We report on Hall effect measurements on YbRh2Si2 single crystals with different residual resistivity and on LuRh2Si2 single crystal. The temperature dependence of the linear-response Hall coefficient of YbRh2Si2 is described by the anomalous Hall effect and the normal contribution incorporating multi-band effects. Sample dependencies at low T are found and explained by slight changes in the charge-carrier concentrations. © 2007 Elsevier B.V. All rights reserved.

The unconventional nature of the quantum criticality in YbRh 2Si 2 is highlighted on the basis of magnetoresistivity and susceptibility measurements. Results obtained under chemical pressure realized by isoelectronic substitution on the rhodium site are presented. These results illustrate the position of the T*-line associated with a breakdown of the Kondo effect near the antiferromagnetic instability in the low-temperature phase diagram. Whereas at zero temperature the Kondo breakdown and the antiferromagnetic quantum critical point coincide in the proximity of the stoichiometric compound, they are seen to be detached under chemical pressure: For positive chemical pressure the magnetically ordered phase extends beyond the T*(B)-line. For sufficiently high negative pressure the T*(B)-line is separated from the magnetically ordered phase. From our detailed analysis we infer that the coincidence is retained at small iridium concentrations, i.e., at small negative chemical pressure. We outline further measurements which may help to clarify the detailed behavior of the two instabilities. © 2010 Springer Science+Business Media, LLC.