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Ultrafast terahertz control of extreme tunnel currents through single atoms on a silicon surface

Abstract

Ultrafast control of current on the atomic scale is essential for future innovations in nanoelectronics. Extremely localized transient electric fields on the nanoscale can be achieved by coupling picosecond duration terahertz pulses to metallic nanostructures. Here, we demonstrate terahertz scanning tunnelling microscopy (THz-STM) in ultrahigh vacuum as a new platform for exploring ultrafast non-equilibrium tunnelling dynamics with atomic precision. Extreme terahertz-pulse-driven tunnel currents up to 107 times larger than steady-state currents in conventional STM are used to image individual atoms on a silicon surface with 0.3 nm spatial resolution. At terahertz frequencies, the metallic-like Si(111)-(7 × 7) surface is unable to screen the electric field from the bulk, resulting in a terahertz tunnel conductance that is fundamentally different than that of the steady state. Ultrafast terahertz-induced band bending and non-equilibrium charging of surface states opens new conduction pathways to the bulk, enabling extreme transient tunnel currents to flow between the tip and sample.

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Figure 1: Control of extreme tunnel currents with THz-STM in ultrahigh vacuum.
Figure 2: Imaging silicon atoms with terahertz-driven scanning tunnelling microscopy (TD-STM).
Figure 3: Modelling TD-STM on Si(111)-(7 × 7) in the extreme tunnel current regime.
Figure 4: Ultrafast control of non-equilibrium tunnelling through single atoms on Si(111)-(7 × 7).
Figure 5: Electric field spatial profiles obtained from 3D electromagnetic simulations.
Figure 6: Hot electrons in the terahertz-induced current.

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Acknowledgements

We acknowledge fruitful discussions with T. Cocker, R. Huber, C. Ropers, R. Wolkow, J. Burgess and D. Jenson. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), the Alberta Science and Research Investments Program (ASRIP), the Alberta Innovates Technology Futures (AITF) Strategic Chairs Program, and the Informatics Circle of Research Excellence (iCORE) Centre for Interdisciplinary Nanoscience (iCiNano). V.J. acknowledges support from NSERC and AITF. K.I. acknowledges support from the Danish Council for Independent Research under Postdoc Project 64092. C.R. acknowledges support from the German Academic Exchange Service (DAAD). We thank R. Wolkow for providing the Si sample, S. Xu (Alberta Centre for Surface Engineering and Science, University of Alberta) for scanning electron microscope imaging of the tungsten tip and A. He for secondary ion mass spectrometry of the Si sample. We are grateful for technical support from G. Popowich, D. Fortin and B. Shi. Electromagnetic simulations were carried out using COMSOL Multiphysics with licence provided by CMC Microsystems.

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V.J. and F.A.H. conceived the UHV THz-STM experiment, interpreted the data and designed the set-up. V.J. and J.R.H. built the set-up. V.J. carried out experiments, analysed the data and wrote the manuscript. V.J. and K.I. developed the Bardeen tunnelling model, hot electron model and fits to the data. P.H.N., C.R. and G.J.H. performed the electromagnetic simulations. P.H.N., G.J.H., H.M.S. and J.R.H. contributed to preliminary measurements with the UHV THz-STM system. M.R.F. contributed to interpretation of the data. F.A.H. initiated and supervised the project. All authors contributed to discussions.

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Correspondence to Vedran Jelic or Frank A. Hegmann.

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Jelic, V., Iwaszczuk, K., Nguyen, P. et al. Ultrafast terahertz control of extreme tunnel currents through single atoms on a silicon surface. Nature Phys 13, 591–598 (2017). https://doi.org/10.1038/nphys4047

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