Nuclear spin relaxation in disordered conductors

The relaxation timesT ₁ andT ₂ for nuclear spins interacting with the conduction electrons of disordered metals are calculated. An explicite expression for Warren's relaxation enhancement in terms of electron spectra is obtained, showing in particular that the proportionality ofT ₁ and the conductivity is a universal feature of high resistivity conductors. A formula for an inhomogeneous line width contribution due to disorder induced static Knight shift fluctuations is found. Two dimensional electron spin diffusion is shown to imply various logarithmic line width corrections.     

The change from paramagnetic resonance to ferromagnetic resonance for iron nanoparticles made by the sol-gel method

Sol-gel glass embedded with iron nanoparticles provides fascinating features inheriting paramagnetic, ferromagnetic, and superparamagnetic resonance properties under various compositional weight ratios and annealing conditions. Two spectra arising from paramagnetic Fe³⁺ ions and ferrimagnetic Fe₂O₃ particles both centered at g_e=2.0 compete with intensities as the annealing temperature T_A increases. Meanwhile, the resonance line width of Fe₂O₃ particles sharply decreases due to the strong exchange narrowing. The asymmetric line shape and the rather broad line width can be elucidated by the ferromagnetic resonance of the single domain Fe₂O₃ nanoparticles.     

AN EXACT SOLUTION OF A MAS FID IN TWO-SITE EXCHANGE PROBLEM

已知两位置交换分子运动速率对于静态     

The effect of GaAs(001) substrate roughness on the magnetic properties of epitaxial Fe films

The effect of GaAs(001) surface roughness on the magnetic properties of MBE-grown Fe films having a thickness t in the interval from 12 to 140 Å is investigated by the ferromagnetic resonance method. The films were deposited at room temperature with rates of 9 and 3 Å/min. For films grown on substrates with the rms deviation of the roughness σ≈10 and 30 Å, the spectrum is essentially dependent on the relationship between t and σ. At t≤σ and t≥3σ, a single absorption line is observed, whereas at σ≤t≤3σ, two absorption lines are present. These features of the spectra are related to the island growth of the films and the influence of roughness on island coalescence.     

Raman scattering in TlInS2xSe2(1−x) layered mixed crystals (0.25≤x≤1): Compositional dependence of the mode frequencies and line widths

The Raman spectra of TlInS_2xSe_2(1−x) layered mixed crystals were studied for a wide composition range (0.25≤x≤1) in the frequency region 10-360 cm⁻¹ at room temperature. The shift of Raman-active phonon frequencies versus mixed crystals composition x were established. The effect of crystal disorder on the line width broadening of three high-frequency Raman-active modes is reported.     

Electron spin resonance study of the dynamic magnetic susceptibility of CuO, Cu1−x Zn x O, and Cu1−x Li x O single crystals

Results are presented of studies of the dynamic magnetic susceptibility of CuO, Cu_1?x Zn _x O (x ≈ 1.5%), and Cu_1?x Li _x O (x ≈ 1%) single crystals. The orientational dependence of the ESR spectra was investigated at room temperature. The results for CuO are analyzed using a model of a quasi-one-dimensional antiferromagnet (S = 1/2) with anisotropic exchange interaction between Cu²⁺ spins in the chains and exchange coupling between the chains allowing for one-dimensional spin diffusion and spinon excitations. The estimated line width is of the same order of magnitude as the experimental data. Substituting Cu with Zn scarcely alters the spin dynamics of the Cu²⁺ ions, as in weakly diluted magnets. Lithium doping substantially increases the ESR line width and this is attributed to excess holes forming rapidly relaxing spin complexes with copper ions.     

Dielectric loss behaviour of cadmium iodide polytypic crystals

The loss spectra of several ordered/disordered parent and polytypic crystals of CdI₂ have been studied. The region of interest is seen to lie in the frequency range 0.1–3 kHz where several peaks are found to occur. While, in general, three prominent peaks occur in the region ～ 1 to ～ 3 kHz, disordered structures exhibit additional larger peaks below 1 kHz. The peaks occuring in the 1–3 kHz region arise due to inherent point defects present in the crystals and those occuring below 1 kHz and singular to the disordered structures are attributed to a resonance absorption effect embodying interfacial polarization which arises due to the occurence of dislocation dipoles which are responsible for the creation of the disorder.     

Dynamic critical phenomena in two-dimensional fully frustrated Coulomb gas model with disorder

The dynamic critical phenomena near depinning transition in two-dimensional fully frustrated square lattice Coulomb gas model with disorders was studied using Monte Carlo technique. The ground state of the model system with disorder σ=0.3 is a disordered state. The dependence of charge current density J on electric field E was investigated at low temperatures. The nonlinear J-E behavior near critical depinning field can be described by a scaling function proposed for three-dimensional flux line system [M.B. Luo, X. Hu, Phys. Rev. Lett. 98 (2007) 267002]. We evaluated critical exponents and found an Arrhenius creep motion for field region Ec/2<E<Ec. The scaling law of the depinning transition is also obtained from the scaling function.     

