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304 Handbook on the Physics and Chemistry of Rare Earths In Section 3, we will discuss scenarios inspired by experiments that challenge this notion of electrons as mere bystanders. 2.4 Experiments: Li(Ho,Y)F4, Ce(Cu,Au)6, and YbRh2Si2 2.4.1 Li(HoY)F4 Once the theoretical considerations for quantum criticality in the transverse field Ising model became known, the search began for a real material to dis- play this QPT. The chosen “ideal” compound was pure LiHoF4, a dipolar- coupled insulating ferromagnet with strong Ising anisotropy and small Curie temperature TC 1⁄4 1.53 K. The first experiments (Bitko et al., 1996) measured ac-magnetic susceptibility down to 0.05 K in H? perpendicular to the Ising axis up to 8 T. The results for both the classical ferromagnetic transition and the QPT (at T % 0 with critical field H?C 1⁄4 4:93 T) showed in both cases a mean-field susceptibility divergence, ie, the susceptibility exponent g was approximately 1. Fig. 4 exhibits the divergences as function of T near H?c and function of ðH? H?C Þ=H?C at constant T, where we see the near linear dependences. FIG. 4 104 103 102 101 10–3 10–2 10–1 100 H⊥ = 49 kOe 10–3 10–2 (H⊥–H⊥c) / H⊥c 10–1 100 T = 0.1 K (T–Tc) / Tc The divergence of the ac magnetic susceptibility as a function of the reduced critical temperature and applied field H? perpendicular to the Ising axis. Tc has been defined as the lowest temperature of measurement 0.114 K while the extrapolated to T 1⁄4 0H? is 4.93 T. All the suscep- tibility divergence are mean field like with critical exponent g 1⁄4 1, meaning no difference between the zero field ferromagnetic transition and the field tuned QPT. According to Bitko, D., Rosenbaum, T.F., Aeppli, G., 1996. Quantum critical behavior for a model magnet. Phys. Rev. Lett. 77 (5), 940. c′ (cm3 / mol Ho)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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