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, the absolute value of $H_{c1}(0)$ is determined to be about 54 Oe, manifesting a low superfluid density for LaO$_{0.9}$F$_{0.1}$FeAs.
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…
The electronic and magnetic properties of the mother material LaOFeAs of new superconductors have been carefully studied using first-principles electronic structure calculations based on the generalized gradient approximation in the density functional theory. The present calculation predicts that the ground state of LaOFeAs is antiferromagnetic with a stripe type magnetic moment alignment leading to orthorhombic symmetry of the crystal. In this particular magnetic state, the density of states at the Fermi level is very small. On the other hand, LaOFeP has turned out to be paramagnetic and a go…
Here we report the first-principle FLAPW-GGA calculations of the elastic properties of two related phases, namely, the ternary arsenide SrFe2As2 and the quaternary oxyarsenide LaOFeAs as the basic phases for the newly discovered "1111" and "122" 38-52K superconductors. The independent elastic constants (Cij), bulk moduli, compressibility, and shear moduli are evaluated and discussed. Additionally, numerical estimates of the elastic parameters of the polycrystalline LaOFeAs and SrFe2As2 ceramics are performed for the first time.
We investigated the recently found superconductor LaO_{1-x}F_xFeAs by X-ray absorption spectroscopy (XAS). From a comparison of the O K-edge with LDA calculations we find good agreement and are able to explain the structure and changes of the spectra with electron doping. An important result from this edge is a limitation of the Hubbard U to values not significantly larger than 1 eV. From experimental Fe L_2,3-edge spectra and charge transfer multiplet calculations we gain further information on important physical values such as hopping parameters, the charge transfer energy Delta, and the on-…
We describe a new methodology of synthesizing the oxypnictide superconductors with potassium fluoride as a source as a source of fluorine instead of LaF3. This route also allows the substitution of potassium at lanthanum sites which leads to an increase in the upper critical field as would be expected for a multiband superconductor. We also report the highest reported Tc onset of 28.5K and highest upper critical field in these superconductors.
The full-potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange-correlation potential has been applied to predict the elastic properties of quaternary oxyarsenide LaOFeAs - the basic phase for the newly discovered 26-52K superconductors. The optimized lattice parameters, independent elastic constants (Cij), bulk modulus B, compressibility betta, and shear modulus G are evaluated and discussed. For the first time the numerical estimates of the elastic parameters for polycrystalline LaOFeAs ceramics are performed. Our analysi…
We synthesized Sr-doped $La_{0.85}Sr_{0.15}OFeAs$ sample with single phase, and systematically studied the effect of oxygen deficiency in the Sr-doped LaOFeAs system. It is found that substitution of Sr for La indeed induces the hole carrier evidenced by positive thermoelectric power (TEP), but no bulk superconductivity is observed. The superconductivity can be realized by annealing the as-grown sample in vacuum to produce the oxygen deficiency. With increasing the oxygen deficiency, the superconducting transition temperature ($T_c$) increases and maximum $T_c$ reaches about 26 K the same as t…
We present scanning tunneling microscopy studies of the LaOFeAs parent compound of iron pnictide superconductors. Topographic imaging reveals two types of atomically flat surfaces, corresponding to the exposed LaO layer and FeAs layer respectively. On one type of surface, we observe strong standing wave patterns induced by quasiparticle interference of two-dimensional surface states. The distribution of scattering wavevectors exhibits pronounced two-fold symmetry, consistent with the nematic electronic structure found in the Ca(Fe1-xCox)2As2 parent state.
We study a spin $S$ quantum Heisenberg model on the Fe lattice of the rare-earth oxypnictide superconductors. Using both large $S$ and large $N$ methods, we show that this model exhibits a sequence of two phase transitions: from a high temperature symmetric phase to a narrow region of intermediate ``nematic'' phase, and then to a low temperature spin ordered phase. Identifying phases by their broken symmetries, these phases correspond precisely to the sequence of structural (tetragonal to monoclinic) and magnetic transitions that have been recently revealed in neutron scattering studies of LaO…
The 57Fe Mössbauer spectroscopy was applied to an iron-based layered superconductor LaO0.89F0.11FeAs with a transition temperature of 26 K and its parent material LaOFeAs. Throughout the temperature range from 4.2 to 298 K, a singlet spectrum with no magnetic splitting was observed as a main component of each Mössbauer spectrum of the F-doped superconductor. No additional internal magnetic field was observed for the spectrum measured at 4.2 K under a magnetic field of 7 T. On the other hand, the parent LaOFeAs shows a magnetic transition at around 140 K, and this temperature is slightly lower …
We present a detailed study on the magnetic order in the undoped mother compound LaOFeAs of the recently discovered Fe-based superconductor LaO$_{1-x}$F$_x$FeAs. In particular, we present local probe measurements of the magnetic properties of LaOFeAs by means of $^{57}$Fe Mössbauer spectroscopy and muon spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5) $μ_B$ at the iron site below T_N = 138 K, well separated from a structural phase transition at …
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 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.
