| Summary: | 博士 === 國立交通大學 === 物理研究所 === 103 === 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.
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