Spin Fluctuation Theory for Quantum Tricritical Point Arising in Proximity to First-Order Phase Transitions: Applications to Heavy-Fermion Systems, YbRh2Si2, CeRu2Si2, and beta-YbAlB4

摘要

We propose a phenomenological spin fluctuation theory for anti ferromagnetic quantum tricritical point (QTCP), where a first-order phase transition changes into a continuous transition at zero temperature. Under magnetic fields, ferromagnetic quantum critical fluctuations develop around the anti ferromagnetic QTCP in addition to antiferromagnetic fluctuations, which is in sharp contrast with the conventional anti ferromagnetic quantum critical point. For itinerant electron systems, we show that the temperature dependence of critical magnetic fluctuations around the QTCP is given as chi(Q) proportional to T-3/2 (chi(0) proportional to T-3/4) at the anti ferromagnetic ordering (ferromagnetic) wave number q = Q (q = 0). The convex temperature dependence of chi(-1)(0) is a characteristic feature of the QTCP, which has never been seen in the conventional spin fluctuation theory. We propose a general theory of quantum tricriticality that has nothing to do with the specific Kondo physics itself, and solves puzzles of quantum criticalities widely observed in heavy-fermion systems such as YbRh2Si2, CeRu2Si2, and beta-YbAlB4. For YbRh2Si2, our theory successfully reproduces quantitative behaviors of the experimentally obtained ferromagnetic susceptibility and magnetization curve when suitable phenomenological parameters are chosen. The quantum tricriticality is also consistent with singularities of other physical properties such as specific heat, nuclear magnetic relaxation time 1/T1T, and the Hall coefficient. For CeRu2Si2 and beta-YbAlB4, we point out that the quantum tricriticality is a possible origin of the anomalous diverging enhancement of the uniform susceptibility observed in these materials.