Undoped LaFeAsO Research Articles

Fast recovery of the stripe magnetic order by Mn/Fe substitution in F-doped LaFeAsO superconductors

$^{75}$As Nuclear Magnetic (NMR) and Quadrupolar (NQR) Resonance were used, together with M\"{o}ssbauer spectroscopy, to investigate the magnetic state induced by Mn for Fe substitutions in F-doped LaFe$_{1-x}$Mn$_{x}$AsO superconductors. The results show that $0.5$% of Mn doping is enough to suppress the superconducting transition temperature $T_c$ from 27 K to zero and to recover the magnetic structure observed in the parent undoped LaFeAsO. Also the tetragonal to orthorhombic transition of the parent compound is recovered by introducing Mn, as evidenced by a sharp drop of the NQR frequency. The NQR spectra also show that a charge localization process is at play in the system. Theoretical calculations using a realistic five-band model show that correlation-enhanced RKKY exchange interactions between nearby Mn ions stabilize the observed magnetic order, dominated by $Q_1=(\pi,0)$ and $Q_2=(0,\pi)$ ordering vectors. These results give compelling evidence that F-doped LaFeAsO is a strongly correlated electron system at the verge of an electronic instability.

Temperature-Dependent As K-Edge EXAFS Studies of LaFe 1−x Co x AsO (x = 0.0 and 0.11) Single Crystals

We report the experimental results of temperature dependent polarized As K-edge extended x-ray absorption fine structure (EXAFS) of LaFe1-xCoxAsO (x=0.0 and 0.11) single-crystals. By aligning the Fe-As bond direction in the direction of the x-ray beam polarization we have been able to identify an anomaly in the Fe-As bond correlations at the tetragonal to orthorhombic transition at 150K, while previous investigations with standard unpolarized EXAFS of undoped LaFeAsO powder samples were not able to detect any such anomaly. Using our approach we have been able to identify in the superconducting doped sample, LaFe0.89Co0.11AsO, a broad anomaly around 60 K. The low temperature anomaly has good correlations with the temperature dependence of several properties like resistivity, magnetic susceptibility, linear thermal expansion, etc indicating the emergence of the dynamical oscillations of the Fe - As pairs

Nematicity in LaFeAsO1−xFx

AbstractOrbital 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 LaFeAsO1−x Fx with a focus on resistivity, Nernst effect, thermal expansion, and As nuclear magnetic resonance (NMR) data. The transport properties at the underdoped level exhibit pronounced anomalies at about the same temperature where undoped LaFeAsO develops long‐range nematic ordering, i.e., at about 160 K. Furthermore, the As‐NMR spin‐lattice relaxation rate 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 which suggests that in LaFeAsO1−x Fx the nematic phase actually competes with superconductivity.Schematic electronic phase diagram of LaFeAsO1−x Fx.

The Structural Distortion in Antiferromagnetic LaFeAsO Investigated by a Group-Theoretical Approach

As experimentally well established, undoped LaFeAsO is antiferromagnetic below 137K with the magnetic moments lying on the Fe sites. We determine the orthorhombic body-centered group Imma (74) as the space group of the experimentally observed magnetic structure in the undistorted lattice, i.e., in a lattice possessing no structural distortions in addition to the magnetostriction. We show that LaFeAsO possesses a partly filled "magnetic band" with Bloch functions that can be unitarily transformed into optimally localized Wannier functions adapted to the space group Imma. This finding is interpreted in the framework of a nonadiabatic extension of the Heisenberg model of magnetism, the nonadiabatic Heisenberg model. Within this model, however, the magnetic structure with the space group Imma is not stable but can be stabilized by a (slight) distortion of the crystal turning the space group Imma into the space group Pnn2 (34). This group-theoretical result is in accordance with the experimentally observed displacements of the Fe and O atoms in LaFeAsO as reported by Clarina de la Cruz et al. [nature 453, 899 (2008)].

