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The magnetization curves, as well as electron spin resonance and nuclear magnetic resonance spectra of 87Rb+ ions, have been measured in the quasi-two-dimensional antiferromagnet RbFe(MoO4)2 on a triangular lattice with the random modulation of the exchange bond network. Random modulation has been performed by means of the partial substitution of nonmagnetic Rb+ ions for K+ ions. It has been shown that random static disorder thus created at an impurity ion concentration of 15% drastically changes the spin structure. A noncollinear structure with significant sublattice magnetization components transverse to the magnetic field occurs in the doped compound. At the same time, the spin structure in the pure compound has the same magnetic moment, but transverse spin components disappear near a magnetic field of one third of the saturation field (three-sublattice structure with two upward sublattices and one downward sublattice). The revealed doping-induced drastic rearrangement of the spin structure of the triangular antiferromagnet has been explained by the interplay between the contribution of dynamic fluctuations, which makes maximally collinear states favorable in free energy, and the contribution from freeze-in disorder, which ensures the energy gain for the maximally noncollinear arrangement of sublattices.

We develop the methods of electromagnetic computer near-field microwave tomography of distributed subsurface inhomogeneities of complex dielectric permittivity and of holography (shape retrieval) of internally homogeneous subsurface objects. The methods are based on the solution of the near-field inverse scattering problem from measurements of the resonance-parameter variations of microwave probes above the medium surface. The capabilities of the proposed diagnostic technique are demonstrated in the numerical simulation for sensors with a cylindrical capacitor as a probe element, the edge capacitance of which is sensitive to subsurface inhomogeneities.

We describe the process of adaptive self-inversion of an image (nonlinear switching) of smallscale opaque object, when the amplitude-modulated laser beam, which illuminates it, is focused in a weakly absorbing medium. It is shown that, despite the nonlocal character of the process, which is due to thermal nonlinearity, the brightness-inverse image is characterized by acceptable quality and a high conversion coefficient. It is shown that the coefficient of conversion of the original image to the inverse one depends on the ratio of the object dimensions and the size of the illuminating beam, and decreases sharply for relatively large objects. The obtained experimental data agree with the numerical calculations. Inversion of the images of several model objects and microdefects in a nonlinear KDP crystal is demonstrated.

A study is made of the spinon continuum fine structure which is observed to occur in the spin-liquid phases of chain- and triangular-lattice magnets at small wave vectors due to the action of a uniform Dzyaloshinsky–Moriya interaction. An ordered phase with a strongly quantum-reduced order parameter is found to exhibit the coexistence of magnon and spin type excitations, the former crossing over to the latter when the excitation energy exceeds that of the exchange interaction.

We analyze the scattering of vortex pairs (the particular case of 2D dark solitons) by a single quantum vortex in a Bose–Einstein condensate with repulsive interaction between atoms. For this purpose, an asymptotic theory describing the dynamics of such 2D soliton-like formations in an arbitrary smoothly nonuniform flow of a ultracold Bose gas is developed. Disregarding the radiation loss associated with acoustic wave emission, we demonstrate that vortex–antivortex pairs can be put in correspondence with quasiparticles, and their behavior can be described by canonical Hamilton equations. For these equations, we determine the integrals of motion that can be used to classify various regimes of scattering of vortex pairs by a single quantum vortex. Theoretical constructions are confirmed by numerical calculations performed directly in terms of the Gross–Pitaevskii equation. We propose a method for estimating the radiation loss in a collision of a soliton-like formation with a phase singularity. It is shown by direct numerical simulation that under certain conditions, the interaction of vortex pairs with a core of a single quantum vortex is accompanied by quite intense acoustic wave emission; as a result, the conditions for applicability of the asymptotic theory developed here are violated. In particular, it is visually demonstrated by a specific example how radiation losses lead to a transformation of a vortex–antivortex pair into a vortex-free 2D dark soliton (i.e., to the annihilation of phase singularities).

