In this paper we calculate the electron-phonon coupling of the newly-discovered superconductor LaO$_{1-x}$F$_x$FeAs from first-principles, using Density Functional Perturbation Theory. For pure LaOFeAs, the calculated electron-phonon coupling constant $λ=0.21$ and logarithmic-averaged frequency $ω_{ln}=206 K$, give a maximum $T_c$ of 0.8 K, using the standard Migdal-Eliashberg theory.
For the $F-$doped compounds, we predict even smaller coupling constants, due to the strong suppression of the electronic Density of States at the Fermi level. To reproduce the experimental $T_c=26 K$, a 5-6 times larger coupling constant would be needed.
Our results indicate that electron-phonon coupling is not sufficient to explain superconductivity in the newly-discovered LaO$_{1-x}$F$_x$FeAs superconductor, probably due to the importance of strong correlation effects.
We present first principles calculations of the atomic and electronic structure of electron-doped LaOFeAs. We find that whereas the undoped compound has an antiferromagnetic arrangement of magnetic moments at the Fe atoms, the doped system becomes non magnetic at a critical electron concentration. We have studied the electron-phonon interaction in the doped paramagnetic phase. For the $A_{1g}$ phonon, the separation between the As and Fe planes induces a non-collinear arrangement of the Fe magnetic moments. This arrangement is anti parallel for interactions mediated by As, and perpendicular fo…
We report density functional calculations of the electronic structure, Fermi surface, phonon spectrum and electron--phonon coupling for newly discovered superconductor LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$. Significant similarity between LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ and LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$ is found, i.e. there is a strong Fermi surface nesting at ($π$,$π$,0), which results in unstable phonon branches. Combining the frozen phonon total energy calculations and an anharmonic oscillator model, we find that the quantum fluctuation prevents the appearance of static long--range order. The calcul…
The electron-phonon coupling of LaO$_{1-x}$F$_x$FeAs is re-investigated on the basis of density functional theory in local density approximation. The implications of the $(π,π)$ nesting of the Fermi surfaces are carefully studied and found to lead to a non-standard electron-phonon coupling of the corresponding Fe in-plane breathing mode, which might make a strong contribution to the high superconducting transition temperature. The semi-metallic behavior of the undoped material is also further illuminated.
We report density functional theory calculations for the parent compound LaOFeAs of the newly discovered 26K Fe-based superconductor LaO$_{1-x}$F$_x$FeAs. We find that the ground state is an ordered antiferromagnet, with staggered moment about 2.3$μ_B$, on the border with the Mott insulating state. We fit the bands crossing the Fermi surface, derived from Fe and As, to a tight-binding Hamiltonian using maximally localized Wannier functions on Fe 3d and As 4p orbitals. The model Hamiltonian accurately describes the Fermi surface obtained via first-principles calculations. Due to the evident pro…
Diamagnetic susceptibility measurements under high hydrostatic pressure (up to 1.03 GPa) were carried out on the newly discovered Fe-based superconductor LaO_{1-x}F_{x}FeAs(x=0.11). The transition temperature T_c, defined as the point at the maximum slope of superconducting transition, was enhanced almost linearly by hydrostatic pressure, yielding a dT_c/dP of about 1.2 K/GPa. Differential diamagnetic susceptibility curves indicate that the underlying superconducting state is complicated. It is suggested that pressure plays an important role on pushing low T_c superconducting phase toward the …
We present results of ab-initio calculations of the electronic band structure, lattice dynamical properties, and elastic constants of LaOFeAs, the parent compound of the recently discovered superconducting iron-oxypnictide (LaO1-xFxFeAs). The total and partial electronic density of states (EDOS) of the undoped LaOFeAs (in the insulating or metallic state) are extracted from the electronic band structure. The phonon dispersion and the phonon density of states (PDOS) are also studied. Possible implications of the band structure, EDOS, and PDOS of LaOFeAs on the eventual appearance of high-Tc sup…
