Quantum diffusion of muons in bismuth

A muon spin relaxation rate in a super-pure sample of bismuth has been measured. The experimental data and the theory of coherent diffusion in crystalline lattices with shifted levels are compared.     

Giant muon knight shifts in antimony and antimony alloys

The Knight shift K_μ at the positive muon has been measured as a function of magnetic field, temperature, crystal orientation and alloyed impurity (bismuth or tin) in antimony. The anomalously large and anisotropic K_μ in pure Sb at low temperature has been confirmed and shown to be independent of magnetic field up to 9 kG; its anisotropic part is found to have the same strong temperature dependence as its isotropic component. The addition of 6.3 at.% Bi significantly reduces both K_μ and its anisotropy, but enhances their temperature dependence. The addition of 12.5 at.% Bi, or, more dramatically, as little as 0.3 at.% Sn to antimony is sufficient to reduce K_μ to a small value, effectively eliminating the anomalous behaviour.     

Phonon autoecho in bismuth and antimony single crystals

Phonon autoecho is observed upon pumping Bi and Sb semimetals with ultrashort high-energy laser pulses. The autoecho is manifested as a revival of reflection oscillations generated by an A_1g coherent phonon after their complete disappearance. The phenomenon of phonon autoecho offers decisive evidence of the nonclassical character of the state of the crystal lattice that is accomplished in pumping-probing experiments by femtosecond laser pulses.     

Positive conductivity type in bismuth film

The Hall effect and thermoelectric power have been measured over the temperature range 77–300K for two kinds of bismuth films (99.9999% purity), one prepared at 150° C and the other at room temperature on a glass substrate. The former films exhibit an enhancement of positive nature of the conductivity as the film thickness is decreased, indicating the presence of a number of local acceptor states. From the present results it becomes apparent that the transport properties of evaporated films are structure-sensitive.     

Combined breakdown in bismuth—Antimony semiconductor alloys

S-shaped current-voltage characteristics for Bi_1?xSb_x alloys are studied theoretically and experimentally. The phenomenon is shown to occur due to the combined interband breakdown, the impact ionization being caused both by the external electric field and the Hall field. The latter is governed by the proper magnetic field of the plasma current. The negative differential resistance (NDR) occurs for the range of currents where the impact ionization is growing intensively but the pinch effect is not yet developed to full extent. The phenomenon is enhanced if the impact ionization rate in the Hall field is greater than in the applied one.     

Positive muons in single crystal sodium fluoride: A simple spin system

We report on the relaxation of positive muons (μ ⁺) stopped in a single crystal of sodium fluoride at 15±0.2K. At this temperature theμ ⁺ is believed to be static, and the observed time dependence of theμ ⁺ spin polarization is seen to deviate from the familiar Kubo-Toyabe form at late times. Specifically these data exhibit oscillations in the long time tail, which are attributed to hyperfine transitions between theμ ⁺ and neighboring spins. Quantitative comparison of these data to exact quantum mechanical calculations indicates that most of the time dependence can be explained by considering only interactions with the first shell (i.e., two fluorine and two sodium nuclei), and suggests strongly that the muon occupies a site along the <110> axis, midway between two fluorine nuclei.     

纳米结构碲化铋合金的制备及电热输运特性

采用惰性气体保护蒸发-冷凝法制备了纳米Bi及Te粉末, 结合机械合金化和放电等离子烧结技术, 在不同烧结温度下制备出了单一物相且具有纳米层状结构及孪晶亚结构的n型Bi₂Te₃块体材料, 并系统研究了块体材料的晶粒尺度、微结构及其对电热传输特性的影响. SEM, TEM分析结果表明, 以纳米粉末为原料, 通过有效控制工艺条件, 可以制备出具有纳米层状结构Bi₂Te₃合金块体材料, 同时纳米层状结构中存在孪晶亚结构; 热电性能测试结果表明, 具有纳米层状结构及孪晶亚结构的块体试样与粗晶材料相比, 热导率大幅度降低, 在423 K附近, 热导率由粗晶材料的1.80 W/mK降至1.19 W/mK, 晶格热导率从1.16 W/mK降至0.61 W/mK, 表明纳米层状结构与孪晶亚结构共存, 有利于进一步提高声子散射, 降低晶格热导率. 其中在693 K放电等离子烧结后的试样于423K附近取得最大值的无量纲热电优值(ZT), 达到0.74.     

Surface morphology and Raman spectroscopy of thin layers of antimony and bismuth chalcogenides

The phonon spectra in thin layers of bismuth telluride and solid solutions of Bi_2–xSb_xTe_3–ySe_y of different composition, belonging to three-dimensional topological insulators, have been investigated by micro-Raman spectroscopy, and the morphology of an interlayer van der Waals (0001) surface in them has been studied by semicontact atomic force microscopy at room temperature. The analysis of the Raman spectra and the intensity ratio of active and inactive longitudinal optical modes depending on the composition, morphology of the interlayer surface, and thickness of the layers enabled the estimation of the effect of topological surface states of Dirac fermions, associated with the strengthening of the electron–phonon interaction as a result of resonance Raman scattering, and the identification of the compositions, in which the contribution of topological surface states becomes dominant.     

