Transverse spin correlations in a random anisotropy system: Amorphous ErCo2

We provide a coherent interpretation of early small angle scattering experiments performed by some of us on amorphous ErCo₂ [9]. At low temperature the zero field transverse spin-spin correlation function is found to fit a simple exponential for large length scales (l >l_c), supporting the lower critical dimensionality d_c=4. For shorter length scales (l<l_c) the correlation function is of the Ornstein-Zernike type. These results are physically understood in terms of the breaking of ferromagnetism into Imry and Ma domains.A further physical interpretation leads us to consider the localization of ferromagnetic spin waves within Imry and Ma domains in zero field, and their delocalization by application of an external field.     

Nuclear magnetic relaxation due to slow molecular reorientation and fast internal rotation interference

The relaxation of the two-proton system longitudinal magnetization M_z(t) is considered theoretically, using the Bloch equations with a diffusion term. The condition dd> τ_c ≥ 1, dd> τ ? 1 is fulfilled here, where dd> is the energy of the proton dipole-dipole interaction, τ is the correlation time of molecular diffusion rotation and τ is the correlation time for the proton rotation relative to the molecule. Unlike the ordinary Bloch equations a two-exponential law for the evolution of M_z(t) is obtained for definite values dd>, τ_c and τ. The results of the present work may be useful for the consideration of the system having mobile proton groups in viscous media.     

Pulsed EPR hole-burning experiment on dangling bond centers in a-Si:H aerosol particles formed by thermal decomposition of silane

Si dangling bond centers in aerosol particles of amorphous hydrogenated silicon formed by thermal decomposition of SiH₄ in Ar were studied by pulsed electron paramagnetic resonance. The hole-burning and inversion-recovery experiments demonstrate that large-scale rapid spectral diffusion takes place in the samples with high spin concentration. Correlation times τ_c of the spectral diffusion and spin-lattice relaxation timesT ₁ were obtained in the temperature range between 77 and 290 K. Above 130 K, τ_c andT ₁ are proportional one to the other. The unusual feature of this spectral diffusion is that the shape of the central part of the spectral hole does not change when the delay time increases. The other paramagnetic centers previously investigated showed a remarkable change of the hole shape which was induced by modulation of dipolar interaction due to spin flips. It is suggested that the observed anomaly in the Si dangling bond centers arises due to cooperative spin flips.     

NMR investigations of dynamical processes in orientationally ordered and disordered NaCN,RbCN and CsCN

The nuclear spin lattice relaxation timeT ₁ of the²³Na,⁸⁵Rb,⁸⁷Rb,¹³³Cs,¹⁴N nuclei is measured in NaCN, RbCN and CsCN as a function of temperature below and above the ferroelastic phase transition temperatureT _c. BelowT _c the behaviour ofT ₁ of the alkali nuclei renders possible to determine the flip frequency of the CN molecules and its temperature dependence. AboveT _c from the¹⁴NT ₁ the correlation time τ_c of the rotational motions of the CN molecules and its temperature dependence is determined. An empirical rule is verified demonstrating that atT _c the correlation times take nearly the same values for all cyanides. For the high and low temperature phases one obtains atT _c about τ_c=5·10^?13s and τ_c=5·10^?11s, respectively. The results are discussed with respect to the mechanism of the phase transition.     

The concept of screening length in lifetime and relaxation semiconductors

It is shown that, contrary to normal practice, the most appropriate criterion for distinguishing between lifetime and relaxation semiconductors in the presence of traps is the ratio of the screening length L_s, to the ambipolar diffusion length L_Da,. L_s, is calculated. Its significance is not limited to zero current, even though it reduces to the conventional Debye length L_D when the trap concentration is zero. (With traps, we always have L_s < L_D.) The dielectric relaxation time itself is unaffected by traps, but in steady state situations, a material behaves as if it had an effective lifetime τ_0s = τ₀η, where η depends on the concentration and energetic position of the traps. τ_o, may be orders of magnitude greater than τ₀, the conventional diffusion length lifetime. Typical values of L_s, are presented as a function of trapping parameters. L_s>L_Da leads to relaxation behavior; L_s < L_Da, to lifetime behavior.     

