Charge density waves pinning

Fig. 33. Far infrared to uv reflectivity of K2Pt(CN)4Bro.3o(H20)8 for light polarized parallel to the conducting axis. The dashed line is for the sample at 300°K, the solid line 40 K. The low frequency structure (50cm i) at 40 °K is assigned to the response of a pinned charge density wave (pinned Frohlich mode) (72).

Optical study indicates that at low temperatures the low-energy electronic properties of some organic metal-like conductors (e.g., TTF-TCNQ) are dominated by charge density wave (CDW) effects. Frequency-dependent conductivity of TTF-TCNQ, obtained from the IR reflectance, at 25 K displays a double-peak structure with a low-frequency band near 35 cm-1 and a very intense band near 300 cm-1 [45]. The intense band may be ascribed to single-particle transitions across the gap in a 2kF (Peierls) semiconducting state, while the 35-cm-1 band is assigned to the Frohlich (i.e., CDW) pinned mode. Low-temperature results based on the bolometric technique [72,73] (Fig. 15) confirm the IR reflectance data. Such a con-... 

The concept of an elastic coherence length was first introduced in the context of charge density waves [28,62] and flux line lattices in superconducters [262], The relevance of this concept for friction was suggested by Baumberger and Carol [54] The contact can be decomposed into pinning centers, also called Larkin domains, with a typical linear size which constitute the BK blocks introduced in the beginning of this section. 

Impurity Pinning of Charge Density Wave in the Peierls-Frohlich State 217... 

IMPURITY PINNING OF CHARGE DENSITY WAVE IN THE PEIERLS-FROHLICH STATE... 

The dynamics of impurity pinning of the charge density wave and the frequency dependence of conductivity are investigated in the one-dimensional Peierls-Frohlich state. 

All of the physical measurments point to the equivalence of all the platinum atoms (in a noninteger oxidation state) in a chain. The results of the numerous measurements on K2Pt(CN)4Bro.3(H20)s, demonstrates this system to be a one-dimensional metal undergoing a metal-insulator transition as the temperature is lowered. The far infrared and optical measurements show that the electronic excitation spectrum is not that of a simple one-dimensional metal but has a complex behavior at low frequencies. The available data from many diverse types of experiments have been analyzed in terms of numerous models. This system is currently best characterized in terms of a one-dimensional metal undergoing a Peierls transition to a semiconductor at low temperatures, with evidence for the presence of a pinned charge density wave. Further careful measurements of the partially oxidized tetracyanoplatinates are necessary to fully understand the applicability of various one-dimensional models to this class of materials. 

J. E. Eldridge and F. E. Bates, Far-infrared optical properties of semiconducting tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ), including the pinned charge-density wave. Phvs. Rev. B 28 6912 (1983). 

Similarly, in a metal/semiconductor junction, in which the metal work function lies between the valence band maximum and the conduction band minimum, the tails of the metallic wave function decrease exponentially in the adjacent semiconductor gap states, inducing transfer of charge, which in turn creates the dipole. The mismatch between the metal Fermi level and E is reduced by this dipole (by a factor inversely proportional to the semiconductor s optical dielectric constant, s o, in first approximation [51]). Only in the case of semiconductors with a large optical dielectric constant or high density of interface states is the Fermi level almost completely pinned at E- In this case, since E is an intrinsic property of the semiconductor, the Schottky barrier height of a particular semiconductor is independent of the metal (and its work function) utilized as contact. In order to define the CNL, one... 

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