Coulomb blockade and negative differential conductance in metallic

double-dot devices

Single-electron tunneling devices based on the Coulomb blockade enable the manipulation of per-electron currents. Because of high charge sensitivity, low-power dissipation, and high packing density, these structures are proposed for future generations of computational technology and are therefore being extensively investigated both experimentally and theoretically.

Recently, Junno et al.¹ have reported interesting data of differential-conductance measurements for a gold double-dot device. The gold dot is so small that the charging energy is as high as 80 me V, and the Coulomb blockade can be well recognized even at a liquid-nitrogen temperature. The honeycomb stability diagram, typical for the pump devices as designed by Pothier et al.,² has been clearly observed at a liquid-helium temperature. The specificity of the device mea-sured is that there exists an essential cross coupling between a dot and the gate belonging to the other dot [cf., the capacitances C₁₂ and C₂₁ in Fig. 1], which results in a diagonal stretching of the honeycomb cell. An asymmetry of the two junctions, coupling dots to leads, i.e., a difference betweencapacitances C_s and C_d in Fig. 1, is also believed to make the cells skewed and to yield a distinct Coulomb staircase in the current-voltage sI -Vd curves. The charging diagrams have been also measured for silicon germanium double-quantum dots³ but with a focus on the role of electrostatic gates.

The aim of this work is (i) to describe systematically the zero-temperature stability diagrams for the device measured in Ref. 1 and (ii) to simulate the I -V characteristics in the same device, taking into account the finite temperature and the offset charge effects. The results obtained, while describ-ing the experimental data quite well, show a rich variety of features on both the stability diagrams and the I -V curves in dependence on the device parameters. In particular, we find that a negative differential conductance (NDC) of different phases, including a second or a multiple Coulomb gap, canbe manipulated at low temperatures when the coupling capacitances are set with the appropriate values.

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