We develop and test (in numerical simulation) methods of computer tomography of distributed under-surface inhomogeneities in the conductivity of a medium and for holography (i.e., reconstruction of the shape) of solid and uniformly composed subsurface objects in the context of the problems of multifrequency electromagnetic diagnostics of the Earth’s crust structure in the extremely and ultra low frequency bands. The methods are based on solution of the inverse scattering problem using the results provided by multifrequency measurements of the distribution of complex amplitudes of the electromagnetic field on the surface of the medium under consideration.

We observe a disappearance of the 1/3 magnetization plateau and a striking change of the magnetic configuration under a moderate doping of the model triangular antiferromagnet RbFe(MoO4)2. The reason is an effective lifting of degeneracy of mean-field ground states by a random potential of impurities, which compensates, in the low-temperature limit, the fluctuation contribution to free energy. These results provide a direct experimental confirmation of the fluctuation origin of the ground state in a real frustrated system. The change of the ground state to a least collinear configuration reveals an effective positive biquadratic exchange provided by the structural disorder. On heating, doped samples regain the structure of a pure compound, thus allowing for an investigation of the remarkable competition between thermal and structural disorder.

We develop a method for resonance near-field microwave sounding of inhomogeneous media. Possibilities of the method are demonstrated for an example of dielectric materials which contain spatially localized foreign inclusions. The procedure for solving the inverse problem of reconstruction of the dielectric permittivity, geometric dimensions, and the depth of location of such inhomogeneities has been developed.

The concept of a resonant tunneling photonic nanotriode that allows the mutual all-optical control of cross-cut light flows is developed. The concept is based on the strong resonant interaction between the quasi-localized eigenmode of nonlinear planar dielectric waveguides and incident beams. It is demonstrated that small variations of input guided wave intensities may result in deep modulation of beam reflection and transmission coefficients.

We show that transverse, with respect to the propagation direction, local narrowing of a metal-dielectricmetal plasmonic slot waveguide leads to a two-dimensional surface plasmon nanolocalization and can squeeze the plasmon eigenmode into a spot with a characteristic size of about several tens of nanometers. We demonstrate that the simultaneous waveguide tapering and decreasing transverse narrowing scale make possible an enhancement of the plasmon propagation distance in comparison with the homogeneous waveguide. We also find the fundamental limit of 2D plasmon nanolocalization, which is of the same order as the depth of penetration of the electromagnetic field into a metal which is actually independent of the field frequency in the near infrared domain.

A method of resonant near-field microwave probing is developed for contactless diagnostics of a high-pressure plasma. The efficiency of this method in measuring the parameters of the plasma of an rf capacitive discharge in argon under atmospheric pressure is demonstrated. The experimental results are compared with the data obtained using the independent method, the microwave radiation “cutoff,” and with theoretical estimates.

The nonlinear stage of modulation instability of dark solitons is studied analytically and numerically. We propose an asymptotic description of the dynamics of these solitons in terms of their local velocity and the curvature of the lines at which solitons are concentrated. The features of the destruction of dark solitons (in particular, the formation of vortex structures from them) are analyzed.

The problem of excitation of a homogeneous Bose-Einstein condensate by axially symmetric potential barriers moving with respect to the condensate with both supersonic and subsonic velocities is considered in terms of the Gross-Pitaevskii equation. The specific features of the structure of the vortex shedding past the barriers are analyzed for both regimes of motion.

In many low-dimensional systems with antiferromagnetic interactions, a magnetic order of the Neel type is absent. These systems remain in a quantum-disordered (spin-liquid) state down to zero temperature. The disordered state appears to be stable to weak perturbations when magnetic excitations are separated from the ground state by an energy gap. The stability of the spin-liquid ground state is destroyed upon introduction of impurities or in a sufficiently strong magnetic field. This paper presents a review of the main results of the experimental investigations performed in order to reveal and identify mesoscopic spin clusters formed in the vicinity of impurity ions, to determine the spatial structure of an impurity-induced magnetic order in spin-gap systems, and to examine the low-frequency excitation spectra of field-induced antiferromagnetic phases.

