介觀物理下的兩個主題之 壹:顆粒鉻薄膜的電導率及穿隧電子能態密度 貳:單重態自旋電子在量子點中的自旋阻滯

Abstract

在本篇論文中，共分有兩個主題，在主題一當中，我們在雙電子垂直雙量子點系統內，觀察到單重態電子自旋阻滯的現象。相對於之前曾被觀察到因三重態電子所導致的包立自旋阻滯 (Pauli spin blockade)，此現象是在高磁場下被觀察到，而其一量子點的雙電子基態在此磁場下已經過單重態-雙重態轉換 (the singlet-triplet transition)。在菱形庫倫阻滯量測中所觀察到自旋阻滯所發生的區域範圍與實驗中雙子點能階頻譜的結果一致，更支持單重態電子自旋阻滯發生於此系統中。此外，我們在單重態自旋阻滯下發現了約為 10 皮安培的漏電流，此數量級的漏電流與電子單重態在核自旋擾動下的維持時間 (lifetime) 符合。 另一方面，在主題二顆粒鉻薄膜的電導率與穿隧電子能態密度的實驗中，我們在低溫下測量了四個 鋁/氧化鋁/鉻 穿隧接點的穿隧微分電導以及鉻電極的電導率。我們所製作的鉻薄膜電極為三維，並具有顆粒性，而其顆粒間的無維穿隧電導 (dimensionless intergrain tunneling conductance spanning) 介於接近一到遠大於一之間，量測到的穿隧電導曲線在零偏壓附近有一甚大的奇異點，且在低偏壓 (幾個微電子伏特) 下，與偏壓的對數有正比關係，而過渡到高偏壓時，則轉變為與偏壓的平方根成正比。同時，鉻電極的電導在某特徵溫度下亦反映了與溫度的對數有對應關係。此實驗結果可以近年來提出的顆粒金屬理論解釋。此外，在無維穿隧電導介於接近一的樣品，除了鉻電極的電導反映了與溫度的對數有對應關係外，我們亦觀察到歸一化的微分電導 ([G(V,T)-G(0,T)]/√T)，與參數 (√(e|V|/k_B T)) 在絕對溫度 2.5 到 32 K 之間有一統一定比 (universal scaling) 行為，但此結果還需要理論上的解釋。

There are two topics in this thesis. In the first topic, spin blockade with spin singlet electrons, we observe a novel spin blockade in two-electron vertical double quantum dots where the single electron transport is blocked for spin singlet electrons. In contrast to the conventional Pauli spin blockade with spin triplet electrons, this singlet spin blockade (SSB) is observed under high magnetic field, where the doubly occupied states in one of the dots go beyond the singlet-triplet ground-state transition. The SSB region in Coulomb diamond measurements is in agreement with the two-electron excitation spectrum. A leakage current of 10 pA order is observed in SSB, which is consistent with the spin singlet lifetime due to random nuclear spin fluctuations. As for the second topic, conductivity and tunneling density of states in granular Cr films, we have measured the tunneling differential conductances, G(V), of four Al/AlOx/Cr planar tunnel junctions as well as the conductivities, σ(T), of the Cr electrodes at liquid-helium temperatures. The Cr electrodes were made to be granular with dimensionless intergrain tunneling conductance spanning from g ≃1 to g ≫ 1, and the dimensionality of the granular array d =3. For the samples with g ≫ 1, we found that the measured G(V) curves display large zero-bias singularities which obey a ln V law at low bias voltages ( ≲ a few meV), while crossing over to a √V law at high bias voltages. Simultaneously, the conductivities of the Cr electrodes reveal lnT dependence below a characteristic temperature. These results are explained in terms of the recent theory of granular metals. In a sample with g ≃1, in addition to the conductivity dependence σ(T)∝ln T, we observed a universal scaling behavior of the normalized differential conductance [G(V,T)-G(0,T)]/√T with the combined parameter √(e|V|/k_B T) in a wide temperature interval of 2.5 to 32 K. This result awaits a theoretical interpretation.