We employ several strategies for local gating, but rely heavily upon a technique in which we use a vertically integrated geometry which was initially developed by researchers at IBM. A carbon nanotube, grown via chemical vapor deposition is selectively contacted with Ti/Au leads forming tunnel contacts. In addition, nanotubes may have intratube tunnel barriers which form a series double quantum dot. Local gates aligned over each section of the nanotube exhibit a differential effect over the two dots: gate one predominantly couples to dot one and gate two predominantly couples to dot two.
This work represents the first instance in which different sections of a single molecule could be controlled independently. This naturally has great implications for the fabrication of complex coherent electronic devices for applications in quantum computing. Our measurments showed both control over the positions of the energy levels of each dot and the ability to tune the amount of electron hybridization between the two dots.
In addition to such “practical” device significance, we were also able to probe the physics of electron interaction in nanotubes. Semiconductor heterostructure based quantum dots are generally considered model systems for basic physical phenomena. Since we were able to replicate many effects in nanotube double dots we have shown the relevance of nanotubes for studies of charge and spin effects in quantum devices. Additionally some of our data show unexpected phenomena with implications for basic theoretical work on transport in carbon nanotubes. Finally, measuring transport on a double quantum dot allows for the full characterization of device parameters including resolving each component of the total device capacitance. Previously, only capacitance ratios were accessible.
The use of carbon nanotubes in this study is particularly important due to their tiny dimensions. The tubes are approximately 1-2 nm in diameter and the device studied here is approximately 1mm in length, which has been broken into two independently controllable sections. Since the nanotubes are so small, quantum mechanical effects are visible at much higher temperatures than those needed for measurements in comparable semiconductor heterostructure devices. Although the measurement of our device still required cryogenic temperatures, the ability to apply our technology to future devices which have been scaled down in length is vitally important. Each of the quantum dots in the double dot we fabricated was approximately 500nm long. If that length is scaled down to 20nm (which is now achievable using electron beam lithography), quantum effects would be accessible at room temperature! This makes them ideal candidates for practical applications of quantum electronics, and their long predicted spin coherence lives make them especially attractive for quantum computation, where universal computation schemes have been theoretically devised using double quantum dots as basic circuit elements.
FIG. 1(A) Schematic of top-gated device. (B) Electron micrograph of a representative device. Arrows indicate the embedded nanotube.
FIG. 2. Experimental charge stability diagrams for the series double quantum dot as a function of two gate voltages, each shifting the energy levels of a single dot. Voltages VG1 and VG2 are divided by five before being applied to the gates of the device. (A) Color scale displays dI/dVG1 calculated from dc current (IDC) at VSD=500mV. White lines are guides to the eye showing the honeycomb pattern of peaks in conductance. Vertex pairs correspond to points of degeneracy between the two dots where resonant transport occurs, while cotunneling may produce finite conductance along the honeycomb edges.
For more information contact Michael Biercuk (biercuk@fas)