Non-superconducting Compounds Research Articles

75As NMR studies on iron-based La1111 superconductors in lightly and heavily electron doped states

We report 75As NMR experiments in heavily electron-doped LaFePnO0.75H0.25 (Pn=As1−x Sb x and As1−x′ P x′ ) compounds with the maximum , and compare with the previous results in lightly electron-doped LaFePn(O,F) compounds. The Tc of this series can be sensitively controlled by the pnictogen height (hPn ) through the substitution at Pn site, and the electron doping level through the substitution at the O site with H or F. In heavily electron-doped LaFePnO(La1111) compounds, we found that spin fluctuations at low-energies were moderately suppressed upon cooling in the Sb-substituted high Tc compounds with high hPn (x ≥ 0), although they are completely suppressed in P-substituted non-superconducting compounds (x′ ≥ 0.2) with lower hPn . This feature is largely different from that in the lightly doped La1111 compounds with the well-nested Fermi surfaces, where the spin fluctuations are critically enhanced upon cooling. Here, we present the characteristics of spin fluctuations over wide doping region of La1111-based compounds, and discuss the relationship with the superconductivity.

Enhanced superconductivity and moderate spin fluctuations suppressed at low energies in heavily electron-doped La1111-based superconductor

To elucidate the origin of re-enhanced high-$T_c$ phase in the heavily electron-doped Fe-pnictides, systematic $^{75}$As NMR studies are performed on heavily electron-doped LaFe$Pn$O$_{0.75}$H$_{0.25}$ by controlling the pnictogen height ($h_{Pn}$) from the Fe plane through the substitution at $Pn$(=As) site with Sb or P. The measurements of nuclear spin relaxation rate (1/$T_1$) and Knight shift ($K$) reveal that the moderate spin fluctuations at high temperatures are suppressed toward low temperatures. Such characteristic spin fluctuations with gap like feature at low energies are more enlarged in higher $T_c$ compounds with higher $h_{Pn}$, while those are totally suppressed in non-superconducting compounds with lower $h_{Pn}$. This implies that the contribution of the finite energy part in the spin fluctuation spectrum is crucial for enhancing $T_c$ in the heavily electron-doped regime. This is in contrast to many cases of typical Fe-based compounds with hole and electron Fermi surfaces of similar sizes, where the spin fluctuations at low energies develop significantly at low temperatures. The features in the heavily electron-doped states are discussed in relation with the characteristics of the faint hole Fermi surface derived from $d_{xy}$ orbital that rises when $h_{Pn}$ is high, together with the enhanced electron correlation effects.

Non-linear lattice response of Sm oxypnictides to hydrostatic pressure

Hydrostatic pressure Raman measurements at room temperature have been carried out on the SmFeAsO (Sm1111) series of oxypnictides with various substitutions (F for O and Co for Fe) and transition temperature in order to investigate lattice modifications and their connection to doping and superconductivity. Synchrotron XRD data on some of these compounds indicated that at low doping the lattice constants vary smoothly with pressure, but with further increasing of the carrier concentration there is a deviation from the normal equation of state and these effects are related with modifications in the superconducting FeAs4 tetrahedra. The hydrostatic pressure Raman measurements indicate that the A1g mode of the rare earth atom for the superconducting compounds deviates from the linear pressure dependence at the same pressures where the XRD results show pressure-induced lattice anomalies. A similar anomaly is found for the As phonon of the same symmetry. As in cuprates, the effect is diminished in the non-superconducting compounds and it is not related with the F substitution being present in the Sm(Fe1−xCox)AsO as well. The calculated Grüneisen parameters indicate a more anharmonic phonon for the Fe atom compared with the Sm and As atoms.