We have performed 75As Nuclear Magnetic Resonance (NMR) measurements on aligned powders of the new LaO0.9F0.1FeAs superconductor. In the normal state, we find a strong temperature dependence of the spin shift and Korringa behavior of the spin lattice relaxation rate. In the superconducting state, we find evidence for line nodes in the superconducting gap and spin-singlet pairing. Our measurements reveal a strong anisotropy of the spin lattice relaxation rate, which suggest that superconducting vortices contribute to the relaxation rate when the field is parallel to the c-axis but not for the p…
In this paper we discuss the normal and superconducting state properties of two pnictide superconductors, LaOFeAs and LaONiAs, using Migdal-Eliashberg theory and density functional perturbation theory. For pure LaOFeAs, the calculated electron-phonon coupling constant $λ=0.21$ and logarithmic-averaged frequency $ω_{ln}=206 K$, give a maximum $T_c$ of 0.8 K, using the standard Migdal-Eliashberg theory. Inclusion of multiband effects increases the Tc only marginally. To reproduce the experimental $T_c$, a 5-6 times larger coupling constant would be needed. Our results indicate that standard elec…
Raman spectra have been measured on iron-based quaternary CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs with varying fluorine doping at room temperatures. A group analysis has been made to clarify the optical modes. Based on the first principle calculations, the observed phonon modes can be assigned accordingly. In LaO$_{1-x}$F$_x$FeAs, the E$_g$ and A$_{1g}$ modes related to the vibrations of La, are suppressed with increasing F doping. However F doping only has a small effect on the E$_g$ and A$_{1g}$ modes of Fe and As. The Raman modes of La and As are absent in rare-earth substituted CeO$_…
Using the density-functional perturbation theory with structural optimization, we investigate the electronic structure, phonon spectra, and superconductivity of BiS2-based layered compounds LaO1-xFxBiS2. For LaO0.5F0.5BiS2, the calculated electron-phonon coupling constant is equal to lambda = 0.8, and obtained Tc = 9.1 K is very close to its experimental value, indicating that it is a conventional electron-phonon superconductor.
We report the first Nernst effect measurement on the new iron-based superconductor LaO$_{1-x}$F$_{x}$FeAs $(x=0.1)$. In the normal state, the Nernst signal is negative and very small. Below $T_{c}$ a large positive peak caused by vortex motion is observed. The flux flowing regime is quite large compared to conventional type-II superconductors. However, a clear deviation of the Nernst signal from normal state background and an anomalous depression of off-diagonal thermoelectric current in the normal state between $T_{c}$ and 50 K are observed. We propose that this anomaly in the normal state Ne…
Using angle-integrated photoemission spectroscopy we have probed the novel LaO$_{0.9}$F$_{0.1}$FeAs superconductor over a wide range of photon energies and temperatures. We have provided the first full characterization of the orbital character of the VB DOS and of the magnitude of the d-p hybridization energy. Finally, we have identified two characteristic temperatures: 90K where a pseudogap-like feature appears to close and 120K where a sudden change in the DOS near E$_F$ occurs. We associate these phenomena with the SDW magnetic ordering and the structural transition seen in the parent compo…
We present point-contact spectroscopy data for junctions between a normal metal and the newly discovered F-doped superconductor LaO$_{0.9}$F$_{0.1-δ}$FeAs (F-LaOFeAs). A zero-bias conductance peak was observed and its shape and magnitude suggests the presence of Andreev bound states at the surface of F-LaOFeAs, which provides a possible evidence of an unconventional pairing symmetry with a nodal gap function. The maximum gap value $Δ_0\approx3.9\pm0.7$meV was determined from the measured spectra, in good agreement with the recent experiments on specific heat and lower critical field.