Flow equations for the ionic Hubbard model

Taking the site-diagonal terms of the ionic Hubbard model (IHM) in one and two spatial dimensions, as H₀, we employ Continuous Unitary Transformations (CUT) to obtain a “classical” effective Hamiltonian in which hopping term has been renormalized to zero. For this Hamiltonian spin gap and charge gap are calculated at half-filling and subject to periodic boundary conditions. Our calculations indicate two transition points. In fixed Δ, as U increases from zero, there is a region in which both spin gap and charge gap are positive and identical; characteristic of band insulators. Upon further increasing U, first transition occurs at U=Uc₁, where spin and charge gaps both vanish and remain zero up to U=Uc₂. A gap-less state in charge and spin sectors characterizes a metal. For U>Uc₂ spin gap remains zero and charge gap becomes positive. This third region corresponds to a Mott insulator in which charge excitations are gaped, while spin excitations remain gap-less.     

Carrier relaxation time from infrared reflection spectra in semiconductors

When the carrier relaxation time in a semiconductor is estimated independently from reflectivity curves (τ₀) and from Hall mobility data (τ_H) it is frequently found that τ₀ and τ_H disagree rather markedly. An explanation is often suggested in terms of statistical effects. Since this requires a sufficiently non-degenerate distribution and a strongly dispersive scattering mechanism, we explore another alternative invoking the role of a space charge boundary layer. Two examples are considered and the analysis yields quite reasonable results.     

Magnetic Excitations in the antiferromagnet CoCl2

Magnetic neutron inelastic scattering has been used to investigate the non degenerate spim waves propagating in the [00.1] direction in CoCl₂. The neutron results are explicable in terms of a model containing isotropic exchange as proposed by Lines [12, 13] with the addition of a small “in plane” anisotropy. At zero wave vector the anisotropy produces an energy gap in the lower magnon branch which is observed by AFMR to be 70 GHz. Since the maximum spin wave frequency at the zone centre and boundary is 0.56 THz and 0.42 THz respectively the calculations were made within the ground doublet of the ⁴T_1g term. The derived exchange parameters are consistent with those obtained from susceptibility and light scattering. Finally there was no direct evidence to suggest the presence of a strong magnon-phonon hybridization in the [00.1] direction of CoCl₂.     

Dynamical theory of polymer melt morphology

A general kinetic equation of the monomer density variables for polymer blends and block copolymer melts is obtained which describes slow morphology variations. The general theory is applied to a polymer blend adopting the biased reptation model of a polymer chain under mean field. We obtain an equation of motion of interfaces in a phase-separated polymer blend, which contains an interface reaction term for length scales shorter than l_c ≡ R²_G/ξ, where R_G is the gyration radius of a polymer chain and ξ the interfacial width. We also discuss some problems associated with the incompressibility requirement for phase separation kinetics of binary systems not limited to polymers. For length scales greater than l_c the interface dynamics involves diffusion in bulk pure phases even in the strong segregation limit in a way different from that for the usual time-dependent Ginzburg-Landau equation for the conserved order parameter. Implications of the existence of the new term on the late stage phase separation kinetics of polymer blend are discussed.A phenomenological model to study morphology dynamics not relying on the reptation model is also proposed.     

