Minky Seo

Quantum point contact with large subband energy spacings

Quantum point contact (QPC) with an extra metallic gate in between the split gates of a conventional QPC was fabricated and studied. Clear conductance quantization was observed at 4.2 K when a proper positive voltage was set to the middle gate of the QPC. The maximum energy spacing between the ground and the first exited state of the QPC was around 7 meV which is at least a few times larger than that of conventional QPCs. Using same approach, a possibility of making a relatively clean and long 1D wire has been tested.

Single electron pumping through a quantum dot-embedded carbon nanotube using surface acoustic wave

We have studied acoustoelectric current through a quantum dot-embedded carbon nanotube induced by a surface acoustic wave. The measurements were carried out on a same device but in two very different quantum dot charging energy Ec regimes (∼50 and ∼5 meV). The results showed dramatic differences in induced acoustoelectric current depending on Ec. The induced acoustoelectric current showed a polarity reversal around the Coulomb blockade peak when Ec is small (5 meV). For a large Ec (50 meV), however, a current plateau was observed as a function of surface acoustic wave powers as well as gate voltages.

Robustness of potential-profile-tunable electron pump

We performed a robustness test of a potential-profile-tunable electron pump for various parameters. Tuning the entrance and exit gates, we measured a flat current plateau within the 2 μA/A uncertainty for a ~40 mV gate-voltage range at B ~ -14 T. For changes of B-field, the 2-μA/A plateau was obtained for -14 T <; B <; -12.7 T. The 2 μA/A plateau was also observed in the overlapped exit-gate voltage range over ~40 mV at T = 300 mK and 1.3 K. The robustness of the pumping demonstrates stable operation and a feasible way to construct parallel pumps.

Potential-tunable quantum dot single-electron pump

We have investigated a metal-gated QD charge pump device whose potential profile can be controlled. The proposed pump is uniquely designed with several metallic gates on a GaAs/AlGaAs two-dimensional electron gas system. We find that the length of 1st current plateau depends on potential profile of the QD. Under optimal conditions, a quantized current plateau with estimated accuracy of ppm level for the output current of ~80 pA at 500 MHz is observed at 4.2 K in the absence of magnetic fields.

Gate-defined quantum dot single electron pump

We report single electron pumping through a metal gate defined quantum dot (QD) fabricated on a GaAs/AlGaAs 2-dimensional electron gas (2DEG) system. The quantum dot used for pumping is uniquely designed to have larger subband energy spacings than those of a conventional quantum dot. Several quantized current steps were observed at liquid helium temperature when the pump was operated at 100 MHz of pumping frequency. The flatness of the current plateaus was measured with different subband energy spacings. It was found that the flatness of the plateau is influenced by the subband energy spacings of the QD.

Saw-induced current through CNT in coulomb blockade regime

Acousto-electric current through a carbon nanotube (CNT) induced by a surface acoustic wave (SAW) was studied. SAW were generated by an inter-digital transducer (IDT) on a quartz substrate and delivered to a CNT to pump electrons along the direction of the SAW propagation. A distinct current plateau was observed as a function of gate voltages and SAW powers. The observed quantized current value was higher than the expected value I=ef by 20 %. A strong interaction between electrons in a 1D wire could be one of the reasons for the discrepancy.

The Frequency Dependence of Quantized Current Plateaus Induced by Surface Acoustic Waves Through a Quantum Point Contact

The acousto-electric current through a split gate induced by surface acoustic waves (SAW) was studied. The current plateaus were observed when proper frequencies of SAW were applied to the device. The resulting current plateau values corresponded to nef, where n is the number of electrons transferred per cycle and f is the induced SAW frequency. Also, the width of the plateaus was found to change periodically as the SAW frequency changed. Unwanted coupling of the RF signal (applied to inter-digitized transducer to produce SAW) to the gate of QPC were found to cause the changes in plateaus width.