High-frequency longitudinal oscillations of the quasi-two-dimensional electron liquid

The specific character of longitudinal collective electromagnetic oscillations in a layered conductor with the quasi-two-dimensional electron energy spectrum has been analyzed.

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Дата:2012
Автори: Gokhfeld, V.M., Kirichenko, O.V., Peschansky, V.G.
Формат: Стаття
Мова:English
Опубліковано: Інститут фізики конденсованих систем НАН України 2012
Назва видання:Condensed Matter Physics
Онлайн доступ:http://dspace.nbuv.gov.ua/handle/123456789/120169
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Цитувати:High-frequency longitudinal oscillations of the quasi-two-dimensional electron liquid / V.M. Gokhfeld, O.V. Kirichenko, V.G. Peschansky // Condensed Matter Physics. — 2012. — Т. 15, № 1. — С. 13704: 1-7. — Бібліогр.: 21 назв. — англ.

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spelling irk-123456789-1201692017-06-12T03:03:38Z High-frequency longitudinal oscillations of the quasi-two-dimensional electron liquid Gokhfeld, V.M. Kirichenko, O.V. Peschansky, V.G. The specific character of longitudinal collective electromagnetic oscillations in a layered conductor with the quasi-two-dimensional electron energy spectrum has been analyzed. Проаналiзовано специфiку поздовжнiх колективних електромагнiтних коливань у шаруватому провiдни-ку з квазi-двовимiрним електронним енергетичним спектром. 2012 Article High-frequency longitudinal oscillations of the quasi-two-dimensional electron liquid / V.M. Gokhfeld, O.V. Kirichenko, V.G. Peschansky // Condensed Matter Physics. — 2012. — Т. 15, № 1. — С. 13704: 1-7. — Бібліогр.: 21 назв. — англ. 1607-324X PACS: 71.20.-r DOI:10.5488/CMP.15.13704 arXiv:1204.5997 http://dspace.nbuv.gov.ua/handle/123456789/120169 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
language English
description The specific character of longitudinal collective electromagnetic oscillations in a layered conductor with the quasi-two-dimensional electron energy spectrum has been analyzed.
format Article
author Gokhfeld, V.M.
Kirichenko, O.V.
Peschansky, V.G.
spellingShingle Gokhfeld, V.M.
Kirichenko, O.V.
Peschansky, V.G.
High-frequency longitudinal oscillations of the quasi-two-dimensional electron liquid
Condensed Matter Physics
author_facet Gokhfeld, V.M.
Kirichenko, O.V.
Peschansky, V.G.
author_sort Gokhfeld, V.M.
title High-frequency longitudinal oscillations of the quasi-two-dimensional electron liquid
title_short High-frequency longitudinal oscillations of the quasi-two-dimensional electron liquid
title_full High-frequency longitudinal oscillations of the quasi-two-dimensional electron liquid
title_fullStr High-frequency longitudinal oscillations of the quasi-two-dimensional electron liquid
title_full_unstemmed High-frequency longitudinal oscillations of the quasi-two-dimensional electron liquid
title_sort high-frequency longitudinal oscillations of the quasi-two-dimensional electron liquid
publisher Інститут фізики конденсованих систем НАН України
publishDate 2012
url http://dspace.nbuv.gov.ua/handle/123456789/120169
citation_txt High-frequency longitudinal oscillations of the quasi-two-dimensional electron liquid / V.M. Gokhfeld, O.V. Kirichenko, V.G. Peschansky // Condensed Matter Physics. — 2012. — Т. 15, № 1. — С. 13704: 1-7. — Бібліогр.: 21 назв. — англ.
