Antiferromagnetic Spin Fluctuations above the Dome-Shaped and Full-Gap Superconducting States ofLaFeAsO1−xFxRevealed byAs

Abstract

We report a systematic study by 75 As nuclear-quadrupole resonance in LaFeAsO1−xFx. The antiferromagnetic spin fluctuation (AFSF) found above the magnetic ordering temperature TN = 58 K for x = 0.03 persists in the regime 0.04 ≤ x ≤ 0.08 where superconductivity sets in. A domeshaped x-dependence of the superconducting transition temperature Tc is found, with the highest Tc = 27 K at x = 0.06 which is realized under significant AFSF. With increasing x further, the AFSF decreases, and so does Tc. These features resemble closely the cuprates La2−xSrxCuO4. In x = 0.06, the spin-lattice relaxation rate (1/T1) below Tc decreases exponentially down to 0.13 Tc, which unambiguously indicates that the energy gaps are fully-opened. The temperature variation of 1/T1 below Tc is rendered nonexponential for other x by impurity scattering.

Enhancement of superconducting transition temperature due to antiferromagnetic spin fluctuations in iron pnictides LaFe(As1−xPx)(Mukuda1, Engetsu2, Yamamoto3 et al. 2014Phys. Rev. B2810330View full textAdd to dashboardCite

“…On the other hand, for the LaFeAs(O 1−y F y ) series, the maximum T c emerges at y=0.1 without any development of AFMSFs upon cooling down to T c 14,15,23 , although AFMSFs can be observed in the vicinity of the AFM ordered phase with a lower T c , i.e., in the range of 0.04 < y < 0.08 [24][25][26] . In this context, we emphasize 27,29,30 .…”

NMR Search for the Spin Nematic State in a LaFeAsO Single Crystal

“…Earlier NMR measurements demonstrated that low frequency spin fluctuations associated with the spin density wave (SDW) instability grow toward T c near optimal doping [2][3][4][5], favoring the scenario of spin-fluctuation induced superconductivity. There is, however, a complication; the slowing of the lattice vibrations accompanies that of spin fluctuations [6][7][8].…”

Quantum criticality in electron-doped BaFe2−xNixAs2

“…Such deviation has been ascribed to the quantum critical fluctuations associated with a quantum critical point (QCP) 3,4 , whose relationship to the occurrence of superconductivity has been one of the central issues in condensed-matter physics in the last decades. In iron-pnictide high-temperature superconductors 5,6 , searching for magnetic fluctuations has also become an important subject 7 . Quantum critical fluctuations of order parameters take place not only in spatial domain, but also in imaginary time domain 8,9 .…”