Wayne K. Dawson

Candidate muon‐probe sites in oxide superconductors

Two independent search methods (potential‐energy and magnetic‐dipole‐field calculations) are used to determine muon stop sites in the RBa2Cu3Ox (x≂7) superconductors. Possible sites, located about 1 A away from oxygen ions, have been found and are prime candidates as muon‐probe locations. The results are discussed in light of existing muon‐spin‐relaxation (μSR) data of these exciting oxides, and compared to H‐ and positron‐oxide superconductor studies. Further work is in progress to establish in detail the muon‐probe sites; this knowledge is an essential ingredient for a correct interpretation of μSR data of high‐temperature superconducting oxides.Two independent search methods (potential‐energy and magnetic‐dipole‐field calculations) are used to determine muon stop sites in the RBa2Cu3Ox (x≂7) superconductors. Possible sites, located about 1 A away from oxygen ions, have been found and are prime candidates as muon‐probe locations. The results are discussed in light of existing muon‐spin‐relaxation (μSR) data of these exciting oxides, and compared to H‐ and positron‐oxide superconductor studies. Further work is in progress to establish in detail the muon‐probe sites; this knowledge is an essential ingredient for a correct interpretation of μSR data of high‐temperature superconducting oxides.

Magnetic ordering in (Y1−xPrx)Ba2Cu3O7 as evidenced by muon spin relaxation

Using the zero‐field‐muon‐spin‐relaxation (μSR) technique clear evidence has been found for antiferromagnetic ordering of Cu moments within the CuO planes of (Y1−xPrx)Ba2Cu3O7. The Neel temperatures are approximately 285, 220, 35, 30, and 20 K for x=1, 0.8, 0.6, 0.58, and 0.54, respectively. For x=0.50 we observe a fast‐relaxing component of the muon polarization in addition to a long‐time tail, reminiscent of spin‐glass behavior. This region of the phase diagram (0.5≤x≤0.54) corresponds to the existence of both superconductivity and magnetism. The fully developed local magnetic field for x>0.54 is found to be ∼16 mT, but decreases to ∼12 mT at T=17 K for the x=1 sample, presumably due to the onset of Pr‐ion ordering. Magnetic ordering also occurs in PrBa2Cu3O6; the Neel temperature is ∼325 K.

Magnetism and candidate muon-probe sites in RBa2Cu3O y

Key results of zero-field (ZF) and transverse-field (TF) muon-spin-relaxation (μSR) experiments on superconducting and insulating RBa2Cu3Oy (R123y, with R=Eu, Gd, Pr and Pr/Y:y=6, 7) are examined. The chemical behavior of the positive muon probe is addressed, and muon-oxygen bonding is shown to occur in all these cuprates. To explain magnetic fields at muon-probe sites in PrxY1−xBa2Cu3Oy (0<=x<0.5,y=7 andx=0,y=6) samples, improvements on the reported magnetic structures from neutron diffraction are necessary. Cu magnetism in Pr123y (y=6,7) is observed belowTN1, which is near RT. The magnetism seen belowTN2 can be interpreted assuming an additional ordering in the Cutt-O chain layers. Alternatively, Pr ordering is also considered as the cause of the second phase transition. Considering the specific muon-probe location, a more detailed interpretation can be provided for the μSR parameters, measured in the normal and mixed states of these unconventional superconductors.

Muon-spin-relaxation study of magnetism in ErBa2Cu3O6.2

The copper magnetism of ErBa2Cu3O6.2 is examined by transverse‐field (TF) and zero‐field (ZF) muon‐spin relaxation (μSR). These data indicate two magnetic phases with TN1≂330 K and TN2∼65 K. The second phase is signaled by deviation of the ZF‐μSR frequencies from a standard magnetization curve and an abrupt change in the TF‐μSR relaxation rate. A relaxation feature indicates a muon depolarization mechanism with a T3/2 dependence in the low‐temperature phase. Observed fields are compared to those calculated for proposed magnetic structures.

