Electron and nuclear magnetic resonances associated with magnetoelectric and antiferroelectric interactions in the exchange-noncollinear (“square”) chiral antiferromagnet Nd<Subscript>2</Subscript>CuO<Subscript>4</Subscript> and their behavior in chiral phase transitions

Experiments with the tetragonal antiferromagnet Nd₂CuO₄ in the temperature range 1.5 K < T < T _N = 245 K show that the magnetic moments of Cu²⁺ possess an exchange-noncollinear magnetic structure of the “square” type, which has the form of an exchange doublet whose components exhibit different chiralities (Γ₄ and Γ₅ phases). Between these phases, consecutive phase transitions Γ₄ ? Γ₅ ? Γ₄ with a change in chirality take place at temperatures T₁ = 30 K and T₂ = 70 K. The electron and nuclear magnetic resonances (natural frequencies and susceptibilities) associated with excitation of magnons (due to the magnetoelectric and antiferroelectric interactions) by an ac electric field E(t), as well as a variable magnetic field H(t) applied in the case of a constant electric field E⁰, are calculated. It is predicted that nuclear magnetic resonance is excited by an ac electric field at frequencies determined by hyperfine fields of the sublattices. The change in the resonance frequencies upon the above chiral phase transitions are analyzed (being first-order phase transitions, these transitions possess a number of features associated with the chirality of the magnetic structures).     

Dynamic phenomena connected with magnetic and antiferroelectric interactions in trirutiles

Dynamic effects caused by the magnetoelectric and antiferroelectric interactions in tetragonal antiferromagnets are studied. The analysis is based on the example of trirutiles that are a series of antiferromagnets with different exchange structures and orientation states. We are mainly dealing with the excitation by an alternating electric field E(t) of spin waves typical of these magnets (antiferroelectric resonance) and the nuclear magnetoelectric resonance connected with these interactions. In the first case, special emphasis is placed on specific magnons (antimagnons), where only the antiferromagnetism vectors L take part in oscillations, whereas the total ferromagnetism vector M remains unchanged. The nuclear magnetoelectric resonance can be generated by oscillations of both L and M caused by field E(t). In this way, the field contributes to the hyperfine field, which acts on the nuclear spins. It is shown that the magnetic and antiferroelectric interactions in the dynamics can manifest themselves both at high (usually, exchange) frequencies ωw_E (antiferroelectric resonance) and at rather low nuclear frequencies of ω_n?ω_E. Particular cases of magnetic structures (phases) are considered where field E(t) can excite not only antimagnons, but also quasiantiferromagnons that have lower eigenfrequencies than those of quasimagnons (relativistic and semirelativistic).     

Diffusion in very chaotic hamiltonian systems

We study nonintegrable hamiltonian dynamics: H(I,θ) = H₀(I) + kH₁(I,θ), for large k, that is, far from integrability. An integral representation is given for the conditional probability P(I,θ, t¦I₀, θ₀, t₀) that the system is at I, θ at t, given it was at I₀, θ₀ at t₀. By discretizing time into steps of size ?, we show how to evaluate physical observables for large k, fixed ?. An explicit calculation of a diffusion coefficient in a two degrees of freedom problem is reported. Passage to ? = 0, the original hamiltonian flow, is discussed.     

Preparation and characterization of slice-like Cu2(OH)3NO3 superhydrophobic structure on copper foil

Superhydrophobic structure was prepared on copper foil via a facile solution-immersion method. Thus slice-like Cu₂(OH)₃NO₃ crystal was prepared on the surface of the copper foil by sequential immersing in an aqueous solution of sodium hydroxide and cupric nitrate. And the superhydrophobic structure was obtained by modifying the slice-like Cu₂(OH)₃NO₃ crystal with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS-17). The morphologies, chemical compositions and states, and hydrophobicity of the surface-modifying films on the copper foil substrates were analyzed by means of scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and water contact angle measurement. Moreover, the thermal stability of the slice-like structure was also evaluated using thermogravimetric analysis (TGA). It was found that roughening of the copper foil surface helped to increase the hydrophobicity to some extent, but no superhydrophobicity was obtained unless the slice-like Cu₂(OH)₃NO₃ crystal formed on the Cu substrate was modified with 1H,1H,2H,2H-perfluorodecyltriethoxysilane. Besides, the superhydrophobicity of the FAS-17-modified slice-like Cu₂(OH)₃NO₃ structure was closely related to the surface morphology. And this hydrophobic structure retained good superhydrophobic stability at elevated temperature and in long-term storage as well, which should be critical to the application of Cu-matrix materials in engineering.     

