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We study instability of plasmons in a dual-grating-gate graphene field-effect transistor induced by dc current injection using self-consistent simulations with the Boltzmann equation. With only the acoustic-phonon-limited electron scattering, it is demonstrated that a total growth rate of the plasmon instability, with the terahertz/mid-infrared range of the frequency, can exceed $4\times10^{12}$ s$^{-1}$ at room temperature, which is an order of magnitude larger than in two-dimensional electron gases based on usual semiconductors. By Comparing the simulation results with existing theory, it is revealed that the giant total growth rate originates from simulataneous occurence of the so-called Dyakonov-Shur and Ryzhii-Satou-Shur instabilities., 5 pages, 5 figures

Array of field-effect transistors (FET) with asymmetric T-gates and floating electrodes fabricated on a single chip was used as terahertz (THz) detector. Nonresonant detection with strong photovoltaic response was realized due to excitation of electron plasma oscillations in the common channel of the FETs array. Voltage responsivities obtained by the array of FETs with floating electrodes surpass the photoresponse reported for the array of FETs connected in series by external wiring.

We study instability of plasmons in a dual-grating-gate graphene field-effect transistor induced by dc current injection using self-consistent simulations with the Boltzmann equation. With ultimately high-quality graphene where the electron scattering is only limited by acoustic phonons, it is demonstrated that a total growth rate of the plasmon instability, with the terahertz/mid-infrared range of the frequency, can exceed 4 X 1012 s-1 at room temperature, which is an order of magnitude larger than in two-dimensional electron gases based on usual semiconductors. We show that the giant total growth rate originates from cooperative promotion of the so-called Dyakonov-Shur and Ryzhii-Satou-Shur instabilities.

The polarization conversion of terahertz radiation by the periodic array of graphene nanoribbons located at the surface of a high-refractive-index dielectric substrate (terahertz prism) is studied theoretically. Giant polarization conversion at the plasmon resonance frequencies takes place without applying external DC magnetic field. It is shown that the total polarization conversion can be reached at the total internal reflection of THz wave from the periodic array of graphene nanoribbons even at room temperature.

We present the responsivity spectra in the 0.2–0.4 THz range at low temperature of a AlGaAs/GaAs field-effect transistor connected to an on-chip integrated antenna. We fabricated asymmetric plasmonic microcavities to obtain down-converted signals from the intrinsic nonlinearity of 2D electron plasma oscillations. Finite-element modeling simulations of the 2D electron oscillations and electromagnetic calculation of the antenna spectral response suggest that the device may work as ultra-broadband nonlinear terahertz detector.

Graphitic porous carbon materials from pyrolysis of wood precursors were obtained by means of a nanosized Fe catalyst, and their microstructure and electrical and thermal transport properties investigated. Thermal and electrical conductivity of graphitized carbon materials increase with the pyrolysis temperature, indicating a relationship between the degree of graphitization and thus in crystallite size with transport properties in the resulting carbon scaffolds. Evaluation of the experimental results indicate that thermal conductivity is mainly through phonons and increases with the temperature in Fe-catalyzed carbons suggesting that the mean free path of phonons in the material is small and defect scattering dominates over phonon-phonon interactions in the range from room temperature to 800 °C.

It is demonstrated theoretically that the effects of lateral electromagnetic screening of plasmons in a two-dimensional electron system by side metallic electrodes modify the plasmon spectrum. The effects of lateral screening become more pronounced at larger values of the plasmon wave vector. This leads to the deviation of the dispersion of laterally screened plasmons from the square-root dispersion law characteristic of unscreened plasmons. The results allow interpreting the recent experimental data of other authors on the dispersion of the plasma oscillations in a laterally screened two-dimensional electron system.

The theory of the plasmon resonance excitation in the plasmonic-crystal structure on a dielectric substrate is presented. The effect of the plasmon resonance intensity oscillation as a function of the substrate thickness is predicted. It is shown that the enhancement of the plasmon resonance intensity occurs in a broad terahertz frequency range in the structure on a membrane substrate.

