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 ...

We use (75)As nuclear magnetic resonance to investigate the local electronic properties of Ba(Fe(1-x)Ru(x))(2)As(2) (x = 0.23). We find two phase transitions: to antiferromagnetism at T(N) ≈ 60 K and to superconductivity at T(C) ≈ 15 K. Below T(N), our data show that the system is fully magnetic, with a commensurate antiferromagnetic structure and a moment of 0.4μ(B)/Fe. The spin-lattice relaxation rate 1/(75)T(1) is large in the magnetic state, indicating a high density of itinerant electrons induced by Ru doping. On cooling below T(C), 1/(75)T(1) on the magnetic sites falls sharply, providing unambiguous evidence for the microscopic coexistence of antiferromagnetism and superconductivity.

Oxidative injury has been implicated in the aetiology of Parkinson's disease (PD) and gypenosides (GP), which are saponins with various bioactivities, have shown antioxidative effects in vitro. The present study was designed to evaluate the effect of GP on a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mouse model of PD. Acute administration of MPTP led to decreased glutathione content and reduced superoxide dismutase activity in the substantia nigra of the mice, which resulted in oxidative stress, loss of nigral dopaminergic neurons and motor dysfunction. Co-treatment with GP attenuated all the injuries induced by MPTP in a dose-dependent manner. The neuroprotective effect of GP may be attributed to increased antioxidation, as manifested by significantly increased glutathione content and enhanced superoxide dismutase activity in the substantia nigra. These results strongly indicate the possible therapeutic potential of GP as an antioxidant in PD.

We report 75As NMR studies on single crystals of rare-earth doped iron pnictide superconductor Ca1−xPrxFe2As2. In both cases of x = 0.075, 0.15, a large increase of νq upon cooling is consistent with the tetragonal-collapsed tetragonal structure transition. A sharp drop of the Knight shift is also seen just below the structure transition, which suggests the quenching of Fe local magnetism, and therefore offers important understanding of the collapsed tetragonal phase. At even low temperatures, the 1/75T1 is enhanced and forms a peak at T ≈ 25 K, which may be caused by the magnetic ordering of the Pr3+ moments or spin dynamics of mobile domain walls.

Ba8CoNb6O24 presents a system whose Co 2+ ions have an effective spin 1/2 and construct a regular triangular-lattice antiferromagnet (TLAFM) with a very large interlayer spacing, ensuring purely two-dimensional character. We exploit this ideal realization to perform a detailed experimental analysis of the S = 1/2 TLAFM, which is one of the keystone models in frustrated quantum magnetism. We find strong low-energy spin fluctuations and no magnetic ordering, but a diverging correlation length down to 0.1 K, indicating a Mermin-Wagner trend towards zero-temperature order. Below 0.1 K, however, our low-field measurements show an unexpected magnetically disordered state, which is a candidate quantum spin liquid. We establish the (H, T ) phase diagram, mapping in detail the quantum fluctuation corrections to the available theoretical analysis. These include a strong upshift in field of the maximum ordering temperature, qualitative changes to both low-and high-field phase boundaries, and an ordered regime apparently dominated by the collinear "up-up-down" state. Ba8CoNb6O24 therefore offers fresh input for the development of theoretical approaches to the field-induced quantum phase transitions of the S = 1/2 Heisenberg TLAFM.

No abstract

No abstract

We report nuclear magnetic resonance (NMR) studies of the intercalated iron selenide superconductor (Tl, Rb)yFe2−xSe2 (Tc = 32 K). Single-crystal measurements up to 480 K on both 77 Se and 87 Rb nuclei show a superconducting phase with no magnetic order. The Knight shifts K and relaxation rates 1/T1T increase very strongly with temperature above Tc, before flattening at 400 K. The quadratic T -dependence and perfect proportionality of both K and 1/T1T data demonstrate their origin in paramagnetic moments. A minimal model for this pseudogap-like response is not a missing density of states but two additive contributions from the itinerant electronic and local magnetic components, a framework unifying the K and 1/T1T data in many iron-based superconductors.