Dale R. Harshman

Magnetism and superconductivity in Sr \(\)YRu \(\)Cu \(\)O \(\) and magnetism in Ba \(\)GdRu \(\)Cu \(\)O \(\)

Abstract:We report magnetization, surface resistance (), and electron spin resonance (ESR) for non-superconducting Ba2GdRu1-uCuuO6, and find that all three magnetic ions (Gd, Ru, and Cu) are ordered at low temperatures. Both ESR (Gd sublattice) and weak ferromagnetic resonance (dopant Cu) are observed, while no magnetic resonance due to either paramagnetic or ordered Ru is detected. In addition, for superconducting ( K) Sr2YRu1-uCuuO6, resistivity, muon spin rotation (SR), and 99Ru Mössbauer absorption are reported. None of the O6 materials (e.g., Sr2YRu1-uCuuO6) have cuprate planes, although Cu is employed as a dopant. In Sr2YRu1-uCuuO6, the Ru moments order at a temperature (23 K) below that for the resistive onset of superconductivity, while the Cu orders at a higher temperature, 86 K. Therefore at low temperatures, this material exhibits magnetic order, coexisting with diamagnetism. The only non-magnetic layers in the superconducting O6 structure, the SrO layers, carry holes and exhibit diamagnetic screening characteristic of type-II superconductivity.

Theory of high-TC superconductivity: transition temperature

After reading over our published manuscript, we noticed that the discussion concerning the determination of σ for the ruthenate Ba2YRu0.9Cu0.1O6 in section 2.3.1 (3rd paragraph) is somewhat terse. Herein we provide an expanded analysis which better explains our estimate of γ (and thus σ) for this compound. All numbers, figures and conclusions remain unaltered. The ruthenate compounds A2YRu1-xCuxO6 (with A = Ba or Sr; x = 0.05–0.15) are double-perovskites containing no cuprate planes and with ν = μ = 1 [1] (reference [82] in the paper). The determination of γ follows from equation (2.5b), wherein rule 1b introduces the factor 1/2. In the lower limit, one expects a minimum of ~2 charges per Cu dopant, which are shared between two charge reservoirs of each layer type (AO and 1/2 (YRu1-xCuxO4)), producing a net factor of unity. Thus, for Ba2YRu0.9Cu0.1O6 (with TC0 ~ 30–40 K), we estimate γ = (1/2)(1) = 1/2, yielding σ = 0.05 as stated by equation (2.5c) in the paper. While one may expect an average effective charge state for Ru near +5, and that of Cu to be between +2 and +3 (post anneal) [2], the lower-limit estimation provided, which places the corresponding data point in figure 2 to the left of the line, appears sufficient to include the ruthenates with the other high-TC compounds found to follow equation (2.6) so far. Owing to the uncertainty in the experimental values for TC0, as well as the Ru and Cu valence states, however, this compound was excluded in the data analyses presented. Future research will attempt a more accurate determination of the charge per doped Cu, and thus σ. We would also like to point out a typographical correction in the definition of the corresponding ruthenate type II reservoir in the last column of table 1, which should read 1/2 (YRu0.9Cu0.1O4). An unrelated item is found in the fourth line of section 2.3.3, where Tb(O0.80F0.20)FeAs should read Tb(O0.80-yF0.20)FeAs. Additionally, reference [132] is now known and has the form: [132] Harshman D R and Fiory A T 2011 J. Phys.: Condens. Matter 23 315702 References [1] Parkinson N G, Hatton P D, Howard J A K, Ritter C, Chien F Z and Wu M-K 2003 J. Mater. Chem. 13 1468 [2] Rao S M, Wu M-K, Ren H C, Chen C L, Guo J-H, Hsu F C, Chen S Y, Chen Y Y, Chang C L and Liu H L 2011 in preparationThe superconducting transition temperatures of high-Tc compounds based on copper, iron, ruthenium and certain organic molecules are discovered to be dependent on bond lengths, ionic valences, and Coulomb coupling between electronic bands in adjacent, spatially separated layers [1]. Optimal transition temperature, denoted as T_c0, is given by the universal expression

Superconductivity and magnetism in Sr2Y(Ru1-uCuu)O6 and in Ba2Gd(Ru1-uCuu)O6

k_BT_c0 = e^2 \Lambda / \ell\zeta

MUON SPIN ROTATION IN SR2YRU1-UCUUO6

;