A study has been made of the dependences of the electrical resistivity and the Hall coefficient on the temperature in the range 1.8-1300K and on magnetic fields of up to 28kOe for the biomorphic SiC/Si (MDF-SiC/Si) composite and biomorphic porous SiC (MDF-SiC) based upon artificial cellulosic precursor (MDF - medium density fiberboards). It has been shown that electric transport in MDF-SiC is effected by carriers of n-type with a high concentration of ∿1020cm-3 and a low mobility of ∿0.4cm2V-1s-1. The specific features in the conductivity of MDF-SiC are explained by quantum effects arising in disordered systems and requiring quantum corrections to conductivity. The TEM studies confirmed the presence of disordering structural features (nanocrystalline regions) in MDF-SiC. The conductivity of MDF-SiC/Si composite originates primarily from Si component in the temperature range 1.8-500K and since ∿500 to 600K the contribution of MDF-SiC matrix becomes dominant. © 2010 Elsevier Ltd., This study was supported by the Russian Basic Research Foundation (project no. 7-03-91353NNF_a), the Presidium of the Russian Academy of Sciences (program no. P-03), and the Spanish Projects MAT 2007-30141-E and PET 2006-0658.

This paper reports on comparative investigations of the structural and electrical properties of biomorphic carbons prepared from natural beech wood, as well as medium-density and high-density fiberboards, by means of carbonization at different temperatures Tcarb in the range 650–1000°C. It has been demonstrated using X-ray diffraction analysis that biocarbons prepared from medium-density and high-density fiberboards at all temperatures Tcarb contain a nanocrystalline graphite component, namely, three-dimensional crystallites 11–14 A in size. An increase in the carbonization temperature Tcarb to 1000°C leads to the appearance of a noticeable fraction of two-dimensional graphene particles with the same sizes. The temperature dependences of the electrical resistivity ρ of the biomorphic carbons have been measured and analyzed in the temperature range 1.8–300 K. For all types of carbons under investigation, an increase in the carbonization temperature Tcarb from 600 to 900°C leads to a change in the electrical resistivity at T = 300 K by five or six orders of magnitude. The dependences ρ(T) for these materials are adequately described by the Mott law for the variable-range hopping conduction. It has been revealed that the temperature dependence of the electrical resistivity exhibits a hysteresis, which has been attributed to thermomechanical stresses in an inhomogeneous structure of the biocarbon prepared at a low carbonization temperature Tcarb. The crossover to the conductivity characteristic of disordered metal systems is observed at Tcarb ≳ 1000°C.

Physics of plasma oscillations and basic principles of plasmonic detection of terahertz radiation in FET structures with two-dimensional electron channels are discussed. Plasmonic devices are practically attractive because they are extremely fast and electrically tunable through the entire terahertz frequency band by changing electric potentials at metal contacts of the device

We present the theory of plasmon excitation in a grating-gate transistor structure with spatially modulated 2D electron channel. The plasmon spectrum varies depending on the electron density modulation in the transistor channel. We report on the frequency ranges of plasmon mode excitation in the gated and ungated regions of the channel and on the interaction of these different types of plasmon modes. We show that a constructive influence of the ungated regions of the electron channel considerably increases the intensity of the gated plasmon resonances and reduces the plasmon-resonance linewidth in the grating-gated transistor structure.

Terahertz photoresponse of a GaAs/InGaAs transistor structure with large-area slit grating gate has been measured. Peaks in the photoresponse curve are assigned to plasmon resonances excited in the structure. More effective excitation of plasmon resonances is achieved in a grating gate structure with narrow slits, which increase the photoresponse amplitude by an order of magnitude.

