Optical and photoemission investigation of structural and magnetic transitions in the iron-based superconductor Sr0.67Na0.33Fe2As

Abstract

We report the temperature dependent optical conductivity and angle-resolved photoemission spectroscopy (ARPES) studies of the multiband iron-based superconductor Sr0.67Na0.33Fe2As2. Measurements were made in the high-temperature tetragonal paramagnetic phase; below the structural and magnetic transitions at TN 125 K in the orthorhombic spin-density-wave (SDW)-like phase, and Tr 42 K in the reentrant tetragonal double-Q magnetic phase where both charge and SDW order exist; and below the superconducting transition at Tc 10 K. The free-carrier component in the optical conductivity is described by two Drude contributions; one strong and broad, the other weak and narrow. The broad Drude component decreases dramatically below TN and Tr, with much of its strength being transferred to a bound excitation in the mid-infrared, while the narrow Drude component shows no anomalies at either of the transitions, actually increasing in strength at low temperature while narrowing dramatically. The behavior of an infrared-active mode suggests zone-folding below Tr. Below Tc the dramatic decrease in the low-frequency optical conductivity signals the formation of a superconducting energy gap. ARPES reveals hole-like bands at the center of the Brillouin zone (BZ), with both electron-and hole-like bands at the corners. Below TN, the hole pockets at the center of the BZ decrease in size, consistent with the behavior of the broad Drude component; while below Tr the electron-like bands shift and split, giving rise to a low-energy excitation in the optical conductivity at 20 meV. The C2 and C4 magnetic states, with resulting spin-density-wave and charge-SDW order, respectively, lead to a significant reconstruction of the Fermi surface that has profound implications for the transport originating from the electron and hole pockets, but appears to have relatively little impact on the superconductivity in this material.

Muon spin rotation and infrared spectroscopy study of Ba1−xNaxFe2As2

“…Our infrared data thus reflect a strong suppression of the superconducting response due to the competition with the i-SCDW order that is more severe than in the o-AF and the assigned SVC states. Finally, note that clear signatures of a SC energy gap have very recently been reported for a similar BNFA sample for which T c was somewhat higher and the measurements were performed to a lower temperature of 5 K [61].…”

“…Here, 2 SC for the narrow and broad Drude bands amounts to 19.8 and 13 meV at x = 0.32 and 30 and 12 meV at x = 0.34, respectively (see also Table I). Similar to BaFe 2−x Co x As 2 (BCFA) and BKFA [54,55,57,61] and also CaKFe 4 As 4 (BCKFA) [65], the larger SC gap is assigned to the narrow Drude peak, which supposedly originates from the electronlike bands near the X point of the Brillouin zone.…”

“…The most significant difference between the x = 0.26 and x = 0.32 samples concerns the spectral weight loss of the Drude peaks in the AF state that occurs due to the SDW formation, see Figs. 13(a and, recently for an underdoped Sr 1−x Na x Fe 2 As 2 crystal that undergoes a corresponding transition from o-AF to i-SCDW order [61]. To the contrary, for the x = 0.32 sample in the assigned hedgehod SVC state, the major spectral weight loss involves the narrow Drude peak (D 1 ), whereas the SW of the broad Drude peak (D 2 ) remains almost constant.…”

Making a Hedgehog Spin-Vortex State Possible: Geometric Frustration on a Square Lattice

“…In particular, computational studies support this trend for iron arsenides, ,, but the interplay of magnetic fluctuations and spin–orbit coupling (SOC) may also play a major role . Experimentally, magnetic phases with C 4 symmetry (and changes to C 2 symmetry and orthogonal/nematic magnetic states) have been observed to occur in a number of metallic iron arsenides upon careful variation of the chemical composition. ,,− Interestingly, orthogonal magnetic states have also been reported in disorder-free Mott insulators, such as LaO 2 Co 2 OSe 2 , REO 2 Fe 2 OSe 2 (RE = La, Ce, Nd), , and SrF 2 Fe 2 OS 2 . However, none of these systems exhibits a hedgehog SVC state, detected in iron arsenides.…”