Electrical transport properties of F-doped LaFeAsO oxypnictide

A series of LaFeAsO 1− x F x ( x = 0–0.225) oxyarsenides have been synthesized by a solid-state reaction method in order to optimize electrical transport properties through appropriate F doping. Both electrical resistivity and Seebeck coefficient of undoped LaFeAsO show an anomaly at about 150 K, which is related to a structural phase and/or spin-density-wave (SDW) transition. Seebeck coefficient seems to be determined by two competitive factors: it is enhanced by suppressing the structural phase and/or SDW transition, and reduced by increasing carrier concentration. Seebeck coefficient is significantly enhanced just after suppressing the anomaly, and the maximum Seebeck coefficient reached −142 μV/K for the sample with F doping x = 0.075 from −58 μV/K for undoped LaFeAsO, and then decreased with further increasing carrier concentration through F doping. Meanwhile, the electrical resistivity is decreased with increasing F doping, resulting in a maximum power factor value of 1.2 mW/mK 2 at 80 K for polycrystalline LaFeAsO 0.85F 0.15 sample , this value is the same order as that of the best low temperature thermoelectric Bi 88Sb 12 compounds, and could be significantly higher in single crystals.

Magnetic properties and electronic structure of LaFeAsO0.85F0.1

The magnetic properties of the compound LaFeAsO0.85F0.1 were investigated by measurements of the dc magnetization for different values of the magnetic field H=0.02, 1.0 and 2.0T in the temperature range 4.2–300K. Superconducting behavior was found below 26K, whereas a distinct peculiarity in the low-field dependence of the magnetic susceptibility χ(T) was clearly observed at TM≃135K, which resembles a weak ferromagnetic (FM) response with saturation magnetic moment of about 10−4μB per formula unit at 50K. The transition at TM is presumably not governed by magnetic impurities but rather correlated with the antiferromagnetic (AFM) transition in undoped LaFeAsO at about the same temperature. We suggest that the observed magnetic properties of the LaFeAsO0.85F0.1 sample are due to an interplay of FM and AFM transitions, and are presumably related with an intrinsic feature of a small portion of the undoped LaFeAsO phase inherent in our sample. In order to shed light on the problem of magnetic instability of the LaFeAsO, ab initio DFT calculations of the electronic structure and paramagnetic susceptibility were performed within the local spin density approximation. It is shown that a V-shaped peculiarity in the density of electronic states close to the Fermi level can govern magnetic properties of LaFeAsO with fluorine doping and/or oxygen deficiency.

75As and 139La NMR studies on LaFeAs(O1-xFx)

We report experimental results of 75As and 139La nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs(O1-xFx) (x=0.0, 0.04, 0.07 and 0.11) where superconductivity occurs in x greater than 0.04. In the undoped LaFeAsO, 1/T1 of 139La exhibits a distinct peak at TN~142K below which the La NMR spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the x=0.04 sample, 1/T1T of 75As exhibits a Curie–Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the x=0.11 sample, in contrast, pseudogap behavior is observed in 1/T1T at the 75As site with a gap value of ΔPG~175K. The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure by the electron-doping. As for the superconducting properties for x=0.04, 0.07 and 0.11, 1/T1 of 75As in all compounds does not exhibit a coherence peak just below Tc and follows a T3 dependence at low temperatures. These results seemingly suggests that unconventional superconductivity with zero gap along lines, whereas the lack of the residual density of states at the low temperatures is incompatible with the presence of the line-nodes. We discuss similarity and difference between LaFeAs(O1-xFx) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F-doping dependence of Tc and 1/T1.

