E. Lippelt

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-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〉.

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 .

Static Magnetic Correlations in CeAI3 below 0.7 K

We report the observation of a spontaneous muon-spin precession frequency in zero external magnetic field in CeAl3 below 0.7 K. At 35 mK the µ+ frequency reflects a local magnetic field of 220 G at the muon site. This is compatible with a static moment of about 0.05 µB per Ce ion. Specific heat measurements on the same sample reveal no anomaly in cp(T) at the temperature where the observation of the spontaneous precession frequency sets in.

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.

μSR studies on CeAl2

AbstractμSR studies on REAl2 type compounds have so far given rather inconclusive results since no μSR frequency has been observed in the ordered magnetic states. Therefore, the results from the paramagnetic region [1,2] have been interpreted without detailed knowledge of the muon site or the mobility of the muons. In the present study of a single crystal sample of CeAl2 we investigated in some detail the paramagnetic temperature range including the transition region to magnetic ordering around 3.6 K. The ordered magnetic state is antiferromagnetic with a modulated structure [3], and the absence of a spontaneous μSR precession signal belowTN is therefore not unexpected.

Muon knight shift between 75 and 300 K in the high temperature superconductor YBa2Cu3O7

Abstract First μ+ Knight shift results (Kμ) in the system Ba2Cu3O7 between 75 and 300 K are reported. Above Tc ≈ 90 K Kμ is composed of a temperature independent part of 24±7 ppm and a Curie-like component. The absolute value of the relative field shift at 75 K is found to be smaller than expected by ∼ 8 3 π∥M∥ , where M is the macroscopic magnetization of the sample.

Positive muons in bismuth: determination of their sites, detection of a large local lattice contraction and giant associated electric field gradients, evidence for short and long range diffusion

Using a high purity Bi single crystal the temperature and orientation dependence of the zero and transverse field muon spin relaxation rate has been studied in detail. The results imply that the μ+ occupies one of the two possible interstitial sites in the distorted rhombohedral crystal structure of Bi below 10 K and the other site above 80 K. At both sites the nearest neighbor Bi atoms are found to be shifted towards the μ+ by ≈10% of their nominal distance, implying a large local lattice contraction. In concomitance extremely strong electric field gradients are manifest at the nn Bi nuclei. An almost temperature independent reduced relaxation rate in the temperature range from 20 K to 60 K is interpreted in terms of short range diffusion along a limited chain of alternating types of sites. Above 100 K both long range and short range diffusion are indicated.

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.