Effects of electron correlations,dynamical screening and plasmon excitations on muon diffusion in metals

An investigation is made of the effects of electron-electron interaction on muon diffusion in metals. It is shown that electron-electron correlation plays an important role in the motion of muons. The equation-of-motion method is used to calculate the correlation function. It is shown that electron correlations effectively reduce the muon hopping rate at low temperatures. It is also shown that the effect of dynamic screening increases the hopping rate. We found that due to plasmon excitation, the hopping rate is reduced by a factor which can be as larger as one to two orders of magnitude.     

Spin correlations in REAl2 compounds aboveT c

Transverse fieldSR measurements in the paramagnetic region on polycrystalline samples of CeAl₂, NdAl₂, PrAl₂, GdAl₂, ErAl₂, HoAl₂, DyAl₂ and TmAl₂ are reported. We have observed increased damping rate and frequency shifts of the muon precession signal asT _c is approached from above. The shifts are linear in applied magnetic field, while the damping rates are more complicated functions of magnetic field strength and temperature.We interpret the damping as being partly due to inhomogeneous broadening proportional to the magnetic susceptibility, and partly due to apparent slowing down of RE spin fluctuations. These slow fluctuations are ascribed to the existence of correlations between the RE spins also at temperatures well aboveT _c. The variations of shifts and damping rates within the series of REAl₂ compounds will be discussed.     

A remark on coherent Raman beats in an inhomogeneous broadened system

We have pointed out that coherent Raman beats (CRB) in a three-level system with inhomogeneous broadening of optical transition do not suffer from power broadening and AC Stark shift due to the probe laser field if the sublevel coherence from which the CRB are stimulated is prepared over a sufficiently wide frequency range of the inhomogeneous broadening.     

Diffusion of positive mouns in copper detected by zero-field μSR

Using the Zero-Field μSR method coupled with the unique pulsed muon beam, new systematic measurements of diffusion (hopping) rate of positive muon were performed for the two ultra-pure copper samples (residual resistivity ratio = 18,000 and 7,350) and for the copper doped with 95 ppm iron. For these measurements a new detection system with an improved time resolution was installed to reduce the distortion of μ-e decay time spectrum due to the counting loss of positrons. A preliminary result suggests that the leveling-off of the hopping rate below 0.5 K is not affected by the purity for the ultra-pure sample, while it is strongly modified for the doped copper.     

Frequency power law of material damping

The experimental studies often show that the damping in various solid materials increases with frequency over a finite bandwidth, and the increase is weak if the damping is low. A frequency power law is suggested and discussed in this paper to describe this damping behaviour with special respect to the low loss resilient materials used for sound and vibration control. The dynamic modulus as a function of frequency is determined from the loss modulus through the Kramers-Kronig dispersion relations to satisfy the causality requirement. It is proved that the dynamic modulus obeys the frequency power law of the same type as the loss modulus. In addition, it is proved that the weak frequency dependences of damping properties are inevitably associated with the low loss factor. Examples of fitting the frequency power law to experimental data on some materials of acoustical purposes are presented.     

Quantum diffusion of positive muon in pure tantalum

Quantum diffusion of positive muons has been studied in high purity Ta by zero and longitudinal field muon spin relaxation techniques. We found that the muon hopping rate in Ta is more or less the same as in pure Cu (i.e., \nu\simeq10⁵\sim10⁶\ s^™1), showing a characteristic temperature dependence proportional to T^™\alpha with \alpha ranging from 2 to 0.2 below 20 K. In addition, a step‐like change of the nuclear dipolar width was observed at 60 K associated with the sharp peak of hopping rate, suggesting a change of stable muon sites. This revised version was published online in August 2006 with corrections to the Cover Date.     

Damping and modal structure of patterned magnetic nanoarrays

A number of studies have shown that ferromagnetic resonance linewidths in magnetic thin films contain a significant contribution due to processes, such as inhomogeneous broadening and two-magnon scattering followed by decay to the phonon thermal bath. We present a classical theoretical approach that permits the calculation of such processes for arrays of arbitrarily shaped magnetic nanoparticles and, by comparison with experimental results, demonstrates that the linewidth broadening and damping are dominated by inhomogenities that strongly depend on the equilibrium state of magnetization and can be used to identify the latter.     

Muon spin relaxation in solid3He

Muon spin relaxation in solid³He depends non-monotonously on the temperature. It is shown that this is entirely due to the magnetic dipole-dipole interactions. The observed line narrowing cannot be explained by the mechanism of positive charge hopping described by an Arrhenius-like law. The relaxation rate at low temperatures was found to increase under the influence of an external electric field.     