μSR investigation of magnetism and superconductivity in (Y1−xPrx)Ba2Cu3O7

Abstract Muon spin rotation and relaxation techniques have been used to study the superconductivity and magnetism in (Y 1− x P x )Ba 2 Cu 3 O 7 (0⩽ x ⩽1) and PrBa 2 Cu 3 O 6 . Clear evidence for magnetic ordering of the Cu moments within the Cu-O planes is seen. Additionally, a lower magnetic transition is observed which, based upon previous work, has been associated with the ordering of Pr moments on the Y sublattice of the YBa 2 Cu 3 O 7 structure. For x = 1, the upper Neel temperature T N1 is ∼270 K and the magnitude of the fully developed local magnetic field is ∼16 mT. Below the lower Neel temperature T N2 = 17 K, the magnitude of the static field is reduced to ∼ 12mT. For 0.4⩽ x ⩽0.54, there appears to be a coexistence region of long range magnetism and superconductivity.

Zero‐field muon‐spin relaxation in (Y1−xPrx)Ba2Cu3Oy

Results of zero‐field muon‐spin‐relaxation (μSR) experiments of Y1−xPrxBa2Cu3Oy [(Y1−xPrx)1237; 0.5<x≤1.0, y=7; x=1, y=6] are examined in light of the observed magnetism and the candidate μ‐probe site. A μSR frequency signal near 2 MHz (extrapolated to 0 K) has been found for 0.5≪x≤1, y=7, and z=0.08, y=7, representing a local magnetic field of about 0.15 kOe at the established B2 μ‐probe site. For x near 1.0 (z=0), a shift in μSR frequency of about −0.4 MHz has been observed near TN2(x). Between TN2 and TN1, the magnetism is due to antiferromagnetic ordering of the Cu moments in the CuO2 planes. No frequency shifts have been observed for Pr1236 at its TN2. The anomalous frequency shift for (PrxY1−x)237 can be due to additional Cu‐chain antiferromagnetism and/or attributable to additional Pr magnetism.

Magnetic frustration, muon probing, and hydrogen bonding in RBa2Cu3Oy

Abstract Recent muon-spin-relaxation (μSR) studies on (Pr x Y 1−x )Ba 2 Cu 3 O y and H z YBa 2 Cu 3 O 7 have confirmed the muon-probe site location near the BaO planes. Similar behavior for H + in Hydrogen-doped YBa 2 Cu 3 O 7 is seen as has been observed for the μ + , namely bonding with oxygen. Thus, the knowledge of the muon-probe site in R123y improves the interpretations of the μSR magnetic studies in (Pr x Y 1−x ) 1237 and the μSR vortex studies in R1237. Magnetic frustration appears to be the key to resolve the physical origin of the CuO-based superconductivity.

Transverse-and zero-field μSR investigation of magnetism and superconductivity in (Y1−xPr x )Ba2Cu3O7

Zero-field muon-spin-rotation (μSR) measurements on (Y1−xPrx)Ba2Cu3O7 [x=1.0, 0.8, 0.6, and 0.54] show evidence for antiferromagnetic ordering of the Cu moments within the Cu−O planes, with Néel temperatures 285, 220, 35. 30 and 20 K, respectively. Forx=1.0 the local muon magnetic field is ≈16 mT, but decreases to ≈12 mT at 17K, due to additional magnetic ordering. The zero-field data, in conjunction with transport data, allow construction of a complete phase diagram for this system. Transverse-field (1 kOe) μSR data forx=0.2 (Tc=75 K) show that the muon depolarization is determined primarily by the Cu nuclear moments forT>Tc, and by the vortex state forT<Tc. Fitting the superconducting-state data to a BCS model yields an extrapolated zero-temperature magnetic penetration depth of 2170 Å.

Muon and hydrogen bonding, and magnetic resonance in pure and doped 519-01519-01519-01

Results of three magnetic resonance studies on pure and doped YBa2Cu3O7 (Y1237) are compared. The muon and the proton, in low concentrations, behave similarly in these cuprates; both magnetic probes bond to oxygens just below the BaO planes. Phase changes from superconducting to magnetic behavior of H-doped Y1237 correspond to similar transitions observed for either 0 depletion or (Pr,Y) mixing. Chemical phase-separation effects are unlikely to be responsible for the simultaneous observation of superconductivity and magnetism. The similarity of the physical effects caused by H doping, O depletion, or (Pr, Y) mixing in the 1/2/3 cuprates points toward a plausible magnetic origin of CuO-based superconductivity.