series Condensed Matter Physics
work_keys_str_mv AT gokhfeldvm highfrequencylongitudinaloscillationsofthequasitwodimensionalelectronliquid
AT kirichenkoov highfrequencylongitudinaloscillationsofthequasitwodimensionalelectronliquid
AT peschanskyvg highfrequencylongitudinaloscillationsofthequasitwodimensionalelectronliquid
first_indexed 2025-07-08T17:21:38Z
last_indexed 2025-07-08T17:21:38Z
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fulltext Condensed Matter Physics, 2012, Vol. 15, No 1, 13704: 1–7 DOI: 10.5488/CMP.15.13704 http://www.icmp.lviv.ua/journal High-frequency longitudinal oscillations of quasi-two-dimensional electron liquid V.M. Gokhfeld1, O.V. Kirichenko2, V.G. Peschansky2 1 Donetsk Institute for Physics and Engineering named after O.O. Galkin of the National Academy of Sciences of Ukraine, Donetsk, Ukraine 2 B.I. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, Kharkiv, Ukraine Received August 29, 2011, in final form January 30, 2012 The specific character of longitudinal collective electromagnetic oscillations in a layered conductor with the quasi-two-dimensional electron energy spectrum has been analyzed. Key words: layered conductors, fermi-liquid, longitudinal oscillations PACS: 71.20.-r 1. Introduction Numerous low-dimensional conductors differ by their layered structure and highly anisotropic elec- trical conductivity. Their in-plane conductivity is much higher than that in perpendicular direction, i.e. along the OZ -axis. Such an anisotropy is typical of cuprates, e.g. YBaCuO in a nonsuperconducting phase, transition metal dichalcogenides (NbSe2, TaS2), graphite and its intercalates in particular, as well as a broad family of tetrathiafulvalene salts of (BEDT-TTF)2I3 type [1–5]. A common feature of many layered conductors is their weak electron-energy dependence on the momentum projection pz . This is due to the fact that their interlayer distance a is much higher than the in-plane crystal lattice parameter. Then, the wave functions of appropriate electrons weakly overlap, and the energy of single-particle charged excitations may be represented by rapidly converging series ε(p) = ∞∑ n=0 εn (p⊥)cos anpz ħ , (1.1) where the functions εn decrease with increasing n and ε1(p⊥) is much less than ε0(p⊥). Here ħ is the Planck constant, p⊥ = [pn] is the momentum projection onto the layer plane. In conductors with high charge carriers density allowance for the electron-electron interaction is of great importance. The system of quasiparticles carrying a charge should be regarded as the Fermi liquid. According to the Landau-Silin theory [6, 7], the interaction between quasiparticles may be taken into account in the form of self-consistent field. In this case, the quasiparticle energy depends on the distribution function n(p,r, t) for the other quasiparticles. As a result, the energy of an electron acquires the correction δε to the dispersion law (1.1) δε= ∫ 2d3p ′ (2πħ)3 L(p,p′)n1(p′,r, t), (1.2) where n1 is the correction to the equilibrium Fermi distribution function n0(ε). In quasi-two-dimensio- nal conductors, the Landau correlation function L(p,p′), as well as the quasiparticle energy in the “gas” approximation (1.1), depends weakly on pz . © V.M. Gokhfeld, O.V. Kirichenko, V.G. Peschansky, 2012 13704-1 http://dx.doi.org/10.5488/CMP.15.13704 http://www.icmp.lviv.ua/journal V.M. Gokhfeld, O.V. Kirichenko, V.G. Peschansky The specificity of quasi-two-dimensional electron energy spectrum provides the most favorable con- ditions for a detailed study of the electron-electron Fermi liquid correlations. If the initial dispersion law (1.1) is unknown, investigation of nonequilibrium processes in the electron system in a stationary and uniform external field does not provide novel information about electron-electron Fermi liquid cor- relations. The solution of the inverse problem of restoring the electron energy spectrum from experi- mental data makes it