Stress–strain behavior under static loading in Gd123 high-temperature superconductors at 77 K

Mechanical properties of melt-growth GdBa2Cu3Ox (Gd123) superconducting samples with 10wt.% Ag2O and 0.5wt.% Pt were evaluated at 77K through flexural tests for specimens cut from the samples in order to estimate the mechanical properties of the Gd123 material without metal substrates, buffer layers or stabilization layers. We discuss the mechanical properties; the Young’s modulus and flexural strength with stress–strain behavior at 77K. The results show that the flexural strength and fracture strain of Gd123 at 77K are approximately 100MPa and 0.1%, respectively, and that the origin of the fracture is defects such as pores, impurities and non-superconducting compounds. We also show that the Young’s modulus of Gd123 is estimated to be 160–165GPa.

Disappearance of Superconductivity in the Solid Solution between (Ca4Al2O6)(Fe2As2) and (Ca4Al2O6)(Fe2P2) Superconductors

The effect of alloying the two perovskite-type iron-based superconductors (Ca(4)Al(2)O(6))(Fe(2)As(2)) and (Ca(4)Al(2)O(6))(Fe(2)P(2)) was examined. While the two stoichiometric compounds possess relatively high T(c)'s of 28 and 17 K, respectively, their solid solutions of the form (Ca(4)Al(2)O(6))(Fe(2)(As(1-x)P(x))(2)) do not show superconductivity over a wide range from x = 0.50 to 0.95. The resultant phase diagram is thus completely different from those of other typical iron-based superconductors such as BaFe(2)(As,P)(2) and LaFe(As,P)O, in which superconductivity shows up when P is substituted for As in the non-superconducting "parent" compounds. Notably, the solid solutions in the non-superconducting range exhibit resistivity anomalies at temperatures of 50-100 K. The behavior is reminiscent of the resistivity kink commonly observed in various non-superconducting parent compounds that signals the onset of antiferromagnetic/orthorhombic long-range order. The similarity suggests that the suppression of the superconductivity in the present case also has a magnetic and/or structural origin.

Two-dimensional Mapping of Superconducting Materials by Valence Electron Parameters

Two dimensional mappings on superconductive elements A and AB compounds are constructed by using the valence electron parameters (the difference between pseudopotential radii (r (s), r (p) and r (d)) of s and p electrons or of s and d electrons, Δ(sp) or Δ(sd), and the orbital electronegativity of s electron, χ(s)=(Z/r (s))1/2, where Z is valence) as the coordinates. From the relation of band gap against χ(s), the change of metal to semiconductor in sp-bonded elemental substances occurs at the threshold value of (Z/r (s))1/2=2.098. It is found that the boundary for superconductivity in sp-bonded elemental substances and AB compounds appears around (Z/r (s))1/2≈2.3. On the other hand, superconductivity appears in the region with higher (Z/r (s))1/2 value in the transition metals and AB compounds containing transition metals. Lower limit in the abscissa scale is observed around (Z/r (s))1/2=1.3, suggesting the limitation due to electron density in metallic substances and AB compounds. Non-superconducting late 3d transition elements are clearly divided by a polynomial curve. Non-superconducting sp-bonded AB compounds are well separately placed in the region with higher (Z/r (s))1/2 value than 2.3 and lower Δ(sp) value than 0.150.

Evidence of spin-density-wave order inRFeAsO1−xFxfrom measurements of thermoelectric power

Data on the magneto-thermopower and specific heat of three compounds belonging to '1111' oxypnictides family are reported. One specimen (SmAsFeO0.8F0.2) is a superconductor with Tc = 53 K, while two others (SmAsFeO and NdAsFeO) are nonsuperconducting parent compounds. Our results confirm that spin density wave (SDW) order is present in SmAsFeO and NdAsFeO. In these two samples a strict connection between the thermoelectric power and electronic specific heat is found in the vicinity of SDW transition, what indicates that the chemical potential of charge carriers strongly depends on temperature in this region. Low temperature data suggest presence of significant contribution magnon-drag to the thermoelectric power.