The steady-state stability of a dual-wavelength vertical-external-cavity surface-emitting laser has been analyzed. It is shown that, in the laser with independent active regions containing quantum wells (QWs) of different depths, this state is always stable. The increment of growth and the frequency of oscillations of small deviations from the steady state are determined in the presence of coupling between the active regions, which is caused by the absorption of short-wavelength radiation in deep QWs. In the region of violation of the steady-state stability, the generation proceeds in the form of short pulses, with the almost simultaneous excitation of the optical fields at both frequencies.

It is shown that the gated plasmons in a multichannel nanoheterotransistor with two-dimensional electron gas can be trapped within two-dimensional gated nanocavities due to total internal reflection of these plasmon modes from the boundaries of gated nanocavities. This leads to tremendous shrinking of the gated plasmon resonance linewidth.

The electrical and galvanomagnetic properties of high-porosity biocarbon preforms prepared from white pine wood by pyrolysis at carbonization temperatures Tcarb = 1000 and 2400°C have been studied. Measurements have been made of the behavior with temperature of the electrical resistivity, as well as of magnetoresistance and the Hall coefficient in the 1.8–300-K temperature interval and magnetic fields of up to 28 kOe. It has been shown that samples of both types (with Tcarb = 1000 and 2400°C) are characterized by high carrier (hole) concentrations of 6.3 × 1020 and 3.6 × 1020 cm−3, respectively. While these figures approach the metallic concentration, the electrical resistivity of the biocarbon materials studied, unlike that of normal metals, grows with decreasing temperature. Increasing Tcarb brings about a decrease in electrical resistivity by a factor 1.5–2 within the 1.8–300-K temperature range. The magnetoresistance also follows a qualitatively different pattern at low (1.8–4.2 K) temperatures: it is negative for Tcarb = 2400°C and positive for Tcarb = 1000°C. An analysis of experimental data has revealed that the specific features in the conductivity and magnetoresistance of these samples are described by quantum corrections associated inherently with structural characteristics of the biocarbon samples studied, more specifically with the difference between the fractions of the quasi-amorphous and nanocrystalline phases, as well as with the fine structure of the latter phase forming at the two different Tcarb.

The effect of pump wave reflections on the carrier generation rate and uniformity of carrier population in quantum wells (QWs) of a dual-wavelength vertical-cavity surface-emitting laser has been numerically analyzed. The laser’s active region has been described within a mathematical model allowing any number of QWs and arbitrary distribution of carrier generation rate. It is shown that the optimal arrangement of blocking layers in the active region of a dual-wavelength vertical-cavity surface-emitting laser allows one to obtain a very uniform QW population. It is established that pump wave reflections significantly affect the local carrier generation rate and, therefore, the distribution of excited carriers in the laser structure.

The shape of the gated plasmon resonance absorption line in a field-effect heterotransistor with a remote InGaAs/InAlAs gate has been calculated. It is shown that the experimentally observed giant broadening of the plasmon resonance linewidth is caused by the effect of intermodal plasmon-plasmon scattering.

Several aspects of the theory of plasmon resonant DC photoconduction are discussed here, in connection with recent observations involving a THz-irradiated grid-gated double-quantum-well FET.1 In this, we construct a classical model of nonlinear polarizability to second order in the THz field using a “hydrodynamic” type formulation including the roles of a stress-tensor and friction/viscosity. The resulting second order polarizability exhibits resonant behavior when the THz frequency matches plasmon frequencies of the system, sharply reducing the effectiveness of screened impurity scattering potentials which can admit resonant DC photoconduction. Furthermore, we also show that an asymmetric double-quantum-well system with lateral periodicity can mix optical and acoustic plasmons, giving rise to an interlayer THz field which becomes very strong when tuned by gate voltage into the “mode-mode-repulsion” regime wherein the optical and acoustic modes equally share amplitude. This can enhance interlayer electron tunneling and may contribute to photoconductivity.