The electron spin resonance spectrum of a quasi-1D $S=1/2$ antiferromagnet ${\mathrm{K}}_{2}{\mathrm{CuSO}}_{4}{\mathrm{Br}}_{2}$ was found to demonstrate an energy gap and a doublet of resonance lines in a wide temperature range between the Curie-Weiss and Ne\`el temperatures. This type of magnetic resonance absorption corresponds well to the two-spinon continuum of excitations in $S=1/2$ antiferromagnetic spin chain with a uniform Dzyaloshinskii-Moriya interaction between the magnetic ions. A resonance mode of paramagnetic defects demonstrating strongly anisotropic behavior due to interaction with spinon excitations in the main matrix is also observed.

Various types of magnetic resonance signals observed in quantum-disordered magnets of the Haldane and dimer types are reported. The collective states of correlated spins near the breaks of the S = 1 spin chains in the Haldane magnet PbNi 2 V 2 O 8 demonstrate magnetic response of an effective spin S = 1 / 2 , while the excited states of long chains reveal the ESR spectrum of an effective spin S = 1 in a crystal field. A magnetic resonance spectrum of the latter type is also observed for the collective excited states in the system of coupled S = 1 2 dimers in crystals of TlCuCl 3 . Further, the interaction of triplet excitations and defects in PbNi 2 V 2 O 8 is shown to result in a hybrid spin resonance mode.

Single-ion anisotropy is of importance for the magnetic ordering of the frustrated pyrochlore antiferromagnets Gd2Ti2O7 and Gd2Sn2O7. The anisotropy parameters for the Gd2Sn2O7 were measured using the electron spin resonance (ESR) technique. The anisotropy was found to be of the easy plane type, with the main constant D=140mK. This value is 35% smaller than the value of the corresponding anisotropy constant in the related compound Gd2Ti2O7.

The static and resonance properties of a quasi-two-dimensional antiferromagnet phase on a distorted triangular lattice of KFe(MoO4)2 have been experimentally studied. Magnetization curves exhibit features corresponding to the spin-flop transition in a collinear biaxial antiferromagnet and simultaneously show a magnetization plateau characteristic of a triangular spin structure. The magnetic resonance spectra also display absorption lines corresponding to the spin structures of both types. The experimental data are described in terms of a model comprising alternating weakly bound magnetic layers, in which the main two exchange integrals have different values. Below the Neel temperature (TN=2.5 K), some of these layers possess a collinear antiferromagnetic structure, while the other layers have a triangular or spiral structure.

The magnetic properties of defects were studied in spin-gap magnets such as spin-Peierls magnet CuGeO3, Haldane magnet PbNi2V2O8, and charge-ordered ladder magnet NaV2O5. Doping of these systems with nonmagnetic impurities leads to additional magnetic degrees of freedom, which manifest themselves at low temperatures, where the intrinsic magnetic susceptibility of a spin-gap system is close to zero. Magnetic susceptibility appears due to the local destruction of the singlet ground state as a result of impurity-induced breakage of spin chains. Antiferromagnetically correlated areas arise near the spin-chain breaks. The sizes of these areas and the effective spin of these specific spin clusters are estimated. The order parameter and its spatially modulated depth are determined for impurity-induced magnetically ordered phases. The magnetic properties of defects for the NaV2O5 ladder structure are explained in the model of electrons “hopping” near the chain break. The hopping degree of freedom effectively influences the total spin of a spin-chain fragment and magnetization of the system.

When the conscription of noncommissioned officers and enlisted personnel for service in the Russian Armed Forces and other forces was converted to volunteer or contract recruitment at the end of 1992, women were given equal rights with men to join the military. This brought many women into the military; the percentage of women in the armed forces rose from 3.5 percent in 1990 to almost 10 percent in 1999. A similar phenomenon can be observed in many countries besides Russia, countries that differ from one another in their economic and political development, customs, culture, and religious traditions. The increasing involvement of women in military service is evidence that this trend is becoming a regular process, typical of the late twentieth century. As military service becomes more intellectual, fewer physical demands are made of service personnel, missions are changed and broadened, and the utilization of human resources is gradually extended.