The decay of the vacuum in the field of superheavy nuclear systems

In the strong Coulomb field of a nucleus or quasimolecule withZ?172 a change of the QED vacuum has been predicted, signalled by the spontaneous emission of positrons if holes in theK-shell are available. The dynamical semiclassical theory of positron excitation in heavy ion scattering is presented and extended to collisions with nuclear contact. Interference patterns in the energy spectrum of the emitted positrons and, for sufficiently long reaction timeT, the emerging of a characteristic line at the position of the 1sσ-resonance is predicted. The position and width of the 1sσ-resonance is calculated in dependence on the nuclear configuration, the effect of electron screening is taken into account. Recent experimental indications [1, 2] for structures in the spectra of positrons emitted in U-U collisions at energies close to the Coulomb barrier are discussed in terms of spontaneous positron creation. The observed spectra could be explained under the assumption of long lasting nuclear reactions (T?4·10^?20 s) with a cross section in the millibarn region. Various consequences of this interpretation are discussed.     

Explanation for the effect of natural strong narrowing of Mössbauer lines on long-lived isomers. Variety of nuclei and media with this effect

The effect of natural strong narrowing (NSN) of Mössbauer lines on long-lived isomers has been explained. This effect was doubted for more than thirty years because its mechanism was unknown. The mechanism of NSN has been revealed. Its threshold condition has been established. The theory of inhomogeneous broad-ening of a line in the case of NSN has been developed. Broadenings owing to magnetic, quadrupole, and 2 ^L -pole hyperfine interactions are suppressed almost to zero at NSN. The mechanism of NSN provides a width about the natural width. The linewidth at NSN is independent of the magnitude and direction of the magnetic field if it is lower than ～100 G. The variety of nuclei and media with NSN or with the collapse of the hyperfine interaction (responsible for NSN) is outlined.     

UV absorption of RbAg4I5-RE (Sm, Yb) thin-film systems

UV spectra of samples prepared by vacuum deposition of Sm and Yb thin films on 100–200-nm thick films of the RbAg₄I₅ solid electrolyte (SE) at 300–350 K contain strong absorption bands peaking at about 4.3 and 5.0 eV. After deposition of ～5 nm of Sm, the ionic conductivity σ of the samples decreases from σ ₀ to ≈0.9 σ ₀, and the SE lattice parameter, from 11.24 to ≈11.15 Å, with the x-ray reflection halfwidth increasing from 0.5 to 0.8°. Further growth of Sm concentration in the samples changes the x-ray diffraction pattern, the absorption at 4.3 and 5.0 eV increases, a new absorption edge forms at 3.8 eV, and σ decreases down to ～10^?2 σ ₀. It is conjectured that the strong UV absorption bands in heavily defected silver halides of the RbAg₄I₅-Sm(Yb) system is genetically related to the 4d ¹⁰→4d ⁹5s electronic transitions in free Ag⁺ ions.     

准一维多链无序体系跳跃电导特性

在单电子紧束缚近似下,建立了准一维多链无序体系直流、交流电子跳跃输运模型,通过计算探讨了无序模式、维度效应、温度及外场对其直流、交流电导率的影响.计算结果表明:准一维多链无序体系的直流、交流电导率随着格点能量无序度的增大而减小,非对角无序具有增强体系电子输运能力的作用.随着链数的增加,体系的直流、交流电导率增大,但格点能量无序度较小时,维度效应的影响不明显.在对角无序情况下准一维多链无序体系的交流电导率随温度的升高而增大,而在非对角无序模式下却随温度的升高而减小,但对于直流情况,体系的直流电导率随温度的升     

The primary investigation of optical-limiting effects of optical active phthalocyanine derivative TMBO-SiPc

The optical-limiting property of an optically active phthalocyanine derivative with different configuration in chloroform solution was measured at 532 nm with 10 ns pulses. The excited state absorption cross-section σ_ex of the derivative was determined with the Z-scan technique. The results showed that the phthalocyanine derivative possesses larger σ_ex than the ground state absorption cross-section σ₀, indicating that it is a material for reverse saturable absorption. And it is specially noticed that the change in the optically active configuration affects its optical-limiting property significantly.     

Electronic spectra and structure of the hydrogen halides: Characterization of the electronic structure of DI lying between 67 800 and 74 400 cm−1 above X1Σ+

High-resolution absorption spectra of DI were reinvestigated in the 1475-Å (67 800 cm^?1) to 1344-Å (74 400 cm^?1) region. This spectral region was found to contain bands attributable to (1) V¹Σ⁺-X¹Σ⁺(v′-0) with v′ > 0, (2) (v′-0) transitions with v′ ≥ 0 from X¹Σ⁺ to states associated with the (σ²π³)cπ and (σ²π³)cσ configurations, and (3) (v′-0) transitions with v′ ≥ 0 from X¹Σ⁺ to states associated with the previously unreported configurations (σ²π³)dσ, dπ, aδ, eσ, and fσ.     

