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1.
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   

2.
Single crystals of (Bi1−xPbx)2Sr2Ca2Cu3O10+δ (x = 0 and 0.16) (sizes up to 3 × 2 × 0.1 mm3) have been grown by means of a newly developed “vapour-assisted travelling solvent floating zone” technique (VA-TSFZ). Post-annealing under high pressure of O2 (up to 10 MPa at T = 500 °C) was applied to enhance Tc (up to 111 K) and improve the homogeneity of the crystals (ΔTc  1 K). The structure of both Pb-free and Pb-doped Bi-2223 was refined for the first time from single crystal X-ray diffraction (XRD) data. The unit cell of the average structure is pseudo-tetragonal with a = 5.4210(7), b = 5.4133(6) and c = 37.010(7) Å, and a = 5.395(1), b = 5.413(1) and c = 37.042(11) Å, for the Pb-free and the Pb-doped phase, respectively. An incommensurate modulation in the direction of one of the short cell vectors has been defined (q  0.21 a1), however, the structure can be conveniently described in a supercell with a fivefold volume (a = 27.105(4) Å). With respect to the “non-modulated” structure, one additional oxygen atom for ten initial O was found to be inserted into the BiO layers. The superconducting anisotropy of Bi-2223 was found to be ∼50, from measurements of the lower critical field. The anisotropy of Bi-2223 is significantly reduced compared to that of Bi-2212, and this accounts for the enhanced irreversibility fields in Bi-2223. Furthermore, Bi-2223 has a higher critical current density, and a reduced magnetic relaxation rate compared to Bi-2212, which are both signatures of more effective pinning in Bi-2223 due to its reduced anisotropy.  相似文献   

3.
Ba1?xKxBiO3 with x from 0.315 to 0.6 were successfully synthesized by molten salts method and characterized by XRD and magnetic susceptibility measurements. It is found that Ba1?xKxBiO3 powders could directly be precipitated from KOH melts. Superconductivity has been observed in all samples and the highest superconducting transition temperature was found to be Tc = 30.6 K with x = 0.4. The lattice constant linearly depended on the potassium content in accord with the equation of a = 4.3548–0.1743x, and the decrease of the mole ratio of Bi3+/Bi5+ resulted in the increase of the potassium content, which suggested the disproportionation of Bi valence.  相似文献   

4.
The superconducting R1.4Ce0.6RuSr2Cu2O10  δ(R = Sm, Eu and Gd) withTc  28, 32 and 42 K are also magnetically ordered atTN  220, 122 and 180 K, respectively, thus,TN  Tc. This is in contrast to intermetallic magnetic superconductors (such as RNi2B2C) in whichTc  TN. Magnetic susceptibility and Mossbauer spectroscopy show that superconductivity is confined to the CuO2planes, whereas magnetism is due to the Ru sublattice. Irreversibility phenomena and magnetic anomalies, observed at low magnetic fields originate from antisymmetric exchange coupling of the Dzyaloshinsky–Moria type, and from spin reorientation of the Ru moments. The shielding fraction is about 100%, supporting the conclusion that the materials consist of a single phase, manifesting both magnetism and superconductivity at once.  相似文献   

5.
Isoelectronic Tellurium (Te) substitution for Selenium (Se) in the tetragonal phase of FeSe (β-FeSe) increases the superconducting transition temperature (Tc) by applying a negative pressure on the lattice. However, the normal state resistivity increases and shows semi-metallic behavior for samples with higher Te concentration. With increasing Te concentration, the Tc increases and reaches a maximum for FeSe0.5Te0.5 and then decreases with further increase of Te. We have investigated the effect of Cobalt (Co) and Nickel (Ni) doping in FeSe0.5Te0.5 in the nominal composition range Fe1?xTMxSe0.5Te0.5 (TM = Co (x = 0.05, 0.1, 0.15, 0.2) and Ni (x = 0.05, 0.1)). Both Co and Ni doping suppress Tc and drives the system to metal–insulator transition. The in-plane (‘a’) and out-of-plane (‘c’) lattice constants decrease with increasing dopant concentration.  相似文献   