A series of layered (Sr$_{1-x}$K$_x$)Fe$_2$As$_2$ compounds with nominal x=0 to 0.40 are synthesized by solid state reaction method. Similar to other parent compounds of iron-based pnictide superconductors, the pure SrFe$_2$As$_2$ shows a strong resistivity anomaly near 210 K, which was ascribed to the spin-density-wave instability. The anomaly temperature is much higher than those observed in LaOFeAs and BaFe$_2$As$_2$, the two prototype parent compounds with ZrCuSiAs- and ThCr$_2$Si$_2$-type structures. K-doping strongly suppresses this anomaly and induces superconductivity. Like in the case…
From first-principles calculations, we have studied the electronic and magnetic structures of the ground state of LaOFeAs. The Fe spins are found to be collinear antiferromagnetic ordered, resulting from the interplay between the strong nearest and next-nearest neighbor superexchange antiferromagnetic interactions. The structure transition observed by neutron scattering is shown to be magnetically driven. Our study suggests that the antiferromagnetic fluctuation plays an important role in the Fe-based superconductors. This sheds light on the understanding of the pairing mechanism in these mate…
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 …
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…
In order to assist the search of new superconductors in iron selenide materials by intercalation, we calculate the crystal and electronic structures of MgOFeSe using the first-principles density functional theory. MgOFeSe is isotructural to the parent compound of iron pnictide superconductor LaOFeAs. In LaOFeAs, the anion O$^{2-}$ is located at the center of each LaO tetrahedra. But for MgOFeSe, we find that the crystal structure with the cation Mg$^{2+}$ as the tetrahedral center in the MgO layer is energetically more stable. The low energy band structures around the Fermi surfaces of MgOFeSe…
Within the framework of density functional theory we investigate the nature of magnetism in various families of Fe-based superconductors. (i) We show that magnetization of stripe-type antiferromagnetic order always becomes stronger when As is substituted by Sb in LaOFeAs, BaFe$_2$As$_2$ and LiFeAs. By calculating Pauli susceptibilities, we attribute the magnetization increase obtained after replacing As by Sb to the enhancement of an instability at $(π,π)$. This points to a strong connection between Fermi surface nesting and magnetism, which supports the theory of the itinerant nature of magne…
Here we report the superconductivity in the iron-based oxyarsenide Sm[O1-xFx]FeAs, with the onset resistivity transition temperature at 55.0 K and Meissner transition at 54.6 K. This compound has the same crystal structure as LaOFeAs with shrunk crystal lattices, and becomes the superconductor with the highest critical temperature among all materials besides copper oxides.