Electrical conductivity and NMR studies in paraelastic phase of Tl2SeO4

We have carried out the electrical conductivity and NMR measurements and investigated the characteristic features of the electrical conductivity in a paraelastic phase of Tl₂SeO₄. It was found from electrical conductivity measurement that the activation energy in the paraelastic phase (above T_c(= 661 K)) is 0.98 eV. We also found that ²⁰⁵Tl-NMR line width drastically decreases above T_c and becomes approximately 0.5 Gauss. This result indicates that in the paraelastic phase the mobile Tl ions exist. Moreover this result is consistent with the existence of the anomalously large hopping motion of Tl ions observed in X-ray diffraction measurement. Furthermore the activation energy estimated from the motional narrowing of ²⁰⁵Tl-NMR absorption line is 0.92 eV and is in agreement with that obtained from the conductivity measurement. From these results, it is deduced that the mobile Tl ions play an important role in the appearance of electrical conductivity in the paraelastic phase.     

On the theory of linear absorption line shapes in gases

A detailed theory of the line shape in linear absorption spectroscopy of low-pressure gases is developed. The goal is to take into account all effects that come into play in the determination of Boltzmann's constant from measurements of the Doppler width. We demonstrate that there is no additional broadening from finite transit time across the laser beams. The molecular recoil and the second-order Doppler effect are included in the line shape thanks to a complete quantum treatment. The Mössbauer–Lamb–Dicke narrowing of Doppler lines by collisions is also included and the special cases of Galatry and Nelkin–Ghatak profiles are presented. To cite this article: C.J. Bordé, C. R. Physique 10 (2009).     

Diffusive EPR line width behaviour in two-dimensional Cu(Hippurate)2 4H2O single crystal

X-band EPR measurements were performed at room temperature on layered Cu(Hippurate)₂4H₂O single crystals. Despite the dimeric molecular structure the EPR spectra are characteristic for individual Cu-complexes with square-pyramidal structure and g-factors: g_x=2.045, g_y=2.085 and g_z=2.346. The anticipated zero-field splitting from dimers with S=1 is averaged out by interdimer exchange coupling within the layers. The dimers in adjacent layers are not exchange coupled as we determined from the two-component EPR spectra. Thus, the crystal is an ideal 2D magnetic system and shows a strong spin diffusion effect in the EPR line width. The spin diffusion contribution to the line width is described as P(3cos²Θ−1)² with which is much higher compared to other 2D copper(II) crystals. The background line width is due to dipolar coupling and non-resolved hyperfine structure. Exchange coupling was determined from the exchange narrowing effect as of about 0.1 cm⁻¹.     

Electronic spectra and structure of the hydrogen halides: States associated with the (σ2π3)cσ and (σ2π3)cπ configurations of HI and DI

The high resolution absorption spectra of HI and DI have been investigated in the ～ 1425 Å to 1570 Å region. The majority of the bands observed in the region are assigned to transitions from X¹Σ⁺ to states associated with the excited configurations (σ²π³)cσ and (σ²π³)cπ. This region also contains bands assigned to transitions from X¹Σ⁺ to states associated with more highly excited configurations and to excited vibrational levels of the V¹Σ⁺, b²Π, and C¹Π states. The states associated with the (σ²π³)cσ and cπ configurations exhibit $\text{Ω}$, ω coupling. Effective molecular constants are presented for the bands observed within the above wave-length region.     

Austausch und Spinresonanz in den Gadolinium-Pniktiden GdP,GdAs und GdSb

Powder samples of mixed crystals Gd _x Y_1?x P, Gd _x Y_1?x As and Gd _x Y_1?x Sb have been studied by electron-spin-resonance for Gd-concentrations 0.01≦x≦1 between the ordering temperatures and room temperature. All measurements show a single exchange-narrowed and asymmetric absorption line of Lorentzian shape in the centre region. Theg-factors are 1.995±0.008 in agreement with the expected value for a⁸ S _7/2-state of the half-filled 4f-shell. The linewidths are explained by dipolar and exchange interactions. It is shown that the effect of exchange narrowing is proportional to the sum of the absolute values of the exchange parameters. Therefore a new method for studying the magnetic coupling far above the ordering temperature is proposed. The dependence of exchange on changes of the lattice parameters has been evaluated. The observed asymmetry of the resonance line gives a measure for the electric conductivity. In particular the beginning of critical scattering near the ordering temperature is observed.     

Electronic spectra and structure of the hydrogen halides: Characterization of the electronic structures of HBr and DBr lying between 79 500 and 83 900 cm−1 above X1Σ+

The high-resolution absorption spectra of HBr and DBr were reinvestigated in the 1258-Å (79 500 cm^?1) to 1192-Å (83 900 cm^?1) region. This spectral region was found to contain bands attributable to (1) V¹Σ⁺-X¹Σ⁺(v′ - 0) with v′ > 0, (2) (v′ - 0) transitions with v′ > 0 from X¹Σ⁺(v″ = 0) to states associated with the (σ²π³)cπ and (σ²π³)cσ configurations, and (3) (v′ - 0) transitions with $\text{v′ ≧ 0}$ from X¹Σ⁺(v″ = 0) to states associated with the previously unreported configurations (σ²π³)dσ, dπ, aδ, eσ, and fσ.