Dynamic force spectroscopy of conservative and dissipative forces in an Al-Au(111) tip-sample system

The conservative and dissipative interaction between an aluminum tip and a gold (111) surface were investigated using dynamic force spectroscopy in UHV. Complete force vs distance curves and friction coefficient vs distance curves were obtained quantitatively. The force curves were compared to the model by Muller, Yushenko, and Derjaguin, and long and short range interactions were subsequently quantified without fit parameters. A short range conservative interaction was separated from longer range van der Waals forces. The long range behavior of the damping coefficient obeys an inverse power law of third order.     

Effects of cross correlations between exchange and magnetic-anisotropy inhomogeneities on the spectrum and damping of spin waves

Effects of cross correlations between inhomogeneities of the exchange and magnetic-anisotropy parameters on the spectrum and damping of spin waves in a ferromagnet are investigated. One- and three-dimensional inhomogeneities are considered. It is demonstrated that the positive cross correlations lead to an increase in the modification of the dispersion law and the damping of spin waves. The negative cross correlations result in the opposite effects: a decrease in the modification of the dispersion law and a decrease in the damping of spin waves. A comparison of the specific features revealed in this work and the results of targeted experimental investigations of modifications of the dispersion laws and damping in inhomogeneous magnets would make it possible to determine the contribution of the cross correlations to the formation of the stochastically inhomogeneous ground state in amorphous magnetic alloys.     

Third-order equation for two-component spinors

We present the properties of a two-component spinor field that obeys a third-order equation. It is separated into a massive part that corresponds closely to a Dirac field, and a massless part that obeys the Weyl equation. We discuss the interaction of such a field with an external electromagnetic field and the (weak) interactions of two such fields. They can be considered both in terms of relativistic quantum mechanics and quantum field theory. We conclude that this formulation has some attractive features, such as a unified treatment of electrons and muons with their neutrinos, a special role of thePC transformation, a more convergent propagator and a new approach to interactions. It also has some serious difficulties, aside from those generally associated with higher-order equations. These are mainly related to inconsistencies in the simultaneous considerations of electromagnetic and weak interactions. The approach also suggests a further unification of the electron and muon fields into a single bispinor field.     

Third-order equation for bispinors

We present the general features of a bispinor field that obeys a third-order equation. It separates into two massive fields that obey the Dirac equation and a four-component massless field. We discuss briefly its electromagnetic interactions and a leptonic interaction that introduces a mass difference. This field can thus describe the electron, the muon and both neutrinos. The difficulties related to inconsistencies between electromagnetic and weak interactions for the two-component spinors are still present for the bispinor field.     

Linewidth Analysis of Spin Labels in Liquids: I. Theory and Data Analysis

We present a method of simulating the EPR spectra of spin labels in liquids using direct convolution of hyperfine splitting with Lorentzian linewidths. The aim is to simulate the experimental lineshape by considering all spectrometer characteristics as well as inhomogeneous and homogeneous linewidth effects. A major advance in this method is the correction for the broadening produced by Zeeman modulation commonly used to obtain EPR signals; this allows experimenters much more freedom to optimize their experimental conditions for the best signal-to-noise ratio. Microwave power broadening (saturation) effects on the EPR lines are significant even at very low observer levels. Successful simulation requires that all contributions from unresolved hyperfine splittings be explicitly included. Inhomogeneous broadening is dealt with by including all spins that interact with the electron (as a set of superhyperfine interactions); there is no "effective Gaussian" to substitute for the correct superhyperfine interactions. The effects of spin exchange on the linewidth and lineshape can be observed and must be taken into account in order to extract the fundamental linewidths.     

A quantum field theory of viscosity near the liquid-glass transition

The two band model is considered for randomly distributed atoms in the harmonic potential approximation. Taking into account the scattering processes due to random eigenfrequencies and random hopping matrices contributing to the self-energy parts and vertex parts in the correlation functions of the density fluctuations in the interband, we obtain the generalized frequency-dependent viscosity in the viscoelastic theory. The viscosity is proportional to the relaxation time of the atoms, the inverse of which consists of the mean-square deviation of random eigenfrequencies and that of random hopping matrices. The former is almost constant and the latter obeys the Vogel-Fulcher law. In the vicinity of the transition they cross over.     

