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The nematic phase in iron based superconductors (IBSs) has attracted attention with a notion that it may provide important clue to the superconductivity. A series of angle-resolved photoemission spectroscopy (ARPES) studies were performed to understand the origin of the nematic phase. However, there is lack of ARPES study on LaFeAsO nematic phase. Here, we report the results of ARPES studies of the nematic phase in LaFeAsO. Degeneracy breaking between the $$d_{xz}$$ d xz and $$d_{yz}$$ d yz hole bands near the $$\Gamma$$ Γ and M point is observed in the nematic phase. Different temperature dependent band splitting behaviors are observed at the $$\Gamma$$ Γ and M points. The energy of the band splitting near the M point decreases as the temperature decreases while it has little temperature dependence near the $$\Gamma$$ Γ point. The nematic nature of the band shift near the M point is confirmed through a detwin experiment using a piezo device. Since a momentum dependent splitting behavior has been observed in other iron based superconductors, our observation confirms that the behavior is a universal one among iron based superconductors.

The nematic phase in iron based superconductors (IBSs) has attracted attention with a notion that it may provide important clue to the superconductivity. A series of angle-resolved photoemission spectroscopy (ARPES) studies were performed to understand the origin of the nematic phase. However, there is lack of ARPES study on LaFeAsO nematic phase. Here, we report the results of ARPES studies of the nematic phase in LaFeAsO. Degeneracy breaking between the $$d_{xz}$$ d xz and $$d_{yz}$$ d yz hole bands near the $$\Gamma$$ Γ and M point is observed in the nematic phase. Different temperature dependent band splitting behaviors are observed at the $$\Gamma$$ Γ and M points. The energy of the band splitting near the M point decreases as the temperature decreases while it has little temperature dependence near the $$\Gamma$$ Γ point. The nematic nature of the band shift near the M point is confirmed through a detwin experiment using a piezo device. Since a momentum dependent splitting behavior has been observed in other iron based superconductors, our observation confirms that the behavior is a universal one among iron based superconductors.

The nematic phase in iron based superconductors (IBSs) has attracted attention with a notion that it may provide important clue to the superconductivity. A series of angle-resolved photoemission spectroscopy (ARPES) studies were performed to understand the origin of the nematic phase. However, there is lack of ARPES study on LaFeAsO nematic phase. Here, we report the results of ARPES studies of the nematic phase in LaFeAsO. Degeneracy breaking between the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$d_{xz}$$\end{document}dxz and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$d_{yz}$$\end{document}dyz hole bands near the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Gamma$$\end{document}Γ and M point is observed in the nematic phase. Different temperature dependent band splitting behaviors are observed at the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Gamma$$\end{document}Γ and M points. The energy of the band splitting near the M point decreases as the temperature decreases while it has little temperature dependence near the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Gamma$$\end{document}Γ point. The nematic nature of the band shift near the M point is confirmed through a detwin experiment using a piezo device. Since a momentum dependent splitting behavior has been observed in other iron based superconductors, our observation confirms that the behavior is a universal one among iron based superconductors.

We report a systematic elastoresistivity study on ${\mathrm{LaFe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{AsO}$ single crystals, which have well separated structural and magnetic transition lines. All crystals show a Curie-Weiss--like nematic susceptibility in the tetragonal phase. The extracted nematic temperature is monotonically suppressed upon cobalt doping, and changes sign around the optimal doping level, indicating a possible nematic quantum critical point beneath the superconducting dome. The amplitude of the nematic susceptibility shows a peculiar double-peak feature. This could be explained by a combined effect of different contributions to the nematic susceptibility, which are amplified at separated doping levels of ${\mathrm{LaFe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{AsO}$.

Angle-resolved photoemission spectroscopy (ARPES) is used to study the energy and momentum dependence of the inelastic scattering rates and the mass renormalization of charge carriers in LiFeAs at several high symmetry points in the Brillouin zone. A strong and linear-in-energy scattering rate is observed for sections of the Fermi surface having predominantly Fe $3d_{xy/yz}$ orbital character on the inner hole and on electron pockets. We assign them to hot spots with marginal Fermi liquid character inducing high antiferromagnetic and pairing susceptibilities. The outer hole pocket, with Fe $3d_{xy}$ orbital character, has a reduced but still linear in energy scattering rate. Finally, we assign sections on the middle hole pockets with Fe $3d_{xz,yz}$ orbital character and on the electron pockets with Fe $3d_{xy}$ orbital character to cold spots because there we observe a quadratic-in-energy scattering rate with Fermi-liquid behavior. These cold spots prevail the transport properties. Our results indicate a strong $\it{momentum}$ dependence of the scattering rates. We also have indications that the scattering rates in correlated systems are fundamentally different from those in non-correlated materials because in the former the Pauli principle is not operative. We compare our results for the scattering rates with combined density functional plus dynamical mean-field theory calculations. The work provides a generic microscopic understanding of macroscopic properties of multiorbital unconventional superconductors., 6 pages, 2 figures

Muon spin rotation ($\ensuremath{\mu}\mathrm{SR}$) and $^{19}\mathrm{F}$ nuclear magnetic resonance (NMR) measurements were performed to investigate the effect of Mn for Fe substitutions in ${\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Y}}_{y}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors. While for $y=0$ a very low critical concentration of Mn ($x=0.2%$) is needed to quench superconductivity, as $y$ increases the negative chemical pressure introduced by Y for La substitution stabilizes superconductivity and for $y=20%$ it is suppressed at Mn contents an order of magnitude larger. A magnetic phase arises once superconductivity is suppressed both for $y=0$ and for $y=20%$. Low-energy spin fluctuations give rise to a peak in $^{19}\mathrm{F}$ NMR $1/{T}_{1}$ with an onset well above the superconducting transition temperature and whose magnitude increases with $x$. Also the static magnetic correlations probed by $^{19}\mathrm{F}$ NMR linewidth measurements show a marked increase with Mn content. The disruption of superconductivity and the onset of the magnetic ground state are discussed in the light of the proximity of ${\mathrm{LaFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$ to a quantum critical point.

$^{19}\mathrm{F}$ NMR measurements in optimally electron-doped ${\mathrm{La}}_{1\text{\ensuremath{-}}y}{\mathrm{Y}}_{y}{\mathrm{Fe}}_{1\text{\ensuremath{-}}x}{\mathrm{Mn}}_{x}{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors are presented. The effect of Mn doping on the superconducting phase is studied for two series of compounds $(y=0$ and $y=0.2)$ where the chemical pressure is varied by substituting La with Y. In the $y=0.2$ series superconductivity is suppressed for Mn contents an order of magnitude larger than for $y=0$. For both series a peak in the $^{19}\mathrm{F}$ NMR nuclear spin-lattice relaxation rate $1/{T}_{1}$ emerges upon Mn doping and becomes significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. $^{19}\mathrm{F}$ NMR linewidth measurements show that for similar Mn contents magnetic correlations are more pronounced in the $y=0$ series, at variance with what one would expect for $\stackrel{P\vec}{Q}=(\ensuremath{\pi}/a,0)$ spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at $\stackrel{P\vec}{Q}=(\ensuremath{\pi}/a,0)$ and to enhance other low-frequency modes. The effect of this transfer of spectral weight on the superconducting pairing is discussed along with the charge localization induced by Mn.