V. M. Yakovenko

Conversion of terahertz wave polarization at the boundary of a layered superconductor due to the resonance excitation of oblique surface waves.

We predict a complete TM↔TE transformation of the polarization of terahertz electromagnetic waves reflected from a strongly anisotropic boundary of a layered superconductor. We consider the case when the wave is incident on the superconductor from a dielectric prism separated from the sample by a thin vacuum gap. The physical origin of the predicted phenomenon is similar to the Wood anomalies known in optics and is related to the resonance excitation of the oblique surface waves. We also discuss the dispersion relation for these waves, propagating along the boundary of the superconductor at some angle with respect to the anisotropy axis, as well as their excitation by the attenuated-total-reflection method.

Terahertz transverse-electric- and transverse-magnetic-polarized waves localized on graphene in photonic crystals

We predict the coexistence of both TE- and TM-polarized localized electromagnetic waves that can propagate \emph{in the same frequency range} along a graphene layer inserted in a photonic crystal. In addition, we studied the excitation of these modes by an external wave and have shown that the resonance peaks of the sample transmissivity should be observed due to the excitation of the localized waves, independently of the polarization of the exciting wave. The simplicity of the derived dispersion relations for the localized modes and the possibility to excite waves of both polarizations provide a method for measuring graphene conductivity.

Magnetoresistance of magnetic tunnel junctions with low barrier heights

The magnetoresistance of low-barrier magnetic tunnel junctions (MTJs) was studied within a two-band model of free electrons in ferromagnetic electrodes, taking into consideration image forces. For MTJs with an MgO insulator, explanations are given of the giant tunneling magnetoresistance (TMR) effect and the effect of increasing TMR as the width of the MgO barrier increases. It is shown that TMR and the electron current density through MTJs depend strongly on the dielectric constant of the MgO insulator. It is found that the TMR of low-barrier MTJs reaches a maximum at a particular value of the applied bias voltage. It is demonstrated that the electron current density through low-barrier MTJs can be high enough to switch the magnetization of a ferromagnetic electrode.

Whispering gallery modes in a hemispherical isotropic dielectric resonator with a perfectly conducting planar surface

The eigenmodes of a hemispherical dielectric resonator placed on a perfectly conducting planar surface are studied. The resonator is shown to support independent E-and H-type oscillations. For E and H oscillations, the sums n + m of the polar and azimuthal indices are, respectively, even and odd. Accordingly, E and H oscillations exhibit, respectively, n-and (n + 1)-fold frequency degeneracy in azimuthal index m. The distributions of the whispering gallery mode fields on the surface of the hemisphere and on the external conducting plane are obtained. The energy distribution among the eigenmodes with polar index n=36 is studied. The polar angle that corresponds to the maximum of the whispering gallery mode field and the width of the energy localization region in terms of the polar angle are calculated. The number of field variations in the spherical coordinates are correlated with the indices of the resonator’s eigenmodes. The eigenfrequencies and Q factors of the resonator are plotted versus the permittivity of the isotropic environment in the 8-mm wavelength range.

Oblique surface Josephson plasma waves in layered superconductors

We have theoretically studied oblique surface waves (OSWs) which propagate along the interface between a dielectric and a layered superconductor. We assume that this interface is perpendicular to the superconducting layers, and OSWs at the interface can propagate at an arbitrary angle with respect to them. The electromagnetic field of the OSWs in a layered superconductor is a superposition of an ordinary wave (with its electric field parallel to the layers) and an extraordinary wave (with its magnetic field parallel to the layers). We have derived the dispersion equation for the OSWs and shown that the dispersion curves have end points where the extraordinary mode transforms from evanescent wave to bulk wave, propagating deep into the superconductor. In addition, we have analytically solved the problem of the resonance excitation of the OSWs by the attenuated-total-reflection method using an additional dielectric prism. Due to the strong current anisotropy in the boundary of the superconductor, the excitation of the OSWs is accompanied by an additional important phenomenon: The electromagnetic field component with the orthogonal polarization appears in the wave reflected from the bottom of the prism. We show that, for definite optimal combinations of the problem parameters (the wave frequency, the direction of the incident wave vector, the thickness of the gap between dielectric prism and superconductor, etc.), there is a complete suppression of the reflected wave with its polarization coinciding with the polarization of the incident wave. Contrary to the isotropic case, this phenomenon can be observed even in the dissipationless limit. In such a regime, the complete transformation of the incident wave into a reflected wave with orthogonal polarization can be observed.

