二雜質近藤效應量子相圖之完整實驗對應

Abstract

本研究中，我們藉由製造一系列(銅/鋁)/氧化鋁/釔之平面穿隧接面來觀測二雜質近藤效應相圖中的不同基態。在(銅/鋁)/氧化鋁/釔穿隧接面中，釔原子會擴散進入氧化鋁能障中，形成自旋½之磁性雜質，因而導致近藤效應。我們可藉由改變能障性質來調整磁性雜質與電極間的耦合強度。 我們測量了十幾個(銅/鋁)/氧化鋁/釔穿隧接面的微分電導G(V,T)。在所有樣品中皆可觀測到了一段弱耦合近藤效應區間，即微分電導G(V,T)有-logT行為，溫度約在10 – 35 K之間。然後溫度在約2 – 25 K時，可觀測到一段量子臨界區域，展現了非費米液體行為。而此區間中微分電導會遵守二雜質近藤效應的普適尺度形式。 隨著溫度持續降低，各穿隧接點表現的行為會過渡進入三種不同的基態。第一種是近藤屏蔽態(費米液體)，第二種是RKKY局域自旋單重態(費米液體)，最後一種則坐落於量子臨界點附近。在這三種基態中，微分電導在溫度、外加偏壓或磁場中表現出截然不同的行為。此外，以這種穿隧接面的方式可以達到非常高的近藤溫度(約40 K)，可藉此來更廣闊地探索量子相變，而且對於許多強關聯電子系統可能很有研究潛力。

In this work, we fabricated a series of (Cu/Al)/AlOx/Y planar tunnel junctions which are on the different location of the phase diagram for two-impurity Kondo effect. In (Cu/Al)/AlOx/Y tunnel junctions, the Y atoms will diffuse into AlOx barrier to be treated as the spin-½ magnetic impurities and result in Kondo effect. We tune the coupling strength between magnetic impurity and electrode by modulating barrier properties. We measured the differential conductance G(V,T) in a dozen of (Cu/Al)/AlOx/Y tunnel junctions. For all junctions, we can observe the weak coupling behavior, i.e. G(0,T) is -logT dependence, between about 10 – 35 K. And a quantum critical regime also can be observed in the range about 2 – 25 K, in which shows non-Fermi liquid behavior. G(V,T) obeys the two-impurity Kondo universal scaling form. As the temperature further decreases, the G(V,T) of tunnel junctions will cross over to three different behaviors. First one is the Kondo screened ground state (Fermi liquid behavior), second one is the RKKY local spin singlet state (Fermi liquid behavior), and the other one is in the vicinity around quantum critical point. G(V,T) clearly shows three different behavior with varying temperature, bias voltage and applied magnetic field. Furthermore, the versatile tunnel junction can achieve very high TK (~ 40 K) to explore quantum phase transition in a wide window, and may be of potential relevance to many strongly correlated electron systems.