possible to determine the dependence of the electron energy ε̃(p) = ε(p)+δε(p) renormalized by the Fermi liquid interaction, at the Fermi surface only. However, wave processes are very sensitive to the form and magnitude of the Fermi liquid correla- tions. The gas approximation does not provide for propagation of the spin waves in nonmagnetic metals, predicted by Silin [8], as well as for a series of Fermi liquid effects. Spin waves and transverse electro- magnetic waves in the Fermi liquid have been studied in detail not only in metals (see, for example, the monograph [9, 10]) but also in low-dimensional conductors [11–17]. Here we consider longitudinal oscillations in the quasi-two-dimensional Fermi liquid. Following Vla- sov [18] and Landau [19] we should solve the set of equations including the Poisson equation div E = 4πρ′, (1.3) the continuity equation ∂ρ′ ∂t +div j = 0 (1.4) and the constitutive equation relating the electric field density J to the electric field E ∼ exp(−iωt) with allowance for the Fermi liquid correlations. For the sake of brevity in calculations, we shall confine ourselves to the simplest case where the elec- tron dispersion law contains only zero and first harmonics and the expression (1.1) takes the following form: ε(p) = ε0(p⊥)+ t⊥ cosθ. (1.5) Here ε0(p⊥) is an arbitrary function, t⊥ is the constant value, θ = apzħ−1. We consider the case where the electric field and the wave vector k are parallel to the normal to the layers, i.e. to the hard direction for the electric current. We shall derive the dielectric function with allowance for the Fermi liquid interaction (FLI), ascertain the dispersion law for longitudinal plasma oscillations and find out the distribution of the high-frequency electric field in a half-infinite sample. We shall also consider low-frequency collective excitations in a layered conductor which are possible in the presence of two bands of the type (1.5) in its electron spectrum. 2. Dielectric function In order to determine the relation between the current density j = ∫ 2d3p (2πħ)3 e ∂ε̃ ∂p n(p,r, t) (2.1) and the electric field of the wave, it is necessary to solve the kinetic equation ∂n ∂t + ∂n ∂r dr dt + ∂n ∂p dp dt = Ŵcoll{n} . (2.2) Naturally, the collision operator Ŵcoll{n} vanishes working on the equilibrium Fermi function n0(ε̃)= [ 1+exp ( ε̃−µ T )]−1 (2.3) which depends on the energy ε̃ = ε(p)+δε(p,r, t) renormalized by the Fermi liquid correlations. In the case of a weak perturbation of the system of charge carriers, i.e. when the value of the electric field of the wave is small, the collision integral is the linear integral operator working on the function n1(p,r, t) = n(p,r, t)−n0(ε̃)=−Φ(p,r, t) ∂n0(ε̃) ∂ε̃ . (2.4) 13704-2 High-frequency longitudinal oscillations We shall confine ourselves to the τ-approximation for the collision integral, where Ŵcoll corresponds to the operator of multiplication nonequilibrium part of the distribution function for conduction elec- trons by the the frequency of their collisions 1/τ: Ŵcoll{n} = 1 τ Φ(p,r, t) ∂n0(ε̃) ∂ε̃ . (2.5) In the kinetic equation, it should be taken into account that besides the electric field, conduction electrons experience the force of the self-consistent field of interacting quasiparticles dp dt = F = eE− ∂ ∂r δε(p,r, t). (2.6) The current density depends only on the nonequilibrium part of the distribution function of charge carriers j(r, t) =−e ∫ m∗dε ∫ dθ ∫ dϕ ∂n0(ε) ∂ε vΦ(ε,θ,ϕ,r, t) 1 2π3ħ2a . (2.7) Here m∗ is the cyclotron effective mass, ϕ is the variable of integration over charge carriers states with constant values of energy and momentum projection on the normal to the layers, v = ∂ε(p)/∂p. The kinetic equation for the case under consideration takes the form kvzΦ−ωΨ+ ieEvz = i Φ τ , (2.8) where Ψ=Φ−δε, the relaxation time τ is supposed to be sufficiently large. Within the chosen model for energy spectrum (1.5), electron velocity across the layers