Dynamic phenomena in layer superconducting cuprate and organic metals

Dynamic electron spin resonance (ESR) and extended x-ray absorption edge fine structure (XAFS) measurements suggest that layered organic metals and cuprate superconductors behave similarly. The response to microwave radiation in a modulated external magnetic field indicates that: (i) triplet state, T * ESR is observed below T_c for both; (ii) the condensation of free spin doublet D to T* occurs above the transition temperature to superconductivity T_c (10 ± 1 K for the organic metal (BEDT-TTF)₃Ta₂F₁₁ and 92 to 12 K for YBa₂Cu₃O_7-δ and its rare earth derivatives); (iii) antiferomagnetic (AF) resonance is detected above T_c for the organic metal. Here the exchange field between the aligned AF domains: J_AF(150 K) = 130.7 mT (153 mK) is greater than the exchange term J(150 K) ≈ 15 mT (20 mK) between free spins (S = 1/2) leading to T* states; the lifetime of AF domains τ_AF decreases below 150 K and resonance is not detected below 44 K (i.e. τ_AF < 10^-10 s) allowing a superconducting transition to appear below 10 K; (iv) the relaxation time τ₁ for the half field, triplet state ESR absorption increases fourfold near 10 K for the organic metal and, (v) the onset of superconductivity is detected in all superconductors by the appearance of an energy loss at exactly H=0 and, magnetization oscillations observed versus H below T_c when the samples are cooled in a non-zero field H. The spin-lattice relaxation time for the organic metal triplet state, half field ESR near 10 K is interpreted using the Gorter phenomenological relation τ₁ = C_H /α_H, C_H and α_H are respectively the heat capacity and the thermal contact coefficient to the lattice by the spin system, at constant field H . Complementary changes in x-ray edge widths near T_c are correlated to electron-phonon interactions.     

漫散射光自相关用于生物介质成象的实验研究

本文用背散射光时间自相关函数研究了随机介质中隐一生物组织时的成象,由相关时间τ_c确定出隐含物体的位置.还研究了生物组织在随机介质内不同深度时自相关函数的性质.     

On the dynamic conductivity for an electron-impurity system

The correlation function formula for the dynamic conductivity of a system of non-interacting electrons in the field of impurities is analyzed in terms of proper connected diagrams. By selecting those diagrams appropriate in the region of weak coupling and low impurity concentration, a set of coupled equations for the energy broadening γ (ω, ε, n_s) and the energy shift Δ(ω, ε, n_s) is derived, where both γ and Δ depend on the frequency ω of a probing field, the energy ε of the electron, and the concentration, n_g, of impurities. With the assumption of a finite range potential, these equations are solved. It is found that γ (ω, n_s) is smaller than that extrapolated value which the conventional expression γ₀ for the low-concentration collision frequency would predict, in the entire region studied, that the difference γ₀-γ becomes appreciable when the ratio of the average time between scatterings, τ_c, to the average duration of a scattering, τ_d, is 100 or less, that γ (ω, n_s) decreases monotonically from its static value γ (0, n_s), and becomes vanishingly small in the region ω≈1/τ_d, and that in the static limit (ω=0), γ=γ₀[1?(2/π) (γ₀τ_d)+…], that the energy shift Δ is positive, and increases from 0 and reach a peak of magnitude γ₀ as ω is raised from 0. By using the γ and Δ obtained, the dynamic conductivity σ(ω, n_s) for degenerated electrons is calculated. The deviation, σ-σ₀, from the conventional expression σ₀=(?i) (n_ee²/M) [ω-iΓ₀]^?1, (n_e]=number density of electrons), for 0°K, is appreciable when the ratio τ_c/τ_d is 100 or less. The field-term correction, which arises from the modification of the scattering due to the probing field, is found to be negligible in the entire region studied.     

Relaxationsphänomene in Mössbauerspektren von Magnetisierten paramagnetischen Substanzen. I [(Fe x,Al1-x )NH4(SO4)2· 12H2O]

The Fe³⁺-spin in alums of type (Fe _x , A1_1-x )NH₄(SO₄)₂ · 12 H₂O interacts (i) with the crystal lattice viaLS-coupling, and (ii) with the spins of the adjacent Fe³⁺-ions via magnetic dipole-dipole interaction. These interactions lead to a time fluctuation of the spin direction, characterized by correlation times τ _c and τ′ _c of increasing order. The times may be deduced from the⁵⁷Fe-Mössbauer spectra of the alums, τ _c from the width, and τ′ _c ≈τ _c from the position of the hyperfine structure lines. The theoretical interpretation of the Mössbauer spectra is relatively simple, when (i) the spin-lattice interaction gets frozen in, and (ii) a strong applied magnetic fieldH _a decouples the spins of the Fe³⁺-ion and the⁵⁷Fe-nucleus. The spectra were taken, therefore, at 4.2 °K and 8 kOe≦H _a ≦ 54 kOe. According to the 1/r ³-dependence of the magnetic dipole-dipole interaction τ _c should be related tox, the Fe-concentration, τ _c ·x≈τ₀=const. Forx≧0.5 our experimental results are in agreement with this rule when τ₀=(1.5±0.5) · 10^?9 s. For an alum withx=0.26, however, the observed spectra cannot be explained in terms of temporal spin fluctuations, at least not in the framework of the models which are available now. Here, presumedly, the electron spins of adjacent Fe³⁺-ions are coupled to more or less isolated and, consequently, relatively stationary spin clusters of various sizes, leading to many time independent internal magnetic fields. A treatment of this proposal is in preparation.     

