介觀及奈米尺寸結構中電子動力學及相互作用之研究 (II)

Abstract

本計畫擬研究四個介觀與奈米尺寸以及無序系統的基礎物理課題：（一）擬探討無序系統中的dynamical structural defects（或兩能級隧穿系統two-level tunneling systems）。我們計畫尋找適當的材料樣品，研究兩能級隧穿系統與導電電子耦合從而造成的低溫傳輸性質。兩能級隧穿系統常常造成類似（非磁性）近藤效應的物理現象，同時也可能造成電子相位相干時間的破壞，是凝態物理學中的一個極基礎而重要的多體物理課題。（二）擬研究金屬/絕緣體/金屬隧道結的低溫穿隧電流現象。在nano-granular金屬隧道結中，無序的影響會使得費米能級處的電子能態密度降低，因而造成微分電導（differential conductance或dI/dV）在零偏壓處有一個很明顯的極小值。但是當有（磁性或軌道）近藤效應存在時，零偏壓處的微分電導反而會有一個極大值。我們將在低溫下以及高磁場中進行測量，以深入研究產生極小或極大值微分電導的微觀物理機制；尤其將探討後者由近藤效應產生的dI/dV對溫度以及偏壓的scaling行為。（三）擬持續介觀系統中電子相位相干時間的量測，將繼續研究無序系統中的電子—聲子散射與電子—聲子—雜質干涉現象，探討該項干涉在介觀及奈米樣品中產生的新電、熱效應，並釐清退相干（dephasing）機制。（四）擬進行單根金屬及半導體奈米線的低溫電性與量子傳輸特性之量測，和新物理現象或非費米液行為之探索。 兩能級隧穿系統和電子退相干機制的研究往往需要在極低溫下和極高磁場中進行，為了進行超低溫的量測，本計畫擬加強與日本理化學研究所（RIKEN）的「低溫物理實驗室」的國際合作，使用其超大型3He-4He稀釋制冷機，以期得到最確切而無疑意的數據。同時開展交大—RIKEN—東京大學三方在垂直雙量子點之自旋依賴傳輸課題的合作研究。本計畫也將加強與理論學家的密切討論與合作。

In this program, we propose to study several very fundamental physics topics in mesoscopic and nanoscale structures as well as in disordered systems. (1) We propose to investigate the effects of dynamical structural defects (i.e., defects with internal degrees of freedom) on the electronic transport in disordered samples. The transport behavior due to interactions between conduction electrons and dynamic defects (often modeled as two-level tunneling systems) will be investigated. The roles of dynamic defects in governing the electron dephasing time at very low temperatures will be studied. (2) We will measure the differential conductances of metal/insulator/metal tunnel junctions. The suppression of electronic density of states at the Fermi level in disordered nano-granular metals, which results in profound zero bias conductance dips, will be studied. In addition, we will study the important case of tunnel junctions which show zero-bias conductance peaks. Such conductance peaks may arise from an enhanced tunneling current due to magnetic or orbital Kondo effect. The scaling behavior of dI/dV with temperature and bias voltage will be explored at low temperatures and in high magnetic fields. (3) We plan to continue our measurements of electron dephasing time at ultra-low temperatures to clarify the issue of whether the dephasing time near the absolute zero temperature should diverge or saturate to a finite value. The electron-phonon scattering time in the dirty limit as well as the electron-phonon-impurity interference effect in disordered metals will also be pursued. (4) We plan to carry out electrical measurements on individual metal as well as semiconductor nanowires to investigate the fundamental electrical transport properties, bearing in mind to search for possible evidence of non-Fermi liquid states in nanostructures. In order to clarify the roles of dynamical structural defects and to measure the electron dephasing time at ultra-low temperatures and in high magnetic fields, we propose to further strengthen our ongoing international collaborations with Dr. Kimitoshi Kono, Chief Scientist of the Low Temperature Physics Laboratory at RIKEN, Japan. Dr. Kono’s Laboratory is equipped with several high-cooling-power 3He-4He dilution refrigerators, and is one of the most prestigious low temperature physics laboratories in the world. We will also initiate a “NCTU-RIKEN-Tokyo” collaborative project on spin-dependent transport in vertical double quantum dots. We shall establish close collaborations with theorists to understand the underlying physics of our experimental results.