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1.
We have successfully synthesized Ce based oxypnictide superconductors with fluorine doping (CeO1?xFxFeAs) by a two step solid state reaction method. Detailed XRD and EDX confirm the crystal structure and chemical compositions. We observe that an extremely high Hc2(0) of 94 T can be achieved in the x = 0.1 composition. This increase in Hc2(0) is accompanied by a decrease in transition temperature (38.4 K in x = 0.1 composition) from 42.5 K for the x = 0.2 phase. The in-plane Ginzburg–Landau coherence length is estimated to be ~27 Å at x = 0.2 suggesting a moderate anisotropy in this class of superconductors. The Seebeck coefficient confirms the majority carrier to be electrons and strong dominance of electron–electron correlations in this multiband superconductor.  相似文献   

2.
First-principles calculations are performed for hydrogen-doped iron-based superconductors, LaFeAsOHx, which exhibits higher transition temperature than hydrogen-free LaFeAsO. We find that hydrogen atoms favor the positions near FeAs layers and induce FeAs4 tetrahedrons to regular ones, which are considered to bring about higher transition temperature. However, hydrogen doping more than x  0.25 breaks typical Fermi surface and therefore we estimate the optimal doping as x  0.2.  相似文献   

3.
A comparative local structure study of pnictide superconductors Ca0.82La0.18FeAs2 (112-type, Tc∼ 40 K) and Ba0.64K0.36Fe2As2 (122-type, Tc∼ 37 K), using room temperature x-ray total scattering measurements is reported. The Fe–As superconducting active layer is found to be globally similar in both the systems consisting of edge-sharing FeAs4/4 tetrahedra as in all the iron-pnictide superconductors discovered so far. Although optimally superconducting, the active layer in these compounds is found to sustain a large local inhomogeneity. These results thus imply that a nanoscopic manipulation of the Fe–As active layer, rather than its isotropic structural tuning, is the key parameter to control the superconducting properties of the iron-based systems.  相似文献   

4.
We discuss the novel superconducting characteristics and unusual normal-state properties of iron (Fe)-based pnictide superconductors REFeAsO1?y (RE = La, Pr, Nd) and Ba0.6K0.4Fe2As2 (Tc = 38 K) by means of 57FeNMR and 75AsNQR/NMR. In the superconducting state of LaFeAsO0.7 (Tc = 28 K), the spin component of the 57Fe-Knight shift decreases to almost zero at low temperatures, which provide firm evidence of the superconducting state formed by spin-singlet Cooper pairing. The nuclear spin–lattice relaxation rates (1/T1) in LaFeAsO0.7 and Ba0.6K0.4Fe2As2 exhibit a T3-like dependence without a coherence peak just below Tc, indicating that an unconventional superconducting state is commonly realized in these Fe-based pnictide compounds. All these events below Tc are consistently argued in terms of an extended s±-wave pairing with a sign reversal of the order parameter among Fermi surfaces. In the normal state, 1/T1T decreases remarkably upon cooling for both the Fe and As sites of LaFeAsO0.7. In contrast, it gradually increases upon cooling in Ba0.6K0.4Fe2As2. Despite the similarity between the superconducting properties of these compounds, a crucial difference was observed in their normal-state properties depending on whether electrons or holes are doped into the FeAs layers. These results may provide some hint to address a possible mechanism of Fe-based pnictide superconductors.  相似文献   

5.
A brief review of optical and Raman studies on the Fe-based superconductors is given, with special emphasis on the competing phenomenon in this system. Optical investigations on ReFeAsO (Re = rare-earth element) and AFe2As2 (A = alkaline-earth metal) families provide clear evidence for the gap formation in the broken symmetry states, including the partial gaps in the spin-density wave states of parent compounds, and the pairing gaps in the superconducting states for doped compounds. Especially, the superconducting gap has an s-wave pairing lineshape in hole-doped BaFe2As2. Optical phonons at zone center detected by Raman and infrared techniques are classified for several Fe-based compounds. Related issues, such as the electron–phonon coupling and the effect of spin-density wave and superconducting transitions on phonons, are also discussed. Meanwhile, open questions including the T-dependent mid-infrared peak at 0.6–0.7 eV, electronic correlation, and the similarities/differences between high-Tc cuprates and Fe-based superconductors are also briefly discussed. Important results from other experimental probes are compared with optical data to better understand the spin-density wave properties, the superconductivity, and the multi-band character in Fe-based compounds.  相似文献   