High-temperature superconductivity: the roles of oxide layers

\ell

Theory of High-Tc Superconductivity: Accurate Predictions of Tc

is the spacing between interacting charges within the layers, \zeta is the distance between interacting layers and \Lambda is a universal constant, equal to about twice the reduced electron Compton wavelength (suggesting that Compton scattering plays a role in pairing). Non-optimum compounds in which sample degradation is evident typically exhibit Tc < T_c0. For the 31+ optimum compounds tested, the theoretical and experimental T_c0 agree statistically to within +/- 1.4 K. The elemental high Tc building block comprises two adjacent and spatially separated charge layers; the factor e^2/\zeta arises from Coulomb forces between them. The theoretical charge structure representing a room-temperature superconductor is also presented.

Nature of high-temperature superconductivity

Abstract Muon spin rotation and electron spin resonance data on sintered samples of superconducting Sr 2 Y(Ru 1− u Cu u )O 6 and non-superconducting Ba 2 Gd(Ru 1− u Cu u )O 6 are reported, both for u =0.1. In the case of Sr 2 Y(Ru 1− u Cu u )O 6 , the SrO layers are found to be p-type and to exhibit an onset for superconductivity at ≈45 K – a temperature considerably lower than the spin-ordering temperature of the Cu ions (≈86 K), indicating that the Cu ions themselves do not play a significant role in the superconductivity. Below T c , the fluctuating Ru moments begin to slow down and freeze, so that at about ≈29.3 K a spin-glass state is observed, which gives way to ferromagnetic ordering of the Ru ions in the Y(Ru 1− u Cu u )O 4 planes, with the magnetization alternating direction in the a – b plane from one magnetic layer to the next. These data confirm our earlier discovery that fluctuating moments (in this case, Ru moments) interfere with pairing. Ba 2 Gd(Ru 1− u Cu u )O 6 shows no evidence of superconductivity, which we interpret as due to pair breaking by the L =0 magnetic Gd ions, which are not crystal-field split.

THERMAL CONDUCTIVITY OF YBa2Cu3O7: s-WAVE INTERPRETATION

The magnetic and superconducting behaviors of sintered Sr{sub 2}YRu{sub 1{minus}u}Cu{sub u}O{sub 6} (for u = 0.05, 0.10, 0.15) were probed using transverse- and zero-field muon spin rotation ({mu}{sup +}SR). In general, positive muons are attracted to oxygen ions in the high-{Tc} oxides, and so, Sr{sub 2}YRu{sub 1{minus}u}Cu{sub u}O{sub 4} layers and those associated with the SrO-layer oxygen. The transverse- and zero-field data for all three stoichiometries u exhibit a sudden onset of magnetic structure at T{sub N} {approximately} 30 K, with a static local field of {approximately}3 kG. This transition is marked by a dramatic increase in the relaxation rate as the temperature decreases below T{sub N}, corresponding to an increased static disordering of the magnetic moments. Above T{sub N} no static fields are observed. Instead the data exhibit a slow dynamic depolarization, presumably due to the rapid fluctuation of paramagnetic moments. Both transverse- and zero-field data also indicate a smaller second component ({approximately}10%) which the authors associate with the SrO layer, exhibiting superconducting behavior in transverse field with an observed {Tc} {approx} T{sub N} {approximately} 30 K.The magnetic and superconducting behaviors of sintered Sr2YRu1-uCuuO6 (for u=0.05, 0.10, 0.15) were probed using transverse- and zero-field muon spin rotation (μ+ SR). In general, positive muons are attracted to oxygen ions in the high-Tc oxides, and so, Sr2YRu1-uCuuO6 should (and does) present two types of μ+ sites, those associated with the oxygen in the YRuO4 layers and those associated with the SrO-layer oxygen. The tranverse- and zero-field data for all three stoichiometries u exhibit a sudden onset of magnetic structure at TN~30 K, with a static local field of ~3 kG. This transition is marked by a dramatic increase in the relaxation rate as the temperature decreases below TN, corresponding to an increased static disordering of the magnetic moments. Above TN no static fields are observed. Instead the data exhibit a slow dynamic depolarization, presumably due to the rapid fluctuation of paramagnetic moments. Both transverse- and zero-field data also indicate a smaller second component (~10%) which we associate with the SrO layer, exhibiting superconducting behavior in transverse field with an observed Tc≈TN~30 K.

