A. J. Greer

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.

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

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.

MUON SPIN ROTATION IN SR2YRU1-UCUUO6

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.

Eu2−zCezSr2Cu2RuO10 superconducts in its SrO layers, not in its cuprate-planes

Abstract Eu 2−z Ce z Sr 2 Cu 2 RuO 10 superconducts in its SrO layers at an onset temperature of ∼43K. Its CuO 2 planes and its RuO 2 planes are magnetic at and below ∼100K and ∼200K, respectively.

Muon spin rotation in GdSr2Cu2RuO8: implications

Muon spin rotation measurements are reported for GdSr2Cu2RuO8, a material with an onset temperature for superconductivity of about 45 K (which is virtually the same for its superconducting sister compounds Sr2YRu1− u Cu u O6 and Gd2− z Ce z Sr2Cu2RuO10). The data indicate two magnetic ordering transitions, at about 15 K and about 130 K, in addition to the Gd ordering transition known to occur at about 2.6 K. We tentatively attribute the 130 K transition to Ru and the 15 K transition to Cu ordering, effectively ruling out any superconducting mechanism based on fluctuating magnetic moments, which are frozen below about 15 K. If there is only one mechanism of high-temperature superconductivity, then the three facts that (i) all three sister compounds have essentially the same onset T c for superconductivity and (ii) all three of these compounds contain SrO layers but (iii) only two of the three compounds, GdSr2Cu2RuO8 and Gd2− z Ce z Sr2Cu2RuO10 (and not Sr2YRu1− u Cu u O6) contain cuprate planes imply that the superconducting layers of all three compounds must be the common SrO layers, and not the cuprate planes (which do not occur in Cu-doped Sr2YRuO6). Otherwise the coincidence of onset temperatures must be an accident, and there must be at least two mechanisms of high-T c superconductivity: one for doped Sr2YRuO6 and another for cuprate-plane superconductivity.

Muon spin rotation in GdSr 2 Cu 2 RuO 8 : implications

Muon spin rotation measurements are reported for GdSr2Cu2RuO8, a material with an onset temperature for superconductivity of about 45K (which is virtually the same for its superconducting sister compounds Sr2YRu1 uCuuO6 and Gd2 zCezSr2Cu2RuO10). The data indicate two magnetic ordering transitions, at about 15K and about 130K, in addition to the Gd ordering transition known to occur at about 2.6K. We tentatively attribute the 130K transition to Ru and the 15K transition to Cu ordering, effectively ruling out any superconducting mechanism based on fluctuating magnetic moments, which are frozen below about 15K. If there is only one mechanism of high-temperature superconductivity, then the three facts that (i) all three sister compounds have essentially the same onset Tc for superconductivity and (ii) all three of these compounds contain SrO layers but (iii) only two of the three compounds, GdSr2Cu2RuO8 and Gd2 zCezSr2Cu2RuO10 (and not Sr2YRu1 uCuuO6) contain cuprate planes imply that the superconducting layers of all three compounds must be the common SrO layers, and not the cuprate planes (which do not occur in Cu-doped Sr2YRuO6). Otherwise the coincidence of onset temperatures must be an accident, and there must be at least two mechanisms of high-Tc superconductivity: one for doped Sr2YRuO6 and another for cuprate-plane superconductivity. Philosophical Magazine ISSN 1478–6435 print/ISSN 1478–6443 online # US Government http://www.tandf.co.uk/journals DOI: 10.1080/1478643031000156399 kkEmail: drh@physikon.net. Permanent address. }

Nodeless Pairing State in Yba2Cu3O7

Muon spin rotation (μ+SR) results are reported on single-crystal YBa2Cu3O7, having a transition temperature of Tc = 91.3 K with ΔTc < 0.5 K (in zero applied field). Flux motion and de-pinning can mask intrinsic properties such as the true underlying pairing state, and has led to misinterpretations of data. The present data exhibit a non-monotonic behavior for the second moment of the internal field distribution as a function of field as T→0, which rules out any single pairing state explanation of the data, without including other extrinsic effects. The data are, however, consistent with s-wave (or extended s-wave) pairing, provided that field-dependent and temperature-activated vortex de-pinning are first taken into account. Applying a self-consistent vortex de-pinning model, the data are found to be best described by an underlying two-fluid model, yielding a London penetration depth value of λab(T=0,H=0) = 127.6 nm. Attempts to fit the data using BCS theory with varying coupling strengths and a d-wave model produced much poorer fits. In fact, the probability that the d-wave model gives a better fit than the two-fluid model is less that 4×10−6. This work reveals that the d-wave interpretations are incorrect, confirms earlier work (Refs. 1–3) which first established s-wave pairing in YBa2Cu3O7 powders and (heavily twinned) crystals, and re-establishes s-wave pairing for superconductivity in this material.

VERIFICATION OF NODELESS SUPERCONDUCTING PAIRING IN SINGLE-CRYSTAL YBa2Cu3O7

The temperature and field dependence of the penetration depth was determined from muon spin rotation (μ+SR) measurements on a single crystal of YBa2Cu3O7 having a superconducting transition at Tc ≈ 91.3 K. Data were acquired at applied magnetic fields of 0.05, 1.0, 3.0, and 6.0 Tesla, yielding results inconsistent with any pairing state requiring nodes, including d-wave pairing. These data are, however, completely consistent with s-wave (or extended s-wave) superconductivity, with clear evidence of field-dependent, temperature-activated vortex pinning. Our results confirm the s-wave character originally observed in 1989, and show that the features of μ+SR (and microwave) data used by other authors as evidence for d-wave superconductivity are instead due to temperature- and field-dependent vortex pinning/reordering, resulting in significant distortion of the flux lattice.

Location and properties of the superconducting hole-condensate in Sr2YRu1−uCuuO6☆

Abstract The superconductivity in the compound Sr 2 YRu 1−u Cu u O 6 is in the SrO layers.