6.
Inelastic neutron scattering has been performed on powder sample of an iron-based superconductor BaFe2(As0.65P0.35)2 with superconducting transition temperature (Tc) = 30 K, whose superconducting (SC) order parameter is expected to have line node. In the normal state, constant-E scan of dynamical structure factor, S(Q, E), exhibits a peak structure centered at momentum transfer Q  1.20 Å?1, corresponding to antiferromagnetic wave vector. Below Tc, the redistribution of the magnetic spectral weight takes place, resulting in the formation of a peak at E  12 meV and a gap below 6 meV. The enhanced magnetic peak structure is ascribed to the spin resonance mode, evidencing sign change in the SC order parameter similar to other iron-based high-Tc superconductors. It suggests that fully-gapped s± symmetry dominates in this superconductor, which gives rise to high-Tc (=30 K) despite the nodal symmetry.  相似文献   

7.
Magnetic and dielectric properties have been investigated for Bi2CuO4, which has the same chemical formula as that of the parent materials of cuprate superconductors R2CuO4 (R: rare earths). Magnetization measurements show the antiferromagnetic transition of the Cu2+ spins at ~42 K, as reported previously. Dielectric measurements for the frequencies of 1 kHz to 1 MHz show that the dielectric constants are 100–500 at room temperature. The dielectric dispersion reveals that the dielectric response lacks spatial coherence, a property which indicates the possible existence of phase separation as suggested for La2CuO4. The imaginary part of dielectric response gives the activation energy of 0.22 eV, suggesting that the dielectric response is governed by the electron hopping between the Cu ions.  相似文献   

8.
The vortex structure in p-wave superconductors is investigated by the Bogoliubov–de Gennes theory on a tight-binding model. We calculate the temperature dependence of the electronic state at each site in the vortex lattice state, and show the difference between sin px+i sin py-wave and sin px−i sin py-wave superconducting state. Furthermore the relation of the electronic structure and the site-dependence of the nuclear magnetic relaxation time is also discussed.  相似文献   

9.
The recent discovery of arsenic-based high temperature superconductors has reignited interest in the study of superconductor: biological interfaces. However, the new superconductor materials involve the chemistry of arsenic and their toxicity remains unclear [Hand, E., 2008. Nature 452 (24), 922]. In this study the possible adverse effects of this new family of superconductors on cells have been examined. Cell culture studies in conjunction with microscopy and viability assays were employed to examine the influence of arsenic-based superconductor PrOxFeAs (x = 0.75) material in vitro. Imaging data revealed that cells were well adhered and spread on the surface of the superconductor. Furthermore, cytotoxicity studies showed that cells were unaffected during the time-course of the experiments, providing support for the biocompatibility aspects of PrOxFeAs-based superconductor material.  相似文献   

10.
The effect of yttrium substitution at the lanthanum site on the superconducting properties of La1?xYxO0.9F0.1FeAs (‘x = 0, 0.10, 0.20, 0.30, 0.50 and 0.60) oxypnictides has been studied. Powder X-ray diffraction studies confirm single phases till x = 0.1 beyond which minor amount of Y2O3 is observed. The temperature dependence of resistivity measurements confirm the superconducting transition temperature (Tc) of 34.8 (±0.05) K and corresponding Meissner transition at 34.3 K in the ‘x = 0.3 composition which is higher than that reported for the parent phase (LaO0.9F0.1FeAs (Tc = 28 K)). Further increase in the concentration of yttrium leads to broadening and suppression of the superconducting transition. The value of Hc2 at zero temperature is estimated to be about 60.5 T. The Seebeck coefficient (S) shows a negative sign indicating that the major contribution to the conductivity is by electrons. The Hall coefficient (RH) also remains negative throughout the temperature range supporting the thermopower results. The lattice parameters (a and c) decreases and the charge-carrier density increases with yttrium doping.  相似文献   

11.
Recent studies of high- Tcsuperconductors have clarified new aspects of tunneling spectroscopy. The unconventional pairing states, i.e. d-wave symmetry in these materials have been established through various measurements. Differently from isotropic s-wave superconductors, d-wave pairing states have an internal phase of the pair potential. The internal phase modifies the surface states due to the interference effect of the quasiparticles. Along these lines, a novel formula of tunneling spectroscopy has been presented that fully takes into account of the anisotropy of the pair potential. The most essential difference of this formula from conventional ones is that it suggests the phase-sensitive capability of tunneling spectroscopy. The formula suggests that the symmetry of the pair potential is determined by the orientational dependence measurements of tunneling spectroscopy. Along these lines, several experiments have been performed on high-Tc superconductors. The observation of the zero-bias conductance peaks (ZBCP) on Y Ba2Cu3O7  δstrongly suggests the dx2  y2-wave pairing states of hole-doped high-Tc superconductors. On the other hand, the absence of ZBCP on (electron-doped)Nd1.85Ce0.15CuO4  δindicates that the pair potential of this material is a nodeless state. In this paper, recent developments of tunneling spectroscopy for anisotropic superconductors are reviewed both on theoretical and experimental aspects.  相似文献   

