The current noise properties of a triphenyl molecule bonded between two gold electrodes were studied ~ to determine the suitability of the system as a terahertz radiation source. The system was modelled as a quantum mechanical oscillator coupled to a tunnel junction, and the equation of motion (EOM) for the density matrix of the rotating middle ring of the triphenyl molecule (with the two end-rings frozen), was derived. Weak coupling between the rotating ring and the tunnelling electrons was assumed, as well as a high bias. The EOM allowed the subsequent derivation of an expression for the current noise of the molecular system, which was then analysed numerically. Peaks in the current noise spectrum were observed at 0, 2 and 4 times the rotational frequency of the middle ring, twice the value normally observed for linear coupling between the two systems, a result of the quadratic coupling that was in place. Peak widths of the two lowest frequencies were observed to be narrow, producing a clean signal that is essential for terahertz sources, while the peak heights demonstrated a quadratic dependence on the applied bias V. Investigation of the signal-to-noise ratio (SNR) showed that SNR increased as V increased in magnitude. The SNR was observed to be sizeable for the two lowest frequencies, further illustrating the potential for the triphenyl molecule to be utilised as a terahertz source. The dependence of the SNR and average current on the ratio of the coupling parameters was also investigated, showing that for a fixed ratio of the parameters, where they individually varied by a rescaling factor, the SNR remained constant while the current scaled as the square of the rescaling factor. As the ratio was varied, the SNR and current increased as the magnitude of the ratio tended to zero.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:557615 |
| Date | January 2012 |
| Creators | Harper, C. L. |
| Publisher | Queen's University Belfast |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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