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…
High resolution photoemission measurements have been carried out on non-superconducting LaOFeAs parent compound and various superconducting R(O1-xFx)FeAs (R=La, Ce and Pr) compounds. We found that the parent LaOFeAs compound shows a metallic character. Through extensive measurements, we have identified several common features in the electronic structure of these Fe-based compounds: (1). 0.2 eV feature in the valence band; (2). A universal 13~16 meV feature; (3). A clear Fermi cutoff showing zero leading-edge shift in the superconducting state;(4). Lack of superconducting coherence peak(s); (5)…
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…
We report the synthesis and characterization of a new electron-doped La-oxypnictide superconductor by partial substitution of lanthanum by thorium. The superconducting transition temperature at about 30.3 K was observed in La0.8Th0.2OFeAs which is the highest in La-based oxypnictide superconductors synthesized at ambient pressure. We find that the decrease in lattice parameters with Th doping in LaOFeAs is more drastic as compared to that obtained by high pressure (6 GPa) synthesis of oxygen deficient LaOFeAs. The resistivity and magnetic susceptibility measurements yield an upper critical fie…
In hole-doped high-Tc superconductors, thermal conductivity increases drastically just below Tc, which has been considered as a hallmark of a nodal gap. In contrast, such a coherence peak in thermal conductivity is not visible in electron-doped compounds, which may indicate a full-gap state such as a (d+is)-wave state. To settle this problem, we study the thermal conductivity in the Hubbard model using the fluctuation-exchange (FLEX) approximation, which predicts that the nodal d-wave state is realized in both hole-doped and electron-doped compounds. The contrasting behavior of thermal conduct…
We report on the synthesis and structural/ microstructural studies of iron based fluorine doped LaOFeAs superconductors. We have successfully synthesized fluorine doped superconducting LaO1-xFxFeAs materials by choosing lower temperature (1150^0C) and longer synthesis duration (60 hours) as compared to the standard values of these employed in the pioneering first contribution [Kamihara et al 2008 J Am Chem Soc 130 3296]. Decrease of lattice parameters as determined by x-ray diffraction confirm the substitution of fluorine. The superconducting transition temperature is 27.5 K which is observed …
We study the emergence of a magnetic resonance in the superconducting state of the electron-doped cuprate superconductors. We show that the recently observed resonance peak in the electron-doped superconductor Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-δ}$ is consistent with an overdamped spin exciton located near the particle-hole continuum. We present predictions for the magnetic-field dependence of the resonance mode as well as its temperature evolution in those parts of the phase diagram where $d_{x^2-y^2}$-wave superconductivity may coexist with an antiferromagnetic spin-density wave.
Within the framework of the kinetic energy driven d-wave superconductivity, the electronic structure of the electron doped cuprate superconductors is studied. It is shown that although there is an electron-hole asymmetry in the phase diagram, the electronic structure of the electron-doped cuprates in the superconducting-state is similar to that in the hole-doped case. With increasing the electron doping, the spectral weight in the $(π,0)$ point increases, while the position of the superconducting quasiparticle peak is shifted towards the Fermi energy. In analogy to the hole-doped case, the sup…
We evaluate the doping dependence of the quasiparticle current and low temperature superfluid density in two slave-particle theories of the tt't''J model -- the slave-boson theory and doped-carrier theory. In the slave-boson theory, the nodal quasiparticle current renormalization factor $α$ vanishes proportionally to the zero temperature superfluid density $ρ_S(0)$; however, we find that away from the $ρ_S(0) \to 0$ limit $α$ displays a much weaker doping dependence than $ρ_S(0)$. A similar conclusion applies to the doped-carrier theory, which differentiates the nodal and antinodal regions of …
An antiferromagnetic (AF) spin fluctuation induced pairing model is proposed for the electron-doped cuprate superconductors. It suggests that, similar to the hole-doped side, the superconducting gap function is monotonic d_{x^2-y^2}-wave and explains why the observed gap function has a nonmonotonic d_{x^2-y^2}-wave behavior when an AF order is taken into account. Dynamical spin susceptibility is calculated and shown to be in good agreement with the experiment. This gives a strong support to the proposed model.
The tunneling spectra of the electron-doped cuprate Pr_2-xCe_xCuO4 as a function of doping and temperature is reported. We find that the superconducting gap, delta, shows a BCS-like temperature dependence even for extremely low carrier concentrations (studied here for the first time). Moreover, delta follows the doping dependence of Tc, in strong contrast with tunneling studies of hole-doped cuprates. From our results we conclude that there is a single superconducting energy scale in the electron-doped cuprates.
We report resistivity, Hall effect, Nernst effect, and magnetoresistance measurements on T'-phase La1.85Y0.15CuO (LYCO) films prepared by pulsed laser deposition under different oxygen conditions. Our results show that superconductivity in LYCO originates from an oxygen-doped Mott-like insulator and not from a weakly correlated, half-filled band metal as proposed previously.
Quantum oscillations in hole doped high temperature superconductors are difficult to understand within the prevailing views. An emerging idea is that of a putative normal ground state, which appears to be a Fermi liquid with a reconstructed Fermi surface. The oscillations are due to formation of Landau levels. Recently the same oscillations were found in the electron doped cuprate, $\mathrm{Nd_{2-x}Ce_{x}CuO_{4}}$, in the optimal to overdoped regime. Although these electron doped non-stoichiometric materials are naturally more disordered, they strikingly complement the hole doped cuprates. Her…
We present magnetotransport evidence for antiferromagnetism in films of the electron-doped cuprates Pr$_{2-x}$Ce$_x$CuO$_4$. Our results show clear signature of static antiferromagnetism up to optimal doping x=0.15, with a quantum phase transition close to x=0.16, and a coexistence of static antiferromagnetism and superconductivity for 0.12$\le$x$\le$0.15.