12.
Here we report the first-principle FLAPW-GGA calculations of the elastic properties of two related layered phases, namely, the ternary arsenide SrFe2As2 and the quaternary oxyarsenide LaFeAsO – basic phases for the newly discovered “1 2 2” and “1 1 1 1” 38–55 K superconductors. The independent elastic constants (Cij), bulk moduli, compressibility, and shear moduli are evaluated and discussed. The numerical estimates of the elastic parameters of the polycrystalline SrFe2As2 and LaFeAsO ceramics are performed for the first time. Additionally, the peculiarities of chemical bonding in these phases are discussed.  相似文献   

13.
In this paper we present, for the first time, cytogenetical data on Latrodectus gr. curacaviensis (Theridiidae) from Brazil, as well as the first data on meiosis and sex chromosome system of this genus. Testes were submitted to colchicine, hypotonic, and fixation treatment, and chromosomal preparations were stained with Giemsa solution. The analysis showed 2n = 26 telo/acrocentric chromosomes in spermatogonial metaphases. Metaphase I exhibited 12 autosomal bivalents and two sex chromosome univalents (12II + X1X2). All bivalents revealed one terminal chiasma. Metaphases II confirmed the sex chromosome system, showing 12 autosomes or 12 autosomes plus two X chromosomes, respectively. Male karyotype prevailing in theridiids is formed by 2n = 22 chromosomes, including sex chromosome system X1X2 in all species. The Latrodectus species of the geometricus clade analyzed until now showed smaller diploid number (2n = 16 and 2n = 18) than the species of the mactans clade (2n = 24 and 2n = 26). Thus, according to the chromosome number, the examined Latrodectus species seems to be related to the mactans clade.  相似文献   

14.
《Solid State Ionics》2006,177(19-25):1807-1810
The crystal chemistry and mixed conductor properties of the n = 2 member of the Ruddlesden–Popper (R–P) phases Sr3−xLaxFe2−yNiyO7−δ with 0  x  0.3 and 0  y  1.0 have been studied at high temperature. High-temperature X-ray diffraction and thermogravimetric measurements of the equilibrium pO2 (10 5  pO2  1 atm) in the temperature range 400  T  1000 °C indicate that the Sr3FeNiO7−δ phase is able to accommodate a large oxygen non-stoichiometry (δ  1.5) without structural transformations. The electrical conductivity and oxygen permeability increase with the substitution of Ni for Fe in the range 550  T  1000 °C. The electrical transport of the Sr3FeNiO7−δ phase is thermally activated and the activation energy decreases with the substitution of Ni for Fe for a given oxygen content. The increase in the oxygen permeation flux with increasing Ni content is due to an increasing oxygen non-stoichiometry and a lower activation energy for permeation.  相似文献   

15.
《Solid State Ionics》2006,177(3-4):237-244
Ongoing studies of the KHSeO4–KH2PO4 system aiming at developing novel proton conducting solids resulted in the new compound K2(HSeO4)1.5(H2PO4)0.5 (dipotassium hydrogenselenate dihydrogenphosphate). The crystals were prepared by a slow evaporation of an aqueous solution at room temperature. The structural properties of the crystals were characterized by single-crystal X-ray analysis: K2(HSeO4)1.5(H2PO4)0.5 (denoted KHSeP) crystallizes in the space group P 1¯ with the lattice parameters: a = 7.417(3) Å, b = 7.668(2) Å, c = 7.744(5) Å, α = 71.59(3)°, β = 87.71(4)° and γ = 86.04(6)°. This structure is characterized by HSeO4 and disordered (HxSe/P)O4 tetrahedra connected to dimers via hydrogen bridges. These dimers are linked and stabilized by additional hydrogen bonds (O–H–O) and hydrogen bridges (O–H…O) to build chains of dimers which are parallel to the [0, 1, 0] direction at the position x = 0.5.The differential scanning calorimetry diagram showed two anomalies at 493 and 563 K. These transitions were also characterized by optical birefringence, impedance and modulus spectroscopy techniques. The conductivity relaxation parameters of the proton conductors in this compound were determined in a wide temperature range. The transport properties in this material are assumed to be due to H+ protons hopping mechanism.  相似文献   

16.
We have performed an ab initio band structure calculation for the new high-Tc related iron-pnictide compounds LaFeXO (X = P, As), BaFe2As2, CaFe2As2 and LiFeAs (X = P, As). We found that LaFeXO and CaFe2As2 have many similarities in their band structures, which is expected by an ionic model. We found that the degree of distortion of FeAs4 tetrahedra in LaFeAsO considerably changes the slope of the density of states near the Fermi level, and this result may explain why REFeAsO (RE = Nd, Sm, …) have higher Tc than LaFeAsO when electrons are doped. For all the above compounds, the density of states at the Fermi level decreases when X atoms approaches to the Fe–Fe plane, which means that the hybridization between Fe and X orbitals considerably expands the Fe d-bands.  相似文献   