In the quasi-optical aberration-free approximation, a parabolic equation for the envelope of an electromagnetic wave packet propagating along a geometric optical ray in a smoothly inhomogeneous isotropic medium with time dispersion is obtained. The corresponding Green’s function is found whose parameters are determined by integrating a system of ordinary differential equations. The effects of the combined influence of the refraction, diffraction, and dispersion on the evolution of the packet are analyzed; in particular, the effects that cause precession of the envelope waves about the binormal to the propagation trajectory are considered.

Magnetic properties of the triclinic copper vanadate $\ensuremath{\alpha}\ensuremath{-}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$ with linear chains of edge-sharing ${\mathrm{CuO}}_{6}$ octahedra were studied by magnetic susceptibility and electron-spin-resonance measurements. The broad maximum of a magnetic susceptibility at ${T}_{M}=44\mathrm{K}$ corresponding to the intrachain exchange interaction $J=34\mathrm{K}$ indicates one-dimensional short-range magnetic ordering within the chains. Three-dimensional long-range antiferromagnetic ordering takes place at ${T}_{N}=24\mathrm{K},$ suggesting a rather large value for the interchain exchange interaction ${J}_{\ensuremath{\perp}}\ensuremath{\sim}16.5\mathrm{K}.$ The energy gap found in the antiferromagnetic resonance spectrum 76 GHz agrees with the spin-flop magnetic-field value of about 2.7 T.

We found anomalies in the temperature dependence of the dielectric and magnetic susceptibility of ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5}$ in the microwave and far-infrared frequency ranges. The anomalies occur at the phase transition temperature ${T}_{c},$ at which the spin gap opens. The real parts of the dielectric constants ${\ensuremath{\epsilon}}_{a}$ and ${\ensuremath{\epsilon}}_{c}$ decrease below ${T}_{c},$ while ${\ensuremath{\epsilon}}_{b}$ remains constant. The change in ${\ensuremath{\epsilon}}_{a}$ is proportional to the square of the ordering parameter below ${T}_{c}.$ The dielectric anomalies found indicate a redistribution of charges at the phase transition. The character of the dielectric constant anomalies is of the antiferroelectric type, which is in agreement with the models assuming the zigzag charge ordering in the $\mathrm{ab}$ plane below ${T}_{c}.$ The anomaly of the microwave magnetic losses is probably related to the coupling between the spin and charge degrees of freedom in vanadium ladders.

The low-temperature magnetic resonance in two related Heisenberg pyrochlores Gd 2 Ti 2 O 7 and Gd 2 Sn 2 O 7 is studied. The gapped acoustic branches are found in both compounds. Weakly dispersive soft modes are observed in the saturated phase of Gd 2 Ti 2 O 7 , in agreement with the predictions of the spin-wave theory.

The low-energy dynamics of the spin-gap magnet TlCuCl 3 is studied. Closing of the energy gap by a magnetic field at the critical field H c leads to the formation of the field-induced transverse antiferromagnetic order. The magnetic resonance technique allows to probe an energy range 0.05–0.5 meV hardly accessible by other means. The measured spin-excitation spectra are described quantitatively within the framework of the effective field theory under an assumption of a weak anisotropy of the exchange interaction. Model calculations demonstrate that low energy dynamics at the critical field is strongly influenced by the longitudinal oscillations of the order parameter.

The influence of thermal and strictional nonlinear effects on the whistler emission in magnetoactive plasma is studied experimentally. It is established that a nonlocal thermal nonlinearity determines the directional pattern of the antenna, while a strictional nonlinearity, which is strongest near the antenna surface, is responsible for the matching of the emitter with the surrounding plasma.

