Field and temperature dependence of the superfluid density in LaFeAsO1-xFx superconductors: a muon spin relaxation study

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor LaFeAsO1-xFx (x=0.075 and x=0.1). The temperature dependence of the deduced superfluid density is consistent with a BCS s-wave or a dirty d-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of lambda ab(0)=254(2) nm for x=0.1 and lambda ab(0)=364(8) nm for x=0.075. Further evidence for unconventional superconductivity is provided by the ratio of Tc versus the superfluid density, which is close to the Uemura line of high-Tc cuprates.     

Field dependence of the electron spin relaxation in quantum dots

The interaction of the electron spin with local elastic twists due to transverse phonons is studied. The universal dependence of the spin-relaxation rate on the strength and direction of the magnetic field is obtained in terms of the electron gyromagnetic tensor and macroscopic elastic constants of the solid. The theory contains no unknown parameters and it can be easily tested in experiment. At high magnetic field it provides a parameter-free lower bound on the electron spin relaxation in quantum dots.     

Zero field muon spin relaxation rate in the superconducting state of Nb3Al

It was shown that at T=12 k the dipole relaxation of a positive muon in Nb₃Al in the zero external magnetic field is well described by the Kubo-Toyabe function for the static distrubution of the dipole magnetic fields in the absence of muon diffusion. The superconducting state of Nb₃Al was used for eliminating the background from muons stopped in the cryostat walls.     

Longitudinal muon spin relaxation in muonium-like systems

Nuclear spin-lattice relaxation in paramagnetic systems is treated using the classic expression for transition probability between the coupled electron and nuclear spin states. The rate equations governing the incoherent occupancies of these states are solved analytically (where possible) and numerically (where not) to construct the relaxation function for the nuclear spin. The method is illustrated for muonium, and the muonium-substituted molecular radicals, for the case of perturbation due to fluctuation of the local field,i.e. modulation of the interaction with a third spin. A slight departure from single exponential behaviour is demonstrated for slow fluctuations.     

Zero-field muon spin relaxation reflecting dynamics of spin glass

The muon spin relaxation function is calculated in a fluctuating random dilute spin system, and the effect of Edwards—Anderson's order parameter is considered in a time-dependent treatment. The results demonstrate the unique capability of zero-field μSR probing spin glasses.     

Zero-field spin relaxation of positive muon in Al-Mg

The muon spin relaxation in the Al-Mg dilute alloy was measured under the zero external field from 34 to 120 K, and the observed spectra were fitted to the calculated spin relaxation function of the two state model which is taking account of both trapping and detrapping mechanisms. Thus, the muon behavior in Al in the presence of impurity traps were able to be visualized: a muon diffuses in the incoherent hopping process with one phonon assistance, while it is then captured by a trap in a diffusion control process, and detrapped in the thermal activation process.     

Longitudinal muon spin relaxation in the organic metal polypyrrole

Longitudinal and zero field measurements of the muon spin relaxation function are reported for muons implanted in the conducting polymer polypyrrole. The observed muon relaxation can be accounted for by interaction with polaron excitations in the conducting polymer. Evidence is found for polaron localisation at temperatures below ∼ 50 K. The temperature dependence of the polaron mobility derived from the muon data is consistent with direct electrical conductivity measurements.     

Zero field muon spin relaxation in simple magnets and spin glasses

A theory is developed for the calculation of zero field muon spin relaxation function for classical simple magnets (Ising, XY, and Heisenberg) in D(=1,2,3) dimensions. The results are different from the Kubo-Toyabe theory, except for Heisenberg system in three dimension. Relation between the relaxation function and random field distribution is dicussed and a new method of analysing experimental data is suggested and discussed in the context of spin glasses.     

Magnetic field dependence of the spin relaxation rate in optically oriented mercury vapour

The magnetic field dependence of the relaxation rate of ¹⁹⁹Hg has been investigated at low fields in the presence of Ne or N₂. It is shown that there is a correlation time, in the interaction of the Hg nuclear spin with the walls, that is equal to the time between wall collisions. The diffusion coefficients of Hg into N₂ and Ne are evaluated, and the possibility of investigating other correlation times is discussed.     

Fast dynamization routine for muon spin relaxation functions

The calculation of dynamic relaxation functions is an essential ingredient for the analysis ofSR, spectra. So far no fastand memory efficient algorithm has been presented to cure this problem. The traditional method is the calculation of large, memory inefficient tables by means of either Monte Carlo simulation or numerical Laplace inversion. An alternate treatment within the strong collision model is presented making use of a Fourier transform.This work was supported by the German Federal Minister for Research and Technology (Bundesminister für Forschung und Technologie [BMFT]) under Contract Nr. 03KA2-TUM-4.     

Longitudinal muon spin relaxation in organic radicals in the vapour phase

The aim of this work is the study of longitudinal field relaxation for organic radicals in the gas or vapour phase, for which the technique of Muon Spin Relaxation proves to be uniquely suitable. Following on from the first such study of the muonium substituted ethyl radical in gaseous ethene [1], the present work represents the beginning of a systematic study of species of various molecular symmetries and still higher molecular weight, with a view to investigating their collisional dynamics. Preliminary data for the muonium substituted cyclohexadienyl radical in benzene vapour, and the results of tests for radical formation in acetylene and carbon monoxide, are presented.     

