We report the 23 Na and 75 As NMR studies on NaFeAs single crystals. The structure transition temperature TS (55 K) and the spin density wave (SDW) transition temperature TSDW (40.5 K) are determined by the NMR line splits. The spin-lattice relaxation rates indicate that the spin fluctuations are strongly enhanced just below TS and drive a second order SDW transition. A fluctuating feature of the SDW ordering is also seen below the TSDW . We further performed highpressure NMR studies on NaFeAs, and found that the TSDW increases by ∼7 K and the magnetic moment increases by ∼30% under 2.5 GPa pressure. The discovery of superconductivity in iron pnictides 1 has attracted intense research interests, and so far hightemperature superconductivity is achieved in many ironbased compounds upon doping. In particular, three classes with similar structures, including the 1111 structure RFeAsO 1−x F x (R=La, Nd, Sm etc.)2-4 , the 122 structure Ba(Fe 1−x Co x ) 2 As 2 /Ba 1−x K x Fe 2 As 2 5 , and the 111 structure LiFeAs/NaFeAs 6-8 have been extensively studied. In their parent compounds, long-range antiferromagnetism (AFM), or the spin-density-wave (SDW), has been reported with a stripe-like magnetic structure 9 . There are renewed concerns on the nature of the magnetism, regarding to whether the SDW follows a local moment or a Fermi surface nesting picture. For the 111 class, the magnetism appears to be very weak. In LiFeAs, superconductivity is observed instead of the SDW ordering 6 , although the SDW fluctuations are seen above T C 10 . In NaFeAs, the SDW order is observed with a low transition temperature 11 . Its magnetic moment is reported to be 0.09 µ B /Fe by neutron scattering 12 , in contrast to the larger values of about 0.4 µ B /Fe in the 1111 and about 1 µ B /Fe in the 122 parent compounds 5,9,13 . From local density approximation (LDA) calculations, however, the band structures of all three classes are similar [14][15][16] .One important fact is that the SDW order only develops at or below the structure transition, namely the hightemperature tetragonal (HTT) to the low-temperature orthorhombic (LTO) transition. It is conjectured that the structure transition is important for the SDW ordering, and may also be important for the superconductivity. For instance, it has been argued that both the structure phase transition and the SDW ordering are driven by a ferro-orbital ordering caused by the d xz and d yz orbitals 17,18 . The structure transition T S and the SDW transition T SDW are well separated in NaFeAs and the 1111 class, while in the 122 class the two transitions occur simultaneously. These distinctive properties open a sight for studying the relation between the structure and the magnetism.In this paper, we present our 23 Na and 75 As NMR studies on nominally undoped NaFeAs single crystals, mainly focusing on the interplay of the structure and the magnetism. First, we determined the sharp SDW transition temperature (T SDW ≈ 40.5 K) and the structure transition temperature (T S ≈ 55 K) directly from the NMR...

In this letter, we reported the results of NMR study on LiFeAs single crystals. We find a strong evidence of the low temperature spin fluctuations; by changing sample preparation conditions, the system can be tuned toward an spin-density-wave (SDW) quantum-critical point. The detection of an interstitial Li(2) ion, possibly locating in the tetrahedral hole, suggests that the off-stoichiometry and/or lattice defect can probably account for the absence of the SDW ordering in LiFeAs. These facts show that LiFeAs is a strongly correlated system and the superconductivity is likely originated from the SDW fluctuations. PACS numbers:The interplay of magnetism and superconductivity is one of the dominant themes in the study of unconventional superconductors, such as high-T c cuprates, organic superconductors and heavy fermions, where the magnetic fluctuations are crucial to the superconductivity in general [1][2][3]. This subject has also been extensively studied both experimentally and theoretically in the recent discovered iron pnictides [4], where high-temperature superconductivity is achieved by suppressing a competing spin-density-wave (SDW) state upon chemical doping or pressure [5][6][7]). Here the superconductivity emerging in proximity to a spin-density-wave (SDW) quantum critical point (QCP), as well as the persisting spin fluctuations shown above T C [8], support strongly that superconducting pairing is mediated by spin fluctuations.However, in an iron pnictide LiFeAs, bulk superconductivity up to 18 K, instead of long-range antiferromagnetism (AFM), is found in the ground state without nominal doping [9][10][11]. Angle-resolved photoemission spectroscopy (ARPES) studies [12][13][14] do not see evidence of spin fluctuations. In particular, the superconducting gap seems to be a single isotropic gap with a moderate amplitude [12][13][14], in contrast to the multiple gaps in other iron pnictides which is likely originated from Fermi surface nesting and spin fluctuations [15][16][17][18][19][20]. The µSR studies show that LiFeAs has a different Uemura relation with other pnictide superconductors [21]. These facts lead to an everlasting proposal that LiFeAs is a conventional superconductor, rather than a strongly correlated superconductor.Theoretically, the local density approximation (LDA) calculations indicate that LiFeAs has a similar band structure with LaFeAsO, BaFe 2 As 2 and NaFeAs, and therefore a similar magnetic ordering in the undoped and a universal origin of superconductivity in the doped materials are expected among all compounds [22][23][24][25]. Indeed, particularly for the sister compound NaFeAs with the same 111 structure, the magnetism [26,27] and superconducting properties [28,29] Therefore the study of whether LiFeAs is a strongly correlated superconductor is certainly important for understanding the correlation among the band structure, the magnetism and the mechanism of superconductivity of the high T c pnictides. In order to resolve this problem, we performed NMR studies on LiFeAs single c...

