Structural collapse and superconductivity in rare-earth-doped CaFe2As2

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 × 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.

High-TcSuperconductivity near the Anion Height Instability in Fe-Based Superconductors: Analysis ofLaFeAsO1−xOnari1, Yamakawa2, Kontani3 2014Phys. Rev. Lett.70101000View full textAdd to dashboardCite

“…(The c-axis length is unchanged in the C 2 structure transition at x ∼ 0.) Similarly, high-T c (∼ 50K) superconductivity is realized near the "collapsed C 4 phase" in rare-earth doped CaFe 2 As 2 [19,20]. In (La,P) co-doped CaFe 2 As 2 , higher-T c state is realized near the anion-height instability, whereas it avoids the AFM phase as clearly shown in Ref.…”

“…In (La,P) co-doped CaFe 2 As 2 , higher-T c state is realized near the anion-height instability, whereas it avoids the AFM phase as clearly shown in Ref. [19,20]. These experiments strongly indicate that the anion-height instability is a key ingredient for higher-T c superconductivity of order 50K.…”

A New Layered Iron Arsenide Superconductor: (Ca,Pr)FeAs2

“…The structures of these compounds indicate that there are possibilities to find new iron-based superconductors with arsenide blocking layers. In addition, Saha et al reported that single crystalline (Ca,RE)Fe 2 As 2 (RE = La, Ce, Pr, Nd) showed superconductivity with high T c exceeding 40 K in resistivity measurement 11 . After this report, superconductivity in RE-doped CaFe 2 As 2 has been reported by several groups 12 .…”

Interface-induced superconductivity at ∼25 K at ambient pressure in undoped CaFe 2 As 2 single crystals

“…The observations offer an avenue to higher T c. this family seems to be rather unusual. One intriguing observation has been the nonbulk superconductivity up to 49 K induced in Ca122 at ambient pressure by slight Ca replacement with a rareearth element, La, Ce, Pr, or Nd (17)(18)(19). A systematic study on the slightly rare-earth-doped Ca122 suggests that the nonbulk superconductivity detected may be associated with the naturally occurring interfaces associated with defects in the samples (20).…”