Absorption and photoemission spectroscopy of rare-earth oxypnictides

The electronic structure of various rare-earth oxypnictides has been investigated by performing Fe L2, 3 x-ray absorption spectroscopy, and Fe 2p and valence band x-ray photoemission spectroscopy. As representative samples the non-superconducting parent compounds LnFeAsO (Ln=La, Ce, Sm and Gd) have been chosen and measured at 25 and 300 K, i.e. below and above the structural and magnetic phase transition at ∼150 K. We find no significant change of the electronic structure of the FeAs layers when switching between the different rare-earth ions or when varying the temperature below and above the transition temperatures. Using a simple two-configuration model, we find qualitative agreement with the Fe 2p3/2 core-level spectrum, which allows for a qualitative explanation of the experimental spectral shapes.

Magnetic order close to superconductivity in the iron-based layered LaO1-xF x FeAs systems

Following the discovery of long-range antiferromagnetic order in the parent compounds of high-transition-temperature (high-T(c)) copper oxides, there have been efforts to understand the role of magnetism in the superconductivity that occurs when mobile 'electrons' or 'holes' are doped into the antiferromagnetic parent compounds. Superconductivity in the newly discovered rare-earth iron-based oxide systems ROFeAs (R, rare-earth metal) also arises from either electron or hole doping of their non-superconducting parent compounds. The parent material LaOFeAs is metallic but shows anomalies near 150 K in both resistivity and d.c. magnetic susceptibility. Although optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed by doping with electrons to induce superconductivity, there has been no direct evidence of SDW order. Here we report neutron-scattering experiments that demonstrate that LaOFeAs undergoes an abrupt structural distortion below 155 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then, at approximately 137 K, develops long-range SDW-type antiferromagnetic order with a small moment but simple magnetic structure. Doping the system with fluorine suppresses both the magnetic order and the structural distortion in favour of superconductivity. Therefore, like high-T(c) copper oxides, the superconducting regime in these iron-based materials occurs in close proximity to a long-range-ordered antiferromagnetic ground state.

Aspects of Electronic Structure Instability in Search for New Superconductors: Superconducting Boride at Liquid Nitrogen Temperature?

It has been shown that electron-phonon coupling in superconductors induces temperature-dependent electronic structure instability which is related to fluctuation of analytic critical point of some bands across the Fermi level. The band fluctuation results in a considerable reduction of chemical potential and to breakdown of the adiabatic Born-Oppenheimer approximation. At critical temperature Tc, superconducting system undergoes transition from the adiabatic electronic ground state into the antiadiabatic state at broken symmetry, which is stabilized due to the effect of nuclear dynamics. This effect is absent in non-superconducting compounds. In a good agreement with the experimental Tc of superconducting state transition, the critical temperature of the adiabatic ↔ antiadiabatic state transition has been calculated for three different superconductors. Two hypothetical compounds, LiB and ZnB2, are predicted to be potential superconductors with Tc about 17 and 77.5 K, respectively.

Favorable magnetic fluctuation anisotropy for unconventional superconductivity inf-electron systems

We report spin-lattice relaxation time measurements in isostructural ($\mathrm{Ho}\mathrm{Co}{\mathrm{Ga}}_{5}$-type) actinide compounds with different ground states (paramagnet, antiferromagnet, and superconductor) using single-crystal samples. The anisotropy of local magnetic fluctuations due to $LS$ coupling on the actinide (magnetic) site is evaluated. In the superconductor $\mathrm{Pu}\mathrm{Rh}{\mathrm{Ga}}_{5}$, the $XY$-type magnetic anisotropy is strong compared with nonsuperconducting compounds. Favorable magnetic anisotropy for $d$-wave superconductivity in $f$-electron systems is discussed.

Valence Instability and Superconductivity in Heavy Fermion Systems

Many cerium-based heavy fermion (HF) compounds have pressure-temperature phase diagrams in which a superconducting region extends far from a magnetic quantum critical point. In at least two compounds, CeCu2Si2 and CeCu2Ge2, an enhancement of the superconducting transition temperature was found to coincide with an abrupt valence change, with strong circumstantial evidence for pairing mediated by critical valence, or charge transfer, fluctuations. This pairing mechanism, and the valence instability, is a consequence of a f-c Coulomb repulsion term U_fc in the hamiltonian. While some non-superconducting Ce compounds show a clear first order valence instability, analogous to the Ce alpha-gamma transition, we argue that a weakly first order valence transition may be a general feature of Ce-based HF systems, and both magnetic and critical valence fluctuations may be responsible for the superconductivity in these systems.