Terahertz (THz) response spectra of GaN-based field-effect transistor (FET) arrays are calculated in a self-consistent electromagmetic approach. Two types of FET arrays are considered: (i) FET array with a common channel and a large-area grating gate, and (ii) array of FET units with separate channels and combined intrinsic source and drain contacts. It is shown that the coupling between plasmons and THz radiation in the FET array can be strongly enhanced as compared to a single-unit FET. The computer simulations show that the higher-order plasmon modes can be excited much more effectively in the array of FET units with separate channels and combined source and drain contacts then in FET array with a common channel and a large-area grating gate.

We present a simple physical model describing the optical properties of periodic planar metallic nanostructures in terms of equivalent oscillating-current resonant circuits, which explain the total light absorption phenomenon in different plasmonic nanostructures on a common physical ground by referring it to the impedance matching condition at the plasmon resonance. This model can be easily adapted to describe different resonant nanostructures exhibiting total absorption of light at plasmon resonances.

Using a rigorous electrodynamic approach, the spectra of the terahertz response of periodically ordered arrays of field-effect transistors with two-dimensional electron channel were calculated. It was shown that the higher-order plasmon resonances can be excited in such structures at frequencies above 10 THz.

An array of GaAs/InGaAs/AlGaAs field-effect transistors with an asymmetric T-gate in each transistor and floating electrodes was fabricated on a single chip and tested as a detector of terahertz (THz) radiation. Principle of detection was based on excitation of plasma oscillations in the common electron channel of the FETs array. Strong terahertz photovoltaic response was demonstrated without any supplementary antenna. Voltage re-sponsivities above 1000 V/W and up to 2000 V/W were obtained at zero (unbiased mode) and positive (directed from drain to source — biased mode) dc currents in the FETs array channel, respectively, surpassing the photorespponse demonstrated by the array of FETs connected in series by external wiring [5].

International audience; We report on the intrinsic responsivity of an asymmetric dual-grating-gate plasmonic detector over 100 kV/W at 200 GHz and 50 kV/W at 300 GHz measured at room temperature with zero source-drain bias. We demonstrate that broadband characteristics of the responsivity depend much on the geometrical parameters of the detectors.

We introduce and theoretically study the plasmon-photogalvanic effect in a planar noncentrosymmetric plasmonic crystal containing a homogeneous two-dimensional electron system gated by a periodic metal grating with an asymmetric unit cell. The plasmon-photogalvanic dc current arises due to the two-dimensional electron drag by the noncentrosymmetric plasmon modes excited under normal incidence of terahertz radiation. We show that the collective plasmon modes of the planar plasmonic crystal become strongly noncentrosymmetric in the weak-coupling regime of their anticrossing. A large plasmon wave vector (which is typically by two-three orders of magnitude greater than the terahertz photon wave vector) along with strong near-field enhancement at the plasmon resonance make the plasmonic drag a much stronger effect compared to the photon drag observed in conventional two-dimensional electron systems.

This paper reviews recent advances in the research and development toward the graphene-based terahertz (THz) lasers. Mass-less Dirac Fermions of electrons and holes in gapless and linear symmetric band structures in graphene enable a gain in a wide THz frequency range under optical or electrical pumping. The excitation of the surface plasmon polaritons in the population-inverted graphene dramatically enhances the THz gain. Photon-emission-assisted resonant tunneling in a double-graphene-layered nano-capacitor structure also strongly enhances the THz gain. Novel graphene-based heterostructures using these physical mechanisms for the current-injection driven THz lasing are discussed. Their superior gain-spectral properties are analyzed and the laser cavity structures for the graphene THz laser implementation are discussed.

This paper reviews recent advances in terahertz wave generation using graphene and compound semiconductor nano-heterostructures. The excitation of two-dimensional (2D) plasmons in high-electron mobility transistors (HEMTs) and related semiconductor nano-heterostructures has been used for emission of THz electromagnetic radiation. Plasmons in graphene (which is one or several monolayers of a honeycomb carbon lattice) have a higher velocity and peculiar transport properties owing to the massless and gapless energy spectrum of graphene. Optical and/or injection pumping of graphene results in a negative-dynamic conductivity in the THz spectral range, which may enable new types of THz lasers. Fundamental physics behind the device operation mechanisms and experimental results are demonstrated including coherent monochromatic THz radiation from InP-based HEMT-type emitters and stimulated emission of THz radiation with a giant gain via excitation of surface plasmon polaritons in optically pumped monolayer intrinsic graphene.