Effect of chemical pressure on spin density wave and superconductivity in undoped and 15% F-dopedLa1−yYyFeAsOcompounds

We present a study concerning the partial substitution of yttrium at the lanthanum site of the undoped LaFeAsO and superconducting ${\text{LaFeAsO}}_{0.85}{\text{F}}_{0.15}$ compounds. We prepared samples with a nominal yttrium content up to 70% producing simultaneous shrinkage of both the $a$- and $c$-lattice parameters by 1.8% and 1.7%, respectively. The chemical pressure provided by the partial substitution with this smaller ion size causes a lowering of the spin density wave temperature in the undoped compounds, as well as an increase in the superconducting transition temperatures in the doped ones. The 15% fluorine-doped samples reach a maximum critical temperature of 40.2 K for the 50% yttrium substitution. Comparison with literature data indicates that chemical pressure cannot be the only mechanism which tunes drastically both ${T}_{\text{SDW}}$ and ${T}_{c}$ in 1111 compounds. Our data suggest that the structural disorder induced by the partial substitution in the La site or by doping could play an important role as well.

High-Pressure Studies on Superconductivity in LaFeAsO1−x F x and SmFeAsO1−x F x

The pressure dependence on the superconducting transition temperature (Tc) was investigated for the iron-based superconductors LaFeAsO1−xFx and SmFeAsO1−xFx. The Tc’s increase largely for LaFeAsO1−xFx with a small increase of pressure, while a sharp decrease of Tc was observed for SmFeAsO1−xFx. The electrical resistivity measurements reveal pressure-induced superconductivity for undoped LaFeAsO and SmFeAsO. These pressure effects seem to be related to an anisotropic decrease of the lattice constants under high pressure from the x-ray diffraction measurements up to 10 GPa for the LaFeAsO1−xFx system.

An NMR study on the F-doping evolution of the iron oxypnictide LaFeAs(O1-xFx)

We report the experimental results of 75As and 139La nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs(O1- xFx) (x=0.0, 0.04, 0.07, 0.11 and 0.14) where superconductivity occurs at x≳0.03. In the undoped LaFeAsO, 1/T1 of 139La exhibits a distinct peak at TN∼142 K below which the La-NMR spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the x=0.04 sample, 1/T1T of 75As exhibits a Curie–Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with a decrease in temperature. At x=0.11 and 0.14, in contrast, pseudogap behavior is observed in 1/T1T at the 75As site with a gap value of ΔPG∼175 and 165 K, respectively. The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure because of the electron doping. As for the superconducting properties for x=0.04, 0.07 and 0.11, the 1/T1 of 75As in the three samples does not exhibit a coherence peak just below Tc and follows a T3 dependence at low temperatures. These results may suggest unconventional superconductivity with a zero gap along the lines, but neither the field-induced extra relaxation rate nor the residual density of states at the low temperatures is incompatible with the presence of the line nodes. We discuss the similarities and differences between LaFeAs(O1- xFx) and cuprates and also discuss the relationship between spin dynamics and superconductivity on the basis of the F-doping dependence of Tc and 1/T1.

Pressure studies of (La, Sm)FeAsO 1−xF x and LaFePO

Pressure dependence on superconducting transition temperature ( T c) was examined for iron-based superconductor LaFeAsO 1− x F x , SmFeAsO 1− x F x and LaFePO. The T c’s increase largely for LaFePO and LaFeAsO 1− x F x with a small increase of pressure. The electrical resistivity measurements reveal the pressure-induced superconductivity in undoped LaFeAsO and SmFeAsO. X-ray diffraction measurements were also performed under high pressure up to 10 GPa for LaFePO and LaFeAsO 1− x F x system, where the anisotropic decrease of the lattice constants was observed with applying pressure.

Electromagnetic properties of undoped LaFePnO (Pn = P, As)

Undoped LaFePnO (Pn = P, As) shows metallic resistivity and a tetragonal phase at room temperature. The electrical resistivity (ρ) of undoped LaFeAsO exhibits a sudden decrease around Tanom ∼160 K with decreasing the temperature and increases with decreasing the temperatures (T) below another threshold temperature (Tmin). It makes a contrast to undoped LaFePO, which shows neither of Tanom nor Tmin. The Tanom corresponds to a crystallographic phase transition: undoped LaFeAsO shows an orthorhombic phase at temperatures < 160 K. A positive magnetoresistance (MR = (ρ-ρ0) / ρ0) also appears below Tanom. DFT calculations indicate that spin moment of LaFeAsO is 1.56 μB / Fe in an antiferromagnetic spin configuration in a tetragonal phase, while spin moment of LaFePO is almost quenched.

