Abstract
Using non-resonant Fe K-beta x-ray emission spectroscopy, we reveal that Sr-doping of CaFe2As2 decouples the Fe moment from the volume collapse transition, yielding a collapsed-tetragonal, paramagnetic normal state out of which superconductivity develops. X-ray diffraction measurements implicate the c-axis lattice parameter as the controlling criterion for the Fe moment, promoting a generic description for the appearance of pressure-induced superconductivity in the alkaline-earth-based 122 ferropnictides (AFe2As2). The evolution of the superconducting critical temperature with pressure lends support to theories for superconductivity involving unconventional pairing mediated by magnetic fluctuations.
Highlights
Whether the mechanism for moment loss is driven by changes in the atomic volume of Fe, as suggested for mantle minerals,[35] or some c-axis-dependent internal coordination remains ambiguous. These experimental results challenge the notion that the Fe moment is universally quenched in the collapsed tetragonal (CT) phase of the 122 systems, and instead promote a generic description of the 122 systems under pressure
As-As bonding develops across the mirror plane of the crystal structure, isostructurally collapsing the structure and truncating the antiferromagnetically ordered (AFM) state
While the Fe moments in the CT phase of undoped CaFe2As2 are quenched,[23,24] we have shown that the CT phase can support a substantial Fe moment (∼0.5 μB) that appears to be strongly coupled to the c-axis lattice parameter, which is controlled by the size of the alkaline earth atom, thermal contraction, and the volume change induced by the CT phase
Summary
In the research on iron pnictide superconductors, theoretical calculations strongly suggested that conventional, phonon-mediated superconductivity was incompatible with observed critical temperatures Tc.[1,2,3,4] several systems have shown an empirical correlation between Tc and the structural parameters.[5,6,7] Since iron-based superconductors have proven a fertile playground for understanding how structural and magnetic degrees of freedom affect superconductivity.[8,9,10,11] While there are no fewer than five systems sharing similar structural building blocks, the most widely studied ferropnictide superconductors are those that crystallize in the tetragonal ThCr2Si2 crystal structure. In order to tune the structural and magnetic degrees of freedom without charge doping, we have performed high-pressure experiments on Ca0.67Sr0.33Fe2As2, which provides a larger-volume system that decouples the quenching of Fe moments from the volume-collapse transition. Consistent with theoretical calculations, the magnitude of the Fe moments appears to be largely controlled by the c-axis lattice parameter, providing a natural mechanism to explain the observed superconductivity in the 122 systems under pressure
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