[Paper Review] Giant overlap between the magnetic and superconducting phases of CeAu2Si2 under pressure

Zhi Ren, Leonid V. Pourovskii+4 more|arXiv (Cornell University)|Jun 5, 2014

Rare-earth and actinide compounds37 references8 citations

TL;DR

This study discovers a giant overlap between superconducting and magnetic phases in CeAu2Si2 under high pressure, with superconductivity (Tc up to 2.5 K) coexisting with antiferromagnetism over a broad pressure range (11.8–22.3 GPa). The coexistence is driven by a pressure-induced orbital crossover in Ce 4f electrons, leading to enhanced Tc and TM simultaneously, peaking near a critical pressure where magnetism vanishes, suggesting orbital physics as a key driver of unconventional superconductivity in heavy fermion systems.

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 CeAu2Si2 (TN ~ 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 Tc and TM are strongly enhanced when increasing the pressure from 16.7 to 20.2 GPa. Tc reaches a maximum at a pressure slightly below pc ~ 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 endpoint lying just above pc. We show that the maximum Tc of CeAu2Si2 actually occurs at almost the same unit-cell volume as that of CeCu2Si2 and CeCu2Ge2, and when the Kondo and crystal field splitting energies becomes comparable. Dynamical mean-filed theory calculations suggest that the peculiar behavior in pressurized CeAu2Si2 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.

Motivation & Objective

• To investigate the interplay between superconductivity and magnetism in CeAu2Si2 under high pressure.
• To determine whether the observed coexistence of superconducting and magnetic phases is linked to electronic correlations or orbital degrees of freedom.
• To explore the role of Ce 4f electron delocalization and orbital occupancy in driving unconventional superconductivity.
• To compare the pressure evolution of Tc and TM in CeAu2Si2 with that of related Ce-based heavy fermion compounds like CeCu2Si2 and CeCu2Ge2.
• To identify the microscopic origin of the enhanced Tc and TM in the intermediate pressure regime (16.7–20.2 GPa).

Proposed method

• High-pressure transport and calorimetry measurements were performed on high-quality single crystals of CeAu2Si2 up to 27.4 GPa.
• Resistivity scaling analysis was used to identify a critical end point associated with continuous delocalization of Ce 4f electrons.
• First-principles dynamical mean-field theory (DMFT) calculations were employed to analyze the pressure dependence of Ce 4f orbital occupancy and electronic structure.
• Unit-cell volume was calculated and compared across CeAu2Si2, CeCu2Si2, and CeCu2Ge2 to identify common scaling behavior.
• The evolution of Kondo temperature (TK) and crystal field splitting energy was analyzed to assess their role in pairing mechanisms.
• The pressure dependence of magnetic transition temperature (TM) and superconducting transition temperature (Tc) was mapped to identify correlations and coexistence regions.

Experimental results

Research questions

• RQ1 What is the extent and nature of the coexistence between superconductivity and magnetism in CeAu2Si2 under high pressure?
• RQ2 Why is the superconducting transition temperature Tc enhanced in the same pressure range where the magnetic transition temperature TM increases?
• RQ3 What is the microscopic origin of the critical end point observed in resistivity scaling, and how is it related to 4f electron delocalization?
• RQ4 How does the orbital occupancy of Ce 4f electrons evolve with pressure, and what role does it play in mediating superconductivity?
• RQ5 Why does the maximum Tc in CeAu2Si2 occur at nearly the same unit-cell volume as in CeCu2Si2 and CeCu2Ge2, despite differences in chemical composition?

Key findings

• A broad pressure interval of 11.8–22.3 GPa exhibits a giant overlap between superconducting and magnetic phases in CeAu2Si2, unprecedented in Ce-based heavy fermion systems.
• The superconducting transition temperature Tc reaches a maximum of 2.5 K just below the critical pressure pc ≈ 22.5 GPa, where magnetic order disappears.
• Both Tc and TM are strongly enhanced from 16.7 to 20.2 GPa, with Tc ∝ TM observed over a wide pressure range, indicating a correlated evolution.
• Resistivity scaling reveals a continuous delocalization of Ce 4f electrons, signaling a critical end point located just above pc.
• First-principles DMFT calculations show a distinct intermediate state in Ce 4f orbital occupancy under pressure, suggesting orbital physics as a key factor in the observed behavior.
• The maximum Tc occurs at a unit-cell volume nearly identical to that of CeCu2Si2 and CeCu2Ge2, and when Kondo and crystal field energies become comparable, hinting at a universal pairing mechanism.