We use neutron spectroscopy to determine the nature of the magnetic excitations in superconducting BaFe 1.9 Ni 0.1 As 2 ͑T c =20 K͒. Above T c the excitations are gapless and centered at the commensurate antiferromagnetic wave vector of the parent compound, while the intensity exhibits a sinusoidal modulation along the c axis. As the superconducting state is entered a spin gap gradually opens, whose magnitude tracks the T dependence of the superconducting gap as observed by angle-resolved photoemission. Both the spin-gap and magnetic-resonance energies are temperature and wave-vector dependent, but their ratio is the same within uncertainties. These results suggest that the spin resonance is a singlet-triplet excitation related to electron pairing and superconductivity.
We used high-resolution scanning tunneling spectroscopy to study the hole-doped iron pnictide superconductor Ba(0.6)K(0.4)Fe(2)As(2) (T(c)=38 K). Features of a bosonic excitation (mode) are observed in the measured quasiparticle density of states. The bosonic features are intimately associated with the superconducting order parameter and have a mode energy of ~14 meV, similar to the spin resonance measured by inelastic neutron scattering. These results indicate a strong electron-spin excitation coupling in iron pnictide superconductors, similar to that in high-T(c) copper oxide superconductors.
A determination of the superconducting (SC) electron pairing symmetry forms the basis for establishing a microscopic mechanism for superconductivity. For iron pnictide superconductors, the s ± -pairing symmetry theory predicts the presence of a sharp neutron spin resonance at an energy below the sum of hole and electron SC gap energies (E 2 ) below T c . On the other hand, the s ++ -pairing symmetry expects a broad spin excitation enhancement at an energy above 2 below T c . Although the resonance has been observed in iron pnictide superconductors at an energy below 2 consistent with the s ± -pairing symmetry, the mode has also been interpreted as arising from the s ++ -pairing symmetry with E 2 due to its broad energy width and the large uncertainty in determining the SC gaps. Here we use inelastic neutron scattering to reveal a sharp resonance at E = 7 meV in SC NaFe 0.935 Co 0.045 As (T c = 18 K). On warming towards T c , the mode energy hardly softens while its energy width increases rapidly. By comparing with calculated spin-excitation spectra within the s ± and s ++ -pairing symmetries, we conclude that the ground-state resonance in NaFe 0.935 Co 0.045 As is only consistent with the s ± pairing, and is inconsistent with the s ++ -pairing symmetry.
We report the discovery of superconductivity on high-quality single crystals of transition-metal pnictides WP grown by chemical vapor transport (CVT) method.Bulk superconductivity is observed at T c = 0.84 K under ambient pressure by electrical resistivity and AC magnetic susceptibility measurements. The effects of magnetic field on the superconducting transitions are studied, leading to a large anisotropy parameter around 2 with the in-plane and out-of-plane upper critical fields of 2,∥ =172 Oe and 2,⊥ =85 Oe, respectively. Our finding demonstrates that WP is the first superconductor in 5d transition-metal at ambient pressure in MnP-type, which will help to search for new superconductors in transition-metal pnictides.
The origin of nematic order remains one of the major debates in iron-based superconductors. In theories based on spin nematicity, one major prediction is that the spin-spin correlation length at (0,π) should decrease with decreasing temperature below the structural transition temperature T_{s}. Here, we report inelastic neutron scattering studies on the low-energy spin fluctuations in BaFe_{1.935}Ni_{0.065}As_{2} under uniaxial pressure. Both intensity and spin-spin correlation start to show anisotropic behavior at high temperature, while the reduction of the spin-spin correlation length at (0,π) happens just below T_{s}, suggesting the strong effect of nematic order on low-energy spin fluctuations. Our results favor the idea that treats the spin degree of freedom as the driving force of the electronic nematic order.
Background:Parkinson disease (PD) is a neurodegenerative disease characterized by chronic and progressive loss of dopaminergic neurons in substansia nigra pars compacta. Oxidative stress is proposed to play a critical role in the pathogenesis of PD. Uric acid (UA), as an important physiological antioxidant, is identified a molecular predictor associated with a decreased risk and a slower disease progression for PD and potential neuroprotectant of PD by increasing epidemiological and clinical evidences. Within this review, we will present a comprehensive overview of the data linking UA to PD in recent years.Methods:We searched PubMed, EMBASE, Web of Science databases for relevant studies. Any observational or experimental studies that evaluated UA and PD were our goal of searching the electric databases.Results:Twelve studies that evaluated UA and PD were identified in this review. We reviewed the roles of UA in the pathogenesis of PD, the association of UA with morbidity, severity/progression, nonmotor symptoms, motor complications of PD, with an attempt to provide new ideas for diagnosis and treatment in PD.Conclusion:Our findings supported that lots of clinical and epidemiological data observed lower UA levels in PD patients. Manipulation of UA or its precursors’ concentration could be effective to treat or prevent PD. However, it is still suspectable that higher UA levels are better enough to PD patients. Furthermore, for the complex nature of PD and its heterogeneous genetic and environmental influences, it is inadequate for just manipulating UA in treating the disease.
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