We present conductance-voltage (G-V) data for point contact junctions between a normal metal and the electron doped cuprate superconductor Pr{2-x}Ce{x}CuO4 (PCCO). We observe a zero bias conductance peak (ZBCP) for the under-doped composition of this cuprate (x=0.13) which is consistent with d-wave pairing symmetry. For optimally-doped (x=0.15) and over-doped (x=0.17) PCCO, we find that the G-V characteristics indicate the presence of an order parameter without nodes. We investigate this further by obtaining point contact spectroscopy data for different barrier strengths and as a function of m…
High-resolution laser-based angle-resolved photoemission measurements have been carried out on the electron-doped (Nd$_{1.85}$Ce$_{0.15}$)CuO$_4$ high temperature superconductor. We have revealed a clear kink at $\sim$60 meV in the dispersion along the (0,0)-($π$,$π$) nodal direction, accompanied by a peak-dip-hump feature in the photoemission spectra. This indicates that the nodal electrons are coupled to collective excitations (bosons) in electron-doped superconductors, with the phonons as the most likely candidate of the boson. This finding has established a universality of nodal electron c…
High Tc superconductors show a rich variety of phases associated with their charge degrees of freedom. Valence charges can give rise to charge ordering or acoustic plasmons in these layered cuprate superconductors. While charge ordering has been observed for both hole- and electron-doped cuprates, acoustic plasmons have only been found in electron-doped materials. Here, we use resonant inelastic X-ray scattering (RIXS) to observe the presence of acoustic plasmons in two families of hole-doped cuprate superconductors [La2-xSrxCuO4 (LSCO) and Bi2Sr1.6La0.4CuO6+d (Bi2201)], crucially completing t…
Using as a model the Hubbard Hamiltonian we determine various basic properties of electron-doped cuprate superconductors like ${Nd}_{2-x}{Ce}_{x}{CuO}_{4}$ and ${Pr}_{2-x}{Ce}_{x}{CuO}_{4}$ for a spin-fluctuation-induced pairing mechanism. Most importantly we find a narrow range of superconductivity and like for hole-doped cuprates $d_{x^{2}-y^{2}}$ - symmetry for the superconducting order parameter. The superconducting transition temperatures $T_{c}(x)$ for various electron doping concentrations $x$ are calculated to be much smaller than for hole-doped cuprates due to the different Fermi surf…
We reformulate the mean-field theory associated with the SU(2) gauge theory of spin-charge separation in doped Mott-insulators of Wen and Lee so that it can deal with the regime of substantial carrier density. We find that it describes remarkably accurately phase separation tendencies. Moreover, for elegant reasons the theory insists that the superconducting state should have a d+s orderparameter with an s-wave component growing in the overdoped regime. This appears to be consistent with both Raman- and tunneling measurements in cuprate superconductors.
Using a phenomenological lattice model of coupled spin and charge modes, we determine the spin susceptibility in the presence of fluctuating stripe charge order. We assume the charge fluctuations to be slow compared to those of the spins, and combine Monte Carlo simulations for the charge order parameter with exact diagonalization of the spin sector. Our calculations unify the spin dynamics of both static and fluctuating stripe phases and support the notion of a universal spin excitation spectrum in doped cuprate superconductors.
We present data analyses of the thermal conductivity and specific heat of electron-doped Pr1.85Ce0.15CuO4-y. The zero-field thermal conductivity and specific heat of this optimally electrondoped system can be only explained by a nodeless gap symmetry. The magnetic-field dependence of the electronic specific heat in the vortex state is in quantitative agreement with an s-wave theory. Our quantitative data analyses provide bulk evidence for a nodeless gap symmetry in optimally electron-doped cuprates.
Neutron-scattering experiments were performed to study the magnetic field effect on the electron-doped cuprate superconductor Nd_{1.86}Ce_{0.14}CuO_4, which shows the coexistence of magnetic order and superconductivity. The (1/2 3/2 0) magnetic Bragg intensity, which mainly originates from the order of both the Cu and Nd moments at low temperatures, shows no magnetic field dependence when the field is applied perpendicular to the CuO_{2} plane up to 10 T above the upper critical field. This result is significantly different from that reported for the hole-doped cuprate superconductors, in whic…
We have analyzed data from angle resolved photoemission spectroscopy, Fourier transform scanning tunnelling spectroscopy, and low-temperature thermal conductivity for optimally doped Bi_2Sr_2CaCu_2O_8+y in order to discriminate between d-wave and extended s-wave pairing symmetry. The combined data are inconsistent with d-wave symmetry, but quantitatively consistent with extended s-wave symmetry with eight line nodes. We also consistently explain all the phase-sensitive experiments.
