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(Color online) (a) Scanning electron microscope image of the etched parallel-coupled graphene double dot sample structure. The bar has a length of 500 nm. The diameters of the two dots are both 100 nm, constriction between the two dots is 35 nm in width and length. The four narrow parts connecting the dot to source and drain parts have a width of 30 nm. Seven in-plane plunger gates labeled as GL, GR, GM, PSL, PDL, PSR, and PDR are integrated around the dot for fine tuning. (b) Schematic picture of the device. N-type heavily doped silicon substrate is used as a global back gate. A layer of overexposed PMMA is used as a bridge to make gate GM separated from the drain part of graphene.
(Color online) (a), (b), and (c) PDQD differential conductance as a function of plunger gate voltage VGL and VGR. The red dash lines are guides to the eyes showing the honeycomb pattern. (N,M) represents the carriers in the left and right dot, respectively. (b) Zoom-in of the area (N,M) of the honeycomb pattern. (c) Zoom-in of a vertex pair with white dash lines. (d) Capacitance model for the analysis of the double dot system. Graphene nano-constrictions behave as tunneling barriers, which are presented, for example, as RSL, CSL (a capacitance and a resistance coupled in parallel). Gate GL and GR are capacitively coupled to the dots; CGL and CGR represent the capacitance.
(Color online) Interdot coupling vs middle gate voltage VGM. Conductance as a function of gate voltage VGL and VGR at VBG = 3 V, Vbias = −1 mV, the scan regions of GL and GR are the same. (a), (b), and (c) represent three different coupling regimes of the two dots. (a) weak coupling regime, VGM = −0.15 V, (b) medium coupling regime, VGM = −0.2 V (c) strong coupling regime, VGM = 0.45 V. (d) shows coupling energy ECM(V) as a non-monotonic function of the middle gate voltage VGM. A, B, C point here represent the corresponding coupling energy in (a), (b), and (c).
(Color online) (a) SEM image of single layer graphene PDQD integrated with two QPCs. The bar has a length of 500 nm. (b) Characteristic honeycomb structure of the conductance through the PDQD as a function of two in-plane plunger gates voltage VGL and VGR, revealed by direct transport measurement of the PDQD at 4.2 K.
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