Quantum Tunneling of Charge-Density Waves in Quasi-One-Dimensional Conductors (Trichalcogenide, Tantalum Trisulfide, Niobium-Triselenide)
Miller, John Harris, Jr.
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https://hdl.handle.net/2142/69309
Description
Title
Quantum Tunneling of Charge-Density Waves in Quasi-One-Dimensional Conductors (Trichalcogenide, Tantalum Trisulfide, Niobium-Triselenide)
Author(s)
Miller, John Harris, Jr.
Issue Date
1985
Department of Study
Electrical Engineering
Discipline
Electrical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Abstract
The charge-density wave (CDW) dynamics of the linear chain compound orthorhombic TaS(,3) is characterized by extensive measurements of dc conductivity, ac admittance, direct mixing, harmonic mixing, second harmonic generation, and third harmonic generation as functions of dc bias voltage, applied frequencies, and, in some cases, the amplitude of an additional ac signal. Measurements of the direct and harmonic mixing responses of NbSe(,3) are also reported.
The results are analyzed in terms of an extension of the tunneling theory of CDW depinning, proposed by John Bardeen, coupled to the theory of photon-assisted tunneling (PAT). Where possible, the results are also compared with predictions of the classical overdamped oscillator model of CDW transport.
The tunneling model is shown to provide a complete and semiquantitative interpretation of the entire small-signal ac dynamics at megahertz frequencies, using only the measured dc I-V curve and an experimentally inferred frequency-voltage scaling parameter, and also accounts for much of the large-signal behavior studied thus far. The observation of both an induced ac harmonic mixing current and a third harmonic generation current whose amplitudes peak at output frequencies far below the measured "cross-over frequency" for ac conductivity agrees with the phenomenological tunneling model, but is in serious disagreement with the classical overdamped oscillator model of CDW motion. Furthermore, the absence of any observed quadrature component in the harmonic mixing response, even though the measured linear response at the applied frequencies has substantial frequency-dependent in-phase and quadrature components, is probably impossible to reconcile with any classical theory. The results reported here thus provide compelling evidence in favor of collective, coherent quantum tunneling as the mechanism of charge-density wave depinning, and indicate that macroscopic quantum effects are observed in the megahertz frequency region at temperatures as high as 200 K.
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