International audience; We study theoretically and experimentally the plasmonic THz detection by the asymmetric dual-grating-gate HEMT at room temperature without source-to-drain bias. We derive the analytical expressions of photocurrents due to the plasmonic drag and ratchet effects, and we discuss about their frequency dependences. We also compare the theory to the experimentally obtained frequency dependence. It is demonstrated that they agree qualitatively well.

We have measured the electric signal downconverted from a terahertz frequency by an unbiased high mobility two-dimensional electron-gas (2DEG) device. The 2DEG was confined in an asymmetric plasmonic microcavity, and the radiation frequency was continuously tuned in the 0.2-0.4 THz range. The presence of resonant peaks at three frequencies corresponding to three plasma oscillation modes of the ungated 2DEG clearly points to the intrinsic nature of the hydrodynamic nonlinearity responsible for the downconversion as opposed to previously proposed plasmonic cavity configurations where the 2DEG oscillates under the metal gate that also acts as the source of the nonlinearity.

We study theoretically the terahertz (THz) response of a bilayer of density-modulated two-dimensional electron gases, which we employ to model the actual double-quantum-well electron channel of a grid-gated field-effect transistor in which strong THz photoresponse was recently observed. We have shown that such a system can be driven into the anticrossing regime between gated and ungated plasma resonances by tuning the gate voltage. The amplitude of the interlayer THz electric field in the ungated (double-layered) portions of the channel increases dramatically in the anticrossing regime. This strong interlayer THz electric field may strongly affect interlayer electron tunneling which, in turn, may contribute to the physical mechanism underlying the strong THz photoresponse observed in recent experiments.

The non-stationary nanoelectrodynamics of polariton mode excitation and radiation in a two-dimensional excitonic layer due to a finite current pulse in the layer are analyzed in this paper. Using dyadic Green's functions we obtain an exact result for the radiated field in closed form. This exact solution exhibits physical features of striking interest: in particular, spatial growth of the radiated inhomogeneous exciton-polariton electromagnetic plane wave field away from the nanolayer, while decaying in time. This analysis also provides insight into the resolution of an old controversy concerning differing approaches to the description of radiative modes of a two-dimensional excitonic system.

Recent experimental observations by X.G. Peralta and S.J. Allen, et al. of dc photoconductivity resonances in steady source-drain current subject to terahertz radiation in a grid-gated double-quantum well FET suggested an association with plasmon resonances. This association was definitively confirmed for some parameter ranges in our detailed electrodynamic absorbance calculations. In this paper we propose that the reason that the dc photoconductance resonances match the plasmon resonances in semiconductors is based on a nonlinear dynamic screening mechanism. In this, we employ a shielded potential approximation that is nonlinear in the terahertz field to determine the nonequilibrium Green's function and associated density perturbation that govern the nonequilibrium dielectric polarization of the medium. This ''conditioning'' of the system by the incident THz radiation results in resonant polarization response at the plasmon frequencies which, in turn, causes a sharp drop of the resistive shielded impurity scattering potentials and attendant increase of the dc source-drain current. This amounts to disabling the impurity scattering mechanism by plasmon resonant behavior in nonlinear screening.

This paper reviews recent advances toward new types of terahertz (THz) lasers using active plasmonics in graphene. Optical gain originated from graphene ultrafast nonequilibrium carrier dynamics and phonon properties in the broadband THz frequency range might pave the way for the THz lasers but the challenge is to overcome the strong losses in the THz range. Graphene plasmons, quanta of the collective charge density waves excited by the two-dimensional carriers in graphene, can substantially enhance the light-matter (THz photons-graphene) interaction, leading to a “giant THz gain”. One possible implementation relies on superradiant plasmonic THz lasing in graphene-metal micro-ribbon array structures.

