Crossover from a heavy fermion to intermediate valence state in noncentrosymmetric Yb2Ni12(P,As)7.

W B Jiang,X Lu,H Ishii,K D Tsuei,Y F Wang,H Q Yuan,L Yang,F Gao,M Smidman,J M Lee,T Shang,J M Chen,C Y Guo,Z W Cheng,Y F Liao,Z Hu,L H Tjeng

doi:10.1038/srep17608

Abstract

We report measurements of the physical properties and electronic structure of the hexagonal compounds Yb2Ni12Pn7 (Pn = P, As) by measuring the electrical resistivity, magnetization, specific heat and partial fluorescence yield x-ray absorption spectroscopy (PFY-XAS). These demonstrate a crossover upon reducing the unit cell volume, from an intermediate valence state in Yb2Ni12As7 to a heavy-fermion paramagnetic state in Yb2Ni12P7, where the Yb is nearly trivalent. Application of pressure to Yb2Ni12P7 suppresses TFL, the temperature below which Fermi liquid behavior is recovered, suggesting the presence of a quantum critical point (QCP) under pressure. However, while there is little change in the Yb valence of Yb2Ni12P7 up to 30 GPa, there is a strong increase for Yb2Ni12As7 under pressure, before a near constant value is reached. These results indicate that any magnetic QCP in this system is well separated from strong valence fluctuations. The pressure dependence of the valence and lattice parameters of Yb2Ni12As7 are compared and at 1 GPa, there is an anomaly in the unit cell volume as well as a change in the slope of the Yb valence, indicating a correlation between structural and electronic changes.

Highlights

IntroductionWhile superconductivity was found in many Ce-based systems, to date the only Yb-based heavy fermion superconductor reported is β-YbAlB4, which undergoes a SC transition at a low temperature of Tc = 0.08 K7
We report measurements of the physical properties and electronic structure of the hexagonal compounds Yb2Ni12Pn7 (Pn = P, As) by measuring the electrical resistivity, magnetization, specific heat and partial fluorescence yield x-ray absorption spectroscopy (PFY-XAS)
Superconductivity in the vicinity of the quantum critical point (QCP) is believed to be mediated by spin fluctuations, there has been particular attention to the role played by fluctuations of the rare-earth valence in determining the ground state properties

Summary

Introduction

While superconductivity was found in many Ce-based systems, to date the only Yb-based heavy fermion superconductor reported is β-YbAlB4, which undergoes a SC transition at a low temperature of Tc = 0.08 K7 This compound is itself close to a QCP and displays NFL behavior, without the need to tune parameters such as pressure or magnetic field[7,8]. For Yb2Fe12P7, an unusual temperature-field phase diagram has been proposed, where a magnetic QCP separates a low-field magnetic ordered NFL region and a high-field NFL state[12] Another example is Yb2Ni12P7 (YNP), which was initially believed to be an intermediate valence (IV) compound[13], it has been described as a heavy fermion compound with an enhanced electronic specific heat coefficient of γ = 195 mJ/Yb-mol K2 14. As of yet the presence of mixed valence behavior has not been directly measured in either of these compounds

Methods

Results

Conclusion