Muon diffusion in metals

The hopping rate W of the positive muon in metals is studied theoretecally. We consider four interactions: muon-electron interaction, linear as well as quadratic muon-lattice interactions and inhomogeneous broadening of the muon levels due to imperfections. The W-vs-T curve consists of four regions. The highest-T region, where the small polaron theory is applicable, is followed by a minimum, below which the Kagan-Klinger damping may be dominant. Then, the damping due to metallic electrons dominates and the hopping rate obeys a power law of a negative power. When kT is smaller than the inhomogeneous broadening, the hopping rate obeys another power law of a positive power, as Sugimoto has shown. We analyze experimental data on muon diffusion in Cu, Al and Fe.     

Anisotropy Engineering Edge Magnetism in Zigzag Honeycomb Nanoribbons

It has been demonstrated that the zigzag honeycomb nanoribbons exhibit an intriguing edge magnetism. Here the effect of the anisotropy on the edge magnetism in zigzag honeycomb nanoribbons is investigated using two kinds of large-scale quantum Monte Carlo simulations. The anisotropy in zigzag honeycomb nanoribbons is characterized by the ratios of nearest-neighbor hopping integrals t_1 in one direction and t_2 in another direction. Considering the electron-electron correlation, it is shown that the edge ferromagnetism could be enhanced greatly as t_2/|t_1|increases from 1 to 3, which not only presents an avenue for the control of this magnetism but is also useful for exploring further novel magnetism in new nano-scale materials.     

Study of the response of the cosmic ray muon flux to nonstationary processes in the Earth’s atmosphere

The results of the study of the correlation between dynamic atmospheric phenomena and cosmic ray muon flux variations are presented. The results of the long-term experiment on continuous measurements of spatioangular variations of the muon flux using the URAGAN muon hodoscope are considered. It was shown that the correlation between weather phenomena and changes in the angular distribution of the muon flux on the Earth’s surface are observed in 80% of cases, both in the general counting rate and in the zenith-azimuthal distribution of the muon flux intensity.     

Hopping promoted by electron-electron coupling

The hopping rate involving electron-electron interaction is estimated for an impurity band and is found to exceed that involving electron-phonon coupling     

Hopping between localized Floquet states in periodically driven quantum dots

The dynamic localization in energy space, suppression of the absorption of energy from an external microwave field due to quantum interference, was analyzed recently for a closed quantum dot in the absence of electron-electron interactions. Here a weak interaction is shown to lead to a finite absorption and heating, which may be viewed as hopping between localized Floquet states. The heating rate grows together with the electronic temperature, eventually destroying the localization.     

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.     

Specific features of the electrical conductivity of V<Subscript>4</Subscript>O<Subscript>7</Subscript> single crystals

The electrical conductivity of V₄O₇ single crystals has been measured over a wide temperature range, including both the region of existence of the metallic phase and the region of the metal-insulator transition. It has been shown that the low conductivity of metallic V₄O₇ is due to the strong electron-electron correlation, whose role increases with decreasing temperature as the phase-transition temperature is approached. The temperature dependence of the conductivity of the insulator phase of V₄O₇ is explained in terms of the theory of hopping conduction taking into account the influence of atomic thermal vibrations on the resonance integral.     

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.     

Competition Between the SDW and CDW in Quasi-One-Dimensional Organic Polymer Ferromagnet

The next-nearest-neighbor hopping interactions of ρ-electrons in quasi-onedimensional organic polymer ferromagnet are considered by Peierls-Hubbard model, and a set of self-consistent equations are established to optimize the system. The competition between the SDW and CDW states, which is determined by the interplay between the electron-electron correlation and the next-nearest-neighbor hopping interaction, is studied. At the CDW state,the SDW along the main chain will be tuned by the CDW. Consequently the ferromagnetic state, in which all the spins of the unpaired electrons at side freeradicals are arranged parallelly,will be no longer a stable ground state of the system.     

μ+ Diffusion in copper below 4 K reconfirmed by LF-μ+SR

Muon spin relaxation in longitudinal magnetic field (LF-μ⁺SR) reveals separately the static width of the local dipolar field distribution at the μ⁺ and the μ⁺ hopping rate in the copper lattice at 4.2 and 0.7 K, confirming that muon hopping between octahedral interstitial sites increases at lower temperatures.     

Quantum diffusion of muonium in GaAs

The diffusion rate of muonium in the III–V compound semiconductor GaAs has been determined from measurements of muon spinT ₁ relaxation induced by motion in the presence of nuclear hyperfine interactions. It is shown for the first time in a semiconductor that (a) there is a crossover of the transport mechanism at about 90 K from stochastic to zero-phonon hopping, as evidenced by a steep rise in the hop rate at lower temperatures, and that (b) the muonium diffuses at the hop rate of 10¹⁰ s⁻¹ (corresponding diffusion constantD≈10⁻⁶ cm²s⁻¹) at lower temperatures as well as at room temperature.     

