Pressure-induced volume-collapsed tetragonal phase of CaFe2As2 as seen via neutron scattering (0807.3032v3)

Abstract: Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe2As2 undergoes a pressure-induced transition to a non-magnetic, volume "collapsed" tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction.

• The paper demonstrates that applying pressure over 0.35 GPa triggers a phase change in CaFe₂As₂ from an antiferromagnetic orthorhombic to a non-magnetic collapsed tetragonal structure.
• The study reports a significant 9.5% collapse of the c-axis and nearly 5% reduction in unit cell volume under pressure.
• Spin-polarized total-energy calculations support the experimental findings, indicating that the structural collapse is crucial for pressure-induced superconductivity.

Pressure-Induced Volume-Collapsed Tetragonal Phase of CaFe₂As₂ As Seen Via Neutron Scattering

The paper by Kreyssig et al. presents an in-depth analysis of the effects of pressure on the electronic and structural properties of CaFe₂As₂, a parent compound of the iron-arsenide superconductors, using neutron scattering techniques. This research elucidates the transition mechanisms to a non-magnetic, volume-collapsed tetragonal phase under pressure, a structural phenomenon that precedes the emergence of superconductivity in these materials.

Neutron scattering experiments were conducted with emphasis on maintaining hydrostatic pressure conditions akin to macroscopic measurements. The findings reveal that at ambient pressure, CaFe₂As₂ displays an orthorhombic structure with antiferromagnetic order, which transforms with pressure increase. Above the 0.35 GPa threshold, the compound transitions to a non-magnetic tetragonal phase characterized by substantial lattice parameter modifications. The collapse of the $c$-axis by 9.5% and the high anisotropic reduction of the unit cell volume by nearly 5% are particularly notable.

Spin-polarized total-energy calculations further support the experimental observations, demonstrating the collapse of the magnetic moment concomitant with the structural phase transition. These calculations highlight an energy minimization in the tetragonal phase and align with the experimental disappearance of antiferromagnetic order.

The implications of these results extend to the broader class of iron-arsenide superconductors, notably in understanding pressure-induced superconductivity. It suggests a correlation between the structural parameters, such as the As-Fe-As bond angles, and the superconducting transition temperatures ($T_c$). The pressure-induced change to the 'collapsed' tetragonal phase appears essential for the superconducting state, emphasizing the delicate interplay between lattice structure and electronic properties.

The core conclusion of the paper indicates that superconductivity emerges not from the ambient orthorhombic phase but from the pressure-stabilized 'collapsed' tetragonal phase. This aligns with broader patterns observed in doped variants of the iron-arsenides, implying potential universalities in the superconducting pairing mechanisms across these materials.

Future trajectories, as suggested by this paper, could involve detailed investigations into the role of chemical pressure through doping and its interaction with mechanical pressure on superconductivity. Understanding the symmetry breaking in the Fe-As layer, particularly through the As-Fe-As bond angle changes, may yield insights into the superconductivity mechanisms, further contributing to the development of high-$T_c$ superconductors in the iron arsenide family.