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Quantum Critical Matter and Phase Transitions Chapter 280 311 specific heat and resistivity. The field-tuned results could then be compared to Figs. 7–9. However, distinctly different power-law exponents were observed for the field-tuned measurements (von L€ohneysen et al., 2001). This behavior is consistent with the HM theories while, in contrast, x and P tuning required a completely different scenario. Additional field tuning experimental were carried out on silver-doped CeCu6xAgx (Heuser et al., 1998). However, slightly larger Agx(%0.2) con- centrations are needed to start in the antiferromagnetic phase. Limited mea- surements were performed only on polycrystalline samples. These authors claim that both doping and field-tuning generate the NFL behavior which is caused by an underlying QCP. One must be careful: NFL properties are not only related to quantum criticality in these strongly correlated intermetallic heavy-fermion compounds. Disorder is also a prime cause of anomalous thermodynamic and transport behavior. Additionally, the magnetic field, as a vector quantity, can induce deviations from T2 resistivity dependences due to magnetic modes and anisotropic scattering. Similarly, discrepancies can be found in the specific heat. So we must be cautious about claiming NFL behavior is solely related to an underlying QCP. Therefore, after all is said and measured, a number of open and fundamen- tal queries remain. Why does the field-tuning appear so much different from the pressure or doping-tuned QPT? What is the role of disorder, specifically in terms of destroying the heavy Fermi liquid (HFL) state? How does the Kondo compensation of the local moments exactly work? This last point is related to a more general question on whether the magnetic state is the result of Fermi surface or local moment effects? Then there is, finally, the question of the observed o/T scaling and the possible two-dimensional spin fluctua- tions. These ruminations represent our present-day questioning of quantum criticality in CeCu6xAux. 2.4.3 YbRh2Si2 From the opposite end of the lanthanide series, another antiferromagnetic heavy-fermion material was discovered in 2000: YbRh2Si2 (Trovarelli et al., 2000). This intermetallic compound requires ultra-low temperatures and small magnetic field (Hc ffi 0.7 T perpendicular to the easy axis) to reach a QPT, from a very low temperature antiferromagnetic phase (TN 1⁄4 0.07 K and 103mB moment). Again, the key signatures of quantum criticality arise through NFL behavior, ie, the unconventional low-T power laws of the resis- tivity, specific heat and susceptibility. For YbRh2Si2, the NFL (quantum critical) regime spans the entire temperature-field phase space, above the QCP and even above the antiferromagnetic phase. Fig. 10 exhibits this phase diagram based on resistivity measurements (Custers et al., 2003). On the left side, the field is applied along the hard direction H k c, which requires a field of 0.7 T to reach the QCP. As indicatedPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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