The experimental method of concentration pulses applied for studying hydrogen permeability of construction materials is considered. An algorithm of parametric identification is constructed. Parameter estimations for hydrogen transport in amorphous and recrystallized ferrum alloys are presented.

Magnetic and electrical properties of nanoporous carbon (NPC) containing atoms of Ni in pores were studied using electron spin resonance (ESR) method, conductance, and Hall measurements. Being introduced in nanopores, nickel significantly changes properties of the NPC. Two kinds of charge carriers are revealed—holes and electrons. EPR spectra show that holes are captured at the localized states at low temperature and become free at temperatures, higher than the ionization energy. This position is justified by conductance and Hall effect measurements.

We have demonstrated theoretically that radiative polaritons in an interface layer having strong excitonic response exhibit appreciable frequency dependence on an in-plane wave vector, k ∥ , in the region of the ω–k ∥ dispersion plane between the “light-lines” of the media to either side of the layer. We predict ultrafast radiative decay of such polaritons with lifetimes as low as 10 fs in slabs of organic–inorganic PbI-based layered perovskite-type semiconductors. We confirm an unusual physical feature of the radiative polaritons by determining the linear electromagnetic response of the excitonic layer to a current δ-pulse.

Light absorption spectra by interface layer with strong excitonic response in asymmetric dielectric environment are calculated for various angles of incidence of the external light. Appreciable frequency shift of the absorption resonances in the total reflection regime is found. It is shown that higher polarition modes may reveal themselves in absorption spectra of high-ordered excitonic layers with strong exciton-photon interaction. In this case, doublet and triplet structures of absorption spectra emerge for s- and p-polarized light, respectively. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have calculated the light absorption spectra of a metal film containing a hexagonal two-dimensional lattice of spherical nano-voids. It is shown that giant resonant absorption of light occurs at the plasma resonance of the void lattice in the visible when the void-to-surface distance and the inter-void spacing are thinner than the skin depth. The maximum absorbance, at plasma resonance, is shown to be 50%. This phenomenon occurs when the void-to-surface distance has a specific value, which ensures optimal coupling of plasmons in the voids to the external light. We have found a physical criterion for the optimal coupling, which reads that the radiative broadening of the plasma resonance must be equal to its dissipative broadening in order to produce maximum light absorption at the resonance.

The terahertz absorption spectra of plasmon modes in a grid-gated double-quantum-well field-effect transistor structure is analyzed theoretically and numerically using the scattering matrix approach and is shown to faithfully reproduce strong resonant features of recent experimental observations of terahertz photo-conductivity in such a structure.