Spin-$1/2$ Heisenberg antiferromagnets ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$ and ${\mathrm{Cs}}_{2}{\mathrm{CuBr}}_{4}$ with distorted triangular-lattice structures are studied by means of electron spin resonance spectroscopy in magnetic fields up to the saturation field and above. In the magnetically saturated phase, quantum fluctuations are fully suppressed, and the spin dynamics is defined by ordinary magnons. This allows us to accurately describe the magnetic excitation spectra in both materials and, using the harmonic spin-wave theory, to determine their exchange parameters. The viability of the proposed method was proven by applying it to ${\mathrm{Cs}}_{2}{\mathrm{CuCl}}_{4}$, yielding $J/{k}_{B}=4.7(2)\text{ }\text{ }\mathrm{K}$, ${J}^{\ensuremath{'}}/{k}_{B}=1.42(7)\text{ }\text{ }\mathrm{K}$, [${J}^{\ensuremath{'}}/J\ensuremath{\simeq}0.30$] and revealing good agreement with inelastic neutron-scattering results. For the isostructural ${\mathrm{Cs}}_{2}{\mathrm{CuBr}}_{4}$, we obtain $J/{k}_{B}=14.9(7)\text{ }\text{ }\mathrm{K}$, ${J}^{\ensuremath{'}}/{k}_{B}=6.1(3)\text{ }\text{ }\mathrm{K}$, [${J}^{\ensuremath{'}}/J\ensuremath{\simeq}0.41$], providing exact and conclusive information on the exchange couplings in this frustrated spin system.

Single crystals of the spin-Peierls compound NaV{sub 2}O{sub 5} were synthesized. Both magnetic susceptibility and the electron-spin-resonance (ESR) intensity measured at 36.2 GHz showed an abrupt decrease at 35 K, indicating transition into the dimerized state. This decrease was much more pronounced for the ESR intensity as compared with the static susceptibility. The temperature dependences of the static and dynamic susceptibilities correspond to an energy gap of 92 K in the magnetic excitation spectrum. At low temperatures significant broadening of the ESR line was observed and a complex signal consisting of three spectral components was found. {copyright} {ital 1997} {ital The American Physical Society}

An adiabatic demagnetization process is studied in the pyrochlore antiferromagnet Gd 2 Ti 2 O 7 . A strong cooling of the sample is observed by decreasing magnetic field in the range 120–60 kOe corresponding to a crossover between saturated and spin–liquid phases. This phenomenon indicates that a considerable part of the magnetic entropy associated with a macroscopic number of local soft modes survives in the strongly correlated paramagnetic state. Monte Carlo simulations demonstrate good agreement with the experiment. The cooling power of the process is experimentally estimated with a view to possible technical applications. The results on Gd 2 Ti 2 O 7 are compared to those for Gd 3 Ga 5 O 12 , a related material for low-temperature magnetic cooling.

The effect of an electric field on the magnetic susceptibility in a pure CuGeO3 single crystal at microwave frequencies is investigated. A quadratic effect of the electric field on the magnetic susceptibility, which increases with decreasing temperature, is observed in the spin-Peierls state at temperatures below 5 K. The observed effect is tentatively attributed to residual magnetism, due to distortions of the regular dimerized structure at domain walls with different values of the dimerization phase, and to the effect of the electric field on the interchain exchange interaction.

The adiabatic T(H) curves measured in a Heisenberg pyrochlore Gd2Ti2O7 prove the existence of a macroscopic number of local soft modes in this compound. A spin gap of 25 GHz, developing in Gd2Ti2O7 on cooling down to 1.3 K (in the collective paramagneti phase), was observed by ESR spectroscopy. ESR of diamagnetically diluted crystals revealed the single-ion anisotropy energy to be about a quarter of Curie-Weiss temperature. This might be responsible for the observed energy gap. Below 1 K, in the magneticaly ordered phase, the pin excitations have a threebranch spectum with two enegy gaps

The spin dynamics of a quasi one dimensional $S=1$ bond alternating spin-gap antiferromagnet Ni(C$_9$H$_{24}$N$_4$)NO$_2$(ClO$_4$) (abbreviated as NTENP) is studied by means of electron spin resonance (ESR) technique. Five modes of ESR transitions are observed and identified: transitions between singlet ground state and excited triplet states, three modes of transitions between spin sublevels of collective triplet states and antiferromagnetic resonance absorption in the field-induced antiferromagnetically ordered phase. Singlet-triplet and intra-triplet modes demonstrate a doublet structure which is due to two maxima in the density of magnon states in the low-frequency range. A joint analysis of the observed spectra and other experimental results allows to test the applicability of the fermionic and bosonic models. We conclude that the fermionic approach is more appropriate for the particular case of NTENP., Comment: 11 pages, 11 figures, published in Phys.Rev.B

