Evidence for a two component magnetic response in UPt3

The magnetic response of the heavy fermion superconductor UPt3 has been investigated on a microscopic scale by muon Knight shift studies. Two distinct and isotropic Knight shifts have been found for the field in the basal plane. While the volume fractions associated with the two Knight shifts are approximately equal at low and high temperatures, they show a dramatic and opposite temperature dependence around T(N). Our results are independent on the precise muon localization site. We conclude that UPt3 is characterized by a two component magnetic response.     

Longitudinal field quenching studies in KCl and KBr — Evidence for a precursor muonium state

The repolarization of μ⁺ in a longitudinal field as a function of field strength has been measured at various temperatures in KCl and KBr single crystals. The data at room temperature in KCl are essentially in agreement with Ivanteret al. /1/ and Nishiyamaet al. /5/. The obtained quenching curves can be reasonably fitted by assuming that the long lived muonium state, seen directly in transverse field experiments by Kieflet al. /4/, is preceded by another short lived muonium state with reduced hf-coupling. It is also necessary to allow for electron spin exchange in the precursor state. The model fits yield the hyperfine frequency of the precursor state, its chemical lifetime and an electron spin flip rate. It is speculated that the precursor state and the temperature dependence of the fitted parameters reflect the early history of the μ⁺ in its terminal spur.     

Homogeneous VS. Inhomogeneous spin freezing in pure and au-dopedCuMn

Zero-field μSR provides clear evidence for a spatially inhomogeneous spin freezing in the spin glass systemCuMn. To test a spin glass with very different properties with respect to this characteristic, we investigatedCuMn samples doped with Au. The freezing process, as observed by μSR, was found unaffected.     

μSR in UAs

UAs has the NaCl structure and undergoes a first order transition into a type I (single k) antiferromagnetic state at 123 K, followed by a second first order transition at 62 K into a type IA (double k) antiferromagnetic structure. μSR spectra of a powder sample were taken in zero and transverse fields up to 0.3 T. They cover the paramagnetic and the two antiferromagnetic states. The most significant features of our data are: i) a first increase of relaxation rate below T=180 K; ii) a sudden jump in both, relaxation rate and frequency shift at T=123 K, together with a small decrease in initial asymmetry (≈15%); iii) no μ⁺ spin rotation in zero field in the type I state; iv) an overlay, of 3 spectra in the type IA state. Two of these spectra show spin rotation in zero field. Their frequencies are clearly temperature dependent. In a transverse field of 5 and 10 mT the external field adds nearly fully to the internal field. Work supported in part by the Bundesministerium fur Forschung und Technologie, Federal Republic Germany.     

μSR studies of borocarbides

The muon‐spin‐relaxation technique has been utilized to characterize the magnetic properties of the borocarbide compounds RNi₂B₂C ( R=\,Tm, Er, Ho and Dy), which exhibit coexistence of superconductivity and magnetism. Some general features of their magnetic ground‐state and unusual results obtained in the Ho, Tm and Dy systems are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

μ+SR frequency shifts in the van‐Vleck paramagnet PrIn3

As part of an ongoing μ⁺SR study on cubic van‐Vleck paramagnets of the PrM₃ series ( M\ =\ In,Pb,Tl,Sn), in which the CEF level scheme varies systematically, we present preliminary static μ⁺SR results on a single crystal of PrIn₃. From the angular dependence and from the appearance of the μSR spectra, we could determine that stopped μ⁺ occupy octahedral sites of the (1/2, 0, 0) type, i.e. sites with 2 Pr and 4 In atoms as neighbors. The spectral frequency shifts can be accounted for by assigning to the Pr sites essentially unperturbed 4f dipole moment values, as derived from bulk magnetization measurements. The temperature dependence of the shifts scales to a good accuracy with the bulk susceptibility. It is concluded that unlike previous observations with PrNi₅, the local perturbations induced by an interstitial μ⁺ on the CEF levels of neighboring Pr in the PrIn₃ system are small compared with the unperturbed level splitting itself. This revised version was published online in July 2006 with corrections to the Cover Date.     

Anomalous field dependence of muon Knight shift and line width in the singlet ground state compound PrCu2

We report on ZF and TF‐studies of PrCu₂ above the induced Jahn–Teller transition at 7.3 K. Generally a two‐component signal is found, one showing inhomogeneous the other one homogeneous, temperature dependent line broadening. In ZF the former component is well represented by a Gaussian Kubo–Toyabe function with \varDelta \simeq 6.5\,μs^-1 at 7.5 K, corresponding to a field width of 76 G. This is about 30 times larger than what is calculated to arise from the ¹⁴¹Pr‐nuclear dipole moments alone, pointing to strong hyperfine enhanced features. TF‐field scans at 12 K revealed that the enhancement is suppressed in external fields exceeding 1 kG. In parallel the Knight shift drops from very large values well above 10% at 100 G to shifts of the order of 1% above 1 kG. A scaling of the Knight shifts with the corresponding relaxation rates seems to imply that the strange field dependence below 1 kG is associated with the magnetic susceptibility of the muons’s nearest neighbour Pr³⁺‐ions, a result for which we have no explanation yet to offer. This revised version was published online in July 2006 with corrections to the Cover Date.     

Conjugative Stabilization in an “Antiaromatic” System: The Conformational Mobility of 1,5-Bisdehydro[12]annulene

The isodynamical interconversion between the quasiplanar equilibrium conformers 1b and 1c of 1,5-bisdehydro[12]annulene requires a free energy of activation ΔG^# = 4.5 ± 0.2 kcal/mol (120 K) which was determined by line shape analysis of its temperature-dependent NMR. spectrum. Force field calculations indicate that this barrier reflects the energy needed to disrupt the cyclic π-conjugation in going to the nonplanar transition state 1a .