The valence fluctuations in materials are essentially charge fluctuations between the orbitals and it is highly nontrivial how the magnetic field affects them. We clarified the mechanism of how critical end points of first-order valence transitions are controlled by a magnetic field [1]. We show that by applying a magnetic field, the critical temperature is suppressed to be a quantum critical point (QCP) and unexpectedly the QCP exhibits nonmonotonic field dependence in the ground-state phase diagram, giving rise to emergence of metamagnetism even in the intermediate valence-crossover regime. A remarkable result is that the magnetic susceptibility diverges at the QCP of the valence transition. Namely, at the QCP, critical valence fluctuation, i.e., charge-transfer fluctuation between f and conduction electrons, diverges. At the same time, uniform spin fluctuation diverges. This offers essentially new mechanism of metamagnetism, which differs from ever-existing mechanisms of the metamagnetism.
The driving force of the field-induced QCP is clarified to be cooperative phenomena of Zeeman effect and Kondo effect, which create a distinct energy scale from the Kondo temperature. This mechanism explains peculiar magnetic response in CeIrIn5 and metamagnetic transition in YbXCu4 for X=In as well as sharp contrast between X=Ag and Cd [1,2].
Our theoretical prediction has been experimentally verified in YbAgCu4 by X-ray absorption measurement under magnetic field, which succeeded in detecting sharp increase in the Yb-valence around the metamagnetic field B ∼40 T [3].
[1] S. Watanabe, A. Tsuruta, K. Miyake and J. Flouquet: Phys. Rev. Lett. 100 (2008) 236401.
[2] S. Watanabe, A. Tsuruta, K. Miyake and J. Flouquet: J. Phys. Soc. Jpn. 78 (2009) 104706.
[3] Y. H. Matsuda et al., J. Phys. Soc. Jpn. 81 (2012) 015002.
Back to Shinji Watanabe's home page
Last updated: May 16 2016