In-plane ferromagnetic spin fluctuations in novel superconducting system [formula omitted

Na 23 NMR was measured for newly synthesized superconducting and non-superconducting bilayer-hydrated (BLH) Na x CoO 2 · y H 2 O compounds systematically prepared by time-controlling duration-effect method of the sample preparation in high humidity atmosphere. The nuclear spin-lattice relaxation rate 1 / T 1 of Na 23 shows a ferromagnetic power-law behavior ( T 0.7 ) for both superconducting and non-superconducting BLH compounds in the low temperature regime, which has been also observed in Na 0.7 CoO 2 , while the order of amplitude of 1 / T 1 is substantially suppressed in BLH cases. Therefore, the in-plane ferromagnetic fluctuation still remains after the intercalation of water molecules although the inter-plane antiferromagnetic correlation is weakened, suggesting that the superconductivity occurs in the vicinity of the quantum critical point of the A-type spin fluctuations in the phase diagram of the novel Na x CoO 2 · y H 2 O system.

Fermiology of PuCoGa 5 and of related Pu-115 compounds

We report computational investigations of the electronic structures of the superconducting Pu-compounds PuCoGa 5 as well as of the non-superconducting compounds PuFeGa 5 and PuNiGa 5. To capture the localization behavior of the Pu 5f electrons, we apply two computational approaches, which are both rooted in the density-functional theory: the local spin-density approximation (LSDA) and the around mean field (AMF) LSDA + U approach. The latter is applicable to moderately localized 5f electrons while the former is applicable to delocalized 5f electrons. Our LSDA calculations show that the Fermi surfaces of the three Pu-115 compounds are sensitive to the amount of band filling, i.e., the number of electrons of the 3d element. Precisely at the electron filling corresponding to PuCoGa 5 the Fermi surface has a particularly two-dimensional shape. AMF–LSDA + U calculations (with a Coulomb U of about 3 eV and exchange J of 0.6 eV) lead to a non-magnetic ground state for PuCoGa 5, in which the 5f states are shifted to a higher binding energy, in better agreement with photoemission data. The Fermi surface of PuCoGa 5 computed with the AMF–LSDA + U approach is nonetheless rather two-dimensional and similar to the LSDA Fermi surface. The AMF–LSDA + U approach with a Coulomb U of ≈ 3 eV would thus predict an electronic structure for PuCoGa 5 in accord with several experimental data.

Thermal Conductivity of Ce Doped Bi-2212 Superconductors

The temperature dependence of the thermal conductivity inBi2Sr2Ca1−xCexCu2Oy x = 0.1, 0.2, 0.3, 0.4is presented. With increasing Ce-doping level, the thermalconductivity peak under TC issuppressed then disappears, while another peak appears at lowtemperatures for the non-superconducting compounds. The numericalanalysis shows that the thermal conductivity peak under TC can bewell described by the normal electron relaxation-time contributionmodel, and the phonon-induced thermal conductivity peak could be welldescribed within the Debye approximation of the phonon spectrum. Theexistence and variation of these two thermal conductivity peaks indicatethe adjustability between the superconducting and insulating componentsin the samples with different Ce-doping levels.

次世代超電導線材の開発と特性評価 改良型ＴＦＡ‐ＭＯＤ法におけるＹ１２３膜組織形成機構と水蒸気分圧の影響

YBa2Cu3O7-δ (Y123) films were deposited under various water vapor partial pressures (P(H2O)) in the crystallization process by an advanced metalorganic deposition (MOD) method using trifluoroacetates. Microstructures of the films were evaluated by means of transmission electron microscopy. As a result, in the Y123 film crystallized under lower P(H2O) such as 2.1%, large pores were remarkably observed. Furthermore, there were many amorphous layers of non-superconducting phases in the film. In contrast, small pores were distributed in the films deposited under higher P(H2O) of 4.2, 6.3 and 13.5%. Considering that the critical current (Ic) values become higher as P(H2O) increases, the existence of large pores and non-superconducting phase compounds causes a reduction in Ic. By investigating the microstructures of the films quenched during the crystallization process, it was found that the pores in the films are generated by the reaction of the non-superconducting phases trapped in the growing Y123 film, which then transform to Y123 afterwards accompanied by volume reduction of about 30%. The sizes of the entrapped phase crystals in the Y123 film depend on the P(H2O), and this may determine the pore sizes in the final film.

