Giant Overlap between the Magnetic and Superconducting Phases ofCeAu2Si2under Pressure

Abstract

High pressure provides a powerful means for exploring unconventional superconductivity which appears mostly on the border of magnetism. Here, we report the discovery of pressure-induced heavy-fermion superconductivity up to 2.5 K in the antiferromanget CeAu 2 Si 2 (T N ≈ 10 K). Remarkably, the magnetic and superconducting phases are found to overlap across an unprecedentedly wide pressure interval from 11.8 to 22.3 GPa. Moreover, both the bulk T c and T M are strongly enhanced when increasing the pressure from 16.7 to 20.2 GPa. T c reaches a maximum at a pressure slightly below p c ≈ 22.5 GPa, at which magnetic order disappears. Furthermore, the scaling behavior of the resistivity provides evidence for a continuous delocalization of the Ce 4f electrons associated with a critical end point lying just above p c . We show that the maximum T c of CeAu 2 Si 2 actually occurs at almost the same unit-cell volume as that of CeCu 2 Si 2 and CeCu 2 Ge 2 , and when the Kondo and crystal-field splitting energies become comparable. Dynamical mean-filed theory calculations suggest that the peculiar behavior in pressurized CeAu 2 Si 2 might be related to its Ce-4f orbital occupancy. Our results not only provide a unique example of the interplay between superconductivity and magnetism, but also underline the role of orbital physics in understanding Ce-based heavy-fermion systems.

High-Pressure Study of the Ground- and Superconducting-State Properties of CeAu2Si2

“…With magnetic fields applied along the c-axis, we examined the pressure dependence of the superconducting upper critical field H c2 and its initial slope dH c2 /dT | T →Tc , as well as the pressure dependence of the normal-groundstate properties for H > H c2 . The present work confirms the existing phase diagram [1] and reveals the following major novelties. i) A new transition line T mod M , presumably due to the reconstruction of magnetic order, is observed inside the AF phase [see Fig.…”

“…[1]. To generate pressure, we used a Bridgman-type sintered-diamond-anvil pressure cell with a pyrophyllite gasket and steatite as a soft-solid pressure medium.…”

“…In this context, the antiferromagnet CeAu 2 Si 2 presents new challenges to the current understanding of HF superconductivity. In CeAu 2 Si 2 [1], the magnetic and superconducting phases overlap across an unprecedentedly broad pressure range from p = 11.8 GPa up to the critical pressure p c ∼ 22.5 GPa, where the superconducting transition temperature T c is highest, as shown in the schematic phase diagram in Fig. 1(a).…”

“…Among the Ce-based HF superconductors, CeAu 2 Si 2 displays one of the highest T c of ≈ 2.5 K. Superconductivity (SC) appears to involve HF quasiparticles with an effective mass m*≈ 100 m e , as inferred from the large * gernot.scheerer@unige.ch initial slope of the upper critical field [1]. Furthermore, the strong pair-breaking effect of non-magnetic impurities attests unconventional SC [2].…”

“…1(a)] [1,2]. The rapid collapse of magnetism and the SC enhancement may be driven by critical valence [16] or even orbital [1] fluctuations.…”