6.
In this paper, sulfur substitution and pressure effect on superconductivity of α-FeSe has been investigated in Fe(Se1 ? xSx)0.88 (x = 0.1, 0.2). For x = 0.1, the critical temperature Tc is slightly larger than that of non-substituted sample, in consistent with the pressure effect on the superconductivity of α-FeSe. However, with further increasing S content to x = 0.2, Tc decreases. Temperature dependent of specific heat showed that the structural transition seems to be suppressed. Tc for x = 0.2 can be further decreased by applying pressure of 5 kbar, in contrary to the pressure effect on α-FeSe. We suggest that, in addition to the suppression of structural transition, other factors like the increase of carrier concentration should be considered for understanding the pressure effect on the superconductivity of α-FeSe.  相似文献   

7.
Annealing effects of FeSe1?xTex (0.6  x  1) single crystals have been investigated from measurements of the powder X-ray diffraction and specific heat. Through the annealing, several peaks of powder X-ray diffraction have become sharp and a clean jump of the specific-heat at the superconducting (SC) transition temperature, Tc, has been observed for x = 0.6–0.9, indicating bulk superconductivity. For annealed single-crystals of x = 0.6–0.8, the SC condensation energy, U0, and the SC gap, Δ0, at 0 K have been estimated as ~1.8 J/mol and 2.3–2.5 meV, respectively. The value of 2Δ0/kBTc is 3.9–4.5, indicating a little strong-coupling superconductivity. Both the electronic specific-heat coefficient in the normal state, γn, and the residual electronic specific-heat coefficient in the SC state, γ0, have been found to show significant x dependence. The values of γn are much larger than those estimated from the band calculation.  相似文献   

8.
A series of superconducting cuprates with the nominal composition YBa2Cu3  xCdxO7  yand the effect of Cd substitution on Cu sites in this compound is presented. X-ray powder diffraction patterns for these cadmium cuprates with reduced diamagnetism indicate an orthorhombic unit cell like-perovskite structure for (0  x  0.15), while for higher Cd concentration, i.e.x = 1.0 the material is polyphasic. The observed superconducting transition temperature of the samples is nearly the same ([formula] K), except for (x = 1.0) whereTcdrops to 72 K and a transition from metallic to semiconducting behavior of the normal state of the resistivity is observed. Such a decrease inTcfor higher Cd concentration could be attributed to the presence of the green phase in this composition.  相似文献   

9.
We realized highly underdoped Bi-2212 crystals by using a Bi substitution into a Sr site (x), together with the excess oxygen (δ) control. The samples with x = 0–0.3 were grown by a traveling solvent floating zone method. For each x, δ was varied using the precision annealing method [1]. Under several assumptions, the effective Bi valence was evaluated to be +2.2, which is much lower than the formal valence +3. This indicates that the Bi substitution accompanies additional uptake in excess oxygen. The x dependent δ-doping level p relation and the observed shrink in the c-axis length are consistent with this assumption. Based on the decomposition phase diagram obtained in this study, Tc of 22 K was realized by careful annealing treatments. The samples showed a superconducting volume fraction of over 50%.  相似文献   