Magnetization of the frustrated $S=1/2$ chain compound LiCuVO$_4$, focusing on high magnetic field phases, is reported. Besides a spin-flop transition and the transition from a planar spiral to a spin modulated structure observed recently, an additional transition was observed just below the saturation field. This newly observed magnetic phase is considered as a spin nematic phase, which was predicted theoretically but was not observed experimentally. The critical fields of this phase and its dM/dH curve are in good agreement with calculations performed in a microscopic model (M. E. Zhitomirsky and H. Tsunetsugu, preprint, arXiv:1003.4096v2)., Comment: 5 pages, 4 figures

The s-wave pion-nucleus repulsion problem may be solved by taking into account nuclear relativism within the Walecka model.

Magnetization, specific heat, and neutron diffraction measurements are used to map out the entire magnetic phase diagram of KFe(MoO4)2. This stacked triangular antiferromagnet is structurally similar to the famous multiferroic system RbFe(MoO4)2. Because of an additional small crystallographic distortion, it contains two sets of inequivalent distorted magnetic triangular lattices. As a result, the spin network breaks down into two intercalated yet almost independent magnetic subsystems. One is a collinear antiferromagnet that shows a simple spin-flop behavior in applied magnetic fields. The other is a helimagnet that instead goes through a series of exotic commensurate-incommensurate phase transformations. In the various phases one observes either true three-dimensional or unconventional quasi-two-dimensional ordering.

The strongly correlated disordered phase of two highly frustrated pyrochlore magnets Gd2Ti2O7 and Gd2Sn2O7 is probed using electron-spin resonance in the temperature range 1.3-30 K. The deviation of the absorption line from the paramagnetic position nu=gamma H observed in both compounds below the Curie-Weiss temperature Theta(CW)similar or equal to 10 K suggests an opening up of a gap in the excitation spectra. On cooling to 1.3 K (which is above the ordering transition T-N similar or equal to 1.0 K) the resonance spectrum is transformed into a wideband of excitations with the gap amounting to Delta similar or equal to 26 GHz (1.2 K) in Gd2Ti2O7 and 18 GHz (0.8 K) in Gd2Sn2O7. The gaps increase linearly with the external magnetic field. For Gd2Ti2O7 this branch coexists with an additional nearly paramagnetic line absent in Gd2Sn2O7. These low-lying excitations with gaps, which are preformed above the ordering transition, may be interpreted as collective spin modes split by the single-ion anisotropy.

Relaxation rates and coupling coefficients of spin waves have been determined in the layered ferromagnet ( CH 3 NH 3 ) 2 CuCl 4 . The parameters could be measured by observing the transient response of the parallel pumped microwave absorption to rapid magnetic field changes.

The saturation of magnetization was studied in pulsed magnetic fields $(H)$ for the triangular lattice antiferromagnet $\mathrm{Rb}\mathrm{Fe}{(\mathrm{Mo}{\mathrm{O}}_{4})}_{2}$ at temperatures $(T)$ below the magnetic ordering point ${T}_{N}$. The field-driven transition from the antiferromagnetic to the paramagnetic phase was studied by specific-heat measurements in a steady magnetic field. The specific heat shows a sharp peak at a value of the magnetic field below which a singularity in the magnetization curve occurs, indicating a splitting of the field-induced transition to the paramagnetic phase. This splitting vanishes at low temperatures. A complete $T\text{\ensuremath{-}}H$ phase diagram is derived experimentally both for the magnetic field lying in the spin plane and perpendicular to the plane. A spin-flip magnetic-resonance mode with the softening of the spin-wave spectrum at the saturation field has been observed. The gap of the spin-wave spectrum was found to be reduced by zero-point fluctuations by a factor of $0.82\ifmmode\pm\else\textpm\fi{}0.08$ with respect to the prediction of the molecular-field theory.