The problem of radiative decay of coherent polariton modes in a two-dimensional excitonic system with strong exciton-photon coupling is analyzed here. We show, theoretically, that inhomogeneous plane electromagnetic waves are spontaneouslyemitted from such a system. The dispersion branches of radiative exciton-polaritons penetrate into the slow-wave region of thev2k k dispersion plane where they exhibit appreciable renormalization of their frequency. We conclude that slow radiativepolaritons can be observed in time-resolved photoluminescence experiments on PbI-based self-organized quantum-wellstructures at grazing angles of emission.q 2003 Elsevier Ltd. All rights reserved. PACS: 73.21. 2 b; 78.47. þ p; 78.67. 2 nKeywords: A. Quantum wells; A. Semiconductors; D. Optical properties 1. IntroductionThe properties of radiative (resonant) polaritons in a two-dimensional (2D) excitonic system have been studiedtheoretically in a number of papers [1–7] using bothsemi-classical [2–4,7]and quantum-mechanical [1,5,6]approaches. There were three optically active exciton-polariton modes found, which were called T, L and Z modes[2]. These modes are distinguished by their electric fieldpolarization. The T mode has s (or TE) polarization whereasL and Z modes have p (TM) polarization. In a symmetricaldielectric environment, L and Z modes exhibit even and odddistributions of in-plane components of electric field across2D excitonic layer, respectively. Radiative exciton-polar-itons were experimentally observed in single and multipleGaAs/AlGaAs quantum well (QW) structures [8–10].Anenhanced (superradiant) spontaneous radiative decay rate offree excitons in QWs was predicted [11,12] and observed intime-resolved photoluminescence (PL) experiments [13,14]in the picosecond domain.In earlier theoretical papers [1–7], a perturbativeapproach was employed, with representing a photon quasi-continuum by a spectrum of electromagnetic (EM) eigen-modes of a cavity of infinite size. This involves the presenceof both incoming and outgoing homogeneous plane EMwaves at the plane of the 2D excitonic system. Resonantexciton-polaritons acquire a finite lifetime due to couplingto the quasi-continuum of photons in the infinite cavity.Within the framework of this model, it was concluded [2,6,7] that resonant polariton branches, which are essentiallyexciton-like having almost no dispersion, terminate at thepoints of their crossing with the free photon dispersion lineof the surrounding dielectric medium (in the barrier).In other words, the conclusions of the earlier theories [2,6,7] can be summarized as follows: (i) only polaritonswithin the light cone, i.e. those having in-plane phasevelocity exceeding the speed of light in the barrier, whichcan be thus referred as ‘fast polaritons’, can be opticallyactive and (ii) the frequency of luminescence from the

The terahertz absorption spectrum of plasmon modes in a grid-gated double-quantum-well (DQW) field-effect transistor structure is analyzed theoretically and numerically using a first principles electromagnetic approach and is shown to faithfully reproduce important physical features of recent experimental observations. We find that the essential character of the response—multiple resonances corresponding to spatial harmonics of standing plasmons under the metal grating—is caused by the static spatial modulation of electron density in the channel. Higher order plasmon modes become more optically active as the depth of the electron density modulation in the DQW tends towards unity. The maximum absorbance, at plasma resonance, is shown to be 1/2. Furthermore, the strongest absorption also occurs when the standing plasmon resonance coincides with the fundamental dipole mode of the ungated portion of the channel.

Measurements of the electrophysical properties of nanoporous carbon (NPC) samples (conductivity and Hall effect), as well as studies of the same samples using electron spin resonance (ESR), are summarized. Free holes are shown to play the major part in charge transport in such materials, thus permitting identification of the ESR signal with free holes. An analysis of the shape of the resonance lines, as well as of their dependence on temperature and the technology employed in sample preparation, established the ESR signals to consist of two superimposed resonance lines associated with carriers, free or localized to various extents, whose magnetic susceptibility obeys Pauli and Curie-Weiss laws, respectively. The temperature dependences of the ESR signal parameters were studied, and the experimental relations were compared with theory to determine the model-parameters. An analysis of the temperature behavior of these parameters suggests the conclusion that NPC samples are heterophase porous systems whose properties are dominated by structural characteristics.

Samples of the opal + HgSe nanocomposite with 100% filling of the first-order opal pores by mercury selenide were prepared. The effective thermal conductivity κeff and electrical resistivity ρeff were measured in the temperature range T=5–200 K, and the thermopower coefficient α was measured in the interval 80–300 K. The coefficient α of HgSe in opal was shown to remain the same as that in bulk mercury selenide samples with similar carrier concentrations. The mechanism of carrier scattering in the HgSe loaded in opal also did not change. The total thermal conductivity κ tot 0 and electrical resistivity ρ0 were isolated from κeff and ρeff, and the electronic (κ e 0 ) and lattice (κ ph 0 ) components of thermal conductivity of HgSe in opal were determined. The magnitude of κ ph 0 was found to be considerably smaller than κph of bulk HgSe with the same carrier concentration throughout the temperature interval studied (5–200 K). For T>20 K, this behavior of κ ph 0 (T) is accounted for by the presence of specific impurities and defects forming in HgSe, and for T