Change in the resonance frequency of surface plasmons in copper nanoparticles excited by femtosecond laser pulses

Changes in the optical density ΔD(t), halfwidth ΔH/2(t), and spectral position of the maximum Δλ_SP(t) of the surface plasmon band in Cu nanoparticles after their excitation by femtosecond laser pulses have been investigated. The Δλ_SP(t) dependence appears to be alternating and is accompanied by a nonmonotonic variation in ΔH/2(t) in the time interval 0–5 ps. The results are explained in a model based on the evolution of the dielectric response of such a composite medium excited by intense laser pulses.     

Account of the fine structure of hydrogen atom levels in the effective emission cross sections of Balmer lines excited by electron impact in gases and plasma

The possibility of a correct account of the fine structure was shown for two limiting cases observed in beam and plasma experiments. A significant difference was found in the emission cross sections and the rate coefficients of direct and dissociative excitation of the H _α and H _β lines by electron impact in two limiting cases (e.g., this difference reaches an order of magnitude for dissociative excitation of the H _β line). The most reliable data on the cross sections and the rate coefficients of direct and dissociative excitation of the H _α and H _β lines by electron impact were found for both limiting cases. It was shown that, among the first four lines of Balmer series (rather easily detected), only the first two lines, i.e., H _α and H _β, can be currently employed in plasma spectroscopy studies because of the absence of data on the partial excitation cross sections of hydrogen atom nl-sublevels with n≥5.     

Approximately linear relations between two-nucleon and three-nucleon parameters

The correlation between the triton binding energyE _t and then-d scattering length² a and the zero energy singlet and triplet wound integral I_os and I_ot is investigated, with phaseequivalent separableNN interactions obeying all the important off-shell constraints. It is found that the correlation betweenE _t and² a is stronger than betweenE _t (² a) andI _os ,I _ot separately. A strong correlation is also found between the percentageD-state in the deuteronP _d andI _ot . Fixing all the other off-shell parameters but allowing variations ofI _os andI _ot leads to surprisingly accurate linear relations betweenE _t(² a) andI _os ,I _ot (P _d). These relations depend strongly on the other off-shell parameters.     

A stable superconducting state at 8 K and ambient pressure in\alpha _t - (BEDT - TTF^ \star )_2 I_3 ^1

We report bulk superconductivity at 8 K and ambient pressure in crystals of α _t (BEDT-TTF)₂I₃. In contrast to the earlier observed metastable superconducting state at 8 K in crystals of β-(BEDT-TTF)₂I₃ here the superconducting state is stable and the crystals can be prepared by tempering α-(BEDT-TTF)₂I₃ above 70 °C for several days. ac-susceptibility measurements show that the observed superconducting state at 8 K is a bulk property of the crystals. Resistivity measurements indicate a sharp superconducting transition at 8 K with an onset temperature of about 9 K. The upper critical fields H_c2 at 1.3 K lie between 3 and 11 T depending on the direction of the magnetic field with respect to the crystal axes. ESR- as well as NMR-measurements indicate a total transformation of the α-phase crystals into the new superconducting α _t -crystals after tempering.     

Influence of magnetic field on exciton luminescence

The magnetic field dependence of the exciton emission intensity I_ex(H) has been investigated in Ge crystals stressed along the direction near 〈100〉. In the low field limit the magnetic field correction has been evaluated to the wave functions of the ground and some excited states of an isotropic exciton. The calculated dependence I_ex(H) in the case of Ge is in a good agreement with the experimental one at H ? 0.5 T.     

