Muon spin rotation in superconductors

By means of the muon spin rotation technique (⁺SR), the temperature dependence of the magnetic field inside the normal-conducting domains of high-purity tantalum crystals in the intermediate state has been measured in the temperature range 2.36K+SR. Possible applications of these findings to the study of long-range diffusion of positive muons at low temperatures are indicated.     

Muon spin rotation studies at SIN

Recently the accelerator and the muon channel at SIN became operational. We report here on some of the first positive muon spin rotation experiments at SIN. The first experiment we discuss concerns the chemistry of muonium. We have observed for the first time a muonium or radical signal in pure water. Next, we discuss the application of the positive muon for the study of ferromagnetic metals and alloys. First measurements on a single crystal of iron around liquid helium temperature seem to indicate that at low temperature the muon does not diffuse. We further report on the first stroboscopic observation of the muon spin rotation which will allow one to take full advantage of the high stopping density at SIN.     

Muon spin rotation in liquid noble gases

A new mechanism of slow muon depolarization in liquid noble gases is suggested. The muon spin polarization in [A-μ-A]⁺ complexes is calculated. The results allow one to observe [A-μ-A]⁺ experimentally and to measure hyperfine interaction constants.     

Muon spin relaxation studies of magnetic-field-induced effects in high-Tc superconductors

Muon spin relaxation measurements in high transverse magnetic fields [FORMULA: SEE TEXT] revealed strong field-induced quasistatic magnetism in the underdoped and Eu-doped (La,Sr)2CuO4 and La1.875Ba0.125CuO4, existing well above Tc and TN. The susceptibility counterpart of Cu spin polarization, derived from the muon spin relaxation rate, exhibits a divergent behavior towards T approximately 25 K. No field-induced magnetism was detected in overdoped La1.81Sr0.19CuO4, optimally doped Bi2212, and Zn-doped YBa2Cu3O7.     

Muon spin rotation in orthoferrites ErFeO3 and DyFeO3

SR measurements on the rare earth orthoferrites ErFeO₃ and DyFeO₃ are reported. Experiments have been made as a function of temperature (8–460 K) and the external magnetic field. Above 90 K a single internal field on the order of 2 kG parallel to the c-direction is observed. Zero external field measurements between 50 K and 84 K showed in ErFeO₃ two distinct local fields. The external field dependence of the observed frequencies demonstrates that the local field is related to the weak ferromagnetism in both orthoferrites.Work supported by the Swiss National Science Foundation.     

The classification of superconductors using muon spin rotation

In this paper we explore the role of muon spin rotation (μSR) techniques in the characterization and classification of superconducting materials. In particular we focus upon the Uemura classification scheme which considers the correlation between the superconducting transition temperature,T _c, and the effective Fermi temperature,T _F, determined from μSR measurements of the penetration depth. Within this scheme strongly correlated “exotic” superconductors, i.e, hightT _C cuprates, heavy fermions, Chevrel phases and the organic superconductors, form a common but distinct group, characterized by a universal scaling ofT _C withT _F such that 1/100 <T _C/T _F<1/10. For conventional BCS superconductorsT _C/T _F<1/1000. The results of new μSR measurements of the penetration depth in superconducting Y(Ni_1?x Co _x )₂B₂C and YB₆ are also presented. In Y(Ni_1?x Co _x )₂B₂C the decrease ofT _C with increasing Co concentration is linked to a marked decrease in the carrier density from 2.9·10²⁸ m^?3 atx=0 to 0.6·10²⁸ m^?3, atx=0.1, while the carrier mass enhancement remains almost constant at approximately 10. For YB₆ we find evidence of a modest enhancement of the carrier mass (m ^*/m=3), and a relatively low carrier density of 0.24·10²⁸ m^?3. These results are discussed within the Uemura classification scheme. It is found that neither Y(Ni_1?x Co _x )₂B₂C withT _c/T _F>>1/250 nor YB₆ withT _C/T _F>>1/340 can be definitively classified as either “exttic” or “conventional”, but instead the compounds display behavior which interpolates between the two regimes.     

