Behavior of higher harmonics of the granular HTSC response to a low-frequency magnetic field

A theoretical explanation is proposed for the experimental results on the behavior of higher harmonics of the granular HTSC response to a variable magnetic field of frequencies ～10² Hz. The theory explains the periodicity in the dependence of the harmonics amplitudes on a static external magnetic field; the dependence of the period on the harmonics number and on the amplitude of the variable magnetic field; the existence of the threshold in the dependence of the odd-harmonics amplitudes on the amplitude of the variable magnetic field; and the possible formation of pairs for these dependences for the neighboring odd harmonics. It is shown that the experimental dependences can be explained without a detailed analysis of microprocesses in individual Josephson loops only by treating HTSC as a macroscopic medium characterized by the dependence Φ(H) of the magnetic flux on the external magnetic field, which is typical of type II superconductors.     

Self-localization of excitons and nonlinear optical properties of J-aggregates in a periodically modulated thin film

Nonlinear optical properties of dye J-aggregates embedded in a one-dimensional periodic medium are analyzed theoretically and numerically. The effect of strain-induced exciton self-localization in a chain of monomers on nonlinear optical properties of the system is investigated for weakly and highly ordered J-aggregates. It is found that exciton self-localization in a periodic medium enhances light absorption and effective optical susceptibility. Optical bistability thresholds are examined for weakly and strongly excited dye J-aggregate thin films. It is shown that the threshold intensity of incident electromagnetic field is lower by more than an order of magnitude as compared to that for a homogeneous medium.     

Effect of magnetic field on dislocation-induced unelasticity and plasticity of LiF crystals with various impurities

The effect of a magnetic field of 0.04–0.8 T on the amplitude dependence of the internal friction in LiF crystals with various impurities is studied using a two-component resonant-oscillator method at a frequency of 80 kHz. The state of the samples was controlled in situ from the current-voltage characteristics of a compound oscillator. It is found that the state of the dislocations-pinning-centers system changes in the presence of a magnetic field; this leads to an increase in the internal friction and plasticity of the samples. The effect of the magnetic field is sensitive to the impurity composition in the crystal.     

Partial differential cross section of inelastic magnetic neutron scattering in the paramagnetic phase of LaCoO3

The partial differential cross section of inelastic magnetic neutron scattering from the compound LaCoO₃ in the paramagnetic phase is studied theoretically. The contribution to scattering from the high-spin state of an ion in zero magnetic field and the modification of this contribution upon application of a magnetic field are calculated using the effective Hamiltonian for the ⁵ D term. The amplitude of the peak in the dependence of the scattering cross section on the energy of scattered neutrons, which corresponds to the transition from the low-spin to the intermediate-spin state, is estimated.     

Orientational and fluctuation polarizations of langevin dipoles in a random electric field

The polarization and dielectric susceptibility of polar regions composed of Langevin microdipoles in a random electric field are considered. It is shown that the polarization of this system has orientational and fluctuation components in addition to the conventional phonon contribution. Both components diverge when the amplitude of the random field tends to zero but can be stabilized in finite internal fields. The orientational susceptibility does not depend on the external field if its amplitude is smaller than the amplitude of the random field and rapidly decreases in the opposite case. The field dependence of the fluctuation susceptibility exhibits a maximum. With an increase in the field, the fluctuation susceptibility quadratically increases as const + AE² in weak fields and decreases as E^?1/2 in strong fields. The equation of state takes the form P² ～ E in relatively strong fields (as compared to the internal field) and P ～ E in weak fields.     

Atomic beam preparation for hydrogen maser operation with unpolarized atoms

The conventional atomic beam preparation technique for hydrogen maser operation consists of a six pole magnetic field focussing in combination with adiabatically changing magnetic fields in the transition region between focusser and maser cavity, which results in a state selected, polarized ensemble of hydrogen atoms. Hydrogen maser operation with state selected, but unpolarized atoms has the advantage to reduce considerably the sensitivity of hydrogen maser frequency on coherently excited, low frequency,Δm _F =±1 Zeeman transitions. The influence of the magnetic field pattern in the transition region on the energy level population probabilities for the focussed hydrogen beam has been analysed. Atomic beam preparation techniques using non-adiabatically changing magnetic field configurations in the transition region are described. The dependence of hydrogen maser oscillation on the energy level population within the atomic beam is theoretically investigated and the results are used for the analysis of the experimentally achieved atomic beam populations. It has been shown that a pulsed magnetic spin guidance field in the transition region, which switches between adiabatically changing magnetic field configurations and a sudden magnetic field reversal, results in an ensemble of hydrogen atoms with vanishing time averaged polarization, which does not deteriorate the conventionally achieved maser oscillation amplitude.     

Electromagnetic scattering as a radiation reaction problem

Electromagnetic scattering is treated as a dynamical problem, by equating the external torque exerted by the incident wave on the sphere to the self torque due to the radiated (scattered) wave. ForI _mech=0,our scattering amplitude is equal to the usualP-wave amplitude of the electromagnetic scattering on an infinitely conducting sphere. The poles of the scattering amplitude, in particular their dependence onI _mech, are studied. For example, a pole on the positive imaginary axis, which usually corresponds to a bound state, corresponds to a runaway solution in our case. Non-decaying resonances are also discussed.     