Momentum space distribution of electrons in an atom using hydrogenic wave functions

A formulation is developed to derive exact analytic expressions for electron-electron correlation and density of electrons in momentum space using hydrogenic wave functions. It is shown that for large atoms the expression for density of electrons has a simple form.     

Effects of exchange currents on muon capture

It is shown that the one-pion exchange current renormalizes the axial and induced pseudoscalar form factors of the weak axial current. The effect decreases total muon capture rate about 13% and the branching ratio of radiative-to-ordinary muon capture rate about 12%.     

Neutral (V + A) currents and weak lepton-lepton scattering

Symmetrical neutral (V + A) currents are introduced to describe weak-scattering processes not accompanied by change in the nature of the primary particles; a study is made of the spin correlations in neutrino-electron (muon) and electron-electron (positron, muon) scattering by means of a current-current neutral (V + A) scheme.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 64–68, August, 1970.     

Frictional accumulation of negative muons and its application

A novel method is proposed for the efficient conversion of intermediate energy negative muons into a low-energy muon beam. It is based on using an electric field to eject muons from a moderator consisting of a large number of thin carbon foils placed perpendicularly to the axis of a high-field solenoid. High-energy muons are made to slow down within the moderator to an energy where further slowing down is inhibited by the electric field acceleration between the foils. The muons accumulate at low energy within the moderator hopping from one foil to the next until they come out as a low-energy muon beam. The resulting phase compression factor exceeds 1000. Efficient initial injection of the muons into the moderator is obtained either by letting the muons enter it in a direction opposite to the acceleration force or by producing the muons within a magnetic trap containing the moderator. A practical configuration based on the second scheme is presented. By implementing the method into the most intense muon production configurations a new pathway is opened that may ultimately compete with other schemes in the selection of the optimal source for high-energy muon colliders.     

Role of next-nearest-neighbour hopping in the internal structure of the ground state and finite temperature quantities of 2D t-J model

An exact diagonalization calculation for a small cluster in the two-dimensional t-J model has been studied to calculate two-hole correlation. Calculations reveal dominant hole-hole correlation for holes sitting on next-nearest-neighbour (NNN) sites and critical coupling occurs at J/t = 0.8. With the increase in negative-type NNN hopping, correlation decreases at NNN sites whereas it increases at other sites. The thermodynamic properties such as entropy and specific heat are studied as functions of temperature with various NNN hopping strength. Results show that with the inclusion of negative NNN hopping, the system becomes more ordered. A qualitative transition temperature region has been estimated. It is shown that with the increase in NNN hopping strength, T _c increases. Specific heat results show non-Fermi liquid-type behaviour of the system. All our calculations establish the importance of negative-type NNN hopping.     

The electrical property dependence of disordered copper oxide on oxygen content

We have deposited disordered copper oxide films on glass substrates, with varying oxygen to copper ratios, by the reactive sputtering method. The variation in the ratio is enough to take the system from having disordered metallic like conduction at low oxygen content to hopping like behaviour at higher oxygen content. The hopping like behaviour is described by an unusual band structure, where Mott-variable range hopping is seen at lower temperatures and electron-electron variable range hopping at higher temperatures in some samples. The metallic region shows the expected effects of quantum corrections to the conductivity, with the often seen influence of spin-orbit scattering in copper. The transition between the hopping and metallic state is of the percolation type, where the conduction path between Cu islands is broken as the oxygen content increases.     

Empirical information on quantum diffusion

The applicability of the quantum theory of diffusion to experiments on muon motion in selected metals is examined. Thereby the conventional picture of immediate self-trapping of the muon is employed. Small-polaron hopping of muons at intermediate temperatures seems to be established. There are indications for coherent diffusion in several metals at low temperatures. The quantitative behavior of the diffusion coefficient or transfer rate at low temperatures found in Al and Cu is in disgreement with the theoretical predictions.Many discussions with O. Hartmann, E. Karlsson, L.O. Norlin, T.O. Niinikoski, D. Richter, J.M. Welter, and A. Yaouanc are grateful acknowledged.     

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.     

Track reconstruction in the BESⅢ muon counter

The reconstruction algorithm for BESⅢ Muon Counter, MucRecAlg, is developed with the object-oriented language C++ in BESⅢ offline software environment. MucRecAlg consists of the following functions: to find track seeds either from extrapolation of tracks in the main drift chamber or from the fired strips in muon counter, to select fired strips associated to the candidate tracks, to fit the candidate tracks with a linear or quadratic function and to calculate other parameters of the tracks for muon identification. Monte Carlo samples are generated to check the performance of the reconstruction package, such as reconstruction efficiency, muon remaining rate and pion rejection rate, etc. The preliminary results show that the pion rejection rate is around 3%-4% while the muon remaining rate is better than 90% in 0.4-1.6 GeV/c momentum region, which meets the requirement as shown in the design report.