The Novel Magnetic and Valence Instabilities Tuned by Pressure in YbNi3Ga9
Uwatoko Group
In heavy fermion systems, non-Fermi liquid state and/or unconventional superconductivity often appear near a quantum critical point (QCP) where a second order phase transition is suppressed to zero temperature [1]. Therefore, pressure tuning of the ground state from a nonmagnetic state to a magnetic state or vice versa has attracted attention. The anomalous properties in the vicinity of QCP are described by the spin fluctuation theory, especially for most of Ce-based compounds, however, the unconventional critical behavior has been recently reported in several Yb-based heavy fermion compounds. Soon after, several theories have been proposed to clarify the origin of the unconventional criticality, nevertheless, it remains an open question. Since the pressure variation of magnetism in Yb systems is accompanied by a valence change from the divalent to the trivalent state of Yb ions, direct observations of Yb valence toward the magnetic QCP is highly desirable. Here we show the realization of the pressure-induced valence crossover and the novel metamagnetic behavior possibly due to the valence instability near the magnetic quantum phase transition in intermediate valence YbNi3Ga9 [2,3].
Fig. 1. Contour plot of the Yb valence in the temperature-pressure phase diagram of YbNi3Ga9. The transition and crossover temperatures are deduced from resistivity (circles) and ac magnetic susceptibility (squares) measurements. Here, closed squares below and above Pc indicate the CP and TCP, respectively. The dashed lines are guides to the eye.
Figure 1 shows the temperature-pressure-magnetic field phase diagram of YbNi3Ga9 together with a contour plot of the Yb valence value in the T-P plane. We have revealed a clear evolution of the Yb valence toward the magnetic trivalent state as well as the change from the nonmagnetic to the magnetic ground state with increasing pressure. The resulting phase diagram suggests that the occurrence of the pressure-induced first-order magnetic transition in YbNi3Ga9 is associated with enhanced valence fluctuations. This interpretation is consistent with the observations of a first-order metamagnetic transition below Pc, which is associated with the field-induced valence change from the mixed-valent state to the trivalent state with magnetic degrees of freedom. Our results suggest that the proximity to the critical end point of the first-order valence transition gives rise to the unconventional critical behavior in other Yb systems.
References
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