庫倫交互作用與電子躍遷效應對串接耦合量子點熱電特性的影響

Effects of Coulomb blockade and interdot hopping on the thermoelectric properties of serially coupled quantum dots

熊子綱(Tzu Kang)

指導教授 ： 郭明庭

國立中央大學/資訊電機學院/電機工程學系/碩士(2012年)

摘要

在這篇論文裡，我們理論性地利用雙能階安德生模型探討了一個串接耦合量子點(由內含雙量子點的奈米線連接金屬電極所構成)的熱電特性，在庫倫阻塞區域內的電流及熱流公式可以由凱帝旭格林函數的技巧計算求得。我們探討了底下效應對系統熱電優值的影響: 1)量子點間電子跳躍強度以及庫倫交互作用，2)量子點能階位於電極費米能階以上或以下時 。我們也探討了席貝克係數(Seebeck coefficient)隨溫度變號的現象。當量子點能階位在費米能階以上時，ZT值的最大值會因庫倫交互作用而受壓抑;而當量子點能階位在費米能階以下時，最大ZT值與電子庫倫交互作用有關。而在考量聲子熱導時，最佳ZT值較容易發生於量子點間電子跳躍強度大於電極與量子點的穿隧率之處。我們證明了ZT不是電子跳躍強度的單調遞增函數。除此之外，當量子點能階位在費米能階以上時(遠離費米能階)電子庫倫交互作用對席貝克係數的影響並不大，而當量子點能階位在費米能階以下時，我們發現席貝克係數的變號與溫度有關，這意味著我們可以透過溫度的控制來調整系統的雙極效應。

關鍵字

電子躍遷效應，庫倫交互作用，熱電，費米能階，席貝克係數，格林函數，聲子熱導，電子熱導

並列摘要

In this thesis, we theoretically investigate the thermoelectric properties of a serially coupled quantum dot system (double quantum dots embedded in a nanowire connected to metallic electrodes ) by a two-level Anderson model. The charge and heat currents in the Coulomb blockade regime are calculated by the Keldysh-Green function technique. We study the following effects on the figure of merit (ZT) of system:1) electron interdot hopping strengths (t_AB) and Coulomb interactions, and 2) quantum dot energy levels above and below the Fermi energy (E_F) of electrodes. We also study the sign variation of Seebeck coefficient with respect to equilibrium temperature. When QD energy levels are aboveE_F , the maximum ZT is suppressed by the electron Coulomb interactions. When QD energy levels are below EF, the maximum ZT is attributed to the electron Coulomb
interactions. The optimization of ZT prefers that the interdot electron hopping strengths are larger than electron tunneling rates arising from the coupling between the QDs and the electrodes in the presence of phonon thermal conductance. We demonstrate that ZT is not a monotonic increasing function of interdot electron hopping strength (t_AB). In addition, the Seebeck coefficient is not sensitive to the electron Coulomb interactions when QD energy levels are above E_F (far away from E_F). When QD energy levels are below E_F, we find the sign changed in the Seebeck coefficient with respect to temperature, which indicates that we can manipulate temperature to control the bipolar effect of junction system.

並列關鍵字

interdot electron hopping ； thermoelectric ； electron thermal conductance ； phonon thermal conductance ； Coulomb interaction ； Fermi level ； Seebeck coefficient

參考文獻

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