vz = v1 sin(θ), with v1 =−at⊥ħ−1, is dependent on θ only andmuch less than the characteristic Fermi velocity vF for the motion along the layers. Then, in order to find out the dielectric permeability and finally the spectrum of longitudinal vibrations, the averaging over φ of the nonequilibrium distribution function of the charge carriers along with the Lanadau-Silin correlation function is sufficient. In accordance with the symmetry of the problem, the Landau correlation function L(p,p′), which connects the effective and the real distribution of charge carriers, may be represented in the form L(θ,θ′) = L0 +2L1 cos(θ−θ′). (2.9) In this case Φ(θ) =Ψ(θ)+ π∫ −π dθ′ 2π L(θ,θ′)Ψ(θ′) (2.10) and one can easily solve the equations (2.8)–(2.10). In particular, for Ψ0 = 〈Ψ〉/〈1〉, averaging over θ we have Ψ0 = ieE k ω̃W ω+W [ iτ−1 + ω̃ ( L0 +λω2/k2v2 0 )] , (2.11) where W (k,ω) = 〈 kvz kvz + ω̃ 〉 〈1〉−1 = 1− ω̃ √ ω̃2 −k2v2 1 , (2.12) λ= 2L1 (1+2L1) , ω̃=ω+ i τ , 〈1〉 = 2 (2πħ)3 ∫ ∂n0 ∂ε d3p . With regard to (2.7) and (2.8), the dielectric function ǫ= 1+4π〈Ψ〉/ikE takes the form ǫ(k,ω) = 1+ κ2 k2 ω̃W ω+W [ iτ−1 + ω̃ ( L0+λω2 k2v2 0 )] , (2.13) where κ2 = 4πe2〈1〉 = 4m2e2 aħ2 13704-3 V.M. Gokhfeld, O.V. Kirichenko, V.G. Peschansky is the square of the decrement of the static screening in the gas approximation. As is easily seen from (2.13) and the dispersion equation ǫ(k,ω) = 0, the decrement with allowance for FLI equals κ (p 1+L0 )−1 . For the activation frequency of the plasma oscillations we have ω2 p = 2me2v2 1 aħ2 (1+L1) = η 4πNe2 m (1+L1) , (2.14) where η = (v1/vF)2. Due to the smallness of η, the plasma frequency in the quasi-two-dimensional con- ductor essentially reduces in comparison with the case of a conventional metal. Making use of the for- mula (2.14) we obtain the dispersion law for longitudinal plasmons: ω2(k) = ω2 p λ   k2 2κ2 −K +λK + √( K − k2 2κ2 )2 +λK k2 κ2   , (2.15) K (k) = 1+ (1+L0)k2 κ2 . When the Landau correlation function has only a zero harmonic, the dispersion relation becomes notice- ably simpler: ω(k) = ev1 ħ √ 2m a κ2 + (1+L0)k2 κ √ κ2 + (1/2+L0)k2 . (2.16) 3. Distribution of the electric field in a specimen Having calculated the dielectric function for the infinite specimen, we can solve the boundary prob- lem for the half-space z Ê 0. Landau [19] was the first to consider the penetration of the high-frequency longitudinal electric field into the Maxwell plasma. Propagation of the longitudinal waves in a degener- ated isotropic conductor was studied in [20] where a detailed analysis of the boundary conditions for an electron distribution function was made. In the layered conductor, the angles of incidence of electrons on the boundary do not exceed v1/vF ≪ 1, which allows us to suppose that charge carriers are reflected specularly. The electric field outside the conductor (i.e. between the capacitor plates, one of which is the specimen) is supposed to be given by E (z É 0) = (0,0,E0 exp(−iωt)). Then, the field distribution in the specimen is described by the following expression E (z)= E0 iπ ∞∫ −∞ dk kǫ(k,ω) exp(ikz). (3.1) The k = 0 pole residue gives the field value in the bulk of the conductor E (∞) which is much less than E0, if ω≪ωp : E (∞) = E0 ǫ(k,ω) , ǫ(0,ω) = 1− ω2 p ω(ω̃+ iL1/τ) . (3.2) However, at ωτ≫ 1, the field attenuates in a nonmonotonous manner because the integral (3.1) contains an oscillating component E1(z) =−4E0F 2 π ∞∫ 1 dxx p x2 −1exp(ik1xz) ( x2 −1 ) [ 2+ (1+L0)F 2x2 ]2 + ( 2+L0F 2x2 )2 (3.3) originating from the branching point k1 = ω̃/v1. At z ≫ v1/|ω̃|, the ratio of the oscillating component to E (∞) takes the form E1(z) E∞ = 1−F 2 p 2π(1+L0F 2/2)2 ( v1 ω̃z )3/2 exp(izω̃/v1). (3.4) Here F = ω̃/ωp. In the layer of the order of the electron free path lz = v1τ, the field oscillates with the period 2πv1/ω. In the layered conductor, both scale lengths reduce in comparison with the isotropic metal, remaining, however, microscopic for non-high frequencies ω and τ−1. 