Field dependence of the muon spin relaxation rate in MnSi

Muon spin rotation/relaxation measurements have been performed in the itinerant helical magnet MnSi at ambient pressure and at 8.3 kbar. We have found the following: (a) the spin-lattice relaxation rate 1/T(1) shows divergence as T1T proportional, variant (T-T(c))(beta) with the power beta larger than 1 near T(c); (b) 1/T(1) is strongly reduced in an applied external field B(L) and the divergent behavior near T(c) is completely suppressed at B(L)> or =4000 G. We discuss that (a) is consistent with the self-consistent renormalization theory and reflects a departure from "mean-field" behavior, while (b) indicates selective suppression of spin fluctuations of the q=0 component by B(L).     

A muon‐spin‐rotation study of the superconducting condensate density in high temperature superconductors

Muon‐spin‐rotation studies on a variety of polycrystalline cuprate high‐T_ c superconductors reveal a generic dependence of the superconducting transition temperature T_ c upon condensate density throughout the entire range of hole doping. Upon full oxygenation the CuO chains in YBa₂Cu₃O_7-\delta become metallic and superconducting and the condensate density is dramatically enhanced. The very rapid suppression of the condensate density n_ s upon Zn substitution in YBa₂(Cu_1-xZn_x)₃O_6+\delta is inconsistent with s‐wave pairing and magnetic scattering but points towards a d‐wave pairing state with non‐magnetic scattering in the unitarity limit. This revised version was published online in August 2006 with corrections to the Cover Date.     

Commensurate spin density wave in LaFeAsO: a local probe study

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor LaFeAsO1-xFx. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of 57Fe M?ssbauer spectroscopy and muon-spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5)muB at the iron site below T(N)=138 K, well separated from a structural phase transition at T(S)=156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below T(N) are reproduced.     

Even parity, orbital singlet, and spin triplet pairing for superconducting LaFeAsO1-xFx

We examine the spin-triplet superconducting state of even parity mediated by ferromagnetic Hund's coupling between electrons in two almost degenerate orbital bands. This state may be realized in the recently discovered LaFeAsO(1-x)F(x). It is robust against orbital-independent disorder. The splitting of the orbital degeneracy suppresses superconductivity and leads to an anisotropic spectrum in the Bogoliubov quasiparticle. The former predicts a strong pressure dependence of T(c) and the latter predicts Fermi pockets, which may be tested in angle resolved photoemission spectra.     

Coexistence of magnetic fluctuations and superconductivity in the pnictide high temperature superconductor SmFeAsO1-xFx measured by muon spin rotation

Muon spin rotation experiments were performed on the pnictide high temperature superconductor SmFeAsO1-xFx with x=0.18 and 0.3. We observed an unusual enhancement of slow spin fluctuations in the vicinity of the superconducting transition which suggests that the spin fluctuations contribute to the formation of an unconventional superconducting state. An estimate of the in-plane penetration depth lambda ab(0)=190(5) nm was obtained, which confirms that the pnictide superconductors obey an Uemura-style relationship between Tc and lambda ab(0);(-2).     

Unconventional superconductivity with a sign reversal in the order parameter of LaFeAsO1-xFx

We argue that the newly discovered superconductivity in a nearly magnetic, Fe-based layered compound is unconventional and mediated by antiferromagnetic spin fluctuations, though different from the usual superexchange and specific to this compound. This resulting state is an example of extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets. The main role of doping in this scenario is to lower the density of states and suppress the pair-breaking ferromagnetic fluctuations.     

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.     

Unconventional pairing originating from the disconnected Fermi surfaces of superconducting LaFeAsO1-xFx

For a newly discovered iron-based high T_{c} superconductor LaFeAsO1-xFx, we have constructed a minimal model, where inclusion of all five Fe d bands is found to be necessary. The random-phase approximation is applied to the model to investigate the origin of superconductivity. We conclude that the multiple spin-fluctuation modes arising from the nesting across the disconnected Fermi surfaces realize an extended s-wave pairing, while d-wave pairing can also be another candidate.     

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.     

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.     

Antiferromagnetic spin fluctuations above the dome-shaped and full-gap superconducting states of LaFeAsO1-xFx revealed by (75)As-nuclear quadrupole resonance

We report a systematic study by (75)As nuclear-quadrupole resonance in LaFeAsO(1-x)F(x). The antiferromagnetic spin fluctuation found above the magnetic ordering temperature T(N) = 58 K for x = 0.03 persists in the regime 0.04 ≤ x ≤ 0.08, where superconductivity sets in. A dome-shaped x dependence of the superconducting transition temperature T(c) is found, with the highest T(c) = 27 K at x = 0.06, which is realized under significant antiferromagnetic spin fluctuation. With increasing x further, the antiferromagnetic spin fluctuation decreases, and so does T(c). These features resemble closely the cuprates La(2-x)Sr(x)CuO(4). In x = 0.06, the spin-lattice relaxation rate (1/T(1)) below T(c) decreases exponentially down to 0.13T(c), which unambiguously indicates that the energy gaps are fully opened. The temperature variation of 1/T(1) below T(c) is rendered nonexponential for other x by impurity scattering.     

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.     

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.     

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.     

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.     

Effect of equilibrium fluctuations on superfluid density in layered superconductors

Using a phenomenological approach based on the fluctuation-dissipation theorem we calculate suppression of superconducting currents due to phase fluctuations and find that, in contrast to a recent prediction, the effect of thermal fluctuations cannot account for linear temperature dependence of the superfluid density in high-T(c) superconductors at low temperatures. Quantum fluctuations are found to dominate over thermal fluctuations at low temperatures. Near T(c) sizable thermal fluctuations are found to suppress the critical current in the stack direction stronger than in the direction along the layers. The spectral density of voltage fluctuations at small frequencies is nonzero, in contrast to what may be expected from a naive interpretation of the Nyquist formula.     

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.     

Low-temperature spin diffusion in a highly ideal S=1/2 Heisenberg antiferromagnetic chain studied by muon spin relaxation

The organic radical-ion salt DEOCC-TCNQF4 contains linear chains of stacked molecules with significant Heisenberg antiferromagnet interactions along the chain and extremely weak interactions between the chains. Zero-field muSR has confirmed the absence of long-range magnetic order down to 20 mK and field-dependent muSR is found to be consistent with diffusive motion of the spin excitations. The anisotropic spin dynamics and the upper boundary for magnetic ordering temperature both indicate interchain magnetic coupling /J'/<7 mK. As the intrachain coupling J is 110 K, /J'/J/ is significantly less than 10(-4). This system therefore provides one of the most ideal examples of the one-dimensional S=1/2 Heisenberg antiferromagnet yet discovered.     

Hydrogen bonding in sodium alanate: a muon spin rotation study

We have detected the occurrence of hydrogen bonding involving an interstitial positive muon situated between hydrogen atoms of two independent alanate anions in sodium alanate (NaAlH4). Ti doping, which is known to dramatically improve the hydrogen cycling performance of NaAlH4, reduces the kinetic barrier of the transition of the muon from the muon-dialanate state to a mobile interstitial state. This observation strongly suggests that hydrogen bonding is the primary bottleneck for hydrogen release or uptake in sodium alanate, which might be common to other complex hydrides.