The effect of traps on the muon spin relaxation function

Summary In summary it is clear that SR in the presence of traps has led to a significant generalization of prior NMR theories of spin relaxation to include the effects of a non-stationary distribution of diffusing particles. In both the high field transverse geometry and in zero field it is possible to extract significant information regarding the concentrations and binding energies of traps, and, in the latter case, to unambiguously measure the temperature dependence of the relaxation rate to traps.At the same time it is also clear that significant problems still remain to be solved. Many of these are concerned with our present inadequate knowledge regarding the diffusion of muons in even pure materials, let alone the disordered systems upon which we have concentrated here. However, given the growth in our understanding of trapping phenomena over the past several years, it is probably not too much to hope that in another few years many of these present questions will also be answered.     

Spin dynamics of geometrically frustrated spin systems observed by muon spin relaxation

μSR is shown to be a sensitive probe of fluctuating internal magnetic fields in geometrically frustrated magnets. The usefulness of μSR in these systems is illustrated in the case of pyrochlores, in which the antiferromagnetically coupled ions occupy a lattice of corner sharing tetrahedra. Remarkably, one observes a type of spin freezing in Y₂Mo₂O₇ and Tb₂Mo₂O₇ which is similar to that seen in conventional spin glasses, even though there is no detectable structural disorder. Unlike ordinary spin glasses these geometrically frustrated antiferromagnets display unusual low temperature spin dynamics which persist down to the lowest accessible temperatures.     

Nuclear spin lattice relaxation in antiferromagnets: Field dependence of the dipolar-induced two-magnon process

We consider the role of a magnetic field on the dipolar-induced two-magnon process for the nuclear spin-lattice relaxation in antiferromagnets.     

Spin dynamics in magnetic materials investigated by muon spin relaxation

We present a quantitative analysis of the temperature dependence of the muon spin relaxation rate measured in simple magnets. We consider the low temperature, critical and high temperature regimes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spin-glass dynamics determined from muon spin relaxation and neutron spin echo measurements

Muon spin relaxation (SR) and neutron spin echo (NSE) measurements of magnetic ion correlation times and correlation functions in the spin glass systemsCuMn,AgMn, andAuFe are compared. It is found that theSR and NSE measurements are in excellent agreement both above and below the spin-glass freezing temperatures. The experimental results are compared to recent theories of spin-glass dynamics.We are grateful to D.L. Huber and R.E. Walstedt for stimulating discussions. This work was performed under the auspices of the U.S. Department of Energy, and was also supported by the U.S. National Science Foundation, Grant No. DMR-8115543.     

Observation of the anisotropic spin-glass transition and transverse spin ordering in pseudo-brookite through muon spin relaxation

Zero-field longitudinal muon-spin-relaxation (μSR) experiments have been performed on single crystals of pseudo-brookite (Fe₂−_xTi_l+x ^O ₅; x=0.25), an anisotropic spin-glass system. The spinglass temperature (Tg) is determined to be 44.0±0.5K. Above Tg, a distinct exponential muon-spin-relaxation rate (λ) is observed, while below Tg a square-root exponential decay is seen, indicating fast spin fluctuations in the ‘frozen’ state. Near 8K, a maximum in λ is observed, which is due to transverse spin ordering at these low temperatures. Even near Tg, λ is very low (<1 μs⁻¹), likely due to a well-defined muon-oxygen state in the single crystals. The sharp λ-increase (with decreasing temperatures) above Tg allows a comparison between spinfreezing models like the Vogel-Fulcher law and a power law. The results of these initial measurements indicate that dynamic (and static) magnetism in oxide spin glasses can be directly monitored through μSR.     

Theory of muon spin relaxation by critical fluctuations in antiferromagnetic salts

Relaxation rates for positive muons implanted in antiferromagnetic salts are predicted on the basis of calculations performed with a coupled-mode theory of critical fluctuations in isotropic and uniaxial magnets. Anisotropy suppresses fluctuations transmitted by the isotropic (hyperfine) relaxation mechanism. The dipolar mechanism leads to a relaxation rate which increases on lowering the temperature to the critical point; the predicted form, proportional to the critical correlation length, yields the same temperature dependence as for the NMR linewidth. For a magnet with isotropic spin interactions, both contributions (hyperfine and dipolar) to the relaxation rate diverge with a temperature dependence given by the square-root of the correlation length.     

MuSR技术在凝聚态物理中的应用

缪子自旋弛豫/旋转(MuSR)实验技术利用缪子的自旋,在原子尺度上研究样品内部磁场的静态分布和动态涨落。文章介绍MuSR实验技术在凝聚态物理中的应用,阐述了MuSR在磁性、超导材料中发现的独特信息,包括磁有序、内部磁场分布、具有特定频率范围的自旋涨落,以及在半导体材料中获得杂质的电子状态和位置。这些应用表明 MuSR 对于理解关联电子材料复杂的物理现象及其物理机制具有重要意义。