Calculation of the correlation time for motional narrowing of the 181Ta Mössbauer line due to hydrogen diffusion

The correlation time for motional narrowing of a Mössbauer resonance line (¹⁸¹Ta) by hydrogen diffusion has been calculated by applying the random walk theory. The correlation times τ_c are 12.546 541τ and 3.756 711τ for tetrahedral and octahedral occupancies, respectively, where τ is the mean residence time of a hydrogen atom.     

Dielectric dispersion in d-ADP

Complex electric permittivity in paraelectric phase close to T_c of d-ADP over the frequency range 0.8–38 GHz has been measured. Molecular relaxation time τ₀ at 239 K equals to 2.2 or 4.0 ps depending on the internal field model and is very close to that found for d-KDP.     

The measurement and verification of parameters of pulse electromagnetic radiation generated by a large-radius ring current

Experimental and analytic studies of the generation and propagation of electromagnetic radiation due to repetitive current pulses of a nanosecond duration (peak power to 1 MW, current slew rate of 3.5 A/ns) are presented. The radiation source was a fine-wire ring antenna of large radius (ρ_a=1.4 m). The antenna was driven along its full length instantaneously within the time τ shorter than the time of wave travel along the ring diameter (τ≤2ρ_a/c). Parameters of the emitted wave were measured. The experimental data are consistent with the calculated emitted-wave parameters that take into account radiation reflection from the conducting walls of the laboratory. The efficiency of transformation of drive pulse energy into ultra-wideband radiation was found to be approximately 15%. A ring antenna driven by repetitive current pulses (within the time τ≤2ρ_a/c) is suggested to be used as a reference ultra-wideband emitter.     

Chaotic boundary of a Hamiltonial map

The half-plane maps are a class of complex Hamiltonial maps whose invariant curves at most fixed irrational frequencies can be obtained as a convergent Taylor series expansion. For these maps the boundary between regular ordered motion on invariant curves and irregular chaotic motion is given by the radius of convergence of the series. The successive terms of the series oscillate wildly, due to the presence of small divisors. Methods are presented for taming the series, based on the conversion of the convergenceexponentC = -lnα_c into the integral of a continuous but nondifferentiable lambda function, whose graph whows a similarity structure on small scales. Self-similarity properties are illustrated for the chaotic boundary function α_c, where v is the frequency.     

Neutron inelastic scattering study of transverse spin fluctuations in CsNiF3: A soliton-only central peak

We have observed a quasi-elastic contribution to the spectrum of the transverse spin fluctuations S^perp;(Q, ω), perpendicular to an applied magnetic field in the easy plane of the one-dimensional ferromagnet CsNiF₃. According to the present theoretical understanding this contribution is due solely to soliton quasi-particles and it should not contain two-magnon scattering. The observed dependence on momentum transfer is as expected for soliton scattering with zero intensity at q_c = 0 rising through a maximum with increasing q_c.     

Metal insulator transition due to surface roughness scattering in a quantum well

We calculate the critical width a_c for a quantum well structure as function of the two-dimensional electron density. For a > a_c the electron gas has a finite dc conductivity at temperature zero, and for a < a_c the dc conductivity is zero. Homogeneous background doping, remote doping, and surface roughness scattering are considered. Due to surface roughness we find a strong increase of a_c at high electron concentration, in novel contrast to heterostructures. Explicit results are presented for GaAs and InAs quantum wells and compared with experimental results. Experiments are suggested to test the predictions of the localization theory.