Static Magnetic Order and Superfluid Density ofRFeAs(O,F)(R=La,Nd,Ce) and LaFePO Studied by Muon Spin Relaxation: Unusual Similarities with the Behavior of Cuprate Superconductors

Muon spin relaxation measurements in iron-oxypnictide systems have revealed: (1) commensurate long-range order in undoped LaFeAsO; (2) a Bessel function line shape in LaFeAs(O0.97F0.03) which indicates possible incommensurate or stripe magnetism; (3) anomalous weak magnetism existing in superconducting LaFePO, CeFeAs(O0.084F0.16), and NdFeAs(O0.88F0.12) but absent in superconducting LaFeAs(O0.92F0.08); and (4) scaling of the superfluid density with T_{c} in the Ce-, La-, and Nd-FeAs superconductors following a nearly linear relationship found in cuprates.

Effect of external pressure on the Fe magnetic moment in undoped LaFeAsO from density functional theory: Proximity to a magnetic instability

We investigate the effect of external pressure on the Fe magnetic moment in undoped LaFeAsO within the framework of density-functional theory and show that this system is close to a magnetic instability. The Fe moment is found to drop by nearly a factor of 3 within a pressure range of $\ifmmode\pm\else\textpm\fi{}5\text{ }\text{GPa}$ around the calculated equilibrium volume. While the Fe moments show an unusually strong sensitivity to the spin arrangement (type of antiferromagnetic structure), the low-temperature structural distortion is found to have only a minor influence on them. Analysis of the Fermi-surface topology and nesting features shows that these properties change very little up to pressures of at least 10 GPa. We discuss the magnetic instability in terms of the itinerancy of this system.

Signatures of Electronic Correlations in Optical Properties ofLaFeAsO1−xFx

Spectroscopic ellipsometry is used to determine the dielectric function of superconducting LaFeAsO0.9F0.1 (T_{c}=27 K) and undoped LaFeAsO polycrystalline samples in the wide range 0.01-6.5 eV at temperatures 10< or =T< or =350 K. The charge carrier response in both samples is heavily damped. The spectral weight transfer in LaFeAsO associated with an opening of the pseudogap at about 0.65 eV is restricted to energies below 2 eV. The spectra of superconducting LaFeAsO0.9F0.1 reveal a significant transfer of spectral weight to a broad optical band above 4 eV with increasing temperature. Our data may imply that the electronic states near the Fermi surface are strongly renormalized due to electron-phonon and/or electron-electron interactions.

X-ray spectra and electronic structures of the iron arsenide superconductorsRFeAsO1−xFx(R=La,Sm)

The electronic structures of undoped and fluorine-doped LaFeAsO superconductors and fluorine-doped SmFeAsO superconductors are studied using soft x-ray absorption and emission spectroscopy combined with full potential linearized augmented planes waves (FP LAPW) calculations. The comparison between the numerical simulations and the experimental $\text{Fe}\text{ }L$-emission spectra shows that the Fe states are concentrated near ${E}_{F}$, suggesting that the materials are not highly correlated systems. The comparison of the O $K$-edge and $\text{F}\text{ }K$-edge spectra with the calculated density of states shows that the F dopants do not directly participate in the electronic structure near ${E}_{F}$, and so they serve the same purpose as an oxygen vacancy leading to an increase in the number of available charge carriers. Increasing the amount of fluorine doped into the structure of SmFeAsO is shown to cause a narrowing of the bandwidth of occupied O valence states.

