Quantum criticality in YbNiGe3 and off-center Rattling in the Thermoelectric Ferromagnet Eu8Ga16Ge30 : Pressure-Dependent Spectr
We highlight the main results of recent collaborative works [1-3] involving pressure-dependent spectroscopic investigations on our flux-grown single crystals of the title compounds. YbNiGe3 and YbNiSi3 form a model quantum critical system, in which the Ge compound shows strong Yb valence fuctuations and the Si compound is an anti-ferromagnetic Kondo lattice with TN = 5.1 K and moderate heavy fermion behavior. The evolution of the Yb valence state in both compounds under pressure up to 15.6 GPa at 300 K was investigated by means of x-ray absorption spectroscopy in La1 partial fuorescence yield mode (PFY-XAS) and resonant x-ray emission spectroscopy (RXES) around the Yb L3 absorption edge . A mean valence of n = 2.52 ±
0.01 is obtained for YbNiGe3 at ambient pressure. The Yb valence increases with pressure until an almost saturated value of n ~ 2.87 is reached at 15.6 GPa, implying that the system should cross a quantum critical point (estimated at around 8 GPa) without achieving a full trivalent state. In contrast, the Yb valence in YbNiSi3 is nearly 3 at ambient pressure, with almost no temperature dependence.
b-Eu8Ga16Ge30 is a unique ferromagnetic clathrate compound (TC = 36
K) which features rattling and tunneling phenomena related to the Eu2+ ions off-center, anharmonic vibrations, with consequent glass-like thermal conductivity. Temperature- and pressure-dependent x-ray powder diffraction (XRD), x-ray absorption near-edge structure (XANES), and x-ray magnetic circular dichroism (XMCD) measurements have been conducted on the compound at two different synchrotron radiation facilities, using different experimental procedures [2,3]. In both cases a pressure-induced amorphization around 18 GPa and simultaneous collapse of the magnetic order has been found, although unexpected diverences were observed in terms of presence/absence of recrystallization after pressure release, and presence/absence of Eu valence shift accompanying the amorphization transition.
We thank the financial support of FAPESP, CNPq and UFABC.
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