Effects of c/a Anisotropy and Local Crystal Structure on Superconductivity in AFe₂(As_1−xP_x)₂ (A = Ba_1−ySr_y, Sr_1−yCa_y and Eu)

Abstract

We investigated the effects of c/a anisotropy and local crystal structure on superconductivity (SC) in As/P solid solution systems, AFe 2 (As 1−x P x ) 2 (A122P) with various A ions. With decreasing A site atomic size from A=Ba to Eu, the structural anisotropy decreases, and the rate of decreasing with x also increases. The rapid narrowing of the region of antiferromagnetic composition (x) can be considered to be a result of this anisotropy change due mainly to the change in the Fermi surface (FS) nesting condition. By contrast, although the structural anisotropy systematically changes, the maximum T c values are almost the same in all A122P systems except for Eu122P. These results indicate that the modification of the FS topology via the structural anisotropy does not affect SC. However local structural parameters, such as pnictogen height, are crucial for T c .The discovery of iron based superconductors in 2008 has stimulated much discussion. 1) Their rich phase diagrams have suggested various bosonic fluctuations, such as spin, orbital and charge, possibly acting as a glue between electrons. Many theories have been proposed to explain the superconducting mechanism in terms of these fluctuations. However, which fluctuation plays the most important role in superconductivity (SC) remains in dispute.In some theories, Fermi surface (FS) nesting plays a crucial role. For example, spin fluctuation is enhanced when the condition of nesting between the hole and electron FSs is good, J. Phys. Soc. Jpn. which induces unconventional SC. [2][3][4] In the case of BaFe 2 (As 1−x P x ) 2 (Ba122P), the experimental results of inelastic neutron scattering and nuclear magnetic resonance (NMR) studies revealed that spin fluctuation was clearly enhanced in the optimally doped x-region. 5, 6) Furthermore, the study of angle-resolved photoemission spectroscopy (ARPES) 7) demonstrated that the observed FSs fulfill the nesting condition between the electron and hole FSs, which was consistent with the spin fluctuation theory in Ba122P. 8,9) From the crystallographic viewpoint, the appearance of SC would be ascribed to the optimization of the pnictogen (Pn) height from the Fe layer (h Pn ) 10) or the Pn-Fe-Pn bond angle (α), 11) which was also explained by nesting-based theories. 9,12) On the other hand, according to the nesting scenario, the distance between neighboring Fe layers, which must be correlated with interlayer hybridization and thus with anisotropy, should have a distinct influence on antiferromagnetism (AFM) and SC. Nevertheless, both Néel temperature (T N ) and superconducting transition temperature (T c ) are not higher in Ba122P than in Sr122P, although the ratio of a-and c-axes lattice constants, c/a, which is an index of structural anisotropy, is larger in Ba122P than in Sr122P. These behaviors are inconsistent with the nesting-based model. 13,14) The change in the FS with structural anisotropy has been confirmed in a recent ARPES study. 15) According to that study, the d z 2 FS at the Brillouin zone center was war...

Visualization of the magnetic flux structure in phosphorus-doped EuFe2As2 single crystals

“…Superconductivity in these compounds can be induced by doping with phosphorus. 24 Superconductivity in EuFe 2 (As 1−x P x ) 2 single crystals occurs in a rather narrow doping range x = 0.14 − 0.25 (or in the phosphorus content range 7.0 − 12.5 at%) with the maximum superconducting transition temperature T max SC = 27 K. 25,26 The magnetic transition in the Eu 2+ subsystem is observed at temperatures T C ∼ 17 − 20 K and depends moderately on the phosphorus content (doping level) in the specified range of contents. 25,[27][28][29][30][31][32] Previously, the magnetic flux structure was visualized with the MFM on artificial thin-film superconductor/ferromagnet (Nb/FeNi) hybrid structures, 15 where the domain structure and Abrikosov vortices frozen in the superconductor were observed simultaneously.…”

Electronic Structure of Sr1−yCayFe2(As1−xPx)2 (x = 0.25, y = 0.08) Revealed by Angle-Resolved Photoemission Spectroscopy

“…Thus, the spin fluctuation of SrCa122P should be weaker than that of Ba122P, while T c ( = 32 K) is almost the same as those of the other phosphorus substituted 122 2/23 systems. 15) By ARPES measurements using synchrotron radiation light, we found that the FSs of SrCa122P have three dimensional shapes. One of the hole FSs is strongly warped between the Γ and Z points, and the innermost FS observed at the Z point disappears at the Γ point, which is similar to the FS features of Sr122P.…”

“…Single crystals of SrCa122P were synthesized by the self-flux method as described elsewhere. 15) The crystals were annealed at 500 • C for 4 days in order to minimize disorder effect which broaden ARPES spectra and affects the determination of a SC gap values. 5) The highest T c value in this system (T c = 32 K with ∆T c ∼ 3 K) was confirmed by the magnetic susceptibility measurement.…”

$^{31}$P NMR studies of an iron-based superconductor Ba$_{0.5}$Sr$_{0.5}$Fe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ with $T_\mathrm{c}$ = 29 K

“…Since T N increases from 135 K (BaFe 2 As 2 ) to 199 K (SrFe 2 As 2 ) [14,36], the Sr substitution may enhance an interlayer coupling. In passing, BaFe 2 (As 1−x P x ) 2 , Ba 0.5 Sr 0.5 Fe 2 (As 1−x P x ) 2 , and SrFe 2 (As 1−x P x ) 2 have in common the optimal T c = 30−33 K [7]. No remarkable effect of the random potentials was found in the 31 P NMR spectra and relaxation rates for the present Ba 0.5 Sr 0.5 Fe 2 (As 1−x P x ) 2 with x ∼ 0.4.…”

“…An itinerant antiferromagnet BaFe 2 As 2 (Ba122) has the Néel temperature T N = 135 K [4]. The high-T c superconductivity with the optimal T c ∼ 30 K has been found in isovalent P substituted BaFe 2 (As 1−x P x ) 2 [5], Ba 0.5 Sr 0.5 Fe 2 (As 1−x P x ) 2 [6,7], and SrFe 2 (As 1−x P x ) 2 [7,8]. In the underdoped regime with respect to the P concentration for BaFe 2 (As 1−x P x ) 2 , NMR measurements revealed that the superconductivity emerges in the weakly antiferromagnetic state at T c < T N [9].…”