Low Temperature Fluorination of Superconducting and Non-superconducting Compounds Using NH4F

Compounds Y1Ba2Cu3, Pr1Ba2Cu3O7 and La2CuO4 were fluorinated by a low temperature fluorination method using NH4F.The fluorinated samples were analysed by x-ray diffraction and resistivity measurements. The x-ray diffractogram of the fluorinated samples indicated broadening of the peaks. None of the fluorinated samples showed any superconducting behaviour.

Magnetic and structural features of RNi2B2C and RNiBC (R=Er, Ho, Dy, Tb, Gd) superconducting compounds

Temperature dependent Mössbauer spectroscopy on 57Fe doped (1 at % of Ni) RNi2B2C and RNiBC provided clear evidence of a pair-breaking field at the Ni site for non-superconducting compounds. This field is not present in the superconducting collinear AF DyNi2B2C, however it appears when this compound is diluted with non-magnetic Lu (Dy1-xLu xNi2B2C). Important local information on the spin structure of the R magnetic moments is obtained for both systems. The local symmetries of RNi2B2C and RNiBC compounds, measured through the deltaE Q, were found to be linearly correlated with the c<FONT FACE=Symbol>¢/</FONT>a ratio (c´ is the distance of the R-C layers between which the Ni2-B2 layers are sandwiched). A scaling of Tc with the local symmetry (c<FONT FACE=Symbol>¢/</FONT>a) was found to be the same for all the RNi2B2C and RNiBC (except for Y) compounds.

Investigation of empirical laws for superconductivity of alloy and compound superconductors

By studying the average effect of the electronegativity of alloy and compound superconductors, some empirical laws are found. The average value of the electronegativity of alloy superconductors is from 1.3 to 1.9, most of which is concentrated in the region 1.5 < < 1.8. The of many higher Tc superconductors is nearly 1.60. All these characteristics are similar to element superconductors. The average value of the electronegativity of positive valence elements in oxide superconductors is from 1.3 to 1.6 and the average effect of an electronegativity of zero on every element is from 1.75 to 2.00. The mean of and is from 1.5 to 1.8. When both and are near the limit value (1.3 or 1.6 and 1.75 or 2.00), the material has a very low Tc, even is non-superconductive. The of other compound superconductors is either too high or too low and the is too low so that the Tc of these superconductors is quite low. Studying the average effect of the electronegativity of non-superconducting compounds further confirms the laws.

Effects of nano-ZrO 2 particles on the superconductivity of Pb-doped BSCCO

The major limitations of Bi-system superconductor application are the intergrain weak links and weak flux pinning capability. In order to promote the practical application of Bi-system superconductor, effective flux pinning centers must be artificially introduced into HTS to enhance the flux pinning capability. With those goals, we have investigated the effects of nanometer-sized ZrO 2 addition on flux pinning capability of the bulk Pb-doped Bi-2223 by using of X-ray diffraction (XRD), DC resistivity, ρ, AC susceptibility, c, and magnetic hysteresis loops measurements. We have prepared samples by Bi 1.8Pb 0.4Sr 2Ca 2.2Cu 3O 10+ y (nano-ZrO 2) x with x range from 0 to 0.5. The results show that, with the increasing of x, the T c (onset) drops down gradually. However, the measurements of the critical current density, J c, show that appropriate amount-doped nano-ZrO 2 greatly enhance the values of J c under magnetic fields. According to the results of XRD patterns, there are slight reflection peaks of ZrO 2-related nonsuperconducting compounds. It is believed that these nonsuperconducting ZrO 2-related compounds act as effective flux pinning centers.