P. Birrer

Study of La2CuO4 and related compounds by μSR

The spontaneous muon precession frequency and relaxation rate were measured in several La-based Cu-oxide samples of La2CuO4 with different stoichiometry in order to study the influence of La-vacancy, Sr-doping and O-doping on magnetism and super-conductivity. From the measured frequency at low temperatures in the magnetically ordered samples and the relaxation rate in the normal state of superconducting samples the muon site was determined. We find the muon to be on the face of the unit cell of La2CuO4. 1 Å away from the apical oxygens ions O(2), favoring the typical bond length of 1 Å for O−H.

Identification ofμ +-sites in the 1-2-3 compoundin the 1-2-3 compound

Possibleμ+ sites have been identified by a comparison of measured spontaneous internal fields in YBa2Cu3O6+δ and REBa2Cu3O7−δ and of measured second moments and width of transverse field powder spectra with the corresponding calculated quantities. In the YBa2Cu3O7 system only one possibleμ+ site emerges at a distance of ∼1.05 A from a chain oxygen O(4) at the position (0.15(1), 0.44(1), 0.071(1)). In the system YBa2Cu3O6 the only possibleμ+ sites are near to an oxygen O(1) with z/c=0.133 and the same distance as above. The analysis leads also to improved valuesμHo = 2.2(1)μB andμCu = 0.67(4)μB and to a determination of the latters direction:μCu ‖〈110〉.

Magnetic penetration depth in Bi2(Sr,Ca)2CuO8−δ

Abstract The muon-spin relaxation was measured in the high- T c superconductor Bi 2 Sr 1.3 Ca 0.7 CuO 6.15 in transverse external magnetic fields of 72 G, 2 kG and 3 kG (TF-μSR) in field-cooling temperature scans between 300 and 2 K. Below T c = 84 K the relaxation rate increases almost field-independently, as expected for a superconducting material in the mixed state. The data allow to determine the London penetration depth λ⊥ perpendicular to the basal plane, after taking into account various corrections. Its value at zero temperature, λ⊥ (0) ∼ 3500 A, is significantly larger than in YBa 2 Cu 3 O 7 (∼ 1600 A), although both system possess nearly the same T c .

Measurements of the London penetration depth in Bi-based high-T c compounds

The muon spin relaxation was measured in the high-Tc superconductors Bi2Sr1.3Ca0.7 CuO6.15, Bi2Sr2CaCu2O8 and Bi2PbxSr2Ca2Cu3O10. We present our method to determine the London penetration depth perpendicular toc-axis λ⊥ (0) taking also contributions from the anisotropic magnetization and nuclear dipoles to the muon frequency spectrum into account.

Magnetic penetration depth in the Chevrel phase superconductors SnMo6SySe8−y and PbMo6SzSe8−z

We report on transverse field (300 mT)μ+ SR measurements in the Chevrel phase superconductors SnMo6SySe8−y (y=1, 4, 7, 8) and PbMo6SzSe8−z (z=4.8) using the field cooling procedure (from room temperature down to 2.2 K). The London penetration depth λL was determined by the analysis of the fits with a single Gaussian relaxation function and by comparing the measured field distributions with calculated ones, where a broadening due to small random displacements of the vortices was taken into account.

Discovery of novel magnetic features in the heavy-electron compound U2Zn17

Abstract Transverse field μ + SR-measurements on a single crystal sample of U 2 Zn 17 reveal a strikingly non uniform magnetic behavior. While part (∼30%) of the U-5f moments remain paramagnetic below T N = 9.7 K (at least for external fields ≥0.2 T) another part (∼20%) enters into a complex order below 9.7 K which is associated with huge internal field spreads at the μ + sites. Above T N this part causes an anisotropic μ + Knight shift involving direction cosines up to 8th order. Only. the remaining fraction is compatible with the simple AF order suggested by neutron diffraction studies.

Zero-field μSR study of ErBa2Cu3Ox for various oxygen contentx

Results from zero-field μSR experiments are presented, performed on the high-Tc compounds ErBa2Cu3Ox for various oxygen contentx with 6≤x≤7 at temperatures between 40 mK and 300 K. The aim was to study the magnetic ordering behavior of the Cu moments and the Er moments, and its interplay with superconductivity.

μ+ sites and local moments in Van-Vleck paramagnet: PrNi5

Multi-frequencyμ+ SR spectra were observed in PrNi5 at 23 K in an external transverse field of 3010 G. Three types of interstitial sites are identified as the most likely muon stopping positions. A substantial amount of the magnetic moment sampled by the μ is found to be localized on the Ni sites.

Transverse fieldμ+ SR investigations of rare earth substituted 1-2-3 compoundsSR investigations of rare earth substituted 1-2-3 compounds

Transverse field studies (0.3 T) in HoBa2Cu3O7, DyBa2Cu3O7 and ErBa2Cu3O7 (x=6.40. 6.45, 6.53, 7) were undertaken with the aim of detecting a possible 4f-moment inducedμ+ Knight shift which could elucidate the nature of the magnetic coupling among the 4f-moments and a possible interference with superconductivity. AboveTc no shift could be detected in any of the systems except in HoBa2Cu3O7 where a split signal appears involving a positive and a negative shift. Unexpected shifts may be present belowTc.

μ+ Knight shift and spin relaxation in indium single crystalKnight shift and spin relaxation in indium single crystal

Abstractμ+ SR measurements have been performed in a single crystal indium sample between 12 K and 300 K with a stroboscopic μSR spectrometer. The muonic Knight shiftKμ and the muonic depolarization rate σ were obtained for various angles θ between the tetragonal crystallinec-axis and the direction of the external field. The isotropic part ofKμ is only weakly temperature dependent and is consistent with the estimated Pauli spin susceptibility value. At a temperature of 12 K the angular dependence ofM2 (the second moment of the field distribution at the muon, obtained from the measured σ(θ) values) allows a clear determination of the muon location — the symmetric tetrahedral site. The observed anisotropicKμ cannot be explained by the dipoles at the In atoms responsible for the bulk magnetic susceptibility but probably originates from an anisotropic Pauli spin susceptibility.