Muon spin rotation (μSR) in a metal-cluster carbonyl compound

μSR spectra from a metal carbonyl compound are reproted for the first time. The metal cluster compound Cs₂[Os₁₀C(CO)₂₄] shows a temperature dependent signal for implanted muons with the onset of motional narrowing at ca. 200 K. Also observed is a probable change in the width within the temperature range 40–140 K. A tentative assignment is provided for the muon sites.     

Muon spin rotation in GdSr2Cu2RuO8: Implications

Muon spin rotation measurements are reported for GdSr₂Cu₂RuO₈, a material with an onset temperature for superconductivity of about 45 K (which is virtually the same for its superconducting sister compounds Sr₂YRu_{1? u} Cu _u O₆ and Gd_{2? z} Ce _z Sr₂Cu₂RuO₁₀). The data indicate two magnetic ordering transitions, at about 15 K and about 130 K, in addition to the Gd ordering transition known to occur at about 2.6 K. We tentatively attribute the 130 K transition to Ru and the 15 K transition to Cu ordering, effectively ruling out any superconducting mechanism based on fluctuating magnetic moments, which are frozen below about 15 K. If there is only one mechanism of high-temperature superconductivity, then the three facts that (i) all three sister compounds have essentially the same onset T _c for superconductivity and (ii) all three of these compounds contain SrO layers but (iii) only two of the three compounds, GdSr₂Cu₂RuO₈ and Gd_{2? z} Ce _z Sr₂Cu₂RuO₁₀ (and not Sr₂YRu_{1? u} Cu _u O₆) contain cuprate planes imply that the superconducting layers of all three compounds must be the common SrO layers, and not the cuprate planes (which do not occur in Cu-doped Sr₂YRuO₆). Otherwise the coincidence of onset temperatures must be an accident, and there must be at least two mechanisms of high-T _c superconductivity: one for doped Sr₂YRuO₆ and another for cuprate-plane superconductivity.     

Muon spin echo

A muon spin echo method is proposed for determining the contributions of the static and dynamic local fields in muon experiments. It is shown that if for each muon which has stopped in the sample a rf pulse of fixed duration is applied to the sample at a time τ after the muon entered the sample, then after a sufficient number of muon-positron decays have been accumulated a muon spin echo can appear at time 2τ. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 500–503 (10 October 1996)     

Muon spin rotation experiments on a silicon crystal doped with13C

A crystal of silicon doped with carbon enriched to 60.1% in¹³C was studied bySR to determine whether¹³C hyperfine structure could be observed in the frequency spectra of normal muonium, Mu, or anomalous muonium, Mu^*. Measurements at 100 G and 100 K with 40 million good events yielded extremely weak Mu^* signals and no Mu in these data or in measurements at 10 G and 150 K. Transmission electron micrographs of this sample contained small regions showing strain contrast and structure factor contrast. Annealing the sample at 900°C for 84 hours led to featureless electromicrographs. SubsequentSR measurements yielded a strong Mu^* signal but still no Mu. No broadening due to¹³C was observed.     

Muon spin relaxation in ferromagneticPdMn

Positive-muon ( ⁺) spin relaxation experiments have been carried out in the dilute ferromagnetic alloy Pd+2 at.% Mn (T _c=5.8 K). In the paramagnetic state the inhomogeneous ⁺ linewidth is proportional to the bulk magnetization. BelowT _c the ⁺ linewidth and the width of the ⁺ local field distribution in zero applied field are both in qualitative accord with the Sherrington-Kirkpatrick theory of disordered magnets.This work was performed under the auspices of the U.S. Department of Energy, and was supported by the U.S. National Science foundation, grant nos. DMR-7909223 and DMR-8115543, and by the Netherlands Stichting voor Fundamenteel Onderzoek der Materie (FOM).     

Muon spin rotation studies of spin dynamics at avoided level crossings in LiY0.998Ho0.002F4

We have studied the Ho3+ spin dynamics for LiY0.998Ho0.002F4 via the positive muon (mu+) transverse field depolarization rate lambdaTF as a function of temperature and magnetic field. We find sharp minima in lambdaTF(H) at fields for which the Ho3+ ion system has field-induced (avoided) level crossings. The reduction scales with calculated level repulsions, suggesting that mu+ depolarization by slow fluctuations of nonresonant Ho3+ spin states is partly suppressed when resonant tunneling opens new fluctuation channels at frequencies much greater than the muon precession frequency.