We report the theoretical study of the flux-lattice melting in the novel iron-based superconductor $LaO_{0.9}F_{0.1}FeAs$ and $LaO_{0.925}F_{0.075}FeAs$. Using the Hypernetted-Chain closure and an efficient algorithm, we calculate the two-dimensional one-component plasma pair distribution functions, static structure factors and direct correlation functions at various temperatures. The Hansen-Verlet freezing criterion is shown to be valid for vortex-liquid freezing in type-II superconductors. Flux-lattice meting lines for $LaO_{0.9}F_{0.1}FeAs$ and $LaO_{0.925}F_{0.075}FeAs$ are predicted throu…
We present the first experimental results of the lower critical field $H_{c1}$ of the newly discovered F-doped superconductor LaO$_{0.9}$F$_{0.1}$FeAs (F-LaOFeAs) by global and local magnetization measurements. It is found that $H_{c1}$ showed an clear linear-$T$ dependence down to a temperature of 2 K, indicative of an unconventional pairing symmetry with a nodal gap function. Based on the d-wave model, we estimated a maximum gap value $Δ_0=4.0 \pm 0.6$ meV, in consistent with the recent specific heat and point-contact tunneling measurements. Taking the demagnetization factor into account, th…
We discuss the electronic structure, lattice dynamics and electron-phonon interaction of newly discovered superconductor LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ using density functional based calculations. A strong Fermi surface nesting at $\mathbf{k}$=($π$,$π$,0) suggests a proximity to charge density wave instability and leads to imaginary harmonic phonons at this $\mathbf{k}$ point associated with in-plane displacements of S atoms. Total energy analysis resolves only a shallow double-well potential well preventing the appearance of static long-range order. Both harmonic and anharmonic contributions t…
The densities of the valence and conduction band electronic states of the newly discovered layered superconductors LaOFeAs, LaO$_{0.87}$F$_{0.13}$FeAs (Tc=26 K), SmO$_{0.95}$F$_{0.05}$FeAs and SmO$_{0.85}$F$_{0.15}$FeAs (Tc=43 K) are studied using soft X-ray absorption and emission spectroscopy combined with FP LAPW calculations of LaOFeAs, LaO$_{0.875}$FeAs and LaO$_{0.875}$F${0.125}$FeAs. The Fe 3d-states are localized in energy near the top of the valence band and are partially hybridized with p-type O (2p), As (4p), and La (6p) states approximately 3 eV below the Fermi energy. The Fe L3 X-…
We have studied the newly found superconductor compound LaO$_{1-x}$F$_x$FeAs through the first-principles density functional theory calculations. We find that the parent compound LaOFeAs is a quasi-2-dimensional antiferromgnetic semimetal with most carriers being electrons and with a magnetic moment of $2.3μ_B$ located around each Fe atom on the Fe-Fe square lattice. Furthermore this is a commensurate antiferromagnetic spin density wave due to the Fermi surface nesting, which is robust against the F-doping. The observed superconduction happens on the Fe-Fe antiferromagnetic layer, suggesting a…
A recent experiment reported the first rare-earth binary oxide superconductor LaO ($T_c $ $\sim$ 5 K) with a rock-salt structure [K. Kaminaga et al., J. Am. Chem. Soc. 140, 6754 (2018)]. Correspondingly, the underlying superconducting mechanism in LaO needs theoretical elucidation. Based on first-principles calculations on the electronic structure, lattice dynamics, and electron-phonon coupling of LaO, we show that the superconducting pairing in LaO belongs to the conventional Bardeen-Cooper-Schrieffer (BCS) type. Remarkably, the electrons and phonons of the heavy La atoms, instead of those of…
Neutron scattering measurements have been performed on polycrystalline samples of the newly discovered layered superconductor LaO$_{0.5}$F$_{0.5}$BiS$_{2}$, and its nonsuperconducting parent compound LaOBiS$_{2}$. The crystal structures and vibrational modes have been examined. Upon F-doping, while the lattice contracts significantly along $c$ and expands slightly along $a$, the buckling of the BiS$_2$ plane remains almost the same. In the inelastic measurements, a large difference in the high energy phonon modes was observed upon F substitution. Alternatively, the low energy modes remain almo…
Using state-of-the-art first-principles calculations we study the magnetic behaviour of CeOFeAs. We find the Ce layer moments oriented perpendicular to those of the Fe layers. An analysis of incommensurate magnetic structures reveals that the Ce-Ce magnetic coupling is rather weak with, however, a strong Fe-Ce coupling. Comparison of the origin of the tetragonal to orthorhombic structural distortion in CeOFeAs and LaOFeAs show marked differences; in CeOFeAs the distortion is stabilized by a lowering of spectral weight at the Fermi level, while in LaOFeAs by a reduction in magnetic frustration.…
We have employed a new route to synthesize single phase F-doped LaOFeAs compound and confirmed the superconductivity above 20 K in this Fe-based system. We show that the new superconductor has a rather high upper critical field of about 54 T. A clear signature of superconducting gap opening below T$_c$ was observed in the far-infrared reflectance spectra, with 2$Δ/\textit{k}T_c\approx$3.5-4.2. Furthermore, we show that the new superconductor has electron-type conducting carrier with a rather low carrier density.
