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Excited-state spectroscopy on a quantum dot side coupled to a quantum wire

Introduction

To investigate the states in a quantum dot electrically, we connect the dot to leads. In conventional measurement, we connect the dot to two leads and measure the current through the dot. Recently, it also becomes possible to measure with a side coupled quantum dot, which has only one lead.

But the measurement methods have not been fully established. For example, no method to evaluate the accurate excitation energy has been demonstrated in the side coupled quantum dot.

Here, we present a new scheme to evaluate the energy in the side coupled geometry.


Experiment

The right picture shows the scanning electron micrograph of the device. A quantum dot side couples to a quantum wire on the right side.

A quantum point contact is also formed on the left side and this works as a charge detector of the dot.





Elzerman et al. demonstrated the detection of the excited states by applying square wave voltages on the quantum dot in 2004.

The excited state enhances the electron injection as shown in the right schematics. This is observed as the deepest region in the right image plot.

The width of the shallow dip shown by an arrow in the figure reflects the excitation energy.








The remaining work is to convert the width to the energy. To realize this, we need a reference of the energy scale.

We utilized the non-equilibrium energy distribution which was formed when we applied bias voltage across the short quantum wire as the reference.

The right graph shows the signal comes from the distribution. By using this signal, we evaluated the accurate excitation energy.








To check the validity of the present method, we applied the method to spin excited states. The right graph shows the evaluated excitation energy as a function of the magnetic field. The energy increases linearly with the magnetic field.

The slope is consistent with the reported values in different experiments and this certifies the validity of the present method.




Conclusion

We demonstrated the evaluation of the accurate excitation energy utilizing a biased short quantum wire as a lead.

We applied the method to the spin exited states and checked the validity of the method.


References

"Excited-state spectroscopy on a nearly closed quantum dot via charge detection", J. M. Elzerman, R. Hanson, L. H. Willems van Beveren, L. M. K. Vandersypen, and L. P. Kouwenhoven, Applied Physics Letters 84, 4617 (2004).

"Excited-state spectroscopy on a quantum dot side coupled to a quantum wire", T. Otsuka, E. Abe, Y. Iye, and S. Katsumoto, Applied Physics Letters 93, 112111 (2008).