## Dielectric properties of multiband electron systems: II- Collective modes (1996)

### BibTeX

@MISC{Zupanović96dielectricproperties,

author = {P. ˇ Zupanović},

title = {Dielectric properties of multiband electron systems: II- Collective modes},

year = {1996}

}

### OpenURL

### Abstract

Starting from the tight-binding dielectric matrix in the random phase approximation we examine the collective modes and electron-hole excitations in a two-band electronic system. For long wavelengths (q → 0), for which most of the analysis is carried out, the properties of the collective modes are closely related to the symmetry of the atomic orbitals involved in the tight-binding states. In insulators there are only inter-band charge oscillations. If atomic dipolar transitions are allowed, the corresponding collective modes reduce in the asymptotic limit of vanishing bandwidths to Frenkel excitons for an atomic insulator with weak on-site interactions. The finite bandwidths renormalize the dispersion of these modes and introduce a continuum of incoherent inter-band electronhole excitations. The possible Landau damping of collective modes due to the presence of this continuum is discussed in detail. 1 In conductors the intra-band charge fluctuations give rise to plasmons. If the atomic dipolar transitions are forbidden, the coupling of inter-band collective modes and plasmons tends to zero as q → 0. On the contrary, in dipolar conductors this coupling is strong and nonperturbative, due to the long range monopole-dipole interaction between intra-band and inter-band charge fluctuations. The resulting collective modes are hybrids of intra-band plasmons and inter-band dipolar oscillations. It is shown that the frequency of the lower hybridized longitudinal mode is proportional to the frequency of the transverse dipolar mode when the latter is small. The dielectric instability in a multi-band conductor is therefore characterized by the simultaneous softening of a transverse and a longitudinal mode, which is an important, directly measurable consequence of the present theory. PACS: 71.45.-d Key words: collective modes, Landau damping, dielectric instability

### Citations

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Classical Electrodynamics
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- 1975
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Nuovo Cimento 9
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Concepts in Solids
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- 1963
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Citation Context ...light generalization to the band structure which has a cubic symmetry [1], these expressions reproduce completely the longitudinal and the transverse excitonic collective branches for a cubic crystal =-=[5, 6]-=-. Moreover, they include two distinct types of dipolar terms treated separately in Ref.[5], i.e. those representing the simultaneous excitation and deexcitation of two atomic electron-hole pairs and t... |