C. E. Stronach

The elastic scattering of intermediate energy protons from 40Ca and 208Pb

Abstract Elastic scattering of protons from 40 Ca and 208 Pb has been studied using the MRS facility at TRIUMF. Cross section and analyzing power ( A y ) angular distributions to 50° in the center of mass have been measured for incident proton energies of 200, 300, 400 and 500 MeV. Comparisons of the experimental results are made with calculations based on relativistic and non-relativistic phenomenological models, and with relativistic and non-relativistic microscopic models. The differences between the results of the non-relativistic and relativistic models manifest themselves in the spin observables. They are found to be smaller in the phenomenological approach than in the microscopic one and they depend on energy.

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.

Correlations between charge ordering and local magnetic fields in overdoped YBa2Cu3O6+x

Zero-field muon spin-relaxation (ZF-μSR) measurements were undertaken on under- and overdoped samples of superconducting YBa 2 Cu 3 O 6 + x to determine the origin of the weak static magnetism recently reported in this system. The temperature dependence of the muon spin-relaxation rate in overdoped crystals displays an unusual behavior in the superconducting state. A comparison to the results of NQR and lattice structure experiments on highly doped samples provides compelling evidence for strong coupling of charge, spin, and structural inhomogeneities.

Site of the positive muon in YBa2Cu3O7

A combination of μSR and17O nuclear quadrupolar μLCR has been used to obtain detailed information on the muon site in YBa2Cu3O7. The rms magnitude of the nuclear dipolar fields in YBa2Cu3O7 enriched with 38%17O is about twice that in the natural16O sample. In addition the17O μLCR spectrum shows that the electric field gradient tensor at the oxygen closest to the muon is almost equal to that measured by NMR for the CuO2 plane sites O(2,3). These results suggest that the muon in fully oxygenated YBa2Cu3O7 is located at a single site 1.0 Å from an oxygen ion, probably O(2,3).

Expansion of the vortex cores in YBa 2 Cu 3 O 6.95 at low magnetic fields

Muon spin rotation spectroscopy has been used to measure the effective size of the vortex cores in optimally doped YBa2Cu3O6.95 as a function of temperature and magnetic field deep in the superconducting state. While the core size at H=2T is close to 20 angstroms and consistent with that measured by STM at 6T, we find a striking increase in the core size at lower magnetic fields, where it approaches an extraordinarily large value of about 100 angstroms. This suggests that the average value of the superconducting coherence length in cuprate superconductors may be larger than previously thought at low magnetic fields.

Comparison between muon spin rotation and neutron scattering studies on the 3-dimensional magnetic ordering of La2CuO(4-y)

Muon spin rotation and neutron scattering studies on powder and single-crystal specimens of La 2 CuO 4 - y are compared. The apparent difference between the muon and neutron results for the ordered moment in the antiferromagnetic state is interpreted as the signature of increasingly short-ranged spatial spin correlations with increasing oxygen content.

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.

Static magnetic ordering of CeCu2.1Si2 found by muon spin relaxation

Abstract Zero- and longitudinal-field muon spin relaxation measurements on a poly-crystal sample of a heavy-fermion superconductor CeCu2.1Si2 ( Tc = 0.7 K) have revealed an onset of static magnetic ordering below T ∼ 0.8 K. The line shapes of the observed spectra in zero field indicate a wide distribution of static random local fields at muon sites, suggesting that the ordering is either spin glass or incommensurate spin-density-wave state. The observed width of the random local field at T = 0.05 K corresponds to a small averaged static moment of the order of 0.1μB per formula unit.

Ground-based simulations of cosmic ray heavy ion interactions in spacecraft and planetary habitat shielding materials

This paper surveys some recent accelerator-based measurements of the nuclear fragmentation of high energy nuclei in shielding and tissue-equivalent materials. These data are needed to make accurate predictions of the radiation field produced at depth in spacecraft and planetary habitat shielding materials and in the human body by heavy charged particles in the galactic cosmic radiation. Projectile-target combinations include 1 GeV/nucleon 56Fe incident on aluminum and graphite and 600 MeV/nucleon 56Fe and 290 MeV/nucleon 12C on polyethylene. We present examples of the dependence of fragmentation on material type and thickness, of a comparison between data and a fragmentation model, and of multiple fragments produced along the beam axis.