Strong electron and spin correlations in a double-quantum-dot (DQD) can give rise to different quantum states. We observe a continuous transition from a Kondo state exhibiting a singlepeak Kondo resonance to another exhibiting a double peak by increasing the inter-dot-coupling (t) in a parallel-coupled DQD. The transition into the double-peak state provides evidence for spinentanglement between the excess-electron on each dot. Toward the transition, the peak splitting merges and becomes substantially smaller than t because of strong Coulomb effects. Our device tunability bodes well for future quantum computation applications. The double-quantum-dot (DQD) is emerging as a versatile system for studying a variety of strongly correlated behaviors [1,2,3,4,5,6,7]. Following the experimental demonstration of the Kondo impurity-spin screening effect in single quantum dots [8,9,10,11,12,13], recent theoretical investigations of the coupled-DQD system is uncovering new correlated behaviors [1,2,3,4,5,6,7]. These works suggest that the DQD enables a realization of the two-impurity Kondo problem first discussed in the context of metallic systems [1,14,15,16] in which a competition between Kondo correlations and antiferromagnetic (AF) impurity-spin correlation leads to a quantum critical phenomenon. In a different regime of parameters, a related quantum critical phenomenon can occur driven by a competition between intra-dot Kondo coupling to leads and the inter-dot-coupling [2,3]. In each scenario, a transition is predicted to occur between a quantum state characterized by a single-peaked Kondo resonance, and a different state with a double-peaked resonance. Depending on model and DQD geometry-whether series or parallel coupled-both a continuous or discontinuous [6] behavior in the Kondo peak characteristics have been predicted. The quantum transition in the two-impurity Kondo problem has received wide attention in the theoretical literature in the past two decades, to a large extent because the Kondo to antiferromagnetic transition involves an unusual non-Fermi liquid fixed point. Experimental investigation of this problem thus far has not been reported.Here we describe transport properties of an artificial molecule formed by two-path, parallel-coupled doublequantum-dots, where the inter-dot-tunnel-coupling, t, can be tuned. In the Kondo regime the differentialconductance, dI/dV, exhibits a single peak centered at zero-bias for t comparable to the lead-coupling induced * Electronic address: yingshe@physics.purdue.edu level broadening. Increasing t by less than 10% resulted in a continuous evolution into a split Kondo resonance. At the same time the conductance at zero-bias exhibits a maximum in the vicinity of the transition. This peak splitting behavior in conjunction with distinct temperature dependences in the different regimes demonstrates a direct observation of an inter-dot-coupling-induced quantum transition. Moreover, on the double peak side the zero-bias conductance becomes suppressed; this suppression represents ...
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