We studied low-frequency spin dynamics near the impurity-induced superconductor-to-insulator transition for underdoped high-Tc superconductor YBa2(Cu1-xMx)4O8 (M=Ni, Zn) using the Cu nuclear quadrupole resonance (NQR) spin-echo technique. We observed remarkable suppression of the normal-state pseudo spin-gap and a loss of Cu NQR spectrum intensity at low temperatures around the critical impurity concentration.
On the basis of the Hubbard model, we extend the fluctuation-exchange (FLEX) approach to investigating the properties of antiferromagnetic (AF) phase in electron-doped cuprate superconductors. Furthermore, by incorporating the effect of scatterings due to the disordered dopant-atoms into the FLEX formalism, our numerical results show that the antiferromagnetic transition temperature, the onset temperature of pseudogap due to spin fluctuations, the spectral density of the single particle near the Fermi surface, and the staggered magnetization in the AF phase as a function of electron doping can…
We have developed a real space spin-parallel theory of superconductivity based on the minimum principle in energy. This theory has successfully provided coherent explanations to a number of complicated problems in conventional and non-conventional superconductors. In this paper, we report the study the optimal doping problem in the new oxypnictide high-temperature superconductors using aforementioned theory. In FeAs family, it is shown that there are three optimum (or quasi-optimum) doped phases at doping levels x1=1/3, 1/6 and 1/8, where the vortex lattice forms square or triangular stable co…
Electronic nematicity is often found in unconventional superconductors, suggesting its relevance for electronic pairing. In the strongly hole-doped iron-based superconductors, the symmetry channel and strength of the nematic fluctuations, as well as the possible presence of long-range nematic order, remain controversial. Here, we address these questions using transport measurements under elastic strain. By decomposing the strain response into the appropriate symmetry channels, we demonstrate the emergence of a giant in-plane symmetric contribution, associated with the growth of both strong ele…
We show that, at weak to intermediate coupling, antiferromagnetic fluctuations enhance d-wave pairing correlations until, as one moves closer to half-filling, the antiferromagnetically-induced pseudogap begins to suppress the tendency to superconductivity. The accuracy of our approach is gauged by detailed comparisons with Quantum Monte Carlo simulations. The negative pressure dependence of Tc and the existence of photoemission hot spots in electron-doped cuprate superconductors find their natural explanation within this approach.
Hydrogen (H)-doped LaFeAsO is a prototypical iron-based superconductor. However, its phase diagram extends beyond the standard framework, where a superconducting (SC) phase follows an antiferromagnetic (AF) phase upon carrier doping; instead, the SC phase is sandwiched between two AF phases appearing in lightly and heavily H-doped regimes. We performed nuclear magnetic resonance (NMR) measurements under pressure, focusing on the second AF phase in the heavily H-doped regime. The second AF phase is strongly suppressed when a pressure of 3.0 GPa is applied, and apparently shifts to a highly H-do…
The pairing state symmetry of the electron-doped cuprate superconductors is thought to be s-wave in nature, in contrast with their hole-doped counterparts which exhibit a d-wave symmetry. We re-examine this issue based on recent improvements in our electron-doped materials and our measurement techniques. We report microwave cavity perturbation measurements of the temperature dependence of the penetration depth of Pr_(2-x)Ce_(x)CuO_(4-y) and Nd_(2-x)Ce_(x)CuO_(4-y) crystals. Our data strongly suggest that the pairing symmetry in these materials is not s-wave.
This paper describes the first 63-Cu NMR study of an electron-doped infinitely-layered high Tc superconductor Sr$_0.9$La$_0.1$CuO$_2$ (Tc=42K). The spin dynamics in the normal state above Tc exhibits qualitatively the same behavior as some hole-doped materials with significantly enhanced spin fluctuations. Below Tc, we observed no signature of a Hebel-Slichter coherence peak, suggesting an unconventional nature of the symmetry of the superconducting order parameter.