Using state-of-the-art first-principles calculations we have elucidated the complex magnetic and structural dependence of LaOFeAs upon doping. Our key findings are that (i) doping results in an orthorhombic ground state and (ii) there is a commensurate to incommensurate transition in the magnetic structure between $x=0.025$ and $x=0.04$. Our calculations further imply that in this system magnetic order persists up to the onset of superconductivity at the critical doping of $x=0.05$. Finally, our investigations of the undoped parent compound reveal an unusually pronounced dependence of the magn…
By using a two-step method, we successfully synthesized the iron based new superconductor LaO$_{0.9}$F$_{0.1-x}$FeAs. The Hall effect and magnetoresistance were measured in wide temperature region. A negative Hall coefficient R_H has been found implying a dominant conduction by electron-like charge carriers in this material. The charge carrier density determined at 100K is about 9.8E20 cm-3, which is close to the cuprate superconductors. It is further found that the magnetoresistance does not follow the Kohler's law. Meanwhile, the Hall coefficient, the resistivity at 0T, and magnetoresistance…
We report the specific heat measurements on the newly discovered Fe-based layered superconductor LaO_0.9F_{0.1-delta}FeAs with the onset transition temperature T_c \approx 28 K. A nonlinear magnetic field dependence of the electronic specific heat coefficient gamma(H) has been found in the low temperature limit, which is consistent with the prediction for a nodal superconductor. The maximum gap value Delta_0 \approx 3.4$\pm$0.5 meV was derived by analyzing gamma(H) based on the d-wave model. We also detected the electronic specific heat difference between 9 T and 0 T in wide temperature region…
We have studied the reflectance of the recently discovered superconductor LaO_0.9F0.FeAs in a wide energy range from the far infrared to the visible regime. We report on the observation of infrared active phonons, the plasma edge (PE) and possible interband transitions. On the basis of this data and the reported in-plane penetration depth lambda_L(0) about 254 nm [H. Luetkens et al., Phys. Rev. Lett. v. 101, 0970009 (2008)] a disorder sensitive relatively small value of the total electron electron-boson coupling constant lambda_tot=lambda_e-ph+lambda_e-sp ~ 0.6 +- 0.35 can be estimated adoptin…
We use group theory to classify the superconducting states of systems with two orbitals on a tetragonal lattice. The orbital part of the superconducting gap function can be either symmetric or anti-symmetric. For the orbital symmetric state, the parity is even for spin singlet and odd for spin triplet; for the orbital anti-symmetric state, the parity is odd for spin singlet and even for spin triplet. The gap basis functions are obtained with the use of the group chain scheme by taking into account the spin-orbit coupling. In the weak pairing limit, the orbital anti-symmetric state is only stab…
Competing ferroelectric and charge density wave phases have been proposed to be present in the electron-phonon coupled LaO$_{1-x}$F$_{x}$BiS$_{2}$ superconductor. The lattice instability arises from unstable phonon modes that can break the crystal symmetry. Upon examination of the crystal structure using single crystal diffraction, we find a superlattice pattern arising from coherent in-plane displacements of the sulfur atoms in the BiS$_{2}$ superconducting planes. The distortions morph into coordinated ferro-distortive patterns, challenging previous symmetry suggestions including the possibl…
In this study, the superconducting gap symmetry of LaO$_{0.5}$F$_{0.5}$BiSSe is elucidated through specific heat measurements with one single crystal of LaO$_{0.5}$F$_{0.5}$BiSSe. A clear jump is observed at 3.95 K in LaO$_{0.5}$F$_{0.5}$BiSSe, suggesting bulk superconductivity. From specific heat measurements, $ΔC_\mathrm{e}/γT_\mathrm{c}$ and 2$Δ$(0)/$k_\mathrm{B}T_\mathrm{c}$ are obtained as 2.31 and 4.5. These values indicate strong electron-phonon coupling superconductivity. The temperature ($T$) and magnetic-field ($H$) dependence of the electronic specific heat $C_\mathrm{e}(T, H)$ sugg…