Crystallographic phase transition andhigh-Tc superconductivity in LaFeAsO:F

Undoped LaFeAsO, the parent compound of the newly foundhigh-Tc superconductor, exhibits a sharp decrease in the temperature-dependent resistivity at∼160 K. The anomaly can be suppressed by F doping with simultaneous appearance ofsuperconductivity appears correspondingly, suggesting a close association of the anomalywith the superconductivity. We examined the crystal structures, magnetic propertiesand conductivity of undoped (normal conductor) and 14 at.% F-doped LaFeAsO(Tc = 20 K) by synchrotron x-ray diffraction (XRD), DC magnetic measurements, and ab initiocalculations demonstrated that the anomaly is associated with a phase transition from tetragonal(P4/nmm) toorthorhombic (Cmma) phases at ∼160 K as well as an antiferromagnetic spin ordering transition at∼140 K. These transitions can be explained by spin configuration-dependent potential energysurfaces derived from the ab initio calculations. The suppression of the transitions is ascribed tointerrelated effects of geometric and electronic structural changes due to doping byF− ions.

Evolution from Itinerant Antiferromagnet to Unconventional Superconductor with Fluorine Doping in LaFeAs(O1-xFx) Revealed by 75As and 139La Nuclear Magnetic Resonance

We report experimental results of 75 As and 139 La nuclear magnetic resonance (NMR) in the iron-based layered LaFeAs(O 1- x F x ) ( x = 0.0, 0.04, and 0.11). In the undoped LaFeAsO, 1/ T 1 of 139 La exhibits a distinct peak at T N ∼142 K below which the spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the 4% F-doped sample, 1/ T 1 T exhibits a Curie–Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the 11% F-doped sample, in contrast, pseudogap behavior is observed in 1/ T 1 T both at the 75 As and 139 La site with a gap value of Δ PG ∼172 K. The spin dynamics vary markedly with F doping, which is ascribed to the Fermi-surface structure. As for the superconducting properties for the 4 and 11% F-doped samples, 1/ T 1 in both compounds does not exhibit a coherence peak just below T c and follows a T 3 dependence at low temperatures, which suggests unconventional super...

Tetragonal–Orthorhombic Phase Transition and F-Doping Effects on the Crystal Structure in the Iron-Based High-TcSuperconductor LaFeAsO

Crystal structure refinements of undoped and 14 at.% F-doped LaFeAsO ( T c =20 K) were performed over the temperature region of 25 to 300 K by synchrotron X-ray diffraction. The electronic structures of the crystals were also examined using density functional theory (DFT). Undoped LaFeAsO was observed to undergo a crystallographic transition from the tetragonal ( P 4/ n m m ) to orthorhombic ( C m m a ) phases at ∼160 K. This crystallographic transition was accompanied with an antiferromagnetic spin ordering at ∼140 K. The DFT calculation results suggest that both transitions were caused by the “Cooperative Jahn–Teller effect”. In contrast, an F-doped sample maintains the non-magnetic tetragonal phase down to 25 K. F-ion substitutions for the O sites changed the bond length and angle in the distorted LaO tetrahedron, whereas weaker geometric effects were observed in the FeAs layer. Further, the F-doping induced large decreases in the c -axis length and the La–As distance, suggesting an enhancement of the ...

High-Pressure Studies on Superconducting Iron-Based LaFeAsO1-xFx, LaFePO and SrFe2As2

Electrical resistivity and magnetic susceptibility measurements under high pressure were performed on iron-based superconductor LaFePO and LaFeAsO 1- x F x system. A steep increase in superconducting transition temperature ( T c ) was observed for LaFePO and “optimum-doped” and “over-doped” LaFeAsO 1- x F x . Pressure-induced superconductivity was confirmed in undoped LaFeAsO and SrFe 2 As 2 by electrical resistivity measurements under high pressure. X-ray diffraction measurements were also performed under high pressure up to 10 GPa for LaFePO and LaFeAsO 1- x F x system, where the anisotropic decrease of the lattice constants was observed with applying pressure.