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Register a new userEnhanced anisotropic spin fluctuations below tetragonal-to-orthorhombic transition in LaFeAs(O_{1-x}F_x) probed by ^{75}As and ^{139}La NMR
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$^{75}$As and $^{139}$La NMR results of LaFeAs(O$_{1-x}$F$_x$) ($x$=0, 0.025, and 0.04) were reported. Upon F-doping, the tetragonal-to-orthorhombic structural phase transition temperature $T_S$, antiferromagnetic transition temperature $T_N$ and internal magnetic field $mu_0H_{rm int}$ are gradually reduced for $x<0.04$. However, at $x=0.04$, $T_N$ is abruptly suppressed to be 30 K along with a tiny $mu_0H_{rm int}$, which is distinct from the continuous disappearance of the ordered phases in the Ba122 systems of Ba(Fe,Co)$_2$As$_2$ and BaFe$_2$(As,P)$_2$. The anisotropy of the spin-lattice relaxation rate $T_1^{-1}$, $(T_1)^{-1}_{Hparallel ab}/(T_1)^{-1}_{Hparallel c}$, in the paramagnetic phase of $x = 0$ and 0.025 is constant ($sim 1.5$), but increases abruptly below $T_S$ due to the enhancement of $(T_1)^{-1}_{Hparallel ab}$ by the slowing down of magnetic fluctuations. This indicates that the tetragonal-to-orthorhombic structural distortion enhances the anisotropy in the spin space via magnetoelastic coupling and/or spin-orbit interaction.
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In the present paper, we propose the parity even,orbital singlet and spin triplet pairing state as the ground state of the newly discovered super-conductor $LaO_{1-x}F_xFeAs$.The pairing mechanism involves both the special shape of the electron fermi surface and the strong ferromagnetic fluctuation induced by Hunds rule coupling.The special behavior of the Bogoliubov quasi-particle spectrum may leads to Fermi arc like anisotropy super-conducting gap, which can be detected by angle resolved photo emission(ARPES).The impurity effects are also discussed.
Recent discovery of oxypnictide superconductor LaFeAs(O,F) (LFAO-F) with the critical temperature (Tc) of 26 K and succeeding revelation of much increased Tc upon substitution of La for other rare earth elements (such as Sm, leading to ~43 K) and application of pressure for LFAO-F (~ 43 K) has triggered broad interest in the mechanism yielding relatively high Tc in this new class of compounds. While they share a feature with high-Tc cuprates that superconductivity occurs upon carrier doping to pristine compound which exhibits magnetism, they also resemble the heavy-fermion compounds in the sense that superconductivity appears in the vicinity of magnetic phase. Investigation of electronic states near the boundary between these two phases might provide some useful information on the mechanism of superconductivity, as it has been proved to be the case in many exotic superconductors. Here we show by muon experiment in the LFAO-F compound that a macroscopic phase separation into superconducting and spin glass-like magnetic phases occurs at x=0.06 that is near the phase boundary, where both the magnetism and superconductivity develop simultaneously below a common Tc ~ 18 K. This accordance strongly suggests intimate relationship between magnetism and superconductivity typically found in heavy-fermion systems near the quantum critical point.
The parent compounds of iron-based superconductors are magnetically-ordered bad metals, with superconductivity appearing near a putative magnetic quantum critical point. The presence of both Hubbard repulsion and Hunds coupling leads to rich physics in these multiorbital systems, and motivated descriptions of magnetism in terms of itinerant electrons or localized spins. The NaFe$_{1-x}$Cu$_x$As series consists of magnetically-ordered bad metal ($x=0$), superconducting ($xapprox0.02$) and magnetically-ordered semiconducing/insulating ($xapprox0.5$) phases, providing a platform to investigate the connection between superconductivity, magnetism and electronic correlations. Here we use X-ray absorption spectroscopy and resonant inelastic X-ray scattering to study the valence state of Fe and spin dynamics in two NaFe$_{1-x}$Cu$_x$As compounds ($x=0$ and 0.47). We find that magnetism in both compounds arises from Fe$^{2+}$ atoms, and exhibits underdamped dispersive spin waves in their respective ordered states. The dispersion of spin excitations in NaFe$_{0.53}$Cu$_{0.47}$As is consistent with being quasi-one-dimensional. Compared to NaFeAs, the band top of spin waves in NaFe$_{0.53}$Cu$_{0.47}$As is slightly softened with significantly more spectral weight of the spin excitations. Our results indicate the spin dynamics in NaFe$_{0.53}$Cu$_{0.47}$As arise from localized magnetic moments and suggest the iron-based superconductors are proximate to a correlated insulating state with localized iron moments.
The Nd-doped cuprate La_{2-y-x}Nd_ySr_xCuO_4 displays a first-order phase transition at T_d (= 74 K for x=0.10, y = 0.60) to a low-temperature tetragonal (LTT) phase. A magnetic field H applied || the a-axis leads to an increase in T_d, whereas T_d is decreased when H || c. These effects show that magnetic ordering involving both Nd and Cu spins plays a key role in driving the LTO-LTT transition. Related anisotropic effects are observed in the uniform susceptibility and the in-plane magnetoresistance.
Motivated by the recent observation of ferromagnetic spin correlations in the filled skutterudite SrFe$_4$As$_{12}$ [Ding et al., Phys. Rev. B 98, 155149 (2018)], we have carried out $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the role of magnetic fluctuations in a newly discovered isostructural superconductor SrOs$_4$As$_{12}$ with a superconducting transition temperature of $T_{rm c}$ $sim$ 4.8 K. Knight shift $K$ determined by the NQR spectrum under a small magnetic field ($le$ 0.5 T) is nearly independent of temperature, consistent with the temperature dependence of the magnetic susceptibility. The nuclear spin-lattice relaxation rate divided by temperature, 1/$T_1T$, is nearly independent of temperature above $sim$ 50 K and increases slightly with decreasing temperature below the temperature. The temperature dependence is reasonably explained by a simple model where a flat band structure with a small ledge near the Fermi energy is assumed. By comparing the present NMR data with those in SrFe$_4$As$_{12}$, we found that the values of $|K|$ and $1/T_1T$ in SrOs$_4$As$_{12}$ are smaller than those in SrFe$_4$As$_{12}$, indicating no obvious ferromagnetic spin correlations in SrOs$_4$As$_{12}$. From the temperature dependence of 1/$T_1$ in the superconducting state, an $s$-wave superconductivity is realized.
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