Parametric instability in a two-hole species semiconductor-plasma

The parametric excitation of a low frequency wave has been investigated analytically in a two-hole species semiconductor-plasma in the region of kl ? 1 using the hydrodynamic model of the plasmas in the presence of a high frequency oscillatory electric field (E₀ cos ω₀t applied along the X-axis) and a d.c. magnetic field B₀ normal to the electric field (along the Z-axis), the low frequency wave propagating in the X–Z plane making a very small angle θ with the X-axis. The system supports a purely growing unstable mode. The variation of the growth rate of the unstable mode has been studied over a wide range of system parameters for the specific case of an intrinsic GaAs crystal at 300 K. The oscillatory electric field can be obtained by irradiating the crystal with a 119μm H₂O laser.     

Direct measurement of the nuclear magnetic dipole moment of Ho165 with the atomic beam magnetic resonance method

With an atomic beam magnetic resonance apparatus four rf transitions between different Zeeman levels of the⁴ I _15/2 ground state of Ho¹⁶⁵ have been measured in an external magnetic field of about 3000 Gauss. The interaction between the nuclear magnetic dipole moment and the external field could be deduced from these measurements. Because the magnetic field was measured by calibration transitions in K³⁹, Rb⁸⁵ and Rb⁸⁷, the following value could be determined for the nuclear magnetic dipole moment: μ _I (Ho¹⁶⁵)=4.094(44) μ _n (uncorrected for diamagnetic shielding). Thegj-factor of the ground state of Ho¹⁶⁵ was measured to begj(⁴ I _15/2, Ho¹⁶⁵)=1.1951445(40).     

Polarization effects in theP 3/2→P 1/2 fine structure transition cross section forK(4P)-He collisions

Taking the energy dependence of theP _3/2→P _1/2 fine structure transition cross section ofK(4P) colliding with He as an example, theoretical and experimental studies have been made to illustrate the large possibilities opened by the combination of beam experiments with laser excitation in the investigation of the atomic collisional processes. Explicit calculations, experimentally confirmed, show a strong dependence of the above process on the laser excitation schema for producing the excited stateK(4² P _3/2).     

Influence of atomic alignment on crossover signals in saturation spectroscopy

The crossover resonance between the atomic transitions² S _1/2(F=2)→² P _1/2(F′=1, 2), observed in the saturation spectrum of sodium, was found to be sensitive to the atomic orientation of the ground state, produced by velocity selective optical pumping. For zero magnetic field and the same linear polarization of saturating and probing beam, a negative signal was recorded for the crossover, corresponding to an increase in absorption of the probing beam. Application of a sufficiently strong magnetic field perpendicular to the polarization vector causes the destruction of the alignment, leading to a change in sign for the crossover intensity, i.e., to an enhanced transmission of the probing beam. It was shown experimentally that an increase in the atomic transit time has the same effect. Using optical pumping theory all qualitative features could be accounted for.     

Determination of the242m Am nuclear moments

The hyperfine structure of Am atoms was investigated in an atomic beam by laser spectroscopy. The observed splittings were evaluated with respect to the magnetic dipole and electric quadrupole moments of^242m Am. The results are: μ _I (^242m Am)=+0.97(5)nm,Q(^242m Am)=+6.5(2.0)b.     

Fixed-J level densities beyond spin cut-off approximation

It is well known that the expression for fixed-J level densityI _?(E, J)=?I(E) $\frac{\partial }{{\partial M}}$ I(M|E)| _M=J+1/2 reduces to familiar Bethe’s formula provided the conditionalM-distributionI(M|E) is approximated by a Gaussian form, so called spin cut-off approximation (M is thez-component of total angular momentumJ andE is the excitation energy). After a detailed analysis, we find that the Bethe’s formula which overestimatesI?(E, J) at highJ values, in particular near the yrast line, can be significantly improved by including a few higher-order moment terms in a suitable expansion forI(M|E) with the lowest-order term to be a Gaussian, e.g., Edgeworth expansions. We also find that, except at very low excitation energies, reasonable values (close to exact) of the moments ofI(M|E) can easily be obtained when multiple Laplace-back transform of the partition function for grand canonical ensemble is evaluated within the saddle point approximation. Furthermore, we study the effects of shell structure as well as residual interaction on the excitation energy dependence of these moments.     

