F. N. Gygax

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.

The detection of muonium in water

Abstract We report the first direct observation of the muonium atom (μ + e − ) in a liquid sample. Using the transverse field μSR technique muonium spin precession signals were detected in water at six different fields between 4 and 80 G.

Bimolecular rate constants for reactions of muonium in aqueous solutions

Abstract Rate constants have been obtained from the rates of disappearance of the muonium precession signal in aqueous solutions of KMnO 4 , NaNO 3 , NaOH, NaCl, HCl, HClO 4 , maleic acid and ethanol. They disagree with the results of earlier, indirect determinations. Comparison with the rate constants for hydrogen atoms reveals large kinetic isotope effects.

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.

Magnetic Screening and Flux Distribution in the High-Tc Superconductor La1.85Sr0.15CuO4

The magnetic screening and flux distribution within a polycrystalline sample of La1.85Sr0.15CuO4 has been measured by means of muon spin rotation spectrocopy (μSR) between 7.6 K and Tc = 34 K in applied fields of 10 Oe to 2 kOe. Above 500 Oe the transverse field relaxation rate of the μ+ spin polarization is quasi-field-independent, as expected for a superconducting material in the mixed state. At low field, a clear reduction of the relaxation rate is observed, interpreted as the disappearance of the mixed state. However, the average internal field at the muon sites, as deduced from the μ+ Larmor frequency does not reveal a sizeable Meissner effect in clear contradiction with the experimental results of the d.c. magnetization carried out on the same sample.

Precession of μ+ in single crystal nickel

Abstract We have observed long-lived μ + precession in single crystal nickel at 300 and 77K. Inhomogeneous local fields which cause rapid depolarization in polycrystalline samples are avoided. Muon precession in a magnetically saturated sample is reported.

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.

μ+SR-spectroscopy in intermetallic compounds containing heavy-electrons

Abstract μ + SR-investigations of various heavy-electron systems with different electronic ground states are presented. These experiments have revealed the presence of hitherto unknown weak magnetic correlations in some of these materials. The correlations partly coexist with superconductivity and can even develop without a distinct phase transition. They are quasistatic on the muon timescale in CeAl 3 , CeCu 2.1 Si 2 , U 2 Zn 17 , UCu 5 , URu 2 Si 2 , UPt 3 and U 0.977 Th 0.033 Be 13 , but not in the case of UCd 11 . The magnitude of the local magnetic fields at the muon sites points to small effective electronic f-moments ( B ) and/or small coherence lengths. This weak magnetism seems to be an important feature of the electronic ground state of heavy-electron materials and deserves further investigations both from the experimental and theoretical point of view.

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〉.