Fine structure of splitting of the separatrix of a nonlinear resonance

This report is a continuation of an analysis, initiated elsewhere V.V. Vecheslavov and B. V. Chirikov, Zh. éksp. Teor. Fiz. 114, 1516 (1998) [JETP 86, 823 (1998)], of the effect of splitting of the separatrix of a nonlinear resonance for the model of standard mapping, based on results of direct measurements of the splitting angle α(K), where K is the system parameter. Measurements were made in the previously used wide range 0.1≳α≳10⁻²⁰⁸ (1⩾K⩾0.0004), but with significantly higher relative (better than 10⁵⁰) and average (∼10⁻⁵⁵) accuracy. This procedure made it possible to substantially refine the effects observed in Ref. 1 and construct qualitatively new empirical dependences providing reliable extrapolation of the data obtained for the angle and the invariant in the intermediate asymptotic limit K≲10⁻² beyond the limits of the investigated region. The results obtained by us can be useful for further development of the theory of separatrix splitting and formation of the stochastic layer of a nonlinear resonance. Zh. éksp. Teor. Fiz. 116, 336–346 (July 1999)     

Multiple-quantum radio-frequency spectroscopy of atoms: Application to the metrology of geomagnetic fields

The evolution of the radio-frequency magnetic resonance spectrum of optically oriented potassium atoms in terrestrial magnetic fields as a function of the amplitude of the alternating magnetic field H ₁ is investigated theoretically and experimentally. It is shown that among the set of observed n-quantum resonances which satisfy the selection rules ΔF=0 and Δm _F±n (n is an integer), there is one resonance with the highest multiplicity n=4 (m _F=2⇔m _F=−2), which has exceptional properties. These qualities make this resonance a candidate for use in a weak-field quantum magnetometer with record-high characteristics. A preliminary investigation of an experimental model of a four-quantum potassium magnetometer is performed. Zh. Tekh. Fiz. 69, 27–30 (September 1999)     

Boltzmann spectral distribution or “infrared catastrophe” in the resonance radiation of a gas

The purely thermal infrared emission spectra of a resonance medium (sodium vapor) are investigated experimentally. It is shown that the emission intensity in the 2–3 μm range at temperatures of 600–1200 K is several orders of magnitude higher than the intensity obtained from the standard theory of resonance radiation transfer. This phenomenon can be conventionally termed an “infrared catastrophe.” The form of the recorded spectra and the absolute intensity of the emission in both the infrared and visible regions of the spectrum are in agreement with the theory developed by Yu. L. Zemtsov and A. M. Starostin, Zh. éksp. Teor. Fiz. 103, 345 (1993) [JETP 76, 186 (1993)], in which the Boltzmann spectral distribution of the population of the resonance level is proportional to exp(−ħω/T). Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 807–811 (10 June 1997)     

Interaction of long-wavelength excitations in crystals

The conditions of resonance interaction of phonons for a scalar model of a crystal in the region of the initial linear section of the dispersion curve of acoustic phonons are investigated using two-particle Green's functions. Fiz. Tverd. Tela (St. Petersburg) 41, 130–133 (January 1999)     

On the detection of finite-frequency current fluctuations

We consider a measurement of finite-frequency current fluctuations, using a resonance circuit as a model for the detector. We arrive at an expression for the measurable response in terms of the current-current correlators which differs from the standard (symmetrized) formula. The possibility of detection of vacuum fluctuations is discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 280–284 (10 February 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Ferromagnetic resonance in multilayer [Fe/Cr]n structures with noncollinear magnetic ordering

The excitation spectrum in an [Fe/Cr]n multilayer structure with non-collinear magnetic ordering was studied by the ferromagnetic resonance (FMR) method in the frequency interval 9.5–37 GHz at room temperature. Besides an acoustic branch, several additional modes were observed under parallel excitation of resonance. The FMR spectrum was calculated analytically in a biquadratic exchange model, neglecting in-plane anisotropy, for an infinite number of layers in the structure and numerically for a finite number of layers contained in real samples. It was shown that the observed modes correspond to excitation of standing spin waves with wave vectors perpendicular to the film plane. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 690–695 (10 May 1998)     

Nonlinear interactions of acoustic modes in ferromagnets near magnetoacoustic resonance: Effective elastic constants