10.
Substitution of Ge4+ in place of Cu in Tl0.85Cr0.15Sr2CaCu2?xGexO7?δ (x = 0–0.6) showed initial increase in zero critical temperature value, Tc zero from 98 K (x = 0) to 100 K (x = 0.1) and in the range of 85–86 K for x = 0.2–0.3. The slow decrease in Tc zero is unexpected as tetravalent Ge4+ substitution is expected to strongly reduce hole concentration in the samples and suppress Tc zero. Excess conductivity analyses of resistance versus temperature data based on Asmalazov–Larkin (AL) theory revealed that the substitution induced 2D-to-3D transition of fluctuation induced conductivity with the highest transition temperature, T2D3D observed at x = 0.1. FTIR spectroscopy analysis indicates Ge4+ substitution cause reduction in CuO2/GeO2 interplanar distance while our calculation based on Lawrence–Doniach model revealed highest superconducting coherence length, ξc(0) and interplanar coupling, J at x = 0.3. On the other hand, substitution of divalent Mg2+ for Ca2+ in (Tl0.5Pb0.5)(Sr1.8Yb0.2)(Ca1?yMgy)Cu2O7 (y = 0–1.0), which is not expected to directly vary hole concentration, surprisingly caused Tc zero to increase from 89.6 K (y = 0) to an optimum value of 95.9 K (y = 0.6) before decreasing with further increase in y. Excess conductivity analyses showed 2D-to-3D transition of fluctuation induced conductivity for all samples where the highest T2D3D was at y = 0.4. Similar calculation revealed highest values of ξc(0) and J also at y = 0.4. FTIR analysis of the samples indicates inequivalent Cu(1)O(2)Pb/Tl lengths and possible tilting of CuO2 plane as a result of Mg2+ substitution. The increased ξc(0) and J as a result of the Ge4+ and Mg2+ substitutions are suggested to contributed to sustenance of superconductivity above 80 K in the samples.  相似文献   

11.
Low-energy spin excitations have been studied on polycrystalline LaFeAsO1?xFx samples by inelastic neutron scattering. The Q-integrated dynamical spin susceptibility χ″(ω) of the superconducting samples is found to be comparable to that of the magnetically ordered parent sample. On the other hand, χ″(ω) almost vanishes at x = 0.158, where the superconducting transition temperature Tc is suppressed to 7 K. In addition, χ″(ω) in optimally doped LaFeAsO0.918F0.082 with Tc = 29 K exhibits a spin resonance mode. The peak energy, Eres, when scaled by kBTc is similar to the value of about 4.7 reported in other high-Tc iron-based superconductors. This result suggests that there is intimate relationship between the dynamical spin susceptibility and high-Tc superconductivity in iron-based superconductors, and is consistent with a nesting condition between Fermi surfaces at the Γ and M points.  相似文献   

12.
《Solid State Ionics》2006,177(9-10):907-913
The electrode reaction of the perovskite phases Sr1−xLaxCo0.8Fe0.2O3−δ (x = 0.1 and 0.6) on Ce0.9Gd0.1O1.95 has been investigated by impedance spectroscopy in the temperature range 600  T  800 °C. Thick porous electrodes (t 20 μm) were sprayed on Ce0.9Gd0.1O1.95 and ac impedance spectra were recorded on symmetrical cells at the equilibrium. The analysis of the complex impedance diagrams clearly indicates the presence of two contributions. The low frequency one was assigned to the gas phase oxygen diffusion through the porous electrode and a finite length diffusion (Warburg) impedance was used to describe the high frequency (HF) data. The polarization resistance of the HF impedance contribution (Rw) is higher for x = 0.1 while the activation energy of Rw is higher for x = 0.6. The variations of Rw versus the La content, temperature and thickness indicate that the Warburg-type impedance contains information of both bulk oxygen diffusion and surface processes.  相似文献   

13.
Pt doped 122 iron arsenide SrFe1?xPtxAs2 (0 ? x ? 0.4) was successfully synthesized. The tetragonal unit-cell volume and the lattice constant a increase with increasing the Pt content, while c decreases, suggesting that the Fe ions are indeed replaced by Pt ions. By the Pt doping, the magnetic order of the parent phase is suppressed, and superconductivity emerges at approximately x = 0.15. Tc reaches the maximum of 16 K at x = 0.2. The compounds series can be a suitable subject to investigate role of the doped 5d state in the superconducting 3d Fe–As layer.  相似文献   

