Direct Measurement of One-Dimensional Plasmon Dispersion and Damping

Abstract

of remarkably reproducible thin potential wells and barriers, essentially rectangular and uniform to the order of a monolayer, can be created with molecular-beam epitaxy. The coupling behavior of the wells proves that synthetic superlattices can indeed be created. The molecularbeam-epitaxy technique for fabrication and the optical technique for energy-level determination should be applicable to additional configurations and compositions of interest for both basic and applied studies.We wish to acknowledge L. Kopf for the determination of the Al content of the layers and for expert technical assistance. We thank C. H. Henry and M. B. Panish for helpful discussions.Plasmons in the one-dimensional organic metal tetrathiafulvalene-tetracyanoquinodimethane were directly measured by high-energy Inelastic electron scattering in thin crystalline films at 300°K. For plasmons propagating along the conducting b axis the plasmon energy decreases from 0.75 to 0.55 eV and the width increases linearly as the plasmon momentum increases. Plasmons at 45° to b have an energy of 0.6 eV and show no dispersion.The organic charge-transfer salt tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) has been shown to have highly anisotropic electrical conductivity, and above 60°K has been characterized as a one-dimensional metal. ^ Since at high frequencies the room-temperature electronic behavior is certainly metallic in one dimension as established by the plasma edge reported in the normal-incidence reflectivity,^""* TTF-TCNQ provides an opportunity to study the elementary excitations of a one-dimensional electron gas. We report here the first known direct measurements of plasmon dispersion and damping in such a material. The experiment, per-formed at Princeton University, utilized high-energy inelastic-electron-scattering spectroscopy as the most sensitive and direct method to study dispersion and damping of plasma oscillations. NEpitaxial films of TTF-TCNQ (1000 A thick) were grown on the (100) cleaved face of NaCl and had the same bioriented nature as previously observed.^ That is, the film consisted of irregularly shaped regions (20 ]um across) within each of which the conducting b axis of the crystals point-ecj^long one of the two orthogonal [llO] directions in the face of the salt substrate. Our sample thus was equivalent to two TTF-TCNQ crystals at right angles to each other (both having the 1330

Collective mode dispersions of organic chain compounds

“…Further analyses of Williams and Bloch, as well as of Kahn et al, within the extended models [4,5] with two one-dimensional electron bands, each on a different type of chain, have predicted a second excitation. Williams and Bloch have found that the second coupled plasmon mode has been acoustic, but still both obtained modes have been placed above the electron-hole quasi-continuum in contrast to the experimental findings which did not resolve the two branches [2], whereas Kahn et al have shown that coupling between sublattice plasmons leads to the appearance of the acoustic collective mode in the narrow region between the quasi-continuums of two sublattices where its weak strength makes it unobservable in agreement with experiment.…”

“…Modes Ω 1 and Ω 2 can be directly associated with the optical data of TTF-TCNQ. Mode Ω 1 can be assigned to the excitation at ≈ 10 meV observed at 100K in the infrared measurements [6][7][8], while mode Ω 2 corresponds to the excitation at ≈ 0 75 eV [2]. The supporting reasons include the following: The measurements show [6,8] that the low-lying excitation at ≈ 10 meV disappears below 50 K, which is the temperature of the Peierls transition for TTF-TCNQ.…”

“…Early measurements of the optical properties of the quasione-dimensional organic conductor TTF-TCNQ [2] have shown a strong angular dependence of excitation at 0.75 eV which is in qualitative agreement with the dispersion of the plasmon mode predicted by Williams and Bloch within the simple model of a quasi-one-dimensional conductor with one one-dimensional electron band per chain in the random phase approximation (RPA) [3]. Further analyses of Williams and Bloch, as well as of Kahn et al, within the extended models [4,5] with two one-dimensional electron bands, each on a different type of chain, have predicted a second excitation.…”

Excitation spectrum of TTF-TCNQ: a finite temperature calculation

“…The approach has been verified in [6,7], by assigning the renormalised plasmon to the excitation at 10 meV [2][3][4], while the renormalised dipolar mode is assigned to the excitation at 0.75 eV [1]. According to this approach, the origin of such a combination of collective modes is a strong threedimensional dipole Coulomb electron-electron interaction which leads to a strong coupling of initial intraband plasmons and interband dipolar modes.…”

“…Collective modes as low energy renormalised plasmon and high energy renormalised dipolar modes are the consequence of electronic correlations. Experimentally, collective modes have been observed in this material by electron energy-loss spectroscopy (EELS) [1] and by optical measurements [2][3][4].…”