Effect of field and medium asymmetries on enhancement of signal-to-noise ratio

The effect of field and medium asymmetry on improvement of signal-to-noise ratio (SNR) in respect of homodyne detection technique is analysed theoretically. The analysis is applied to the sample of bulk GaAs irradiated by off-resonant nanosecond pulsed laser. The dependence of SNR on electric field amplitude and local oscillator (LO) phase is examined. It is found that SNR is maximum for the combination of asymmetric field and asymmetric medium.      

应用相位调制驻波光场制备囚禁离子的暗压缩态

基于囚禁离子与驻波激光场相互作用,应用驻波光场的相位调制的方法,我们提出了一种制备离子振动压缩态的新方法.研究了压缩振幅与调制振幅的关系.我们发现:适当选择调制振幅,可把离子制备在最佳压缩振动态上,并且它们的产生伴随着自发辐射的相消.提出了一种实验构想,并进行了讨论.     

Magnon condensation into a Q ball in 3He-B

The theoretical prediction of Q balls in relativistic quantum fields is realized here experimentally in superfluid 3He-B. The condensed-matter analogs of relativistic Q balls are responsible for an extremely long-lived signal of magnetic induction observed in NMR at the lowest temperatures. This Q ball is another representative of a state with phase coherent precession of nuclear spins in 3He-B, similar to the well-known homogeneously precessing domain, which we interpret as Bose-Einstein condensation of spin waves--magnons. At large charge Q, the effect of self-localization is observed. In the language of relativistic quantum fields it is caused by interaction between the charged and neutral fields, where the neutral field provides the potential for the charged one. In the process of self-localization the charged field modifies locally the neutral field so that the potential well is formed in which the charge Q is condensed.     

Squeezing and sub-Poissonian effects up-to fourth order in fifth harmonic generation

The concept of squeezing of the electromagnetic field is investigated in fundamental mode in fifth harmonic generation with the approximation |gt|²?1, where g is coupling constant and t, the interaction time between waves during the process. It has been found that squeezing occurs in amplitude, amplitude-squared, amplitude-cubed and fourth-order amplitude states of the field for selective phase values of field amplitude of fundamental mode. The dependence of squeezing on the photon number has also been investigated and found to be sub-Poissonian in nature. The signal to noise ratio has been studied in different order. It is found that the signal to noise ratio is higher in lower order.     

Self-localization of excitons in a periodically modulated molecular medium

Electromagnetic field propagation is analyzed in a one-dimensional Bragg grating consisting of periodically arranged linear molecules making up a resonant medium. Dye J-aggregates and conjugated polymers are considered as examples of the medium. Both adiabatic and nonadiabatic dynamics of the acoustic waves generated by electromagnetic field in the system are examined. The effects of exciton-phonon and exciton-phonon-photon interactions on the band structure and formation of self-localized excitations are examined on various time scales. A new mechanism for controlling bandgap parameters in a bistable regime is described. Some effects of electromagnetic-field nonuniformity on generation of phonons in molecules and exciton self-localization are investigated.     

Dynamics of the intermediate state and domain walls in an external oscillating magnetic field

We study the dynamics of 90-degree domain walls in the intermediate state of antiferromagnets, the state being realized in a first-order spin-flop transition in an external magnetic field. We show that an additional oscillating external magnetic field leads to a drift in the domain walls and find the dependence of the drift velocity on the amplitude, frequency, and polarization of the oscillating field. Finally, we discuss the possibility of the domain structure drifting as a whole. Zh. éksp. Teor. Fiz. 112, 1374–1385 (October 1997)     

Tunneling magnetoresistance of phase-separated manganites

A simple model of phase separation is used to study the magnetoresistance of manganites in the nonmetallic state. It is assumed that the phase separation corresponds to the emergence of small ferromagnetic metallic droplets (ferrons) in a nonconducting antiferromagnetic or paramagnetic medium, with the metallic phase concentration being far from the percolation threshold. The charge transfer is accomplished by way of electron jumps between droplets. The magnetoresistance in such a system is defined both by the variation of the volume of the metal phase and by the dependence of the probabilities of electron transitions on the magnitude of the magnetic field. It is demonstrated that, in the region of low magnetic fields, the magnetoresistance is quadratic with respect to the field and decreases with temperature by the T ^?n law, where n takes values from 1 to 5 depending on the correlation between the parameters. In the high-field limit, the magnetoresistance increases abruptly with the volume of the metal phase. The crossover of the field dependence from quadratic to a stronger one may be accompanied by the emergence of a platean in the magnetoresistance. The correlation between the obtained results and the available experimental data is discussed.     

Ultimately short optical pulses in carbon nanotubes in dispersive nonmagnetic dielectric media

We consider Maxwell’s equations for an electromagnetic field propagating in carbon nanotubes placed in dispersive nonmagnetic dielectric media. An effective equation having the form of an analog of the classical sine-Gordon equation was obtained and analyzed numerically. The dependence of the pulse on the type of carbon nanotubes, initial pulse amplitude, and dispersion constants of the medium was revealed.     