14.
We have studied the effect of negative chemical pressure in the RuGd1.5(Ce0.5?xPrx)Sr2Cu2O10?δ with Pr content of 0.0 ? x ? 0.2. This is also investigated using the bond length results obtained from the Rietveld refinement analysis. The c parameter and cell volume increase with x for 0.0 ? x ? 0.15. The width of the resistivity transition also increases with Pr concentration, indicating higher inhomogeneity and oxygen deficiency. The difference in the ionic valences of Pr3+,4+ and Ce4+ causing different hole doping, the difference in the ionic radii, and oxygen stoichiometry affect the superconducting transition. The magnetoresistance shows a cusp around 135 K which lies between the antiferromagnetic and ferromagnetic transition temperatures, which is probably due to the presence of a spin glass region. There exist two magnetic transition temperatures for 0.0 ? x ? 0.2 which respectively change from TM = 155 K to 144 K and from Tirr = 115 K to 70 K. The magnetization versus applied magnetic field isotherms at 77 K and 300 K show that the remanent magnetization and coercivity are lower for samples with higher Pr content.  相似文献   

15.
A series of SmFe1?xZnxAsO0.8F0.2 samples with x = 0, 0.05, 0.1, 0.2 and 0.4 have been successfully synthesized using a solid state method. The lattice parameters are found to increase with increasing Zn doping content. The superconductivity has been definitely suppressed by Zn doping at Fe site with the transition temperature Tc being reduced from 52.5 K to 23.3 K for the sample of x = 0.05, and to 18.2 K for the sample of x = 0.1. For the samples with x > 0.1, the superconducting transition vanishes, and, at the meantime, the spin-density-wave anomaly recovers at 140 K. The metal to semiconductor transition is also observed in the SmFe1?xZnxAsO0.8F0.2 system. The behavior of SmFe1?xZnxAsO0.8F0.2 is very different from that of REFeAsO (RE = rare earth metal), which reveals a very strong electron correlation in SmFe1?xZnxAsO0.8F0.2.  相似文献   

16.
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   

17.
The superconducting property of Zr55Co(30?x)Al15Nbx (x = 0–20 at.%) metallic glasses fabricated by rapid solidification was investigated. The Zr55Co(30–x)Al15Nbx (x = 5–20 at.%) metallic glasses with a mixture structure of amorphous and nanocrystal phases exhibited superconductivity of Tc,on = 1.8–2.6 K. The maximum Tc,on = 2.6 K was obtained for the Zr55Co10Al15Nb20 metallic glass. This was attributable to the superconducting property of nanocrystalline particles contained in the Zr55Co10Al15Nb20 alloy. The increase of Nb element in the Zr55Co(30–x)Al15Nbx alloy led to the increase of Tc,on and the decrease of glass transition temperature. The glass transition temperature was between 704 and 749 K for the Zr55Co(30–x)Al15Nbx (x = 0–20 at.%) alloys. The temperature interval of supercooled liquid state was between 51 and 68 K for the Zr55Co(30–x)Al15Nbx (x = 0–20 at.%) alloys.  相似文献   

18.
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   

19.
The superconducting gap in FeAs-based superconductor SmFeAs(O1?xFx) (x = 0.15 and 0.30) and the temperature dependence of the sample with x = 0.15 have been measured by Andreev reflection spectroscopy. The intrinsic superconducting gap is independent of contacts while many other “gap-like” features vary appreciably for different contacts. The determined gap value of 2Δ = 13.34 ± 0.47 meV for SmFeAs(O0.85F0.15) gives 2Δ/kBTC = 3.68, close to the BCS prediction of 3.53. The superconducting gap decreases with temperature and vanishes at TC, in a manner similar to the BCS behavior but dramatically different from that of the nodal pseudogap behavior in cuprate superconductors.  相似文献   

20.
Artificially stacked structures of [(infinite layer CaCuO2)k/(superconducting (Cu, C)Ba2CuOx:(Cu, C)-1201)l]m were fabricated on SrCuO2 buffer layer by means of sequential deposition of each block using pulsed laser deposition. Smooth and epitaxial growth all over the deposition cycles of (Cu, C)-1201 and CaCuO2 were confirmed by streak patterns of in situ RHEED. In comparison with (Cu, C)-1201 single layer films, the artificial stacking resulted in a rise of superconducting properties, Tc-onset and Tc(ρ=0) up to 95 K and 65 K, respectively.  相似文献   

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