Static and dynamic double-exchange effects in doped manganites

It is shown that antiferromagnetic ordering in doped manganites with strong double-exchange interaction is transformed into ferromagnetic canted ordering with residual antiferromagnetic behavior in the basal plane as a result of hopping of mobile electron. The canting angle between the core magnetiztions is controlled by the competition of the Heisenberg antiferromagnetic exchange and double exchange. The temperatures of the paramagnet-antiferromagnet and paramagnet-canted ferromagnetic phase transitions are calculated. The results on the dependence of the magnetization in the canted phase and critical temperatures on the doping degree are in qualitative agreement with experiment. The form of uniform oscillations of core magnetiztions in the canted ferromagnetic phase of a doped manganite sample with hopping conduction is analyzed with and without allowance for relaxation of mobile electrons to the lattice. We propose a mechanism for the ferromagnetic resonance broadening and its resonance frequency shift in a ferromagnetic conducting sample (hopping conduction) of doped manganite due to double exchange. The resonance frequency shift and the ferromagnetic resonance damping constant (linewidth) are calculated in this model. In contrast to other relaxation mechanisms, the model is based on the fact that mobile electrons rapidly relax to the lattice (over a time on the order of the precession period).     

Quantum tunneling of light particles in metals

The motion of a particle in a metallic crystal is studied for low temperatures where transitions between adjacent interstitial sites are caused by quantum tunneling. The influence of electrons and phonons on the hopping rate is taken into account by means of a functional integral method. The electronic influence may effectively be described by Ohmic damping which dominates the low temperature behavior of the defect motion. When subsequent tunneling transitions are statistically independent, the diffusion constant is found to obey a power law, D∼T^2K−1, where K depends on the defect-electron interaction. This power law is limited at low temperatures by the effects of phonon excitations. Near the transition between electron and phonon dominated behavior the diffusion constant has a minimum where the precise temperature dependence of the rate depends not only on phonon spectra but also on the processes limiting phonon lifetimes.     

Phononless hopping conduction in two-dimensional layers of quantum dots

Regularities are studied in charge transport due to the hopping conduction of holes along two-dimensional layers of Ge quantum dots in Si. It is shown that the temperature dependence of the conductivity obeys the Efros-Shklovskii law. It is found that the effective localization radius of charge carriers in quantum dots varies nonmonotonically upon filling quantum dots with holes, which is explained by the successive filling of electron shells. The preexponential factor of the hopping conductivity ceases to depend on temperature at low temperatures (T<10 K) and oscillates as the degree of filling quantum dots with holes varies, assuming values divisible by the conductance quantum e²/h. The results obtained indicate that a transition from phonon-assisted hopping conduction to phononless charge transfer occurs as the temperature decreases. The Coulomb interaction of localized charge carriers has a dominant role in these phononless processes.     

Superfluorescence in the presence of inhomogeneousbroadening and relaxation

In this paper we show how inhomogeneous broadening produces dephasing, inhibits cooperative emission and thus reduces the intensity of the SF pulse. We also show how electronic relaxation or time-dependent hyperfine interactions can mollify the effect of inhomogeneous broadening so that SF can be recovered. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spin-wave resonance in magnetic films under conditions of the skin effect

The spin-wave resonance spectrum of a ferromagnetic film magnetized normally to its surface is investigated as a function of the finite depth of penetration of the high-frequency field into the film with due regard for damping in the spin system and different types of surface-spin pinning. The exact numerical solution of the equation of motion for magnetization is obtained by considering the finite thickness of the skin layer. For a substantially inhomogeneous distribution of the high-frequency field over the layer thickness, the change in the resonance shape at frequencies close to the ferromagnetic resonance frequency is observed in addition to the broadening of all the resonance peaks and the decrease in their amplitudes.     

Magnetic dynamics of dilute iron nano-clusters in silver films from Mössbauer spectroscopy and muon spin rotation

A silver film containing nanometer size clusters of iron (nominal conc. 1 at%) has been studied by Mössbauer spectroscopy and Low-Energy Muon Spin Rotation. Below about 20 K spin glass freezing due to interparticle interactions is found from both methods. Whereas Mössbauer spectra are insensitive to the fast fluctuations of cluster moments above spin glass freezing temperature, muon spin rotation in magnetic fields applied perpendicular to the polarized muon spins allows tracing the fluctuations of superparamagnetic moments. The temperature dependence of the damping of the muon spin rotation signal shows Arrhenius behavior between 10 to 100 K. Depending on the assumed shape of damping the activation energy of superparamagnetic fluctuations of cluster moments ranges between about 20 K ·k _B and 40 K ·k _B . Above about 120 K muon spin depolarization indicates diffusion and trapping of muons.