Charge fluctuations across the pressure-induced quantum phase transition in EuCu_2(Ge_(1-x)Si_x)_2

摘要

Pressurizing strategically selected compositions of the EuCu_2(Ge_(1-x)Si_x)_2 series affords an opportunity for gaining microscopic insight into the ground-state properties and interplay between magnetism and valence fluctuations across a quantum critical point. This is investigated by way of systematic ~(151)Eu Moessbauer spectroscopy measurements on ⅹ = 0 and ⅹ = 0.5 compositions in the series, pressurized up to 7 GPa including variable temperature scans in the range 300-4.2 K. In EuCu_2Ge_2 the temperature and pressure dependences of the hyperfine interaction parameters indicate that both the magnetic and divalent state, Eu~(v+) where ⅴ = 2, are stable up to 6-7 GPa, thus serving as a useful reference. Whereas in the ⅹ = 0.5 composition which initially involves Eu~(2+), collapse of the magnetically ordered state is onset at ～1.3 GPa and there is emergence of a nonmagnetic intermediate valence state coexisting with the magnetically ordered state. This regime of mixed states is a precursor of a quantum phase transition to a nonmagnetic homogeneous intermediate valence state ⅴ ～ 2.45, across a quantum critical point at 3.6 GPa, suggesting a first-order phase transition. X-ray-diffraction pressure studies at 300 K up to 6 GPa of the ⅹ = 0.5 composition indicate there is no change in lattice symmetry from the tetragonal ThCr_2Si_2-type structure. There are also no obvious discontinuities in pressure dependences of the lattice parameters upon evolving through the quantum critical point at 3.6 GPa. Increasing pressure changes the starting Eu~(2+) valence monotonically, until the mean valence attains its largest value ⅴ ～ 2.45 indicative of enhanced charge fluctuations at the quantum critical point and plateaus thereafter. High-pressure resistance measurements at low temperatures down to 40 mK near the quantum critical point reveal no evidence for superconductivity.