13704-4 High-frequency longitudinal oscillations For the sake of completeness, the resonance case ω=ωp should be considered. At such high frequen- cies, the direct stimulation of the monochromatic field is hard to achieve, but the character of the spatial distribution of the resonance harmonics may become apparent in the pulsed mode, as well as in the experiments with electron beams. Near the plasma frequency ωp, the radicals in the expression for the dielectric function may be ex- panded into a series about small k2, and in the main approximation we have ǫ(0,ω) = 1− ω2 p ω(ω̃+ iL1/τ) , |ǫ(0,ω)|≪ 1. (3.5) 4. The spectrum of the short-wave plasmons Consider the case of large values of k in collisionless limit. Strictly speaking, this is the case where the quasiclassical formula for the conductivity σ(k,ω) =−ie2 〈 v2 z kv− ω̃ 〉 (4.1) is inappropriate because it is not invariant with respect to translation k → k + 2π/a. The translation- invariant dielectric function ǫ(k) = 1+4πiσ(k)/ω can be easily constructed if we note that the magnitude ħkv is the result of the expansion in small k of the energy difference in the quantum perturbation theory formulas. In the case of the spectrum (1.5), this difference is ε ( p+ħk 2 ) −ε ( p−ħk 2 ) =ħvz ( 2 a ) sin ( k a 2 ) , i.e., in (4.1) k should be replaced by Q = (2/a)sin(ka/2). As a result, we have ǫ(k,ω) = 1+ κ2 Q2  1− ω √ ω2 −Q2v2 1   . (4.2) Usually, the application of the quasiclassical approximation for the calculation of the plasmon spectrum is limited by the condition ħk ≪ pF,ħ/a (see, for example, [21]). In the case under consideration, this restriction is of no necessity because the dispersion law (1.5) is determined everywhere in the Brillouin zone. From (4.2) we obtain: ω(k) =ωp0 √ 2 b b + sin2(ka/2) √ 2b + sin2(ka/2) , (4.3) where ωp0 = (ev1/ħ) p 2m/a, b = (κa)2/4 = ane2/ħ2 i.e., the ratio of the lattice period to the Bohr radius. As far as Bohr radius (at m ≃ m0) is of the order of 10−8 cm, the parameter b may be very large, especially in artificial superlattices with great separation between conducting layers. In this case, the spectrum (4.3) corresponds to a narrow band ωmax p ω0 −1= b +1 p b2 +b/2 −1 ≃ { 3/4b (b ≫ 1)p 2/b (b ≪ 1) (4.4) and the static screening decrement κ is replaced by κ= 2 a arsinh (p b ) . (4.5) It is easily seen that the results obtained above are valid in an external magnetic field applied across the layers, because it has no effect on the electron velocity along the normal to the layers for the energy spectrum under discussion. 13704-5 V.M. Gokhfeld, O.V. Kirichenko, V.G. Peschansky 5. Two-band model The Fermi surface is supposed to be singly connected. However, the electron structure of “synthetic metals” is quite complicated. It is probable that there exist two charge carrier groups (for example, elec- trons and holes) of the type (1.5) with the different values Vzmax : v (2) 1 = B v (1) 1 and B Ê 1. In the case of two charge carrier groups in an isotropic metal FLI (even in its simplest form L(p,p′) = Lα,β, α,β,= 1,2), there appears a longitudinal collective mode of the type of zero sound at sufficiently low frequencies, τ−1 ≪ω≪ωp. Its phase velocity V = uv (2) 1 satisfies the characteristic equation D(k) = ∑ ζαWα = LW1W2 = ζ1W (Bu)+ζ2W (u)+LW (Bu)W (u) = 0, (5.1) where ζα = mα m1 +m2 , L = ζ1ζ2 (L11 +L22 −L12 −L21) and W (u) = W (ω̃/kv (2) 1 ) is given by the formula (2.12). The equation (5.1) has a real root (u > 0) only at sufficiently intensive FLI, namely at L Ê Lmin(B) =−ζ2/W (B). In the quasi-two-dimensional metal, the threshold value Lmin is much less than that in the isotropic metal for the same B , especially near B = 1. In this case Lmin = 0, and the solution of the equation (5.1) is u(L) = 1+L p 1+2L . (5.2) The wave under consideration corresponds to antiphase partial oscillations of the electron densities for both charge carrier groups. The elastic