We present results of it ab initio LDA calculations of electronic structure of "next generation" layered ironpnictide High-Tc superconductor LiFeAs (Tc=18K). Obtained electronic structure of LiFeAs is very similar to recently studied ReOFeAs (Re=La,Ce,Pr,Nd,Sm) and AFe2As2 (A=Ba,Sr) compounds. Namely close to the Fermi level its electronic properties are also determined ma inly by Fe 3d-orbitals of FeAs4 two-dimensional layers. Band dispersions of LiFeAs are very similar to the LaOFeAs and BaFe2As2 systems as well as the shape of the Fe-3d density o f states and Fermi surface.
We report on semiclassical angle-dependent magnetoresistance oscillations (AMRO) and the Shubnikov-de Haas effect in the electron-overdoped cuprate superconductor Nd$_{2-x}$Ce$_x$CuO$_4$. Our data provide convincing evidence for magnetic breakdown in the system. This shows that a reconstructed multiply-connected Fermi surface persists, at least at strong magnetic fields, up to the highest doping level of the superconducting regime. Our results suggest an intimate relation between translational symmetry breaking and the superconducting pairing in the electron-doped cuprate superconductors.
For electron-doped cuprates, the strong suppression of antiferromagnetic spin correlation by efficient reduction annealing by the "protect-annealing" method leads to superconductivity not only with lower Ce concentrations but also with higher transition temperatures. To reveal the nature of this superconducting state, we have performed angle-resolved photoemission spectroscopy measurements of protect-annealed electron-doped superconductors Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_{4}$ and directly investigated the superconducting gap. The gap was found to be consistent with $d$-wave symmetry, sugges…
We study how manifestations of strong electron-phonon interaction (EPI) depend on the carrier concentration by solving the two-dimensional Holstein model for the spin-polarized fermions using an approximation free bold-line diagrammatic Monte Carlo (BDMC) method. We show that the strong EPI, obviously present at very small Fermion concentration, is masked by the Fermi blockade effects and Migdal's theorem to the extent that it manifests itself as moderate one at large carriers densities. Suppression of strong EPI fingerprints is in agreement with experimental observations in doped high tempera…
The real-space localized hole pair is constructed in the Cu-O plane of the hole-doped cuprate superconductors. We prove analytically and numerically that two electrons, due to the nearest-neighbor Coulomb repulsive confinement effect, can be in pairing inside a single plaquette with the d-wave symmetry. The scenario supports the `no glue' pairing picture for the cuprates. Based on the scenarios, the physical origin of the Fermi pocket (or Fermi arc) and the two pseudogap behavior are provided. Our framework leads directly to a unified linear relationship between the pseudogap temperature T* an…
We consider the screening of an impurity moment in a d-wave superconductor under the influence of a Zeeman magnetic field. Using the Numerical Renormalization Group technique, we investigate the resulting pseudogap Kondo problem, in particular the field-induced crossover behavior in the vicinity of the zero-field boundary quantum phase transition. The impurity spectral function and the resulting changes in the local host density of states are calculated, giving specific predictions for high-field STM measurements on impurity-doped cuprates.
The occurrence of high temperature superconductivity, and the competition with magnetism, in stoichiometric and doped LaOFeAs and isostructural iron-oxypnictides is raising many fundamental questions about the electronic structure and magnetic interactions in this class of materials. There are now sufficient experimental data that it may be possible to identify the important issues whose resolution will lead to the understanding of this system. In this paper we address a number of the important issues. One important characteristic is the Fe-As distance (or more abstractly the pnictogen (Pn) he…
With the aim at understanding the non-monotonic $d_{x^{2}-y^{2}}$-wave gap, we analyze the local electronic structure near impurities in the electron-doped cuprate superconductors. We find that the local density of states near a non-magnetic impurity in the scenario of $d_{x^{2}-y^{2}}$-wave superconductivity with higher harmonics is qualitatively different from that obtained from the $d_{x^{2}-y^{2}}$-wave superconductivity coexisting with antiferromagnetic spin density wave order. We propose that spin-polarized scanning tunneling microscopy measurements can distinguish the two scenarios and …
The pseudogap state is one of the most enigmatic characteristics in the anomalous normal state properties of the high temperature cuprate superconductors. A central issue is to reveal whether there is a symmetry breaking and which symmetries are broken across the pseudogap transition. By performing high resolution laser-based angle-resolved photoemission measurements on the optimally-doped Bi2Sr1.6La0.4CuO6+δ superconductor, we report the observations of the particle-hole symmetry conservation in both the superconducting state and the pseudogap state along the entire Fermi surface. These resul…