Magnetic resonance and Mössbauer studies of GdSr2Cu2RuO8

Abstract On the basis of a model in which the superconductivity does not reside in the cuprate planes, (i) four superconductors have been predicted (PrBa 2 Cu 3 O 7 , Gd 2− z Ce z Sr 2 Cu 2 TiO 10 , Pr 2− z Ce z Sr 2 Cu 2 NbO 10 , and Eu 2− z Ce z Sr 2 Cu 2 TiO 10 ); (ii) the failures of Gd 2− z Ce z CuO 4 , Cm 2− z Th z CuO 4 , and Ba 2 GdRu 1− u Cu u O 6 to superconduct have been explained; (iii) the non-existence of n-type high-temperature oxygen-based superconductors has been proposed; and (iv) the locus of superconductivity has been attributed, not to the cuprate planes, but to oxygen layers such as the BaO layers, SrO layers, or interstitial oxygen layers of various high-temperature superconductors. This non-cuprate-plane model allows a consistent explanation of the ∼45 K superconductivity of all three ruthenate compounds: Sr 2 YRu 1− u Cu u O 6 , GdSr 2 Cu 2 RuO 8 , and Gd 2− z Ce z Sr 2 Cu 2 RuO 10 .

The superconducting transition temperatures of Fe1+xSe1−y, Fe1+xSe1−yTey and (K/Rb/Cs)zFe2−xSe2

After reading over our published manuscript, we noticed that the discussion concerning the determination of σ for the ruthenate Ba2YRu0.9Cu0.1O6 in section 2.3.1 (3rd paragraph) is somewhat terse. Herein we provide an expanded analysis which better explains our estimate of γ (and thus σ) for this compound. All numbers, figures and conclusions remain unaltered. The ruthenate compounds A2YRu1-xCuxO6 (with A = Ba or Sr; x = 0.05–0.15) are double-perovskites containing no cuprate planes and with ν = μ = 1 [1] (reference [82] in the paper). The determination of γ follows from equation (2.5b), wherein rule 1b introduces the factor 1/2. In the lower limit, one expects a minimum of ~2 charges per Cu dopant, which are shared between two charge reservoirs of each layer type (AO and 1/2 (YRu1-xCuxO4)), producing a net factor of unity. Thus, for Ba2YRu0.9Cu0.1O6 (with TC0 ~ 30–40 K), we estimate γ = (1/2)(1) = 1/2, yielding σ = 0.05 as stated by equation (2.5c) in the paper. While one may expect an average effective charge state for Ru near +5, and that of Cu to be between +2 and +3 (post anneal) [2], the lower-limit estimation provided, which places the corresponding data point in figure 2 to the left of the line, appears sufficient to include the ruthenates with the other high-TC compounds found to follow equation (2.6) so far. Owing to the uncertainty in the experimental values for TC0, as well as the Ru and Cu valence states, however, this compound was excluded in the data analyses presented. Future research will attempt a more accurate determination of the charge per doped Cu, and thus σ. We would also like to point out a typographical correction in the definition of the corresponding ruthenate type II reservoir in the last column of table 1, which should read 1/2 (YRu0.9Cu0.1O4). An unrelated item is found in the fourth line of section 2.3.3, where Tb(O0.80F0.20)FeAs should read Tb(O0.80-yF0.20)FeAs. Additionally, reference [132] is now known and has the form: [132] Harshman D R and Fiory A T 2011 J. Phys.: Condens. Matter 23 315702 References [1] Parkinson N G, Hatton P D, Howard J A K, Ritter C, Chien F Z and Wu M-K 2003 J. Mater. Chem. 13 1468 [2] Rao S M, Wu M-K, Ren H C, Chen C L, Guo J-H, Hsu F C, Chen S Y, Chen Y Y, Chang C L and Liu H L 2011 in preparationThe superconducting transition temperatures of high-Tc compounds based on copper, iron, ruthenium and certain organic molecules are discovered to be dependent on bond lengths, ionic valences, and Coulomb coupling between electronic bands in adjacent, spatially separated layers [1]. Optimal transition temperature, denoted as T_c0, is given by the universal expression