temperature range, give direct evidence of the average samarium valence change which goes from 2.60 at 300 K to 2.53 at 4.2 K. Previous work claimed that the Sm2+ : Sm3+ ratio was temperature independent in SmB6. As for SmB6 a samarium valence change has been observed with decreasing temperature below 300 K in the Sm 0.75La0.25B6 solid solution
Spin excitations in the Kondo semiconductor CeNiSn have been studied in a wide Q-range and in the energy range of h(cross) omega =1.2-7 meV by means of single-crystal neutron scattering. The magnetic fluctuation at low temperatures in this energy range is dominated by the easy a-axis component Im chi ad. Below the coherence temperature of 20 K, two dynamic antiferromagnetic correlations develop as excitation peaks at h(cross) omega =2 and 4 meV. The 4 meV excitation appears at Q=(Qa. 1/2 +n, Qc) where Qc and Qc are arbitrary and n is an integer, which indicates that the correlation is quasi-one dimensional along the b-axis. The 2 meV excitation appears around Q=(0,0,I) and (0,1,0), which shows three-dimensional Q-dependence. These two excitations reflect the nature of the Kondo coherent state of CeNiSn.
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