A new layered compound LaOTlF2 is designed and investigated using first-principles calculations in this work. The parent compound is an insulator with an indirect band gap of 2.65 eV. Electron-doping of the parent compound makes the material metallic. In the meantime, several lattice vibrational modes couple strongly to the conduction band, leading to a large electron-phonon coupling constant and conventional superconductivity. The highest superconducting transition temperature Tc is predicted to be approximately 8.6 K with λ about 1.25 in the optimally doped LaO0.95F0.05TlF2, where λ is calcu…
We present zero field and transverse field \muSR experiments on the recently discovered electron doped Fe-based superconductor LaO_{1-x}F_xFeAs. The zero field experiments on underdoped (x=0.075) and optimally doped (x=0.1) samples rule out any static magnetic order above 1.6 K in these superconducting samples. From transverse field experiments in the vortex phase we deduce the temperature and field dependence of the superfluid density. Whereas the temperature dependence is consistent with a weak coupling BCS s-wave or a dirty d-wave gap function scenario, the field dependence strongly evidenc…
We report resistivity and upper critical field B_c2(T) data for As deficient LaO_(0.9)F_(0.1)FeAs_(1-delta) in a wide temperature and high field range up to 60 T. These disordered samples exhibit a slightly enhanced superconducting transition at T_c = 29 K and a significantly enlarged slope dB_(c2))/dT = -5.4 T/K near T_c which contrasts with a flattening of B_(c2)(T) starting near 23 K above 30 T. This flattening is interpreted as Pauli limiting behaviour (PLB) with B_(c2)(0) approx 63 T. We compare our results with B_(c2)(T)-data reported in the literature for clean and disordered samples. W…
Theory of superconductivity generated in one atomic layer thick two dimensional electron gas by a single flat band of high energy longitudinal optical phonons is considered. The polar dielectric $SrTiO_{3}$ (STO) exhibits such an energetic phonon mode and the 2DEG is created both when one unit cell $FeSe$ layer is grown on its $\left( 100\right) $ surface and on the interface with another dielectric like $LaAlO_{3}$ (LAO). We obtain a quantitative description of both systems solving the gap equation for $T_{c}$ without making use of approximations like the Kirzhnits Ansatz for arbitrary chemic…
In two recent articles (cond-mat/0606177 and arXiv:0804.1615), we have suggested a unified theory of superconductivity based on the real-space spin-parallel electron pairing and superconducting mechanism and have shown that the stable hexagonal and tetragonal vortex lattices (the optimal doping phases) can be expected in the newly discovered LaO{1-x}F{x}FeAs (x0=1/7=0.1428) and SmO{1-x}F{x}FeAs (x0=1/6=0.1667), respectively. In this paper, we present a theoretical study of the effects of hydrostatic and anisotropic pressure on the superconducting transition temperature Tc of the Fe-based layer…
High-resolution and high-flux neutron as well as X-ray powder-diffraction experiments were performed on the oxypnictide series LaO(1-x)FxFeAs with 0<x<0.15 in order to study the crystal and magnetic structure. The magnetic symmetry of the undoped compound corresponds to those reported for ReOFeAs (with Re a rare earth) and for AFe2As2 (A=Ba, Sr) materials. We find an ordered magnetic moment of 0.63(1)muB at 2 K in LaOFeAs, which is significantly larger than the values previously reported for this compound. A sizable ordered magnetic moment is observed up to a F-doping of 4.5% whereas there is …
We report the synthesis and characterization of antimony doped oxypnictide superconductor, LaO0.8F0.2FeAs1-xSbx (x = 0.05 and 0.10). The parent compound LaOFeAs with fluorine doping exhibits superconductivity at maximum transition temperature ~ 28.5 K [11]. Here we partially substitute As by Sb (LaO0.8F0.2FeAs1-xSbx) and observe enhancement of the transition temperature to 30.1 K. This is the only instance so far where Tc increases with doping in the conducting layer (FeAs) and this leads to the highest transition temperature in any La-based oxypnictide. XRD and EDAX measurements confirm phase…
We report the temperature dependent x-ray powder diffraction of the FeAs-based superconductors in the range between 300 K and 95 K. In the case of NdOFeAs we have detected the structural phase transition from the tetragonal phase, with P4/nmm space group, to the orthorhombic phase,with Cmma space group, over a broad temperature range from 150 K to 120 K, centered at T0 137K. This transition is reduced, by about 30K, by the internal chemical pressure going from LaOFeAs to NdOFeAs. On the contrary the superconducting critical temperature increases from 27K to 51 K going from LaOFeAs to NdOFeAs d…