Ionisation of phosphorus, arsenic, antimony, and bismuth by electron impact

Total ionization cross sections of neutral phosphorus, arsenic, antimony, and bismuth atoms by electron impact are reported and compared to the only available experimental results by Freund et al. [Phys. Rev. A 41, 3575 (1990)]. These calculations take into account the possibilities that some target atoms used in the experiments were in metastable states close to the ground state, the excitation-autoionization of nsnp⁴ excited states may be substantial, and the ions produced in experiments may be in excited, low-lying metastable states. The cross sections for direct ionization calculations are based on the BEB model by Kim and Rudd [Phys. Rev. A 50, 3954 (1994)]. Plane-wave Born cross sections scaled by the method developed by Kim [Phys. Rev. A 64, 3954 032713 (2001)] are used to determine the contributions from excitation-autoionization. The combination of the BEB model and the scaled Born cross sections is in agreement with the experimental data by Freund et al. These theoretical data are useful to experimentalists and can be used to complete data tables needed for plasma or astrophysical studies.     

Positive muons in bismuth: determination of their sites,detection of a large local lattice contraction and giant associated electric field gradients,evidence for short and long range diffusion

Using a high purity Bi single crystal the temperature and orientation dependence of the zero and transverse field muon spin relaxation rate has been studied in detail. The results imply that the ⁺ occupies one of the two possible interstitial sites in the distorted rhombohedral crystal structure of Bi below 10 K and the other site above 80 K. At both sites the nearest neighbor Bi atoms are found to be shifted towards the ⁺ by 10% of their nominal distance, implying a large local lattice contraction. In concomitance extremely strong electric field gradients are manifest at the nn Bi nuclei. An almost temperature independent reduced relaxation rate in the temperature range from 20 K to 60 K is interpreted in terms of short range diffusion along a limited chain of alternating types of sites. Above 100 K both long range and short range diffusion are indicated.     

Structure and electrical properties of rapidly quenched foils of antimony—copper and antimony—zinc alloys

It has been determined that foils of antimony and Sb-Cu and Sb-Zn alloys obtained by rapid quenching of melts are characterized by a (1012) + (0001) texture. The nonequilibrium solubilities of copper and zinc in antimony reach 2–3 at. %. Zinc is a donor in antimony while copper acts as a neutral doping element.Translated from Izvestiaya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 72–75, September, 1991.     

Positive muons in spin-crossover complex [Fe(NCS)2(phen)2]

Diamagnetic muon species in a spin-crossover complex diisothiocyanatobisphenanthrolineiron(II) were investigated: two species with different spin relaxation functions and rates were observed over the range from 21 to 293 K. The sum of their yields changed at about the spin transition temperature of the complex, where the spin state of the complex changes from paramagnetic to diamagnetic when it is cooled down across the temperature. The results suggest that paramagnetic muon species could exhibit diamagnetic muon-like SR signals in paramagnetic environment.     

Positive muons as probes in liquid phase free radical chemistry: Radical structures and reaction rates

Muonium-substituted organic free radicals are observed by muon spin rotation when positive muons are stopped in liquid unsaturated compounds. From muon precession frequencies in high external magnetic fields the isotropic muon-electron hyperfine coupling constants are determined. They lead to radical structure assignments. Results of a variety of projects are described which show thatSR can successfully complement conventional physical methods of free radical chemistry: A study of the temperature dependence of the coupling constants of isotopically substituted ethyl radicals yields information on structure and barriers to internal rotation. Rate constants for several radical reactions are extracted from the damping of theSR signals, in particular for unimolecular rearrangements and cis-trans-isomerizations. The theory for the analysis for the case of reversible site exchange is outlined.All results described here were obtained by E. Roduner, P. Burkhard and W. Strub of this laboratory, and B. Webster, M. Ramos and D. McKenna of the University of Glasgow. We all appreciate support from the Swiss National Foundation for Scientific Research, the National Institut for Scientific Investigations of Portugal, the Carnegie Trust for the Universities of Scotland, the Royal Society London and SIN.     

Elastic constants of bismuth polycrystal containing tin,lead, antimony and tellurium impurities

Investigations of ultrasonic velocity and internal friction are carried out in bismuth polycrystals containing Sn, Pb, Sb and Te impurities in the concentration range of 0–1 atomic percent using the composite oscillator technique, and elastic constants are estimated from velocity and density data. The variations of elastic constants are interpreted in terms of changes of lattice parameter, valency of the impurity, the electron-atom ratio, atomic size and Fermi energy. It is observed that the elastic constants are less affected in Bi-Te alloys compared with Bi-Sb, Bi-Pb and Bi-Sn alloys in the above concentration range.