An analytical model for cross-Kerr nonlinearity in a four-level N-type atomic system with Doppler broadening

We present an analytical model for cross-Kerr nonlinear coefficient in a four-level N-type atomic medium under Doppler broadening.The model is applied to87 Rb atoms to analyze the dependence of the cross-Kerr nonlinear coefficient on the external light field and the temperature of atomic vapor.The analysis shows that in the absence of electromagnetically induced transparency(EIT)the cross-Kerr nonlinear coefficient is zero,but it is significantly enhanced when the EIT is established.It means that the cross-Kerr effect can be turned on/off when the external light field is on or off.Simultaneously,the amplitude and the sign of the cross-Kerr nonlinear coefficient are easily changed according to the intensity and frequency of the external light field.The amplitude of the cross-Kerr nonlinear coefficient remarkably decreases when the temperature of atomic medium increases.The analytical model can be convenient to fit experimental observations and applied to photonic devices.     

Investigation of cold rubidium Rydberg atoms in a magneto-optical trap

This paper reports on the results of experiments with cold rubidium Rydberg atoms in a magneto-optical trap. The specific feature of the experiments is the excitation of Rydberg atoms in a small volume within a cloud of cold atoms and the sorting of measured signals and spectra according to the number of detected Rydberg atoms. The effective lifetime of the 37P Rydberg state and its polarizability in a weak electric field are measured. The results obtained are in good agreement with theoretical calculations. It is demonstrated that the localization of the excitation volume in the vicinity of the zero-magnetic-field point makes it possible to improve the spectral resolution and to obtain narrow microwave resonances in Rydberg atoms without switching off the quadrupole magnetic field of the trap. The dependence of the amplitude of dipole-dipole interaction resonances in Rydberg atoms on the number of atoms is measured. This dependence exhibits a linear behavior and agrees with the theory for a weak dipole-dipole interaction.     

Harmonic generation in TlBa2Ca3Cu4Oy superconductor

We have studied harmonic generation in the magnetic response of bulk samples of the ceramic superconductor TlBa₂Ca₃Cu₄O_y. Dependence of the odd and even harmonics of magnetization on the DC magnetic field, amplitude of the AC magnetic field and temperature was investigate experimentally. We have used a critical state model recently developed by Müller et al. to calculate the full harmonic response. At fixed temperature, the dependence of the odd order harmonic response on the amplitude of the AC magnetic field H_m shows an unambiguous crossover from an H²_m behaviour (Bean-like) at small amplitudes to an H³_m behaviour for larger H_m, in quantitative agreement with the modified critical state model.     

Second harmonic wave generation from Joule heating in layered organic conductors

The amplitude of a second harmonic wave (SHW) generated from Joule heating as a heat source in organic conductor β-(BEDT-TTF)₂IBr₂ is analyzed as a function of the magnetic field strength and its orientation with respect to the plane of the layers. Angular oscillations of the SHW amplitude are correlated with the angular changes of in-plane conductivity that arise from the periodic dependence of charge carriers velocity on the field orientation. It was found that the nonlinear effect of wave generation leads to a shift between the position of the peaks of the wave amplitude and in-plane conductivity. This allows an important information on the parameter values of organic conductors as well as wave velocity to be obtained. Magnetic field dependence shows that the wave is not strongly attenuated with increasing field and might give insights on the interactions between the electromagnetic, temperature and acoustic oscillations. We found that these observations are completely different compared to those of linear acoustic wave generation. It has been shown that the necessary conditions for observing the nonlinear acoustic wave generation are fulfilled in a wide range of fields and angles that allow the acoustic properties of organic conductors to be studied in detail.     

Effect of thermal nonlinearity in high-absorption media on the parameters of the photoacoustic signal detected by the gas microphone method: The fundamental and second harmonics

A perturbation theory is put forward that describes the effect of thermal nonlinearity due to the temperature dependence of the thermophysical parameters of high-absorption systems with a low thermal conductivity on the parameters of the photoacoustic signal detected by the gas microphone technique. It is found that the dependence of the photoacoustic signal amplitude on incident beam intensity I ₀ stems from the dependence of the illuminated surface temperature on I ₀. This dependence is a complicated function instead of being a simple quadratic function as was expected. In the limiting cases (μ_sβ ? 1 and μ_sβ ? 1), this contribution to the photoacoustic signal amplitude is described by simple expressions, which are convenient for determining the thermal coefficients of the thermophysical parameters of the medium. It is found that the thermal nonlinearity significantly affects the photoacoustic signal phase in the frequency region meeting the condition μ_sβ ～ 1. In the above limiting cases, its effect is insignificant. A theory of generation of the photoacoustic signal second harmonic is proposed. The second harmonic is related to the temperature dependence of the thermophysical parameters of the buffer gas and sample. It is shown that the amplitude of the signal is a quadratic function of the incident beam intensity and varies with its frequency as ω^?3/2 for μ_sβ ? 1 and ω^?5/2 for μ_sβ ? 1.