Superconductivity in novel bismuth-sulphur superconductors has attracted large research efforts, both experimental and theoretical, but a consensus on the nature of superconductivity in these materials has yet to be reached. Using density functional theory for superconductors, we study the electron-phonon pairing mechanism in LaO$_{0.5}$F$_{0.5}$BiS$_2$. We first confirm the presence of a commensurate charge density wave instability, in accordance with previous studies. Using a recently developed integration scheme for the electron-phonon coupling, we found that its strength is much lower than…
Polycrystalline samples of RO1-xFxFeAs (0 < x < 0.25) (R = La, Sm, Gd) were investigated by wavelength-dispersive X-ray spectroscopy (WDX) in the electron microscope to determine the composition of the samples, in particular the fluorine content. It was found that the measured fluorine content can deviate considerably from the initial weight. In the lanthanum compound LaO1-xFxFeAs, we found good agreement mainly for x > 0.05, but for x < 0.05 the fluorine hardly goes into the sample. For the samarium compound we measured less than half the fluorine in the sample as initially weighed at all flu…
Very recently a new family of layered materials, containing BiS2 planes was discovered to be superconducting at temperatures up to Tc=10 K, raising questions about the mechanism of superconductivity in these systems. Here, we present state-of-the-art first principles calculations that directly address this question and reveal several surprising findings. The parent compound LaOBiS2 possesses anharmonic ferroelectric soft phonons at the zone center with a rather large polarization of $\approx 10 μC/cm^2$, which is comparable to the well-known ferroelectric BiFeO3. Upon electron doping, new unst…
We used the proximity effect as a tool to achieve an ideal (barrier - free) NS boundary for quantitative evaluation of transport phenomena that accompany converting dissipative current into supercurrent in NS systems with unconventional superconductors, single - crystal chalcogenide FeSe and granulated pnictide LaO(F)FeAs. Using features (limitations) of Andreev reflection in the NS systems with dispersion of the electron spin subbands, we revealed direct evidence for spin - polarized nature of transport and the absence of residual magnetization in iron - based superconductors in the normal st…
We study the influence of inelastic rare-earth impurity scattering on electron-phonon mediated superconductivity and mass renormalization in (La$_{1-x}$Pr$_{x}$)Os$_{4}$Sb$_{12}$ compounds. Solving the strong coupling Eliashberg equations we find that the dominant quadrupolar component of the inelastic scattering on Pr impurities yields an enhancement of the superconducting transition temperature T$_c$ in LaOs$_{4}$Sb$_{12}$ and increases monotonically as a function of Pr concentration. The calculated results are in good agreement with the experimentally observed T$_c (x)$ dependence. Our anal…
The specific heat $C(T)$ of new iron-based high-$T_c$ superconductor SmO$_{1-x}$F$_x$FeAs ($0 \leq x \leq 0.2$) was systematically studied. For undoped $x$ = 0 sample, a specific heat jump was observed at 130 K. This is attributed to the structural or spin-density-wave (SDW) transition, which also manifests on resistivity as a rapid drop. However, this jump disappears with slight F doping in $x$ = 0.05 sample, although the resistivity drop still exists. The specific heat $C/T$ shows clear anomaly near $T_c$ for $x$ = 0.15 and 0.20 superconducting samples. Such anomaly has been absent in LaO$_{…
We report measurements of the phonon density-of-states in iron oxypnictide superconductors by inelastic x-ray scattering. A good agreement with ab-initio calculations that do not take into account strong electronic correlations is found, and an unpredicted softening of phonon branches under F doping of these compounds is observed. Raman scattering experiments lead us to conclude that this softening is not related to zone center phonons, and consequently imply an important softening of the relevant phonon branches at finite momentum transfer Q.
Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial-phonons in an iron-based superconductor Sr$_2$VO$_3$FeAs ($T_c \approx$ 33 K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulate…
We report experimental results of ^{75}As and ^{139}La nuclear magnetic resonance (NMR) in the iron-based layered La(O_{1-x}F_{x})FeAs (x= 0.0, 0.04 and 0.11) system. In the undoped LaOFeAs, 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_1T 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 …
We have synthesized a quaternary fluoroarsenide SrFeAsF with the ZrCuSiAs-type structure (P4/nmm, a = 0.3999 and c = 0.8973 nm), which is composed of an alternately stacked (FeAs)- and (SrF)+ layers, analogous to the FeAs-based superconductor LaFeAsO. SrFeAsF shows metallic type conduction with the anomaly at ~180 K. The partial replacement of the Fe with Co suppresses the anomaly and induces the superconductivity, while the maximal Tc (4 K for SrFe0.875Co0.125AsF) is much lower than that of the Co-substituted LaFeAsO. Replacement of (LaO)+ layers with (SrF)+ layers results in a enlargement of…
Unlike the ferropnictide superconductors, which crystallize in a tetragonal crystal structure, binary FeAs forms in an orthorhombic crystal structure, where the local atomic environment resembles a highly distorted variant of the FeAs4 tetrahdedral building block of the ferropnictide superconductors. However, like the parent compounds of the ferropnictide superconductors, FeAs undergoes magnetic ordering at low temperatures, with no evidence favoring a superconducting ground state at ambient pressure. We employ pressure-dependent electrical transport and x-ray diffraction measurements using di…
By partially substituting the tri-valence element La with di-valence element Sr in $LaOFeAs$, we introduced holes into the system. For the first time, we successfully synthesized the hole doped new superconductors $(La_{1-x}Sr_x)OFeAs$. The maximum superconducting transition temperature at about 25 K was observed at a doping level of x = 0.13. It is evidenced by Hall effect measurements that the conduction in this type of material is dominated by hole-like charge carriers, rather than electron-like ones. Together with the data of the electron doped system $La(O_{1-x}F_x)FeAs$, a generic phase …
To shed light on the role of magnetism on the superconducting mechanism of the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering on phonon dynamics in the low-temperature orthorhombic parent compounds, which present a spin-density wave. The study covers both the 122 (AFe2As2; A=Ca, Sr, Ba) and 1111 (AFeAsF; A=Ca, Sr) phases. We extend our recent work on the Ca (122 and 1111) and Ba (122) cases by treating computationally and experimentally the 122 and 1111 Sr compounds. The effect of magnetic ordering is investigated through detailed non-magnetic and magnetic lattice d…
We report first principles calculations of the electronic structure, phonon dispersions and electron phonon coupling of LaNiPO. These calculations show that this material can be explained as a conventional electron phonon superconductor in contrast to the FeAs based high temperature superconductors.
