NMR evidence for an intimate relationship between antiferromagnetic spin fluctuations and extendeds-wave superconductivity in monocrystallineSrFe2(As1−x

Abstract

We report systematic 31 P-NMR study on iron (Fe)-based superconductors SrFe2(As1−xPx)2 (Sr122AsP), in which a superconducting (SC) transition temperature Tc at x=0.35 increases from Tc=26 K up to 33 K by annealing an as-grown mono-crystalline sample. The present NMR study has unraveled that Tc reaches a highest value of 33 K at x=0.35 around a quantum critical point at which antiferromagnetic (AFM) order disappears. When noting that the SC transition disappears at x=0.6 where the AFM spin fluctuations (SFs) are no longer present, we remark that the onset and increase of Tc are apparently associated with the emergence and enhancement of AFM-SFs, respectively. In the SC state, the residual density of state (RDOS) at the Fermi energy EF in the SC state becomes much smaller for the annealed sample than for the as-grown one, suggesting that some inhomogeneity and/or imperfection for the latter increases RDOS as expected for unconventional SC state with nodal gap. These findings in Sr122AsP are consistent with the unconventional s±-wave Cooper pairing state that is mediated by AFM-SFs. We also discuss other key-ingredients besides the AFM-SFs to increase Tc further.

Multiple Antiferromagnetic Spin Fluctuations and Novel Evolution of Tc in Iron-based superconductors LaFe(As1-xPx)(O1-yFy) revealed by 31P-NMR Studies

“…Here, note data at temperatures higher than T ∼160 are used the contribution of AFMSFs in (1/T 1 ) is negligible. The value of K chem is comparable to those evaluated in previous 10,16 K s (T is proportional to χ(q=0) the s = A hf (0)χ(q=0) ∝ A hf (0)N (E F ). Here, χ(q=0) is the static spin susceptibility, A hf (0) is the hyperfine coupling constant at q=0, and N (E F ) is the density of states (DOS) at the Fermi level (E F ).…”

“…This contrasted behavior between (1/T 1 T ) and K s (T ) demonstrates the development of AFMSFs with finite wave vectors as temperature decreases. To deduce the development of AFMSFs following the previous studies, 8,10,16 we assume that (1/T 1 T ) is decomposed as…”

“…11,12 On the other hand, the AFMSFs are not so distinct at low energies for the compounds with a high T c (∼50 K), which are characterized by the local lattice parameters of FeAs 4 close to the regular tetrahedron. [13][14][15][16] It has been reported that once further electrons are doped in the hydrogen-substituted LaFeAs(O 1−y ′ H y ′ ), the new phases of SC and AFM orders are uncovered and denoted as SC3 and AFM3 in Fig. 1, respectively.…”

$^{31}$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

“…31 K ab is the sum of the spin shift K spin and the chemical shift K chem , that is 31 K ab = K spin + K chem . The chemical shift K chem is estimated to be 0.018 % for BaFe 2 (As 1−x P x ) 2 [10] and 0.04 % for SrFe 2 (As 1−x P x ) 2 [11]. Then, K spin is estimated to be 0.10 or 0.12 %.…”

“…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]. Systematic 31 P NMR studies have been performed for BaFe 2 (As 1−x P x ) 2 [10] and SrFe 2 (As 1−x P x ) 2 [11]. The successive phase transitions from spin-density-wave (SDW) antiferromagnets to unconventional superconductors (T c < T N ) have been studied with theoretical models [12,13].…”