The magnetoelastic contribution ΔĈ ₍₃₎ to the effective third-order elastic constants Ĉ ₍₃₎ ^ef is determined; it describes the additional elastic anharmonicity induced by nonlinear spin-spin and spin-phonon interactions in ferromagnets. In the vicinity of magnetoacoustic resonance, this anharmonicity can be manifested in three-frequency elastic wave interactions, producing magnetoacoustic mode-frequency transformation effects. It is shown that these effects are magnified in resonance as the result of a huge increase (by several orders of magnitude) in the dynamic elastic constants ΔĈ ₍₃₎. Quantitative estimates are obtained for yttrium iron garnet. Fiz. Tverd. Tela (St. Petersburg) 40, 2080–2084 (November 1998)     

Quadrupole effects in the NMR spectra of 27Al in disordered mixed compounds CaREAlO4 (RE=La, Pr, Eu, Y)

The NMR spectra of ²⁷Al are measured in several disordered rare-earth aluminates CaREAlO₄. A change in the shape of the resonance line of the transition 1/2↔−1/2 in the transverse orientation in the rare-earth series RE=La, Pr, Eu, Y is observed, from a narrow (∼16 kHz), structureless line in the La compound to a wide (∼130 kHz), three-component line in the Y compound. This is attributed to a nonuniform distribution of the electric field gradient tensor at the ²⁷Al site. The electric field gradient tensor at the ²⁷Al site in a cluster (AlO₆)⁻⁹ is calculated by a nonempirical method (MO LCAO SCF) in the “deformable oxygen octahedron” model. The results are found to be in satisfactory agreement with the experimental values of 〈V _zz〉 and can be used to obtain the experimental splitting of the resonance line in the Y compounds for reasonable values of the parameters of the local deformations of the AlO₆ octahedron. The nature of the strong axial correlations in the distribution of the Ca and Y atoms in CaYAlO₄ is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 618–623 (April 1997)     

Observation of stochastic resonance in a monostable magnetic system

A stochastic resonance on mixed harmonics is observed experimentally in a monostable magnetic system. The curve of the stochastic resonance at higher-order harmonics in systems with a weak nonlinearity is calculated. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 10, 788–792 (25 May 1997)     

Magnetic resonance in pure and diamagnetically diluted spin-Peierls CuGeO3

Magnetic resonance at frequencies 22–75 GHz is investigated in pure single-crystalline CuGeO₃ in the temperature range 1.2–25 K. At temperatures below 4 K the magnetic resonance line splits into four spectral components. The line close to the ESR of the paramagnetic phase is associated with impurities or defects. The other three lines have the same intensity in different samples and are apparently characteristic for pure single crystals at low temperature. An antiferromagnetic resonance with two resonance absorption branches and a spectrum characteristic for an antiferromagnet with easy, average, and hard anisotropy axes was observed and investigated in the Néel compound Cu_0.98Zn_0.02GeO₃ with diamagnetic dilution of the spin subsystem. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 277–282 (25 August 1996)     

Inhomogeneous ferromagnetic resonance modes in [Fe/Cr]n superlattices with a high biquadratic exchange constant

In a set of [Fe/Cr]_n superlattices, magnetization curves and spectra of ferromagnetic resonance under an in-plane magnetic field have been studied at room temperature. Along with the acoustic branch, several additional branches have been observed in resonance spectra. Resonance spectra have been calculated analytically for a structure with an infinite number of layers and numerically for finite numbers of layers in real samples using a model of biquadratic exchange taking account of the fourth-order magnetic anisotropy. A possibility of describing both static and resonance properties of the system in terms of this model has been demonstrated. Zh. éksp. Teor. Fiz. 116, 1817–1833 (November 1999)     

NMR line shift of gallium in GaAs crystals in the temperature range 160–360 K

The shift in the nuclear magnetic resonance (NMR) line of ⁷¹Ga isotopes in GaAs crystals is investigated in the temperature range 160–360 K. It is observed that the position of the resonance line is temperature-independent. The value of the shift relative to an isolated nucleus is 440 and 420 ppm for the three samples investigated. The roles of various contributions to the NMR line shift are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1418–1419 (August 1998)     

Electron spin resonance and nuclear magnetic resonance of sodium macrostructures in strongly irradiated NaCl-K crystals: Manifestation of quasi-one-dimensional behavior of electrons

Data from an investigation of electron spin resonance and nuclear magnetic resonance of NaCl-K (∼1 mole%) crystals strongly irradiated with electrons imply the observation of a metal-insulator transition with decreasing temperature and the manifestation of quasi-one-dimensional electron motion in sodium macrostructures. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 178–183 (10 February 1998)     