We present a systematic photoemission study of the newly discovered high Tc superconductor class (Sr/Ba)1-xKxFe2As2. By utilizing a unique photon energy range and scattering geometry we resolve the details of the single particle dynamics of interacting electrons on the central Fermi surfaces of this series which shows overall strong coupling behavior (2D/kBTc = 6). Quasiparticle dispersion kinks are observed in a binding energy range of 15 to 50 meV which matches the magnetic excitation energy scales (parameterized by J1,J2). The size of the Cooper pair wavefunction is found to be less than 20…
We report inelastic x-ray scattering experiments on the lattice dynamics in SmFeAsO and superconducting SmFeAsO$_{0.6}$F$_{0.35}$ single crystals. Particular attention was paid to the dispersions along the [100] direction of three optical modes close to 23 meV, polarized out of the FeAs planes. Remarkably, two of these modes are strongly renormalized upon fluorine doping. These results provide significant insight into the energy and momentum dependence of the coupling of the lattice to the electron system and underline the importance of spin-phonon coupling in the superconducting iron-pnictide…
Since the discovery of high-Tc LaO_1-xF_xFeAs, and other such systems based on FeAs layers, several proposals have been made for the superconducting order parameter Delta_k, on both phenomenological and microscopic grounds. Here we discuss how the symmetry of Delta_k in the bulk can be determined, assuming that single crystals will soon be available. We suggest that a measurement of the dependence of the low temperature specific heat on the angle of a magnetic field in the FeAs plane is the simplest such method, and calculate representative specific heat vs. field angle oscillations for the va…
We propose a new Rashba-free mechanism to realize topological superconductivity with electron-phonon interaction. In the presence of a magnetic field, electron-phonon interaction with small momentum transfer is found to favor spin-triplet Cooper pairing. This process facilitates the formation of chiral topological superconductivity even when Rashba spin-orbital coupling is absent. As a proof of concept, we propose an experimentally feasible heterostructure to systematically study the entangled relationship among forward-phonon scatterings, Rashba spin-orbital couplings, pairing symmetries, and…
This paper explains the multi-orbital band structures and itinerant magnetism of the iron-pnictide and chalcogenides. We first describe the generic band structure of an isolated FeAs layer. Use of its Abelian glide-mirror group allows us to reduce the primitive cell to one FeAs unit. From density-functional theory, we generate the set of eight Fe $d$ and As $p$ localized Wannier functions for LaOFeAs and their tight-binding (TB) Hamiltonian, $h(k)$. We discuss the topology of the bands, i.e. allowed and avoided crossings, the origin of the d6 pseudogap, as well as the role of the As $p$ orbita…
Based on results from femtosecond time-resolved photoemission, we compare three different methods for determination of the electron-phonon coupling constant λ in Eu and Ba-based 122 FeAs compounds. We find good agreement between all three methods, which reveal a small λ < 0.2. This makes simple electron-phonon mediated superconductivity unlikely in these compounds.
The electronic structures of FeAs-compounds strongly depend on the Fe-As bonding, which can not be described successfully by the local density approximation (LDA). Treating the multi-orbital fluctuations from $ab$-$initio$ by LDA+Gutzwiller method, we are now able to predict the correct Fe-As bond-length, and find that Fe-As bonding-strength is 30% weaker, which will explain the observed "soft phonon". The bands are narrowed by a factor of 2, and the $d_{3z^2-r^2}$ orbital is pushed up to cross the Fermi level, forming 3-dimensional Fermi surfaces, which suppress the anisotropy and the ($π,π$)…
The newly discovered superconductor La(O,F)FeAs (Tc = 26 K) was investigated using the neutron scattering technique. No spin-density-wave (SDW) order was observed in the normal state nor in the superconducting state, both with and without an applied magnetic field of 9 T, consistent with the proposal that SDW and superconductivity are competing in the laminar materials. While our inelastic measurements offer no constraints on the spin dynamic response from d-wave pairing, an upper limit for the magnetic resonance peak predicted from an extended s-wave pairing mechanism is provided. Our measure…
The discovery of the new family of oxypnictide superconductors, such as LaFeAsO0.89F0.1, with critical temperature, Tc, over 26 K, has brought new impetus to the fields of high temperature superconductivity and strongly correlated electron systems. These materials showed high upper critical fields, Hc2, which have been estimated to be higher than 55 or 63-65 T in LaO0.9F0.1FeAs with Tc of 26 K, 70 T in PrFeAsO0.85F0.15, and over 100 T in SmO0.85F0.15FeAs with Tc of 46 K. The two gap superconductivity proposed for LaO0.9F0.1FeAs suggests that the Hc2 could be further increased to a significant …
