A. Schenck

Magnetic and superconducting properties of the heavy-fermion superconductor UPd2Al3

Zero- and transverse-field muon-spin rotation (μ+ SR) spectroscopy, as well as critical-field investigations were performed on the newly discovered antiferromagnetic superconductor UPd2Al3. In contrast to the situation in UNi2Al3, no spontaneous frequency could be observed in the zero-field spectroscopy, suggesting a symmetric muon stopping site. The coexistence between magnetism and superconductivity could unambiguously be confirmed by the μ+ SR data. The magnetic-penetration depth λ(0) has been obtained from the increase of the μ+ SR line width below Tc and from the slopes of the critical field at Tc. The results are compatible with a value of λ(0) = (6250 ± 1250) and suggest that UPd2Al3 is a BCS-type weak-coupling superconductor.

Inhomogeneous magnetism in URu2Si2 studied by muon spin relaxation under high pressure

Abstract We have studied the low-temperature phase below 17.5 K in the heavy-fermion superconductor URu 2 Si 2 , using the zero-field μSR technique under hydrostatic pressures P up to 0.85 GPa . The measurements qualitatively confirm the previous results of NMR and neutron scattering that pressure induces unusual evolution of an inhomogeneous antiferromagnetic (AF) phase which spatially competes with some nonmagnetic ‘hidden order’. The AF volume fraction, however, develops abruptly at around a pressure of 0.6 GPa , in contrast to the P -linear variations in the NMR results. Moreover, the onset temperature of the AF-volume evolution is clearly lower than 17.5 K , whereas the nominal Neel temperature seems to be about 20 K . These experimental results suggest that the AF phase exists as a metastable state.

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.

μ+SR studies of UM2Al3, M=Ni, Pd

Positive-muon spin rotation (μ+ SR) measurements have been carried out in the new heavy-fermion superconductors UM2Al3, M=Ni and Pd. In UNi2Al3 the observedμ+ frequencies in zero applied field indicate commensurate antiferromagnetic (AF) ordering of U moments ∼0.1μB/U atom. These moments coexist with superconductivity and have the highest values observed in an AF heavy-fermion superconductor. The absence of well-defined frequencies in zero-fieldμ+ SR in the AF state of UPd2Al3 suggests symmetricμ+ stopping sites. In this systemμ+ SR linewidths belowTc yield a preliminary value of ∼8000 Å for the London penetration depth.

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〉.

Weak magnetism phenomena in heavy-fermion superconductors: selected µSR studies

The behaviour of the so-called weak-moment antiferromagnetic states, observed in the heavy-fermion superconductors UPt3 and URu2Si2, is discussed in view of recent ?SR results obtained as functions of control parameters such as chemical substitution and external pressure. In UPt3, the Pd substitution for Pt reveals the dynamical character of the weak-moment order. On the other hand, ?SR measurements performed on samples in which Th substitutes U suggest that crystallographic disorder on the magnetic sites deeply affects the fluctuation timescale. In URu2Si2, a phase separation between the so-called hidden order state, present at ambient pressure, and an antiferromagnetic state, occurring under pressure, is observed. In view of the pressure?temperature phase diagram obtained by ?SR, it is deduced that the respective order parameters have different symmetries.

Evidence for static ultrasmall-moment magnetism in CeRu2Si2

Abstract We report on zero-, transverse- and longitudinal-field μ + SR studies on a high-quality single crystal of CeRu 2 Si 2 . The development of some sort of static magnetism (Ce-moment ≅0.001μ B ) below 1.5 K is observed in the zero-field relaxation rate. Measurements in longitudinal fields demonstrate the static character of this magnetism and reveal in addition the presence of dynamic 4f spin fluctuations (also observed in the transverse-field spectra) which coexist with the static magnetism below 1.5 K.

Magnetic quantum critical point and superconductivity in UPt3 doped with Pd.

Transverse-field muon spin relaxation measurements have been carried out on the heavy-fermion superconductor UPt (3) doped with small amounts of Pd. We find that the critical Pd concentration for the emergence of the large-moment antiferromagnetic phase is approximately 0.6 at. %Pd. At the same Pd content, superconductivity is completely suppressed. The existence of a magnetic quantum critical point in the phase diagram, which coincides with the critical point for superconductivity, provides evidence for ferromagnetic spin-fluctuation mediated odd-parity superconductivity, which competes with antiferromagnetic order.

μ+SR study in CeCu6 single crystal

Abstract μ + SR measurements have been carried out down to 40 mK on a high-quality single crystal. The zero-field data show no sign of magnetic ordering down to the lowest temperature investigated. Transverse-field data indicate a large orientational dependence of the muon Knight-shift and point to a breakdown of the relation between atomic and bulk susceptibility.

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.