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Register a new userNew Superconductivity Dome in LaFeAsO$_{1-x}$F$_{x}$ Accompanied by Structural Transition
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High temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO$_{1-x}$F$_{x}$ ($x leq$ 0.2) and many other iron-based superconductors, which leads to proposals that superconductivity is mediated by fluctuations associated with the nearby magnetism. Here we report the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO$_{1-x}$F$_{x}$ with 0.25$leq x leq$0.75, where the maximal critical temperature $T_c$ at $x_{opt}$ = 0.5$sim$0.55 is even higher than that at $x leq$ 0.2. By nuclear magnetic resonance and Transmission Electron Microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for $x>$ 0.5 above $T_c$. Our results point to a new paradigm of high temperature superconductivity.
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We report $^{75}$As nuclear magnetic resonance (NMR) / nuclear quadrupole resonance (NQR) and transmission electron microscopy (TEM) studies on LaFeAsO$_{1-x}$F$_{x}$. There are two superconducting domes in this material. The first one appears at 0.03 $leq$ $x$ $leq$ 0.2 with $T_{rm c}$$^{max}$ = 27 K, and the second one at 0.25 $leq$ $x$ $leq$ 0.75 with $T_{rm c}$$^{max}$ = 30 K. By NMR and TEM, we demonstrate that a $C4$-to-$C2$ structural phase transition (SPT) takes place above both domes, with the transition temperature $T_{rm s}$ varying strongly with $x$. In the first dome, the SPT is followed by an antiferromagnetic (AF) transition, but neither AF order nor low-energy spin fluctuations are found in the second dome. In LaFeAsO$_{0.97}$F$_{0.03}$, we find that AF order and superconductivity coexist microscopically via $^{75}$As nuclear spin-lattice relaxation rate (1/$T_1$) measurements. In the coexisting region, 1/$T_1$ decreases at $T_{rm c}$ but becomes to be proportional to $T$ below 0.6$T_{rm c}$, indicating gapless excitations. Therefore, in contrast to the early reports, the obtained phase diagram for $x leq$ 0.2 is quite similar to the doped BaFe$_{2}$As$_{2}$ system. The electrical resistivity in the second dome can be fitted by $rho = {{rho }_{0}}+A{{T}^{n}}$ with $n$ = 1 and a maximal coefficient $A$ at around $x_{opt}$ = 0.5$sim$0.55 where $T_{rm s}$ extrapolates to zero and $T_{rm c}$ is the maximal, which suggest the importance of quantum critical fluctuations associated with the SPT. We have constructed a complete phase diagram of LaFeAsO$_{1-x}$F$_{x}$, which provides insight into the relationship between SPT, antiferromagnetism and superconductivity.
Low energy spin fluctuations are studied for the electron-doped Fe-based superconductor LaFeAsO(1-x)F(x) by inelastic neutron scattering up to the energy transfer of w = 15 meV using polycrystalline samples. Superconducting samples (x=0.057, Tc=25 K and x=0.082, Tc=29 K) show dynamical spin susceptibility chi(w) almost comparable with the parent samples. However chi(w) is almost vanished in the x=0.158 sample where the superconductivity is highly suppressed. These results are compatible with the theoretical suggestions that the spin fluctuation plays an important role for the superconductivity.
Orbital ordering has recently emerged as another important state in iron based superconductors, and its role for superconductivity as well as its connection to magnetic order and orthorhombic lattice distortion are heavily debated. In order to search for signatures of this so-called nematic phase in oxypnictides, we revisit the normal state properties of the pnictide superconductor LaFeAsO$_{1-x}$F$_x$ with a focus on resistivity, Nernst effect, thermal expansion, and $^{75}$As NMR data. The transport properties at the underdoped level $x=0.05$ exhibit pronounced anomalies at about the same temperature where undoped LaFeAsO develops long-range nematic ordering, i.e. at about 160 K. Furthermore, the $^{75}$As-NMR spin-lattice relaxation rate $1/(T_1T)$ reveals a progressive slowing down of spin fluctuations. Yet, long-range magnetic order and also a detectable orthorhombic lattice distortion are absent. Thus, we conclude from the data that short-range orbital-nematic ordering or a slowly fluctuating form of it sets in near 160 K. Remarkably, all anomalies in the transport and also the indications of slow spin fluctuations disappear close to optimal doping $x=0.1$ which suggests that in LaFeAsO$_{1-x}$F$_x$ the nematic phase actually competes with superconductivity.
We use neutron scattering to study the structural distortion and antiferromagnetic (AFM) order in LaFeAsO$_{1-x}$F$_{x}$ as the system is doped with fluorine (F) to induce superconductivity. In the undoped state, LaFeAsO exhibits a structural distortion, changing the symmetry from tetragonal (space group $P4/nmm$) to orthorhombic (space group $Cmma$) at 155 K, and then followed by an AFM order at 137 K. Doping the system with F gradually decreases the structural distortion temperature, but suppresses the long range AFM order before the emergence of superconductivity. Therefore, while superconductivity in these Fe oxypnictides can survive in either the tetragonal or the orthorhombic crystal structure, it competes directly with static AFM order.
We have investigated the electronic structure of LaFeAsO$_{1-x}$F$_{x}$ (x = 0; 0.1; 0.2) by angle-integrated photoemission spectroscopy and local density approximation (LDA) based band structure calculations. The valence band consists of a low energy peak at E = -0.25 eV and a broad structure around E = -5 eV in qualitative agreement with LDA. From the photon energy dependence of these peaks we conclude that the former derives almost exclusively from Fe 3d states. This constitutes experimental evidence for the strong iron character of the relevant states in a broad window around EF and confirms theoretical predictions.
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