Unlike the parent phases of the iron-arsenide high-Tc superconductors, undoped FeSe is not magnetically ordered and exhibits superconductivity with Tc approximately 9 K. Equally surprising is the fact that applied pressure dramatically enhances the modest Tc to approximately 37 K. We investigate the electronic properties of FeSe using 77Se NMR to search for the key to the superconducting mechanism. We demonstrate that the electronic properties of FeSe are very similar to those of electron-doped FeAs superconductors, and that antiferromagnetic spin fluctuations are strongly enhanced near Tc. Furthermore, applied pressure enhances spin fluctuations. Our findings suggest a link between spin fluctuations and the superconducting mechanism in FeSe.

We report the first NMR investigation of spin dynamics in the overdoped nonsuperconducting regime of Ba(Fe1-xCox)2As2 up to x=0.26. We demonstrate that the absence of interband transitions with large momentum transfer Q{AF} approximately (pi/a,0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x greater than or approximately 0.15. Our experimental results provide direct evidence for a correlation between T{c} and the strength of Q{AF} antiferromagnetic spin fluctuations.

We report a systematic investigation of Ba(Fe 1Àx Co x ) 2 As 2 based on transport and 75 As NMR measurements, and establish the electronic phase diagram. We demonstrate that doping progressively suppresses the uniform spin susceptibility and low frequency spin fluctuations. The optimum superconducting phase emerges at x c ' 0:08 when the tendency toward spin ordering completely diminishes. Our findings point toward the presence of a quantum critical point near x c between the SDW (spin density wave) and superconducting phases.KEYWORDS: iron pnictide superconductor, high temperature superconductivity, NMR, quantum criticality DOI: 10.1143/JPSJ.78.013711The recent discovery of iron-pnictide high T c superconductors 1-6) poses a new intellectual challenge in condensed matter physics. Although the superconducting mechanism remains unknown, it has become apparent that ironpnictides share remarkable similarities with high T c cuprates: the undoped parent phases LaFeAsO and BaFe 2 As 2 have FeAs square-lattice sheets, and itinerant electrons in these layers undergo antiferromagnetic long range order (AFLRO) below modest temperatures, T SDW $ 140 K; [7][8][9] electron or hole doping suppresses the AFLRO and induces superconductvity. There exist clear dissimilarities, too. For example, all five 3d orbitals of Fe atoms contribute to the multiple Fermi surfaces, hence to superconductivity, in ironpnictides. 10,11) In contrast, only the Cu 3d x 2 Ày 2 orbital plays a role in curates. Furthermore, substitution of Co atoms into Fe sites of the parent phases results in electron doping and induces superconductivity [12][13][14] without creating localized moments, 15) while Zn 2þ ions doped into Cu 2þ sites induce localized moments and destroy superconductivity in cuprates. Sorting out these similarities and dissimilarities may lead us to an understanding the mechanism of high T c superconductivity in iron-pnictides as well as in cuprates.In this letter, we utilize transport and NMR techniques to probe the evolution of the electronic properties of Ba(Fe 1Àx -Co x ) 2 As 2 single crystals from the undoped SDW phase (x ¼ 0 with T SDW ¼ 135 K), underdoped SDW phase (x ¼ 0:02 with T SDW ¼ 100 K, and x ¼ 0:04 with T SDW ¼ 66 K), optimally doped superconducting phase (x ¼ 0:08 with T c ¼ 22 K), to the slightly overdoped regime (x ¼ 0:105 with T c ¼ 15 K). We establish the electronic phase diagram which has a quantum critical point near the optimum concentration x c $ 0:08, and explore the possible relation between paramagnetic spin fluctuations and the mechanism of superconductivity. From NMR Knight shift and spin-lattice relaxation rate measurements, we show that electron doping progressively suppresses the uniform spin susceptibility spin and low frequency spin fluctuations. The optimally doped superconducting phase emerges at x c $ 0:08 as soon as doped electrons completely suppress the tendency toward AFLRO.We grew single crystals of Ba(Fe 1Àx Co x ) 2 As 2 from FeAs flux, 13) and determined the actual Co concentration x by energ...

Diluted magnetic semiconductors have received much attention due to their potential applications for spintronics devices. A prototypical system (Ga,Mn)As has been widely studied since the 1990s. The simultaneous spin and charge doping via hetero-valent (Ga 3 þ ,Mn 2 þ ) substitution, however, resulted in severely limited solubility without availability of bulk specimens. Here we report the synthesis of a new diluted magnetic semiconductor (Ba 1 À x K x )(Zn 1 À y Mn y ) 2 As 2 , which is isostructural to the 122 iron-based superconductors with the tetragonal ThCr 2 Si 2 (122) structure. Holes are doped via (Ba 2 þ , K 1 þ ) replacements, while spins via isovalent (Zn 2 þ ,Mn 2 þ ) substitutions. Bulk samples with x ¼ 0.1 À 0.3 and y ¼ 0.05 À 0.15 exhibit ferromagnetic order with T C up to 180 K, which is comparable to the highest T C for (Ga,Mn)As and significantly enhanced from T C up to 50 K of the '111'-based Li(Zn,Mn)As. Moreover, ferromagnetic (Ba,K)(Zn,Mn) 2 As 2 shares the same 122 crystal structure with semiconducting BaZn 2 As 2 , antiferromagnetic BaMn 2 As 2 and superconducting (Ba,K)Fe 2 As 2 , which makes them promising for the development of multilayer functional devices.

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In a prototypical ferromagnet (Ga,mn)As based on a III-V semiconductor, substitution of divalent mn atoms into trivalent Ga sites leads to severely limited chemical solubility and metastable specimens available only as thin films. The doping of hole carriers via (Ga,mn) substitution also prohibits electron doping. To overcome these difficulties, masek et al. theoretically proposed systems based on a I-II-V semiconductor LiZnAs, where isovalent (Zn,mn) substitution is decoupled from carrier doping with excess/deficient Li concentrations. Here we show successful synthesis of Li 1 + y (Zn 1 − x mn x )As in bulk materials. Ferromagnetism with a critical temperature of up to 50 K is observed in nominally Li-excess (y = 0.05-0.2) compounds with mn concentrations of x = 0.02-0.15, which have p-type metallic carriers. This is presumably due to excess Li in substitutional Zn sites. semiconducting LiZnAs, ferromagnetic Li(Zn,mn)As, antiferromagnetic LimnAs, and superconducting LiFeAs systems share square lattice As layers, which may enable development of novel junction devices in the future.

We report ^{75}As NMR measurements on the new quasi-one-dimensional superconductor K_{2}Cr_{3}As_{3} (T_{c}∼6.1  K) [J. K. Bao et al., Phys. Rev. X 5, 011013 (2015)]. We found evidence for strong enhancement of Cr spin fluctuations above T_{c} in the [Cr_{3}As_{3}]_{∞} double-walled subnanotubes based on the nuclear spin-lattice relaxation rate 1/T_{1}. The power-law temperature dependence, 1/T_{1}T∼T^{-γ} (γ∼0.25), is consistent with the Tomonaga-Luttinger liquid. Moreover, absence of the Hebel-Slichter coherence peak of 1/T_{1} just below T_{c} suggests an unconventional nature of superconductivity.

We report on 19 F nuclear magnetic resonance ͑NMR͒ investigation of the high-temperature superconductor LaFeAsO 0.89 F 0.11 ͑T c ϳ 28 K͒. We demonstrate that low-frequency spin fluctuations exhibit pseudogap behavior above T c . We also deduce the London penetration depth from NMR line broadening below T c .