force generated by non-equilibrium charge carrier distribu- tion allows one to detect the electron zero sound by means of the concomitant elastic wave. This effect was observed in a series of ordinary metals (W, Al, Ga) [21]. It may be described by jointly solving the electron kinetic equation and the elastic theory equation for the half-space with the given oscillation of the boundary u0 exp(−iωt). As a result, for the displacement of ions in the k-representation, we obtain U (k) = 2U0 ik [ 1+ ω2 s2k2(1+R)−ω2 ] , (5.3) R(k) = ωΛ2〈1〉 ω̃ρs2 ζ1ζ2(1+L) [ 1−W1W2 1+L D(k) ] , where ρ is the mass density, s is the velocity of the longitudinal sound. We assume that s ≪ v1. In this case, U (k) has two different poles: the pole ks ≃ ω/s is connected to the ordinary sound; the other one, k1s , is close to the root of the equation (5.1) and describes an extra elastic wave. It is easy to see that in the degenerated case (v (1) 1 = v (2) 1 = v) we have: U0s ≃U0 ωΛ2〈1〉 ω̃ρv2 1 ζ1ζ2 2L 1+L exp [ iω̃z p 1+2L v1(1+L) ] . (5.4) This expression is proportional to v−2 1 instead of v−2 F in an ordinary metal. 6. Conclusions The analysis given above shows that the electrodynamic characteristics of the layered conductor es- sentially differ from those of the isotropic metal with the same charge carrier density. In particular, the activation frequency and the velocity of the longitudinal waves, propagating along the normal to the lay- ers, decrease essentially. Moreover, the case of quasi-two-dimensional charge carrier spectrum is most favorable in observing the electron zero sound and in studying the electron phenomena in various mod- ifications of carbon. 13704-6 High-frequency longitudinal oscillations References 1. Wosnitza J., Fermi Surfaces of Low-Dimensional Organic Metals and Superconductors. In: Springer Tracts in Modern Physics, Springer, 1996. 2. Singelton J., Rep. Prog. Phys., 2000, 63, 1111; doi:10.1088/0034-4885/63/8/201. 3. Kartsovnik M.V., Chem. Rev., 2004, 104, 5737; doi:10.1021/cr0306891. 4. Kartsovnik M.V., Peschansky V.G., Fiz. Nizk. Temp., 2005, 31, 249 [Low Temp. Phys., 2005, 31, 185–202; doi:10.1063/1.1884422]. 5. The Physics of Organic Superconductors and Conductors, ed. by A.G. Lebed, Springer Series in Material Sciences, Springer Verlag, Berlin, 2008. 6. Landau L.D., Zh. Eksp. Teor. Fiz., 1957, 32, 59 [Sov. Phys. JETP, 1957, 5, 40]. 7. Silin V.P., Zh. Eksp. Teor. Fiz., 1957, 33, 495 [Sov. Phys. 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B, 2005, 71, 045304; doi:10.1103/PhysRevB.71.045304. 17. Kirichenko O.V., Peschansky V.G., Stepanenko D.I., Condens. Matter Phys., 2005, 8, 835. 18. Vlasov A.A., Zh. Eksp. Teor. Fiz., 1938, 8, 201. 19. Landau L.D., Zh. Eksp. Teor. Fiz., 1946, 16, 574. 20. Gokhfeld V.M., Kaganov M.I., Lyubarskii G.Ya., Zh. Eksp. Teor. Fiz., 1987, 92, 523 [Sov. Phys. JETP, 1987, 65, 423]. 21. Bezugly E.V., Burma N.G., Deineka E.Yu., Fil’ V.D., Sverkhprovodimost’, 1991, 4, 661 (in Russian). Високочастотнi поздовжнi осциляцiї квазi-двовимiрної електронної рiдини В.М. Гохфельд1,2, О.В. Кириченко2, В.Г. Пiщанський2 1 Донецький фiзико-технiчний iнститут iм. А.А. Галкiна, Донецьк, Україна 2 Фiзико-технiчний iнститут низьких температур iм. Б.I. Вєркiна НАН України, Харкiв, Україна Проаналiзовано специфiку поздовжнiх колективних електромагнiтних коливань у шаруватому провiдни- ку з квазi-двовимiрним електронним енергетичним спектром. Ключовi слова: шаруватi провiдники, фермi-рiдина, поздовжнi коливання 13704-7 http://dx.doi.org/10.1088/0034-4885/63/8/201 http://dx.doi.org/10.1021/cr0306891 http://dx.doi.org/10.1063/1.1884422 http://dx.doi.org/10.1063/1.593703 http://dx.doi.org/10.1063/1.1374724 http://dx.doi.org/10.1134/1.1625734 http://dx.doi.org/10.1134/1.1854813 http://dx.doi.org/10.1103/PhysRevB.71.045304 Introduction Dielectric function Distribution of the electric field in a specimen The spectrum of the short-wave plasmons Two-band model Conclusions