17.
The effect of calcium doping on the superconducting properties of top seeded melt growth (TSMG) processed Y1.5Ba2?xCaxCu3Oy superconductors was studied in terms of calcium content (Xca). YBa2?xCaxCu3O7?δ (Xca = 0, 0.005, 0.01, 0.02, 0.04, 0.1, 0.3) powders were synthesized by the powder calcination method. YBa2?xCaxCu3O7?δ powders were mixed with 0.25 mole Y2O3 powder and 1 wt.% CeO2 as Y2BaCuO5 (Y211) refiner, and finally made into Y1.5Ba2?xCaxCu3Oy (Y1.5) + 1 wt.% CeO2 composition. The single Y123 growth on the top surface was observed up to Xca = 0.1, while the multiple Y123 growth was observed at Xca ? 0.1. The superconducting transition temperature (Tc) and critical current density (Jc) of TSMG processed Y1.5 samples were inversely proportional to Xca. The Y211 size increased with increasing Xca due to the enhancement of Y211 coarsening by calcium doping. No Y211 refining effect by CeO2 was observed in the calcium doped samples. The Tc and Jc decrease by calcium doping are likely to be due to the calcium incorporation with the Y123 lattice and formation of coarse Y211 particles.  相似文献   

18.
The hydrostatic pressure effect on the dielectric properties of (NH4)2H2P2O6 ferroelectric crystal was studied for pressures from 0.1 MPa to 360 MPa and for temperatures from 100 to 190 K. The pressure–temperature phase diagram obtained is linear with increasing pressure. The paraelectric–ferroelectric phase transition temperature decreases with increasing pressure with the pressure coefficient dTc/dp=?5.16×10?2 K MPa?1. Additionally, the pressure dependences of Curie–Weiss constants for the crystal in paraelectric (C+) and ferroelectric (C?) phases are evaluated and discussed. The possible mechanism of paraelectric–ferroelectric phase transition is also discussed.  相似文献   

19.
The N2 and O2 pressure broadening coefficients of the pure rotational transitions at 625.66 GHz (NKaKc=101?9–100?10, J=10.5–10.5) and 649.70 GHz (NKaKc=102?9–92?8, J=9.5–8.5) in the vibronic ground state X2A′ of the perhydroxyl (HO2) radical have been determined by precise laboratory measurements. For the production of HO2, the mercury-photosensitized reaction of the H2 and O2 precursors was used to provide an optimum condition for measurement of the pressure broadening coefficient. The Superconducting Submillimeter-wave Limb Emission Sounder (SMILES) was designed to monitor the volume mixing ratio of trace gases including HO2 in the Earth's upper atmosphere using these transitions. The precise measurement of pressure broadening coefficient γ in terms of the half width at half maximum is required in order to retrieve the atmospheric volume mixing ratio. In this work, γ coefficients of the 625.66 GHz transition were determined for N2 and O2 at room temperature as γ(N2)=4.085±0.049 MHz/Torr and γ(O2)=2.578±0.047 MHz/Torr with 3σ uncertainty. Similarly, the coefficients of the 649.70 GHz transition were determined as γ(N2)=3.489±0.094 MHz/Torr and γ(O2)=2.615±0.099 MHz/Torr. The air broadening coefficients for the 625.66 GHz and 649.70 GHz lines were estimated at γ(air)=3.769±0.067 MHz and 3.298±0.099 MHz respectively, where the uncertainty includes possible systematic errors. The newly determined coefficients are compared with previous results and we discuss the advantage of the mercury-photosensitized reaction for HO2 generation. In comparison with those of other singlet molecules, the pressure broadening coefficients of the HO2 radical are not much affected by the existence of an unpaired electron.  相似文献   

20.
The solid solutions BaAl1−xSi1+x (0  x  0.5) were prepared. The compound with the stoichiometric composition (x = 0) did not show superconductivity as reported by other investigators, but the solid solutions with x > 0 became superconductors with a transition temperature Tc = 2.8 K. The comparison of the lattice parameters with those of the other isotypic ternary superconductors MAlSi (M = Ca, Sr) suggested that the superconductivity could be related to the lattice parameter within the (AlSi) plane rather than the interlayer spacing. The band structures near the Fermi level of MAlSi (M = Ca, Sr, Ba) were measured using soft X-ray photoelectron spectroscopy, which were in good agreement with the calculated ones, confirming that the contribution of the d orbitals of the alkaline-earth metals were predominant in the conduction bands.  相似文献   

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