Structural collapse and superconductivity in rare earth-doped CaFe2As2

TL;DR

This study demonstrates how rare earth doping in CaFe2As2 induces a controllable structural collapse at ambient pressure, significantly affecting its electronic and magnetic properties, and stabilizing high-temperature superconductivity up to 47 K.

Contribution

It reveals a novel isostructural collapse mechanism driven by ion size, leading to high-temperature superconductivity in a rare earth-doped CaFe2As2 system.

Findings

Induced structural collapse via rare earth substitution.

Superconductivity with T_c up to 47 K observed.

Collapse affects electronic structure and magnetic properties.

Abstract

Aliovalent rare earth substitution into the alkaline earth site of CaFe2As2 single-crystals is used to fine-tune structural, magnetic and electronic properties of this iron-based superconducting system. Neutron and single crystal x-ray scattering experiments indicate that an isostructural collapse of the tetragonal unit cell can be controllably induced at ambient pressures by choice of substituent ion size. This instability is driven by the interlayer As-As anion separation, resulting in an unprecedented thermal expansion coefficient of $180\times 10^{-6}$ K$^{-1}$. Electrical transport and magnetic susceptibility measurements reveal abrupt changes in the physical properties through the collapse as a function of temperature, including a reconstruction of the electronic structure. Superconductivity with onset transition temperatures as high as 47 K is stabilized by the suppression of antiferromagnetic order via chemical pressure, electron doping or a combination of both. Extensive investigations are performed to understand the observations of partial volume-fraction diamagnetic screening, ruling out extrinsic sources such as strain mechanisms, surface states or foreign phases as the cause of this superconducting phase that appears to be stable in both collapsed and uncollapsed structures.