Measurements of ferromagnetic resonance (FMR) in both in-plane (H‖) and out-of-plane (H⊥) magnetic field configurations have been performed on a series of epitaxial (La0.7Ca0.3)1−xMn1+xO3 films. The FMR linewidths have a local maximum near the Curie point and increase continuously at low temperatures. The effective anisotropy of the films has the easy-plane character. The angular dependences of Hres were found to be in reasonable agreement with the theoretical ones, taking into account the second- and fourth-order terms of the magnetic anisotropy energy. A reasonable agreement between ΔHres(α) and Bloch type dissipation was obtained only for deviations of the external field from the normal to the film plane up to α=20°. From the temperature and angular dependences of the FMR spectra, the main magnetic parameters such as the effective magnetization, effective field Heff, anisotropy field HA2, and FMR linewidth were estimated.

Nanoporous carbon (NPC) material prepared from polycrystalline powders of SiC, TiC, or B 4 C carbides, using different techniques, was studied by means of ESR, Hall and conductance measurements for three groups of each type of sample—control group, and those annealed at T =960 and 1180 °C. It is found, according to Hall measurements, that holes are the majority charge carriers in NPC. The concentration of free carriers is found to be rather high (about 10 19 –10 21 cm −3 ). The ESR spectrum has the so-called ‘Dyson line shape’ with a large signal asymmetry. Two groups of carriers with strongly different electronic and magnetic properties are found. The values of the g -factor and their variability are related to the existence of two different kinds of particles: small carbon nanoclusters (1–3 nm) and large-scale carbon skeleton structural elements (up to 1000 nm) of complex shape, where the holes move along graphite-like sp 2 bonds. The temperature dependence of the integral intensities shows the existence of two non-separable spin subsystems—charge carriers and localised spins. A role of oxygen atoms in the electronic system of NPC is studied.

The transformation of the spectrum of plasma oscillations with the zero reduced wave vector in the spatially modulated two-dimensional electron system moving to the regime of isolated quasi-one-dimensional electron channels is theoretically investigated. The results provide an explanation of the well-known experimental observations of the plasma resonance transformation when a two-dimensional electron system crosses the continuity violation threshold.

A complex of studies of the physical properties of thin lanthanum manganite La0.67Ca0.33MnO3 films were performed for a monolithic layered structure consisting of a LiNbO3 substrate and a La0.67Ca0.33MnO3 thin epitaxial film. For the first time, not only ordinary acoustoelectric (AE) current of charge carriers dragged by a surface acoustic wave, but also longitudinal anomalous AE current, which flowed in a distinguished direction independent of the direction of the surface acoustic wave propagation, was observed. The anomalous AE effect predominated close to the metal-insulator transition, whereas the odd AE effect predominated at high and low temperatures. The sign of the ordinary odd AE effect corresponded to hole conduction of the film. A theoretical analysis showed that the anomalous AE effect is due to a strong film conduction modulation caused by deformation created by the surface acoustic wave. The theoretical results were in close agreement with experiment. The temperature dependences of resistivity ρ(both in the absence of a magnetic field and in fields of up to 3 T), high-frequency magnetic susceptibility, and thermoelectric power were studied.

This paper reports on a study of the temperature dependences of the electrical resistivity, Hall coefficient, and thermopower of nanoporous carbon prepared from polycrystalline carbides (α-SiC, TiC, Mo2C) and 6H-SiC single crystals in the temperature range 1.5–300 K. The structural units responsible for the character of charge transport in these materials are carbon nanoclusters measuring ∼10–30 A. The conductivity in all the samples studied was found to be p type with a high carrier concentration (nh ∼ 1020 cm−3). The behavior of the transport coefficients at low temperatures is discussed.