We report a high-pressure 75 As NMR study on the heavily hole-doped iron pnictide superconductor KFe2As2 (Tc ≈ 3.8 K). The low-energy spin fluctuations are found to decrease with applied pressure up to 2 GPa, but then increase again, changing in lockstep with the pressure-induced evolution of Tc. Their diverging nature suggests close proximity to a magnetic quantum critical point at a negative pressure of P ≃ −0.6 GPa. Above 2.4 GPa, the 75 As satellite spectra split below 40 K, indicating a breaking of As site symmetry and an incipient charge order. These pressure-controlled phenomena demonstrate the presence of nearly-critical fluctuations in both spin and charge, providing essential input for the origin of superconductivity. Heavily doped FeSCs, whose Fermi surfaces are quite different from optimally doped materials, challenge the existing understanding. KFe 2 As 2 has large hole doping (0.5 hole/Fe), but far from being a regular metal it shows heavy-fermion characteristics below a low coherence temperature of order 60 K [8] and superconductivity at a low but finite T c of 3.8 K [9,10]. The absence of electron pockets around (π, π) [11] suggests that spin fluctuations from interband nesting are unlikely, but low-energy electronic correlations are surprisingly strong. Similarly strong low-energy spin fluctuations [12,13] Recent high-pressure studies of KFe 2 As 2 discovered an anomalous reversal of T c , which has a minimum at 1.8 GPa [18]. Scenarios proposed to explain this include a change of pairing symmetry [18,19] and a k z modulation of the superconducting gap [20]. Although spin fluctuations are essential to FeSC superconductivity, no measurements under pressure have yet been reported.In this Letter, we present a high-pressure study of KFe 2 As 2 by nuclear magnetic resonance (NMR). The 75 As spectra and spin-lattice relaxation rate (1/ 75 T 1 T ) are measured under pressures up to 2.42 GPa, revealing three surprising features. First, 1/ 75 T 1 T is dominated by strong low-energy spin fluctuations, suggesting incipient antiferromagnetic order at a quantum critical point near −0.6 GPa. Second, the spin fluctuations show exactly the same reversal behavior as T c , and indeed identical evolution at all pressures. Third, a line splitting of the 75 As satellite spectra below 40 K for pressures above 2.4 GPa indicates a breaking of four-fold symmetry. This effect is caused by charge order, whose fluctuations we propose are strong around the d 5.5 electron filling of KFe 2 As 2 . This emergent charge order is accompanied by the enhancement of spin fluctuations and hence of T c , demonstrating the importance of nearly-critical charge fluctuations in heavily hole-doped FeSCs.Our KFe 2 As 2 single crystals were synthesized by the self-flux method [21]. We measure very large residual resistivity ratios of 1390, indicating extremely high sample quality. We performed high-pressure NMR measurements using a NiCrAl clamp cell, which reaches a maximum pressure of 2.42 GPa at T = 2 K; to obtain a maximally hydrostatic ...

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A mathematical model is developed for the molecular weight distribution (MWD) of free-radical styrene polymerization in a simulated semi-batch reactor system. The generation function technique and moment method are employed to establish the MWD model in the form of Schultz-Zimmdistribution. Both static and dynamic models are described in detail. In order to achieve the closed-loop MWD shaping by output probability density function (PDF) control, the dynamic MWD model is further developed by a linear B-spline approximation. Based on the general form of the B-spline MWD model, iterative learning PDF control strategies have been investigated in order to improve the MWD control performance. Discussions on the simulation studies show the advantages and limitations of the methodology