On the occurrence of an excitation-effect in pyromellitimide thin film

The effect of a preliminary illumination on the amount of polarization generated in the subsequently dark-polarized Kapton-H samples has been investigated by using the uv and visible radiations as an excitation source. The magnitude of this effect is governed by the two excitation parameters, viz. duration of the illumination (t _ex) and the time interval (Δt) between the end of illumination and the outset of the polarizing electric field. A rise in the operating temperature makes the appearance of the phenomenon more pronounced. The uv radiation has been found more efficient than the visible one for producing the excitation effect.     

The nuclear magnetization as the origin of transient changes in the magnetic field in pulsed NMR experiments

The very measurement of the NMR signal in a pulsed NMR experiment disturbs the nuclear equilibrium magnetization. A small change in the magnetic field results, which can be of the order of −0.0025 ppm in protonated solvents. It is shown that the resonances in pulsed high-field NMR consequently are non-Lorentzian. The FID is properly described by S(t) = S(0)exp(−iω_0tt)exp(−iΔωf{t}) is the small resonance shift which is experienced immediately following the pulse, and f {t} characterizes the subsequent return toward zero by the response of the lock system and by spin-lattice relaxation. In particular in samples containing large concentrations of protons, shifted and distorted resonance lines can be observed.     

Effect of the intergrain interactions on the coercivity and its angular dependence for Nd(FeCo)B sintered magnets

Effects of interactions between grains with different alignment degrees on the coercivity and its angular dependence for Nd₁₆(Fe_0.8Co_0.2)₇₈B₆ sintered magnets have been studied. The experiments show that the intrinsic coercivity _jH_c decreases with enhancing grain alignment (decreasing alignment coefficient σ), the coercivity _jH_c(θ) increases with increasing angle θ between the applied field and the texture axis of the magnets and the variation ratio is larger for the magnets with better grain alignment. The coercivity of the magnets should be determined by the critical field making the moment of individual grains reverse and the interactions between the grains. For the sintered magnets composed of the grains with μm size, the magnetostatic interaction between the grains is stronger than the exchange coupling interaction and it makes the coercivity of magnet increase with increasing alignment coefficient σ. Taking into account the intergrain interactions, the starting field theory of coercivity is in good agreement with the experimental results for Nd₁₆(Fe_0.8Co_0.2)₇₈B₆ sintered magnets.     

Hyperfine structure measurements in the4 I 9/2 ground state of141Pr

Using the atomic beam magnetic resonance method, the five hyperfine structure separations in the 4f ³ 6s ^{2 4}I_9/2 ground state of ₅₉ ¹⁴¹ Pr have been measured. The results are:F F′ E _FF′ ^* /h (MHz) 7 6 6477.913423(17) 6 5 5556.359848 (6) 5 4 4633.023306 (2) 4 3 3708.201146 (5) 3 2 2782.190601(15) From these quantities, the multipole interaction constantsA _k,k=1, 2, 3, 4 between the nucleus and the electron shell have been calculated.A ₄ especially then served to give the following limit for the intrinsic hexadecapole moment: ¦Q ₄₀¦<0.4eb ². Furthermore, theg _J -factors of the⁴ I multiplet have been measured at magnetic fields of 300 Oe. The results are:g _J(⁴ I _9/2)=0.7310371(15)g _J(⁴ I _11/2)=0.9651476(20)g _J(⁴ I _13/2)=1.1063197(40)g _J(⁴ I _15/2)=1.197963 (30) Small corrections due to perturbations by neighbouring fine structure levels are included.