The challenges of developing neuromorphic vision systems inspired by the human eye come not only from how to recreate the flexibility, sophistication, and adaptability of animal systems, but also how to do so with computational efficiency and elegance. Similar to biological systems, these neuromorphic circuits integrate functions of image sensing, memory and processing into the device, and process continuous analog brightness signal in real-time. High-integration, flexibility and ultra-sensitivity are essential for practical artificial vision systems that attempt to emulate biological processing. Here, we present a flexible optoelectronic sensor array of 1024 pixels using a combination of carbon nanotubes and perovskite quantum dots as active materials for an efficient neuromorphic vision system. The device has an extraordinary sensitivity to light with a responsivity of 5.1 × 107 A/W and a specific detectivity of 2 × 1016 Jones, and demonstrates neuromorphic reinforcement learning by training the sensor array with a weak light pulse of 1 μW/cm2.

-75 As nuclear magnetic resonance (NMR) experiments were performed on Ba(Fe 1-x Mn x ) 2 As 2 ( x Mn = 2.5%, 5% and 12%) single crystals. The Fe layer magnetic susceptibility far from Mn atoms is probed by the 75 As NMR line shift and is found similar to that of BaFe 2 As 2 , implying that Mn does not induce charge doping. A satellite line associated with the Mn nearest neighbours (n.n.) of 75 As displays a Curie-Weiss shift which demonstrates that Mn carries a local magnetic moment. This is confirmed by the main line broadening typical of a RKKY-like Mn-induced staggered spin polarization. The Mn moment is due to the localization of the additional Mn hole. These findings explain why Mn does not induce superconductivity in the pnictides contrary to other dopants such as Co, Ni, Ru or K. [7]. The resulting phase diagram is very similar in all these cases. It looks as if the only requirement to obtain superconductivity is to destroy sufficiently the antiferromagnetic (AF) state, whatever the mechanism at play.Remarkably, there is an exception to this generic behavior: the substitution of Fe by hole-dopant atoms, namely Cr or Mn. Indeed, Mn and Cr substituted at Fe site do not lead to a superconducting state [8][9][10][11]. Neutron diffraction and magnetotransport measurements reveal that at low concentration, Mn or Cr substitutions reduce the ordering temperature T N by 6 K/% and the ordered moment amplitude remains unaffected [9][10]. Above x = 30% for Cr and x = 12% for Mn, new types of AF states appear whose T N even increases with further substitution. It is a G-type AF ordering for Cr doping [9], whereas Mn doping invokes an unusual tetragonal AF state with no orthorhombic distortion as in the parent compound [10]. Similar qualitative features are also observed in SrFe 2 As 2 with Mn doping [11]. This asymmetry between the effects of hole (Mn, Cr) and electron (Co, Ni) dopants is surprising and still unexplained. The question arises as to why

We report results of inelastic-neutron-scattering measurements of low-energy spin-wave excitations in two structurally distinct families of iron-pnictide parent compounds: Na 1−δ FeAs and BaFe 2 As 2 . Despite their very different values of the ordered magnetic moment and Néel temperatures, T N , in the antiferromagnetic state both compounds exhibit similar spin gaps of the order of 10 meV at the magnetic Brillouin-zone center. The gap opens sharply below T N , with no signatures of a precursor gap at temperatures between the orthorhombic and magnetic phase transitions in Na 1−δ FeAs. We also find a relatively weak dispersion of the spin-wave gap in BaFe 2 As 2 along the out-of-plane momentum component, q z . At the magnetic zone boundary (q z = 0), spin excitations in the ordered state persist down to ∼ 20 meV, which implies a much smaller value of the effective out-of-plane exchange interaction, J c , as compared to previous estimates based on fitting the high-energy spin-wave dispersion to a Heisenberg-type model. The discovery of unconventional superconductivity in iron-pnictide compounds [1] with critical temperatures, T c , as high as 56 K has fostered a tremendous interest in these materials in recent years [2]. There are several structurally distinct families of iron-based superconductors with similar phase diagrams [2], governed by an interplay of antiferromagnetism, persistent in pure compounds under ambient pressure, and superconductivity that can be induced by pressure or chemical doping [3]. Although the highest values of T c are usually found in doped compounds with a nonstoichiometric composition, our physical understanding of these systems undoubtedly depends on the detailed knowledge of magnetic properties in the respective parent compounds, which are also easier to treat theoretically due to their simple crystal structure with no substitutional disorder.Among the variety of such stoichiometric materials serving as parent phases for numerous iron-based superconductors, only a few have so far received proper experimental treatment, especially by inelastic neutron scattering (INS), due to miscellaneous reasons related to the availability of sizeable single crystals or their chemical stability. For instance, to the best of our knowledge, direct measurements of spin-wave excitations in the antiferromagnetic (AFM) state of iron pnictides have so far remained limited to a few members of the so-called '122' family with the ThCr 2 Si 2 -type structure, whose single crystals are typically stable in air and are readily available in the large sizes necessary for INS experiments. In particular, high-energy spin-wave modes have been mapped out in CaFe 2 As 2 [4, 5] and BaFe 2 As 2 [6] using time-of-flight (TOF) neutron spectroscopy, which enabled estimations of the effective magnetic exchange interactions in the framework of a localized Heisenberg-type J 1a -J 1b -J 2 -J c model. These results are complemented by INS measurements at lower energies, performed on polycrystalline BaFe 2 As 2 [7] and on single c...

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We demonstrate the preparation of large, free standing iron pnictide single crystals with a size up to 20 x 10 x 1 mm 3 using solvents in zirconia crucibles under argon atmosphere. Transport and magnetic properties are investigated to study the effect of potassium doping on the structural and superconducting property of the compounds. The spin density wave (SDW) anomaly at T s ~138 K in BaFe 2 As 2 single crystals from self-flux shifts to T s ~85 K due to Sn solvent growth. We show direct evidence for an incorporation of Sn on the Fe site. The electrical resistivity data show a sharp superconducting transition temperature T c~3 8.5 K for the single crystal of Ba 0.68 K 0.32 Fe 2 As 2.A nearly 100% shielding fraction and bulk nature of the superconductivity for the single crystal were confirmed by magnetic susceptibility data. A sharp transition T c~2 5 K occurred for the single crystal of Sr 0.85 K 0.15 Fe 2 As 2 . There is direct evidence for a coexistence of the SDW and superconductivity in the low doping regime of Sr 1-x K x Fe 2 As 2 single crystals. Structural implications of the doping effects as well as the coexistence of the two order parameters are discussed.