Aharonov-Bohm oscillations in a ring with a quantum well

Aharonov-Bohm oscillations in a ring with a quantum well are investigated in the ballistic regime. It is shown that when trajectories with multiple circuits around the ring are taken into account, the maxima in the conductivity correspond to resonance levels of an isolated ring. The results obtained are in qualitative agreement with the experiment performed by Yakoby, Heiblum, Mahalu, and Shtrikman [Phys. Rev. Lett. 74, 4047 (1995)]: Although the scattering phase of an electron scattered by a quantum well changes by π on passage through each resonance, the Aharonov-Bohm curves for the centers of neighboring resonances are identical. In the simplified interpretation employed by Yakoby et al. the latter result looks like an identical scattering phase in neighboring resonances. Pis’ma Zh. ωksp. Teor. Fiz. 66, No. 9, 605–610 (10 November 1997)     

Experimental study of the energy gap in the spin-wave spectrum in Co/Pd multilayer films

The energy gap appearing in the spin-wave spectrum as a result of Bragg scattering by the modulation period q=2π/(d ₁+d ₂) of a one-dimensional superlattice is observed by the method of spin-wave resonance in Co/Pd multilayer films. It is shown that this gap is asymmetric: The “positive” deviation is from two to three times greater than the “negative” deviation. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 938–941 (25 June 1996)     

Calculation of the characteristics of the radiation from a generator with a virtual cathode

A three-dimensional computer model, described by a system of Maxwell-Vlasov equations, for the interaction of a plasma with the electromagnetic field was used to calculate the excitation of a field by a relativistic electron beam with a virtual cathode oscillating in a resonance chamber. The characteristics of the generator radiation are investigated. Zh. Tekh. Fiz. 69, 87–92 (February 1999)     

Magnetic properties and energy transfer in the luminophors CaS:Eu,Cl

Studies have been performed on the magnetic properties and electron paramagnetic resonance (EPR) spectra of the small-crystal luminophors CaS:Eu,Cl. By comparing the EPR and magnetic susceptibility data we have determined that Eu enters into the CaS matrix primarily in the trivalent state Eu³⁺. We have found that the magnetic susceptibility of the sample with the lowest europium content (5×10⁻³ at. %) has a sharp peak at T≈5 K. A model is proposed of clusters into which the europium ions in these luminophors can associate. On the basis of this model an explanation is given for the anomalous temperature dependence of the magnetic susceptibility as well as a long list of other experimental facts (including peculiarities of the thermal luminescence). Zh. éksp. Teor. Fiz. 113, 1698–1707 (May 1998)     

Analysis of angular distributions in γN → πeta;N

Angular distributions in the final state of π⁰η photoproduction on nucleons are considered. As a formal base the familiar isobar model is used in which the π⁰ηN state is a product of the resonance decay into ηΔ(1232) and πS ₁₁(1535) channels. One of the principal assumptions used is that in the actual energy region the reaction is dominated by a single resonance state. The developed formalism can serve as a tool for testing spin and parity of that resonance.     

Resonant scattering of three-level Rydberg atoms in a microwave field

We examine the theory of potential scattering of Rydberg atoms in a microwave field. The model of a three-level atom is employed to calculate the radiative force emerging in the resonant coherent interaction with the microwave field for the case of a two-photon resonance and high intensities, using the method of quasienergies of the system consisting of the atom and the field. We determine the probabilities of Landau-Zener transitions in the spatial regions where under two-photon resonance conditions the quasienergies of the atoms approach one another by a small quantity. We also study the dynamics of the variation of the spatial profile of a beam of Rydberg atoms caused by resonant scattering. Finally, we give the results of the first experimental observation of the variation of the transverse beam profile when Rydberg atoms pass through a nonuniform microwave field formed in a rectangular waveguide and in resonance with the two-photon 36P–37P transition. Zh. éksp. Teor. Fiz. 111, 796–815 (March 1997)     

Donors in a strong magnetic field and elastic magnetic-impurity resonance in diamondlike semiconductors

The possibility of resonance during elastic intravalley scattering in n-type semiconductors is investigated in connection with the crossing (due to anisotropy of the effective mass) of the energy levels of excited states of a shallow donor as functions of the magnetic field. The hybridization of states of different frequencies in the vicinity of a crossing is attributed to the emergence of a nonzero dipole moment of the excited impurity atom and, accordingly, a long-range potential, which creates carrier-transport anomalies. The lower part of the donor spectrum is calculated as a function of the magnetic field in Si with B∥〈001〉 and in Ge with B∥〈111〉 or B∥〈110〉. A crossing occurs in Ge in the field range 9.9 T<B<16.7 T and in Si in the field range 10.5 T<B<37.7 T. The characteristic longitudinal relaxation time and the transverse conductivity, which are determined by scattering at excited donors in the presence of the hybridization of states, are calculated. Zh. éksp. Teor. Fiz. 112, 975–1010 (September 1997)