Surface electron states produced by a Rayleigh wave

The electronic properties of a semiconductor bounded by an uneven surface representing an infinitely high potential barrier are investigated. The surface irregularities are produced by a Rayleigh acoustic wave. It is shown that, on the boundary of a semiconductor, surface electron states (waves) may arise whose dispersion laws are obtained under the conditions when conduction electrons are located either in or outside the field of the acoustic wave. Existence domains of surface electron states are found that are distinguished by their physical properties. These domains are separated by a band gap whose width is determined by the height of irregularities.

Surface electromagnetic states in the photonic crystal–ferrite–plasma-like medium structure

Frequencies of the surface electromagnetic states in the photonic crystal–ferrite–plasma-like medium structure were studied both theoretically and experimentally as a function of the ferrite layer thickness, external dc magnetic field and temperature. The photonic crystal was a periodic stack of a finite number of unit cells, each consisting of two different nonmagnetic dielectrics. A nonmagnetic semiconductor played the role of a plasma-like medium. A dc magnetic field was applied parallel to the ferrite layer. The propagation direction of an electromagnetic wave was perpendicular to the dc magnetic field with the electrical component of a microwave field parallel to the magnetic field. An analytical expression relating the frequencies of the surface electromagnetic states to the structural parameters and the magnitude of the dc magnetic field value was derived within the framework of the model of non-conducting and magnetized to saturation ferrite. It was predicted that the states under study are multim...

Josephson plasma oscillations in confined layered superconductors

Intrinsic electromagnetic oscillations were investigated in layered superconductors of finite dimensions, filling a rectangular resonator. Spectra of both ordinary and extraordinary eigenmodes were obtained. A nonlinear effect of decreasing eigenfrequencies of extraordinary modes was analyzed and generation of the third harmonic of oscillations was studied. A nonlinearity of the system is related to a nonlinear dependence of the Josephson current density across superconducting layers on interlayer phase difference of the order parameter. Josephson plasma waves running along a waveguide filled with a layered superconductor were investigated as well as nonlinear effects appearing during propagations of these waves. In addition, in the work, an effect of the slowing down of terahertz waves in waveguides, which is caused by a mutual effect of nonlinearity and damping of waves is predicted.

Fast charge energy losses in structures with a 2D electron gas

Fast charge energy losses for the excitation of natural oscillations in media with a 2D plasma sheath are studied. A semiconducting (dielectric) cylinder-2D plasma sheath-vacuum system is considered. Dispersion relations describing natural oscillations in such a system are derived. Energy losses of a point charge rectilinearly and uniformly moving over and inside the cylinder are found. The loss of energy by a moving charged ring embracing the system (specifically, a nanotube) is considered.

Charged-particle energy loss by the excitation of surface magnetoplasmons in a structure with two- and three-dimensional plasmas

Electron energy loss by the excitation of surface magnetoplasma oscillations by an electron moving along a static magnetic field in vacuum over a two-dimensional plasma layer on the surface of three-dimensional plasma half-space has been studied theoretically. Electron energy loss by the excitation of surface magnetoplasmons has been calculated in the electrostatic approximation. It has been shown that the type of the dispersion law of electrons in such a plasma (quadratic for a two-dimensional Drude gas or linear for graphene) can be determined from the qualitative character of the dependence of the maximum of the spectral density of this loss on the electron density in the two-dimensional plasma.